Spectroelectrochemical study of carbon structural and functionality characteristics on vanadium redox reactions for flow batteries

Vanadium redox flow batteries have applications for large-scale electricity storage. This paper reports the influence of carbon structural characteristics of sustainable walnut shell-derived carbons in carbon/polyvinylidene fluoride composite electrodes on vanadium redox reactions. Pyrolysis, gasification, and chemical treatment procedures were used to modify the structural characteristics of carbons. Carbon functional groups were modified by chemical treatment with HNO3, heat treatment with K2CO3, and high-temperature NH3 treatment. Carbon porous structures were characterized using gas adsorption studies. Raman spectroscopy and X-ray diffraction were used to characterize the carbon molecular structure. Functional groups were characterized using X-ray photoelectron spectroscopy, acid/base titrations, temperature-programmed desorption, and Fourier transform infrared spectroscopy. The influence of carbon structure, porosity, and surface functional groups on the redox reactions of vanadium was investigated using cyclic voltammetry and electrical impedance spectroscopy. The VO2+/VO2+ and V2+/V3+ couples had well-defined peaks in cyclic voltammetry, with the former being the most intense, but the V3+/VO2+ couple was not observed for samples carbonized under nitrogen. The results show that V2+/V3+ and VO2+/VO2+ couples observed in cyclic voltammograms were enhanced for carbonization temperatures up to 800 °C. Electrical impedance spectroscopy also showed impedance trends. The electrochemistry results are primarily related to changes in carbon structure and the catalysis of V3+ oxidation by surface functional groups in the carbon structure. The V3+/VO2+ couple was limited by slow kinetics, but it occurs on specific oxygen and nitrogen sites in the carbon structure. The oxidation of V(iii) to V(iv) only occurs on a limited number of surface sites, and the outer-sphere electron transfer to oxidize V(iii) takes place at much more positive potentials. The coulombic, voltage, and energy efficiency of the carbon electrodes were suitable for batteries.


Introduction
2][3] The main advantages of VRFBs include high safety (nonflammable, no risk of thermal runaway), good scalability, and low contamination between cathode and anode due to only one metal being used in electrolyte solutions.5][6] Graphite or graphite composite materials employed as bipolar plates/electrodes are one of the main components in VRFBs due to their low cost, high electric conductivity, wide potential window, and high chemical resistance to both acidic and oxidizing environments. 7,8][11] Biomass-derived activated carbons 12 have received attention as a potential renewable carbon source for VRFBs since biomassderived carbon is more cost-effective than its counterparts and provides high porosity and oxygen functional group contents. 13,14herefore, using biomass-derived carbon can help reduce the cost of VRFBs and potentially enhance performance.
Functional groups can act as adsorption sites and modify the dispersion of surface catalytic particles. 15,16The impact of functional groups, porous structure, and carbon structure on the kinetics of vanadium redox reactions have been studied on many forms of carbon (carbon nanotubes, carbon blacks, graphite flakes, graphite felt, etc.).Nitrogen and phosphorus functional groups in pristine polyaniline graphite felt (a nonporous structure with very low surface area (o1 m 2 g À1 ) demonstrated catalytic effects for both VO 2+ /VO 2 + , V 3+ /VO 2+ , and V 2+ /V 3+ increasing the reversibility of vanadium reactions and reducing interface transfer resistance compared to the original electrodes. 9An increase in the surface area of graphite felt (18 m 2 g À1 ) compared to untreated graphite felt (3 m 2 g À1 ) resulted in a reduction in overpotential, higher energy efficiency, and electrolyte utilization in VRFBs. 17Density functional theory calculations on graphite electrodes also showed that nitrogen, phosphorus, and boron-doped graphite increased the wettability and electronic conductivity towards vanadium redox reactions. 10Multiwalled carbon nanotubes were used to ascertain the impact of phosphorus content on the electrocatalytic activity of the VO 2+ /VO 2 + couple with phosphorus enhancing the electrocatalytic properties of the activity of electrodes, which led to higher energy efficiency than nondoped electrodes. 18iomass-derived carbons are complex with a wide range of pore size distributions, surface areas, and functional groups, which may vary widely depending on the biomass precursor and carbonization conditions.Carbons derived from orange peel, 13 coconut shell, 14 and sal wood sawdust 19 had high mesopore and micropore contents and BET surface areas up to B1900 m 2 g À1 .The high mesopore contents in orange peelderived carbon 13 and coconut shell-derived carbon 14 significantly enhanced the kinetics of vanadium redox reactions and increased the energy efficiency of VRFBs compared to pristine graphite plates.However, it was shown that coffee bean-derived carbon had the highest micropore and mesopore, but the biochar did not have improved electron transfer kinetics or battery efficiency for vanadium redox flow batteries. 20Maharjan et al. showed that sal wood-derived carbon with a micropore volume of B0.42 cm 3 g À1 and a mesopore volume of B0.93 cm 3 g À1 improved the kinetics of vanadium redox reactions, and the energy efficiency of this activated carbon was the same as pristine graphite plate. 192][23] The surface oxygen functional groups in these biomass-derived carbons were identified using X-ray photoelectron spectroscopy (XPS) analysis, 13,14,19 which provides the surface composition (up to B5 nm) rather than bulk oxygen content for organic materials. 24Furthermore, the oxygen functional group distribution in carbon micropores and mesopores may have different influences on the kinetics of vanadium redox reactions due to the accessibility of vanadium ions.In addition, the role of carbon structure p-p* in electron transfer is also a critical factor influencing electrical conductivity in carbons. 25n VRFBs energy is stored as redox pairs in the electrolyte solutions.The carbon electrodes function as catalysts but do not undergo redox reactions during discharge/charge operations. 26The complexity of vanadium flow battery reactions is well established, especially at highly acid conditions, and there are usually mixtures of surface functional groups present on heterogeneous carbon surfaces.Most studies of the electrochemistry of vanadium reactions and surface chemistry are for dilute electrolyte solutions, and more work is required on concentrate solutions where hydrolytic, polymerization, and ion association processes may occur. 27The aim of the research was to increase insight into the impact of carbonaceous structure and surface functional groups in sustainable biomassderived carbons on the catalysis of vanadium redox reactions for concentrated electrolyte solutions.Currently, VFRBs are constrained by the electrochemical activity of carbon electrodes and a limited understanding of surface redox reactions.The kinetics of vanadium species are often slow on carbon surfaces, and hence there is interest in functionalization of carbon surfaces to improve operation.It is not clear if electrode oxidation treatment improves the performance of both electrodes to the same extent. 28In this paper, the carbon structure and surface functional group characteristics of biomass carbons derived from walnut shells were varied systematically using heat treatment temperature (HTT), hold time at maximum heat treatment temperature, and carbonization atmosphere (N 2 and CO 2 ).Chemical treatment methods were also used to incorporate nitrogen and oxygen functional groups into the carbons to increase understanding of their role in redox reactions in vanadium flow batteries.This multifactorial study investigates carbon structure-reactivity relationships necessary for understanding and improving electrode performance in vanadium redox flow batteries.The series of carbons used were predominantly ultramicroporous and had very similar porous structures thereby eliminating the porous structure as a factor in the electrochemistry.The fundamental issues are the mechanisms through which changes in carbon molecular structure and the functionalization of carbon surfaces affect the electrochemistry of vanadium under electrolyte and electrode conditions reasonably close to operating conditions but simplified to concentrate on the details of the electrochemistry.nitrogen or carbon dioxide at heat treatment temperatures (HTTs) of 600-1000 1C with a heating rate of 8.5 1C min À1 to vary the structures of carbon samples.The samples were labeled as X/T-h, in which X indicates the carbonization atmosphere (X = N 2 or CO 2 ), T is the final pyrolysis temperature in 1C, and h is the hold time in hours after the system reached the heat treatment temperature.An example of the nomenclature is carbon CO 2 /800-1 prepared by pyrolysis of the biomass in a CO 2 atmosphere at an HTT of 800 1C with a 1 h hold time.

Experimental
2.1.2.K 2 CO 3 -treated carbon.15 g of carbon CO 2 /800-1 was soaked in 50 mL of deionized (DI) water mixed with 15 g K 2 CO 3 for 24 h.This carbon was dried overnight in an oven at 60 1C and then heat treated at 800 1C for 2 h under a CO 2 atmosphere (100 mL min À1 ).The carbon was then washed with HCl (0.2 M) and DI water until the pH was B7 and then extracted using a Soxhlet apparatus with 400 mL DI water to remove residual water-soluble compounds.This carbon was denoted as CO 2 / 800-1-K 2 CO 3 /800.
2.1.3.HNO 3 -treated carbons.10 g of carbon CO 2 /800-1 was refluxed in 200 mL HNO 3 (7.5 M) at 95 1C for 6 h.This carbon was washed with DI water until the pH was B6-7 and then extracted using a Soxhlet apparatus with 400 mL of DI water for at least 72 h to remove residual acid and water-soluble compounds in the AC.This carbon was denoted as CO 2 /800-1-HNO 3 .This carbon was treated at temperatures of 400 and 800 1C in an H 2 atmosphere (40 mL min À1 ) for 2 h, and the final carbons were named CO 2 /800-1-HNO 3 /400 and CO 2 /800-1-HNO 3 /800, respectively.

Material characterization
2.2.1.Chemical analyses.Elemental analyses for C, H, N, and O were determined by Microanalytical Services, Okehampton, UK.Oxygen analysis was obtained using the Unterzaucher pyrolysis method. 29.2.2.Porous structure characterization.The porous structure was analyzed using CO 2 adsorption at 0 1C and N 2 adsorption at À196 1C.The CO 2 adsorption/desorption isotherms were obtained using an intelligent gravimetric analyser (IGA) supplied by Hiden Isochema Ltd, Warrington, UK.The Dubinin-Radushkevich (DR) ultramicropore volume was obtained from CO 2 adsorption at 0 1C within the relative pressure range p/p 0 = 2.7 Â 10 À7 -3 Â 10 À2 .N 2 adsorption and desorption isotherms at À196 1C were recorded using a Thermo Fisher Scientific Surface Analyzer, and the isotherms were used to determine the Brunauer-Emmett-Teller (BET) surface area using the linear region of the BET graph up to p/p 0 = B0.1.The Dubinin-Radushkevich (DR) micropore volume was obtained from N 2 adsorption at À196 1C within the relative pressure range up to p/p 0 = 3 Â 10 À2 .The amount of N 2 adsorbed at À196 1C and p/p 0 = 0.995 was used to calculate the total pore volume.
2.2.3.X-ray photoelectron spectroscopy (XPS).The spectra were recorded using a Thermo Fisher Scientific K-alpha + spectrometer and a micro-focused monochromatic Al X-ray source (72 W) with an area of B400 mm 2 .Spectra were recorded at pass energies of 150 eV for survey scans with 1 eV step sizes and 40 eV for the high-resolution scans with 0.1 eV step sizes.The C 1s spectra were recorded over the binding energy ranges 275-300 or 279-300 eV.The O 1s and N 1s spectra were recorded over the binding energy ranges 525-545 eV and 392-410 eV, respectively.Charge neutralization of the sample was attained using a combination of low-energy electrons and argon ions.Curve fitting analysis was performed in CasaXPS software using a Shirley-type background and Scofield cross sections, with an energy dependence of À0.6.
2.2.4.Temperature programmed desorption (TPD).The TPD profiles were obtained using a Netzsch Jupiter STA 449C thermogravimetric analyzer interfaced with an Aeolos QMS 403 quadrupole mass spectrometer.The sample was subjected to a series of depressurization/pressurization cycles with He to remove air from the system before analysis.The sample (16.0 AE 0.5 mg) was placed in an Al 2 O 3 crucible and covered with an aluminum perforated mesh to ensure no sample was lost during the heating procedure.The biochar sample was heated at 10 1C min À1 until 1250 1C with He as the carrier gas (35 mL min À1 ).The gas evolution profiles were monitored as a function of temperature for mass/charge (m/z) ratios: 12, 14, 17, 18, 28, 30, 32 and 44.
2.2.5.Titration studies.The oxygen functional group contents of the carbons were investigated using the acid/base titration method developed by Boehm. 300.2 g aliquots of carbon were reacted at ambient temperature with 25 mL of NaOH (0.1 N), NaHCO 3 (0.1 N), Na 2 CO 3 (0.1 N), and HCl (0.1 N) for 72 h.The excess acid and base were determined by titration with NaOH (0.1 N) and the excess base by titration with HCl (0.1 N), respectively.The concentrations of acidic sites were calculated using the assumptions that NaOH reacts with carboxylic, lactonic, and phenolic groups, Na 2 CO 3 reacts with carboxylic and lactonic groups, and NaHCO 3 reacts only with carboxylic groups.The total content of basic carbon sites was determined from titration with HCl (0.1 N).
2.2.6.Raman spectroscopy.A Horiba LabRAM HR Evolution Raman spectrometer was used to analyze the carbonaceous molecular structure (exciting line wavelength, l = 532 nm, source power 455 mW).A non-dispersive filter was used to reduce the power at the sample to B1% of the laser power.The samples were examined using an Olympus LMPLFLN20Â magnification objective.Each Raman spectrum was obtained as ten accumulations of 10 s acquisitions.OriginPro software was used to curve fit the Raman spectra in the range 800-1800 cm À1 (first-order region).The background was subtracted by forming a linear baseline between the spectral regions 800-950 cm À1 and 1750-1850 cm À1 , where the signals were constant.
2.2.7.Fourier transform infrared spectroscopy (FTIR).The carbon spectra were measured using a PerkinElmer Spectrum 2 Fourier transform infrared spectrometer fitted with a diamond attenuated total reflectance (ATR) accessory.Samples were dried and compacted on the diamond crystal.Spectra were

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Paper recorded for the 400-4000 cm À1 range by averaging 4 scans.The spectral resolution was 4 cm À1 .2.2.8.Powder X-ray diffraction (PXRD).Profiles were measured using a PANalytical X'Pert Pro MPD instrument with a Philips PW3040/60 X-ray source (Cu K a radiation, l = 1.540598 nm) and an X'Celerator* detector.The PXRD profiles were scanned between 5-801 with a scan step of 0.033421.The Scherrer equation was used to determine the carbon crystallite thickness (L c ) and apparent crystallite diameter (L a ), 31,32 which are described by the following equations: where l XRD is the wavelength of the incident X-ray radiation, b 002 and b 100 are the full-width at half peak maximum of the (002) and ( 100) peaks, respectively, and y 002 and y 100 are the diffraction angles of the (002) and ( 100) peaks.The interlayer space (d 002 ) was determined using the Bragg equation: 2.2.9.Transmission electron microscopy.Transmission electron microscopy bright-field (TEM-BF), transmission electron microscopy dark-field (TEM-DF), high-resolution TEM (HR-TEM), and electron diffraction were performed on an FEI Tecnai T20 AEM microscope.

Electrode preparation in nitrogen.
A sample of carbon (90 wt%) and polyvinylidene fluoride as the binder (10 wt%) were thoroughly mixed, and a homogeneous slurry was fabricated by dispersing the mixture in N-methyl-2-pyrrolidone solution (100 mg mL À1 ).The electrodes were made by coating the slurry (30 mL cm À2 ) on one surface of a 0.6 mm thick expanded bipolar plate (TF6, SGL Carbon).Electrodes had a coated area of 1 cm 2 (1 cm Â 1 cm) and a mass of 0.105 g.

Cyclic voltammetry (CV).
Cyclic voltammetry was recorded using a three-electrode cell with a PARSTAT potentiostat (PMC-1000) using 1.6 M V 3.5+ in 4.5 M total sulfate electrolyte solution (GFE, Nuremberg, Germany).The counter electrode (CE) was a platinum wire, and the reference electrode (RE) was a saturated mercury electrode in 1 M H 2 SO 4 (MSE).The applied voltage window was between À1.3 and 1 V vs. RE.The CV graphs reported in this article are steady-state CVs obtained after a full cycle scan with the voltage window mentioned above.Scan rates of 5, 10, and 20 mV s À1 were investigated, but studies mainly used a scan rate of 20 mV s À1 .All measurements were carried out under a nitrogen atmosphere at 20 1C.

Electrical impedance spectroscopy (EIS)
. A V 4.5+ solution (50% V 4+ and 50% V 5+ ) was obtained from the commercial 1.6 M V 3.5+ in a 4.5 M total sulfate electrolyte solution (GFE, Nuremberg, Germany).This solution was electrolyzed in a two-compartment flow cell with a cation exchange membrane (Fumatech, GmbH, Bietigheim, Germany) consisting of PANbased carbon felt (SIGRACELL GFD 4.6 EA, SGL Carbon) with thickness 4.6 mm and a bipolar plate (TF6, SGL Carbon).The electrolysis studies were carried out under an N 2 atmosphere to prevent the oxidation of electrolyte ions.A V 4.5+ solution was used for electrochemical impedance spectroscopy (EIS), and the electrode preparation method used for the CV experiments was also used for EIS.The EIS was also recorded in a three-electrode cell with the same experimental setup used in the CV experiments.An MGP-2 battery test station potentiostat was used to record EIS, using the frequency range from 100 mHz to 20 kHz at an alternating current signal of 10 mV.

Static cell configuration performance
The charge-discharge performance of the carbons was evaluated in a static cell using the procedure reported previously at 20 1C.The PAN-based carbon felt was heat treated at 700 1C (hold time 5 h) in air and allowed to cool down naturally.The treated carbon felt was cut to the precise size of the working electrode 4 cm 2 pieces (2 Â 2 cm) and soaked into 1.6 M V 3.5+ in a 4.5 M total sulfate electrolyte solution (GFE, Nuremberg, Germany).The AC-coated bipolar plate was in contact with the treated carbon felt and the current collector, similar to the VRFB configuration.The two half-cells were separated with an ion exchange membrane.The static cell setup was assembled carefully, with the thickness of the whole cell measured in the range of 23-24 mm to ensure the forces applied during the preparation of different samples were equal.In this setup, carbon felt, ion exchange, and current collector were kept constant to measure the influence of different activated carbons.Additional details of the cell are included in ESI.† Static cell charge-discharge was recorded using a PARSTAT potentiostat (PMC-1000) at 25 1C in an environmental chamber.The cell was stabilized for 1 hour before recording.The chargedischarge tests were performed at constant current densities of 10, 15, and 20 mA cm À2 in the potential cut-off window of 0.9-1.65 V.The coulombic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) were calculated using the following equations: where V d and V c are discharge and charge voltages, respectively, and I d and I c are discharge and charge currents, respectively.

Results and discussion
The structure and surface properties of carbon in carbon/PVDF composite electrodes were varied systematically to study factors

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and process timescales that influence vanadium redox reactions.The primary factors determining carbon structure are the sustainable biomass carbon precursor and the experimental carbonization conditions, mainly heat treatment temperature (HTT), with secondary experimental factors such as hold time, heating rate, and gaseous atmosphere.This study was limited to walnut shell as the carbon precursor, and the carbonization heating rate was constant.The following carbonization variables were investigated: HTT, hold time, and gaseous atmosphere.Chemical treatment, gasification, and thermolysis processes were also used to modify the carbon surface functionality.Three series of samples were used in this study to explore the role of carbonization conditions and chemical treatment procedures on structural carbon characteristics and the impact on carbon/PVDF composite electrode properties.These series were as follows: (1) Series 1: carbonization in a relatively inert nitrogen atmosphere at various HTTs (600-1000 1C) (2) Series 2: carbonization in a carbon dioxide atmosphere at various HTTs where some partial gasification occurs via the Boudouard reaction (700-1000 1C).This results in incorporating some functional groups and activating the porous structure.
(3) Various chemical treatment procedures incorporated functional groups into carbons with HTT 800 1C.The treatment procedures used in this study were (a) oxidation with nitric acid combined with heat treatment in H 2 up to 800 1C to remove oxygen functional groups progressively, 33,34 (b) treatment with K 2 CO 3 at 800 1C, and (c) NH 3 treatment at 600 and 800 1C. 34,35he chemical treatment, gasification, and thermolysis procedures were limited to a maximum of 800 1C so as not to introduce structural differences due to higher carbonization temperatures.

Analytical data
Extensive analytical studies of biomass decomposition in an inert atmosphere to form biochars have established a relationship between the H/C ratio and heat treatment temperature, aromatic clusters, and sorption characteristics.HTT was the main factor, with the H/C ratio decreasing from 1.45-1.8at 150 1C to 0.2-0.45 at 600 1C.The H/C ratio and HTT relationship have a quantitative reverse sigmoidal shape up to 700 1C, independent of the biochar precursor. 36Table 1 gives the elemental analysis for carbon, hydrogen, nitrogen, and oxygen on a dry ash-free (daf) basis for the carbons used in this study.The walnut shell used had an ash content of 0.65 wt% db.The ash contents of the carbons were in the range of 0.26-3.86wt% db.The H/C ratios for carbons N 2 /600-1 and CO 2 /700-1 were 0.29 and 0.26, respectively.These values are within the H/C ranges obtained for the HTTs for biochars prepared from various precursors. 36The O/C ratios for carbon N 2 /600-1 (0.055) and carbon CO 2 /700-1 (0.088) were also similar to those of biochars with the same HTT. 37The H/C and O/C ratios for Series 1 (N 2 atmosphere) carbons decrease with increasing HTT (600-1000 1C).In contrast, the H/C and O/C ratios for Series 2 (CO 2 atmosphere) have a different trend with only small changes in the H/C ratio, and the O/C ratio increases markedly for HTT of 1000 1C.The latter is attributed to the Boudouard reaction increasing the surface oxygen groups.The effect of incorporating oxygen and nitrogen functional groups into the carbon structure was studied by combining chemical treatment, gasification, and thermolysis procedures.

Gas adsorption-desorption characteristics
9][40] The N 2 adsorption at À196 1C was very low compared with CO 2 adsorption at 0 1C for carbon samples prepared by carbonization under a nitrogen atmosphere (Series 1).Nitrogen adsorption measurements at À196 1C are sometimes unsuitable for ultramicroporous carbons due to activated diffusion effects. 41herefore, CO 2 adsorption at 0 1C was used to obtain an ultramicropore volume of porous carbons using the Dubinin-Radushkevich (DR) equation.The CO 2 adsorption and desorption isotherms at 0 1C were reversible and Type I in the IUPAC Classification Scheme 42 with no hysteresis for carbons N 2 /600-1, N 2 /700-1 and N 2 /800-1 (Fig. S1, ESI †).The DR model fitted the adsorption isotherms for the relative pressure range p/p 0 0-0.03 (Fig. S2, ESI †), indicating that the

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pore size distributions in the carbons were Gaussian 43 and the ultramicropore volumes are given in Table 2.The CO 2 adsorption kinetic profile measurements at 0 1C for the adsorption and desorption isotherms confirmed slow diffusion into the porous structures for carbons N 2 /600-1, N 2 /700-1, and N 2 /800-1.
The kinetic profiles were fitted to a stretched exponential model (Fig. S3, ESI †).The kinetics are faster than those reported for CO 2 adsorption on carbon molecular sieves used for air separation. 44However, the higher carbonization temperature for carbon N 2 /1000-1 annealed the carbon structure, and CO 2 adsorption, even at 30 1C was very slow with equilibration times per isotherm point estimated to be B1.5 Â 10 5 s, which prevented isotherm measurements (see ESI †, Fig. S3d).This slow diffusion of CO 2 into the porous structure confirms activated diffusion effects and has implications for liquid phase diffusion of electrochemical species into the porous structure, which are much slower.
In contrast to the series of carbons prepared under a nitrogen atmosphere (Series 1), the corresponding carbons prepared in a CO 2 atmosphere (Series 2) adsorb nitrogen at À196 1C and have a Type I/Type II isotherm with a steep uptake at low p/p 0 corresponding to micropore filling (Fig. S4, ESI †).The N 2 isotherms at À196 1C have minimal adsorption/ desorption hysteresis (Type H4 in the IUPAC classification scheme) 42 consistent with small amounts of mesoporosity in the carbons.The hysteresis loops close at p/p 0 B0.4-0.45,corresponding to a pore size of B4 nm.The mesopore size distributions are shown in Fig. S4 and S9 (ESI †).All the samples have very similar mesopore size distributions with peaks in the range 3.5-4 nm with a sharp decrease in the pore diameter range 4-5 nm.The mesopore distributions show that all carbon samples have only a minimal mesoporosity o6 nm.Both BET and DR graphs were linear (Fig. S5 and S6, ESI †), and the surface area and micropore volumes are given in Table 2.][40] Carbon CO 2 /1000-1 had a high DR CO 2 micropore volume of 0.2948 AE 0.0059 cm 3 g À1 due to partial gasification activating the porous structure, whereas carbon N 2 /1000-1 had very narrow porosity due to thermal annealing.The DR CO 2 graphs are linear (Fig. S2 and S8, ESI †), and CO 2 micropore volumes increased with increasing carbonization temperature, from 600 1C to 800 1C for carbons prepared under both nitrogen and carbon dioxide atmospheres, and this is attributed to the development of porous carbon structure with loss of volatiles in the case of N 2 atmosphere and in addition, some gasification in a CO 2 atmosphere, which increases with increasing temperature.The N 2 DR micropore volumes (o2 nm) are 76-95% of the corresponding DR CO 2 micropore volumes (o0.7 nm) for Series 2 carbons made by carbonization in a CO 2 atmosphere.The values are closer to 1 for higher HTT and longer hold time, indicating an increasing activation level.This can be attributed to activated diffusion limitations in the ultramicroporosity.
Carbon CO 2 /800-1 was used as a starting carbon material for functionalization studies.K 2 CO 3 was used to modify the structure of carbon CO 2 /800-1.This treatment increased the DR CO 2 ultramicropore volume from 0.2376 to 0.3140 cm 3 g À1 (Fig. S8 and S13, ESI †).The corresponding total pore volumes (TPV N 2 ) increased from 0.266 cm 3 g À1 to 0.334 cm 3 g À1 (Fig. S4 and S9, ESI †).The BET surface area increased from 444.8 m 2 g À1 for carbon CO 2 /800-1 to 673.5 m 2 g À1 for carbon CO 2 /800-1-K 2 CO 3 (Fig. S5 and S10, ESI †).K 2 CO 3 has been reported as an activating agent for increasing the porosity of carbons. 46,47xidation with HNO 3 increased the DR N 2 micropore volumes from 0.1806 cm 3 g À1 for CO 2 /800-1 to 0.2229 cm 3 g À1 CO 2 /800-1-HNO 3 indicating an increase in ultramicroporosity.Heat treatment of CO 2 /800-1-HNO 3 decomposed the labile functional groups, giving CO 2 /800-1-HNO 3 /400, which had a DR N 2 micropore volume of 0.2172 cm 3 g À1 .Carbon CO 2 /800-1-HNO 3 /800 had a DR N 2 micropore volume of 0.2884 cm 3 g À1 , : not determined due to activated diffusion effects for N 2 adsorption at À196 1C; DR VCO 2 ultramicropore volume was determined using DR plot for CO 2 adsorption at 0 1C using a density of 1.023 g cm À3 for adsorbed CO 2 ; 40,45 SA CO 2 ultramicropore surface area, determined from DR V CO 2 using an area of 1.9 Â 10 À19 m 2 for the area of a CO 2 molecule; SA BET was determined from the linear region of the BET graph for N 2 adsorption at À196 1C; DR VN 2 determined using the DR graph for p/p 0 range 0-0.03 and a density of 0.8081 g cm À3 for adsorbed N 2 at À196 1C; TPV N 2 was determined from N 2 adsorption at À196 1C at p/p 0 = 0.995 and a density of 0.8081 g cm À3 for adsorbed N 2 ; V meso was calculated from The error bars for the parameters were derived from linear regression analysis of the corresponding CO 2 and N 2 Dubinin-Radushkevich and BET N 2 graphs.

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indicating that a combination of oxidation and heat treatment increases microporosity.The CO 2 ultramicropore volumes for these samples were similar, with no clear trend.Oxidation of carbon CO 2 /800-1-HNO 3 gave carbons with N 2 total pore volumes in the order.
There is minimal adsorption-desorption hysteresis in the N 2 adsorption isotherms at À196 1C (Fig. S9, ESI †).It is evident that there is a small increase in microporosity in the series, but the overall changes in porous structure are relatively small.
BET surface areas were determined from the linear region of the BET graph that included point B, which covered adsorption data up to a maximum p/p 0 of 0.07-0.12(Fig. S10, ESI †).The C parameter in the BET equation was very high (41000 in all cases), indicating significant filling of micropores.The BET surface areas are very strongly correlated with the DR N 2 micropore volume (R 2 = 0.99) (Fig. S14b, ESI †) but weakly with the N 2 total pore volume (R 2 = 0.74) (Fig. S14a, ESI †).The ratios of DR N 2 micropore volume to CO 2 ultramicropore volume were in the range of 89-114% for chemical treatment procedures.The carbons prepared in a CO 2 atmosphere had ultramicropore volumes 475% of the corresponding total pore volume.However, activated diffusion effects for carbons prepared in a nitrogen atmosphere did not allow a total pore volume to be measured using N 2 adsorption at À196 1C.The functionalized carbons had ultramicropore volumes that were 467% of the total pore volume.Carbons prepared in a CO 2 atmosphere (Series 2) had ultramicropore volumes 475% of the total pore volumes.All the carbons used in this study were predominantly ultramicroporous (o0.7 nm).Therefore, differences in the porous carbon structures in both these series of carbons do not contribute significantly to the CV characteristics of vanadium redox reactions due to diffusion limitations in the liquid phase.

Functional group characterization
Characterization of functional groups in heterogenous amorphous carbons requires a range of techniques.XPS provides surface analysis of the carbon to a depth of B5 nm 51 but requires curve resolution to quantify the components.ATR covers depths of 0.1 to 1 mm depending on wavelength, 52 and the spectra of carbons with HTTs of 800 1C are very weak and broad.Titration measurements provide information on oxygen functional groups (carboxyl, anhydrides, lactone, lactol, and phenolic) accessible to aqueous solutions. 30Temperatureprogrammed desorption assumes that functional groups decompose to a specific product.However, this is not always the case since surface species can react.TPD and chemical analysis give information on functional groups in bulk samples.The carbons used in this study were primarily microporous, and the functional group analytical methods provided information on different sampling depths and environments.These issues must be considered when comparing characterization data from various characterization techniques.
3.3.1.X-ray photoelectron spectroscopy (XPS).The XPS surface elemental analyses (at%) obtained from the survey scan of the samples used in this study are given in Table 3. Minimal amounts of K, Ca, and Si (average o2 at% for Series 1 and 2 carbons and absent from the functionalized carbons except carbon N 2 /800-1-NH 3 /800 (0.28 at%)) originating from the walnut shell were also detected.These values are similar to the ash contents for the bulk samples reported in Table 1.Potassium was the primary metal on the surface (see Table 3).A comparison of XPS O 1s peaks with potassium-containing reference materials 53 showed that none could be identified in the XPS spectra of the carbons.Therefore, the XPS O 1s spectra could be used without possible interference of peaks from metal oxide species.Therefore, oxygen surface groups in the carbons were analyzed using both O 1s and C 1s spectra.The C 1s spectra represent carbon functional groups, whereas, for O 1s spectra, some oxygen functional groups have two oxygen types, contributing to different XPS O 1s peaks.Binding energy ranges

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exist for each functional group because of the varying structural environments in heterogeneous carbons.The C 1s, O 1s, and N 1s spectra were analyzed using curve fitting to determine the distribution of surface species as the percentage area of the total area under the peak.The quality of the curve fitting is critical to obtaining accurate distributions of surface groups. 54The normalized residuals (R i ) are defined as follows.
Comparison of the normalized residuals, experimental data, and curve fitting of components for the XPS spectra are shown in Fig. 3-5, providing a visualization of the quality of the curve fitting.The Abbe criterion was also used to analyze the residuals.The Abbe criterion is defined by the equation below.
R(i) and R(i + 1) are the residuals for the i and i + 1 points, respectively, and n is the number of data points.Random noise and statistically distributed residuals should give Abbe = 1.The curve fitting results for the spectra are provided in ESI † (Tables S4 and S5, ESI †), which includes details of the w 2 parameters obtained from the Casi software, the Abbe criteria, and repeatability information.
Carbons are complex heterogeneous materials with a wide range of functional groups located at the edge of graphene layers in varying chemical environments, and carbon has amphoteric characteristics.The functional groups include carboxylic anhydride, phenol, hydroquinone, lactone, quinone, and pyrone. 30he surface oxygen functional group species are only minor components, typically 7-10% of the C 1s profiles for Series 1 and 2 carbons and are observed on the high energy side of the asymmetric C 1s peak.The C 1s XPS spectra of a wide range of carbons have peaks in the following ranges: C sp 2 (284.2-284.6 eV), C sp 3 (284.8-285.4eV), C-O groups (phenols and ethers), 285.9-286.6 eV, carbonyl (ketones and quinones) and carbons bonded to two oxygens at 286.7-287.5 eV and carboxyl, carboxylic anhydrides, and esters (O-CQO) at 288.3-288.9eV and shake-up peaks of carbon in aromatic compounds (p-p* transition) at 290-294 eV. 55These values are consistent with curve-fitting protocols for organic compounds. 56he C 1s spectra for Series 1 carbons (N 2 /700-1 to N 2 /1000-1) showed that the functional group distributions were similar: C-O/OH (58.0 AE 2.2% at 286.49 AE 0.06 eV), CQO (27.0 AE 1.7% at 287.94 AE 0.11 eV) and O-CQO (15.0 AE 1.6% at 289.34 AE 0.27 eV).Series 2 carbons (CO 2 /700-1 to CO 2 /1000-1) also had very similar oxygen functional group distributions: CO/OH (58.7 AE 2.6% at 286.51 AE 0.03 eV), CQO (26.6 AE 1.6% at 288.0 AE 0.1 eV) and O-CQO (14.7 AE 1.9% at 288.96 AE 0.30 eV).Therefore, it is apparent that there are no significant differences observed in C 1s spectra of oxygen functional group distributions between the two series of carbons in the HTT range 700-1000 1C.
Carboxylic, anhydrides, lactones, esters, and pyrone groups present in carbons have both C-O bonds and CQO bonds, and these groups will contribute to different peaks in the O 1s spectra.Two peaks were observed in the XPS spectrum of polymer polyethylene terephthalate (PET) containing carboxyl/ ester groups where the OQC peak occurs at 531.6 eV and the O-C peak at 533.2 eV. 57,58The O 1s peaks for both Series 1 and 2 carbons were curve fitted using five peaks: (1) carbonyl, lactone, and ester CQO (531.0-531.6 eV), (2) C-O ether and hydroxyl bonded to aliphatic (532.5-532.9eV), (3) C-O ether and OH bonded to aromatic carbon, O-C in carboxyl/ester C-O aromatic (533.0-533.6 eV), (4) chemisorbed water/oxygen (534.8-536.20][61][62] Peaks 4 and 5 were 3-8% of the total O 1s peak area and did not show systematic changes with HTT or in functionalization studies.Therefore, these peaks are not discussed in detail. Series 1 carbons (N 2 /700-1 to N  S23.These comparisons show that the functional group distributions are similar.Fig. S22 (ESI †) shows that carbonization hold time did not influence oxygen surface groups in Series 1 and 2 carbons.There is agreement from the C 1s and O 1s spectra that the Series 1 and 2 carbons with HTTs in the range 700-1000 1C have very similar oxygen functional group distributions irrespective of HTT, carbonization atmosphere, and hold time.
The oxygen functional distributions from the C 1s spectra for carbons N 2 /600-1 and N 2 /600-3 were not statistically different based on the C 1s spectra, but the O 1s spectra did show minor differences.The O 1s spectra show a decrease from 33.6 AE 0.6% for carbons N 2 /600-1 and N 2 /600-3 to 23.5 AE 2.1% in the O-C aromatic groups at 533.75 eV and an increase from 24.7 AE 0.6 for N 2 /600-1 and N 2 /600-3 to 35.5 AE 3.4% for HTT 700-1000 1C in the 531.2 eV carbonyl peak.This difference is consistent with the small loss of O-C bonded to aromatics in carboxyl, lactone, etc., and the increase in various carbonyl groups over the temperature range of 600-700 1C.
Carbons CO 2 /800-1 and N 2 /800-1 were used as precursors for treatment to incorporate functional groups into the carbon structure.HNO 3 treatment of carbon CO 2 /800-1 mainly incorporates oxygen functional groups into the carbon structure.The

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total surface oxygen of carbon CO 2 /800-1 increased from 8.1 at% to 13.54 at% for CO 2 /800-1-HNO 3 (see Table 3).In comparison, elemental analysis of the bulk samples gave 7.54 wt% for CO 2 /800-1 and 23.84 wt% in CO 2 /800-1-HNO 3 (see Table 1).The XPS C 1s and O 1s spectra of carbons CO 2 /800-1 and CO 2 /800-1-HNO 3 are compared in Fig. 1a and b Heat treatment of carbon CO 2 /800-1-HNO 3 decreased the surface oxygen content from 13.54 at% for carbon CO 2 /800-1-HNO 3 to 8.4 at% for carbon CO 2 /800-1-HNO 3 /400, and to 2.85 at% for CO 2 /800-1-HNO 3 /800 (Table 3).The XPS survey scan showed that metal species were absent (see Table 3).Therefore, possible O 1s peaks from residual inorganic species do not contribute to the O 1s spectra.The C 1s spectra showed that heat treatment of carbon CO 2 /800-1-HNO 3 to 800 1C to form carbon CO 2 /800-1-HNO 3 /800 increased the C-O/C-OH peak distribution from 55.5% to 93% of the oxygen functionality and decreased the O-CQO peak from 44.5% to 6.7%.These results are consistent with the decomposition of carboxylic groups and the formation of C-O/OH during heat treatment.The O 1s spectra showed that heat treatment of carbon CO 2 / 800-1-HNO 3 decreased the CQO peak at 531.5 eV and the larger peak in the O-C attached to aromatic groups at 533.3 eV and a marked increase in the C-O bonded to aliphatic carbon at 532.2 eV.This change in the distribution of XPS peaks is consistent with the decomposition of carboxylic and the formation of phenolic and ether groups.The O 1s spectra for this series show evidence for the component peaks as shoulders (Fig. 1d-f), thereby supporting the curve resolution scheme.Thermally labile carboxylic groups in CO 2 /800-1-HNO 3 are also confirmed by the loss of CO 2 in TPD, as discussed later.
HNO 3 oxidation results in the incorporation of small amounts of thermally labile nitrogen species (0.8 at%) into the carbon structure (Fig. 1g) that decomposes on heat treatment (Fig. S27 and S28, ESI †).The XPS N 1s spectrum for carbon CO 2 /800-1-HNO 3 was curve-fitted, revealing peaks at 399.9, 401.9, and 405.8 eV.The most prominent XPS peak (47.1 AE 0.7%) at 405.8 eV was tentatively assigned to Noxide. 63,64The other N 1s peaks were at 399.9 eV (33.2%) assigned to pyrrolic or N sp 3 nitrogen and 401.9 eV (19.7 AE 0.8%) assigned to quaternary nitrogen, but there was no evidence for significant amounts of pyridinic nitrogen.The TPD supports the N-oxide assignment, which showed NO desorption at HTT o400 1C (see TPD section).The N 1s peak at 398.5 eV (21.5%) in carbon CO 2 /800-1-HNO 3 /400 was assigned to N sp 2 (pyridinic groups).The peak at 399.5 eV (49.5%) and 400.6 eV (21.1%) were assigned to pyrrolic, with the peak at 402.3 eV (8%) to quaternary nitrogen by analogy with high-temperature carbons, but other labile nitrogen species could also contribute to the peaks.The decomposition of thermally labile surface groups was evident from the absence of the 405.8 eV N-oxide peak in the XPS N 1s spectrum of CO 2 / 800-1-HNO 3 /400 (Fig. S27b, ESI †) and the absence of nitrogen surface species in the N 1s XPS spectra of CO 2 /800-1-HNO 3 /800 (Fig. S27c, ESI †).
Treatment of carbon CO 2 /800-1 with K 2 CO 3 at 800 1C resulted in a small decrease in surface oxygen content from 8.3 to 7.1 at%.The XPS survey scan showed that metal species and residual K 2 CO 3 were absent (see Table 3).A comparison of the XPS C 1s spectra does not show any apparent changes in the distribution of oxygen surface species.However, the O 1s spectrum showed a decrease in the CQO peak at B531 eV from 30.6% in carbon CO 2 /800-1 to 11.7% in carbon CO 2 /800-1-K 2 CO 3 /800.This change was accompanied by smaller increases in the intensities of C-O/C-OH bonded to aliphatic (532.2 eV) and O-C/OH bonded to aromatics (533.5 eV) (Fig. 1c and 2, and Table S5b, ESI †).The titration results showed increased acidic character in aqueous solution, attributed to increased phenolic and carboxylic groups.The TPD results show that more thermally labile oxygen groups in the carbon structure gave CO 2 evolution starting at B550 1C.
Curve fitting was used to analyze the C 1s, O 1s, and N 1s XPS spectra.The C 1s spectra showed that the distribution of oxygen surface groups was not changed very much by NH 3 treatment at 600 and 800 1C (Table S5a, ESI †).However, the XPS O 1s spectra showed that NH 3 treatment changed the distribution of surface oxygen species, as shown by the relative  4).The XPS N 1s showed that pyridinic and pyrrolic groups were the two primary forms of nitrogen, and the N-6/N-5 ratios (B2 : 1) were similar for both carbons (Table 5).Previous studies of the high-pressure carbonization of pure organic compounds with well-defined pyridinic and pyrrolic nitrogen groups as a function of HTT have shown 63 that the order of stability of surface groups is as follows:

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quaternary 4 pyridinic 4 pyrrolic NH 3 treatment of carbon N 2 /800-1 incorporated mainly pyridinic groups with smaller amounts of pyrrolic and quaternary nitrogen, changing the surface chemistry and influencing electrochemistry.
3.3.2.Acid and base titrations.Activated carbons have amphoteric characteristics due to various types of functional groups.Nitrogen and oxygen functional groups exist in carbon structures, and the p-electron system of the carbon basal planes has basic characteristics by binding protons from an aqueous solution. 30A comparison of potentiometric titration methods with the Boehm titration methods has shown good agreement, provided the proper common metric is used. 65HCl neutralizes basic surface groups and the carbon basal plane p electron systems. 66It was proposed that the basic properties of carbon surfaces are due to a combination of redox reactions and proton transfer to/from the surface.In this study, titration with sodium ethoxide was not used to determine reactive carbonyls. 30Therefore, some oxygen groups accessible in aqueous solutions were not quantified.
The titration results in Table 6 showed similar trends to those observed previously for coconut-derived carbons, HNO 3 oxidized carbon, and heat treatment studies of HNO 3 oxidized carbons under a nitrogen atmosphere up to 800 1C. 33,34he inorganic contents of Series 1 and 2 carbons are low (1.5-3.9 wt%), and the functionalized carbons were much lower (B0.5 wt%).Therefore, the inorganic material only makes a minimal contribution to the basic characteristics of the carbons.The acid and base titration results for carbons CO 2 /800-1 and CO 2 /800-1-HNO 3 showed that HNO 3 oxidation incorporated carboxylic, lactone/lactol, and phenolic groups into the carbon structure, which was consistent with both the O 1s and C 1s XPS data.In addition, a small amount of nitrogen (0.8 at%) was also incorporated into the carbon CO 2 /800-1-HNO 3 structure.The XPS spectrum shows that N-oxide and pyrrolic functionalities are the primary forms of nitrogen in oxidized carbon CO 2 /800-1-HNO 3 .Titration results showed that carbons CO 2 / 800-1, N 2 /800-1, and N 2 /1000-1 were basic carbons.CO 2 /700-1 contained small quantities of phenolic, lactone/lactol groups, carboxylic groups, and some basic groups.The titration results of CO 2 /800-1-K 2 CO 3 /800 showed that K 2 CO 3 treatment of carbon CO 2 /800-1 at 800 1C also introduced a range of carboxyl, phenolic, and lactone/lactol functional groups and increased the acidity compared to CO 2 /800-1.These results are consistent with the XPS and TPD results discussed later.Titration studies show that N 2 /800-1-NH 3 /800 shows some weakly acidic character and basic character, whereas carbon N 2 /800-1 only has basic character.However, the surface oxygen was decreased by NH 3 treatment at 800 1C to 1.8 at% (Table 3).

Temperature programmed desorption (TPD).
Temperature-programmed desorption provides information on the thermal stability of carbon functional groups.Surface oxygen species exist in various heterogeneous environments in carbons and decompose to form H 2 O, CO, and CO 2 over a wide temperature range (150-1250 1C).CO 2 is evolved from the decomposition of carboxyl groups, lactones, and reactions of surface oxygen species.A good correlation was observed between NaOH titration measurements and CO 2 formation from thermal desorption studies. 67CO desorption results from the decomposition of carbonyl, quinones, hydroquinones, phenols, and ether functional groups.H 2 O results from the decomposition of hydroquinones and phenolic groups.Carboxylic acid groups usually decompose below 400 1C.Acid anhydrides, carbonyls, lactones, and lactones (carboxylic group derivatives) decompose between 600-700 1C, and hydroquinones/semiquinones and ethers decompose up to 1000 1C. 22,68H 2 O desorption results from the decomposition of carboxylic and phenolic groups and surface reactions of hydrogen species with oxygen surface groups.H 2 , N 2 , and NO are also desorbed at high temperatures and result from the decomposition of surface groups and the reaction of mobile surface species.This characterization method provides insight into the surface functional groups present in carbon samples by comparing gas evolution profiles for various species in TPD.
The TPD profiles for CO, CO 2 , and H 2 O for carbons in He (Series 1) and CO 2 (Series 2) carbonization atmospheres are shown in Fig. S29 (ESI †).The low-temperature regions (o150 1C) of the TPD profiles show the desorption of H 2 O and N 2 physisorbed in the porous structures with peaks at B80 1C.Physisorbed O 2 was also desorbed, but this is not shown.The higher temperature peaks are due to the decomposition of functional groups.The TPD profiles for Series 1 of

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H 2 O, CO, and CO 2 carbon decomposition products shift to higher temperatures with increasing HTT (Fig. S29a, ESI †).
Carbon N 2 /600-1 has a significant weight loss of 4.35 wt% on heat treatment from 600 to 800 1C but much lower weight losses of 1.57 wt% between 800 and 1000 1C and 0.72 wt% between 1000 and 1200 1C (Fig. S30, ESI †).The TPD profiles of H 2 O, CO, and CO 2 decomposition products for Series 2 carbons also shift to higher temperatures with increasing carbon HTT

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(Fig. S29b, ESI †).Carbon CO 2 /700-1 loses 2.32 wt% on heating from 700 to 800 1C in TPD but smaller weight loss values of 1.95 and 0.69 wt% over the temperature ranges 800-1000 1C and 1000-1200 1C, respectively.The weight loss during the carbonization process occurs above the corresponding HTT for the carbon (Fig. S29, ESI †), leading to changes in the bulk carbon structure.HNO 3 treatment of carbon CO 2 /800-1 incorporated additional oxygen and nitrogen functionality into the carbon structure to form oxidized carbon CO 2 /800-1-HNO 3 .The TPD CO 2 profile of carbon CO 2 /800-1-HNO 3 had a wide temperature range of (150-900 1C) with broad and overlapping desorption peaks (Fig. 4).The low temperature CO 2 TPD peak at B239 1C overlapped with peaks at B231 1C for m/z 18 (H 2 O), at B207 1C for m/z 30 (NO) and B221 1C for m/z 28 (CO/N 2 ).The desorption of H 2 O and NO are consistent with the decomposition of carboxylic acid and N-oxide surface groups, respectively.The low-temperature decomposition of N-oxides was reported previously by Xiao et al. 35 The XPS results show that N-oxide functional groups (Fig. 1g and Table 5) were decomposed by heat treatment to 400 1C, but pyrrolic groups were still present (carbon CO 2 /800-1-HNO 3 /400, Fig. S27b, ESI †).Further heat treatment to 800 1C showed that all nitrogen surface groups were absent (carbon CO 2 /800-1-HNO 3 /800, Fig. S27c, ESI †).The other broad overlapping CO 2 peaks are due to the desorption of carboxylic anhydride, lactone, and carbonyl surface sites, but H 2 O desorption is minimal over the temperature range 450-780 1C with a multicomponent peak in the temperature range (780-1250 1C), which is assigned to the decomposition of phenolic groups and surface reactions.The m/z 28 (CO) desorption peaks were observed at 675 and 878 1C, which are attributed to anhydride, lactones, etc.Heat treatment of oxidized carbon CO 2 /800-1-HNO 3 to 800 1C removed oxygen functional groups, which decompose below 800 1C, forming carbon CO 2 /800-1-HNO 3 /800 (see XPS data in Table 4).The TPD profile of carbon CO 2 /800-1-HNO 3 /800 was comparable to carbon  CO 2 /800-1, thereby showing the effect of HTT (Fig. 4).5]69 Comparison of the TPD profiles of carbons CO 2 /800-1-K 2 CO 3 /800 and CO 2 /800-1 shows that the reaction has modified the profiles (Fig. 5).Carbon CO 2 /800-1-K 2 CO 3 /800 showed a shift of the m/z 18 water peak to a higher temperature, and the m/z 28 CO peak was negligible.The K 2 CO 3 treatment procedure produced strong bimodal CO 2 desorption peaks at 725 and 875 1C.The low-temperature CO 2 peak was attributed to lactones/carboxyl, while the high-temperature CO 2 peak, which coincides with the start of H 2 O evolution, was assigned to the decomposition of phenolic and lactol groups and the reaction of surface species.The O 1s XPS results indicate carbon CO 2 / 800-1-K 2 CO 3 /800 has lower CQO contents than CO 2 /800-1 (Table 4 and Table S5, ESI †).The titration results for carbons CO 2 /800-1-K 2 CO 3 /800 show increases in acidic carbon-oxygen surface groups compared with carbon CO 2 /800-1 with surface groups in the order: phenolic 4 carboxyl 4 lactone/lactol.The TPD results are consistent with the XPS (Table 4) and titration results (Table 6).
High-temperature ammonia treatment of carbons led to the incorporation of nitrogen functional groups into the carbon structure.The TPD m/z 28 (CO) peak profile for the starting carbon material N 2 /800-1 did not have a corresponding peak for m/z 14 (Fig. 6).Chemical analysis and the XPS spectrum of carbon N 2 /800-1 show minimal bulk and surface nitrogen contents.Therefore, the comparison of m/z 14 and m/z 28 TPD profiles allows the desorption of N 2 to be distinguished from CO for carbons N 2 /800-1-NH 3 /600 and N 2 /800-1-NH 3 /800.
The TPD profiles of both these samples had weak m/z 28 desorption peaks at B850-900 1C.The m/z 14 and 28 gas evolution profiles show N 2 desorption and some CO desorption from 850-1200 1C (Fig. 6).N 2 is the major nitrogen-containing desorption product.Nitrogen surface functional groups in carbon are mobile, reacting on the surface to form N 2 . 35Both NH 3 treated carbons N 2 /800-1-NH 3 /600 and N 2 /800-1-NH 3 /800 showed small TPD peaks for m/z 30 (NO) at B740 1C (Fig. 6).The thermolysis of nitrogen functional groups shows that pyridine N-oxide is the least stable.Pyrrolic-N gradually transforms into pyridinic-N and subsequently into quaternary-N with increasing HTT. 35,63These nitrogen-containing desorption products are consistent with the nitrogen species detected in XPS (Fig. 1g and 3, Fig. S27 and Table 5, Table S5, ESI †) discussed previously.
3.3.4.Fourier transform infrared spectroscopy.1][72] In this study, the 900-1550 cm À1 spectral region provides a spectral fingerprint region, and the 1550-1800 cm À1 shows absorption bands due to aromatic CQC and CQO stretching vibrations.The ATR spectra for Series 1 carbons N 2 /600-1 and N 2 /800-1 show that the intensity of the band at 1579 cm À1 decreases markedly at 800 1C (Fig. S41a, ESI †).However, the infrared spectrum of carbon CO 2 /800-1 in Series 2 has a more substantial band at 1580 cm À1 than carbon N 2 /800-1, indicating that carbonization in a CO 2 atmosphere has incorporated some surface oxygen groups into the carbon structure (see Fig. S41b, ESI †).Comparison of the ATR spectra of carbons CO 2 /800-1 and CO 2 /800-1-K 2 CO 3 /800 shows that the spectra are very similar, but the latter have higher intensity (see Fig. S41c, ESI †).The main ATR peaks at 1580 and 1713 cm À1 for CO 2 /800-1-HNO 3 , as shown in Fig. S41d (ESI †), are similar to the diffuse reflectance spectrum reported for nitric acid oxidation of a steam-activated carbon,

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which had bands at 1576 and 1717 cm À1 . 35The analytical data for carbon CO 2 /800-1-HNO 3 is similar to that reported for nitric acid oxidation of a steam-activated carbon. 35The peak at 1713 cm À1 is consistent with the presence of carboxylic groups.
Heat treatment studies for HNO 3 oxidation of the steamactivated carbon resulted in progressive decomposition of the surface oxygen groups, with lactone and lactol groups forming at 400-600 1C.The spectra for the carbon treated at high temperatures with ammonia are shown in Fig. S41e (ESI †).
There are relatively small differences between the infrared spectra of carbons N 2 /800-1 and N 2 /800-1-NH 3 /600.However, the infrared spectrum of carbon N 2 /800-1-NH 3 /800 is markedly different with the loss of the 1007 cm À1 peak and increased intensity for the 1190 and 1577 cm À1 peaks.
3.4.Carbonaceous molecular structure 3.4.1.Raman spectroscopy.Raman spectra of disordered carbons have two prominent peaks: the D peak, which is attributed to the A 1g breathing mode in disordered carbonring clusters (near the K-zone boundary) with more than five aromatic rings, and the G peak is due to the E 2g breathing mode of carbon aromatic sites. 73,74Various curve fitting methods using up to 9 peaks have been used to analyze the Raman spectra of carbons.The Raman spectra of carbons used in this study showed a shoulder on the D peak at 1180 cm À1 , designated as the S peak.The S peak has been assigned to the symmetric breathing mode of small polyaromatics and rings containing seven or more carbons. 75A small peak at B1510 cm À1 (A peak) was also required for curve fitting, and this has been attributed to amorphous carbon linked to oxygen functional groups. 31,75,76The Raman spectra were curve-fitted using D, G, A, and S peaks using the method described previously. 75The A and S Raman peaks were fixed at 1510 and 1180 cm À1 in the curve fitting of all spectra.Typical examples of the peak fitting of the Raman spectra, the peak components, and residuals for the fitting and the w 2 values are shown in Fig. S33-S39 (ESI †), and the data are given in Table 7. Repeatability studies are shown in Table S7 and Fig. S32 (ESI †).It is evident from the residuals for the peak curve fitting procedure that the four peaks provide good fits to all the Raman spectra studied.The contributions of the S and A peaks to the Raman spectra were 9.5-14.5 and 12-19% of the spectra, respectively.The contribution of the A peak increases significantly over the carbonization temperature of 600-700 1C but has a constant contribution to the overall spectral profile thereafter.
The I D /I G ratio (peak intensity ratio of the D and G peaks) is used widely as a measure of the degree of ordered carbon in carbon materials, while the peak area ratio for the D and G peaks (A D /A G ) are less commonly reported. 77,78The Raman spectra for carbons prepared in N 2 (Series 1) and CO 2 (Series 2) atmospheres show markedly increased I D /I G and A D /A G ratios with increasing HTT from 600 1C up to 800 1C and slower increase from 800 1C to 1000 1C (Fig. S40a and b, ESI †).Previous studies of the Raman spectra of carbons derived from a wide range of biomass materials have shown that I D /I G increases for the temperature range 800 to B2000 1C but decreases at higher temperatures. 79In contrast, changes with carbonization hold time were small, indicating that temperature is the primary experimental carbonization parameter influencing carbon structure.The full width at half maximum of the D peak (FWHM D ) decreased gradually with increasing HTT (Fig. S40c, ESI †), and this carbon structural rearrangement resulted in a more ordered carbon structure. 80The full width at half maximum intensity of the G peak (FWHM G ), D, and G peak positions are included in Table 7, but none of these parameters show marked changes with carbonization hold time (0-3 h) or carbonization gaseous atmosphere.The changes in D and G peak Raman shifts with HTT were small.Therefore, significant conclusions regarding carbon structure could not be obtained from these parameters. 81he incorporation of functional groups produced minor changes in the Raman spectra.HNO 3 oxidation reduced the Curve fitting peaks for samples N 2 /800-1-NH 3 /600 and N 2 /800-1-NH 3 /800 gave several very weak peaks in the binding energy range 402-407 eV with inconsistent binding energies and amounting to B12-15% of the total peak area.

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FWHM D of the D band, I D /I G , and A D /A G ratio in the Raman spectra of carbon CO 2 /800-1-HNO 3 compared with carbon CO 2 / 800-1.Heat treatment of carbon CO 2 /800-1-HNO 3 resulted in substantial weight loss (Fig. S30b, ESI †) due to the decomposition of the functional groups in TPD (Fig. 4).The A D /A G ratio in the Raman spectrum of carbon CO 2 /800-1-HNO 3 /800 was very similar to that of the starting material carbon CO 2 /800-1.The values of D bandwidth (FWHM D ) were slightly lower for the heat-treated carbon series CO 2 /800-1-HNO 3 , CO 2 /800-1-HNO 3 / 400, and CO 2 /800-1-HNO 3 /800 than for the initial carbon CO 2 / 800-1.The high-temperature treatment of carbon CO 2 /800-1 with K 2 CO 3 at 800 1C and treatment of carbon N 2 /800-1 with NH 3 at 800 1C reduced FWHM D slightly compared to the corresponding precursor materials.The changes in the Raman D/G peak intensity ratios, peak area ratios, and D bandwidths show that carbonization temperature has a much more significant effect on the carbon structure than the chemical treatment and thermolysis to incorporate functional groups.
3.4.2.Powder X-ray diffraction (PXRD).The PXRD profiles of the carbon samples had two broad peaks at B22-231 and B431 representing (002) and ( 100) reflections of carbon crystallites (Fig. S31, ESI †).The PXRD profiles in Series 1 and 2 carbons had very small peaks at 2y B 29.51 due to inorganic mineral content (Fig. S31a and b, ESI †).This crystalline phase present in the biochar was not present in the ash. 82Crystallite thickness (L c ), apparent crystallite diameter (L a ), and interlayer distance (d 002 ) are given in Table 8.Repeatability studies are shown in Table S6 (ESI †).An increase in HTT for both Series 1 (600-1000 1C) and Series 2 (700-1000 1C) carbons increased L a significantly (see Fig. S40d, ESI †), indicating the presence of larger apparent crystallite sizes in the carbon structure, which is attributed to the dewrinkling of graphene layer sheets.Previous studies of 5 coal chars and a carbonized pitch with HTTs 500-2800 1C showed that L a increased over the range 18-66 Å with L a had values in the range 18-33 Å for 600-1000 1C, which is similar to the L a range for the carbons used in this study (Fig. S40e and f, ESI †).The Raman D bandwidth (FWHM D ) for these chars decreased with increasing HTT (500-1900 1C), and this correlated with an increase in the apparent crystallite diameter (L a ). 80This correlation was also observed for the carbons used in this study (Fig. S40e and f,  ESI †).It has been reported that CO 2 gasification removed more aliphatic carbon groups, leading to higher L a values and dewrinkling of carbon graphene sheets. 83However, the L a data for carbons CO 2 /800-0, CO 2 /800-1, and CO 2 /800-3 were not statistically different (Table 8).The L a data for carbons N 2 / 800-0, N 2 /800-1, and N 2 /800-3 increased from 20.09 Å for carbon N 2 /800-1 to 23.57Å for carbon N 2 /800-3.No clear trends in stack layer height (L c ) and interlayer spacing (d 002 ) of the graphene basal planes with HTT were observed for both Series 1 and 2 carbons.
Carbons with the same HTT (CO 2 /800-1 and N 2 /800-1) were used for the carbon functionalization studies with maximum treatment temperatures of 800 1C to eliminate structural changes due to HTT.Carbon molecular structure changes during functionalization studies were investigated by comparison of X-ray diffraction parameters d 002 , L c , and L a (see Table 8).The nitric acid oxidation of carbon CO 2 /800-1 produced carbon CO 2 /800-1-HNO 3 , and L c , d 002 , and L a were increased, but the values of L c and d 002 were close to three standard deviations.HNO 3 oxidation of carbon CO 2 /800-1 increased the crystallite size (L a ) from 23.53 Å for carbon CO 2 / 800-1 to 25.65 Å in carbon CO 2 /800-1-HNO 3 .This is consistent with the dewrinkling of the graphene layers.Heat treatment studies of carbon CO 2 /800-HNO 3 to form CO 2 /800-1-HNO 3 /800 led to a reduction in L a to 24.01 Å, similar to the starting carbon material.

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Treatment of carbon CO 2 /800-1 with K 2 CO 3 at 800 1C decreased L c and increased d 002 and L a (see Table 8).The apparent crystallite size (L a ) increased from 23.53 Å for carbon CO 2 /800-1 to 27.87 Å for carbon CO 2 /800-1-K 2 CO 3 /800.The increase in the apparent crystallite size results from the dewrinkling of the graphene layers.Treatment of carbon N 2 / 800-1 with NH 3 at 800 1C to form carbon N 2 /800-1-NH 3 /800 did not change L c , d 002 was decreased, and L a increased.The apparent crystallite size increased from 21.03 Å for carbon N 2 / 800-1 to 26.99 Å for carbon N 2 /800-1-NH 3 /800.This is also consistent with the dewrinkling of the graphene layers.Therefore, all three treatment procedures increased apparent crystallite size (L a ).Heat treatment of carbon CO 2 /800-1-HNO 3 increased L c and decreased L a , while there was no clear trend in d 002 .
Many carbons, including various graphite, glassy carbons, fibers, carbon black, and other carbons, have been used as electrode materials. 7The carbons used in this study were nongraphitizable biochars and have the following PXRD parameter ranges: crystal thickness (L c ) 8.55-10.75Å, apparent crystallite diameter (L a ) 18.33-28.71Å and interlayer spacing (d 002 ) 3.66-4.14Å.The ranges of values for L a and L c for the biomass carbons are similar to glassy carbon and Spheron 6 carbon black used previously, but the d 002 values are significantly higher for the carbons used in this study than the maximum (3.55 Å) reported previously. 7Carbons with HTTs r 1000 1C consist of units of no more than 10-12 aromatic rings with stacks of 2-3 units.The units have no general organization except for a possible local organization.This is consistent with a more significant structural disorder in the biomass carbons.
3.5.Electrochemical characteristics 3.5.1.Carbon structure.The cell electrodes used for cyclic voltammetry and electrical impedance spectroscopy of vanadium redox reactions were identical carbon/PVDF polymer composites.The reactions occur on the carbon but might be affected by the porous structure of the composite electrode.The primary factors determining carbon structure are the biomass carbon precursor and the experimental carbonization conditions, mainly heat treatment temperature (HTT), with secondary experimental factors such as gaseous atmosphere, hold time, and heating rate.The carbonization heating rate was held constant throughout this study.The effect of hold time on carbon structural characteristics was examined for carbonization at 800 1C under a carbon dioxide atmosphere, and the cyclic voltammograms (CVs) are shown in Fig. 7.The differences between the cyclic voltammograms are minimal, and this is consistent with the similarities in the carbon structural characterization data for the porous structure characteristics from gas adsorption, surface analysis from XPS, and molecular structure from Raman and XRD (see Tables 2-5, 7, and 8).Therefore, the carbon hold time at the HTT was held constant at 1 h for all the carbons used in the detailed electrochemistry studies.
Table 7 Raman spectroscopy parameters I D /I G (peak intensity ratio for D and G peaks), A D /A G (area ratio for D and G peaks), full bandwidths at half maximum of D and G peaks, and D and G peak Raman shifts of carbons from curve fitting Carbon sample

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Three series of carbon samples were used in this study to explore the role of carbonization conditions and chemical treatment procedures on carbon in carbon/PVDF composite electrode properties.Cyclic voltammetry was studied at three sweep rates: 5, 10, and 20 mV s À1 .Consistent trends were obtained for all three sweep rates for carbons prepared under nitrogen and carbon dioxide atmospheres, with the highest currents obtained for 20 mV s À1 (Fig. S18, ESI †).Therefore, the 20 mV sweep rate was used for cyclic voltammetry.
The cyclic voltammograms with a sweep rate of 20 mV s À1 for electrodes made from carbons with HTTs 600, 800, and 1000 1C in a nitrogen atmosphere (Series 1) are shown in Fig. 8a.The CVs have a very weak V 2+ -V 3+ peak, a V 3+ /VO 2+ was absent, and the strongest peak was the VO 2+ -VO 2 + peak.
An increase in carbon HTT enhances the VO 2+ -VO 2 + peak at an HTT of 800 1C for Series 1 (N 2 atmosphere) carbons, but an increase of HTT to 1000 1C appears to have, at most, a small effect.Similar trends were observed at CV sweep rates of 5 and 10 mV s À1 (Fig. S16, ESI †).A weak V 2+ -V 3+ peak was observed, but no V 3+ -VO 2+ peak was observed.Fig. 8b shows the corresponding EIS plots, showing a very distinct difference between carbons with HTTs of 600 and 800 1C.The modeling studies for the EIS are shown in Table S1 (ESI †).Overall, the electrodes with carbon HTT 800 1C were shifted to lower electrode resistance than either carbon 600 1C or carbon 1000 1C electrodes.
The CVs for electrodes made from carbons in Series 2 by carbonization at 700, 800, and 1000 1C in a carbon dioxide atmosphere are shown in Fig. 8c.The CVs for these carbon electrodes are slightly enhanced for the same temperatures compared to the carbon samples prepared in a nitrogen atmosphere (see Fig. 8a).An increase in HTT results in enhancement of the V 2+ -V 3+ peak in the CVs and the V 3+ /VO 2 + couple is observed as an intermediate weak broad peak in carbon CO 2 / 1000-1.There are only very small differences for VO 2+ -VO 2 + peak for HTTs of 800 and 1000 1C.The CVs for carbonization in N 2 and CO 2 at 800 and 1000 1C are shown in Fig. S18 (ESI †).
The oxidation of V 3+ to VO 2+ was observed as a small peak at B0.1 V (vs.MSE) for carbons prepared under a CO 2 atmosphere (Fig. 8c).This phenomenon has been reported previously. 13,14,84The corresponding Nyquist plots are shown in Fig. 8d.Comparison of the Nyquist plots for electrodes made from carbons prepared under N 2 (Series 1) and CO 2 (Series 2) at 800 and 1000 1C shows that the carbons with HTT 1000 1C are shifted to higher resistance (see Fig. S15b and d, ESI †).The Nyquist graphs show the same trend, with plots shifted to the lowest resistance for HTT 800 1C for carbonization in N 2 and CO 2 atmospheres.The electrical resistivity properties of highdensity polyethylene/carbon black composite materials depend on carbon structure and surface functional groups.The resistivity properties of the composites can be modified using gasification and liquid phase chemical treatment of the carbon black. 85,86It is evident that the carbonization temperature for the carbon component of the carbon/PVDF electrodes has a major effect on cyclic voltammograms and Nyquist plots for vanadium redox reactions.
A weak shoulder on the low-frequency side of the Nyquist graph was observed at B15 Hz for the CO 2 carbonization Series 2 and lower (B2 Hz) for the corresponding N 2 carbonization Series 1.The peak was very weak in carbon N 2 /800-1.The frequency shift for the N 2 Series 1 compared with the CO 2 Series 2 carbons is possibly due to differences in diffusion for these electrodes.The EIS of carbon N 2 /600-1 has a well-defined high-frequency peak, indicating a single time constant.However, electrodes prepared from carbons N 2 /800-1 and N 2 /1000-1 have two weak peaks (B750 and 10 000 Hz) indicative of the presence of two time constants.The carbons in Series 2 prepared under the CO 2 atmosphere show a similar trend with HTT for electrodes prepared from carbons CO 2 /800-1 and CO 2 / 1000-1 with very weak peaks in the high-frequency region.
The effect of HTT on CV and EIS is similar for carbonization in both N 2 (Series 1) and CO 2 (Series 2).Gas adsorption studies showed that both series of carbons were predominantly ultramicroporous (o0.7 nm), and therefore, there are liquid phase diffusion limitations into the porous structures (Section 3.2).Hence, changes in CV and EIS with HTT are not directly related to the carbon pore structure.However, porosity may be formed in the carbon/PVDF composite electrode, which may be influenced by differences in carbon surface chemistry.XPS shows that the surface oxygen functional groups are very similar for both carbon series for HTTs in the range of 700-1000 1C.However, minor differences were observed for Series 1 (N 2

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atmosphere) with HTTs 600 1C (Section 3.3.1.).Titration studies show that carbon CO 2 /700-1 has amphoteric properties, while at an HTT of 800 1C, there were virtually only basic characteristics (Table 6).The Raman spectra and PXRD data show that the amorphous carbon molecular structures change with increasing HTT (see Tables 7 and 8).The change in PXRD shows that crystallite size L a increases with increasing HTT (Section 3. Therefore, the change in CV and EIS for carbons in Series 1 and 2 are attributed to changes in carbon molecular structure, giving rise to changes in electrode resistance.
Carbon CO 2 /800-1-HNO 3 has a range of surface functional groups, including carboxylic, anhydride, lactone/lactol, and phenolic groups.Heat treatment in H 2 to form carbon CO 2 / 800-1-HNO 3 /400 resulted in the removal of mainly carboxylic and N-oxide groups, as shown by XPS, TPD, and titration studies.Heat treatment in H 2 to form carbon CO 2 /800-1-HNO 3 /800 progressively removes labile oxygen functional groups.The pore structures were predominantly ultramicroporous (Table 2), limiting liquid phase diffusion.Only minor differences in Raman and XRD characterization data for this series of carbons were observed for the PXRD and Raman spectra of this series of carbons (Tables 7 and 8).Therefore, the differences in electrochemistry could not be accounted for by significant changes in either porous or molecular structures.This series of carbons allows an investigation of the role of oxygen surface functional groups in the vanadium redox reactions.
Therefore, reversible changes in CV for the V 3+ -VO 2+ couple were established by combining HNO 3 oxidation and heat treatment in hydrogen.These changes follow the trend with oxygen functional group concentrations since the ultramicroporous structures limit diffusion, and changes in molecular structures are minimal.The progressive reversible changes in surface oxygen functional groups explain the differences in the CVs and Nyquist plots.
Carbon CO 2 /800-1 was treated with K 2 CO 3 at 800 1C to form carbon CO 2 /800-1-K 2 CO 3 /800, and the cyclic voltammograms and Nyquist plots for the electrodes prepared for these carbons are shown in Fig. 10a and b.The modeling studies showed that the electrode resistances of carbons CO 2 /800-1 and CO 2 /800-1-K 2 CO 3 /800 were similar (Table S1, ESI †).The two weak high frequency Nyquist peaks at 750 and 12000 Hz observed for CO 2 / 800-1 have been replaced by a single peak at 23000 Hz in CO 2 / 800-1-K 2 CO 3 /800.The weak low frequency peak mentioned earlier at 15 Hz corresponds to a shoulder at B10 Hz in CO 2 / 800-1-K 2 CO 3 /800.The increased surface homogeneity is a possible explanation of the change in the high frequency region of the Nyquist plots of CO 2 /800-1-K 2 CO 3 /800 compared with CO 2 / 800-1.Carbon CO 2 /800-1-K 2 CO 3 /800 had a more pronounced CV peak for V 3+ /VO 2+ than carbon CO 2 /800-1.
K 2 CO 3 treatment at 800 1C only leads to small changes in the porous structure, and both samples are predominantly ultramicroporous (Table 2).The PXRD and Raman results for the carbon do not show significant differences (Tables 7 and 8).These results indicate minimal changes in the porous and molecular structures.The titration results showed that carbon CO 2 /800-1-K 2 CO 3 /800 has amphoteric surface characteristics containing both acidic (phenolic, lactone/lactol, and carboxyl) groups and basic groups (Table 6), whereas carbon CO 2 /800-1 had predominantly basic surface characteristics.
Carbon N 2 /800-1 was treated with NH 3 to form carbon N 2 / 800-1-NH 3 /800, and the cyclic voltammograms and Nyquist plots for the corresponding electrodes for vanadium redox reactions are shown in Fig. 11a and b, respectively.

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The V 3+ -VO 2+ transition is absent for the CV profile of carbon N 2 /800-1 but was observed as a very broad peak at B0.1 V in N 2 / 800-1-NH 3 /800.Also, the V 2+ -V 3+ CV peak increased more than the VO 2+ -VO 2 + peak.The EIS profile of the N 2 /800-1-NH 3 /800 electrode was shifted to a higher resistance compared to the N 2 /800-1 electrode (Table S1, ESI †).The two highfrequency weak peaks in the EIS of the N 2 /800-1 electrode coincide with a peak for the N 2 /800-1-NH 3 /800 electrode, consistent with a more homogeneous surface in the NH 3 treated carbon.Also, a weak low frequency peak at B2 Hz EIS associated with diffusion/kinetics was present as a clear shoulder.Ammonia treatment had a marked effect on carbon functional groups but a minimal effect on the predominantly ultramicroporous and molecular structures compared with carbon N 2 /800-1.The original carbon N 2 /800-1 had very low surface and bulk nitrogen contents, while the surface oxygen and bulk contents were much higher.The NH 3 treatment of carbon N 2 /800-1 at 600 and 800 1C incorporates mainly pyridinic and pyrrolic nitrogen groups (ratio B2 : 1) into the carbon structure, with the surface analysis giving 2.56 and 4.86 at% for carbons N 2 /800-1-NH 3 /600 and N 2 /800-1-NH 3 / 800, respectively (Table 5).

Electrode performance
VRFB static cell charge-discharge analysis was performed at current densities of 10, 15, and 20 mA cm À2 to evaluate the electrochemical performance of the carbons (see Fig. 12) Repeatability studies are shown in ESI, † Tables S2 and S3.The coulombic efficiency increases with increasing current density whereas the voltage efficiency decreases with increasing current density.It is apparent that the four carbon samples (N 2 / 800-1, N 2 /800-1-NH 3 /800, CO 2 /800-1-HNO 3 , and CO 2 /800-1-HNO 3 /800), which had a range of functional groups, had similar voltage, coulombic and overall energy efficiencies.

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Stable energy efficiencies of all carbon samples were observed in the range of 89.5-90.9% at current densities of 10-15 mA cm À2 before slightly decreasing to 88.4-90.1% at 20 mA cm À2 , but the overall efficiency differences are comparable to the experimental uncertainties.The slight decrease in energy efficiencies with increasing current densities can be ascribed to ohmic polarization, 87 dynamic voltage (IR) drop at higher current densities, 13,19 or electrolyte ions passing through the membrane. 88It also explains the decline of voltage efficiencies with increasing current densities.The CV and EIS results for electrodes prepared from carbons N 2 /800-1-NH 3 /800 and CO 2 /800-1-HNO 3 demonstrated the influences of nitrogen and oxygen carbon functionality on the V 2+ /V 3+ and VO 2+ /VO 2 + couples.The microporosity in these carbons (ultra-micropores account for 475% of total pore volume) restricts the liquid phase diffusion of vanadium ions into the porous structure of the carbons, which explains the negligible differences in charge-discharge performance in these samples.The performance of walnut shell carbons prepared in this study is compared with that of other carbon materials using VRFB static cells in Table 9.The slightly lower energy efficiency of walnut shell carbons compared to orange peel carbon 13 and spent coffee bean carbon 20 is attributed to the high ultramicropore content limiting diffusion of vanadium ions in the carbon structure.The energy efficiency for walnut shell carbon demonstrated that it is suitable for VRFB electrodes.Carbon N 2 /800-1 had the highest efficiency and was subjected to extended tests for 20 cycles at each current density of 10, 15, and 20 mA cm À2 , which are presented in Fig. 12d.The coulombic efficiency increases whereas the voltage efficiency decreases with increasing current density.The overall energy efficiency decreases with increasing current density, possibly due to the side reactions at each half-cell. 89The stable performance of walnut shell carbons suggests their suitability for use in VRFBs.The coulombic, voltage, and overall energy efficiencies of all the carbons (N 2 / 800-1, N 2 /800-1-NH 3 /800, CO 2 /800-1-HNO 3 and CO 2 /800-1-HNO 3 /800) studied were not significantly different within experimental error, and therefore longer-term stability tests were not undertaken.

The influence of carbon structural characteristics on vanadium redox reactions
Developing relationships between carbon surface chemistry, molecular structure, and porosity, with electrochemical characteristics, is essential for understanding the mechanism.9][20]27, In concentrated VRFB-relevant electrolyte solutions, polymerization, hydrolytic, and ion association processes are often extensive.Also, the capacities of VFBRs decay with use, and surface reactions are thought to be involved. 26,116The role of surface carbon groups on the electrode is unclear. 27The problem with investigating the roles of surface chemistry and carbon structure for vanadium redox electrochemical characteristics is that there is often a mixture of oxygen surface functional groups and a variety of carbon sites in an amorphous porous structure.Oxygen functional groups could enhance the wettability of carbons and act as electro-catalytic sites for redox reactions, [100][101][102]105,114 but the influence of oxygen functional groups was only observed for carbon materials with low surface area or well-defined structure. Studiesof the vanadium redox electrochemistry of polyacrylonitrile electrospun fibers led to the proposal that functional groups were the dominant factor for carbonization temperatures in the range 1000-1300 1C, but carbon edge sites were more critical over the temperature range 1300-1500 1C. 100 Nitrogen and oxygen functional groups were incorporated into graphite felt by wet impregnation of urea followed by heat treatment in nitrogen to 500-700 1C.Electrochemical results suggested that pyrrolic and pyridinic functional groups were active centers for vanadium redox reactions.95 Li et al. attributed enhanced kinetics for V 2+ /V 3+ to C-OH groups incorporated on the surface of carbon felt following thermal activation.101 This commercial carbon felt had a low surface

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area and was thermally resilient, and the improved kinetics of V 2+ /V 3+ were attributed to the increase of oxygen functional groups.The enhanced kinetics of vanadium redox reactions have been ascribed to carboxyl groups present in multi-walled carbon nanotubes. 102imulation studies showed that nitrogen incorporated in the carbon structure had a positive impact on vanadium redox reactions. 10Shao et al. attributed the enhanced VO 2+ /VO 2 + couple (the positive side of VRFB) to nitrogen doping in the mesoporous carbon.However, the BET surface area could have contributed significantly to their performance as it increased from 500 m 2 g À1 for the untreated material to 1100 m 2 g À1 for the NH 3 treated carbon. 104The improvement in CV profiles of biomass-derived carbon was attributed to the mesopore structure. 13,14Costa de Oliveira et al. used voltammetry and EIS methods to reveal that N-functionalities significantly enhance the intrinsic activity of carbon electrode surfaces. 117yridinic/pyrrolic groups improved charge transfer rates and reversibility of the vanadyl oxidation reaction.These factors are crucial for important applications such as vanadium redox flow batteries.In comparison, oxygen functional groups may negatively impact vanadyl redox reactions. 118teimecke et al. used scanning electrochemical microscopy (SECM) to investigate the VO 2+ /VO 2 + reaction for various oxidized carbon materials. 119Electron transfer coefficients of glassy carbon (GC) and graphite substrates and their oxidized derivatives were comparable to cyclic voltammetry.In the case of oxidized samples, the results depended on the material.A small improvement was observed for oxidized GC whereas no effect was observed for graphite where the oxygen content increases from 4.0 to 13.0 at%.A combined in situ-Raman-SECM was also performed for a graphene layer.The graphene edge sites had increased activity at relatively low overpotentials for VO 2+ oxidation and at high overpotentials for VO 2 + reduction compared with basal plane carbon sites.The relationship between vanadium redox reactions and carbon properties, the active electrode component of the

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Materials Advances carbon/PVDF electrode, is a multifactorial problem and is not well understood.In this study, the effects of carbonization or heat treatment temperature, porous structure, and functional groups have been considered in detail.The carbons in Series 1 and 2 used in this study were predominantly ultramicroporous (o0.7 nm), and, therefore, carbon porosity was not a factor influencing the CV measurements of vanadium redox reactions due to diffusion limitations.However, the porosity in the composite electrode formed between the polymer and the carbon is difficult to quantify.Carbon surface chemistry may influence porosity formation in the carbon/PVDF electrodes.
There is no evidence that differences in the porous structure of the carbons used in this study are significant in determining EIS characteristics.Low frequency peaks in EIS may be associated with porosity in the carbon/PVDF electrodes.Carbon surface chemistry is known to alter the resistivity in carbon black/polymer composites. 85,86yclic voltammograms reported in the literature show that the relative peaks for V 2+ -V 3+ , V 3+ -VO 2+ , and VO 2+ -VO 2 + vary markedly depending on the structure of the carbon electrodes and the CV sweep rates, indicating kinetic effects. 13,14,110he vanadium V 3+ /VO 2+ couple has potentiometric reversibility, but there is a large activation energy, and therefore, the kinetics are slow.Sufficiently negative potentials are required to bring the reduction of V(IV) to V(III), but in most cases, the negative potential is low enough to cause the reduction all the way to V(II).The V 3+ /VO 2 + couple influences the initial pre-charging of the electrolyte. 109Previous studies of pre-oxidized glassy carbon

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electrodes showed that increased surface roughness was beneficial, while functional groups such as CQO and C-O-C tend to reduce the activity of glassy carbon. 114Three concurrent processes occur, (1) slow V(III) -V(IV), (2) oxidation V(IV)-V(V), and (3) reaction of V(III) and V(V) to form V(IV) species. 109The reversibility of the CV for V 3+ /VO 2 + was improved by increased vanadium concentrations or oxygen functional groups. 109Various hydrolytic and ion association processes may occur in concentrated vanadium electrolyte solutions, and chemical stability is a factor. 27,111,112The connection between vanadium speciation in the electrolyte and surface groups in carbon electrodes is unclear.The relationship between vanadium species present in the electrolyte, electrode surface groups, and electrochemical kinetics needs systematic investigation.
It has been proposed that the anodic reaction limits kinetics even at low current densities, and the cathodic reaction has mass transport effects at high current densities. 27,113his study investigated the use of sustainable biochar electrodes for vanadium redox reactions.Biomass pyrolysis involves the overlapping decomposition of lignocellulosic materials to form a carbonaceous char by an HTT of 600 1C.As the temperature increases, structural change leads to carbons with heterogeneous surfaces, which contain a wide range of oxygen and nitrogen surface sites and graphene layer sites, including zig-zag and armchair sites with unsatisfied valences at the edges of these layers.The heterogeneous amorphous structure of carbon leads to difficulties in structural characterization, and the techniques available have limitations.Therefore, it is very challenging to quantify the carbon structure of biochars.
The carbons used in this study were sustainable, amorphous, and non-graphitizable biochars, and quantifying the carbon structure is complicated.All carbons in Series 1 and 2 had a high proportion of ultramicroporosity (o0.7 nm), indicating diffusion limitations for liquid phases.XPS C 1s and O 1s studies indicate that surface oxygen functional groups are similar within experimental error for heat-treated carbons in Series 1 and Series 2 (Table S5, ESI †), although some differences are observed in the characterization data using titration methods (Table 6).Therefore, differences in porous structure characteristics and surface oxygen functional groups are not responsible for the significant changes in CV and EIS with HTT.The effect of HTT on carbon structure involves the loss of volatile decomposition products, as shown in the thermogravimetric and TPD data.The Raman characterization parameters show more significant changes in peak intensity (I D /I G ) and area (A D /A G ) ratios and D bandwidths for the HTT range 600-800 1C than for 800-1000 1C.The apparent crystallite size (L a ) changes progressively with HTT.The most significant changes in the CVs for vanadium redox reactions were observed for both Series 1 and Series 2 carbons for HTTs in the 600-800 1C temperature range with minor changes for HTT 800-1000 1C, and this change was independent of CV sweep rate.The electrical impedance measurements show that the electrode resistance reaches a minimum for carbons in Series 1 and 2 with a HTT of 800 1C (see Fig. 8b and d).These changes in carbon electrochemical properties coincide with the carbon HTT for maximum CVs.The electrodes used are carbon/PVDF polymer composites, and the resistivity of these materials depends on carbon structure, polymer matrix, and the volume fraction of carbon.Resistivity is related to the extent of conductive pathways between carbon particles, and carbon composite electrodes must be defect-free.The volume resistivity of the composite may be affected by both the surface groups and the carbon structure.Previous studies have reported carbon/ polymer composite volume resistivity measurements, which change with the extent of carbon black gasification and surface functional groups incorporated by HNO 3 treatment. 85,86The CV reaches a maximum, and the electrode resistance from EIS measurements a minimum at B800 1C.Therefore, it is proposed that the electrical conductivity of the carbon particles in the electrode is responsible for the major changes in CV with HTT in Series 1 and 2 carbons.
A requirement for assessing the impact of oxygen and nitrogen functional groups incorporated into carbon structure on electrochemistry was that the HTTs should not be exceeded by the treatment temperature since this could cause further carbon molecular structural change, which could obscure the impact of functional groups.Carbons N 2 /800-1 and CO 2 /800-1 were chosen because the cyclic voltammograms do not change markedly for HTT 4 800 1C.All the carbons used were predominantly ultramicroporous, and the NH 3 and K 2 CO 3 functionalization treatment conditions were also 800 1C.The maximum heat treatment of carbon CO 2 /800-1-HNO 3 was also limited to 800 1C.All the treatment procedures had minimal effects on the porous and molecular structures.The main difference observed in the CV for the functionalized carbons was the enhancement of the V 3+ /VO 2+ couple, and this was ascribed to the interaction of the nitrogen and oxygen surface groups with V(H 2 O) 6 3+ and VO(H 2 O) 5 2+ .
Oxidation of carbon CO 2 /800-1 with HNO 3 to form carbon CO 2 /800-1-HNO 3 increased the CV V 3+ /VO 2+ couple, and this was reversed by heat treatment in H 2 at 400 and 800 1C.The EIS showed that the electrode resistances were only changed slightly by functionalization.In this series of carbons, surface oxygen groups were incorporated by HNO 3 oxidation, and labile oxygen surface oxygen groups with different thermal stabilities were removed by heat treatment in H 2 .The changes in CV followed corresponding changes in surface oxygen functional groups, with only minor molecular structure changes observed.The series had a wide range of bulk oxygen contents from chemical analysis (7.54-23.84wt%, see Table 1) and surface oxygen contents from XPS (2.85-13.54at%, see Table 3), and the surface and bulk oxygen contents have a linear correlation (R 2 = 0.976) (Fig. S43, ESI †).Fig. 13 shows that the CV changes linearly with surface oxygen (R 2 = 0.999) and the oxygen groups with peaks at 531.1 eV (CQO) (R 2 = 0.999) and 533.5 eV (O-C attached to aromatic groups) (R 2 = 0.999).However, the peak at 532.3 eV, ascribed to O-C aliphatic groups, does not change markedly (R 2 = 0.624), which suggests that O-C aliphatic groups are not responsible while carboxylic groups are involved.Various functional groups, for example, carboxylic, lactone, etc.,

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Materials Advances have these types of oxygen.The XPS peak at B535.6 eV also showed a linear correlation (R 2 = 0.992) with CV current, but this only represented B5% of the total O 1s peak area.The XPS C 1s peak (289 eV) shows that the carboxylic acid groups were 6 at% in carbon CO 2 /800-1-HNO 3 , 1.3 at% in CO 2 /800-1-HNO 3 / 400, and 0.19 at% in CO 2 /800-1-HNO 3 /800 (Fig. S43, ESI †) and this is due to thermal decomposition.The intensity of this XPS C 1s peak has a linear correlation with carboxylic groups determined by titration methods (R 2 = 0.999, See Section 3.3.2and Fig. S43, ESI †).The decomposition of the COOH groups is supported by the loss of CO 2 in TPD (see Fig. 4) and the loss of an FTIR CQO carboxylic peak observed at 1713 cm À1 for carbon CO 2 /800-1-HNO 3 (Fig. S41d, ESI †).The titration measurements showed that the weakly acidic phenolic groups had little or no effect on the CV for the carbon CO 2 /800-1-HNO 3 oxidation series.The TPD results show the decomposition of N-oxide groups, carboxylic, and other oxygen groups with CO, CO 2 , and NO loss.Comparison of the XPS N 1s of carbons CO 2 /800-1-HNO 3 and CO 2 /800-1-HNO 3 /400 showed that the N-oxide peak had decomposed at 400 1C.(Fig. S27 and Table 5, ESI †).
Previous studies have shown that the adsorption of metal ions on carbon was dramatically increased by HNO 3 oxidation, and carboxylic acid groups were shown to strongly adsorb various M 2+ ions, where M 2+ = Cd, Pb, Hg, and Ca through ion exchange. 33,120The adsorption of metal ions was decreased significantly by heat treatment of the carbon.This established a relation between carboxylic acid functional groups and metal ion adsorption. 33Adsorption of vanadium species on surface carboxylic oxygen groups is a possible interaction mechanism that explains trends in the CVs and Nyquist plots.
The underlying process at high potential is most likely the kinetically limited V(III) to V(IV) oxidation and then proceeding to V(V).The oxidation of V(III) to V(IV) only occurs on a limited number of oxygen surface sites, and the outer-sphere electron transfer to oxidize V(III) takes place at much more positive potentials.K 2 CO 3 treatment of carbon CO 2 /800-1 at 800 1C enhanced the CV peaks, particularly the V 3+ /VO 2+ couple, and EIS showed that the electrode resistance increased slightly.Since both carbons CO 2 /800-1 and CO 2 /800-1-K 2 CO 3 /800 have similar ultramicroporous (Table 2) and molecular structures (Tables 7  and 8), the changes in CV are attributed to differences in functional groups.The XPS showed a small decrease in surface oxygen groups from 8.3 at% for carbon CO 2 /800-1 to 7.07% for CO 2 /800-1-K 2 CO 3 /800.Comparison of XPS C 1s spectra of carbons CO 2 /800-1 and CO 2 /800-1-K 2 CO 3 /800 did not show significant differences in the distribution of surface functional groups.The XPS O 1s scan showed a decreased CQO intensity (531 eV), while the CO/OH attached aliphatic (532.2 eV) and C-O attached to aromatic (533.5 eV) increased significantly with the K 2 CO 3 treatment.In comparison, the HNO 3 oxidation series showed that the 531.1 and 533.5 eV peaks correlated with a change in CV, whereas the 532.3 eV peaks did not.However, this must be considered in the context that some of the functional groups incorporated by chemical oxidation are thermally labile, whereas the K 2 CO 3 treatment was carried out at 800 1C.A comparison of carbons with the same HTT shows that carbons CO 2 /800-1-HNO 3 /800 and CO 2 /800-1 have main TPD CO 2 peaks at 825 and 818 1C.The TPD for carbon CO 2 /800-1-K 2 CO 3 /800 shows CO 2 desorption starting at B550 1C with peaks at 725 and 875 1C.The TPD peak at 725 1C does not coincide with the desorption of H 2 O or CO.The titration results showed carbon CO 2 /800-1-K 2 CO 3 /800 had acidic (mainly phenolic and carboxylic with a small amount of lactone/lactol) and basic groups, whereas carbons CO 2 /800-1 only had basic groups and CO 2 /800-1-HNO 3 /800 had phenolic and some basic amphoteric characteristics.This suggests that the difference is due to the CO 2 peak at 725 1C being associated with the strongly acidic groups as characterized by titration are responsible for changes in CV.The FTIR results do not show strong CQO stretching bands in the 1600-1800 cm À1 region, indicating the absence of carboxylic and anhydrides, but the spectra are weak.The characterization techniques show that the TPD CO 2 desorption peak at 725 1C, the presence of acidic functional groups, and the increased intensity XPS O 1s 533.5 eV peak are related to changes in CV.The oxygen functional groups act as sites in carbon CO 2 /800-1-K 2 CO 3 /800 for oxidizing V(III) to V(IV).
Treatment of carbon N 2 /800-1 with ammonia at 800 1C to form carbon N 2 /800-1-NH 3 /800 has a significant effect in increasing the V 3+ -VO 2+ CV peak, which was not observed for the starting carbon N 2 /800-1 and increases the V 2+ -V 3+ CV peak more than the VO 2+ -VO 2 + peak.The EIS showed that NH 3 treatment increased electrode resistance.NH 3 treatments decreased the surface oxygen from 8.1 at% in carbon N 2 /800-1 to 2.7 and 1.8 at% in carbons N 2 /800-1-NH 3 /600 and N 2 /800-1-NH 3 /800, respectively.Curve resolution of the XPS C 1s and O 1s peaks show that the distribution of the oxygen functional groups for carbons N 2 /800-1, N 2 /800-1-NH 3 /600, and N 2 /800-1-NH 3 /800 were similar.XPS N 1s studies show that the NH 3 reaction incorporates mainly pyridinic and pyrrolic nitrogen groups into the carbon structure with a ratio B2 : 1 with total Fig. 13 Comparison of cyclic voltammetry for the HNO 3 functionalized carbon series and XPS oxygen surface analysis and functional groups obtained from O 1s spectra.

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nitrogen 2.56 at% for 600 1C and 4.86 at% for 800 1C.The titration measurements show that the surface of carbon N 2 /800-1-NH 3 /800 had amphoteric characteristics with weakly acidic phenolic groups and basic pyridinic groups on the surface of carbon N 2 /800-1-NH 3 /800.The basic characteristics were slightly higher for N 2 /800-1-NH 3 /800 carbon than N 2 /800-1 due to the pyridinic groups.Surface nitrogen functional groups in activated carbons incorporated by high-temperature ammonia treatment increased the adsorption of capacities of M 2+ metal ion species (M = Cd, Ni, and Cu), and it was proposed that pyridinic groups were responsible. 34A similar interaction of vanadium species with pyridinic groups may explain the differences between the CVs and Nyquist plots before and after NH 3 treatment.The changes in electrochemistry can be attributed to incorporating nitrogen as pyridinic and pyrrolic groups into the carbon structure while reducing surface oxygen groups markedly, which catalyzes V 3+ oxidation on the surface of electrodes.Pyridinic groups are the major nitrogen functional groups in carbon N 2 /800-1-NH 3 /800.

Conclusions
The influence of the structural and surface chemical characteristics of sustainable biomass-derived carbons on vanadium redox reactions has been explored in this study.The two series of walnut shell-derived carbons made in N 2 and CO 2 atmospheres were predominantly ultramicroporous, and therefore, the porous structures had little or no effect on the vanadium redox electrochemical characteristics because of diffusion limitations.The XPS C 1s and O 1s showed minimal changes in surface oxygen functional groups.The effect of carbonization temperature on carbon molecular structure was shown by changes in Raman and X-ray diffraction parameters.The most intense couple in the CV for the vanadium redox electrochemistry was VO 2+ /VO 2 + , where the current increased markedly for carbonization under N 2 and CO 2 in the temperature range of 600-800 1C.However, only minimal changes were observed when increasing the HTT from 800 1C to 1000 1C.These changes are the most significant in cyclic voltammetry.Electrical impedance measurements showed that the electrode resistance reached a minimum at HTT 800 1C.Biochar carbonization temperature has the primary effect on electrochemical properties.This effect is attributed to the modification of the carbon molecular structure, increasing electrical conductivity over the HTT range of 600-800 1C.
The role of surface groups in carbon structures was investigated by chemical oxidation and high-temperature treatment methods.Carbons with an HTT of 800 1C were used for functionalization studies since the cyclic voltammograms are close to the maximum CV current and avoid the effects of molecular structural change due to HTT.Oxidation of carbon CO 2 /800-1 with nitric acid resulted in incorporating a mixture of labile acidic carboxylic, lactone, lactol, and anhydride functional groups into the carbon structure, and the CV current of the V 3+ /VO 2+ couple also increased significantly.Minor changes in CV were also observed for the V 2+ /V 3+ and VO 2+ /VO 2 + couples.
These changes in CV were partly reversed when the oxidized carbon sample was heat treated in H 2 to 400 1C, removing carboxylic groups.Thermolysis at 800 1C in H 2 removed further oxygen functional groups, with the CV and Nyquist graphs being similar to the original carbon CO 2 /800-1, indicating some reversibility.The changes in the CV for the V 3+ /VO 2+ couple are related to changes in surface oxygen in the carbons.The relationship between surface carboxylic groups and CV changes, as shown by XPS, is supported by titration, TPD, and FTIR measurements.The underlying process at high potential is most likely the kinetically limited V(III) to V(IV) oxidation and then to V(V).The oxidation of V(III) to V(IV) only occurs on a limited number of oxygen surface sites, and the outer-sphere electron transfer to oxidize V(III) takes place only at much more positive potentials.
Similarly, electrodes prepared with carbon CO 2 /800-1-K 2 CO 3 /800 had small increases in the CV currents for V 2+ /V 3+ , V 3+ /VO 2+ , and VO 2+ /VO 2 + couples compared with electrodes made from carbon CO 2 /800-1.XPS, titration, and TPD studies showed that K 2 CO 3 treatment incorporated thermally stable acidic surface oxygen functional groups in the carbon structure.The changes in cyclic voltammetry were attributed to changes in oxygen functional groups.NH 3 treatment of carbon N 2 /800-1 at 600 and 800 1C resulted in the incorporation of nitrogen mainly as surface pyridinic and pyrrolic groups in the carbon structure and the removal of a large proportion of surface oxygen groups.The CV for carbon N 2 /800-1-NH 3 /800 showed the presence of a broad V 3+ /VO 2+ peak absent in the starting material carbon N 2 /800-1.Incorporating nitrogen functional groups into the carbon structure has a similar effect to incorporating oxygen functional groups into the carbon structure and involves catalysis of V 3+ /VO 2+ oxidation on the nitrogen surface sites of the electrodes.
The coulombic, voltage, efficiency, and energy efficiency for walnut shell carbons were suitable for VFRBs, and good stability was observed.The functionalization of the carbons only had minor effects on voltage, efficiency, and energy efficiency for both the HNO 3 oxidized and NH 3 treated carbons.The study of carbon structure and functional group relationships with vanadium redox electrochemistry supports the development of improved electrode materials for vanadium redox flow batteries.
4.2.)The changes in Raman parameters (I D /I G , A D /A G , and FWHM D ) change markedly over the HTT range 600-800 1C (Section 3.4.1),but only to a smaller extent up to 1000 1C.The cyclic voltammograms change markedly in the HTT range of 600-800 1C but only slightly over the temperature range (800-1000 1C).This transition temperature coincides with the minimum carbon electrode resistance measured by EIS and the change in carbon surface characteristics from acidic to basic.

Table 1
Elemental and ash analysis for the carbons used in this study

Table 4
Oxygen functional group surface analysis from XPS for the carbon samples used in this study

Table 6
Functional group concentrations from titration studies for carbons (mequiv.g À1 )

Table 8
Crystallite thickness L c , interlayer distance d 002 , and apparent crystallite diameter L a from PXRD a Average values.