Abstract

1. Introduction

Black liquor is a toxic, dark-colored byproduct of paper and pulp production, which can adversely affect water resources¹1 Fu K, Yue Q, Gao B, Sun Y, Zhu L. Preparation, characterization and application of lignin-based activated carbon from black liquor lignin by steam activation. Chem Eng J. 2013;228:1074-82.. This alkaline effluent is primarily composed of dissolved lignin, minerals²2 Boucard H, Weiss-Hortala E, Gueye F, Espitalier F, Barna R. Insights in mechanisms of carbonaceous microparticles formation from black liquor hydrothermal conversion. J Supercrit Fluids. 2020;161:104817., cellulose, hemicellulose, sodium salts, sulfur compounds, and other components³3 Amaral-Labat G, da Silva EL, Cuña A, Malfatti CF, Marcuzzo JS, Baldan MR, et al. A sustainable carbon material from kraft black liquor as nickel-based electrocatalyst support for ethanol electro-oxidation. Waste Biomass Valoriz. 2021;12(5):2507-19.. Black liquor is abundantly produced during the Kraft pulping process, wherein the production of 1 ton of pulp generates approximately the same amount of black liquor³3 Amaral-Labat G, da Silva EL, Cuña A, Malfatti CF, Marcuzzo JS, Baldan MR, et al. A sustainable carbon material from kraft black liquor as nickel-based electrocatalyst support for ethanol electro-oxidation. Waste Biomass Valoriz. 2021;12(5):2507-19.. The considerable production of black liquor warrants research into economically viable and environmentally friendly methods of disposal¹1 Fu K, Yue Q, Gao B, Sun Y, Zhu L. Preparation, characterization and application of lignin-based activated carbon from black liquor lignin by steam activation. Chem Eng J. 2013;228:1074-82., which otherwise would undergoes incineration in industrial boilers for energy generation⁴4 Diehl BG, Brown NR, Frantz CW, Lumadue MR, Cannon F. Effects of pyrolysis temperature on the chemical composition of refined softwood and hardwood lignins. Carbon N Y. 2013;60:531-7.. The lignin, cellulose, and hemicellulose from black liquor may react with formaldehyde during polymerization reactions under alkaline conditions at room temperature³3 Amaral-Labat G, da Silva EL, Cuña A, Malfatti CF, Marcuzzo JS, Baldan MR, et al. A sustainable carbon material from kraft black liquor as nickel-based electrocatalyst support for ethanol electro-oxidation. Waste Biomass Valoriz. 2021;12(5):2507-19.. An alternative approach to augment and regulate the environmental impact of black liquor is its use for the production of new porous carbonaceous materials¹1 Fu K, Yue Q, Gao B, Sun Y, Zhu L. Preparation, characterization and application of lignin-based activated carbon from black liquor lignin by steam activation. Chem Eng J. 2013;228:1074-82., as lignin is composed of phenylpropane units, which are highly reactive⁵5 Seo J, Park H, Shin K, Baeck SH, Rhym Y, Shim SE. Lignin-derived macroporous carbon foams prepared by using poly(methyl methacrylate) particles as the template. Carbon N Y. 2014;76:357-67. and have a high carbon yield⁶6 Zhu J, Yan C, Zhang X, Yang C, Jiang M, Zhang X. A sustainable platform of lignin: from bioresources to materials and their applications in rechargeable batteries and supercapacitors. Pror Energy Combust Sci. 2020;76:100788..

New carbon-based materials, such as graphene derivatives, have extensive applications, for example, as electrodes in supercapacitor⁷7 Kumar R, Sahoo S, Tan WK, Kawamura G, Matsuda A, Kar KK. Microwave-assisted thin reduced graphene oxide-cobalt oxide nanoparticles as hybrids for electrode materials in supercapacitor. J Energy Storage. 2021;40:102724.^,88 Kumar R, Abdel-Galeil MM, Ya KZ, Fujita K, Tan WK, Matsuda A. Facile and fast microwave-assisted formation of reduced graphene oxide-wrapped manganese cobaltite ternary hybrids as improved supercapacitor electrode material. Appl Surf Sci. 2019;481:296-306. and materials for microwave electromagnetic interference⁹9 Kumar R, Sahoo S, Joanni E, Singh RK, Tan WK, Kar KK, et al. Recent progress on carbon-based composite materials for microwave electromagnetic interference shielding. Carbon N Y. 2021;177:304-31., energy conversion and storage¹⁰10 Kumar R, Joanni E, Singh RK, Singh DP, Moshkalev SA. Recent advances in the synthesis and modification of carbon-based 2D materials for application in energy conversion and storage. Pror Energy Combust Sci. 2018;67:115-57.^,1111 Kumar R, Sahoo S, Joanni E, Singh RK, Maegawa K, Tan WK, et al. Heteroatom doped graphene engineering for energy storage and conversion. Mater Today. 2020;39:47-65., and gas sensors¹²12 Hashtroudi H, Kumar R, Savu R, Moshkalev S, Kawamura G, Matsuda A, et al. Hydrogen gas sensing properties of microwave-assisted 2D Hybrid Pd/rGO: effect of temperature, humidity and UV illumination. Int J Hydrogen Energy. 2021;46(10):7653-65.. However, these materials are generally produced from non-renewable resources through multiple steps, thereby increasing their final cost. Therefore, cost-effective carbonaceous materials derived from black liquor have been used for electromagnetic wave absorption¹³13 Vergara DEF, Lopes BHK, Quirino SF, Silva GFBL, Boss AFN, Amaral-Labat GA, et al. Frequency selective surface properties of microwave new absorbing porous carbon materials embedded in epoxy resin. Mater Res. 2019;22(Suppl. 1):e20180834.^,1414 Bispo MC, Lopes BHK, Fonseca BCS, Portes RC, Matsushima JT, Yassuda MKH, et al. Electromagnetic properties of Carbon-Graphene Xerogel, Graphite and Ni-Zn Ferrite composites in polystyrene matrix in the X-Band (8.2 – 12.4 GHz). Materia. 2021;26(2):e12967., as an electrocatalyst support for fuel cells³3 Amaral-Labat G, da Silva EL, Cuña A, Malfatti CF, Marcuzzo JS, Baldan MR, et al. A sustainable carbon material from kraft black liquor as nickel-based electrocatalyst support for ethanol electro-oxidation. Waste Biomass Valoriz. 2021;12(5):2507-19., for adsorption of heavy metals¹⁵15 Tian Y, Zhou H. A novel nitrogen-doped porous carbon derived from black liquor for efficient removal of Cr(VI) and tetracycline: comparison with lignin porous carbon. J Clean Prod. 2022;333:130106.^,1616 Gao Y, Yue Q, Gao B, Sun Y, Wang W, Li Q, et al. Preparation of high surface area-activated carbon from lignin of papermaking black liquor by KOH activation for Ni(II) adsorption. Chem Eng J. 2013;217:345-53.^-1717 Zhao Y, Tian Y, Zhou H, Tian Y. Hydrothermal conversion of black liquor to phenolics and hydrochar: characterization, application and comparison with lignin. Fuel. 2020;280:118651. and antibiotics¹⁵15 Tian Y, Zhou H. A novel nitrogen-doped porous carbon derived from black liquor for efficient removal of Cr(VI) and tetracycline: comparison with lignin porous carbon. J Clean Prod. 2022;333:130106., in electrical systems¹⁸18 Foulet A, Birot M, Backov R, Sonnemann G, Deleuze H. Preparation of hierarchical porous carbonaceous foams from Kraft black liquor. Mater Today Commun. 2016;7:108-16., and for energy storage¹⁹19 Plavniece A, Volperts A, Dobele G, Zhurinsh A, Kaare K, Kruusenberg I, et al. Wood and black liquor-based N-doped activated carbon for energy application. Sustainability. 2021;13(16):9237..

Simultaneously, Cu-based electrocatalysts can be obtained using a simple, fast, and low-cost electrochemical deposition process, while producing non-toxic products in an aqueous medium at room temperature and minimizing environmental impact. This procedure is cost-effective compared to other processes, such as evaporation deposition, chemical reduction, and mechano-chemical synthesis²⁰20 Saini K, Shree Pandey R. Concentration-dependent electrochemical synthesis of quantum dot and nanoparticles of copper and shape-dependent degradation of methyl orange. Adv Mater Lett. 2017;8(11):1080-8.. In this technique, lower potential facilitates electrodissolution of Cu into Cu²⁺ ions in the presence of ascorbic acid, which acts as the reducing agent²⁰20 Saini K, Shree Pandey R. Concentration-dependent electrochemical synthesis of quantum dot and nanoparticles of copper and shape-dependent degradation of methyl orange. Adv Mater Lett. 2017;8(11):1080-8.. Briefly, a potential is applied that induces the oxidation of the Cu plate to Cu²⁺ species, and the reducing agent promotes the reduction of Cu²⁺ to Cu⁰, leading to the nucleation step, followed by the growth phase, to generate Cu particles²⁰20 Saini K, Shree Pandey R. Concentration-dependent electrochemical synthesis of quantum dot and nanoparticles of copper and shape-dependent degradation of methyl orange. Adv Mater Lett. 2017;8(11):1080-8..

Electrochemical CO₂ reduction has been extensively studied to convert CO₂ into chemicals and fuels²¹21 Li L, Huang Y, Li Y. Carbonaceous materials for electrochemical CO2 reduction. EnergyChem. 2020;2(1):100024.

22 Han X, Wang M, Le ML, Bedford NM, Woehl TJ, Thoi VS. Effects of substrate porosity in carbon aerogel supported copper for electrocatalytic carbon dioxide reduction. Electrochim Acta. 2019;297:545-52.

23 Wang Y, Niu C, Wang D. Metallic nanocatalysts for electrochemical CO2 reduction in aqueous solutions. J Colloid Interface Sci. 2018;527:95-106.

24 Sun Z, Ma T, Tao H, Fan Q, Han B. Fundamentals and challenges of electrochemical CO2 reduction using two-dimensional. Mater Chem. 2017;3(4):560-87.

25 Fan Q, Zhang M, Jia M, Liu S, Qiu J, Sun Z. Electrochemical CO2 reduction to C2+ species: heterogeneous electrocatalysts, reaction pathways, and optimization strategies. Mater Today Energy. 2018;10:280-301.

26 Ma T, Fan Q, Li X, Qiu J, Wu T, Sun Z. Graphene-based materials for electrochemical CO2 reduction. J CO2 Util. 2019;30:168-82.^-2727 Yu J, Liu H, Song S, Wang Y, Tsiakaras P. Electrochemical reduction of carbon dioxide at nanostructured SnO2/carbon aerogels: the effect of tin oxide content on the catalytic activity and formate selectivity. Appl Catal A Gen. 2017;545:159-66.. This process includes a simple experimental setup, with great potential for a large-scale applications²¹21 Li L, Huang Y, Li Y. Carbonaceous materials for electrochemical CO2 reduction. EnergyChem. 2020;2(1):100024., besides the possibility to use renewable energy to support the system²²22 Han X, Wang M, Le ML, Bedford NM, Woehl TJ, Thoi VS. Effects of substrate porosity in carbon aerogel supported copper for electrocatalytic carbon dioxide reduction. Electrochim Acta. 2019;297:545-52.^,2828 Li Q, Rao X, Sheng J, Xu J, Yi J, Liu Y, et al. Energy storage through CO2 electroreduction: a brief review of advanced Sn-based electrocatalysts and electrodes. J CO2 Util. 2018;27:48-59.

29 Bashir S, Hossain SS, Rahman SU, Ahmed S, Al-Ahmed A, Hossain MM. Electrocatalytic reduction of carbon dioxide on SnO2/MWCNT in aqueous electrolyte solution. J CO2 Util. 2016;16:346-53.

30 Wang S, Kou T, Baker SE, Duoss EB, Li Y. Recent progress in electrochemical reduction of CO2 by oxide-derived copper catalysts. Mater Today Nano. 2020;12:100096.

31 Yuan J, Yang MP, Zhi WY, Wang H, Wang H, Lu JX. Efficient electrochemical reduction of CO2 to ethanol on Cu nanoparticles decorated on N-doped graphene oxide catalysts. J CO2 Util. 2019;33:452-60.^-3232 Gang Y, Pan F, Fei Y, Du Z, Hu YH, Li Y. Highly efficient Nickel, Iron, and Nitrogen codoped carbon catalysts derived from industrial waste petroleum coke for electrochemical CO2 reduction. ACS Sustain Chem& Eng. 2020;8(23):8840-7.. However, to ensure commercial viability, it is essential to identify ways to overcome high thermodynamic stability of the CO₂ molecule²¹21 Li L, Huang Y, Li Y. Carbonaceous materials for electrochemical CO2 reduction. EnergyChem. 2020;2(1):100024.^,3333 Lu Q, Jiao F. Electrochemical CO2 reduction: electrocatalyst, reaction mechanism, and process engineering. Nano Energy. 2016;29:439-56.^,3434 Xiao X, Xu Y, Lv X, Xie J, Liu J, Yu C. Electrochemical CO2 reduction on copper nanoparticles-dispersed carbon aerogels. J Colloid Interface Sci. 2019;545:1-7. and direct it towards the formation of a particular product²¹21 Li L, Huang Y, Li Y. Carbonaceous materials for electrochemical CO2 reduction. EnergyChem. 2020;2(1):100024., such as CO, formic acid or formate, CH₄, C₂H₄, ethanol, and acetate²¹21 Li L, Huang Y, Li Y. Carbonaceous materials for electrochemical CO2 reduction. EnergyChem. 2020;2(1):100024.. Another issue with this process is the H₂ evolution reaction, which exhibits rapid kinetics and competes with CO₂ reduction²¹21 Li L, Huang Y, Li Y. Carbonaceous materials for electrochemical CO2 reduction. EnergyChem. 2020;2(1):100024.^,2626 Ma T, Fan Q, Li X, Qiu J, Wu T, Sun Z. Graphene-based materials for electrochemical CO2 reduction. J CO2 Util. 2019;30:168-82.^,3434 Xiao X, Xu Y, Lv X, Xie J, Liu J, Yu C. Electrochemical CO2 reduction on copper nanoparticles-dispersed carbon aerogels. J Colloid Interface Sci. 2019;545:1-7., generating more stable intermediates than those obtained from CO₂ reduction, such as CO or *COOH²⁶26 Ma T, Fan Q, Li X, Qiu J, Wu T, Sun Z. Graphene-based materials for electrochemical CO2 reduction. J CO2 Util. 2019;30:168-82.. The development of active, selective, and durable electrocatalysts²¹21 Li L, Huang Y, Li Y. Carbonaceous materials for electrochemical CO2 reduction. EnergyChem. 2020;2(1):100024. can help overcome these obstacles and increase the efficiency of the CO₂ reduction process.

Cu-based electrocatalysts have reduced cost, high stability, high efficiency for CO₂ electroreduction³⁵35 Yan Y, Ke L, Ding Y, Zhang Y, Rui K, Lin H, et al. Recent advances in Cu-based catalysts for electroreduction of carbon dioxide. Mater Chem Front. 2021;5(6):2668-83., and effectively convert CO₂ into hydrocarbons²²22 Han X, Wang M, Le ML, Bedford NM, Woehl TJ, Thoi VS. Effects of substrate porosity in carbon aerogel supported copper for electrocatalytic carbon dioxide reduction. Electrochim Acta. 2019;297:545-52.^,3636 Pérez-Cadenas AF, Ros CH, Morales-Torres S, Pérez-Cadenas M, Kooyman PJ, Moreno-Castilla C, et al. Metal-doped carbon xerogels for the electro-catalytic conversion of CO2 to hydrocarbons. Carbon N Y. 2013;56:324-31.^,3737 Lee S, Hong S, Lee J. Bulk pH contribution to CO/HCOO− production from CO2 on oxygen-evacuated Cu2O electrocatalyst. Catal Today. 2017;288:11-7.. Furthermore, Cu presents a moderate *CO-intermediate binding energy and can catalyze the conversion of CO₂ to CO, HCOOH, alcohols, and hydrocarbons³⁵35 Yan Y, Ke L, Ding Y, Zhang Y, Rui K, Lin H, et al. Recent advances in Cu-based catalysts for electroreduction of carbon dioxide. Mater Chem Front. 2021;5(6):2668-83., whereas other metals exclusively produce CO and HCOO^-37. Thus, Cu is able to convert CO₂ into C₂–C₃ hydrocarbons, whereas other metals are only capable of converting CO₂ into C₁ products²³23 Wang Y, Niu C, Wang D. Metallic nanocatalysts for electrochemical CO2 reduction in aqueous solutions. J Colloid Interface Sci. 2018;527:95-106.. Moreover, the addition of a porous matrix facilitates the diffusion process of reactive species and alter their selectivity²²22 Han X, Wang M, Le ML, Bedford NM, Woehl TJ, Thoi VS. Effects of substrate porosity in carbon aerogel supported copper for electrocatalytic carbon dioxide reduction. Electrochim Acta. 2019;297:545-52.. Therefore, carbon-based materials can be used to anchor metallic Cu, thereby preventing particle aggregation and favoring catalytic activity³⁵35 Yan Y, Ke L, Ding Y, Zhang Y, Rui K, Lin H, et al. Recent advances in Cu-based catalysts for electroreduction of carbon dioxide. Mater Chem Front. 2021;5(6):2668-83.. Although few carbonaceous supports, such as carbon aerogel²²22 Han X, Wang M, Le ML, Bedford NM, Woehl TJ, Thoi VS. Effects of substrate porosity in carbon aerogel supported copper for electrocatalytic carbon dioxide reduction. Electrochim Acta. 2019;297:545-52.^,3434 Xiao X, Xu Y, Lv X, Xie J, Liu J, Yu C. Electrochemical CO2 reduction on copper nanoparticles-dispersed carbon aerogels. J Colloid Interface Sci. 2019;545:1-7., graphene³¹31 Yuan J, Yang MP, Zhi WY, Wang H, Wang H, Lu JX. Efficient electrochemical reduction of CO2 to ethanol on Cu nanoparticles decorated on N-doped graphene oxide catalysts. J CO2 Util. 2019;33:452-60.^,3838 Dongare S, Singh N, Bhunia H. Nitrogen-doped graphene supported copper nanoparticles for electrochemical reduction of CO2. J CO2 Util. 2021;44:101382., and carbon nanotubes³⁹39 Irfan Malik M, Malaibari ZO, Atieh M, Abussaud B. Electrochemical reduction of CO2 to methanol over MWCNTs impregnated with Cu2O. Chem Eng Sci. 2016;152:468-77., have been used previously, their syntheses require multiple steps and is often expensive. Therefore, new carbonaceous supports derived from biomass are being explored⁴⁰40 Gong S, Xiao X, Wang W, Sam DK, Lu R, Xu Y, et al. Silk fibroin-derived carbon aerogels embedded with copper nanoparticles for efficient electrocatalytic CO2-to-CO conversion. J Colloid Interface Sci. 2021;600:412-20.^,4141 Costa RS, Aranha BSR, Ghosh A, Lobo AO, da Silva ETSG, Alves DCB, et al. Production of oxalic acid by electrochemical reduction of CO2 using silver-carbon material from babassu coconut mesocarp. J Phys Chem Solids. 2020;147:109678.. In this regard, the findings of this study will contribute to the production of a carbon structure derived from industrial waste (black liquor) using a sustainable method.

Strategies to lower greenhouse gas emissions to reduce global warming were one of the important themes discussed at COP26 UN Climate Change Conference. This study can help address this issue in the following ways: (i) the employment of a byproduct to produce sustainable raw material for CO₂ reduction, thereby stimulating circular economy and increasing green jobs; (ii) the production of metal-based electrocatalyst using green synthesis; (iii) development of a sustainable strategy for CO₂ sequestration; and (iv) the conversion of CO₂ into value-added products.

This study describes the production of a Cu-based electrocatalyst supported on a bio-based carbon matrix using a simple, sustainable, and low-cost method. The bio-based carbon matrix was first synthesized from raw Kraft black liquor, and then Cu nanoparticles were electrochemically deposited on the carbon matrix without using toxic reagents. Thereafter, the physico-chemical, morphological, textural, and electrochemical properties of the renewable Cu-supported bio-based carbon matrix were determined.

To the best of our knowledge, this is the first study on the use of a porous carbon material, derived from black liquor, as an electrocatalyst support for electrochemical CO₂ reduction, the incorporation of Cu nanoparticles from Cu plates on porous carbon matrix using an environmentally friendly electrodeposition method, and the concentration of black liquor to obtain large surface area. Furthermore, as the spontaneous evaporation of black liquor is a simple method that does not involve activation agents and heating processes.

2. Experimental

2.1. Synthesis of bio-based carbon matrix (C matrix)

Raw Kraft black liquor was obtained from Suzano Papel & Cellulose (Mogi das Cruzes/SP, Brazil) and dried at room temperature for 10 d, which increased the solid content from 16% (w/w) to 25% (w/w). Next, solid content was estimated using gravimetric tests that encompassed weighing the sample before and after drying at 105 °C for 24 h (n = 3)³3 Amaral-Labat G, da Silva EL, Cuña A, Malfatti CF, Marcuzzo JS, Baldan MR, et al. A sustainable carbon material from kraft black liquor as nickel-based electrocatalyst support for ethanol electro-oxidation. Waste Biomass Valoriz. 2021;12(5):2507-19.. Gravimetric tests were also performed for the received liquor sample for comparison.

Sustainable, bio-based, porous carbon was produced from 25.00 g of concentrated black liquor (25% solid content), which was mixed with 3.35 g of resorcinol using a magnetic stirrer at room temperature and pressure (25 °C, 1 atm) until resorcinol was dissolved completely. Then, 11.25 g of polymethylmethacrylate (PMMA) and 11.00 g of 37% aqueous solution of formaldehyde were added to the reaction medium and agitated until a solid gel was obtained. The solid gel was cured and dried under room conditions for 5 d. Thereafter, carbonization of this material was performed in a horizontal furnace at a heating rate of 5 °C min^-1 until 900 °C for 2 h, under inert (N₂) atmospheric conditions. Finally, the bio-based, porous carbon material was washed with deionized water until a neutral pH was achieved, and a fine powder (C matrix) with a particle size of <38 µm was obtained. For comparison, a sample (CBL16) was prepared through the same procedure, but using raw black liquor with 16% solid content.

2.2. Synthesis of bio-based, porous carbon matrix decorated with Cu nanoparticles (C–Cu)

The process of Cu deposition on C matrix was performed at room temperature and pressure. A potential of 30 V was applied using a DC Power Supply (FA-1030 Instrutherm). Electrolytic medium was prepared by mixing 0.55 g of C matrix, 0.20 g of the reducing agent, i.e., vitamin C (ascorbic acid), and 60 mL of deionized water. A magnetic stirrer was used throughout the process. Two Cu plates were immersed in the dark-colored aqueous medium, and Cu deposition occurred on the C matrix within 20 min with N₂ bubbling. At the end of the process, the brownish aqueous medium was oven-dried at 110 °C, and the resulting composite was labeled as C–Cu. Figure 1 presents a schematic illustration of the synthesis of the C matrix and the C–Cu composite.

Figure 1
Schematic illustration of the synthesis of C matrix and C–Cu composite.

2.3. Characterization of the C matrix and the C–Cu composite

The morphology of the samples was evaluated using scanning electron microscope fitted with a field emission gun (FEG-SEM; TESCAN MIRA3N), in the SE mode at a voltage of 5 kV. The primary elements in the composite were quantified using a detector (Oxford X-MAX 50 EDS).

The porosity, surface area, and pore volume of the samples were determined through N₂ adsorption/desorption isotherm measurements at 77 K using the Micromeritics ASAP 2020 Plus equipment. First, the samples were subjected to a 24-h degassing process at 200 °C. Then, Brunauer–Emmet–Teller (BET) method was used to calculate the specific surface area (S_BET)⁴²42 Brunauer S, Emmett PH, Teller E. Adsorption of gases in multimolecular layers. J Am Chem Soc. 1938;60(2):309-19., whereas micropore volume (V_DR) was evaluated using the Dubinin–Radushkevic method⁴³43 Dubinin MM. Fundamentals of the theory of adsorption in micropores of carbon adsorbents: characteristics of their adsorption properties and microporous structures. Carbon N Y. 1989;27(3):457-67.. The mesopore volume was estimated as V_0,97−V_DR, and pore-size distribution was calculated using the density functional theory (DFT)⁴⁴44 Tarazona P. Solid-fluid transition and interfaces with density functional approaches. Surf Sci. 1995;331–333:989-94..

The crystalline structure and phase composition of the samples were analyzed by X-ray diffraction (XRD) using a diffractometer (PANalytical series X’PertPRO), with CuK-α radiation (α = 0.154056 nm) from 10º to 100º in 2-theta at a voltage of 40 kV, current of 30 mA, step size of 0.02º, and a counting time of 15 s per step.

2.4. Catalytic activity

The catalytic activity of the samples for electrochemical CO₂ reduction was evaluated by linear sweep voltammetry (LSV) measurements conducted using a potentiostat (Solartron analytical 1470E) with three-electrode cell configuration. A rectangular Pt electrode was used as the counter electrode, and Ag/AgCl system was used as the reference electrode. The working electrodes (C matrix and C–Cu composite) were deposited on a graphite plate, after blending 15 mg of sample, 100 µL of ethanol, 100 µL of 5% Nafion^®, and 400 µL of deionized water. This suspension was prepared by ultrasonic agitation and dried at 100 °C after deposition. Subsequently, electrochemical measurements were performed using 30 mL of aqueous electrolyte KHCO₃ (1 mol L^-1) in a cell previously purged with N₂, and then CO₂ was added to the medium (45 mL min^-1).

3. Results and Discussion

3.1. Morphology, composition, and structural properties

FEG-SEM was employed to evaluate the morphology of the C matrix and the C–Cu composite. Figures 2a,b show the FEG-SEM images of the C matrix with uniformly distributed pores throughout its rough surface. Spherical Cu nanoparticles (red arrows) were homogenously dispersed on the sustainable carbon source, as shown in Figures 2c,d. Figure 3 presents a broad particle-size distribution of Cu particles on the C–Cu composite, with an average diameter of approximately 161 nm and a great amount of particle sizes <100 nm. The surface morphology of the C–Cu composite was also evaluated using EDS analysis and elemental mapping, as shown in Figures 4 and 5, respectively.

Figure 2
FEG-SEM images of the C matrix (a–b) and the C–Cu composite (c–d).

Figure 3
Particle-size distribution of Cu micro- and nanoparticles on the C–Cu composite.

Figure 4
EDS spectrum of the C–Cu composite.

Figure 5
EDS analysis of the C–Cu composite.

EDS spectrum not only indicated the presence of C and O in the C matrix, with Na and S as impurities from the delignification process³3 Amaral-Labat G, da Silva EL, Cuña A, Malfatti CF, Marcuzzo JS, Baldan MR, et al. A sustainable carbon material from kraft black liquor as nickel-based electrocatalyst support for ethanol electro-oxidation. Waste Biomass Valoriz. 2021;12(5):2507-19., but also verified the presence of Cu (Figure 4). The weight percentage (wt.%) indicates elemental content (Figure 4). In addition, elemental mapping was performed using the EDS spectrum to evaluate the spatial distribution of C, Cu, and O on the C–Cu composite (Figure 5). As expected, C and O were present on the C matrix. Furthermore, the electrochemically decorated Cu nanoparticles were uniformly distributed on the surface of the bio-based, porous carbon material. Figure 6 shows XRD patterns for the C matrix and the C–Cu composite.

Figure 6
XRD spectra of C matrix and C-Cu composite.

The spectrum of the C–Cu composite verified the formation of Cu⁰ (00-002-1225/00-004-0836) in the carbonaceous material after deposition. The diffraction angles were identified at 43.25°, 50.35°, 74.04°, 89.87°, and 95.07°, corresponding to the planes (111), (200), (220), (311), and (222), respectively, as previously reported²²22 Han X, Wang M, Le ML, Bedford NM, Woehl TJ, Thoi VS. Effects of substrate porosity in carbon aerogel supported copper for electrocatalytic carbon dioxide reduction. Electrochim Acta. 2019;297:545-52.^,3131 Yuan J, Yang MP, Zhi WY, Wang H, Wang H, Lu JX. Efficient electrochemical reduction of CO2 to ethanol on Cu nanoparticles decorated on N-doped graphene oxide catalysts. J CO2 Util. 2019;33:452-60.^,3434 Xiao X, Xu Y, Lv X, Xie J, Liu J, Yu C. Electrochemical CO2 reduction on copper nanoparticles-dispersed carbon aerogels. J Colloid Interface Sci. 2019;545:1-7.^,4545 Cao Y, Moniri Javadhesari S, Mohammadnejad S, Khodadustan E, Raise A, Akbarpour MR. Microstructural characterization and antibacterial activity of carbon nanotube decorated with Cu nanoparticles synthesized by a novel solvothermal method. Ceram Int. 2021;47(18):25729-37.^,4646 Dong Y, Wang K, Tan Y, Wang Q, Li J, Mark H, et al. Synthesis and characterization of pure copper nanostructures using wood inherent architecture as a natural template. Nanoscale Res Lett. 2018;13(1):1-8.. These results confirm that ascorbic acid reduced the oxide to its metallic form. The C matrix presents characteristic peaks of amorphous carbon structure around 25° and 44°, corresponding to (002) and (101)¹³13 Vergara DEF, Lopes BHK, Quirino SF, Silva GFBL, Boss AFN, Amaral-Labat GA, et al. Frequency selective surface properties of microwave new absorbing porous carbon materials embedded in epoxy resin. Mater Res. 2019;22(Suppl. 1):e20180834.. Additionally, most of the low intensity peaks in the diffractogram of the C matrix were probably related to inorganic matter from biomass ashes, such as chlorocalcite (KCaCl₃) (00-021-1170)¹³13 Vergara DEF, Lopes BHK, Quirino SF, Silva GFBL, Boss AFN, Amaral-Labat GA, et al. Frequency selective surface properties of microwave new absorbing porous carbon materials embedded in epoxy resin. Mater Res. 2019;22(Suppl. 1):e20180834., and the two remaining peaks can be associated with sulfur compounds (S 01-076-0183/ SCl₂ 01-077-0296/ Na₂S₅ 01-077-0294). These findings agree with those of the EDS spectrum.

3.2. Textural properties

Table 1 shows the textural properties of the C matrix and CBL16 samples and the C–Cu composite. Solid content increased from 16% to 25% by evaporation of the raw black liquor under room conditions, allowing a substantial increase in the surface area (121 m² g^-1). Total porosity constituted 58% of micropores and 42% of mesopores, exhibiting approximately equal proportions.

Previous studies report the use of black liquor as a precursor for different carbon materials, such as char, activated carbon, and porous carbon materials, which can be used for different purposes (Table 2). For increased porosity, these carbon materials undergo several preparatory steps, including hydrothermal processes, chemical reactions and carbonization under varying temperatures, and physical or chemical activation at various temperatures for different periods, which involve the use of gases (O₂), water vapor (H₂O), or chemicals (NaOH and KOH). Moreover, the latter requires several washing steps with HCl solutions and distilled water. The chemical activation process normally results in porous materials with large pore sizes compared to those obtained using physical activation process (Table 2), but multiple steps of preparation, such as the washing step, are always required. As a consequence, the final cost of the material increases.

Herein, the prepared non-activated porous carbon matrix exhibited an S_BET value 10 times higher than that of non-activated char prepared using hydrothermal synthesis. The surface area also exhibited the same order of magnitude compared to physically activated materials. Furthermore, the proposed sustainable material was prepared through a simple process, involving a chemical reaction under room conditions and few chemicals, followed by a standard carbonization process. The preparation did not employ high pressure or temperature and activating agents during the polymerization reaction, and thus reduced the final cost of the catalyst. In addition, the procedure required crude black liquor without lignin extraction or purification, contributing to the completeness of the original polymer network of the macromolecule. Black liquor was only subjected to an air-drying process to improve concentration, and thus increased porosity. The total porosity was further achieved using PMMA and with an increase in solid content in black liquor. Moreover, in this study, no purification process was used. After polymerization reaction, the formed gel was dried under ambient conditions (25 °C, 1 atm), again avoiding the use of high pressures and temperature. Therefore, this method is more viable as it did not involve the use of additional chemicals (CO₂, HCl, and H₂SO₄) and the washing step³3 Amaral-Labat G, da Silva EL, Cuña A, Malfatti CF, Marcuzzo JS, Baldan MR, et al. A sustainable carbon material from kraft black liquor as nickel-based electrocatalyst support for ethanol electro-oxidation. Waste Biomass Valoriz. 2021;12(5):2507-19. for the production of bio-based carbon.

The sustainable C–Cu composite exhibited a surface area (7 m² g^-1) comparable to that of the carbon xerogel-supported Cu (4 m² g^-1)³⁶36 Pérez-Cadenas AF, Ros CH, Morales-Torres S, Pérez-Cadenas M, Kooyman PJ, Moreno-Castilla C, et al. Metal-doped carbon xerogels for the electro-catalytic conversion of CO2 to hydrocarbons. Carbon N Y. 2013;56:324-31.. However, the latter was prepared using synthetic products in a resorcinol–formaldehyde system and metal acetate, while distributing metallic Cu on the external surface of the sample. Despite the non-microporous structure, the synthetic xerogel exhibited electrocatalytic activity during the formation of hydrocarbons, primarily CH₄³⁶36 Pérez-Cadenas AF, Ros CH, Morales-Torres S, Pérez-Cadenas M, Kooyman PJ, Moreno-Castilla C, et al. Metal-doped carbon xerogels for the electro-catalytic conversion of CO2 to hydrocarbons. Carbon N Y. 2013;56:324-31..

Metal loading onto a carbon support generally results in a decrease in the surface area and total pore volume of the matrix, suggesting a partial pore blockage by the metal particles⁴⁸48 Lucas-Consuegra A, Serrano-Ruiz J, Gutiérrez-Guerra N, Valverde J. Low-temperature electrocatalytic conversion of CO2 to liquid fuels: effect of the Cu particle size. Catalysts. 2018;8(8):340.. However, the highest electrocatalytic CO₂ reduction efficiency might be achieved using a sample with the lowest surface area⁴⁸48 Lucas-Consuegra A, Serrano-Ruiz J, Gutiérrez-Guerra N, Valverde J. Low-temperature electrocatalytic conversion of CO2 to liquid fuels: effect of the Cu particle size. Catalysts. 2018;8(8):340.. This behavior has been reported for nanostructured carbon nanotube that is used as an electrocatalyst support. Furthermore, increasing Cu content evidently decreased porosity, but the conversion of CO₂ into methanol was successful⁴⁹49 Safdar Hossain S, Rahman SU, Ahmed S. Electrochemical reduction of carbon dioxide over CNT-supported nanoscale copper electrocatalysts. J Nanomater. 2014;2014:1-10.. Therefore, the decreased surface area of the C–Cu composite suggests that Cu nanoparticles blocked the smallest pores on the carbon material, since the number of Cu particles <100 nm was high (Figure 3).

Figure 7a illustrates the N₂ isotherm of the precursor C matrix. According to the IUPAC report⁵⁰50 Thommes M, Kaneko K, Neimark AV, Olivier JP, Rodriguez-Reinoso F, Rouquerol J, et al. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl Chem. 2015;87(9-10):1051-69., the isotherm corresponds to a combination of types I and IV, suggesting the presence of micro- and mesopores, characteristic of carbonaceous materials derived from carbon-rich residues³3 Amaral-Labat G, da Silva EL, Cuña A, Malfatti CF, Marcuzzo JS, Baldan MR, et al. A sustainable carbon material from kraft black liquor as nickel-based electrocatalyst support for ethanol electro-oxidation. Waste Biomass Valoriz. 2021;12(5):2507-19.^,5151 Amaral-Labat G, Munhoz MGC, Fonseca BCS, Boss AFN, de Almeida-Mattos P, Braghiroli FL, et al. Xerogel-like materials from sustainable sources: properties and electrochemical performances. Energies. 2021;14(23):7977.. Furthermore, hysteresis-loop type H4 has been attributed to micro-mesoporous materials⁵⁰50 Thommes M, Kaneko K, Neimark AV, Olivier JP, Rodriguez-Reinoso F, Rouquerol J, et al. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl Chem. 2015;87(9-10):1051-69..

Figure 7
(a) N₂ adsorption–desorption isotherm of the C matrix, and (b) pore-size distribution estimated using the DFT method.

The pore-size distribution estimated using the DFT model (Figure 7b) revealed micropores centered at 0.9, 1.2, and 1.5 nm, while mesoporosity was identified in two distinct areas, the first being a peak centered at 2.5 nm and the second being a large distribution ranging from 7 nm to 20 nm, which was in agreement with the isotherm. The average pore width, calculated as 4Vp/S_BET⁵²52 Araújo TP, Quesada HB, Santos DF, Fonseca BCS, Barbieri JZ, Bergamasco R, et al. Acetaminophen removal by calcium alginate/activated hydrochar composite beads: batch and fixed-bed studies. Int J Biol Macromol. 2022;203:553-62., was 2.64 nm.

3.3. Catalytic activity for electrochemical CO₂ reduction

LSV measurements were performed to determine the electrocatalytic activity of the C matrix and the C–Cu composite for electrochemical CO₂ reduction. Figure 8a illustrates the LSV curves of the porous C matrix under N₂ bubbling and then under CO₂ saturation in an aqueous solution of 1.0 mol L^-1 KHCO₃. LSV curves under N₂ bubbling are important to compare electrocatalytic activities with and without the presence of CO₂. Furthermore, N₂ bubbling eliminates O₂ in solution and prevents parallel competitive reactions of H₂ evolution, while CO₂ saturation establishes the equilibrium between CO₂ and HCO₃^- species. As shown in the LSV curves, the current values in the potential range of 0.2 V to −1.6 V at 5 mV s^-1 established three regions of negative current labeled (I), (II), and (III). The region (I) corresponds to the typical current of double electric layer charging, whereas the region (III) is related to bulk H₂ evolution, indicating the same current values with a small difference corresponding to region (II). In this region, lower current values were observed for measurements under CO₂ saturation. This can be attributed to the alteration in the double electric layer arrangement owing to the presence of CO₂ or HCO₃^- species. These species are formed as a result of the equilibrium established during CO₂ saturation and implies a decrease in the formation of adsorbed H₂ from water reduction that occurs at more positive potentials.

Figure 8
Linear sweep voltammetry curves of (a) the C matrix and (b) the C-Cu composite under N₂- and CO₂-saturated electrolytes.

A competitive reaction of H₂ evolution is expected in aqueous electrolytes during CO₂ conversion. Simultaneously, the existence of absorbed H₂ onto the catalyst surface is necessary for CO₂ activation⁵³53 Pérez-Rodríguez S, Sebastián D, Lázaro MJ, Pastor E. Stability and catalytic properties of nanostructured carbons in electrochemical environments. J Catal. 2017;355:156-66.. Therefore, for an effective system, the amount of H₂ must be close to the stochiometric value for CO₂ hydrogenation to prevent parallel reactions⁵³53 Pérez-Rodríguez S, Sebastián D, Lázaro MJ, Pastor E. Stability and catalytic properties of nanostructured carbons in electrochemical environments. J Catal. 2017;355:156-66.. Despite the formation of adsorbed H₂ under CO₂ saturation, indicated by the LSV curves (Figure 8a), CO₂ activation onto the porous C electrode was not verified, since CO₂ hydrogenation and, consequently, the suppression of H₂ evolution was not observed, as shown in region III.

The influence of the C–Cu composite was used to compare the current values resulting from the LSV curves illustrated in Figures 8a,b. An increase in current values to approximately 2.4 times was reported for the C–Cu composite, demonstrating increased effectiveness over H₂ evolution, as a well direct influence on CO₂ electroreduction. The effectiveness of Cu nanoparticles for CO₂ reduction is described by the combination of low-index crystal facets and coordination sites such as the corners, edges, and defects in Cu nanoparticles. Regarding CO₂ reduction, those parameters are crucial for the distribution of the gas phase of CO₂ and determines selectivity and sensitivity. Such crystallographic characteristics are expected to be dominant in C₂ products⁵⁴54 Baturina OA, Lu Q, Padilla MA, Xin L, Li W, Serov A, et al. CO2 electroreduction to hydrocarbons on carbon-supported Cu nanoparticles. ACS Catal. 2014;4(10):3682-95.. Tang et al.⁵⁵55 Tang W, Peterson AA, Varela AS, Jovanov ZP, Bech L, Durand WJ, et al. The importance of surface morphology in controlling the selectivity of polycrystalline copper for CO2 electroreduction. Phys Chem Chem Phys. 2012;14(1):76-81. suggested that the electrodeposited 50–110-nm Cu nanoparticles on a Cu electrode demonstrated higher selectivity toward C₂H₄ production upon CO₂ electrolysis in 0.1 mol L^-1 KClO₄. According to Pérez-Cadenas et al.³⁶36 Pérez-Cadenas AF, Ros CH, Morales-Torres S, Pérez-Cadenas M, Kooyman PJ, Moreno-Castilla C, et al. Metal-doped carbon xerogels for the electro-catalytic conversion of CO2 to hydrocarbons. Carbon N Y. 2013;56:324-31., Cu nanoparticles supported on carbon xerogels converted CO₂ into CH₄, C₂H₆, C₃H₄, C₃H_8, and C₃H₆ at −1.65 V using an Ag/AgCl system. Hence, a high quantity of products can be produced using Cu nanoparticles as the catalyst in CO₂-based electrochemical reactions.

As shown in Figure 8b, the C–Cu composite affected the processes occurring in regions II and III. Under CO₂ saturation, a decrease in the current values was associated with these regions. As indicated previously, this decrease might be associated with the presence of adsorbed H₂ that was completely consumed during CO₂ hydrogenation (region II). Consequently, CO₂ hydrogenation probably suppressed H₂ evolution, suggested by the decrease in current values of region III. Thus, LSV results confirm that there was a competition between H₂ evolution and CO₂ reduction for the electrolyte saturated with CO₂, indicating a decrease of H₂ evolution with CO₂ saturation. Similar results have been reported by Kaneco et al.⁵⁶56 Kaneco S, Iiba K, Katsumata H, Suzuki T, Ohta K. Effect of sodium cation on the electrochemical reduction of CO2 at a copper electrode in methanol. J Solid State Electrochem. 2007;11(4):490-5. and Chang et al.⁵⁷57 Chang TY, Liang RM, Wu PW, Chen JY, Hsieh YC. Electrochemical reduction of CO2 by Cu2O-catalyzed carbon clothes. Mater Lett. 2009;63(12):1001-3., wherein Cu electrode exhibited a substantially lower current in CO₂-saturated electrolyte compared to N₂-saturated electrolyte. Moreover, the LSV results provide evidence for the sensitivity and selectivity of the C–Cu composite for CO₂ reduction.

Figure 9 shows the comparison in the activities of the C matrix and C–Cu composite samples under CO₂ saturation. For the C–Cu electrode, a shift in potential of approximately 90 mV was observed from the onset potential of approximately -1.02 V, in contrast to -1.11 V that was observed for the C matrix. This shift was correlated with electrocatalytic activity and suggested an improved CO₂ electroreduction performance. Xiao et al.³⁴34 Xiao X, Xu Y, Lv X, Xie J, Liu J, Yu C. Electrochemical CO2 reduction on copper nanoparticles-dispersed carbon aerogels. J Colloid Interface Sci. 2019;545:1-7. generated Cu nanoparticles dispersed on activated carbon aerogel, which exhibited remarkable CO Faradaic efficiency (75.6%) and the lowest onset potential during LSV analysis under CO₂ saturation. In another work, the onset potential of Cu nanoparticles decorated on pyridoxine modification graphene oxide sheets used for electrochemical CO₂ reduction to ethanol exhibited a significant positive shift. This catalyst showed superior catalytic ability for ethanol generation (FE ethanol = 56.3%) using CO₂-saturated 0.1 mol L^-1 KHCO₃ aqueous solution³¹31 Yuan J, Yang MP, Zhi WY, Wang H, Wang H, Lu JX. Efficient electrochemical reduction of CO2 to ethanol on Cu nanoparticles decorated on N-doped graphene oxide catalysts. J CO2 Util. 2019;33:452-60.. In the previous studies, the Cu nanoparticles were generated by chemical deposition. Therefore, the active sites provided by Cu nanoparticles electrochemically decorated on C matrix assisted the high electrocatalytic ability of the C–Cu composite to catalyze CO₂, compared to the C matrix.

Figure 9
Linear sweep voltammetry curves of the C matrix and the C–Cu composite under CO₂ saturation.

The mechanism underlying electrochemical CO₂ reduction is still debatable³⁷37 Lee S, Hong S, Lee J. Bulk pH contribution to CO/HCOO− production from CO2 on oxygen-evacuated Cu2O electrocatalyst. Catal Today. 2017;288:11-7.; however, some pathways have been suggested. One of them considers CO₂ and HCO₃^- as reactants species and a separation between the formation of HCOO^- (directly from CO₂ or HCO₃^-) and CO (directly from CO₂) to affect electrochemical CO₂ reduction. Furthermore, species, such as C₂H₄, C₂H₆, and CH₄, can also be obtained from CO intermediates³⁷37 Lee S, Hong S, Lee J. Bulk pH contribution to CO/HCOO− production from CO2 on oxygen-evacuated Cu2O electrocatalyst. Catal Today. 2017;288:11-7.. Ma et al.⁵⁸58 Ma M, Djanashvili K, Smith WA. Controllable hydrocarbon formation from the electrochemical reduction of CO2 over Cu nanowire arrays. Angew Chem Int Ed. 2016;55(23):6680-4. proposed a reaction pathway for the electrocatalytic activity of CO₂ supported on Cu nanowire arrays for the formation of HCOO^-, C₂H₄, C₂H₆, CH₄, CO, and CH₃CH₂OH, with evidence from two routes; one with products derived from COH formation and the second with products derived from CO dimerization. It is reinforced that CO is a key intermediate for the formation of hydrocarbons, although the rate-determining step in the conversion of CO₂ to CO is the first electron transfer for the formation of the CO₂^.- intermediate⁵⁸58 Ma M, Djanashvili K, Smith WA. Controllable hydrocarbon formation from the electrochemical reduction of CO2 over Cu nanowire arrays. Angew Chem Int Ed. 2016;55(23):6680-4.. All these products are reported for Cu-based electrodes with previously mentioned starting conditions, thus, the reactions performed using the C–Cu composite possibly follow similar mechanisms.

Therefore, the composite constituted by Cu, a non-noble and cheap electrocatalyst, is a promising material towards a sustainable future. Firstly, Cu foil is a competitively priced material compared to other metals used for CO₂ electroreduction, such as Au and Ag. As shown in Table 3, Cu costs 1.2 times lesser than Ag and 34 times lesser than Au, providing an excellent candidate for electrochemical applications, as the price of the electrocatalyst notably affects the final cost of the material. Secondly, the electrochemical deposition was performed on a bio-based, highly renewable C matrix. Therefore, Cu electrodeposition on a sustainable matrix using Cu foil is a green synthesis method based on a straightforward process that involves low-cost, non-toxic reagents.

4. Conclusions

In this study, a bio-based, porous carbon material was prepared from concentrated Kraft black liquor using a simple, low-cost, and sustainable method, where Cu nanoparticles were electrochemically deposited on the carbon surface using a low-cost environmentally friendly system. We verified that concentrated black liquor increased the surface area and pore volume of the matrix, thereby creating micro- and mesopores in approximately equal proportions. The electrodeposition method homogeneously distributed Cu nanoparticles on the surface of the C matrix. However, the nanoparticles probably obstructed the small pores (<100 nm), indicated by the reduced surface area after Cu deposition. The nanoparticles were mainly composed of metallic Cu, which probably created active sites on the material surface, and consequently, increased the cathodic current in the voltammetric tests. The results suggested an interaction of the C–Cu composite with CO₂ and/or products derived from CO₂ electroreduction, resulting from decreased H₂ evolution, which may have contributed to CO₂ activation. Therefore, the sustainable porous C–Cu composite is an attractive material for CO₂ electroreduction. Furthermore, the material was synthesized using green chemistry, by reusing an industrial waste (black liquor) to generate a new porous material using a simple method and, simultaneously, utilizing one of the most important greenhouse gas, CO₂, in agreement with the COP26 issues.

5. Acknowledgments

This study was supported by the following Brazilian research agencies: Coordenação de Aperfoiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance code 001 (grant n° 88882.444518/2019-01), CNPq, and FINEP.

6. References

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