Surfactant-Promoted Prussian Blue Analogues Fabricated Electrodes for Electrocatalytic Water Oxidation

: Prussian blue analogues (PBAs) have unique structural and chemical behaviour and therefore have applications in various fields of catalysis as energy conversion materials for storage devices and molecular sensing. Herein we focused on the in-situ synthesis of three PBAs comprising cobalt hexacyanoferrate (CoHCF), nickel hexacyanoferrate (NiHCF), and cobalt-nickel hexacyanoferrate (CoNiHCF) through cation i.e. cetyltrimethylammonium bromide (CTAB) assisted drop cast method. The electrocatalysts were characterized through a multitude of spectroscopic techniques and were tested for water oxidation study. It was found that among the three electrocatalysts, CoNiHCF showed comparatively better catalytic performance with an overpotential value of 570 mV (at 1 mA cm -2 )


INTRODUCTION
We are heavily reliant on fossil fuels to meet the annual global energy demand [1].However, the excessive burning of fossil fuels results in the emission of CO 2 into the atmosphere, which can be avoided by substituting non-toxic and renewable fuels for fossil fuels [2][3].To convert solar energy into a usable form, the photovoltaic system has also emerged as a viable solution, but it still has significant drawbacks.As a result, finding a different source of energy is crucial [4][5][6].Given all the options, hydrogen having the highest energy density and producing non-toxic byproducts is regarded as one of the most suitable energy sources [7].Electrochemical water splitting is an intriguing method for producing hydrogen [6,8].However, oxygen evolution reaction (OER) has sluggish kinetics that needs more overpotential, and therefore, effective electrocatalyst that can reduce the activation energy, consequently, the overpotential of the reaction is required.The most common electrocatalysts utilized in the process are metal oxides based on Ir, Ru, and Pt [9][10][11].However, their high cost and scarcity have significantly limited the usage of these metal oxides [11].In the process of splitting water, several transition metals are also used as metal oxide electrocatalysts.Firstrow transition metal oxides, however, have several drawbacks, including the fact that metal oxide performance varies on different experimental conditions, such as morphology, temperature, etc [11].Consequently, it is crucial to control them in order to obtain correct findings.Otherwise, it makes their applicability very challenging [11].Additionally, the first-row transition metal oxides function best in an alkaline environment (Ph ≥ 13) but are less effective in acidic or neutral conditions [12].Different non-oxide materials such as metal phosphides, sulphides, selenides, nitrides, based molecular-based organic metal-organic frameworks (MOFs), and polyoxometalates, etc. are known water oxidation catalysts (WOCs) with advantageous qualities, such as simplicity in synthesis, stability over a wide pH range, and durability during catalytic processes [13].Among different coordination polymers, PBAs have been employed as heterogeneous electrocatalysts for OER [14].The group of Galán-Mascarós have extensively studied the role of PBAs as WOCs [15][16][17][18].The group proved that PBAs are important electrocatalysts in water oxidation because of open metal active sites, the high porosity of the framework, and most importantly the easy oxidization of metal ions to their higher oxidation states [18].The group of Karadas reported the preparation of amorphous PBAs through a novel synthetic route that exhibited high electrocatalytic water oxidation activity [4].Recently we have reported the preparation of amorphous bimetallic PBAs following the pyridinium based surfactant assisted route to prove that surfactants play a significant impact on the film development (binderfree approach), better stability and charge transfer kinetics of OER [19].We further extended the fabrication approach to electrodeposition method as well [20].
Herein, we examined the effect of a cationic surfactant on the electrocatalytic water oxidation performance of synthesized bimetallic and trimetallic PBA films.In the presence of cationic surfactant, PBA films have been deposited on the glassy carbon electrode (GCE).The fabricated electrode ultimately demonstrated a significant improvement in the electrochemical performance of synthesized catalysts toward OER.

Chemicals
The chemicals that include sodium hexacyanoferrate (II) (Na 4 Fe(CN) 6 ; HCF), sodium hydroxide (NaOH), and sodium nitrate (NaNO 3 ) were purchased from Sigma-Aldrich (St. Louis, MO, USA).For cobalt and nickel sources, the nitrate salts were used.Cetyl-trimethyl ammonium bromide (CTAB) as cationic surfactant and DMF as solvent was used.All the solutions were prepared in deionized water at room temperature.

Instrumentation
The Gamry Interface 1010E potentiostat/galvanostat is equipped with a standard three-electrode setup comprising platinum wire as a counter electrode, Ag/AgCl (3 M KCl) as a reference electrode, and modified glassy carbon (2 mm in diameter) as working electrode The transmission spectrum of each powder sample was recorded in the frequency range of 4000 -400 cm -1 and performed on a Thermo Nicolet-6700 FT-IR spectrophotometer.Powdered X-ray diffraction (PXRD) analysis of the synthesized materials was recorded on a PANalytical X'pert instrument equipped with CuKα X-ray source (λ = 1.5418Å) in the range of 20 to 80•.Scanning electron microscopy (SEM) was carried out using FESEM (NOVA-600) coupled with Bruker EDX system at an accelerating voltage of 3 kV.

In-situ Synthesis of Catalyst on the Electrode Surface
Before modification of the GCE, it was initially washed with alumina powder slurry on a polishing pad.The electrode was dipped in acetone and sonicated for about 30 min to remove solid particles from the electrode surface if any.After sonication with acetone, it was sonicated with distilled water.Finally, it was dried in an oven.
To develop the corresponding PBAs films (i.e. of CoHCF, NiHCF, or CoNiHCF) films, a drop of surfactant in DMF was added onto the GCE surface using a micropipette.Once the electrode dried out, a 10 µL of 10 mM aqueous HCF and 15 mM aqueous corresponding metal salt solutions were added simultaneously (schematically presented in Figure 1).

Synthesis and Characterization
The synthesis of PBAs through a surfactantassisted mechanism is the key factor in generating an efficient electrocatalyst for OER in this work.The surfactants form hemimicelles onto the surface Short running title framework, and most importantly the easy oxidization of metal ions to their higher oxidation states [18].The group of Karadas reported the preparation of amorphous PBAs through a novel synthetic route that exhibited high electrocatalytic water oxidation activity [4].Recently we have reported the preparation of amorphous bimetallic PBAs following the pyridinium based surfactant assisted route to prove that surfactants play a significant impact on the film development (binder-free approach), better stability and charge transfer kinetics of OER [19].We further extended the fabrication approach to electrodeposition method as well [20].
Herein, we examined the effect of a cationic surfactant on the electrocatalytic water oxidation performance of synthesized bimetallic and trimetallic PBA films.In the presence of cationic surfactant, PBA films have been deposited on the glassy carbon electrode (GCE).
The fabricated electrode ultimately demonstrated a significant improvement in the electrochemical performance of synthesized catalysts toward OER.

Chemicals
The chemicals that include sodium hexacyanoferrate (II) (Na4Fe(CN)6; HCF), sodium hydroxide (NaOH), and sodium nitrate (NaNO3) were purchased from Sigma-Aldrich (St. Louis, MO, USA).For cobalt and nickel sources, the nitrate salts were used.Cetyl-trimethyl ammonium bromide (CTAB) as cationic surfactant and DMF as solvent was used.All the solutions were prepared in deionized water at room temperature.

Instrumentation
ray source (λ = 1.5418Å) in the range of 20 to 80•.Scanning electron microscopy (SEM) was carried out using FESEM (NOVA-600) coupled with Bruker EDX system at an accelerating voltage of 3 kV.

In-situ Synthesis of Catalyst on the Electrode Surface
Before modification of the GCE, it was initially washed with alumina powder slurry on a polishing pad.The electrode was dipped in acetone and sonicated for about 30 min to remove solid particles from the electrode surface if any.After sonication with acetone, it was sonicated with distilled water.Finally, it was dried in an oven.
To develop the corresponding PBAs films (i.e. of CoHCF, NiHCF, or CoNiHCF) films, a drop of surfactant in DMF was added onto the GCE surface using a micropipette.Once the electrode dried out, a 10 µL of 10 mM aqueous HCF and 15 mM aqueous corresponding metal salt solutions were added simultaneously (schematically presented in Figure 1).At about 1610 cm −1 there is a sharp band that corresponds to OH bending due to water molecules trapped inside the structure.While at 3200-3500 cm −1 a broadband represents OH stretch.At about 3000 cm −1 there is a hump that is because of the C−H stretch due to the presence of CTAB.1500-1000 cm −1 is a fingerprint region, representing alkyl group bending.The band at 500 cm −1 corresponds to M−C stretching.
The crystalline content and phase of the synthesized compounds, PXRD was performed, and the spectra are shown in Figure 3.The spectra reveal that all the samples are iso-structural with the PB crystal system, having a face-centered cubic lattice and Fm3m space group symmetry.All the characteristic 2θ peaks are in good agreement with the reference databases for NiHCF and CoHCF [21].The slight shift of the peaks' positions can be attributed to the presence of CTAB.
The comparative spectra further reveal that the crystallinity of trimetallic CoNiHCF is reduced compared to bimetallic PBAs.
To further study the morphology of the PBAs scanning electron microscopy was performed.It appears in the form of two-dimensional images exhibiting the structural properties of the PBAs.For CoNiHCF Figure 4 shows the regular platelike structures with uniform growth as shown in the close image with a resolution of 2 µm.The EDX spectrum for CoNiHCF confirms the presence of given metal atoms and hints at the atomic ratio of metals (Figure 5).The proposed molecular formula based on the stoichiometric ratio of the metals is Co

Electrocatalytic performance
LSV plots of all the PBAs in the buffer of neutral pH i.e., 7 with a potential range of 0-1.5 V Vs Ag/ AgCl at a scan rate of 50 mV sec -1 were taken after IR compensation.From the graph, as shown in Figure 6 (a), we can say that the activity of NiHCF with CTAB is least toward water oxidation reaction.CoHCF with CTAB performs better towards water oxidation but the best results are obtained for CoNiHCF with CTAB giving the overpotential of 570 mV at 1 mA cm -2 current density.This can be attributed to the amorphous nature of CoNiHCF with CTAB compared to the others, hence providing more surface area for the reaction to occur.A comparison of the overpotential and Tafel slope for all three materials is given in Table 1.
To check the reaction kinetics Tafel plots were drawn between log j vs. overpotential.The slope of Short running title characteristic peaks associated with PB-type systems.Cyanide stretching frequency is the distinctive feature of cyanide-based coordination compounds.All three compounds show sharp bands in the frequency range 2079-2093 cm -1 .The cyanide stretching frequency increases with the increase in the positive character of the metal.That is why it is slightly high for NiHCF and then for CoNiHCF.At about 1610 cm −1 there is a sharp band that corresponds to OH bending due to water molecules trapped inside the structure.While at 3200-3500 cm −1 a broadband represents OH stretch.At about 3000 cm −1 there is a hump that is because of the C−H stretch due to the presence of CTAB.1500-1000 cm −1 is a fingerprint region, representing alkyl group bending.The band at 500 cm −1 corresponds to M−C stretching.The crystalline content and phase of the synthesized To further study the morphology of the PBAs scanning electron microscopy was performed.It appears in the form of two-dimensional images exhibiting the structural properties of the PBAs.For CoNiHCF Figure 4 shows the regular plate-like structures with uniform growth as shown in the close image with a resolution of 2 µm.The EDX spectrum for CoNiHCF confirms the presence of given metal atoms and hints at the atomic ratio of metals (Figure 5).The proposed molecular formula based on the stoichiometric ratio of the metals is Co1.4Ni0.3CTAB0.3[Fe(CN)6]·nH2O.

Short running title
characteristic peaks associated with PB-type systems.Cyanide stretching frequency is the distinctive feature of cyanide-based coordination compounds.All three compounds show sharp bands in the frequency range 2079-2093 cm -1 .The cyanide stretching frequency increases with the increase in the positive character of the metal.That is why it is slightly high for NiHCF and then for CoNiHCF.At about 1610 cm −1 there is a sharp band that corresponds to OH bending due to water molecules trapped inside the structure.While at 3200-3500 cm −1 a broadband represents OH stretch.At about 3000 cm −1 there is a hump that is because of the C−H stretch due to the presence of CTAB.1500-1000 cm −1 is a fingerprint region, representing alkyl group bending.The band at 500 cm −1 corresponds to M−C stretching.

Surfactant-Promoted PBAs as WOCs 551
the graph reflects reaction kinetics.The lower the Tafel slope, the faster will be the reaction kinetics.Cyclic voltammetry (CV) is used to measure the electrochemical surface area (ECSA) of the fabricated electrode in the non-faradic region by varying scan rates and the graph is shown in Figure 6 (c).It is clear from the graph that with the increasing scan rates the value of the current increases.From CV measurements, doublelayer capacitance (C dl ) is calculated.The slope of the graph between scan rates (mV.s - ) vs. the corresponding current (mA) provided double-layer capacitance as 45µF for CoNiHCF, (Figure 6 (d)).From the double-layer capacitance, ECSA reflecting the catalytic performance is determined.More active sites are reflected with higher ECSA possible electrochemical processes.ECSA is found to be 2.25 cm 2 for CoNiHCF.The ECSA further provides the roughness factor (RF) to be 32.1, associated with the electrocatalyst and is another important parameter to determine the catalytic efficiency.A

Electrocatalytic performance
LSV plots of all the PBAs in the buffer of neutral pH i.e., 7 with a potential range of 0-1.5 V Vs Ag/AgCl at a scan rate of 50 mV sec -1 were taken after IR compensation.From the graph, as shown in Figure 6 (a), we can say that the activity of NiHCF with CTAB is least toward water oxidation reaction.CoHCF with CTAB performs better towards water oxidation but the best results are obtained for CoNiHCF with CTAB giving the overpotential of 570 mV at 1 mA cm presence of given metal atoms and hints at the atomic ratio of metals (Figure 5).The proposed molecular formula based on the stoichiometric ratio of the metals is Co   high value of the RF means a more active surface and efficiency for the water oxidation mechanism.Electrochemical Impedance Spectroscopy is used to investigate the changes that take place in the interfacial properties of the electrode upon their encounter with the analyte species [22].The plots in Figure 6(e) illustrated a smaller semicircle in the high-frequency region corresponding to the lower value of the charge transfer resistance (R ct ).CoNiHCF has a very efficient charge transfer process with a very small resistance because of which it has high water oxidation catalytic efficiency.
During the water oxidation process, the stability of the working electrode is tested by using chronoamperometry 1.8 V vs. RHE (constant potential) for the trimetallic CoNiHCF.The electrocatalyst produced a corresponding constant current density for 12 hours as shown in Figure 6 (f).
The comparison of electrochemical performance is given in Table 1.The data shows that the CoNiHCF has a lower overpotential and Tafel slope value which reflects more active sites   The comparison of electrochemical performance is given in Table 1.The data shows that the CoNiHCF has a lower overpotential and Tafel slope value which reflects more active sites available for water oxidation.This can be attributed to the synergistic effect of the presence of Ni and Co, where both are available as active sites for the water oxidation process with faster kinetics and lower R ct .590 mV) and NiHC current density of being a better catal showed an ECSA o factor of 32.1.The s herein provides a PBA-based electroc approach can be exp substrates as well.

ACKNOWLEDG
A.H. and S.A. are than form of project funds

CONFLICT OF
The authors declare n

Fig. 3 .Fig. 2 .
Fig. 3. PXRD patterns for all three synthesized PBAs.To further study the morphology of the PBAs scanning electron microscopy was performed.It appears in the form of two-dimensional images exhibiting the structural properties of the PBAs.For CoNiHCF Figure

Figure 6 (
b) shows the Tafel slope of all three compounds, where CoNiHCF has the lowest value of Tafel slope i.e., 133 mV/dec which indicates the fast reaction kinetics of trimetallic CoNiHCF for the water oxidation reaction considering more active sites on its surface than the other two bimetallic PBAs.
lope of the graph e Tafel slope, the gure 6 (b) shows where CoNiHCF i.e., 133 mV/dec ics of trimetallic ction considering n the other two to measure the of the fabricated arying scan rates ).It is clear from rates the value of rements, double-The slope of the he corresponding acitance as 45µF corresponding constant current density for 12 hours as shown in Figure 6 (f).

Fig. 6
Fig.6(a) LSV curves for all three synthesized PBAs compared with bare GCE, (b) Tafel plots of the three PBAs, (c) CV cycles in the non-faradaic region for CoNiHCF at scan rates varying from 10-150 mV sec -1 , (d) measurement of double-layer capacitance from changing current and scan rate plot for CoNiHCF, (e) Nyquist plots from 0.1Hz to 100KHz frequency range for all three PBAs, and (f) chronoamperometric stability plot at potential correlated with 1 mA cm -2 current density for CoNiHCF.

Fig. 6 .
Fig. 6.(a) LSV curves for all three synthesized PBAs compared with bare GCE, (b) Tafel plots of the three PBAs, (c) CV cycles in the non-faradaic region for CoNiHCF at scan rates varying from 10-150 mV sec -1 , (d) measurement of double-layer capacitance from changing current and scan rate plot for CoNiHCF, (e) Nyquist plots from 0.1Hz to 100KHz frequency range for all three PBAs, and (f) chronoamperometric stability plot at potential correlated with 1 mA cm -2 current density for CoNiHCF.

Table 1 .
-2current density.This can be Comparison of overpotential and Tafel slope for all three catalysts.