Electrochemical Study of Copper Ferrite as a Catalyst for CO 2 Photoelectrochemical Reduction

In this work, p-type CuFe2O4 was synthesized by sol gel method. The prepared CuFe2O4 was used as photocathode catalyst for photoelectrochemical (PEC) CO2 reduction. The XRD, UV-Visible Spectroscopy (UV-Vis), and Mott-Schottky (MS) experiments were done to characterize the catalyst. Linear sweep voltammetry (LSV) was employed to evaluate the visible light (λ>400 nm) effect of this catalyst for CO2 reduction. The band gap energy of the catalyst was calculated from the UV-Vis and was found 1.30 eV. Flat band potential of the prepared CuFe2O4 was also calculated and found 0.27 V versus Ag/AgCl. Under light irradiation in the CO2-saturated NaHCO3 solution, a remarkable current development associated with CO2 reduction was found during LSV for the prepared electrode from onset potential -0.89 V with a peak current emerged at -1.01 V (vs Ag/AgCl) representing the occurrence of CO2 reduction reaction. In addition, the mechanism of PEC was proposed for the photocathode where the necessity of a bias potential in the range of 0.27 to ~ -1.0 V vs Ag/AgCl was identified which could effectively inhibit the electron-hole (e-/h+) recombination process leading to an enhancement of CO2 reduction reactions. Copyright © 2018 BCREC Group. All rights reserved


Introduction
The rapid growth of population and industrialization leads to the generation of huge amount of CO2 gas molecules through the burning of fossil fuels that causes the collapse of the natural carbon cycle and accelerates the climate change.The idea of converting CO2 to hydrocar-bons or oxyhydrocarbons under solar irradiation is taken from nature more specifically from plants [1][2][3] in which CO2 and water combine to form carbohydrate over chlorophyll (catalyst) in presence of sunlight as the energy source.The mimic photosynthesis process gets huge attention in recent years giving the hopes of recycling the CO2 to produce fuels that may have a large impact to solve the two major issues of present days: climate change and energy shortage [4].Photocatalysis (PC) is one of the most promising techniques to convert CO2 into hydrocarbon fuels [5].The use of photon energy in the mimic photosynthesis process requires photoresponsive materials to produce high-energy photogenerated electrons upon light irradiation [6].To produce hydrocarbons or oxygenated hydrocarbons from CO2 requires proton coupled multielectron pathways which suffer from slow reaction kinetics and poor product selectivity [7].The single electron reduction of CO2 to •CO2 - occurs at -1.90 V vs normal hydrogen electrode (NHE) requiring highly reducing equivalents [8].Proton-coupled multielectron reduction processes are used to avoid this high-energy single electron intermediate.But by this approach, it is very difficult to produce a selective product from CO2 reduction [9].The composition of the products of CO2 reduction also depends on the applied potentials [10].In this context, photocatalysis with a properly bias potential may compel the reactions to achieve required products.Moreover, the bias potential effectively reduces the e -/h + recombination rate in the photocatalyst leading to higher quantum efficiency [11].
Metal oxide based photocatalysts have been widely investigated for CO2 reduction, such as TiO2 [12], SrTiO3 [13], CuO-Cu2O [14], Co3O4 [10,15], Cu3Nb2O8 [16], etc.The p-type metal oxides are essential for photoelectrochemical CO2 reduction [5] which acted as a photocathode.In order to increase the visible light absorption efficiency, catalysts with low band gap, high electron conductivity and good stability are preferred.In recent year, CuFe2O4 has been investigated as a photocatalyst for hydrogen evolution, energy storage and decolourization [17][18][19].Various methods have been used to prepare CuFe2O4, such as: hydrothermal method [20], mechanical milling [21], and solgel method [22].Among those, the sol-gel method could produce high homogeneity of CuFe2O4 as well as submicron level crystallite size and higher surface to volume ratio [23].It is well known that, CuFe2O4 possesses inverse spinel configuration with both octahedral and tetrahedral cation sites and the electron conduction is due to the electron hopping of Fe 2+ and Fe 3+ as well as Cu 2+ and Cu + sites in the spinel lattice [23,24].In addition, it shows an excellent chemical stability in basic medium makes it very attractive to be used as catalyst [19].Recently in our research, CuFe2O4 has been synthesized using sol-gel method and used for photocatalytic reduction of CO2 where 220 µmol/gcat.L of methanol was obtained as product [25].Due to the low band gap of CuFe2O4 it is advantageous to be used as visi-ble light active catalyst, however the high e -/h + recombination rate suppressed its photocatalytic efficiency [23,25].To overcome the shortcoming, CuFe2O4 was modified by TiO2 that improved the e -/h + separation, but at the same time, this approach resulted in higher band gap of the composite material [25].The application of bias potential in photoelectrochemical process may effectively interfere the e -/h + recombination and can enhance the photo current leading to significant improvement in CO2 reduction [11].Based on references, there has been no information on photoelectrochemical reduction of CO2 over that p-type CuFe2O4 in aqueous solution.In this context, the current study is focused to evaluate the photoelectrochemical characteristics of CuFe2O4 for CO2 reduction.CuFe2O4 is likely to be a good photocathode due to its p-type behaviour and low band gap and the imposition of a bias potential can efficiently separate the photogenerated e -/h + pairs accelerating the proton coupled multielectron CO2 reduction.The catalyst was characterized by using UV-Vis and XRD.The photoelectrochemical behaviour of the catalyst for CO2 reduction was evaluated by linear sweep voltammetry (LSV) and Mott-Schottky (MS) analysis.

Catalyst preparation
CuFe2O4 was synthesized using sol-gel method with minor adjustment of reaction setting [23,26].Copper nitrate (Cu(NO3)2.3H2O)and ferric nitrate (Fe(NO3)3.9H2O)with a molar ratio of 1:2 (Cu/Fe) were dissolved in 100 mL of distilled water followed by the addition of 10 mL of 65% HNO3 and 16 g agar under vigorous stirring for 3 h at room temperature.Thereafter, the temperature was elevated at 90 °C and stirred continuously for another 3h.At this stage, a green gel was formed.The gel was dried at 130 °C in vacuum oven for 24 h.After that the dried powder was obtained and cal-cined at 900 °C for 14 h.The heating rate during calcination was maintained at 10 °C/min.

Electrode preparation
The electrode was prepared by the method described by Woon et al. [27], Khan et al. [28], and Woon et al. [29].In brief, the catalyst ink was prepared by mixing 22 mg of CuFe2O4 with 140 µL of 5 wt% nafion and 280 µL isopropanol (C3H8O) and subjected to ultra-sonication for 30 min.Thereafter, the ink was evenly brushed on the toray carbon paper with an area of 1 cm 2 .The as prepared electrode was dried in vacuum oven at 90 °C for 6 h.

Catalyst characterization
The XRD results of this powdered catalyst were taken at a room temperature by means of Rigaku Mini FlexII at Bragg angle of 2θ = 10-80° with a scan rate of 0.02 °/min.During this experiment, 30 kV and 15 mA were used at Cu-Kα emission.The crystal pattern of the catalyst was assessed from the XRD results and the crystal size (D) was determined by Scherrer formula as shown in Equation 1 [30][31][32].
(1) where, K is a dimensionless shape factor with a typical value of 0.9, λ is the X-ray wave length of the applied source (0.154118 nm), and B (in rad) denotes by full width of half-maximum (FWHM) of the resulting peak, determined by Gaussian fitting.UV-Visible absorption spectra of the sample were obtained by employed Shimadzu UV 2600 UV-Vis-NIR Spectrophotome-ter.Mott-Schottky analysis was carried out by using an electrochemical analyzer (Autolab Compact PGSTAT 204, Netherland).In this case, the prepared CuFe2O4 electrode, Ag/AgCl and platinum foil were used as working, reference and counter electrode respectively.In this case 0.1 M NaHCO3 solution (pH 6.8) was used as electrolyte.

Photoelectrochemical analysis
The photoelectrochemical CO2 reduction was carried out in a double chamber PEC cell reactor equipped with a quartz window.All the PEC measurements were done in an electrochemical work station (Autolab Compact PGSTAT 204, Netherland) using a three electrode cell consists of working electrode (prepared electrode) counter electrode (Pt-foil) and reference electrode (Ag/AgCl) in NaHCO3 aqueous solution.Prior to starting the reaction, high purity CO2 gas was purged for 30 min at a constant pressure until the solution reached the CO2 saturation and to ensure that all dissolved O2 was completely removed.Linear sweep voltammetry (LSV) was performed in between -0.6 to -1.2 V vs Ag/AgCl under the light on and dark conditions.The light on condition was maintained by using Xenon lamp (Light source: XD-300 High Brightness Cold Light Source, Beijing Perfect light Co., Ltd., China).The light was passed through the filters with different wavelengths (470, 630, and 650 nm).

Characterization of CuFe2O4
The XRD pattern of the CuFe2O4 is presented in Figure 1 ity with JCPDS database (peak position of 101,112, 200, 202, 211, 220, 321, 224, 400, and 422) (Figure 1a).The spectra (JCPDS 110, 200) of the sample also show the existence of trace quantity of CuO phases.According to the Scherrer principle in Equation ( 1), the crystal size of the CuFe2O4 is ~59 nm.To completely identify all the phases, the XRD spectra was refined with Rietveld method and presented in Figure 1b.Phase structure, microstructural parameters and lattice constants are tabulated in Table 1 as obtained from the result of Rietveld refinement with a R% of ~ 16%.Monoclinic CuO (14.67 wt %) was also found along with tetragonal CuFe2O4 (85.33 wt %).The lattice constants of CuFe2O4 and CuO are very close to the reported in the crystallographic database (ICDD 340425) and (ICDD 10706830), respectively.
The UV-Vis spectra of CuFe2O4 in the wavelength of 200-1000 nm are presented in Figure 2(a) and Tauc plot (αhv) 2 versus band gap energy) is shown in Figure 2b to demonstrate the extrapolated intercept band gap energy value [25].The band gap energy for as-prepared CuFe2O4 is found as 1.30 eV which is slightly lower compared to CuFe2O4 synthesized by Kezzim et al. [23] that reported 1.42 eV.The reduction in bandgap might be due to the increased crystallites size of CuFe2O4 (~59 nm) compared to the report of Kezzim et al. [23] (43 nm).The correlation of band gap with the crystallite size was presented by Marotti et al. [33] that revealed that the band gap could be reduced due to the increase in crystal size.Low band gap semiconductors require smaller energy to generate e -/h + pairs which is occurred under visible light irradiation [4].
Mott-Schottky experiments were conducted in 0.1 M aqueous NaHCO3 solution (pH 6.8), and the resulting plot is shown in Figure 3a.Negative slope was observed, suggesting that CuFe2O4 acted as a p-type semiconductor.In ptype semiconductors, normally, the Efb existed in the region of valence band (VB) which can be measured by the x-axis intercept of the plot of 1/C 2 vs E as followed by Equation ( 2) [16].
(2) where, C is the capacitance, e is the electron charge, ϵ is the dielectric constant, ϵ0 is permittivity of vacuum, N is an acceptor density, E is the electrode potential, Efb is the flat band potential, k is the Boltzmann constant, and T is the temperature.As shown in Figure 3a, the xaxis intercept was 0.25 V versus Ag/AgCl.The Efb was determined by using the equation E=Efb-kT/e and was found as 0.27 V versus Ag/AgCl.This result showed that the VB and  CB of p-type CuFe2O4 were approximately 0.27 V and -1.03 V vs Ag/AgCl, respectively.Figure 3b displays the band diagram for p-type CuFe2O4 and the thermodynamic redox potentials for the products of CO2 reduction (V vs NHE).It can be seen that the conduction band position of the p-type CuFe2O4 is located in more negative position compared to the redox potentials of CO, CH4, CH3OH, HCOOH, and HCHO suggesting that under light irradiation the photoexcited electron from the CB of the CuFe2O4 could be transferred to CO2 to occur the proton coupled multielectron CO2 reduction [34,35].In addition, a moderate bias potential in the range of 0.27 to -1.03 V vs Ag/AgCl can be applied to the system to inhibit the e -/h + recombination to facilitate the reaction [11].

Photoelectrochemical activity
Figure 4 shows the voltammogram of CuFe2O4 electrode in N2 and CO2-saturated NaHCO3 solution with and without visible light illumination.It can be seen that the shape of the voltammogram in inert condition presents similar patterns during light on and light off where the cathodic current increases with the increase of applied potential above -0.8V which may arise from water/proton reduction [10].In CO2-saturated solution, a shoulder was observed at -1.04 V vs Ag/AgCl during light off condition, while under light irradiation a peak in cathodic current is evident at -1.01 V vs Ag/AgCl.The cathodic current further increases with the increase in applied potential beyond -1.06V.The positive shift in the onset potential by ~70 mV under light on condition compared to the light off suggests the higher efficiency of CO2 reduction under visible light irradiation.
The voltammogram in Figure 4 is in agreement with Shen et al. [10] and Hori et al. [36] that reported a similar LSV pattern in CO2 saturated and inert conditions.The cathodic peak may arise due to the formation of CO intermediate from CO2 [10,36].As proposed by Schouten et al. [37] the cathodic current during LSV might be due to the water reduction generating proton which at higher potential may combine with adsorbed CO to form different hydrocarbons, such as methanol, methane, formaldehyde, formic acid, etc. Shen et al. [10] reported the production of formate as sole product during photoelectrocatalytic reduction of CO2 at peak potential (-0.66 V vs NHE) over Co3O4 and Cu-Co3O4 nanotube electrodes under visible light irradiation.
Figure 5 shows the LSV voltammogram of CuFe2O4 electrode at different wavelength (470, 630, and 650 nm) along with light off in CO2-saturated 0.1 M NaHCO3 solution.It was found that the onset potential values were almost the same during light on conditions but shifted to more positive values compared to the light off.Furthermore, cathodic peaks were observed during light on condition at all wavelengths.When the electrode was illuminated at 470 nm wavelength the cathodic peak was shifted by 20 mV compared to the electrode illuminated at 650 nm.The result is in accordance with Figure 2a where higher visible light absorption of CuFe2O4 at 470 nm compared to 630 and 650 nm is evident.Apart from that, concentrations of electrolyte play an important role in the reduction of CO2 [38].In Figure 6, it is found that the peak current for higher concentration of NaHCO3 is significantly greater than that of lower concentration.This is due to the in higher concentration of NaHCO3, more HCO -3 ions are available to neutralize the OH -ions formed very close to catalyst surface which comes from the reduction of H2O (Equations 3-4).The pH near to the electrode surface will slightly be increased which is non-equilibrium but the overall pH of the bulk solution remains almost same (Table 2) and exist in equilibrium state [38].As a result, the reduction of H + ions near the catalyst electrode surface are more favourable than that of bulk solution [39].
The possible reduction products of CO2 were carbon monoxide, formic acid, methanol, and methane followed by 2e -, 4e -, and 8e -pathway as shown in Equations 5-8: The production rate and compositions of the hydrocarbons are highly dependent on applied bias potential and catalyst materials.It is necessary to further investigate composition of the products by chromatography method and to elucidate the mechanism of the reaction.In our future work, detail study will be conducted to explore the phenomena proposed in the work.

Conclusion
Nanostructured p-type CuFe2O4 was synthesized by sol-gel method.The prepared CuFe2O4 possesses a band gap of 1.3 eV with the VB and CB edges at 0.27 V and -1.03 V vs Ag/AgCl, respectively, calculated from UV-Vis and Mott-Schottky evidences.The CuFe2O4 photocathode exhibited a strong cathodic peak at -1.01 V with an onset potential of -0.89 V Ag/AgCl under the illumination of visible light.At a fixed potential (-1.01 V vs Ag/AgCl), the

Figure 3 .Figure 4 .
Figure 3. (a) Mott-schottky plot of CuFe2O4 at 2k Hz measured under light off condition; (b) Position of the CB and VB of p-type CuFe2O4 photocathode along with redox potential and products distribution at different potential range at pH 7

Table 1 .
Crystallographic parameters of as-prepared CuFe2O4 extracted from Rietveld analysis

Table 2 .
pH of NaHCO3 aqueous solutions at different concentrations under CO2 saturated cathodic current increased by of 20%, 35%, and 80% at light wavelengths 650, 630, and 470 nm, respectively, compared to light off condition.The maximum light absorbance of CuFe2O4 at 470 nm is in favour of light harvesting allowing higher PEC response for CO2 reduction.An electron flow scheme was proposed to demonstrate the possible mechanism for photoelectrocatalytic reduction of CO2 where the proton coupled multielectron CO2 reduction can produce products, such as: HCOOH, HCHO, CO, CH3OH, C2H4, C2H5OH methane, ethylene, etc.However, the identification of the products was not executed in this study.Thus, further research is required to explore the CO2 reduction by PEC over p-type CuFe2O4 photocathode and understand the CO2 reduction mechanism in depth.