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WO3 composite photoanode tailored for CO2 reduction to fuels
Nano Energy (2015) 15, 153–163
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RAPID COMMUNICATION
Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode tailored for CO2 reduction to fuels Jin Hyun Kima,1, Ganesan Mageshb,1, Hyun Joon Kangc, Marimuthu Banub, Ju Hun Kimc, Jinwoo Leec, Jae Sung Leea,n a
School of Environmental Science & Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea b School of Energy and Chemical Engineering, National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea c Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea Received 16 December 2014; received in revised form 3 April 2015; accepted 16 April 2015 Available online 2 May 2015
KEYWORDS
Abstract
Photoelectrochemical cell; BiVO4; Cobalt carbonate co-catalyst; Heterojunction; CO2 reduction
We report here that cobalt carbonate (Co-Ci) is a tailored oxygen evolution electrocatalyst (OEC) from water on BiVO4/WO3 composite photoanode to drive photoelectrochemical reduction of CO2 to fuels on a Cu cathode. For water oxidation, Co-Ci/BiVO4/WO3 performed best in CO2-saturated KHCO3 (KCi, pH 7) electrolyte recording an exceptional photocurrent of 3.5 mA/cm2 at 1.23 VRHE under 1 sun illumination, and an onset potential of 0.2 VRHE. In the photoanode-driven CO2 reduction, the Co-Ci/BiVO4/WO3 (photoanode)–Cu (cathode) system showed stable photocurrent and 51.9% faradaic efficiency (against water reduction to H2) for CO and C1–C2 hydrocarbons, whereas the best known OEC cobalt phosphate (Co-Pi) was less stable and gave only 22.4% faradaic efficiency. Due to its high stability and CO2 reduction selectivity, the Co-Ci assisted system produced 11 times larger amount of CH4 than the Co-Pi assisted system in a continuous operation. & 2015 Published by Elsevier Ltd.
n
Corresponding author. Tel.: +82 54 279 1552; fax: +82 54 279 1599. E-mail address: [email protected] (J.S. Lee). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.nanoen.2015.04.022 2211-2855/& 2015 Published by Elsevier Ltd.
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J.H. Kim et al. 2H2O + 4h + -O2 + 4H +
Introduction
Co-Ci/BiVO4/WO3 (E0 = 1.23 VRHE)
Solar fuel production from H2O and CO2 is an ideal countermeasure for the energy and environmental concerns the world is facing today [1]. Photoelectrochemical (PEC) method is a promising route for the reduction of CO2 to renewable fuels in combination with the oxidation of water, which resembles the natural photosynthesis of green plants [2]. The PEC CO2-to-fuel system can have different configurations depending on which electrode is the light absorbing photoelectrode [3], i.e. photoanode (PA) [4–6], photocathode (PC) [7,8], or both [9]. Due to their unsuitable band energy positions, these PAs and PCs need bias potentials to drive CO2 reduction under solar irradiation. The PAs, especially made with metal oxide have advantage due to stability, transparency and affordability in PEC-photovoltaic (PV) or PA–PC tandem device, where PV or PC utilizes the photons transmitted through the PA [10]. Moreover, the PAdriven PEC systems are versatile in fuel products due to the availability of various cathode electrocatalysts, which can control the efficiency and selectivity of CO2 reduction [11,12]. In order for a PA-driven PEC to be powered by an inexpensive single junction PV or a PC in tandem, its photocurrent onset potential should be further decreased. This could be achieved by loading an oxygen evolution catalyst (OEC) on PA, which not only decreases the onset potential, but also increases the photocurrent [13]. Monoclinic scheelite BiVO4 is one of the highest performing, visible light-active metal oxide photoanode materials with a direct band gap (Eg) of 2.4 eV [14–16]. High compatibility with modifications such as heterojunction [17–19], OEC [20], and doping [16] makes BiVO4 a subject for extensive current researches worldwide. A thin under-layer of WO3 in the BiVO4/WO3 heterojunction composites improves the performance of BiVO4 by better charge separation [17–19]. Further, BiVO4 decorated with cobalt phosphate (Co-Pi) [21,22] or FeOOH [15,20,23] as an OEC showed near complete surface charge separation resulting in enhanced photocurrents and the lowest photocurrent onset potentials (0.2 VRHE) among known visible light active metal oxide photocatalysts. However, the performance of these OECs depends sensitively on the electrolyte, i.e. Co-Pi works best in a phosphate (KPi) buffer and Co-Bi, Ni-Bi show the best performance in a borate (Bi) buffer. But for CO2 reduction, potassium bicarbonate (KCi) electrolyte gives the best CO2 reduction faradaic efficiency as well as high current density due to its ability to be in chemical equilibrium with CO2 [24]. Since the best known OECs, Co-Pi and FeOOH, do not function its best in the bicarbonate electrolyte, it is crucial to find an OEC that performs well in the CO2-saturated KCi electrolyte to establish an efficient PA–PEC system for CO2 conversion to fuels. Herein, we report for the first time a cobalt-carbonate (CoCi) OEC deposited BiVO4/WO3 (Co-Ci/BiVO4/WO3) composite as a photoanode in bicarbonate electrolyte to absorb solar light and drive the photoelectrochemical reduction of CO2 with a Cu cathode. The new Co-Ci/BiVO4/WO3 (photoanode)–Cu (cathode) system showed an excellent photocurrent stability and high faradaic efficiency (FE) for CO2 reduction with a lower bias potential than those of conventional photocathode-driven PEC systems. The system is presented in Scheme 1 together with operating principle. The main reactions taking place at photoanode and Cu cathode are as follows [6]:
Cu (cathode): CO2 + 8H + + 8e -CH4 + 2H2O (E0 = 0.169 VRHE)
(photoanode):
Overall reaction: H2O +CO2-O2 + fuels (H2, CO, CH4, C2H4…. ) The generation of Co-Ci as an electrochemical OEC in a neutral HCO3 /CO2 system was recently reported by Zhao et al. [25]. However, this is the first time that Co-Ci has been applied to a light absorber to demonstrate its outstanding performance for CO2 reduction.
Experiemental Preparation of BiVO4/WO3 The BiVO4/WO3 film was prepared by a polymer-assisted direct deposition (PADD) for WO3 and a metal organic decomposition method (MOD) for BiVO4 sequentially. A PADD solution was prepared with 1.85 g of ammonium metatungstate and 1 g of polyethylenimine in 10 ml of deionized water. Prepared 5 μl of PADD solution was spread on surface of F-doped SnO2 glass (FTO, TEC-15, Pilkington) and dried at 80 1C, annealed at 550 1C for 1 h. Annealing gave highly transparent film of WO3. BiVO4 film was prepared by a modified MOD method. Thus, 0.2 M Bi(NO3) 5H2O (99.8%; Kanto Chemicals) dissolved in acetic acid (99.7%; Kanto Chemicals) and 0.03 M, VO(acac)2 (98.0%; Sigma Aldrich) in acetylacetone (499.0%; Kanto Chemicals) were prepared as precursor solutions. For fabrication of a BiVO4 film on WO3 layer, 25 μl of stoichiometric precursor solution was dropped on WO3 coated FTO glass (2 cm 2.5 cm) and dried. The precursor film was calcined at 500 1C for 2 h to form a yellow BiVO4/WO3 film. Deposition procedure was repeated twice to achieve optimum performance of composite. The net irradiation area of photoanode was 0.6 cm2 for water oxidation that was connected by silver paste and copper wire and sealed with epoxy resin. For CO2 reduction, a larger FTO (2 cm 2 cm) was used.
Co-Ci deposition on BiVO4/WO3 photoanode The cobalt bicarbonate (Co-Ci) electrocatalyst was deposited with photo-assisted electrodeposition (PED) using illumination (100 mW/cm2) at 0.2 V (vs. SCE) in 0.3 mM Co (NO3)2 6H2O (Z 98%; Aldrich) and 100 ml of 0.1 M potassium bicarbonate (KCi, KHCO3) purged with CO2 to make pH of 7. Optimum total coulomb passed during deposition was about 7 mC/cm2 (deposition time of 10 min). The Co-Ci deposition on FTO glass (without semiconductor layers) was conducted with 1.1 V (vs. SCE) for 10 min with the same solution as the above PED and gave a brown colored film.
Measurements of PEC water oxidation Photoelectrochemical (PEC) measurements of bare and CoCi/BiVO4/WO3 photoanodes were performed with a standard three-electrode configurations; photoanode as the working electrode, Pt mesh rod as the counter electrode and SCE (saturated KCl) as the reference electrode. 0.1 M KCi
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Scheme 1 (a) Illustration of a photoanode-driven photoelectrochemical system for solar fuel production by CO2 reduction. (b) Energy diagram and reactions involved on Co-Ci/BiVO4/WO3 photoanode for oxygen evolution and on Cu cathode for CO2 reduction.
(499.0%, Sigma Aldrich) with (pH 7)/without (pH 9) 30 min of CO2 purge or 0.1 M potassium phosphate (KPi) buffer with 30 min of nitrogen purge were used as electrolyte. All electrodes were illuminated from the front side. The scan rate for the current–voltage curve was 20 mV/s. To measure the degree of charge separation, 0.1 M Na2SO3 (498%, Sigma Aldrich) was added to pH 7 0.1 M KCi. Potentials were recorded with correction by the Nernst relation ERHE = ESCE + 0.0591 pH+ 0.241, in which ESCE is applied bias potential and 0.241 is a conversion factor from SCE electrode to the RHE scale. All electrochemical data were recorded by using a potentiostat (IviumStat, Ivium Technologies). A 300 W Xenon lamp was used to make simulated 1 sun light irradiation condition (AM 1.5 G, 100 mW/cm2) by using a solar simulator (Oriel 91160) with an AM 1.5G filter calibrated with a reference cell certified by the National Renewable Energy Laboratories, USA. Electrochemical impedance spectroscopy (EIS) was conducted with the same configuration of PEC measurements at 0.58 VSCE (1.23 VRHE) with an AC frequency range 10 1–104 Hz under one-sun irradiation. The spectra were analyzed by the Z-View program (Scribner Associates Inc.). The faradaic efficiencies (FE) of H2 and O2 evolution were determined from product analysis in a closed circulation system by a HP 6890 gas chromatography (GC) as follows:
FE= No. of moles formed/ theoretical no. of moles based on measured current.
PEC CO2 reduction The visible light irradiation was carried out using a 500 W Hg lamp (Newport) with a 420 nm cut-off filter (intensity of the radiation was 490 mW/cm2) and water for IR radiation removal. The photoelectrochemical reactor made of polycarbonate consists of two compartments separated by a Nafion-115 proton exchange membrane. The cathode compartment was connected to a gas circulation system with a peristaltic pump (Eyela, Japan) with a Tygon tubing and a six-port valve for on-line sampling to a gas chromatograph (HP 6890) with a Supelco Carboxen 1000 packed column and thermal conductivity detector (TCD). Presence of formic acid, methanol and ethanol in liquid products was analysed using a Hi-plex H column in an Agilent high performance liquid chromatograph. The temperature of the electrolyte was maintained at 10 1C by placing the reactor in a custom built water jacket containing mixture of 20% ethylene glycol in water in an automated chiller Lab Companion).
156 Before the start of the reaction, the electrolyte 0.5 M KHCO3 (pH 7.5) was pre-electrolyzed at 0.025 mA using Pt mesh for 180 min under CO2 purging to remove traces of impurity metal ions in the electrolyte which are known to poison the Cu cathode. The closed circulation system and the reactor were filled with CO2 which was continuously circulated through the electrolyte for the entire reaction period. The reactions were carried out at atmospheric pressure of CO2 and the bias potentials on the photoanode were applied using a Gamry Reference 600 potentiostat. The Gamry potentiostat simultaneously measured the potential between the working electrode and the counter electrode (EWE–ECE) while the reactions were going on in the 3-electrode system.
Results and discussion Fabrication and characterization of photoanodes The fabrication of PA started with the preparation of WO3 on F-doped SnO2 (FTO) by a polymer-assisted direct deposition
J.H. Kim et al. (PADD) method [26] over which BiVO4 was deposited by a modified metal organic decomposition (MOD) resulting in a BiVO4/WO3 film with a good transparency (Figure 1a). An OER electrocatalyst, Co-Ci or Co-Pi was then deposited on the BiVO4/WO3 surface by photo-assisted electrodeposition (PED). In addition, Co-Ci alone was also deposited on FTO by an electrochemical deposition (ED) as a reference. XRD peaks (Figure S1) corresponding to Co-Ci was not observed for Co-Ci/BiVO4/WO3 or Co-Ci/FTO, showing that Co-Ci is in an amorphous phase. UV-visible spectra in Figure S2 indicated that thin WO3 layer showed very small light absorption, whereas BiVO4/WO3 showed good absorption up to 500 nm and 66.6% of light harvesting efficiency (LHE = 1– 10 A, A= absorbance) as expected for BiVO4 with Eg of 2.4 eV. The Co-Ci deposition brought a slight red shift in absorption and much thicker deposition gave a brown colored film. SEM images of BiVO4/WO3 (Figure 1c) showed a porous BiVO4 film (thickness 300 nm, grain size 100 nm) deposited on a densely packed WO3 film (thickness 100 nm, grain of 20 nm, cross-section in Figure 1f). Deposition of Co-Ci resulted in a distinct and homogeneous thin layer of Co-Ci
Figure 1 (a) A photograph of FTO, WO3 and BiVO4/WO3 showing excellent transparency. SEM images of (b) WO3, (c) BiVO4/WO3, (d) thin, (e) thick Co-Ci/BiVO4/WO3 and (f) cross section of BiVO4/WO3 film. The Co-Ci deposition was conducted in 0.2 V (vs. SCE), 10 min (7 mC) for (d) and 20 min (15 mC) for (e).
Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode particles over the surface of BiVO4/WO3 (Figure 1d), and their amount increased with longer deposition time (Figure 1e). The near-surface region of photoanodes was analyzed by XPS. Binding energies of Bi (4f5/2 at 164.3 eV, 4f7/2 at 158.1 eV), and V (2p1/2 at 529 eV, 2p3/2 at 515.9 eV) correspond to standard BiVO4 (Figure S3). Small portion of W (4f5/2 at 37.5 eV, 4f7/2 at 35.0 eV) was detected, probably due to porous structure [19,27]. For Co-Ci/BiVO4/WO3 and Co-Ci/FTO, two peaks corresponding to Co (2p3/2 at 780.7 eV, 2p1/2 at 795.7 eV) indicated presence of both Co (III) and Co(II) as revealed by deconvolution fitting (Figure S4). It has been reported that the oxidation state of Co in Co-Pi varies from II to IV, in the form of amorphous CoOx and Co3O4 [13,28,29]. Carbon C 1s showed two peaks at 284.6 eV (assigned for C–C) and 288.0 eV (assigned to –C(O)O– and –C–OH) [25,30]. BiVO4/WO3 predominantly showed C–C peak from contaminated carbon with a small peak at 284.6.0 eV (Figure S5). However, increase in intensity of C 1s peak and stronger 288.0 eV peak upon Co-Ci deposition indicate the presence of a CO3 2 -like species (Figures 2b,c and S5) [25,30]. More obvious indication of the carbonate species in Co-Ci/BiVO4/ WO3 could be observed from the O 1s spectrum, which gave peaks at 529.3 eV assigned to M-O and at 531 eV assigned to –COx and surface –O–H groups. The BiVO4/WO3 shows predominant M–O peak at 529.3 eV with a very small peak at 531 eV (Figures S5b and 2c). The trend is reversed in Co-Ci/ BiVO4/WO3 (Figures S6b and 2d), which showed a small M–O
Figure 2
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peak and a large non-metal oxide peak at 531 eV [25,30]. Surface elemental composition by XPS in Table S1 showed increase in C, decrease in O, and a large decrease in Bi and V after Co-Ci deposition. All XPS results consistently showed that a film of Co-Ci was deposited on top of BiVO4/WO3 by PED and it had a similar composition as Co-Ci deposited on FTO by ED. Based on XPS results, expected formula is [Co(II/ III)Ox HCO3 ], similar to other complex metal–ligand OECs (Co-Pi, Co-Bi, Ni-Bi) [15,22,29,31–33].
Photoelectrochemical water oxidation Before application to CO2 conversion, the fabricated photoanodes were first tested for PEC water oxidation with Pt mesh rod as a hydrogen-evolving cathode to gauge only its photooxidation activity under simulated 1 sun irradiation (AM 1.5G, 100 mW/cm2). Detailed I–V curves are given in Figures 3a and S6–S8. First, BiVO4/WO3 in CO2-purged 0.1 M KHCO3 (KCi) and argon-purged 0.1 M KPi buffer both at pH 7 gave photocurrents of 2.6 mA/cm2 and 2.0 mA/cm2, respectively, at 1.23 VRHE. Without CO2 purge, KCi showed pH 9 and a lower photocurrent of 2.2 mA/cm2 (Figure S6). This significant difference by different electrolytes is known to be due to the catalytic effect of HCO-3 anion on water oxidation [34]. Thus, dissolved CO2 generates higher concentration of HCO3 ion which is involved in hole transfer to accelerate OER reaction by forming carbonate radicals
C 1s and O 1s XPS spectra of (a,c) BiVO4/WO3 and (b,d) Co-Ci/BiVO4/WO3 photoanode surface.
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Figure 3 (a) Photocurrent data of Co-Ci/ BiVO4/WO3 and BiVO4/WO3 in various electrolytes (under 1 sun illumination, pH 7, (CO2 purged) 0.1 M KHCO3), (b) ηsurf (surface charge separation efficiency) of Co-Ci/BiVO4/WO3 and BiVO4/WO3, (c) IPCE of Co-Ci/BiVO4/ WO3 and BiVO4/WO3 and (d) photocurrent stability of Co-Ci/BiVO4/WO3 (AM 1.5G), and O2/H2 gas evolution (490 mW/cm2, 4420 nm) at pH 7, (CO2 purged) 0.1 M KHCO3.
and peroxocarbonates that provide a beneficial catalytic pathway for O2 evolution. The deposition of Co-Ci on BiVO4/WO3 led to a shift in the photocurrent onset potential of 300 mV from 0.50 VRHE for BiVO4/WO3 to 0.20 VRHE for Co-Ci/BiVO4/WO3. The best performing Co-Ci/BiVO4/WO3 was obtained by PED at 0.2 VSCE for 10 min, which gives an optimum thickness of Co-Ci as evident from the SEM images in Figure 1d. The Co-Ci/BiVO4/ WO3 electrode also showed higher photocurrent in CO2-saturated KCi of pH 7 (3.5 mA/cm2 at 1.23 VRHE) than at pH 9 without CO2 purge (3.3 mA/cm2 at 1.23 VRHE), indicating that higher concentration of HCO3 is favorable for Co-Ci (Figure S7). The performance of Co-Ci/BiVO4/WO3 in KCi was comparable with Co-Pi/BiVO4/WO3 in KPi electrolyte in photocurrent generation (3.5 mA/cm2 at 1.23 VRHE) as well as the cathodic shift of the current onset potential ( 300 mV). This performance is also comparable to that of recently-reported bestperforming BiVO4-based photoanodes using Co-Pi [10,22,35] and FeOOH/NiOOH [20] as OEC when they work in their own optimized electrolytes. A quantitative analysis on the role of OEC could be made through photo-oxidation of sulfite as a hole scavenger, which quantifies the extent of recombination on the surface and in the bulk independently [20,22,23]. The photo-oxidation of sulfite is so facile that it eliminates the hole injection barrier to the electrolyte and charge recombination on the surface. The surface charge separation efficiency (ηsurf) could be calculated by dividing the photocurrent from water oxidation by the photocurrent from sulfite oxidation. The ηsurf of Co-Ci/ BiVO4/WO3 was 85% on average (Figure 3b), which is only
slightly lower than that of Co-Pi/BiVO4/WO3 in KPi electrolyte ( 90%). Bulk charge separation efficiency (ηbulk) was calculated by dividing the photocurrent measured with sulfite in electrolyte by the total absorbed photocurrent density, and it was as high as 72% near 1.23 VRHE (Figure S10). This ηbulk is comparable with recent state-of-the-art BiVO4-based photoanodes [10,19,21,36]. Impedance measurements showed that the resistance at the electrolyte|| semiconductor interface decreased by half upon Co-Ci loading (893 Ω) on to BiVO4/ WO3 (1742 Ω) (Figure S11). Thus, deposition of Co-Ci led to facile charge transfer at electrolyte|| semiconductor interface that usually suffers the highest impedance owing to sluggish water oxidation kinetics. In addition, Co-Ci showed comparable external quantum efficiency (EQE, 1.02%) to that of Co-Pi (1.16%) (Figure S12). The incident photon to current conversion efficiency (IPCE) of the Co-Ci/BiVO4/WO3 electrode was measured. As in Figure 3c, IPCE of Co-Ci/BiVO4/WO3 recorded 60% up to 450 nm, which is comparable to the highest values reported for PA derived from BiVO4 [18–20]. Threshold of IPCE was 510 nm, indicating that Co-Ci does not extend the range of light adsorption but only acts as OEC. Stability tests for Co-Ci/BiVO4/WO3 demonstrated an exceptionally stable photocurrent generation for 3 h in KCi (Figure 3d), whereas in KPi, the photocurrent decayed significantly within 1 h (Figure S13). This indicates that HCO3 is necessary for Co-Ci to be active and stable in solar water oxidation. This is very similar to the role of KPi buffer electrolyte for Co-Pi [22,28,31,32]. The measurement of the evolved gases by gas chromatography showed a
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Figure 4 (a) Photocurrent data of Co-Ci/BiVO4/WO3 with thin (7 mC) and thick (15 mC) deposition (right inset shows morphology of each film). (b) Scheme of hole (blue) and electron (red) transfer for thin and thick Co-Ci deposition.
faradaic efficiency (FE) of 100% for O2 and H2, suggesting that observed photocurrents from Co-Ci/BiVO4/WO3 were fully utilized for the water splitting reactions (Figure 3d). The OEC promotes water oxidation kinetics and reduces its overpotential. But the amount of OEC must be optimized [37] because it adds length of hole diffusion as shown in Figure 4. Porosity of photoelectrode is crucial for charge separation as depletion layer is formed by interaction of electrolyte||semiconductor. Thus through the optimized thin Co-Ci film prepared with 7 mC, holes can transfer without much recombination (ηsurf = 90%), whereas excessively thick Co-Ci deposits (15 mC) forms longer pathway for holes, partially blocks the pores, and shade the light absorber, thus resulting in increased recombination. In addition, higher amounts of Co-Ci resulted in large transient peaks in I–V curves. Identical behavior has been reported for Co-Pi on Fe2O3 and BiVO4, which shows the importance of the proper thickness [33].
Photoelectrochemical CO2 reduction Establishing that Co-Ci/BiVO4/WO3 is as good as the best known water oxidation system in KCi saturated with CO2, we applied it for PEC conversion of CO2 to fuels with the common Cu cathode. Since the study of Co-Pi has been limited to a KPi electrolyte, it is also interesting to investigate its behavior in CO2 saturated KCi electrolyte. As mentioned, it is essential to use the bicarbonate electrolyte for CO2 reduction in general due to its ability for facile supply of CO2 to cathode according to the equilibrium (HCO3 2CO2 + OH ), where OH will be converted back to HCO3 by the dissolved CO2 [24]. In other
electrolytes, the supply of CO2 is limited since diffusion is the only pathway. In addition, the large K + ions adsorb preferentially on the cathode surface, deter the adsorption of H + , and stabilize CO2∙ considered as an intermediate of CO2 reduction [8,38]. Thus, KHCO3 electrolyte favors CO2 reduction over competing water reduction. Hence, Co-Ci/ BiVO4/WO3 was tested as a photoanode in the PA-driven PEC system for CO2 reduction under visible light (4420 nm, 490 mW/cm2) irradiation in CO2-purged 0.5 M KHCO3 electrolyte (pH 7.5). The reactions were carried out in a reactor where the cathode and anode compartments are separated by a proton exchange membrane. The cathode compartment was connected to a closed circulation gas line with provisions for in-situ product analysis using a gas chromatograph. The reactions were carried out in 3-electrode system by applying bias potentials to the photoanode ranging from + 0.30 to + 0.40 VRHE. The time profiles of products are given in Figures 5b and S14. Table 1 and Figure 5c shows FE of the products obtained at different potentials. The loading of Co-Ci shifts the current onset potential of BiVO4/WO3 from + 0.50 V to about + 0.20 V (Figure 3a). Due to this shift, potentials up to + 0.30 V, where BiVO4/WO3 does not produce sufficient photocurrent could now be used for Co-Ci/BiVO4/WO3. In the PA-driven PEC system with CoCi/BiVO4/WO3 as PA and Cu as the cathode, the products were predominantly CH4 with CO and H2 as additional products, and as the potential was increased FE of CH4 increased whereas those of H2 and CO decreased. At a potential of + 0.4 V, FE of CH4 was 46.8%, and a larger C2H4 molecule was also observed. The observed trend shows that products requiring multiple electron reductions are favored at higher potentials whereas 2-electron reduction products
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Figure 5 (a) Chronoamperometry for the PA- Cu cathode PEC CO2 reduction. (b) Amounts of CH4 produced. (c) Faradaic efficiency at 60 min. (d) Faradaic efficiency x charge at 60 min. With potential of 0.4 VRHE and CO2 purging, 0.5 M KCi was used for BiVO4/WO3– Cu, Co-Ci/BiVO4/WO3–Cu and 0.5 M KPi was used Co-Pi/BiVO4/WO3–Cu. Table 1 Faradaic efficiencies (FE) at 60 min during the PA-driven PEC CO2 reduction under visible light (4420 nm) irradiation. PA-cathode for PEC system
BiVO4/WO3–Cu BiVO4/WO3–Cu Co-Ci/BiVO4/WO3–Cu Co-Ci/BiVO4/WO3–Cu Co-Ci/BiVO4/WO3–Cu Co-Pi/BiVO4/WO3–Cu
Potential of photoanode (vs. RHE)
+0.40 +0.35 +0.40 +0.35 +0.30 +0.40
are favored at lower potentials. Co-Ci/BiVO4/WO3 in KCi (pH 7) showed very good photocurrent stability during the PA-driven PEC CO2 reduction (Figure 5a and S15a). Two types of control reactions were carried out using similar applied bias potentials either without irradiation in a CO2 purged reactor or in an Ar purged reactor under irradiation. Both blank reactions did not yield any CO2 reduction products. This confirms that the observed products were only from the reduction of the supplied CO2, and their formation was not possible with only bias potential in the absence of photoelectrons generated at the photoanode by harvesting visible light. The analysis of liquid phase showed the absence of CH3OH or HCOOH. In comparison, Co-Pi/BiVO4/WO3 PA-driven PEC at 0.40 VRHE with Cu cathode in CO2-saturated 0.1 M KPi electrolyte (pH 6.5) gave CO2 reduction FE of 22.4%, which is less than half of the
Charge (Coulomb)
1.08 0.91 3.34 1.75 0.57 1.33
Faradaic efficiency (%) C2H4
CH4
CO
H2
Total
17.7 0 3.7 0 0 0
11.8 12.1 46.8 42.7 13.1 20.4
4.9 6.8 1.4 3.0 12.2 2.0
64.6 72.9 48.1 49.4 64.9 77.3
99.1 91.8 100 95.1 90.2 99.7
51.9% FE obtained at similar potentials from Co-Ci/BiVO4/WO3 in KHCO3 (Table 1 and Figure 5a). This is primarily due to the decreased availability of CO2 in KPi buffer electrolyte. Previous studies showed that with a common cation, the anions in the electrolyte favored CO2 reduction in the order; HCO3 2 4CO3 2 4SO4 2 4PO4.[39] Moreover, the photocurrent and hence the total charge generated from Co-Ci/BiVO4/ WO3 PA-driven PEC is 2.5 times higher than Co-Pi/BiVO4/WO3 PA-driven PEC at 0.4 VRHE (Table 1). When the charge generated and the CO2 reduction faradaic efficiency are taken into account (Faradaic efficiency x charge), Co-Ci/BiVO4/WO3 PAdriven PEC gave 6 fold higher amount of CO2 reduction products and 1.5 times higher H2 than the Co-Pi/BiVO4/WO3 PA-driven PEC (Figures 5d and S14). In addition, Co-Pi/BiVO4/WO3 was unstable and lost most of its activity within 2 h in a CO2 saturated KPi electrolyte (Figure 5a). Due to its rapid loss in
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Table 2 Two-electrode potentials of the PA- or PC-driven PEC CO2 reduction systems measured concurrently during reactions in a 3-electrode system. Working electrode
Counter electrode
Potential of WE vs. RE (VRHE)
∣EWE ECE∣ (V)
Co-Ci/BiVO4/WO3 Co-Ci/BiVO4/WO3 Co-Ci/BiVO4/WO3 CuOx (cathode)
Cu Cu Cu Pt (anode)
0.40 0.35 0.30 0.07
1.5 1.5 1.3 1.8
photocurrent, Co-Pi/BiVO4/WO3 PA-driven PEC stopped producing any CH4 after 30 min and hence the Co-Ci/BiVO4/WO3 PAdriven PEC produced 11 times higher CH4 at the end of 120 min (Figure 5b). As discussed, Co-Pi OEC in KPi buffer electrolyte showed slightly better water oxidation activity than Co-Ci in CO2-purged KCi electrolyte with hydrogen generation at Pt cathode. Yet, in CO2 reduction in CO2-purged KCi electrolyte, Co-Ci showed higher activity, much better stability and superior selectivity to CO2 reduction relative to H2O reduction to H2. Hence, we can conclude that Co-Ci is a tailored OEC for CO2 reduction in PA-driven PEC solar fuel generators. Photoelectrochemical reduction of CO2 to fuels has been conventionally carried out using photocathode (PC)-driven PEC systems in a 3electrode configuration and a bias potential supplied to the photocathode are reported versus the reference electrode (RE) [7]. However, the reported potentials of the working electrode vs. RE in the 3-electrode systems for the PC-driven PEC system cannot be directly compared with those for PAdriven PEC system. Hence, we measured the potential between the working and the counter electrode (∣EWE ECE∣) during CO2 reduction in a 3-electrode system. Since such data are not available for the PC-driven PEC system, we took the same measurement for a reported PC-driven PEC system which used the lowest bias potential in a 3-electrode system [39]. The ∣EWE–ECE∣ offers a way by which PA- and PCdriven systems can be compared on the same ground and represents required external energy supply for both systems. The results show that Co-Ci/BiVO4/WO3 PA-driven PEC at + 0.30 VRHE with Cu as cathode has a ∣EWE–ECE∣ of 1.3 V (Table 2 and Figure S15b). The reference CuOx (Figures S15, S16) PC-driven PEC (with Pt anode) at + 0.07 VRHE, which is the lowest potential reported for a PC-driven PEC CO2 reduction without a co-catalyst has a ∣EWE ECE∣ of 1.8 V (Figure S15c) [39]. These ∣EWE ECE∣ data show that the PAdriven PEC systems are better choice for the reduction of CO2 requiring less external energy input. Another advantage of Co-Ci/BiVO4/WO3 is an excellent transparency (Figure 1a), which allows fabrication of efficient tandem configuration with a multi-junction solar cell or PC to realize a no-bias PEC device.
Conclusion In summary, we developed Co-Ci/BiVO4/WO3 photoanode with outstanding performance in neutral bicarbonate electrolyte – photocurrent of 3.5 mA/cm2 at 1.23 VRHE with 0.2 VRHE onset potential – which is similar to the
performance of Co-Pi/BiVO4/WO3 in phosphate electrolyte. Co-Ci/BiVO4/WO3 showed excellent stability and 100% FE for water splitting. The Co-Ci/BiVO4/WO3 can effectively drive PEC CO2 reduction using a standard Cu cathode in CO2saturated KHCO3 electrolyte producing CO and C1–C2 hydrocarbon products with FE of 51.9% with a stable photocurrent. In comparison, Co-Pi/BiVO4/WO3 driven system in CO2 saturated KPi gave 22.4% FE for CO2 reduction and poor stability. The same Co-Pi system in CO2-saturated KHCO3 electrolyte performed even worse and was extremely unstable. Due to its higher photocurrent and excellent stability, the Co-Ci system produced 11 times more CH4 than the Co-Pi system at similar bias potentials. In addition, Co-Ci/BiVO4/WO3 required less external energy supply than the best known photocathode (CuOx)-driven system. The highly transparent Co-Ci/BiVO4/WO3 could also be utilized to fabricate a no-bias solar fuel generation cell with a PV cell or a photocathode in tandem configuration due to its high transmittance of longer wavelength photons.
Funding sources This work was supported by Brain Korea Plus Program of Ministry of Education, Korean Center for Artificial Photosynthesis (NRF-2011-C1AAA0001-2011-0030278) funded by MISIP and Project no. 10050509 funded by MOTIE (Ministry of Trade, Industry & Energy) of Republic of Korea. It was also supported by Ulsan National Institute of Science and Technology (UNIST).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.04.022.
References [1] N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. 103 (2006) 15729–15735. [2] S. Bensaid, G. Centi, E. Garrone, S. Perathoner, G. Saracco, ChemSusChem 5 (2012) 500–521. [3] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chem. Rev. 110 (2010) 6446–6473. [4] S. Ichikawa, R. Doi, Catal. Today 27 (1996) 271–277. [5] C. Ampelli, G. Centi, R. Passalacqua, S. Perathoner, Energy Environ. Sci. 3 (2010) 292–301.
162 [6] G. Magesh, E.S. Kim, H.J. Kang, M. Banu, J.Y. Kim, J.H. Kim, J.S. Lee, J. Mater. Chem. A 2 (2014) 2044–2049. [7] B. Kumar, M. Llorente, J. Froehlich, T. Dang, A. Sathrum, C.P. Kubiak, Annu. Rev. Phys. Chem. 63 (2012) 541–569. [8] J.-W. Jang, S. Cho, G. Magesh, Y.J. Jang, J.Y. Kim, W.Y. Kim, J.K. Seo, S. Kim, K.-H. Lee, J.S. Lee, Angew. Chem. Int. 53 (2014) 5852–5857. [9] S. Sato, T. Arai, T. Morikawa, K. Uemura, T.M. Suzuki, H. Tanaka, T. Kajino, J. Am. Chem. Soc. 133 (2011) 15240–15243. [10] F.F. Abdi, L. Han, A.H.M. Smets, M. Zeman, B. Dam, R. van de Krol, Nat. Commun. 4 (2013). [11] J. Qiao, Y. Liu, F. Hong, J. Zhang, Chem. Soc. Rev. 43 (2014) 631–675. [12] Q. Lu, J. Rosen, Y. Zhou, G.S. Hutchings, Y.C. Kimmel, J.G. Chen, F. Jiao, Nat. Commun. 5 (2014). [13] M.W. Kanan, Y. Surendranath, D.G. Nocera, Chem. Soc. Rev. 38 (2009) 109–114. [14] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278. [15] Y. Park, K.J. McDonald, K.-S. Choi, Chem. Soc. Rev. 42 (2013) 2321–2337. [16] K.P.S. Parmar, H.J. Kang, A. Bist, P. Dua, J.S. Jang, J.S. Lee, ChemSusChem 5 (2012) 1926–1934. [17] S.J. Hong, S. Lee, J.S. Jang, J.S. Lee, Energy Environ. Sci. 4 (2011) 1781–1787. [18] Y. Pihosh, I. Turkevych, K. Mawatari, T. Asai, T. Hisatomi, J. Uemura, M. Tosa, K. Shimamura, J. Kubota, K. Domen, T. Kitamori, Small 10 (2014) 3692–3699. [19] P.M. Rao, L. Cai, C. Liu, I.S. Cho, C.H. Lee, J.M. Weisse, P. Yang, X. Zheng, Nano Lett. 14 (2014) 1099–1105. [20] T.W. Kim, K.-S. Choi, Science 343 (2014) 990–994. [21] F.F. Abdi, N. Firet, R. van de Krol, ChemCatChem 5 (2013) 490–496. [22] D.K. Zhong, S. Choi, D.R. Gamelin, J. Am. Chem. Soc. 133 (2011) 18370–18377. [23] J.A. Seabold, K.-S. Choi, J. Am. Chem. Soc. 134 (2012) 2186–2192. [24] J. Wu, F.G. Risalvato, F.-S. Ke, P.J. Pellechia, X.-D. Zhou, J. Electrochem. Soc. 159 (2012) F353–F359. [25] Q. Zhao, Z. Yu, W. Yuan, J. Li, J. Int, Hydrog. Energy 38 (2013) 5251–5258. [26] S.J. Hong, H. Jun, J.S. Lee, Scr. Mater. 63 (2010) 757–760. [27] S.K. Cho, H.S. Park, H.C. Lee, K.M. Nam, A.J. Bard, J. Phys. Chem. C 117 (2013) 23048–23056. [28] T.H. Jeon, W. Choi, H. Park, Phys. Chem. Chem. Phys. 13 (2011) 21392–21401. [29] M.W. Kanan, D.G. Nocera, Science 321 (2008) 1072–1075. [30] K.S. Joya, K. Takanabe, H.J.M. de Groot, Adv. Energy Mater. 16 (2014) 1400252. [31] S.K. Choi, W. Choi, H. Park, Phys. Chem. Chem. Phys. 15 (2013) 6499–6507. [32] C. Ding, J. Shi, D. Wang, Z. Wang, N. Wang, G. Liu, F. Xiong, C. Li, Phys. Chem. Chem. Phys. 15 (2013) 4589–4595. [33] D.K. Zhong, D.R. Gamelin, J. Am. Chem. Soc. 132 (2010) 4202–4207. [34] K. Sayama, N. Wang, Y. Miseki, H. Kusama, N. OnozawaKomatsuzaki, H. Sugihara, Chem. Lett. 39 (2010) 17–19. [35] J.H. Kim, J.W. Jang, H.J. Kang, G. Magesh, J.Y. Kim, J.H. Kim, J. Lee, J.S. Lee, J. Catal. 317 (2014) 126–134. [36] S.K. Pilli, R. Janarthanan, T.G. Deutsch, T.E. Furtak, L.D. Brown, J.A. Turner, A.M. Herring, Phys. Chem. Chem. Phys. 15 (2013) 14723–14728. [37] G.M. Carroll, D.K. Zhong, D.R. Gamelin, Energy Environ. Sci. 8 (2015) 577–584.
J.H. Kim et al. [38] M. Jitaru, D.A. Lowy, M. Toma, B.C. Toma, L. Oniciu, J. Appl. Electrochem. 27 (1997) 875–889. [39] G. Ghadimkhani, N.R. de Tacconi, W. Chanmanee, C. Janaky, K. Rajeshwar, Chem. Commun. 49 (2013) 1297–1299. Jin Hyun Kim received his undergraduate degree at The University of Seoul (South Korea) in 2012. He joined Jae Sung Lee's group in Pohang University of Science and Technology as Ph.D. student. He is working for development of photocatalytic materials and systems for photoelectrochemical solar energy conversion.
Ganesan Magesh is currently working as a postdoctoral researcher at the Ulsan National Institute of Science and Technology, South Korea. He completed Ph.D. in Chemistry from Indian Institute of Technology Madras (2010), and M.Sc. in Chemistry from the Anna University, India (2002). His current research interests are semiconductor photoelectrochemical energy conversion, water splitting and carbon dioxide reduction.
Hyun Joon Kang is currently an Integrated M.S./Ph.D. course student in the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH). He received his B.S. in the same department and university in 2009. His study is mainly focused on the doping effects on BiVO4 for photocatalytic water splitting.
Marimuthu Banu is currently working as a postdoctoral researcher at the Ulsan National Institute of Science and Technology, South Korea. She completed Ph.D. in Chemistry (2011) and M.Sc. in Chemistry (2006) from Bharathidasan University, India. Her current research interests are heterogeneous catalysis, materials science and theoretical chemistry.
Ju Hun Kim received his B.S. (2007) and M. S. (2010) degree in the Dept. of Mechanical and Control System Engineering, Dept. of Computer Science and Electronic Engineering at Handong Global University (HGU), Korea. He is currently Ph.D. course student at POSTECH in Chemical Engineering and works with Prof. Jae Sung Lee at UNIST. His current research involves synthesis and development of nanostructured metal oxide photoelectrodes for photoelectrochemical water splitting and artificial photosynthesis.
Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode
Prof. Jinwoo Lee obtained his B.S. (1998) and Ph.D. (2003) from the Department of Chemical and Biological Engineering of Seoul National University, Korea. After his postdoctoral research at Seoul National University (with Prof. Taeghwan Hyeon, 2003–2005) and Cornell University (with Prof. Ulrich Wiesner, 2005–2008), he joined the faculty of the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH) in June, 2008. His research field includes synthesis and applications of ordered functional mesoporous materials and shape controlled nanocrystals in energy conversion and storage devices.
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Professor Jae Sung Lee received his BS degree from Seoul National University (1975), M.S. degree from Korea Advanced Institute of Science and Technology (1977), PhD degree at Stanford University (1984). After postdoctoral carrier at Catalytica, he returned to Korea in 1986 to become a professor of chemical engineering at Pohang University of Science and Technology. He and his group recently moved to Ulsan National Institute of Science and Technology in 2013. He has been leading a laboratory of eco-friendly catalysis and energy, and working on photocatalytic water splitting, fuel cell electrocatalysis, and heterogeneous chemical catalysis.
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RAPID COMMUNICATION
Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode tailored for CO2 reduction to fuels Jin Hyun Kima,1, Ganesan Mageshb,1, Hyun Joon Kangc, Marimuthu Banub, Ju Hun Kimc, Jinwoo Leec, Jae Sung Leea,n a
School of Environmental Science & Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea b School of Energy and Chemical Engineering, National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea c Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea Received 16 December 2014; received in revised form 3 April 2015; accepted 16 April 2015 Available online 2 May 2015
KEYWORDS
Abstract
Photoelectrochemical cell; BiVO4; Cobalt carbonate co-catalyst; Heterojunction; CO2 reduction
We report here that cobalt carbonate (Co-Ci) is a tailored oxygen evolution electrocatalyst (OEC) from water on BiVO4/WO3 composite photoanode to drive photoelectrochemical reduction of CO2 to fuels on a Cu cathode. For water oxidation, Co-Ci/BiVO4/WO3 performed best in CO2-saturated KHCO3 (KCi, pH 7) electrolyte recording an exceptional photocurrent of 3.5 mA/cm2 at 1.23 VRHE under 1 sun illumination, and an onset potential of 0.2 VRHE. In the photoanode-driven CO2 reduction, the Co-Ci/BiVO4/WO3 (photoanode)–Cu (cathode) system showed stable photocurrent and 51.9% faradaic efficiency (against water reduction to H2) for CO and C1–C2 hydrocarbons, whereas the best known OEC cobalt phosphate (Co-Pi) was less stable and gave only 22.4% faradaic efficiency. Due to its high stability and CO2 reduction selectivity, the Co-Ci assisted system produced 11 times larger amount of CH4 than the Co-Pi assisted system in a continuous operation. & 2015 Published by Elsevier Ltd.
n
Corresponding author. Tel.: +82 54 279 1552; fax: +82 54 279 1599. E-mail address: [email protected] (J.S. Lee). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.nanoen.2015.04.022 2211-2855/& 2015 Published by Elsevier Ltd.
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J.H. Kim et al. 2H2O + 4h + -O2 + 4H +
Introduction
Co-Ci/BiVO4/WO3 (E0 = 1.23 VRHE)
Solar fuel production from H2O and CO2 is an ideal countermeasure for the energy and environmental concerns the world is facing today [1]. Photoelectrochemical (PEC) method is a promising route for the reduction of CO2 to renewable fuels in combination with the oxidation of water, which resembles the natural photosynthesis of green plants [2]. The PEC CO2-to-fuel system can have different configurations depending on which electrode is the light absorbing photoelectrode [3], i.e. photoanode (PA) [4–6], photocathode (PC) [7,8], or both [9]. Due to their unsuitable band energy positions, these PAs and PCs need bias potentials to drive CO2 reduction under solar irradiation. The PAs, especially made with metal oxide have advantage due to stability, transparency and affordability in PEC-photovoltaic (PV) or PA–PC tandem device, where PV or PC utilizes the photons transmitted through the PA [10]. Moreover, the PAdriven PEC systems are versatile in fuel products due to the availability of various cathode electrocatalysts, which can control the efficiency and selectivity of CO2 reduction [11,12]. In order for a PA-driven PEC to be powered by an inexpensive single junction PV or a PC in tandem, its photocurrent onset potential should be further decreased. This could be achieved by loading an oxygen evolution catalyst (OEC) on PA, which not only decreases the onset potential, but also increases the photocurrent [13]. Monoclinic scheelite BiVO4 is one of the highest performing, visible light-active metal oxide photoanode materials with a direct band gap (Eg) of 2.4 eV [14–16]. High compatibility with modifications such as heterojunction [17–19], OEC [20], and doping [16] makes BiVO4 a subject for extensive current researches worldwide. A thin under-layer of WO3 in the BiVO4/WO3 heterojunction composites improves the performance of BiVO4 by better charge separation [17–19]. Further, BiVO4 decorated with cobalt phosphate (Co-Pi) [21,22] or FeOOH [15,20,23] as an OEC showed near complete surface charge separation resulting in enhanced photocurrents and the lowest photocurrent onset potentials (0.2 VRHE) among known visible light active metal oxide photocatalysts. However, the performance of these OECs depends sensitively on the electrolyte, i.e. Co-Pi works best in a phosphate (KPi) buffer and Co-Bi, Ni-Bi show the best performance in a borate (Bi) buffer. But for CO2 reduction, potassium bicarbonate (KCi) electrolyte gives the best CO2 reduction faradaic efficiency as well as high current density due to its ability to be in chemical equilibrium with CO2 [24]. Since the best known OECs, Co-Pi and FeOOH, do not function its best in the bicarbonate electrolyte, it is crucial to find an OEC that performs well in the CO2-saturated KCi electrolyte to establish an efficient PA–PEC system for CO2 conversion to fuels. Herein, we report for the first time a cobalt-carbonate (CoCi) OEC deposited BiVO4/WO3 (Co-Ci/BiVO4/WO3) composite as a photoanode in bicarbonate electrolyte to absorb solar light and drive the photoelectrochemical reduction of CO2 with a Cu cathode. The new Co-Ci/BiVO4/WO3 (photoanode)–Cu (cathode) system showed an excellent photocurrent stability and high faradaic efficiency (FE) for CO2 reduction with a lower bias potential than those of conventional photocathode-driven PEC systems. The system is presented in Scheme 1 together with operating principle. The main reactions taking place at photoanode and Cu cathode are as follows [6]:
Cu (cathode): CO2 + 8H + + 8e -CH4 + 2H2O (E0 = 0.169 VRHE)
(photoanode):
Overall reaction: H2O +CO2-O2 + fuels (H2, CO, CH4, C2H4…. ) The generation of Co-Ci as an electrochemical OEC in a neutral HCO3 /CO2 system was recently reported by Zhao et al. [25]. However, this is the first time that Co-Ci has been applied to a light absorber to demonstrate its outstanding performance for CO2 reduction.
Experiemental Preparation of BiVO4/WO3 The BiVO4/WO3 film was prepared by a polymer-assisted direct deposition (PADD) for WO3 and a metal organic decomposition method (MOD) for BiVO4 sequentially. A PADD solution was prepared with 1.85 g of ammonium metatungstate and 1 g of polyethylenimine in 10 ml of deionized water. Prepared 5 μl of PADD solution was spread on surface of F-doped SnO2 glass (FTO, TEC-15, Pilkington) and dried at 80 1C, annealed at 550 1C for 1 h. Annealing gave highly transparent film of WO3. BiVO4 film was prepared by a modified MOD method. Thus, 0.2 M Bi(NO3) 5H2O (99.8%; Kanto Chemicals) dissolved in acetic acid (99.7%; Kanto Chemicals) and 0.03 M, VO(acac)2 (98.0%; Sigma Aldrich) in acetylacetone (499.0%; Kanto Chemicals) were prepared as precursor solutions. For fabrication of a BiVO4 film on WO3 layer, 25 μl of stoichiometric precursor solution was dropped on WO3 coated FTO glass (2 cm 2.5 cm) and dried. The precursor film was calcined at 500 1C for 2 h to form a yellow BiVO4/WO3 film. Deposition procedure was repeated twice to achieve optimum performance of composite. The net irradiation area of photoanode was 0.6 cm2 for water oxidation that was connected by silver paste and copper wire and sealed with epoxy resin. For CO2 reduction, a larger FTO (2 cm 2 cm) was used.
Co-Ci deposition on BiVO4/WO3 photoanode The cobalt bicarbonate (Co-Ci) electrocatalyst was deposited with photo-assisted electrodeposition (PED) using illumination (100 mW/cm2) at 0.2 V (vs. SCE) in 0.3 mM Co (NO3)2 6H2O (Z 98%; Aldrich) and 100 ml of 0.1 M potassium bicarbonate (KCi, KHCO3) purged with CO2 to make pH of 7. Optimum total coulomb passed during deposition was about 7 mC/cm2 (deposition time of 10 min). The Co-Ci deposition on FTO glass (without semiconductor layers) was conducted with 1.1 V (vs. SCE) for 10 min with the same solution as the above PED and gave a brown colored film.
Measurements of PEC water oxidation Photoelectrochemical (PEC) measurements of bare and CoCi/BiVO4/WO3 photoanodes were performed with a standard three-electrode configurations; photoanode as the working electrode, Pt mesh rod as the counter electrode and SCE (saturated KCl) as the reference electrode. 0.1 M KCi
Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode
155
Scheme 1 (a) Illustration of a photoanode-driven photoelectrochemical system for solar fuel production by CO2 reduction. (b) Energy diagram and reactions involved on Co-Ci/BiVO4/WO3 photoanode for oxygen evolution and on Cu cathode for CO2 reduction.
(499.0%, Sigma Aldrich) with (pH 7)/without (pH 9) 30 min of CO2 purge or 0.1 M potassium phosphate (KPi) buffer with 30 min of nitrogen purge were used as electrolyte. All electrodes were illuminated from the front side. The scan rate for the current–voltage curve was 20 mV/s. To measure the degree of charge separation, 0.1 M Na2SO3 (498%, Sigma Aldrich) was added to pH 7 0.1 M KCi. Potentials were recorded with correction by the Nernst relation ERHE = ESCE + 0.0591 pH+ 0.241, in which ESCE is applied bias potential and 0.241 is a conversion factor from SCE electrode to the RHE scale. All electrochemical data were recorded by using a potentiostat (IviumStat, Ivium Technologies). A 300 W Xenon lamp was used to make simulated 1 sun light irradiation condition (AM 1.5 G, 100 mW/cm2) by using a solar simulator (Oriel 91160) with an AM 1.5G filter calibrated with a reference cell certified by the National Renewable Energy Laboratories, USA. Electrochemical impedance spectroscopy (EIS) was conducted with the same configuration of PEC measurements at 0.58 VSCE (1.23 VRHE) with an AC frequency range 10 1–104 Hz under one-sun irradiation. The spectra were analyzed by the Z-View program (Scribner Associates Inc.). The faradaic efficiencies (FE) of H2 and O2 evolution were determined from product analysis in a closed circulation system by a HP 6890 gas chromatography (GC) as follows:
FE= No. of moles formed/ theoretical no. of moles based on measured current.
PEC CO2 reduction The visible light irradiation was carried out using a 500 W Hg lamp (Newport) with a 420 nm cut-off filter (intensity of the radiation was 490 mW/cm2) and water for IR radiation removal. The photoelectrochemical reactor made of polycarbonate consists of two compartments separated by a Nafion-115 proton exchange membrane. The cathode compartment was connected to a gas circulation system with a peristaltic pump (Eyela, Japan) with a Tygon tubing and a six-port valve for on-line sampling to a gas chromatograph (HP 6890) with a Supelco Carboxen 1000 packed column and thermal conductivity detector (TCD). Presence of formic acid, methanol and ethanol in liquid products was analysed using a Hi-plex H column in an Agilent high performance liquid chromatograph. The temperature of the electrolyte was maintained at 10 1C by placing the reactor in a custom built water jacket containing mixture of 20% ethylene glycol in water in an automated chiller Lab Companion).
156 Before the start of the reaction, the electrolyte 0.5 M KHCO3 (pH 7.5) was pre-electrolyzed at 0.025 mA using Pt mesh for 180 min under CO2 purging to remove traces of impurity metal ions in the electrolyte which are known to poison the Cu cathode. The closed circulation system and the reactor were filled with CO2 which was continuously circulated through the electrolyte for the entire reaction period. The reactions were carried out at atmospheric pressure of CO2 and the bias potentials on the photoanode were applied using a Gamry Reference 600 potentiostat. The Gamry potentiostat simultaneously measured the potential between the working electrode and the counter electrode (EWE–ECE) while the reactions were going on in the 3-electrode system.
Results and discussion Fabrication and characterization of photoanodes The fabrication of PA started with the preparation of WO3 on F-doped SnO2 (FTO) by a polymer-assisted direct deposition
J.H. Kim et al. (PADD) method [26] over which BiVO4 was deposited by a modified metal organic decomposition (MOD) resulting in a BiVO4/WO3 film with a good transparency (Figure 1a). An OER electrocatalyst, Co-Ci or Co-Pi was then deposited on the BiVO4/WO3 surface by photo-assisted electrodeposition (PED). In addition, Co-Ci alone was also deposited on FTO by an electrochemical deposition (ED) as a reference. XRD peaks (Figure S1) corresponding to Co-Ci was not observed for Co-Ci/BiVO4/WO3 or Co-Ci/FTO, showing that Co-Ci is in an amorphous phase. UV-visible spectra in Figure S2 indicated that thin WO3 layer showed very small light absorption, whereas BiVO4/WO3 showed good absorption up to 500 nm and 66.6% of light harvesting efficiency (LHE = 1– 10 A, A= absorbance) as expected for BiVO4 with Eg of 2.4 eV. The Co-Ci deposition brought a slight red shift in absorption and much thicker deposition gave a brown colored film. SEM images of BiVO4/WO3 (Figure 1c) showed a porous BiVO4 film (thickness 300 nm, grain size 100 nm) deposited on a densely packed WO3 film (thickness 100 nm, grain of 20 nm, cross-section in Figure 1f). Deposition of Co-Ci resulted in a distinct and homogeneous thin layer of Co-Ci
Figure 1 (a) A photograph of FTO, WO3 and BiVO4/WO3 showing excellent transparency. SEM images of (b) WO3, (c) BiVO4/WO3, (d) thin, (e) thick Co-Ci/BiVO4/WO3 and (f) cross section of BiVO4/WO3 film. The Co-Ci deposition was conducted in 0.2 V (vs. SCE), 10 min (7 mC) for (d) and 20 min (15 mC) for (e).
Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode particles over the surface of BiVO4/WO3 (Figure 1d), and their amount increased with longer deposition time (Figure 1e). The near-surface region of photoanodes was analyzed by XPS. Binding energies of Bi (4f5/2 at 164.3 eV, 4f7/2 at 158.1 eV), and V (2p1/2 at 529 eV, 2p3/2 at 515.9 eV) correspond to standard BiVO4 (Figure S3). Small portion of W (4f5/2 at 37.5 eV, 4f7/2 at 35.0 eV) was detected, probably due to porous structure [19,27]. For Co-Ci/BiVO4/WO3 and Co-Ci/FTO, two peaks corresponding to Co (2p3/2 at 780.7 eV, 2p1/2 at 795.7 eV) indicated presence of both Co (III) and Co(II) as revealed by deconvolution fitting (Figure S4). It has been reported that the oxidation state of Co in Co-Pi varies from II to IV, in the form of amorphous CoOx and Co3O4 [13,28,29]. Carbon C 1s showed two peaks at 284.6 eV (assigned for C–C) and 288.0 eV (assigned to –C(O)O– and –C–OH) [25,30]. BiVO4/WO3 predominantly showed C–C peak from contaminated carbon with a small peak at 284.6.0 eV (Figure S5). However, increase in intensity of C 1s peak and stronger 288.0 eV peak upon Co-Ci deposition indicate the presence of a CO3 2 -like species (Figures 2b,c and S5) [25,30]. More obvious indication of the carbonate species in Co-Ci/BiVO4/ WO3 could be observed from the O 1s spectrum, which gave peaks at 529.3 eV assigned to M-O and at 531 eV assigned to –COx and surface –O–H groups. The BiVO4/WO3 shows predominant M–O peak at 529.3 eV with a very small peak at 531 eV (Figures S5b and 2c). The trend is reversed in Co-Ci/ BiVO4/WO3 (Figures S6b and 2d), which showed a small M–O
Figure 2
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peak and a large non-metal oxide peak at 531 eV [25,30]. Surface elemental composition by XPS in Table S1 showed increase in C, decrease in O, and a large decrease in Bi and V after Co-Ci deposition. All XPS results consistently showed that a film of Co-Ci was deposited on top of BiVO4/WO3 by PED and it had a similar composition as Co-Ci deposited on FTO by ED. Based on XPS results, expected formula is [Co(II/ III)Ox HCO3 ], similar to other complex metal–ligand OECs (Co-Pi, Co-Bi, Ni-Bi) [15,22,29,31–33].
Photoelectrochemical water oxidation Before application to CO2 conversion, the fabricated photoanodes were first tested for PEC water oxidation with Pt mesh rod as a hydrogen-evolving cathode to gauge only its photooxidation activity under simulated 1 sun irradiation (AM 1.5G, 100 mW/cm2). Detailed I–V curves are given in Figures 3a and S6–S8. First, BiVO4/WO3 in CO2-purged 0.1 M KHCO3 (KCi) and argon-purged 0.1 M KPi buffer both at pH 7 gave photocurrents of 2.6 mA/cm2 and 2.0 mA/cm2, respectively, at 1.23 VRHE. Without CO2 purge, KCi showed pH 9 and a lower photocurrent of 2.2 mA/cm2 (Figure S6). This significant difference by different electrolytes is known to be due to the catalytic effect of HCO-3 anion on water oxidation [34]. Thus, dissolved CO2 generates higher concentration of HCO3 ion which is involved in hole transfer to accelerate OER reaction by forming carbonate radicals
C 1s and O 1s XPS spectra of (a,c) BiVO4/WO3 and (b,d) Co-Ci/BiVO4/WO3 photoanode surface.
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Figure 3 (a) Photocurrent data of Co-Ci/ BiVO4/WO3 and BiVO4/WO3 in various electrolytes (under 1 sun illumination, pH 7, (CO2 purged) 0.1 M KHCO3), (b) ηsurf (surface charge separation efficiency) of Co-Ci/BiVO4/WO3 and BiVO4/WO3, (c) IPCE of Co-Ci/BiVO4/ WO3 and BiVO4/WO3 and (d) photocurrent stability of Co-Ci/BiVO4/WO3 (AM 1.5G), and O2/H2 gas evolution (490 mW/cm2, 4420 nm) at pH 7, (CO2 purged) 0.1 M KHCO3.
and peroxocarbonates that provide a beneficial catalytic pathway for O2 evolution. The deposition of Co-Ci on BiVO4/WO3 led to a shift in the photocurrent onset potential of 300 mV from 0.50 VRHE for BiVO4/WO3 to 0.20 VRHE for Co-Ci/BiVO4/WO3. The best performing Co-Ci/BiVO4/WO3 was obtained by PED at 0.2 VSCE for 10 min, which gives an optimum thickness of Co-Ci as evident from the SEM images in Figure 1d. The Co-Ci/BiVO4/ WO3 electrode also showed higher photocurrent in CO2-saturated KCi of pH 7 (3.5 mA/cm2 at 1.23 VRHE) than at pH 9 without CO2 purge (3.3 mA/cm2 at 1.23 VRHE), indicating that higher concentration of HCO3 is favorable for Co-Ci (Figure S7). The performance of Co-Ci/BiVO4/WO3 in KCi was comparable with Co-Pi/BiVO4/WO3 in KPi electrolyte in photocurrent generation (3.5 mA/cm2 at 1.23 VRHE) as well as the cathodic shift of the current onset potential ( 300 mV). This performance is also comparable to that of recently-reported bestperforming BiVO4-based photoanodes using Co-Pi [10,22,35] and FeOOH/NiOOH [20] as OEC when they work in their own optimized electrolytes. A quantitative analysis on the role of OEC could be made through photo-oxidation of sulfite as a hole scavenger, which quantifies the extent of recombination on the surface and in the bulk independently [20,22,23]. The photo-oxidation of sulfite is so facile that it eliminates the hole injection barrier to the electrolyte and charge recombination on the surface. The surface charge separation efficiency (ηsurf) could be calculated by dividing the photocurrent from water oxidation by the photocurrent from sulfite oxidation. The ηsurf of Co-Ci/ BiVO4/WO3 was 85% on average (Figure 3b), which is only
slightly lower than that of Co-Pi/BiVO4/WO3 in KPi electrolyte ( 90%). Bulk charge separation efficiency (ηbulk) was calculated by dividing the photocurrent measured with sulfite in electrolyte by the total absorbed photocurrent density, and it was as high as 72% near 1.23 VRHE (Figure S10). This ηbulk is comparable with recent state-of-the-art BiVO4-based photoanodes [10,19,21,36]. Impedance measurements showed that the resistance at the electrolyte|| semiconductor interface decreased by half upon Co-Ci loading (893 Ω) on to BiVO4/ WO3 (1742 Ω) (Figure S11). Thus, deposition of Co-Ci led to facile charge transfer at electrolyte|| semiconductor interface that usually suffers the highest impedance owing to sluggish water oxidation kinetics. In addition, Co-Ci showed comparable external quantum efficiency (EQE, 1.02%) to that of Co-Pi (1.16%) (Figure S12). The incident photon to current conversion efficiency (IPCE) of the Co-Ci/BiVO4/WO3 electrode was measured. As in Figure 3c, IPCE of Co-Ci/BiVO4/WO3 recorded 60% up to 450 nm, which is comparable to the highest values reported for PA derived from BiVO4 [18–20]. Threshold of IPCE was 510 nm, indicating that Co-Ci does not extend the range of light adsorption but only acts as OEC. Stability tests for Co-Ci/BiVO4/WO3 demonstrated an exceptionally stable photocurrent generation for 3 h in KCi (Figure 3d), whereas in KPi, the photocurrent decayed significantly within 1 h (Figure S13). This indicates that HCO3 is necessary for Co-Ci to be active and stable in solar water oxidation. This is very similar to the role of KPi buffer electrolyte for Co-Pi [22,28,31,32]. The measurement of the evolved gases by gas chromatography showed a
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Figure 4 (a) Photocurrent data of Co-Ci/BiVO4/WO3 with thin (7 mC) and thick (15 mC) deposition (right inset shows morphology of each film). (b) Scheme of hole (blue) and electron (red) transfer for thin and thick Co-Ci deposition.
faradaic efficiency (FE) of 100% for O2 and H2, suggesting that observed photocurrents from Co-Ci/BiVO4/WO3 were fully utilized for the water splitting reactions (Figure 3d). The OEC promotes water oxidation kinetics and reduces its overpotential. But the amount of OEC must be optimized [37] because it adds length of hole diffusion as shown in Figure 4. Porosity of photoelectrode is crucial for charge separation as depletion layer is formed by interaction of electrolyte||semiconductor. Thus through the optimized thin Co-Ci film prepared with 7 mC, holes can transfer without much recombination (ηsurf = 90%), whereas excessively thick Co-Ci deposits (15 mC) forms longer pathway for holes, partially blocks the pores, and shade the light absorber, thus resulting in increased recombination. In addition, higher amounts of Co-Ci resulted in large transient peaks in I–V curves. Identical behavior has been reported for Co-Pi on Fe2O3 and BiVO4, which shows the importance of the proper thickness [33].
Photoelectrochemical CO2 reduction Establishing that Co-Ci/BiVO4/WO3 is as good as the best known water oxidation system in KCi saturated with CO2, we applied it for PEC conversion of CO2 to fuels with the common Cu cathode. Since the study of Co-Pi has been limited to a KPi electrolyte, it is also interesting to investigate its behavior in CO2 saturated KCi electrolyte. As mentioned, it is essential to use the bicarbonate electrolyte for CO2 reduction in general due to its ability for facile supply of CO2 to cathode according to the equilibrium (HCO3 2CO2 + OH ), where OH will be converted back to HCO3 by the dissolved CO2 [24]. In other
electrolytes, the supply of CO2 is limited since diffusion is the only pathway. In addition, the large K + ions adsorb preferentially on the cathode surface, deter the adsorption of H + , and stabilize CO2∙ considered as an intermediate of CO2 reduction [8,38]. Thus, KHCO3 electrolyte favors CO2 reduction over competing water reduction. Hence, Co-Ci/ BiVO4/WO3 was tested as a photoanode in the PA-driven PEC system for CO2 reduction under visible light (4420 nm, 490 mW/cm2) irradiation in CO2-purged 0.5 M KHCO3 electrolyte (pH 7.5). The reactions were carried out in a reactor where the cathode and anode compartments are separated by a proton exchange membrane. The cathode compartment was connected to a closed circulation gas line with provisions for in-situ product analysis using a gas chromatograph. The reactions were carried out in 3-electrode system by applying bias potentials to the photoanode ranging from + 0.30 to + 0.40 VRHE. The time profiles of products are given in Figures 5b and S14. Table 1 and Figure 5c shows FE of the products obtained at different potentials. The loading of Co-Ci shifts the current onset potential of BiVO4/WO3 from + 0.50 V to about + 0.20 V (Figure 3a). Due to this shift, potentials up to + 0.30 V, where BiVO4/WO3 does not produce sufficient photocurrent could now be used for Co-Ci/BiVO4/WO3. In the PA-driven PEC system with CoCi/BiVO4/WO3 as PA and Cu as the cathode, the products were predominantly CH4 with CO and H2 as additional products, and as the potential was increased FE of CH4 increased whereas those of H2 and CO decreased. At a potential of + 0.4 V, FE of CH4 was 46.8%, and a larger C2H4 molecule was also observed. The observed trend shows that products requiring multiple electron reductions are favored at higher potentials whereas 2-electron reduction products
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Figure 5 (a) Chronoamperometry for the PA- Cu cathode PEC CO2 reduction. (b) Amounts of CH4 produced. (c) Faradaic efficiency at 60 min. (d) Faradaic efficiency x charge at 60 min. With potential of 0.4 VRHE and CO2 purging, 0.5 M KCi was used for BiVO4/WO3– Cu, Co-Ci/BiVO4/WO3–Cu and 0.5 M KPi was used Co-Pi/BiVO4/WO3–Cu. Table 1 Faradaic efficiencies (FE) at 60 min during the PA-driven PEC CO2 reduction under visible light (4420 nm) irradiation. PA-cathode for PEC system
BiVO4/WO3–Cu BiVO4/WO3–Cu Co-Ci/BiVO4/WO3–Cu Co-Ci/BiVO4/WO3–Cu Co-Ci/BiVO4/WO3–Cu Co-Pi/BiVO4/WO3–Cu
Potential of photoanode (vs. RHE)
+0.40 +0.35 +0.40 +0.35 +0.30 +0.40
are favored at lower potentials. Co-Ci/BiVO4/WO3 in KCi (pH 7) showed very good photocurrent stability during the PA-driven PEC CO2 reduction (Figure 5a and S15a). Two types of control reactions were carried out using similar applied bias potentials either without irradiation in a CO2 purged reactor or in an Ar purged reactor under irradiation. Both blank reactions did not yield any CO2 reduction products. This confirms that the observed products were only from the reduction of the supplied CO2, and their formation was not possible with only bias potential in the absence of photoelectrons generated at the photoanode by harvesting visible light. The analysis of liquid phase showed the absence of CH3OH or HCOOH. In comparison, Co-Pi/BiVO4/WO3 PA-driven PEC at 0.40 VRHE with Cu cathode in CO2-saturated 0.1 M KPi electrolyte (pH 6.5) gave CO2 reduction FE of 22.4%, which is less than half of the
Charge (Coulomb)
1.08 0.91 3.34 1.75 0.57 1.33
Faradaic efficiency (%) C2H4
CH4
CO
H2
Total
17.7 0 3.7 0 0 0
11.8 12.1 46.8 42.7 13.1 20.4
4.9 6.8 1.4 3.0 12.2 2.0
64.6 72.9 48.1 49.4 64.9 77.3
99.1 91.8 100 95.1 90.2 99.7
51.9% FE obtained at similar potentials from Co-Ci/BiVO4/WO3 in KHCO3 (Table 1 and Figure 5a). This is primarily due to the decreased availability of CO2 in KPi buffer electrolyte. Previous studies showed that with a common cation, the anions in the electrolyte favored CO2 reduction in the order; HCO3 2 4CO3 2 4SO4 2 4PO4.[39] Moreover, the photocurrent and hence the total charge generated from Co-Ci/BiVO4/ WO3 PA-driven PEC is 2.5 times higher than Co-Pi/BiVO4/WO3 PA-driven PEC at 0.4 VRHE (Table 1). When the charge generated and the CO2 reduction faradaic efficiency are taken into account (Faradaic efficiency x charge), Co-Ci/BiVO4/WO3 PAdriven PEC gave 6 fold higher amount of CO2 reduction products and 1.5 times higher H2 than the Co-Pi/BiVO4/WO3 PA-driven PEC (Figures 5d and S14). In addition, Co-Pi/BiVO4/WO3 was unstable and lost most of its activity within 2 h in a CO2 saturated KPi electrolyte (Figure 5a). Due to its rapid loss in
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Table 2 Two-electrode potentials of the PA- or PC-driven PEC CO2 reduction systems measured concurrently during reactions in a 3-electrode system. Working electrode
Counter electrode
Potential of WE vs. RE (VRHE)
∣EWE ECE∣ (V)
Co-Ci/BiVO4/WO3 Co-Ci/BiVO4/WO3 Co-Ci/BiVO4/WO3 CuOx (cathode)
Cu Cu Cu Pt (anode)
0.40 0.35 0.30 0.07
1.5 1.5 1.3 1.8
photocurrent, Co-Pi/BiVO4/WO3 PA-driven PEC stopped producing any CH4 after 30 min and hence the Co-Ci/BiVO4/WO3 PAdriven PEC produced 11 times higher CH4 at the end of 120 min (Figure 5b). As discussed, Co-Pi OEC in KPi buffer electrolyte showed slightly better water oxidation activity than Co-Ci in CO2-purged KCi electrolyte with hydrogen generation at Pt cathode. Yet, in CO2 reduction in CO2-purged KCi electrolyte, Co-Ci showed higher activity, much better stability and superior selectivity to CO2 reduction relative to H2O reduction to H2. Hence, we can conclude that Co-Ci is a tailored OEC for CO2 reduction in PA-driven PEC solar fuel generators. Photoelectrochemical reduction of CO2 to fuels has been conventionally carried out using photocathode (PC)-driven PEC systems in a 3electrode configuration and a bias potential supplied to the photocathode are reported versus the reference electrode (RE) [7]. However, the reported potentials of the working electrode vs. RE in the 3-electrode systems for the PC-driven PEC system cannot be directly compared with those for PAdriven PEC system. Hence, we measured the potential between the working and the counter electrode (∣EWE ECE∣) during CO2 reduction in a 3-electrode system. Since such data are not available for the PC-driven PEC system, we took the same measurement for a reported PC-driven PEC system which used the lowest bias potential in a 3-electrode system [39]. The ∣EWE–ECE∣ offers a way by which PA- and PCdriven systems can be compared on the same ground and represents required external energy supply for both systems. The results show that Co-Ci/BiVO4/WO3 PA-driven PEC at + 0.30 VRHE with Cu as cathode has a ∣EWE–ECE∣ of 1.3 V (Table 2 and Figure S15b). The reference CuOx (Figures S15, S16) PC-driven PEC (with Pt anode) at + 0.07 VRHE, which is the lowest potential reported for a PC-driven PEC CO2 reduction without a co-catalyst has a ∣EWE ECE∣ of 1.8 V (Figure S15c) [39]. These ∣EWE ECE∣ data show that the PAdriven PEC systems are better choice for the reduction of CO2 requiring less external energy input. Another advantage of Co-Ci/BiVO4/WO3 is an excellent transparency (Figure 1a), which allows fabrication of efficient tandem configuration with a multi-junction solar cell or PC to realize a no-bias PEC device.
Conclusion In summary, we developed Co-Ci/BiVO4/WO3 photoanode with outstanding performance in neutral bicarbonate electrolyte – photocurrent of 3.5 mA/cm2 at 1.23 VRHE with 0.2 VRHE onset potential – which is similar to the
performance of Co-Pi/BiVO4/WO3 in phosphate electrolyte. Co-Ci/BiVO4/WO3 showed excellent stability and 100% FE for water splitting. The Co-Ci/BiVO4/WO3 can effectively drive PEC CO2 reduction using a standard Cu cathode in CO2saturated KHCO3 electrolyte producing CO and C1–C2 hydrocarbon products with FE of 51.9% with a stable photocurrent. In comparison, Co-Pi/BiVO4/WO3 driven system in CO2 saturated KPi gave 22.4% FE for CO2 reduction and poor stability. The same Co-Pi system in CO2-saturated KHCO3 electrolyte performed even worse and was extremely unstable. Due to its higher photocurrent and excellent stability, the Co-Ci system produced 11 times more CH4 than the Co-Pi system at similar bias potentials. In addition, Co-Ci/BiVO4/WO3 required less external energy supply than the best known photocathode (CuOx)-driven system. The highly transparent Co-Ci/BiVO4/WO3 could also be utilized to fabricate a no-bias solar fuel generation cell with a PV cell or a photocathode in tandem configuration due to its high transmittance of longer wavelength photons.
Funding sources This work was supported by Brain Korea Plus Program of Ministry of Education, Korean Center for Artificial Photosynthesis (NRF-2011-C1AAA0001-2011-0030278) funded by MISIP and Project no. 10050509 funded by MOTIE (Ministry of Trade, Industry & Energy) of Republic of Korea. It was also supported by Ulsan National Institute of Science and Technology (UNIST).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.04.022.
References [1] N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. 103 (2006) 15729–15735. [2] S. Bensaid, G. Centi, E. Garrone, S. Perathoner, G. Saracco, ChemSusChem 5 (2012) 500–521. [3] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chem. Rev. 110 (2010) 6446–6473. [4] S. Ichikawa, R. Doi, Catal. Today 27 (1996) 271–277. [5] C. Ampelli, G. Centi, R. Passalacqua, S. Perathoner, Energy Environ. Sci. 3 (2010) 292–301.
162 [6] G. Magesh, E.S. Kim, H.J. Kang, M. Banu, J.Y. Kim, J.H. Kim, J.S. Lee, J. Mater. Chem. A 2 (2014) 2044–2049. [7] B. Kumar, M. Llorente, J. Froehlich, T. Dang, A. Sathrum, C.P. Kubiak, Annu. Rev. Phys. Chem. 63 (2012) 541–569. [8] J.-W. Jang, S. Cho, G. Magesh, Y.J. Jang, J.Y. Kim, W.Y. Kim, J.K. Seo, S. Kim, K.-H. Lee, J.S. Lee, Angew. Chem. Int. 53 (2014) 5852–5857. [9] S. Sato, T. Arai, T. Morikawa, K. Uemura, T.M. Suzuki, H. Tanaka, T. Kajino, J. Am. Chem. Soc. 133 (2011) 15240–15243. [10] F.F. Abdi, L. Han, A.H.M. Smets, M. Zeman, B. Dam, R. van de Krol, Nat. Commun. 4 (2013). [11] J. Qiao, Y. Liu, F. Hong, J. Zhang, Chem. Soc. Rev. 43 (2014) 631–675. [12] Q. Lu, J. Rosen, Y. Zhou, G.S. Hutchings, Y.C. Kimmel, J.G. Chen, F. Jiao, Nat. Commun. 5 (2014). [13] M.W. Kanan, Y. Surendranath, D.G. Nocera, Chem. Soc. Rev. 38 (2009) 109–114. [14] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278. [15] Y. Park, K.J. McDonald, K.-S. Choi, Chem. Soc. Rev. 42 (2013) 2321–2337. [16] K.P.S. Parmar, H.J. Kang, A. Bist, P. Dua, J.S. Jang, J.S. Lee, ChemSusChem 5 (2012) 1926–1934. [17] S.J. Hong, S. Lee, J.S. Jang, J.S. Lee, Energy Environ. Sci. 4 (2011) 1781–1787. [18] Y. Pihosh, I. Turkevych, K. Mawatari, T. Asai, T. Hisatomi, J. Uemura, M. Tosa, K. Shimamura, J. Kubota, K. Domen, T. Kitamori, Small 10 (2014) 3692–3699. [19] P.M. Rao, L. Cai, C. Liu, I.S. Cho, C.H. Lee, J.M. Weisse, P. Yang, X. Zheng, Nano Lett. 14 (2014) 1099–1105. [20] T.W. Kim, K.-S. Choi, Science 343 (2014) 990–994. [21] F.F. Abdi, N. Firet, R. van de Krol, ChemCatChem 5 (2013) 490–496. [22] D.K. Zhong, S. Choi, D.R. Gamelin, J. Am. Chem. Soc. 133 (2011) 18370–18377. [23] J.A. Seabold, K.-S. Choi, J. Am. Chem. Soc. 134 (2012) 2186–2192. [24] J. Wu, F.G. Risalvato, F.-S. Ke, P.J. Pellechia, X.-D. Zhou, J. Electrochem. Soc. 159 (2012) F353–F359. [25] Q. Zhao, Z. Yu, W. Yuan, J. Li, J. Int, Hydrog. Energy 38 (2013) 5251–5258. [26] S.J. Hong, H. Jun, J.S. Lee, Scr. Mater. 63 (2010) 757–760. [27] S.K. Cho, H.S. Park, H.C. Lee, K.M. Nam, A.J. Bard, J. Phys. Chem. C 117 (2013) 23048–23056. [28] T.H. Jeon, W. Choi, H. Park, Phys. Chem. Chem. Phys. 13 (2011) 21392–21401. [29] M.W. Kanan, D.G. Nocera, Science 321 (2008) 1072–1075. [30] K.S. Joya, K. Takanabe, H.J.M. de Groot, Adv. Energy Mater. 16 (2014) 1400252. [31] S.K. Choi, W. Choi, H. Park, Phys. Chem. Chem. Phys. 15 (2013) 6499–6507. [32] C. Ding, J. Shi, D. Wang, Z. Wang, N. Wang, G. Liu, F. Xiong, C. Li, Phys. Chem. Chem. Phys. 15 (2013) 4589–4595. [33] D.K. Zhong, D.R. Gamelin, J. Am. Chem. Soc. 132 (2010) 4202–4207. [34] K. Sayama, N. Wang, Y. Miseki, H. Kusama, N. OnozawaKomatsuzaki, H. Sugihara, Chem. Lett. 39 (2010) 17–19. [35] J.H. Kim, J.W. Jang, H.J. Kang, G. Magesh, J.Y. Kim, J.H. Kim, J. Lee, J.S. Lee, J. Catal. 317 (2014) 126–134. [36] S.K. Pilli, R. Janarthanan, T.G. Deutsch, T.E. Furtak, L.D. Brown, J.A. Turner, A.M. Herring, Phys. Chem. Chem. Phys. 15 (2013) 14723–14728. [37] G.M. Carroll, D.K. Zhong, D.R. Gamelin, Energy Environ. Sci. 8 (2015) 577–584.
J.H. Kim et al. [38] M. Jitaru, D.A. Lowy, M. Toma, B.C. Toma, L. Oniciu, J. Appl. Electrochem. 27 (1997) 875–889. [39] G. Ghadimkhani, N.R. de Tacconi, W. Chanmanee, C. Janaky, K. Rajeshwar, Chem. Commun. 49 (2013) 1297–1299. Jin Hyun Kim received his undergraduate degree at The University of Seoul (South Korea) in 2012. He joined Jae Sung Lee's group in Pohang University of Science and Technology as Ph.D. student. He is working for development of photocatalytic materials and systems for photoelectrochemical solar energy conversion.
Ganesan Magesh is currently working as a postdoctoral researcher at the Ulsan National Institute of Science and Technology, South Korea. He completed Ph.D. in Chemistry from Indian Institute of Technology Madras (2010), and M.Sc. in Chemistry from the Anna University, India (2002). His current research interests are semiconductor photoelectrochemical energy conversion, water splitting and carbon dioxide reduction.
Hyun Joon Kang is currently an Integrated M.S./Ph.D. course student in the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH). He received his B.S. in the same department and university in 2009. His study is mainly focused on the doping effects on BiVO4 for photocatalytic water splitting.
Marimuthu Banu is currently working as a postdoctoral researcher at the Ulsan National Institute of Science and Technology, South Korea. She completed Ph.D. in Chemistry (2011) and M.Sc. in Chemistry (2006) from Bharathidasan University, India. Her current research interests are heterogeneous catalysis, materials science and theoretical chemistry.
Ju Hun Kim received his B.S. (2007) and M. S. (2010) degree in the Dept. of Mechanical and Control System Engineering, Dept. of Computer Science and Electronic Engineering at Handong Global University (HGU), Korea. He is currently Ph.D. course student at POSTECH in Chemical Engineering and works with Prof. Jae Sung Lee at UNIST. His current research involves synthesis and development of nanostructured metal oxide photoelectrodes for photoelectrochemical water splitting and artificial photosynthesis.
Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode
Prof. Jinwoo Lee obtained his B.S. (1998) and Ph.D. (2003) from the Department of Chemical and Biological Engineering of Seoul National University, Korea. After his postdoctoral research at Seoul National University (with Prof. Taeghwan Hyeon, 2003–2005) and Cornell University (with Prof. Ulrich Wiesner, 2005–2008), he joined the faculty of the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH) in June, 2008. His research field includes synthesis and applications of ordered functional mesoporous materials and shape controlled nanocrystals in energy conversion and storage devices.
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Professor Jae Sung Lee received his BS degree from Seoul National University (1975), M.S. degree from Korea Advanced Institute of Science and Technology (1977), PhD degree at Stanford University (1984). After postdoctoral carrier at Catalytica, he returned to Korea in 1986 to become a professor of chemical engineering at Pohang University of Science and Technology. He and his group recently moved to Ulsan National Institute of Science and Technology in 2013. He has been leading a laboratory of eco-friendly catalysis and energy, and working on photocatalytic water splitting, fuel cell electrocatalysis, and heterogeneous chemical catalysis.