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Photoelectrochemical carbon dioxide reduction using a nanoporous Ag cathode Yan Zhang, Wesley W Luc, Gregory S Hutchings, and Feng Jiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09095 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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ACS Applied Materials & Interfaces

Photoelectrochemical carbon dioxide reduction using a nanoporous Ag cathode Yan Zhang, Wesley Luc, Gregory S. Hutchings, Feng Jiao* Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, USA. *To whom correspondence should be addressed: [email protected] Abstract Solar fuel production from abundant sources using photoelectrochemical (PEC) systems is an attractive approach to address the challenges associated with the intermittence of solar energy. In comparison to electrochemical systems, PEC cells directly utilize solar energy as the energy input, and if necessary, an additional external bias can be applied to drive the desired reaction. In this work, a PEC cell composing of a Ni-coated Si photoanode and a nanoporous Ag cathode was developed for CO2 conversion to CO. The thin Ni layer not only protected the Si wafer from photo-corrosion, but also served as the oxygen evolution catalyst. At an external bias of 2.0 V, the PEC cell delivered a current density of 10 mA cm-2 with a CO Faradaic efficiency of ~70%. More importantly, a stable performance up to 3 hours was achieved under photoelectrolysis conditions, which is among the best literature reported performances for PEC CO2 reduction cells. The photovoltage of the PEC cell was estimated to be ~0.4V, which corresponded to a 17% energy saving by solar energy utilization. Post-reaction structural analysis showed the corrosion of the Ni layer at the Si photoanode/catalyst interface, which caused the performance degradation under prolonged operations. A stable oxygen evolution catalyst with a robust interface is crucial to the long-term stability of PEC CO2 reduction cells. Keywords Photoelectrochemical; Carbon dioxide reduction; Electrocatalysis; Solar fuel; Nanoporous

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Introduction Solar energy has been viewed as one of the most promising renewable energy sources because solar energy is abundant, widely spread, and green. Because of these benefits, utilizing solar energy as the predominant energy source to produce fuels and chemical feedstocks, such as H2O and CO2, has attracted significant interests.1,2 Moreover, CO2 is considered as an ideal starting feedstock for solar fuel production since CO2-derived fuel can potentially create a carbon-neutral energy cycle.3-5 In order to realize this energy cycle, a photoelectrochemical (PEC) CO2 reduction system is highly desired because the system directly couples solar energy capture with chemical conversion.6-14 In a typical PEC cell, the photon energy is captured by either a photoanode or a photocathode, while the fuel production occurs at the cathode. Recent efforts have been devoted towards the development of PEC cells for CO2 reduction15-19 that utilizes photocathodes such as p-InP, p-ZnTe, Mg-doped CuFeO2, and CuInS2. . 6,8,12,14 However, the optimization of these photocathodes has proven to be considerably difficult due to the complex nature of the catalytic CO2 reduction reaction as well as the band gap alignment that is needed for a single semiconductor material. Therefore, most of current available photocathodes suffer from a number of issues including high overpotentials, low current densities, and poor selectivity.6,8,12,14 An alternative way to construct a PEC cell for CO2 electrolysis is to couple a photoanode with a CO2 reduction cathode. By doing so, the optimization of light capture and charge separation processes is decoupled from the CO2 reduction chemistry. Recently, this approach has been successfully demonstrated in several reports.7,9,10 For example, Magesh et al. reported a PEC cell for CO2 reduction with a high Faradaic efficiency at low external bias potentials by incorporating WO3 as the photoanode and Cu, Sn/SnOx as the CO2 reduction cathode.10 In the 2-electrode experiment, a current density of 1.9 mA cm-2 was achieved at ~1.9 V. Despite these initial results, further improvement of current density could be achieved by choosing a more suitable cathode electrode for CO2 electro-reduction. Lu et al. recently developed a nanoporous Ag catalyst that is able to reduce CO2 electrochemically with a ~92% CO Faradaic efficiency at moderate overpotentials.20 The superior catalytic properties of nanoporous Ag catalyst make it an attractive cathode candidate for PEC cells. Here, a PEC cell for CO2 conversion to CO was constructed using a nanoporous Ag as the cathode and a Ni-coated Si as the photoanode. Si has been commonly used as a photoanode for water splitting since Si is abundant and is capable of absorbing a large portion of the solar spectrum; and therefore, Si was chosen as the photon adsorber.21-23 One important obstacle for developing such a system rises from the compatibility of the photoanode/OER catalyst with the near-neutral environment required by the CO2 reduction reaction. Recently, stable performance of Si-based photoanode at basic and even near-neutral conditions was achieved by passivating the Si wafer with a metal oxide layer.21,22 When coupled with NiOx oxygen evolution reaction (OER) catalyst, the silicon photoanode was proven to be an efficient photoanode for water splitting at slightly basic (pH = 9.5) conditions.22 This is especially interesting since the pH value of the testing solution is very close to that of CO2 electrolysis conditions (pH ~7). In this work, the nanoporous Ag/Ni-coated Si PEC system was able to deliver a current density of 10 mA cm-2 with a Faradaic efficiency of 70% for CO2 to CO conversion at an external bias of 2.0 V. Remarkably, the system exhibited a good stability up to 3 hours under aggressive test conditions. Experimental Section 2 ACS Paragon Plus Environment

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Synthesis of nanoporous Ag cathode The nanoporous Ag was synthesized using a reported procedure by Lu, et al.20 Briefly, Al-Ag alloy ingot (Al:Ag = 80:20 atom%) was fabricated using arc-melting method. The ingot was cut into thin slices (~300 um). The slices were then annealed at 819 K for 12 hours, followed by quenching in an ice/water bath. The quenched alloy slices were then treated with diluted hydrochloric acid to leach out the Al in the alloy. The resulting nanoporous Ag was rinsed with DI water and dried in a vacuum oven. Synthesis of Ni-coated Si photoanode Double-side polished 400 um-thick phosphorus-doped n-Si(110) wafers were obtained from University Wafer. The as-received wafers were cleaned with acetone, DI water, ethanol, and DI water in sequence before use. The Ni layer was deposited using a magnetron sputtering chamber at room temperature. The deposition rate was calibrated based on X-ray reflection measurement and controlled by the applied power to the gun. The front side of the Si wafer was coated with a 10 nm Ni layer, while the other side was coated with a 20 nm Ti layer as the back contactor. Epoxy (Corning 8490) was used to cover the backside and edges of the electrode to prevent contact with the electrolyte. After the epoxy was cured at 348 K for 2 hours, the photoanodes were stored in an Ar-filled glovebox before anodization and testing. Synthesis of non-photon-responsive anode For the synthesis of a non-photon-responsive anode, the same double-side polished 400 um-thick phosphorus-doped n-Si(110) wafers were used. After surface cleaning, the front side of the Si wafer was first coated with a 50 nm Ti layer followed by a 10 nm Ni layer. In this case, the Si wafer only served as a substrate rather than a light absorber. Material Characterizations X-ray diffraction (XRD) measurements were conducted on a Bruker D8 X-ray diffractometer using Cu Kα radiation. Scanning electron microscopy (SEM) images were collected using a Zeiss Auriga-60. Scanning transmission electron microscopy (STEM) imaging was performed on a JEOL JEM-2010F equipped with a high angle annular dark field (HAADF) detector and operated at 200 kV. For the thickness of Ni layer measurement, the sample was coated with a thin layer of Ga and cut using focused ion beam (FIB). X-ray absorption spectroscopy (XAS) data were collected on beamline 5-BM-D of the Advanced Photon Source at Argonne National Laboratory. The samples were placed front-side up horizontally to allow the beam scanning though the thin Ni layer. Fluorescence data were analyzed using the Demeter software package. Energy measurements were calibrated to the known K-edge position of a pure Ni foil. Photoelectrochemical tests The as-prepared Ni-coated Si photoanode was activated through an anodization process right before photoelectrochemical tests. A customized single-compartment cell (Figure S1a) with a boric glass body and a quartz window for light radiation was used for the photoanode anodization process. A standard Ag/AgCl electrode was used as the reference electrode and a Pt wire was used as the counter electrode. The potential at the photoanode was hold at 1.54 V vs. reversible hydrogen electrode (RHE) for 2 hours. The current density at the photoanode quickly increased from 1 mA cm-2 to 6 mA cm-2 during the first hour and gradually reached a plateau in 3 ACS Paragon Plus Environment


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the second hour. This anodization process converted the as-deposited Ni layer to an active oxygen evolution catalyst layer for the photoelectrochemical tests. A two-compartment cell with a polycarbonate body and a quartz window (Figure S1b) was used for the photoelectrochemical tests. A standard Ag/AgCl electrode was used as the reference electrode and a cation-conducting membrane (Nafion N115) was used to separate the two chambers. The catholyte was a 0.5 M KHCO3 aqueous solution saturated with CO2 gas and the anolyte was prepared by adding 6.2 g of boric acid, 6.4 g of potassium chloride and 0.8 g of potassium hydroxide to 200 mL DI water. A 300 W Xe lamp equipped with a 400 nm filter was used as the light source. A potentiostat (Princeton Applied Research VersaSTAT 3) test station was used to perform the electrochemical experiments. To quantify the product, 30 µL of gas sample was taken using a gas-tight syringe every 30 min during test course and analyzed using a gas chromatograph (GC, Shimadzu, GC-2014). Results and Discussions Fabrication of photoelectrochemical cell

Figure 1. (a) PXRD patterns for Al80Ag20 alloy, as-made nanoporous Ag, and post-reaction nanoporous Ag. (b) HRTEM and (c) SEM images of the as-made nanoporous Ag. 4 ACS Paragon Plus Environment

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The synthesis procedure of nanoporous Ag cathode followed a previous report by Lu et al.20 Before dealloying, the powder X-ray diffraction (PXRD) analysis confirmed that the Al80Ag20 alloy had a pure face-centered cubic structure (Figure 1a). After treating in hydrochloric acid, the alloy lost its Al content while the cubic structure was preserved (Figure 1a). In the PXRD patterns, a slight shift of diffraction peaks to low angles was observed for the as-made nanoporous Ag, indicating the expansion of the unit cell due to the loss of Al atoms (smaller radius compared to Ag atoms) in the structure. The crystal structure of the as-made nanoporous Ag was further confirmed by high-resolution transmission electron microscopy (HRTEM) analysis. A typical image of the sample is shown in Figure 1b and lattice fringes are evident in the image. The distance between two neighbor fringes was 0.329 nm, which corresponded to the d-space of (111) planes of standard face-centered cubic Ag structure. The morphology of the asmade Ag catalyst was examined using scanning electron microscopy (SEM). The SEM image of the resulting Ag electrode (Figure 1c) clearly shows a three-dimensionally nanoporous morphology with an average pore size of ~50 nm, which is in good agreement with our previous reports.

Figure 2. (a) A typical cross-section STEM image taken at the interface of Ni layer and Si wafer. The Ga is deposited to protect the Ni layer before the FIB cutting. (b) Anodic current density profile during the course of anodization of a Ni-coated Si photoanode. Turning to the fabrication of Ni-coated Si photoanode, a commercial phosphorus-doped n-Si(110) wafer was coated with a 10-nm Ni layer on the front side as the OER catalyst and a 20-nm Ti layer on the back as the current collector using magnetron sputtering technique. The SEM image of as-deposited Ni thin film shows that a uniform and continuous Ni layer was formed on the Si wafer (Figure S2a). To check the thickness of the as-made Ni layer, the sample was cut using focused ion beam (FIB) and analyzed by scanning transmission electron microscopy (STEM). A typical STEM image that was taken at the cross-section of the sample (Figure 2a) shows that the as-made Ni film was a continuous layer with a thickness about 12 nm, and this Ni film was later transformed into an active OER catalyst layer using the anodization process as described in the Experimental Section. A stable anodic current ~6.5 mA cm-2 was achieved (Figure 2b), suggesting that the photoanode reached a steady state. The SEM image for post-conditioned photoanode (Figure S2b) shows that the Ni layer remained coherent without any notable change. Photoelectrochemical performance evaluation 5 ACS Paragon Plus Environment


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The performance of the PEC cell composed of a nanoporous Ag cathode and a Ni-coated Si photoanode (denoted as np-Ag/Ni-Si PEC cell) was evaluated in a two-electrode experiment at 1.9 V external bias. As shown in Figure 3, the cell exhibited a clear photo-response to visible light irradiation, suggesting that the photovoltage generated at the photoanode can drive the catalytic oxidation reaction at anode and the catalytic reduction reaction at the cathode simultaneously with the assistance of an external bias.

Figure 3. Cathodic current density profile during photoelectrochemical two-electrode test with 1.9 V external bias under chopped illumination. The PEC tests were further performed at a much longer time scale using a two-electrode set-up in a batch-type two-chamber cell. The products in the cathode chamber were quantified using a GC, while we did not quantify the oxygen product in the anode chamber (based on mass balance, two CO2 molecules should produce one O2 molecule). The applied external biases were altered from 1.7 V to 2.1 V to study the cell performance in terms of cathodic current densities and CO Faradaic efficiencies. With a mild external bias of 1.7 V, a cathodic current density of 5 mA cm-2 (Figure 4a) was obtained in the np-Ag/Ni-Si PEC cell. When a slightly higher external bias was applied, the total current arises accordingly (Figure 4). At an external bias of 2.0 V, the PEC cell was able to deliver a current density of 10 mA cm-2. The gas-phase products were analyzed using a gas chromatograph and CO was identified as the major gas product together with H2 as the minor byproduct. The CO Faradaic efficiency was less than 5% at 1.7 V, which quickly increased to the maximum value of ~69% at 1.9-2.0 V, and then decreased to ~45% at 2.1 V (Figure 4). This trend is consistent with previous studies on CO2 electrolysis using the nanoporous Ag catalyst; however a much higher CO Faradaic efficiency (~90%) was achieved using a pure electrochemical cell.20 In terms of current density and CO Faradaic efficiency, the performance of the np-Ag/Ni-Si PEC cell is among the best PEC cell for CO2 reduction in the literature (Table S1).


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Figure 4. Cathodic current densities and CO Faradaic efficiencies for the np-Ag/Ni-Si PEC cell testing at various external biases: (a) 1.7 V, (b) 1.8 V, (c) 1.9 V, (d) 2.0 V, and (e) 2.1 V. (f) A summary of the performances of the np-Ag/Ni-Si PEC cell at various external biases using the data points at 1 hour of reaction. The np-Ag/Ni-Si PEC cells were stable in terms of current density and CO Faradaic efficiency in the first two hours of photoelectrolysis, and then followed by a performance decay, especially at higher external potentials (Figure 4). Previous studies of nanoporous Ag as the cathode in an electrochemical cell showed excellent stability under an 8-hour long durability test. From these studies, the source of fading was not the nanoporous Ag cathode in current study.20 Furthermore, 7 ACS Paragon Plus Environment


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the spent Ag cathode was also examined by PXRD analysis and no structural change was observed (Figure 1a). Therefore, we suspect that the photoanode may have degraded during the photoelectrolysis, leading to the severe performance fading after 2 hours of reaction. Stability of the Ni-coated Si Photoanode

Figure 5. (a) Typical SEM images of Ni-coated Si photoanode after photoelectrolysis for 3 hours at an external bias of (a) 1.7 V and (b) 2.1 V. (c) Ni K-edge XANES spectra of Ni-coated Si photoanodes and three Ni standards. The vertical dot lines indicate the positions of Ni0 and Ni3+. To confirm the origin of the performance degradation, SEM and X-ray absorption spectroscopy (XAS) were performed on both as-made and post-reaction Ni-coated Si photoanodes. SEM analysis showed a clear morphology change of the NiOx layer after reaction (Figure 5a-b). The continuous Ni layer was transformed into isolated islands with a diameter of 200-300 nm on the surface of the Si photoanode. To examine the oxidation state of Ni in the spent photoanode, the X-ray near-edge structure (XANES) spectroscopy studies were performed and the spectra of the as-made Ni-coated Si photoanode along with Ni standards are shown in Figure 5c. The Ni Kedge of the as-made Ni-Si photoanode was very close to that from the standard Ni foil, confirming the metallic nature of the as-made Ni layer. The Ni K-edge positions of anodized and post-reaction photoanodes shifted clearly to the higher energies (close to Ni3+), suggesting that the active Ni species on the photoanode have an average oxidation state of 3+. In comparison to the anodized sample, the Ni oxidation state of the post-reaction sample did not change during the photoelectrolysis, indicating that the performance fading was due to the damage to the interface between the Ni catalyst and the Si wafer. During the segregation process in which the uniform Ni catalyst layer transformed into large separated islands, the underlying Si surface became exposed to the electrolyte and caused severe corrosion under oxidative conditions (i.e. water oxidation). The photo-corrosion of Si-based anodes in PEC cells has been well documented in the literature.21 Although the Ni-coated Si photoanode has previously been reported to be stable for prolonged reactions in near neutral pH conditions, we did not obtain the same performance using the current configuration. The stability issue may have been caused by a 8 ACS Paragon Plus Environment

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Ni layer (~12 nm) being too thick in comparison to the previous report (~2 nm),22 which could affect the structural stability of Ni layer under working conditions. Further studies are required to identify a stable, active water oxidation catalyst compatible with CO2 electrolysis conditions. Energy efficiency

Figure 6. Cathodic current profiles and CO Faradaic efficiencies of electrochemical cells using a non-photo-responsive anode at various external bias: (a) 2.1 V, (b) 2.2 V, (c) 2.3 V, (d) 2.4 V, and (e) 2.5 V. (f) A summary of the performances of the electrochemical cell at various external biases using the data points at 1 hour of reaction. 9 ACS Paragon Plus Environment


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Evaluating the amount of photo-energy harvested and calculating the overall energy efficiency could identify the potential technical challenges associated with the photoelectrochemical CO2 conversion process. The photo-voltage of the Ni-coated Si photoanode was estimated by measuring the applied voltage difference between the PEC cell and an electrochemical cell with a non-photo-responsive anode. The experiments were carried out under identical test conditions as for the PEC test except for light radiation. The current profiles and CO Faradaic efficiencies are shown in Figure 6. The current density increased with external bias, similar to what was observed in the PEC tests. When 2.4 V external bias was applied, the CO faradaic efficiency reached its peak value at about 80% with a cathodic current density about 10 mA cm-2, in comparison to the maximum of CO Faradaic efficiency of ~70% at 2.0 V. Therefore, the photovoltage of the Ni-coated Si photoanode at CO2 reduction conditions is approximately 0.4 V, corresponding to an energy saving of ~17%.

Figure 7. Current-potential correlation for photoelectrochemical CO2 reduction in the case of nanoporous Ag cathode. The dash lines with two arrows indicate the potential differences. Thermodynamically, CO2 reduction to CO requires a potential of 1.34 V. Clearly, the energy saving of the PEC cell is greatly limited by the properties of photoanode. Some preliminary analysis of energy requirements is shown in Figure 7. Assuming the photoanode is based on single crystal Si, a typical current density is about 30 mA cm-2 with a potential of ~0.4 V. Regardless of the performance of Ni catalyst, a minimum external bias of 1.2 V is required to drive the CO2 reduction reaction in a single cell configuration. In a real cell, a higher potential is required to compensate the voltage losses due to internal resistance and activation barriers of water oxidation and CO2 reduction. Therefore, a photoanode with a photovoltage higher than 1.2 V (ideally ~2.4 V) is desired to drive the photoelectrochemical CO2 reduction with negligible external biases. Unfortunately, such a photoanode is not readily available at current stage. Another approach is to connect Si-based photoanodes in series to create a higher photovoltage as shown in Figure 7. For example, if four phosphorus-doped n-Si electrodes are connected in series, a photovoltage of ~1.8 V can be obtained. A drawback of this approach is the mismatch of electrode sizes. Since the nanoporous Ag is able to deliver a current density of 10 mA cm-2 at a potential of ~1.8 V, the size of photoanode will be approximately 4 times of the size of Ag


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cathode, which could pose serious problems to design a practical photoelectrochemical device for CO2 reduction. In addition to hunting for alternative semiconductor material, a stable oxygen evolution catalyst that can survive in harsh water oxidation environment and exhibit a high activity is crucial. Because the majority of CO2 reduction research are performed in near neutral conditions, the OER catalyst must also be compatible with these conditions. Although a recent study showed the possibility of using an Ir-based catalyst in a practical CO2 electrolyzer,24 non-precious metal based catalysts are preferred for commercial applications because Ir is one of the least abundant elements on Earth. Further research efforts are required to identify potential candidates and strategies to enhance the stability of the photoanode and promote the oxygen evolution activity simultaneously. Conclusion A photoelectrochemical cell for CO2 reduction was developed by the implementation of a Nicoated Si photoanode and a nanoporous Ag cathode. With an external bias of 2.0 V, a current density of 10 mA/cm2 with a CO Faradaic efficiency of ~70% was achieved. The PEC cell exhibited a stable performance up to 3 hours. The source of performance degradation under prolonged testing conditions was caused by interfacial damage between the nickel catalyst layer and the silicon wafer. In comparison to a pure electrochemical cell, the photovoltage of the PEC cell was determined as 0.4 V, and this corresponds to an energy saving of 17%. In order to further increase photovoltage and enhance stability under CO2 reduction conditions, new photoanode material and stable oxygen evolution catalyst are required. Supporting Information Digital photographs of the photoelectrochemical cells, additional SEM images, and a literature survey as Supporting Information. Acknowledgements The authors would like to thank the financial support from the University of Delaware. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. References (1)

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(16) Sahara, G.; Abe, R.; Higashi, M.; Morikawa, T.; Maeda, K.; Ueda, Y.; Ishitani, O. Photoelectrochemical CO2 Reduction Using a Ru(II)-Re(I) Multinuclear Metal Complex on a p-type Semiconducting NiO Electrode. Chem. Commun. 2015, 51, 10722-10725. (17) Lin, W. H.; Chang, T. F. M.; Lu, Y. H.; Sato, T.; Sone, M.; Wei, K. H.; Hsu, Y. J. Supercritical CO2-Assisted Electrochemical Deposition of ZnO Mesocrystals for Practical Photoelectrochemical Applications. J. Phys. Chem. C 2013, 117, 25596-25603. (18) LaTempa, T. J.; Rani, S.; Bao, N. Z.; Grimes, C. A. Generation of Fuel from CO2 Saturated Liquids Using a p-Si Nanowire Parallel to n-TiO2 Nanotube Array Photoelectrochemical Cell. Nanoscale 2012, 4, 2245-2250. (19) Huang, X. F.; Cao, T. C.; Liu, M. C.; Zhao, G. H. Synergistic Photoelectrochemical Synthesis of Formate from CO2 on {12-1) Hierarchical Co3O4. J. Phys. Chem. C 2013, 117, 26432-26440. (20) Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G. G.; Jiao, F. A Selective and Efficient Electrocatalyst for Carbon Dioxide Reduction. Nat. Commun. 2014, 5, 3242. (21) Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S. Amorphous TiO2 Coatings Stabilize Si, GaAs, and GaP Photoanodes for Efficient Water Oxidation. Science 2014, 344, 1005-1009. (22) Kenney, M. J.; Gong, M.; Li, Y. G.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. J. HighPerformance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 2013, 342, 836-840.


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(23) Mayer, M. T.; Du, C.; Wang, D. W. Hematite/Si Nanowire Dual-Absorber System for Photoelectrochemical Water Splitting at Low Applied Potentials. J. Am. Chem. Soc. 2012, 134, 12406-12409. (24) Luc, W.; Rosen, J.; Jiao, F. An Ir-based Anode for a Practical CO2 Electrolyzer. Catal. Today 2016, DOI: 10.1016/j.cattod.2016.06.011.



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Photoelectrochemical Carbon Dioxide Reduction ... - ACS Publications

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