Editorial Cite This: Chem. Mater. 2018, 30, 7325−7327
pubs.acs.org/cm
Chemistry of Materials for Water Splitting Reactions
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MATERIALS FOR WATER OXIDATION/OXYGEN EVOLUTION (OER) The oxygen evolution reaction is the most challenging half reaction of the overall water splitting scheme due to the slow kinetics of this proton-coupled electron transfer reaction.2 The overpotential for the reaction, above the thermodynamic minimum, is wasted energy. Catalysts that can lower this activation barrier and increase the sluggish rate of this reaction are sorely needed.3−7 Alkaline conditions are most commonly considered for the OER reaction, but there is great interest in the development of OER catalysts that are stable in an acidic medium, to render the components of the whole water splitting architecture compatible.8 Materials for photoelectrodes in PEC architectures that absorb light and lead to charge separation and finally OER (and HER, vide infra) are also of great interest, and are being investigated to understand their precise atomic structure to tease out a connection between structure and activity for OER.9−11
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plitting water using energy from renewable sources (solar, wind, hydroelectric) is of great interest to generate hydrogen gas, H2(g), via the reaction 2H2O → 2H2 + O2 (Figure 1). The H2 can then be consumed directly as a clean-
Figure 1. Splitting of water, 2H2O → 2H2 + O2, as a means of generating clean-burning hydrogen fuel from renewable energy sources. Left image from ref 21; right image from ref 2. Reprinted with permission from the American Chemical Society.
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burning fuel, or harnessed as a means of chemically storing energy, in the H−H bond, for later use.1 The intermittent nature of solar and wind necessitates a means to store energy to better match demand, and transient storage of this energy as hydrogen could therefore enable generation of electricity with a fuel cell, or be used for direct combustion, when needed. The materials challenges that must be overcome to enable the efficient splitting of water using renewable energy sources are both fascinating and frustrating. Research groups around the world are tackling various aspects of the technology from a number of different angles, with the hope that an architecture, or architectures, that is/are efficient, scalable to grid scale, and economical, will emerge. At present, it is not clear whether integrated photoelectrochemical (PEC) devices or direct electrolysis of water with electricity generated from a wind turbine or photovoltaic, for example, are more advantageous than the other. In any case, research on both PEC and electrolysis of water is highly complementary since both rely on catalysts to lower the overpotential of the water splitting reaction, among others aspects. This virtual issue looks at very recent progress in the development of materials for the splitting of water for generation of H2, as well as the product of the other half of the balanced reaction, O2. New materials also provide important fundamental insights into the mechanisms of the hydrogen and oxygen evolution reactions, the nuance of charge separation and transfer, and other critical steps. Long-term operational stability is a critical issue, and thus materials that play a protective role or are themselves intrinsically stable under harsh conditions (acid, base, illumination, heat) are necessary. The preference is for materials that are earthabundant to ensure the feasibility of mass manufacture at grid scale, but noble metal catalysts are useful if they are highly active, and thus necessitating only trace quantities; they also provide much-needed basic mechanistic data and leads. We have divided the issue up into the following subsections, as described here. © 2018 American Chemical Society
MATERIALS FOR HYDROGEN EVOLUTION (HER) HER is considered to be the easier reaction of the two halfreactions of water splitting, but the development of highly active photoelectrodes for PEC that absorb light and efficiently produce hydrogen are needed.12−16 The development of earthabundant materials that can effectively and rapidly transform light into charge is of great interest.17 As a result, research into metal oxides, chalcopyrites, graphitic materials, and other semiconductor materials is expanding rapidly. Catalysts for HER are also of interest, and include not only the development of new compositions, such as metal phosphides and composite materials, but also nanostructured materials, to access the advantages of high surface area and size-dependent properties.18−20
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MATERIALS FOR BOTH OXYGEN AND HYDROGEN EVOLUTION REACTIONS (OER AND HER) Semiconductor photoelectrodes that have conduction and valence band edge positions enabling both HER and OER are highly desired to increase the simplicity of the overall water splitting device architecture.21−23 Therefore, efforts have been made to tune band edge positions and accurately predict band alignments at the semiconductor/liquid junction.24 Electrocatalysts that are capable of both HER and OER reactions have also been actively developed.25−27 For these catalysts, carefully examining their surface compositions under the HER and OER conditions and identifying catalytically active surface species, which may be different from their original compositions, will be important. Another active area of research is the development of exquisitely controlled nanocomposite and nanostructured catalysts that are endowed with ideal architectures to facilitate OER and HER.28,29 Control over composition and morphology of these materials is critical to Published: November 13, 2018 7325
DOI: 10.1021/acs.chemmater.8b04419 Chem. Mater. 2018, 30, 7325−7327
Chemistry of Materials
Editorial
(GaN)1−x(ZnO)x for Photodriven Oxygen Evolution. Chem. Mater. 2017, 29, 6525−6535. (10) Lumley, M. A.; Choi, K.-S. Investigation of Pristine and (Mo, W)-Doped Cu11V6O26 for Use as Photoanodes for Solar Water Splitting. Chem. Mater. 2017, 29, 9472−9479. (11) Zhang, W.; Yan, D.; Li, J.; Wu, Q.; Cen, J.; Zhang, L.; Orlov, A.; Xin, H.; Tao, J.; Liu, M. Anomalous Conductivity Tailored by Domain-Boundary Transport in Crystalline Bismuth Vanadate Photoanodes. Chem. Mater. 2018, 30, 1677−1685. (12) Septina, W.; Prabhakar, R. R.; Wick, R.; Moehl, T.; Tilley, S. D. Stabilized Solar Hydrogen Production with CuO/CdS Heterojunction Thin Film Photocathodes. Chem. Mater. 2017, 29, 1735−1743. (13) Ge, J.; Yin, W.-J.; Yan, Y. Solution-Processed Nb-Substituted BaBiO3 Double Perovskite Thin Films for Photoelectrochemical Water Reduction. Chem. Mater. 2018, 30, 1017−1031. (14) Park, J. E.; Hu, Y.; Krizan, J. W.; Gibson, Q. D.; Tayvah, U. T.; Selloni, A.; Cava, R. J.; Bocarsly, A. B. Stable Hydrogen Evolution from an AgRhO2 Photocathode under Visible Light. Chem. Mater. 2018, 30, 2574−2582. (15) Frick, J. J.; Cava, R. J.; Bocarsly, A. B. Chalcopyrite CuIn(S1−xSex)2 for Photoelectrocatalytic H2 Evolution: Unraveling the Energetics and Complex Kinetics of Photogenerated Charge Transfer in the Semiconductor Bulk. Chem. Mater. 2018, 30, 4422− 4431. (16) Pussacq, T.; Kabbour, H.; Colis, S.; Vezin, H.; Saitzek, S.; Gardoll, O.; Tassel, C.; Kageyama, H.; Laberty Robert, C.; Mentré, O. Reduction of Ln2Ti2O7 Layered Perovskites: A Survey of the Anionic Lattice, Electronic Features, and Potentials. Chem. Mater. 2017, 29, 1047−1057. (17) Akaike, K.; Aoyama, K.; Dekubo, S.; Onishi, A.; Kanai, K. Characterizing Electronic Structure near the Energy Gap of Graphitic Carbon Nitride Based on Rational Interpretation of Chemical Analysis. Chem. Mater. 2018, 30, 2341−2352. (18) Liu, Q.; Fang, Q.; Chu, W.; Wan, Y.; Li, X.; Xu, W.; Habib, M.; Tao, S.; Zhou, Y.; Liu, D.; Xiang, T.; Khalil, A.; Wu, X.; Chhowalla, M.; Ajayan, P. M.; Song, L. Electron-Doped 1T-MoS2 via Interface Engineering for Enhanced Electrocatalytic Hydrogen Evolution. Chem. Mater. 2017, 29, 4738−4744. (19) Schipper, D. E.; Zhao, Z.; Thirumalai, H.; Leitner, A. P.; Donaldson, S. L.; Kumar, A.; Qin, F.; Wang, Z.; Grabow, L. C.; Bao, J.; Whitmire, K. H. Effects of Catalyst Phase on the Hydrogen Evolution Reaction of Water Splitting: Preparation of Phase-Pure Films of FeP, Fe2P, and Fe3P and Their Relative Catalytic Activities. Chem. Mater. 2018, 30, 3588−3598. (20) Hu, G.; Tang, Q.; Lee, D.; Wu, Z.; Jiang, D.-E. Metallic Hydrogen in Atomically Precise Gold Nanoclusters. Chem. Mater. 2017, 29, 4840−4847. (21) Govindaraju, G. V.; Wheeler, G. P.; Lee, D.; Choi, K.-S. Methods for Electrochemical Synthesis and Photoelectrochemical Characterization for Photoelectrodes. Chem. Mater. 2017, 29, 355− 370. (22) Prévot, M. S.; Jeanbourquin, X. A.; Bourée, W. S.; Abdi, F.; Friedrich, D.; van de Krol, R.; Guijarro, N.; Le Formal, F.; Sivula, K. Evaluating Charge Carrier Transport and Surface States in CuFeO2 Photocathodes. Chem. Mater. 2017, 29, 4952−4962. (23) Ziani, A.; Le Paven, C.; Le Gendre, L.; Marlec, F.; Benzerga, R.; Tessier, F.; Cheviré, F.; Hedhili, M. N.; Garcia-Esparza, A. T.; Melissen, S.; Sautet, P.; Le Bahers, T.; Takanabe, K. Photophysical Properties of SrTaO2N Thin Films and Influence of Anion Ordering: A Joint Theoretical and Experimental Investigation. Chem. Mater. 2017, 29, 3989−3998. (24) Guo, Z.; Ambrosio, F.; Chen, W.; Gono, P.; Pasquarello, A. Alignment of Redox Levels at Semiconductor−Water Interfaces. Chem. Mater. 2018, 30, 94−111. (25) Zhang, Y.; Rui, K.; Ma, Z.; Sun, W.; Wang, Q.; Wu, P.; Zhang, Q.; Li, D.; Du, M.; Zhang, W.; Lin, H.; Zhu, J. Cost-Effective Vertical Carbon Nanosheets/Iron-Based Composites as Efficient Electrocatalysts for Water Splitting Reaction. Chem. Mater. 2018, 30, 4762− 4769.
enable rational improvement, and to understand the basic mechanisms of the many steps that need to occur efficiently for both OER and HER to proceed with both good kinetics and yield.
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STABILITY OF MATERIALS FOR WATER SPLITTING Long-term stability of materials for water splitting is critical. Thin coatings that protect catalysts and photoelectrodes from the harsh aqueous environment are of interest, but must be perfect, with no pinholes or defects through which reagents can travel and corrode the underlying layers. Atomic layer deposition is emerging as a practical approach to generating layers that are only ∼2 nm thick, with few or no imperfections.30 Other research is examining the intrinsic stability of photoelectrode materials, to learn from a fundamental perspective, what makes some materials stable under these conditions for OER, HER, and related reactions.31,32
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Jillian M. Buriak, Editor-in-Chief Carlos Toro, Managing Editor Kyoung-Shin Choi, Associate Editor AUTHOR INFORMATION
ORCID
Jillian M. Buriak: 0000-0002-9567-4328 Carlos Toro: 0000-0002-8359-462X Kyoung-Shin Choi: 0000-0003-1945-8794 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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REFERENCES
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(26) Sun, H.; Xu, X.; Yan, Z.; Chen, X.; Cheng, F.; Weiss, P. S.; Chen, J. Porous Multishelled Ni2P Hollow Microspheres as an Active Electrocatalyst for Hydrogen and Oxygen Evolution. Chem. Mater. 2017, 29, 8539−8547. (27) Zhang, N.; Shao, Q.; Pi, Y.; Guo, J.; Huang, X. SolventMediated Shape Tuning of Well-Defined Rhodium Nanocrystals for Efficient Electrochemical Water Splitting. Chem. Mater. 2017, 29, 5009−5015. (28) Patra, B. K.; Khilari, S.; Bera, A.; Mehetor, S. K.; Pradhan, D.; Pradhan, N. Chemically Filled and Au-Coupled BiSbS3 Nanorod Heterostructures for Photoelectrocatalysis. Chem. Mater. 2017, 29, 1116−1126. (29) Qureshi, M.; Takanabe, K. Insights on Measuring and Reporting Heterogeneous Photocatalysis: Efficiency Definitions and Setup Examples. Chem. Mater. 2017, 29, 158−167. (30) Hannula, M.; Ali-Löytty, H.; Lahtonen, K.; Sarlin, E.; Saari, J.; Valden, M. Improved Stability of Atomic Layer Deposited Amorphous TiO2 Photoelectrode Coatings by Thermally Induced Oxygen Defects. Chem. Mater. 2018, 30, 1199−1208. (31) Zhang, C. J.; Pinilla, S.; McEvoy, N.; Cullen, C. P.; Anasori, B.; Long, E.; Park, S.-H.; Seral-Ascaso, A.; Shmeliov, A.; Krishnan, D.; Morant, C.; Liu, X.; Duesberg, G. S.; Gogotsi, G.; Nicolosi, V. Oxidation Stability of Colloidal Two-Dimensional Titanium Carbides (MXenes). Chem. Mater. 2017, 29, 4848−4856. (32) Singh, A. K.; Zhou, L.; Shinde, A.; Suram, S. K.; Montoya, J. H.; Winston, D.; Gregoire, J. M.; Persson, K. A. Electrochemical Stability of Metastable Materials. Chem. Mater. 2017, 29, 10159−10167.
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DOI: 10.1021/acs.chemmater.8b04419 Chem. Mater. 2018, 30, 7325−7327
