Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts?
E
limited to) X-ray photoelectron spectroscopy (XPS), synchrotron-based X-ray spectroscopy, and Raman spectroscopy, af ter the OER reactions, to understand what are the true catalytically active species on the surface of such claimed OER catalysts of metal chalcogenides, nitrides, and phosphides. (This point was also emphasized in a previous Editorial in ACS Energy Letters.6) Not surprisingly, some papers have reported such results and confirmed these points above. For example, one of the earlier papers that reported OER catalysis using Ni2P nanoparticles clearly identified that the “high activity is attributed to the core− shell (Ni2P/NiOx) structure that the material adopts under catalytic conditions” because transmission electron microscopy imaging clearly revealed that the NiOx shell is several nanometers thick.7 Another more recent report also showed the Co2P is surface oxidized to CoOx through “in situ transformation” during OER reaction.8 Sometimes, the original compound is completely converted into (amorphous) metal oxide/hydroxide in bulk (i.e., not limited to surface oxide), such as shown in the case of the conversion of nickel sulfide into nickel oxide, which is even evidenced by the bulk characterization technique of powder Xray diffraction (PXRD).9 The reported OER catalysts might have appeared to be stable under OER operation, but that could be because that within one or a few electrochemical scan cycles the conversion to metal oxide/hydroxides either on the surface or in bulk is complete, so the electrochemical performance appears steady afterward. In these cases and many other cases that the authors might or might not have carefully checked, the original metal chalcogenides, nitrides, phosphides, and so forth are really the precursors to the real active OER catalysts or “precatalyst”, as illustrated in Figure 1. Therefore, I advocate that the electrocatalysis research community should adopt the attitude that these compounds
lectrochemical and solar-driven photoelectrochemical water splitting for hydrogen and oxygen production are promising approaches to provide affordable clean energy, reduce our reliance on conventional fossil fuels, and mitigate the impact of climate change. To ensure efficient overall water splitting, highly efficient and robust electrocatalysts with significantly reduced overpotentials for both the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) are needed.1 Therefore, there have been significant recent research interests in developing new electrocatalysts that are not based on platinum and other precious metals that are highly active, cost-effective, and robust.2 This research trend is also well reflected in the papers published in ACS Energy Letters. Compared to the more developed HER catalysts,3 OER is more complex and challenging as it involves four sequential proton-coupled electron transfer steps as well as oxygen−oxygen bond formation. Transition metal oxides or (oxy)hydroxides have been the most common and stable OER catalysts, which have seen significant recent developments, with some now surpassing the catalytic activity and performance by the traditional IrO2 benchmark OER catalyst in alkaline media.4,5 Interestingly, there are also increasingly many reports that claim that transition metal sulfides, selenides, nitrides, phosphides, and so forth are also highly efficient OER catalysts. Many of these reports also show that such OER catalysts can demonstrate stable electrocatalytic performance for hours or even longer without apparent performance decay. Because many of these materials have also been found to be active HER catalysts, they are also popularly referred to as “bifunctional catalysts”, which implies that one catalyst can conveniently provide both HER and OER catalysis (as an aside, it is not clear what applications bifunctional HER/OER catalysts would have given that it is unlikely one would need to make hydrogen and oxygen on the same electrode; for separate electrodes, one would optimize both HER and OER separately to achieve the best performance). However, from the point of view of solid-state chemistry, it is well-known that metal sulfides are thermodynamically less stable than metal oxides under oxidizing potentials and metal nitrides and phosphides are less stable than sulfides and so forth. Therefore, one would expect that metal sulfides, selenides, nitrides, and phosphides, and so forth can be easily oxidized to the corresponding metal oxides/hydroxides, especially in the aqueous and strongly oxidative environments of OER. In fact, these oxidation processes are happening everyday in geochemical environments and are responsible for the transformation and formation of many minerals and rocks. These facts call into question what the OER catalysts really are. Especially because catalysis happens on the surface, it is crucial to carry out surfacesensitive structural characterization analysis, including (but not © 2017 American Chemical Society
Figure 1. Metal chalcogenides, nitrides, and phosphides are often efficient HER catalysts and might also appear to be active for OER, but in most cases, they are oxidized to metal oxides/hydroxides on the surface or even in bulk under the OER conditions in akaline media; therefore, the real catalytically active species are the metal oxides/hydroxides, not the original unstable compounds. Published: August 11, 2017 1937
DOI: 10.1021/acsenergylett.7b00679 ACS Energy Lett. 2017, 2, 1937−1938
Editorial
http://pubs.acs.org/journal/aelccp
ACS Energy Letters
■
are assumed to be oxidized in OER environments (at least on the surface) unless conclusively proven otherwise. It is time to set the record straight: unless such rigorous postcatalysis structural analysis can be provided to establish conclusively that the catalytic active species on the surface are still the original compounds, we cannot and should not refer to these unstable materials as OER catalysts or bifunctional catalysts. This would be an important criterion to consider for manuscripts submitted to ACS Energy Letters. Of course, this does not mean that such (often nanocomposite) OER catalysts derived from these unstable compounds are not interesting and should not be studied. In fact, they often display apparent electrocatalytic performance that is better than that of the simple corresponding metal oxides/ hydroxides synthesized directly. Furthermore, some have now purposefully used metal chalcogenides/phosphides as the precursors or scaffolds to prepare the oxide/hydroxide OER catalysts on the surface of the original nanostructured materials and achieved significantly enhanced catalytic performance using such (nanocomposite) electrocatalysts.10−12 There could be many potential reasons for such enhanced catalytic performance. The derived metal oxides/hydroxides could be formed as high surface area nanostructures that could boost the overall performance. The conversion process from the precursor phases may result in unusual amorphous or metastable metal oxide/ hydroxide phases that are more catalytically active but otherwise difficult to gain access to by conventional synthesis. Perhaps the metal chalcogenides and phosphides are more conductive than the corresponding metal oxides/hydroxides and thus could serve as the conductive scaffolds for the active metal oxide/hydroxide species. Another speculation could be that there are synergistic electronic interactions between the different components that make the composite electrocatalysts better than the simple oxides. There could be many interesting scientific questions to be studied here. Besides the Edisonian approach of making various combinations of nanocomposite catalysts and trying to find which one works better, careful mechanistic studies aided by modern structural or spectroscopic analysis techniques and computational tools to fundamentally understand what make these derived OER catalysts work well could allow us to more rationally prepare more efficient OER catalysts. Hopefully, the research community can collectively put the myriad of sometimes confusing research findings in this rapidly exploding field on a firm scientific footing, work on the unresolved scientific questions, and therefore move the field forward to overcome the significant challenges in developing more efficient and robust catalysts for water splitting.
Editorial
REFERENCES
(1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I. B.; Norskov, J. K.; Jaramillo, T. F. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355, eaad4998. (2) Faber, M. S.; Jin, S. Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications. Energy Environ. Sci. 2014, 7, 3519−3542. (3) Caban-Acevedo, M.; Stone, M. L.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, H. C.; Tsai, M. L.; He, J. H.; Jin, S. Efficient hydrogen evolution catalysis using ternary pyrite-type cobalt phosphosulphide. Nat. Mater. 2015, 14, 1245−1251. (4) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347−4357. (5) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles. Chem. Mater. 2015, 27, 7549−7558. (6) Boettcher, S. W. ACS Energy Letters: Elevating Solar Fuels and Electrocatalysis Research. ACS Energy Lett. 2016, 1, 920−921. (7) Stern, L.-A.; Feng, L.; Song, F.; Hu, X. Ni2P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni2P nanoparticles. Energy Environ. Sci. 2015, 8, 2347−2351. (8) Dutta, A.; Samantara, A. K.; Dutta, S. K.; Jena, B. K.; Pradhan, N. Surface-Oxidized Dicobalt Phosphide Nanoneedles as a Nonprecious, Durable, and Efficient OER Catalyst. ACS Energy Lett. 2016, 1, 169− 174. (9) Mabayoje, O.; Shoola, A.; Wygant, B. R.; Mullins, C. B. The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as “Pre-catalysts. ACS Energy Lett. 2016, 1, 195−201. (10) Xu, X.; Song, F.; Hu, X. L. A nickel iron diselenide-derived efficient oxygen-evolution catalyst. Nat. Commun. 2016, 7, 12324. (11) Chen, W.; Wang, H. T.; Li, Y. Z.; Liu, Y. Y.; Sun, J.; Lee, S. H.; Lee, J. S.; Cui, Y. In Situ Electrochemical Oxidation Tuning of Transition Metal Disulfides to Oxides for Enhanced Water Oxidation. ACS Cent. Sci. 2015, 1, 244−251. (12) Liang, H.; Gandi, A. N.; Xia, C.; Hedhili, M. N.; Anjum, D. H.; Schwingenschlögl, U.; Alshareef, H. N. Amorphous NiFe-OH/NiFeP Electrocatalyst Fabricated at Low Temperature for Water Oxidation Applications. ACS Energy Lett. 2017, 2, 1035−1042.
Song Jin,* Senior Editor, ACS Energy Letters
■
Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Song Jin: 0000-0001-8693-7010 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS. 1938
DOI: 10.1021/acsenergylett.7b00679 ACS Energy Lett. 2017, 2, 1937−1938
