Iron Phosphates as Highly Efficient Precatalysts for Oxygen Evolution Reaction and Their in Situ Electrochemical Conversion

Abstract

Current energy issues have prompted demand for the innovations in constructing a clean and sustainable energy platforms. One of examples for energy carrier to actualize a zero carbon society is hydrogen, which is a promising carbon-free fuel with an excellent weight energy density. Hydrogen production through alkaline water electrolysis using renewable energy resources (light and wind)-derived electricity is an attractive ecofriendly and cost-effective technology because cheap nonprecious electrocatalysts can be employed in alkaline media. However, oxygen evolution reaction (OER) at the anode side in the water electrolysis is basically slow and prevents the widespread applications of water electrolysis. Thus, highly efficient and cheap electrocatalysts for OER are highly desirable.Iron (Fe)-based compounds have recently received much attention as OER electrocatalysts because Fe is a quite earth-abundant metallic element that is a very cheap and non-toxic. Therefore, Fe is a promising candidate as electrocatalyst for OER. Due to the insufficient OER activity on simple Fe oxides, previous studies have attempted to enhance the activity by incorporating Fe and other metallic elements.[1,2] Recently, we established a novel comprehensive structural descriptor for the OER on Fe-based simple and multimetal oxides;[3] i.e., shorter minimum Fe–O bond length in the crystalline structures led to higher OER activity regardless of elemental composition, Fe–O coordination number, and crystal category. Thus, this descriptor enabled us to quickly develop excellent OER catalyst, Ba0.65Ca0.35Fe12O19, comprising extremely short Fe–O bond lengths.[4] Furthermore, we found that post-spinel CaFe2O4 catalyzes OER via "multi-iron-site mechanism" involving a reaction intermediate with O–O direct formation on multiple Fe atoms, and CaFe2O4 consequently displayed outstanding OER activity despite the long Fe–O bond length.[5] However, we speculate that OER activity of Fe-based oxides cannot be further enhanced based on crystal structures, and it necessitates to employ another type of compounds.In this study, we report the OER performances of Fe-based phosphates in alkaline media. We selected three kinds of Fe-based phosphates with distinct crystal structures: trigonal FePO4, trigonal Fe3O3(PO4), and monoclinic Fe4(P2O7)3. The electrochemical measurements and post-characterizations identified the Fe-based phosphates as efficient precatalysts that can be electrochemically converted into highly active Fe-based compounds. Notably, the OER performances of the electrochemically converted Fe-based compounds overcame those of the above-mentioned most active Fe-based multimetal oxides and any previously reported crystalline OER electrocatalysts. Therefore, the Fe-based phosphates are promising candidates as OER precatalysts, which can be in situ converted into excellent OER electrocatalysts, and have further advantages of low price, environmental friendliness, and easy production.[6] The part of this paper is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). This paper was funded in part by the JSPS KAKENHI Grant-in-Aid (18H01786), the JST PRESTO (JPMJPR15S3) and CREST (JPMJCR16P3) programs, the Tokuyama Science Foundation, and the “Creation of Life Innovative Materials for Interdisciplinary and International Researcher Development” program of the MEXT. The authors thank Dr. Takeshi Aihara in Tokyo Institute of Technology for fruitful discussions.