1905501 (1 of 11) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com FULL PAPER NiMoFe and NiMoFeP as Complementary Electrocatalysts for Efficient Overall Water Splitting and Their Application in PV-Electrolysis with STH 12.3% Minki Baek, Guan-Woo Kim, Taiho Park, and Kijung Yong* Dr. M. Baek, Prof. K. Yong Surface Chemistry Laboratory of Electronic Materials Department of Chemical Engineering Pohang University of Science and Technology (POSTECH) 77 Cheongam-Ro, Nam-gu, Pohang, Gyeongbuk 37673, South Korea E-mail: kyong@postech.ac.kr Dr. G.-W. Kim, Prof. T. Park Polymer Chemistry and Electronics Lab Department of Chemical Engineering Pohang University of Science and Technology (POSTECH) 77 Cheongam-Ro, Nam-gu, Pohang, Gyeongbuk 37673, South Korea The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201905501. DOI: 10.1002/smll.201905501 1. Introduction Water splitting produces hydrogen, which is clean and renew- able chemical energy source. The water splitting process con- sists of two half reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Both reactions generally require overpotentials to overcome kinetic activation energies. In particular, the OER is a bottleneck process in water Complementary water splitting electrocatalysts used simultaneously in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can simplify water splitting systems. Herein, earth-abundant NiMoFe (NMF) and phosphorized NiMoFeP (NMFP) are synthesized as complementary overall water splitting (OWS) catalysts. First, NMF is tested as both the HER and OER promoter, which exhibits low overpotentials of 68 (HER) and 337 mV (OER). A quaternary NMFP is then prepared by simple phosphorization of NMF, which shows a much lower OER overpotential of 286 mV. The enhanced OER activity is attributed to the unique surface/ core structure of NMFP. The surface phosphate acts as a proton transport mediator and expedites the rate-determining step. With the application of OER potential, the NMFP surface is composed of Ni(OH) 2 and FeOOH, active sites for OER, but the inner core consists of Ni, Mo, and Fe metals, serving as a conductive electron pathway. OWS with NMF-NMFP requires an applied voltage of 1.452 V to generate 10 mA cm -2 , which is one of the lowest values among OWS results with transition-metal-based electrocatalysts. Furthermore, the catalysts are combined with tandem perovskite solar cells for photovoltaic (PV)-electrolysis, producing a high solar-to-hydrogen (STH) conversion efficiency of 12.3%. splitting due to its complex four proton- coupled electron transfer (PCET) steps and thus needs a higher overpotential than the HER, which proceeds in two steps. Elec- trocatalysts can minimize the overpoten- tial of each reaction. Previous works on the HER and OER electrocatalysts have mainly focused on noble metal catalysts including Pt, Ru, and Ir under acidic operating con- ditions, which are widely used in commer- cialized fuel cells and electrolyzers today. However, due to the high cost of noble metals, earth-abundant catalysts have been thoroughly explored to replace noble metal catalysts in recent years. First row (3d) transition-metal-based catalysts have good catalytic properties due to the facile oxida- tion state changes of 3d transition metals. Among these elements, Ni is considered as the most suitable candidate owing to its good HER and OER activities and high resistance to corrosion. [1,2] Introduction of other elements into Ni catalyst can further improve catalytic activities by controlling the surface adsorption energy of reaction intermediates. With Mo incorporation, the Ni-Mo binary alloy exhibits the extremely high HER activity in both acidic and alkaline media. [1,3] On the other hand, Fe incor- poration into a Ni catalyst significantly improves OER activity. [4] Many studies on finding an optimal Fe ratio and investigating the OER mechanism of Ni-Fe catalysts have been reported. [5–7] Actual overall water splitting (OWS) in an electrolyzer is achieved by proceeding HER and OER in a single electrolyte. However, in general, HER catalysts are most active under acidic conditions, whereas OER catalysts are only active under alkaline conditions. Hence, catalyzing both the HER and OER under the same pH conditions is essential but difficult to accomplish. Recently, complementary (or bifunctional) cata- lysts with excellent catalytic activities in both the HER and OER under the same pH conditions have been investigated as attrac- tive solutions to overcome these difficult restrictions in OWS. [8] In addition, these catalysts can reduce the fabrication cost and simplify the water splitting system due to a single manufac- turing process. As low-cost complementary catalysts, Ni-based phosphides are regarded as the most promising one. Stern et al showed the bifunctional catalytic activities of Ni 2 P for the first time. They proved that Ni 2 P shows high HER activity due to its high density of exposed (001) facets [9] and superior OER Small 2019, 1905501