Electrochimica Acta 366 (2021) 137467 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta The significance of the local structure of cobalt-based catalysts on the photoelectrochemical water oxidation activity of BiVO 4 Mahsa Barzgar Vishlaghi a,b , Abdullah Kahraman a,b , Sinem Apaydin a,b , Emre Usman b,c , Dilan Aksoy a,b , Timuçin Balkan b,c , Shamsa Munir b,1 , Messaoud Harfouche d , Hirohito Ogasawara e , Sarp Kaya a,b,c,∗ a Materials Science and Engineering, Koç University, Istanbul 34450, Turkey b Koç University Tüpra ¸ s Energy Center (KUTEM), Istanbul 34450, Turkey c Department of Chemistry, Koç University, Istanbul 34450, Turkey d Synchrotron-light for Experimental Science and Applications in the Middle East (SESAME), Allan 19252, Jordan e SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA a r t i c l e i n f o Article history: Received 3 June 2020 Revised 13 October 2020 Accepted 10 November 2020 Available online 12 November 2020 Keywords: Water oxidation kinetics BiVO 4 Cobalt-based catalyst Local structure Atomic layer deposition a b s t r a c t The local structures of the water oxidation catalysts play an important role in reaction kinetics and the performance of the photoanodes. In this study, we deposited cobalt-based catalysts on nanoporous BiVO 4 with controlled thicknesses by atomic layer deposition (ALD). Despite the similar oxidation states of cobalt in all depositions, different water oxidation activities in neutral pH conditions were observed. A dramatic photocurrent raise, lowered kinetic overpotential, and smaller charge transfer resistance across the photoanode/electrolyte interface were achieved when a uniform ultrathin Co(OH) 2 layer was formed on BiVO 4 . Photocurrent density for water oxidation showed a 95% enhancement at 0.6 V vs. RHE when the catalyst was in the form of Co(OH) 2 , while an 80% increase was obtained for CoO. Ideal coordina- tion of Co(OH) 2 on hydroxylated BiVO 4 surface assists the charge transfer between the electrolyte and BiVO 4 without increasing surface recombination. The results of this study emphasize the importance of controlling the local structure of the catalysts in the performance of the water splitting photoanodes. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Conventional fossil fuel-based energy is a limited resource and carbon emission to the environment upon combustion severely im- pacts the climate. Therefore, a sustainable and clean alternative is necessary to generate the required power. Photoelectrochemi- cal (PEC) splitting of water is an artificial photosynthesis approach for sustainable solar to chemical energy conversion [1–3] in which electrons and holes are generated in a semiconductor under illu- mination convert H 2 O into H 2 and ½O 2 . The semiconductor must absorb photons with energies larger than 1.23 V, which is the free energy required for splitting water [4], and the conduction and va- lence band edges of the semiconductor should straddle the elec- trochemical potential of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Due to the sluggish ∗ Corresponding author at: Department of Chemistry, Koç University, Istanbul 34450, Turkey. E-mail address: sarpkaya@ku.edu.tr (S. Kaya). 1 Present address: School of Applied Sciences, National University of Technology, Islamabad, Pakistan. kinetics of OER which is rate-limiting in overall water splitting, the development of the semiconductor photoanodes is of more inter- est. Metal oxide semiconductors have been studied extensively due to their higher stability and low cost of fabrication compared to other semiconductor types for water splitting [5–8]. Among the n- type photoanodes [9], BiVO 4 has a suitable bandgap of 2.4 - 2.5 eV for visible light absorption and proper band alignment respective to water redox potentials [10]. Its valence band edge is located at 2.4 eV vs. RHE (reversible hydrogen electrode), which is more pos- itive than the electrochemical potential for OER, providing a suffi- cient driving force for holes to oxidize water [11,12]. Furthermore, its conduction band edge is just below the electrochemical poten- tial for HER [13–15], requiring less external bias for the photoelec- trons to drive the water reduction. However, the photocurrent den- sity drawn is still far from the theoretical value (~7 mA/cm 2 ) for a monoclinic BiVO 4 [16]. The major bottleneck for BiVO 4 activity is its slow kinetics for OER, limited electron and hole mobility, and charge transfer across photoanode-liquid interface [17–19]. More- over, a large number of the photogenerated holes are lost due to the facile charge recombination at the electrolyte interface [20–22]. https://doi.org/10.1016/j.electacta.2020.137467 0013-4686/© 2020 Elsevier Ltd. All rights reserved.