High-Efficiency and Durable Water Oxidation under Mild pH Conditions: An Iron Phosphate−Borate Nanosheet Array as a Non- Noble-Metal Catalyst Electrode Weiyi Wang, †,‡ Danni Liu, ‡ Shuai Hao, ‡ Fengli Qu, § Yongjun Ma, ⊥ Gu Du, ∥ Abdullah M. Asiri, ¶ Yadong Yao,* ,† and Xuping Sun* ,‡ † College of Materials Science and Engineering and ‡ College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China § College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, Shandong, China ⊥ Analytical and Test Center, Southwest University of Science and Technology, Mianyang 621010, China ∥ Chengdu Institute of Geology and Mineral Resources, Chengdu 610064, China ¶ Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia * S Supporting Information ABSTRACT: It is highly desired but still remains a key challenge to develop iron-based large-surface-area arrays as heterogeneous water oxidation catalysts that perform efficiently and durably under mild pH conditions for solar-to-hydrogen conversion. In this work, we report the in situ derivation of an iron phosphate−borate nanosheet array on carbon cloth (Fe−Pi−Bi/CC) from an iron phosphide nanosheet array via oxidative polarization in a potassium borate (KBi) solution. As a 3D catalyst electrode for water oxidation at mild pH, such a Fe−Pi− Bi/CC shows high activity and strong long-term electro- chemical durability, and it only demands an overpotential of 434 mV to drive a geometrical catalytic current density of 10 mA cm −2 with maintenance of its activity for at least 20 h in 0.1 M KBi. This study offers an attractive earth- abundant catalyst material in water-splitting devices toward the large-scale production of hydrogen fuels under benign conditions for application. I ntensive recent attention has focused on electrocatalytic and photocatalytic water splitting toward the storage of intermittent renewable energy as hydrogen fuels. 1−4 To achieve more energy-efficient hydrogen production, active catalysts must be implemented to overcome the large overpotentials. 5−9 Proton-exchange membrane electrolyzers respond well to fluctuations in power inputs but suffer from the use of noble- metal catalysts with low abundance and high cost. 10−13 This issue can be avoided by alkaline electrolysis. 14−16 Such harsh chemical environments for both techniques, however, limit the types of photoelectrodes and cell components. 17 The water oxidation reaction at the anode suffers from sluggish kinetics with a large overpotential. 18−20 So, efficient earth-abundant water oxidation catalysts (WOCs) at mild pH need to be developed. Cobalt and nickel have emerged as interesting iron group elements because of their catalytic power toward environ- mentally friendly water oxidation, and recent years have witnessed the rapid development of cobalt/nickel-based heterogeneous WOCs in neutral or near-neutral electro- lytes. 20−30 Such phosphate or borate catalysts are robust, inexpensive, and self-healing, and they are usually grown on conductive substrates as catalyst films by electrodeposition from metal-ion-containing potassium phosphate (KPi) or potassium borate (KBi) solutions. Compared to cobalt and nickel, iron is more earth-abundant and cheaper with less toxicity and rich redox properties for dioxygen activation in both biological and biomimetic environments. 31,32 Unfortunately, the electrodepo- sition preparation of such an iron-based catalyst film from Fe III is difficult because Fe III ions precipitate out from water under neutral or near-neutral conditions. Although iron-based films can be deposited from Fe II solutions with the presence of proton- accepting buffer anions 33 or in acetate buffer, 34 particular care must be taken to avoid Fe II oxidation during deposition, and such catalyst films need large overpotentials to drive water oxidation (η 7 mA cm −2 = 630 mV 33 and η 1 mA cm −2 = 480 mV 34 ) in 0.1 M phosphate-buffered saline. Herein, we report our recent effort toward this direction in developing an iron phosphate−borate nanosheet array on carbon cloth (Fe−Pi−Bi/CC) via direct electrochemcial topotactic conversion from an iron phosphide nanoarray (FeP/CC) in 0.1 M KBi, with the use of FeP as both iron and phosphorus resources. As a 3D electrode, such an Fe−Pi−Bi/CC can drive a geometrical catalytic current density of 10 mA cm −2 at overpotentials of 434 and 383 mV in 0.1 and 0.5 M KBi, respectively. Remarkably, it also shows superior long-term electrochemical durability with 97.8% Faradaic efficiency (FE) for an oxygen evolution reaction (OER). A Fe−Pi−Bi nanoarray was developed via oxidative polar- ization of a FeP nanoarray on CC (see Figure S1 for preparation details). 35 The X-ray photoelectron spectroscopy (XPS) spectra for FeP are presented in Figure S2, and the peaks at 706.7 and 129.5 eV are close to the binding energies (BEs) for iron and phosphorus in FeP, respectively. 36 Figure 1 shows the XPS spectra in the Fe 2p, B 1s, P 2p, and O 1s regions for Fe−Pi−Bi. As shown in Figure 1a, the BEs of Fe 2p 1/2 and Fe 2p 3/2 appear at 724.6 and 711.2 eV, respectively, suggesting the formation of iron Received: December 31, 2016 Published: March 6, 2017 Communication pubs.acs.org/IC © 2017 American Chemical Society 3131 DOI: 10.1021/acs.inorgchem.6b03171 Inorg. Chem. 2017, 56, 3131−3135