Large-Scale, Low-Cost, and High-Efficiency Water-Splitting System for Clean H 2 Generation Yande Peng, †,‡ Kun Jiang, † Winfield Hill, † Zhiyi Lu, § Hongbin Yao, ‡ and Haotian Wang* ,†,∥ † Rowland Institute, Harvard University, Cambridge, Massachusetts 02142, United States ‡ Department of Applied Chemistry, University of Science and Technology of China, Hefei 230026, China § Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ∥ Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States * S Supporting Information ABSTRACT: Scaling up electrochemical water splitting is nowadays in high demand for hydrogen economy implementation. Tremendous efforts over the past decade have been focused on exploring alternative catalytic materials, including a variety of earth-abundant transition- metal-based catalysts, to replace traditional noble metals such as Pt, Ir, or Ru. Nevertheless, few efforts have been carried out for (1) scalable catalyst synthesis on current collectors and (2) practical device design toward large-scale H 2 generation. Herein, we designed a modular alkaline water-splitting electrolyzer system with scaled-up metal foam electrodes covered by low-cost NiMo alloy and Ni 3 Fe oxide for efficient hydrogen evolution and oxygen evolution, respectively. An electrolyte circulation system facilitates the mass transport and thus can further boost the H 2 generation particularly under large currents. As a result, the overall water-splitting performance of one-unit cell with a dimension of 10 × 10 cm 2 under room temperature presents an early onset voltage of 1.54 V and delivered practical currents of 20 and 55 A (9.1 and 25.0 L/h H 2 generation) under 2.2 and 2.9 V without iR compensations, respectively. This demonstration could stimulate new focuses in water splitting toward more practical applications. KEYWORDS: water splitting, hydrogen generation, large scale, mass transport, earth-abundant transition metal ■ INTRODUCTION With more and more solar panels installed on family houses as well as the fast implementation of solar or wind farms all over the world, a powerful energy storage system is urgently needed to overcome the daily and seasonal mismatch between the generation and usage of clean electricity. 1 Hydrogen (H 2 ), which can be generated via electrocatalytic water splitting (2H 2 O → 2H 2 +O 2 ; E 0 = 1.23 V) and converted back to electricity through fuel cell stacks, is becoming an increasingly attractive energy carrier for clean energy storage, 2,3 with important advantages over battery systems particularly in grid- scale applications: (1) Although in battery systems the three- dimensional (3D) volume of electrode materials needs to be linearly scaled up to accommodate the increased energy capacities, the produced H 2 gas can be accumulated in high- pressure tanks without host materials, dramatically reducing the energy storage cost; (2) excess H 2 gas produced by solar panels during summer can be continuously accumulated and stored for the use in winter, circumventing the self-discharging problems in batteries for long-term energy storage; (3) although stationary battery farms cannot be easily moved, clean H 2 produced from centralized solar or wind farms can be transported via pipelines or gas tanks to the end use in cities. More importantly, the pipeline distribution to gas stations would further facilitate the popularization of H 2 fuel cell vehicles. Therefore, the development of a large-scale water- splitting system, which is made of earth-abundant catalysts and delivers high-energy-conversion efficiencies, holds the key to the deployment of H 2 economy in the future. 4−7 Over the past decade, tremendous efforts in the research community have been focused on exploring alternative catalytic materials to replace traditional noble metal catalysts, such as Pt, Ir, and Ru, in both hydrogen evolution reaction (HER) 8 and oxygen evolution reaction (OER). 9,10 A variety of earth-abundant catalysts have been discovered to be promising candidates, including transition-metal (TM) chalcoge- nides, 11−13 metal phosphides, 14−16 and TM alloys 17−20 for HER in acids or bases as well as perovskite oxides, 21,22 TM oxides, 23,24 and layer double hydroxides 25,26 for alkaline OER. However, although many newly discovered catalysts have shown exciting performances, pairing different HER and OER catalysts in an integrated electrolyzer for practical H 2 Received: November 2, 2018 Accepted: January 3, 2019 Published: January 3, 2019 Research Article www.acsami.org Cite This: ACS Appl. Mater. Interfaces 2019, 11, 3971-3977 © 2019 American Chemical Society 3971 DOI: 10.1021/acsami.8b19251 ACS Appl. Mater. Interfaces 2019, 11, 3971−3977 Downloaded via HARVARD UNIV on February 11, 2019 at 15:00:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.