(721a) High-Efficiency Hydrogen Production By Decoupled Water Splitting in an Electrochemical - Chemical Cycle

As the human population grows, a transition towards clean and renewable energy sources becomes critical in curbing the negative effects of fossil fuel combustion. To facilitate this transition, renewable sources must be combined with energy storage and transportation infrastructures. Hydrogen production by water splitting has long been considered a promising solution to mitigating the intermittency issues of renewable energy, while also providing a high energy fuel for transportation. However, water splitting technologies still face challenges to improve their performance, efficiency and cost, especially when combined with renewable power sources. In conventional electrolyzer technologies, hydrogen is produced simultaneously with oxygen in the same cell, and a membrane or diaphragm is essential for product separation. Nevertheless, a dangerous mix of hydrogen and oxygen may still form under partial load conditions, which are inherent to renewable power source. Furthermore, the four-electron oxygen evolution reaction (OER) that takes place at the anode requires large overpotentials of over 400 mV, resulting in significant efficiency losses. Here, we present a strategy for high-efficiency decoupled water splitting, wherein the hydrogen and oxygen are produced in two separate steps in an electrochemical – thermally-activated chemical (E-TAC) cycle. The conventional OER catalyst anode is replaced by a cobalt-doped nickel hydroxide [Ni_xCo_1-x(OH)₂] anode; in the first step, hydrogen is produced at the cathode while the Ni_xCo_1-x(OH)₂ anode is oxidized to Ni_xCo_1-xOOH in an efficient one-electron reaction. This step is followed by a spontaneous chemical step that reduces the anode to its initial composition while oxidizing water and releasing oxygen under mild heating. This E-TAC process enables high-efficiency water splitting in a membrane-free cell without the risk of product crossover.