John Stuart Mill, Socialist

Abstract

Hydrogen gas is a net zero carbon emission clean fuel with an unmatched high specific energy. Water electrolysis is an important alternative method to produce hydrogen to the traditional fossil hydrocarbon reforming in industry. The main challenges of water electrolysis are the high energy consumption (ca. 5 kWh m − 3 (H 2 ) at 80 ℃) and, if accidentally formed, the explosive nature of any unintended mixing of the produced hydrogen and oxygen gases. In order to solve these problems, alternate water electrolysis has been developed by, for example, decoupling of the hydrogen evolution reaction (HER) from the oxygen evolution reaction (OER) in space or time. This critical review intends to introduce the concept and recent developments of alternate water electrolysis in different schemes, including the alternate thermolysis and electrolysis of water, the alternate water electrolysis by using a liquid or solid redox intermedium and the alternate half-electrolysis of water. All the alternate water electrolysis methods solve the gas mixing problem whilst half-electrolysis and those with a solid redox medium omit the membranes. Specifically, only the alternate half-electrolysis of water can save the energy consumption without compromising the operation life and production rate.