watersplitting

Watersplitting is the chemical process of decomposing water into hydrogen and oxygen, typically by supplying energy. The overall reaction is 2 H2O → 2 H2 + O2. In practical systems, water is oxidized at the anode and protons are reduced at the cathode, producing oxygen and hydrogen that are collected in separate gas streams.

The main approaches are electrolysis, photoelectrochemical splitting, and biological processes. Electrolysis uses electrical energy to drive the reaction and is implemented in alkaline, proton-exchange membrane (PEM), and solid oxide cells. These systems require catalysts, membranes or separators, and careful control of operating conditions to optimize gas purity and efficiency. Photoelectrochemical water splitting combines light absorption with electrochemistry, using semiconductor photoelectrodes to harvest solar energy directly for water splitting. Biological water splitting occurs in photosynthetic organisms, where water oxidation is carried out by photosystem II and, in some cases, hydrogen production is facilitated by hydrogenases.

Applications center on the production of hydrogen for energy storage, fuel cells, and industrial feedstocks. When powered by low-carbon electricity, water splitting can provide a pathway to renewable hydrogen and potentially low-emission energy storage. Key challenges include improving energy efficiency, reducing capital and operating costs, developing durable catalysts (often using precious metals), and integrating systems with variable renewable energy sources. Current electrolysis typically requires on the order of tens of kilowatt-hours of electricity per kilogram of hydrogen, with precise figures depending on technology and conditions.

Research directions focus on reducing costs, enhancing durability, increasing solar or grid integration, and advancing scalable, high-efficiency devices for grid and transportation applications.