Device engineering and mechanisms for electrocatalytic water splitting: conventional to decoupled electrolyzer

The drive for sustainable hydrogen production via water electrolysis demands the convergence of advanced electrocatalysts with systematically engineered electrochemical systems. This review critically examines the progression of electrochemical and photoelectrochemical (PEC) water-splitting technologies, emphasizing the transition from conventional coupled systems such as Alkaline water electrolysis (AWE), Proton Exchange Membrane (PEM), Anion Exchange Membrane (AEM), and PEC cells to emerging decoupled architectures incorporating redox mediators and solar integration. While coupled systems have advanced through innovations in catalyst design, zero-gap configurations, and modular stacks, they face scalability challenges due to gas crossover, stoichiometric imbalance in H2/O2 generation, and limited adaptability to intermittent renewables. To overcome these constraints, decoupled platforms utilizing reversible redox mediators, both liquid-phase and solid-state, offer spatial or temporal separation of HER and OER, enabling safer, membrane-free operation and enhanced system flexibility. This review comprehensively assesses engineering advancements across such architectures, including redox flow cells, solid-state mediator systems, wireless bipolar assemblies, and roll-to-roll shuttling designs. Integration of photovoltaics for unassisted, bias-free solar water splitting is also explored. From a system integration and reaction engineering standpoint, key strategies are discussed to optimize charge transfer kinetics, suppress parasitic side reactions, minimize gas crossover, and enhance long-term operational stability. By unifying insights across materials science, reactor architecture, and process intensification, this review outlines a roadmap for scalable, efficient, and grid-compatible hydrogen production technologies.