Supercapacitive Electrolyzer for Decoupled Hydrogen Production

Abstract

The production of green hydrogen is an energy vector and key component to tackle the decarbonization goals. Electrolysis of water for production of green hydrogen using renewable energy sources is an attractive approach, but the integration of electrolysis technologies with these sources is still facing challenges such as membrane degradation and gas crossover. Decoupled electrolysis is a promising approach to overcome these challenges. In this approach, the generation of hydrogen and oxygen can be separated in time or space, which allows for efficient and reliable integration with renewable energy sources. Symes et.al. developed a two-step electrochemical system that separates the water oxidation and hydrogen evolution reactions using an electron-coupled proton buffer. However, most decoupling designs still require the use of membranes for separation of redox mediator, which can be susceptible to degradation and have limited lifetimes. Researchers have introduced solid-state mediators to build membraneless electrolyzers using battery-like materials. The use of solid-state mediators eliminates the need for membranes and can provide improved performance and stability.In this work, we introduce a membraneless architecture decoupled hydrogen production in a hybrid cell combining the standard electrocatalytic reactions of water splitting with a capacitive storage mechanism. This innovative approach increases the flexibility and robustness of the hydrogen production system, allowing for operation in both acidic and alkaline conditions. The supercapacitive electrolyzer was assembled using the bifunctional catalyst cobalt-iron phosphide prepared by electrodeposition and a commercial activated carbon cloth as a capacitive electrode. The results of this work demonstrate the potential of membraneless electrolysis for green hydrogen production. The hybrid cell achieved an energy consumption of 48 kWh/kg at a nominal current density of 10 mA/cm2 in a 5 x 5 cm2 cell. Faradaic efficiencies over 99% were achieved at 100 mA/cm2. The cycle stability of the single cell was demonstrated over 20 h of operation (100 cycles) at a current density of 20 mA/cm2 with no apparent electrode degradation. Moreover, the simplicity of the cell design in this study not only facilitates quick scaling of production but also offers significant advantages in terms of operational cost and maintenance, as membraneless electrolyzers eliminate the need for expensive and complex components. To further enhance the performance of the hybrid cell for green hydrogen production, it is possible to reduce the ohmic contribution of the capacitive electrode and lower the overpotential required for the bifunctional catalyst to facilitate efficient water splitting.