Efficient Decoupled Alkaline Water Electrolysis Using a Low-Cost Sodium Manganate Solid Redox Mediator

Abstract

As an ideal energy carrier with clean, high efficiency and high energy density (142.351 kJ g-1), hydrogen is the most promising alternative to fossil fuels when it comes to decarbonizing the planet and energy supply. Today, however, the vast majority of hydrogen production occurs through fossil-fuel intensive processes which release vast amounts of CO₂ defeating the promise of decarbonizing the world. Among other hydrogen production processes, hydrogen production by electrolysis of water has broad application prospects as an alternative approach, and is very attractive for green energy development due to its advantages of cleanliness, environmental protection and zero emission.(1) Alkaline water electrolysis has dominated the current electrochemical hydrogen production industry for the past few decades, but issues such as gas product mixing and the use of renewable energy sources (e.g., solar, wind) remain inherent challenges.(2) To this end, in this paper, low-cost Na0.44MnO2 was chosen as a solid redox mediator to decouple the evolution of H2 and O2 in alkaline conditions, which can produce high-purity gas products directly in membrane-less electrolytic cells without gas purification. As an intermediate carrier for charge storage, the decoupling efficiency can reach 97% at a constant current of 2 mA/1.5 cm2. In addition, we performed the evolutionary reactions of hydrogen and oxygen separately under intermittent renewable energy conditions to achieve direct conversion of solar to hydrogen energy, which greatly enhances the flexibility of hydrogen production and further confirms the potential practical application of Na0.44MnO2. Figure 1. (a) CV curve of the Na0.44MnO2 electrode, and LSV curves of the Pt electrode for the HER and the RuO2/IrO2 electrode for the OER at a scan rate of 1 mV s-1 in 1M NaOH. (b) The charging/discharging curves of the Na0.44MnO2 electrode at different current densities. (c) Decoupled water electrolysis with Na0.44MnO2 electrode. (d) The stability performance of the separate H2/O2 generation at a current of 5 mA with a step time of 400 s. (e) Hydrogen purity test. Reference M. G. Schultz, T. Diehl, G. P. Brasseur and W. Zittel, science, 302, 624 (2003).M. Carmo, D. L. Fritz, J. Mergel and D. Stolten, International journal of hydrogen energy, 38, 4901 (2013).