H2Bro – a Novel Decoupled Electrocatalytic-Catalytic Water Splitting Process for Green Hydrogen Production

Abstract

Green hydrogen produced by water splitting using renewable electricity is essential to achieve net-zero carbon emissions. Present water electrolysis technologies are uncompetitive with low-cost grey hydrogen produced from fossil fuels, limiting their scale-up potential. Disruptive processes that decouple the hydrogen and oxygen evolution reactions and produce them in separate cells or different stages emerge as a prospective route to reduce system cost by enabling operation without expensive membranes and sealing components.An elegant and innovative electrocatalytic-catalytic process was suggested for continuous high-current H2 production via membraneless decoupled water splitting (Schematic 1). H2 as the desired product is evolved on the cathode of the electrolytic compartment in near-neutral conditions, Br- is electrochemically oxidized on the anode side to Br2, which reacts in the bulk of the aqueous electrolyte to form bromate anions (with hypobromous acid and hypobromite anions as the intermediate products), whereas a catalytic transformation of the soluble redox couple (BrO3 -/Br-) mediates O2 evolution in the catalytic compartment.Bromate-mediated decoupled water splitting system offers decoupling of the H2 and O2 gases as a safety measure, a membraneless architecture as a cell design simplification, continuous operation mode at low temperature of 60 °C and less corrosive environment along with the potential use of abundant (electro)catalytic materials. The use of the redox mediator also allows bypassing the energetically unfavorable oxygen evolution reaction and, by doing so, decreasing the cell voltage.The concept of the new electrocatalytic-catalytic system was verified experimentally in batch-to-batch mode, resulting in long-term Faradaic efficiency of 98±2% and voltage efficiency of 62% at 1 A cm-2 already at its infancy stage [1]. Nevertheless, the merging of the electrolytic and catalytic parts into an integrated continuous flow system is still in process.In this work, a variety of possible system part designs were suggested and discussed, with an emphasis on integrating the electrolytic and catalytic parts into a continuous flow system. The first iteration of the laboratory scale experimental setup was designed in a continuous stirred reactor mode, using Pt as the cathode material for HER, DSA as the anode material for BER, and RuO2 Adams powder as the catalyst for bromate decomposition, and aiming for a high bromide utilization (BrO3 -/Br- ratio) level at the cell voltage of 1.6 V and the current density values above 0.2 A cm-2. The system was designed to automatically control the electrochemical parameters of the electrolytic process, to adjust the electrolyte flow rate through the electrolytic cell and the bromate/bromide mixture residence time in the catalytic cell, to control the H2 evolution rate, and to detect undesired O2 evolution (if any) in the electrolytic cell, and to optimize and maintain the temperature and pH values. A set of the key parameters affecting Faradaic efficiency, cell voltage and voltage efficiency are analyzed in detail.Some light was also shed on the development of a new type of solid-buffer PGM-free electrocatalysts for hydrogen evolution reaction (HER) in near-neutral conditions, as well as on deciphering the mechanism of bromate decomposition process and the development of novel PGM-free catalyst materials as more affordable alternatives for RuO2 in the catalytic cell.References Slobodkin, E. Davydova, M. Sananis, A. Breytus, A. Rothschild “Electrochemical and chemical cycle for high-efficiency decoupled water splitting in a near-neutral electrolyte", accepted for publication in Nature Materials Funded by the European Union (ERC), Project #101097966 “Continuous electrolytic-catalytic decoupled water electrolysis for green hydrogen production” (H2Bro). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them.” Figure 1