Stepwise strategies for overcoming limitations of membraneless electrolysis for direct seawater electrolysis

Abstract

Direct seawater electrolysis (DSWE) has been investigated as a candidate solution for hydrogen (H2) production using abundant seawater without the water purification requirements of conventional electrolyzers. Costly perfluorinated membranes employed in contemporary water electrolyzers (PEMWE) fail in the presence of trace contaminants due to cation exchange for H+ and scaling resulting from Mg2+/Ca2+ hydroxide precipitation caused by local pH increases. Herein, we disclose a membraneless electrolyzer and explore how symmetric and asymmetric electrolyte recirculation schemes impact pH gradients and performance of the system using in-house developed near-neutral pH electrocatalysts. Under the asymmetric recycle scheme, unbuffered seawater feed can be used whereby pH differentials caused by electrolysis are re-balanced by recirculating the liquid anode and cathode effluent streams to the cathode and anode inlet ports, respectively. Cell voltages of 2.61 and 3.50 V were required to attain 100 mA cm−2 of current density under buffered (0.5 M phosphate buffer; pH 6.71) and unbuffered synthetic seawater operation, respectively, and 50 h of chronoamperometric stability at the same current density was achieved for the former. A mere 0.18 % of H2 crossover from the catholyte to the anolyte at 250 mA cm−2 of current density was recorded at prolonged operation.