Quantification of water transport in a CO2 electrolyzer

Abstract

A sufficient supply of water at the catalyst layer is needed to mediate the CO₂ reduction reaction (CO₂RR), yet too much water favours the competing hydrogen evolution reaction (HER) and lowers the efficiency of CO₂ electrolysis. It is therefore important to quantify water at the cathode, but CO₂ electrolyzers are typically enclosed systems where only the inputs and outputs can be readily measured. We report herein an analytical CO₂RR electrolyzer with sensors in the cathode chamber to directly measure the relative humidity (RH) and temperature during electrolysis. These measurements enable the flow of water to be tracked inside the flow cell, and provide the boundary conditions necessary for a 3D model of mass transport and fluid flow in the cathode chamber. We developed a 3D model which showed that the molar fraction of water at the cathodic GDE/membrane interface, x_H2O,mem, is constant under a range of applied current densities (25–200 mA cm⁻²) and CO₂ flow rates (25–200 sccm), and does not change as a function of humidification of the CO₂ feed. The value of x_H2O,mem is held at parity because more water is drawn across the membrane (from the liquid-fed anode) when insufficient water is supplied to the cathode from the CO₂ feed. This result points to a higher flux of water across the membrane when using drier CO₂ feedstocks at higher flow rates and higher current densities. Consequently, undesirable anolyte crossover (which causes salt precipitation at the cathode) can be suppressed by maintaining a humidified CO₂ feed. This 3D model can be used for a range of operating conditions, materials, and flow fields to help with CO₂RR electrolyzer design and optimization.