Insights into the phenomenon of ‘bubble-free’ electrocatalytic oxygen evolution from water

Water electrolysis powered by renewable electricity will likely be critical to a future hydrogen economy. However, bubbles produced during water electrolysis add to capital and operational costs, due to blocking of the electrode surface and the necessary use of pumps and gas-liquid separators in electrolyser systems. Here ‘bubble-free’ electrocatalytic oxygen evolution from water was studied using a gas diffusion electrode that extracted oxygen as it was evolved, before bubbles were formed. The electrode comprised a gas-generating catalyst-binder layer, a poly(tetrafluoroethylene) (PTFE) interlayer, and a gas-extracting expanded PTFE (ePTFE) membrane (Gore-Tex®). Electrode properties between 20-60 °C, and with excess pressure on the alkaline electrolyte (≤80 kPa), were examined using detailed electrochemical measurements and video recordings. Electrochemical signal drift (dV/dt) was developed as a sensitive probe of bubble formation. ‘Breathability’, or the capacity for evolved oxygen to be directly extracted without forming bubbles, correlated with performance, and improved with temperature. A significant decline in oxygen evolution reaction (OER) activation energy with temperature was also observed. Bubble-free operation proceeded through the transportation of gas along a continuous network of hydrophobic, ‘aerophilic’ PTFE surfaces. The further development of bubble-free electrodes ought to permit revolutionary, low-cost electrolyser designs, adding impetus to the creation of a future hydrogen economy.