It has historically proved challenging to accurately determine the overpotential due to bubble formation in water electrolysis. Vogt and colleagues examined the fraction of an electrode surface that was covered by gas bubbles during operation under different conditions. They found that bubble coverage increased with increasing current density and temperature in 3 M KOH electrolyte at ambient pressure in stagnant electrolyte. At above 1.0 A cm -2 , the bubble coverage became excessive, notably decreasing the active surface area of the electrode. Flowing liquid electrolyte over the electrode significantly decreased the bubble coverage with the fractional coverage dependent on the velocity of flow. In early 2019, Li et al. reported a 'breathable' oxygen-evolving anode that was prepared by sputter coating a thin gold layer onto a porous, hydrophobic polyethylene (PE) membrane, followed by electrodepositing a NiFeOx catalyst onto the gold layer. The 'breathable' nature of the electrode derived from the fact that the resulting NiFeOx/Au/PE electrode produced oxygen from water without producing visible gas bubbles at room temperature; the oxygen was drawn, as it was generated, through the porous PE membrane into a gas chamber at its rear. In so doing, the electrode exhibited a 90 mV lower overpotential at 0.01 A cm -2 than when a non-porous PE substrate was used. The non-porous PE substrate forced the formation of oxygen bubbles as no pathway existed to extract the gas from the NiFeOx catalyst layer and this is why it displayed a higher overpotential.