Electrocatalytic performance enhancement through coalescence-induced bubble dynamics: the case of the anode

Abstract

Electrolytically generated hydrogen and oxygen bubbles can substantially reduce the efficiency of (water) electrolysis by blocking active electrode area and increasing ohmic losses. There is therefore an interest in strategies to improve bubble removal, particularly under microgravity where buoyancy, which favors bubble departure under terrestrial conditions, is absent. Both in alkaline and acidic electrolytes, O₂ bubbles tend to remain at the electrode for longer, in part due to solutal Marangoni effects, and detach at significantly larger sizes compared to H₂ bubbles. As an alternative to buoyancy-driven removal, coalescence between a pair of bubbles can induce bubble detachment from the electrode and is therefore a highly attractive mechanism. Building on our previous work demonstrating that coalescence-induced H₂ bubble departure can enhance electrochemical performance using a dual Pt microelectrode system, we now extend this approach to the oxygen evolution reaction in HClO₄, KOH and H₂SO₄. By combining high-speed imaging with electrochemical measurements, we show that coalescence can trigger substantially earlier O₂ bubble departure as compared to buoyancy effects alone, reducing the anodic overpotential by up to ∼100-200 mV under galvanostatic conditions, which is equivalent to an enhancement of the reaction current by up to a factor of ≈ 2 under potentiostatic conditions. Earlier bubble departure also enables stable operation at substantially higher currents that would otherwise be limited by the bubbles blocking the electrode surface. At elevated currents, however, we find that coalescence between neighbouring bubbles becomes progressively inhibited and delayed, leading to larger detachment size and eventually preventing the formation of a single bubble beyond ≈ 1 mA. Finally, in contrast to previous observations for H₂ in H₂SO₄, our data shows that once departed, O₂ bubbles are unlikely to return to the electrode via repeated near-electrode coalescence. We attribute this contrast between O₂ and H₂ bubbles to the inherently lower O₂ gas production rate, which is half that of H₂ at a fixed current.