Electrochimica Acta 48 (2003) 4119–4125 Water electrolysis under microgravity Part 1. Experimental technique H. Matsushima a , T. Nishida a , Y. Konishi b , Y. Fukunaka a,∗ , Y. Ito a,1 , K. Kuribayashi c a Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan b Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana, IL 61801, USA c Institute of Space and Astronautical Science, Sagamihara 229-8510, Japan Received 13 March 2003; received in revised form 7 July 2003; accepted 9 July 2003 Abstract Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H 2 SO 4 ) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from –0.4 to -0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current–potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte. © 2003 Elsevier Ltd. All rights reserved. Keywords: Water electrolysis; Microgravity; Bubble evolution; Microconvection; Wettability 1. Introduction Gas bubble evolution in water electrolysis is a typical in- terfacial phenomenon. Many studies have been conducted under terrestrial gravity condition from the industrial point of view [1–5]. In a water electrolysis plant, the typical cell voltage of 1.8–2.0 V at an operating current density of 100–300 mA cm -2 is considerably higher than the equilib- rium voltage of 1.23V. This is due to high overpotentials at both anode and cathode [2], which are characteristic for gas evolution processes. Much research aimed at developing lower-overpotential electrodes with higher electrocatalytic capability is now in progress. ∗ Corresponding author. Tel.: +81-75-753-5415; fax: +81-75-753-4719. E-mail address: fukunaka@energy.kyoto-u.ac.jp (Y. Fukunaka). 1 ISE Member. In addition to electrocatalytic rate limitations, the pres- ence of the gas bubble on the electrode causes an increase in ohmic drops (IR drops, where I is current and R solution resistance) and this results in higher energy consumption, although micro- and macro-convection of electrolyte is ex- pected in the vicinity of a gas-evolving electrode. The rate of the electrochemical reaction is controlled by the interfacial phenomena in the three-phase zone where gas bubble, elec- trolyte and electrode surface contact each other. The macro- scopic natural convection due to buoyancy complicates the analysis of the ionic mass transfer rate due to the microcon- vection caused by gas bubble formation and growth. The microgravity environment thus provides an ideal environ- ment to examine the effect of microconvection on the elec- trochemical reaction in the three-phase zone in the absence of macroscopic convection (pumping action). A deeper un- derstanding of the electrochemical interfacial phenomena in a three-phase zone may contribute to a rational design strat- egy for the industrial water electrolysis cells. 0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00579-6