A general mass transfer equation for gas-evolving electrodes

• Published in: International journal of hydrogen energy Vol. 74; pp. 283 - 296
• Main Author: Haverkort, J.W.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 12-07-2024
• Subjects: Electrocapillarity; Gas evolution; Marangoni flows; Mass transfer; Micromixing; Natural convection; Electrocapillarity; Natural convection; Marangoni flows; Micromixing; Gas evolution; Mass transfer
• ISSN: 0360-3199
• DOI: 10.1016/j.ijhydene.2024.06.010

Poor mass transport to or from vertical gas-evolving electrodes can adversely impact energy efficiency and product purity in the production of hydrogen, chlorine, and various metals. A proper description that combines natural convection with micromixing of growing, coalescing, and departing bubbles is presently lacking. This work develops a simple, physically sound analytical model that includes the influence of bubble size, flow regime, and bubble surface coverage. By comprehensively reviewing mass transfer measurements from the water electrolysis literature, we observe that the surface coverage of oxygen bubbles increases much more strongly with increasing current density than an often-used square root scaling predicts. Strong differences are observed in the degree of micromixing of hydrogen and oxygen bubbles in alkaline and acidic electrolytes. These varied results can all be explained by a combination of electrocapillarity, and coalescence induced by either a high surface coverage or Marangoni flows.
[Display omitted]
•A simple formula for adding micromixing and natural convection is derived.•A single micromixing parameter between 0 and 5 describes all literature data.•Micromixing is strongest for oxygen in alkaline and hydrogen in acidic electrolytes.•Marangoni forces are attractive in this case, causing coalescence and larger bubbles.•Bubble coverage increases with the square/square root of current for oxygen/hydrogen.