The formation, growth, and stability of oxygen bubbles inside the porous transport layer (PTL) pores of a polymer electrolyte membrane water electrolysis (PEMWE) cell have been modeled. In a PEMWE cell, O₂ bubbles form and grow inside the anode catalyst layer (ACL) and PTL pores. The most critical impact of the O₂ bubbles on the overall performance is the screening of the ACL surface. Semi-empirical relations for the current density-surface coverage for gas evolving electrodes have previously been developed [1] and were used for our PEMWE modeling [2]. This screening effect has been evaluated as an overpotential term, and the results were used to predict the effects of the catalyst and PTL properties on the electrolysis cell performance. The fluctuations in the cell current density were used to identify different types of bubbles and their detachment frequency from the anode electrode.

In this work, the anode catalyst surface has been described as a smooth, planar and horizontal plane. The PTL pores are described as identical straight hydrophilic cylinders with 1-mm height. Such a structure resembles tunable PTL structures such as those synthesized by Kang et al. [3]. Stable O₂ bubble nuclei form mainly at the intersection of the anode catalyst layer and the PTL pore wall [3,4].

Our modeling results show the effects of three different types of bubbles in a PEMWE cell: i) nucleation-driven, ii) drag-driven, and iii) buoyancy-driven bubbles. Figure 1 shows the relative lifetime and overpotential effects of the O₂ bubbles in a PEMWE with a PTL consisting of 11 μm dia. pores operated at a fixed temperature of 80°C and a balanced pressure of 1 bar [5]. Comparing the current density fluctuations in chronoamperometry experiments with the bubble detachment frequencies obtained through modeling provides a distribution of different types of bubbles in the electrolysis cell. These frequencies vary according to the PTL structure and the PEMWE cell operating conditions. Understanding of bubble growth and detachment mechanisms are of significant importance for reduction of mass transport-related losses and enhancement of PEMWE performance.

References:

1. H. Vogt and K. Stephan, Electrochim. Acta, 155, 348–356 (2015).
2. E. Tabu Ojong et al., Int. J. Hydrogen Energy, 42, 25831-25847 (2017).
3. Z. Kang et al., Energy Environ. Sci., 10, 166-175 (2017).
4. O. F. Selamet et al., Int. J. Hydrogen Energy, 38, 5823–5835 (2013).
5. A. Nouri-Khorasani, E. Tabu Ojong, T. Smolinka, and D. P. Wilkinson, Int. J. Hydrogen Energy, 42, 28665–28680 (2017).