For a broader commercialization of polymer electrolyte water electrolysis (PEWE), it is essential to gain a fundamental understanding of the different components and related losses to increase the overall system efficiency to further decrease the operating cost. Mass transport resistances are the least understood loss category. The use of water feed in the vapor state was utilized to gain insight in the origin of mass transport losses. Vapor feed amplifies governing transport parameters. Limiting current densities were determined and mass transport resistances calculated as a function of temperature (60°C-80°C) and relative gas humidity (60% to >100% rH i.e. oversaturation). The studied configurations with symmetric and asymmetric cell humidification exemplify the major impact of water drag and diffusion on the performance. In addition, a relation between mass transport limitation and high frequency resistance (HFR) measured by electrochemical impedance spectroscopy was established.

The overall mass transport resistance is composed of the sub-resistances of the catalyst layer and the PTL. Partial pressure and PTL type variation were used to isolate these main resistances. The catalyst layer was identified to be the governing resistance in the system such that gas feed electrolysis features two order of magnitude lower current densities in comparison to liquid water electrolysis [1] (see also Figure 1). In the diffusion limited regime of the gas feed cells the limits of liquid water electrolysis are emphasized. The origin of the significant liquid/vapor feed differences was investigated by using different PTL types and IrO₂ loadings. The required CCMs were produced by an automated spray coating process where IrO₂ loading was varied between 1.0 mg/cm² and 3.0 mg/cm².

references

[1] J.M. Spurgeon, N.S. Lewis, Proton exchange membrane electrolysis sustained by water vapor, Energy & Environmental Science, 4 (2011) 2993-2998.