Abstract
The understanding of loss terms in polymer electrolyte water electrolysis (PEWE) cells is essential to maximize efficiency and minimize cost and enable the technology for energy applications, such as “power-to-X”. We adapt the use of the transmission-line model for porous electrodes in conjunction with electrochemical impedance spectroscopy measurements of the cell in the H2/N2 mode known for fuel cells to PEWE cells to quantify proton transport resistance and double layer capacitance of the anode catalyst layer. Anode catalyst loading was varied between 0.05 and 3.2 mgIrO2 cm−2. A non-linear relationship was found between the anode IrO2 loading and the proton transport resistance. Catalyst layers with very low IrO2 loading (0.05–0.16 mgIrO2 cm−2) had ∼4 times higher mass-specific activity and ∼2 times higher mass-specific capacitance, revealing an inhomogeneous utilization of the catalytic material. The kinetic and transport limitations associated with the anode catalyst layer have been correlated with its morphological features.
Highlights
The dynamic capabilities and wide current density operation range of polymer electrolyte water electrolysis (PEWE) make it attractive for converting renewable electricity from the grid to H2 in the context of “power-to-X”.1–6 PEWE is especially attractive in power grids with a high share of fluctuating renewables, where the H2 can be used for seasonal energy storage
In this study we present a detailed methodology for obtaining the proton transport resistance (RCHL+a) and the double layer capacitance of the catalyst layer (CLa) from electrochemical impedance spectra collected in the H2/N2 regime
Understanding the relationship between cell performance and the structural features of the anode catalyst layer in PEWE is necessary for tailoring next-generation catalyst layers for optimal transport properties and activity
Summary
RCHL+a/3 can be extracted from the Nyquist plots of H2/N2 operated cells by projecting the low f impedance vertically to the real axis (Figure 3).[22,32,33] An alternative approach is to determine CDL from Equation 3 and calculate RCHL+a.24-26 Graphical extrapolation of RCHL+a from the Nyquist plot relies on the presence of a vertical, capacitive impedance at low f, which is not always observed in the measured data.[22,33,36]. Graphical extrapolation resulted in higher values of RCHL+a in the case of very low and very high cells with 1.8 loadings, while both methods and 2.6 mgIrO2 cm−2. The measured RCHL+a decreases with higher cell temperatures due to the increased ionic conductivity of the ionomer in the CLa (13.0/10.3/9.1 mΩ cm[2] at 50/60/70oC for 0.8 mgIrO2 cm−2, respectively, based on data from Figure 3). The calculated value is in the range of values typically obtained for a fuel cell catalyst layer under fully humidified conditions at low ionomer to carbon ratio.[26] The reason for a large variation in ρHCL+a in the case of 0.05 mgIrO2 cm−2 is the strong deviation of the calculated RCHL+a from the decreasing trend with the IrO2 loading. It is important to note that RCHL+a is very sensitive to variations in the water content of the ionomer in the CLa.[22,24,25,26] Bernt et al.[11] have suggested that the water distribution might be different for thick and thin CLa, ηmtx (mV) RCH+La/3 (mOhm cm2)
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