Quantification and deconvolution of gas diffusion and conversion losses in solid oxide single-cell testing

Concentration-related changes of the Nernstian voltage inevitably contribute to the resistance observed during DC operation of solid oxide cells. In electrochemical impedance spectroscopy (EIS), concentration losses arising from gas diffusion and gas conversion often overlap with electrochemical polarization processes, which is particularly true for high-capacitance mixed-ionic–electronic conducting electrodes, complicating the interpretation of impedance spectra and extracted activation energies. This work presents a comprehensive in-operando methodology to deconvolute gas diffusion and gas conversion resistances in electrolyte supported cells with screenprinted electrodes. Gas diffusion losses are quantified using an extended inert-diluent variation approach, enabling the determination of an effective microstructure parameter and prediction of diffusion resistances at arbitrary operating conditions. An alternative method based on replacing hydrogen/steam with carbon monoxide/carbon dioxide is introduced to access the same parameter, applicable to electrodes exhibiting a separable low-frequency concentration feature, such as nickel / yttria stabilized zirconia (Ni/YSZ). Furthermore, a novel approach for determining gas conversion resistances is presented, avoiding reliance on idealized analytical models, and enabling accurate prediction of concentration losses over a wide range of gas compositions and temperatures. The combined prediction of diffusion and conversion losses accurately reproduces the total concentration impedance over a broad steam partial pressure range and outperforms existing analytical approaches. The methodology is demonstrated on a commercial full cell, containing a nickel / gadolinia-doped ceria (Ni/GDC) fuel electrode, allowing reliable extraction of intrinsic activation energies even in the presence of increased, setup-induced concentration losses.