Advanced impedance-based diagnostics are increasingly important for understanding electrochemical systems where multiple processes overlap across similar timescales. Among these methods, Distribution of Relaxation Times (DRT) analysis has emerged as a particularly powerful approach because it avoids assumed equivalent circuits and instead resolves electrochemical behaviour directly in the timescale domain. This capability makes DRT especially valuable for systems with poorly characterised interfaces, while remaining complementary to differential and conventional impedance analyses.
Molten carbonate electrolysis (MCE) is a high-temperature electrochemical platform attracting growing interest as a low-carbon route for producing critical materials such as iron1, silicon2, and carbon-based products3. Despite promising demonstrations, mechanistic understanding of interfacial reactions, gas-dependent behaviour, and surface chemistry in MCE remains limited. Impedance spectroscopy is well suited to probing these processes, yet advanced analysis techniques such as DRT have seen almost no application in molten carbonate systems and have never been applied to MCE.
This work presents the first systematic application of DRT to molten carbonate electrolysis, using a dual regression–classification DRT framework4. Graphite oxidation in Li–Na–K eutectic carbonate at 600 °C was selected as a benchmark system due to its multistep reaction pathway and chemical stability5.
DRT resolves up to five distinct relaxation processes that are not discernible from raw impedance spectra alone. These contributions evolve with potential, temperature, and immersion time, highlighting sensitivity to melt wetting and interfacial activation. Most notably, DRT reveals a clear mechanistic divergence under CO2, where adsorption-related features collapse and transport and charge-transfer resistances decrease. These signatures are consistent with a CO2-driven reaction pathway, potentially involving a pyrocarbonate species6, and provide the first impedance-based evidence for such behaviour in MCE.
Overall, this study demonstrates that DRT offers a powerful framework for resolving interfacial mechanisms in high-temperature molten salt electrochemistry and guiding the development of next-generation MCE technologies.