Electrochemical Stability
An electrocatalyst must survive the potential and pH at which it operates, not only be active. Pourbaix diagrams map the thermodynamically stable phases of a material (solid, dissolved ions, oxides or hydroxides) as a function of potential and pH, and the Pourbaix decomposition energy measures how far a given compound lies above the most stable combination of phases under those conditions. A material with a small or zero decomposition energy at the operating point is thermodynamically resistant to dissolution, though kinetics can make a nominally unstable material last much longer. The same idea applied to adsorbate terminations gives surface Pourbaix diagrams, which identify the surface coverage under reaction conditions before activity is computed. Stability is especially limiting for the acidic oxygen evolution reaction, where few oxides other than IrO2 last long. See also Doped oxides and Transition metal oxide thermochemistry.
Stability-guided screening in WhereWulff (2023)
WhereWulff uses stability at two levels to decide what to compute (Sanspeur et al. 2023). Its bulk stage reports the energy above the hull and the Pourbaix decomposition free energy at a chosen pH and potential, and its surface stage builds a surface Pourbaix diagram over clean, OH* and O* monolayer terminations for each prioritized facet, then runs the OER intermediates only on the termination that is most stable at the requested conditions (for example, clean BaSrCo2O6-(001) between 1.23 and 1.6 V vs RHE in alkaline conditions). Summary: sanspeur2023.
Machine learning Pourbaix screen of rutile oxides for acidic OER (2024)
Abed and coworkers trained a crystal graph convolutional network on 36,465 Materials Project oxides to predict formation energy per atom as a proxy for Pourbaix decomposition energy, reaching a mean absolute error of 77 meV per atom from unrelaxed, lattice-scaled structures. They screened 2070 hypothetical rutile oxides and used computed metal-oxygen covalency as a second descriptor for bulk stability. Ti doping of RuO2 gave the largest covalency gain, and the synthesized Ru0.6Cr0.2Ti0.2Ox catalyst ran 200 h at 100 mA cm-2 in a PEM electrolyzer while dissolving Ru about 20 times more slowly than RuOx and suppressing lattice oxygen participation. The authors attribute this to a more rigid, covalent metal-oxygen framework that raises the barriers to dissolution. Surface Pourbaix diagrams showed O* termination as most stable at 1.60 V vs RHE for the (110) and (101) facets (Abed et al. 2024). Summary: abed2024.
Carbon support oxidation under OER conditions (2014)
Stability concerns extend beyond the active phase to the support. In the immobilized iron TAML OER electrodes of Demeter and coworkers, only about 45% of the current at 5 mA in 0.1 M HNO3 could be accounted for as O2, and GC showed CO2 as the other product. XPS before and after electrolysis showed loss of the carbon black or carbon paper carbon signal, a large rise in surface oxygen, and no change in the Nafion carbon peak or the iron signal, pointing to oxidation of the carbon support rather than the binder or the macrocyclic catalyst. The authors propose more oxidatively robust conducting supports as the next step (Demeter et al. 2014). Summary: demeter2014.