2.1. Outline of the Model
- Region A: the (volume-active) OER catalyst film (Cat)—with its proton activity, XCat, and electric overpotential, ΔV.
- Region B: a solvent layer (L) at the outer catalyst-film surface, the “buffer layer”—with its proton activity, XLayer.
- Region C: the bulk solvent (S)—with its proton concentration, cH (pH = −log cH).
2.2. Catalytic Electron Flux
2.3. Proton Flux Mediated by Buffer Molecules
2.4. Current–Voltage Relation of OER Catalysis with Proton-Transport Limitations
- For pH values ranging from about 2 to 12, the OER reaction essentially requires buffer molecules that facilitate proton transport. In the absence of an explicit buffer base, either only water molecules or hydroxide ions could serve as proton-transporting buffer bases; both cannot support significant current densities.
- In the presence of a buffer base, the maximal current density is determined by the concentration of unprotonated buffer molecules and, thus, by the solution pH and the pKa value of the used buffer, in line with experimental findings .
- At intermediate current densities, the Tafel slope is increased in comparison to the intrinsic Tafel slope measured at low current densities. The Tafel slope increase is avoided only when macroscopic mass transport limitations are negligible, which is achievable experimentally, e.g., by employing catalyst films deposited on rotating disc electrodes. This behavior has been observed repeatedly and is, at least, qualitatively well explained by the presented model. Whether quantitative agreement with the here described first-order model can be reached is still unclear; extension of the model to include also (e.g.,) limitations of proton transport within the catalyst or at the catalyst–solvent interface may be required.
- At intermediate current densities, the Tafel slopes reflect the buffering capacity of the BH/B− couple, a prediction that still requires experimental verification.
- The limitations by proton transport are assigned to proton transport in the electrolyte phase. This assumption predicts (macroscopic) acidification of the electrolyte near the electrode surface, a prediction that still awaits experimental verification. Additional factors may contribute to proton transport limitations, specifically rate limitations by proton transfer at the catalyst–electrolyte interface or within the catalyst film. In the present model, these are excluded by assuming equal proton activities within the catalyst film and the near-surface electrolyte.
Conflicts of Interest
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