Abstract
Understanding the reaction pathways for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) is the key to design electrodes for solid acid fuel cells (SAFCs). In general, electrochemical reactions of a fuel cell are considered to occur at the triple-phase boundary where an electrocatalyst, electrolyte and gas phase are in contact. In this concept, diffusion processes of reaction intermediates from the catalyst to the electrolyte remain unconsidered. Here, we unravel the reaction pathways for open-structured Pt electrodes with various electrode thicknesses from 15 to 240 nm. These electrodes are characterized by a triple-phase boundary length and a thickness-depending double-phase boundary area. We reveal that the double-phase boundary is the active catalytic interface for the HOR. For Pt layers ≤ 60 nm, the HOR rate is rate-limited by the processes at the gas/catalyst and/or the catalyst/electrolyte interface while the hydrogen surface diffusion step is fast. For thicker layers (>60 nm), the diffusion of reaction intermediates on the surface of Pt becomes the limiting process. For the ORR, the predominant reaction pathway is via the triple-phase boundary. The double-phase boundary contributes additionally with a diffusion length of a few nanometers. Based on our results, we propose that the molecular reaction mechanism at the electrode interfaces based upon the triple-phase boundary concept may need to be extended to an effective area near the triple-phase boundary length to include all catalytically relevant diffusion processes of the reaction intermediates.
Highlights
For the decarbonization of the energy sector, fuel cells are already a key technology in the transition towards the increasing use of renewable energy sources
The deposition methods included metal-precursor impregnation, metal-organic chemical vapor deposition (MOCVD), atomic layer deposition (ALD) and direct current magnetron sputtering, and these methods were utilized on the current collector as well as the electrolyte particles
The reaction pathways for the hydrogen oxidation reaction (HOR) proceed exclusively via the double-phase boundary
Summary
For the decarbonization of the energy sector, fuel cells are already a key technology in the transition towards the increasing use of renewable energy sources. Fuel cells can convert chemical into electrical energy without producing greenhouse gases. Their construction consists of an electrolyte that is sandwiched between catalyst-coated gas diffusion electrodes. Intermediate temperature fuel cells are of growing interest [1,2,3]. One representative of this class is the solid acid fuel cell (SAFC) which utilizes a proton-conducting solid acid electrolyte, such as CsH2 PO4 , at an operating temperature near 520 K [4,5]. Pt composite electrodes have demonstrated power densities of 415 mW cm−2 and performance degradation of just
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