Role of Reaction Intermediate Diffusion on the Performance of Platinum Electrodes in Solid Acid Fuel Cells

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.

Impact of Pore Structure on Two‐Electron Oxygen Reduction Reaction in Nitrogen‐Doped Carbon Materials: Rotating Ring‐Disk Electrode vs. Flow Cell

“…The GDL and MPL ensure the transport of O 2 to reach the CL [26] . In the CL, the O 2 reacts in the gaseous form, and the ORR takes place at the triple‐phase boundary (TPB), a region where the gas, solid, and liquid contact [27] . Figure 5b shows ohmic‐resistance‐compensated linear sweep voltammograms (LSV) of the eight N/C‐ x % CO 2 catalysts.…”

“…[26] In the CL, the O 2 reacts in the gaseous form, and the ORR takes place at the triple-phase boundary (TPB), a region where the gas, solid, and liquid contact. [27] Figure 5b shows ohmicresistance-compensated linear sweep voltammograms (LSV) of the eight N/C-x% CO 2 catalysts. The FC test gives two-order Test conditions: 1 m KOH for anolyte and catholyte; 30 sccm O 2 is fed to the cathode; 1 mg cm À 2 of N/C catalyst for the cathode and 2 mg cm À 2 of IrO 2 /C for the anode; 5 mV s À 1 of scan rate; at room temperature.…”

Comparing Proton Conduction in Potassium and Ammonium Borosulfate—Isostructural Inorganic Polyelectrolytes Exhibiting High Proton Mobility

“…[ 2–4 ] Since then, the Haile group and others have continued to advance solid acid fuel cell (SAFC) technology. [ 5–13 ]…”

“…[2][3][4] Since then, the Haile group and others have continued to advance solid acid fuel cell (SAFC) technology. [5][6][7][8][9][10][11][12][13] Of the solid acid electrolytes reported over the past two decades, CsH 2 PO 4 has emerged as the material of choice for highperformance SAFCs. While CsH 2 PO 4 is functional, it possesses characteristics that complicate its use as an electrolyte.…”