Platinum-Alloy Cathode Catalyst Degradation in Proton Exchange Membrane Fuel Cells: Nanometer-Scale Compositional and Morphological Changes

Abstract

Electrochemical measurements showed an ≈75% Pt surface area loss and an ≈40% specific activity loss for a membrane electrode assembly (MEA) cathode with acid-treated “ normalPt3Co ” catalyst particles in a normalH2∕normalN2 proton exchange membrane fuel cell after 24h voltage cycling between 0.65 and 1.05V vs reversible hydrogen electrode. Transmission electron microscopy, scanning transmission electron microscopy, associated X-ray energy dispersive spectroscopy, and high angle annular dark-field techniques were used to probe the microstructural changes of the MEA cathode and the compositional changes along the MEA cathode thickness and within individual normalPtxCo nanoparticles before and after voltage cycling. Further Co dissolution from acid-treated normalPtxCo particles that leads to an increased thickness of a Pt-enriched surface layer and the development of core/shell normalPtxCo particles was largely responsible for the reduction in the specific activity of normalPtxCo nanoparticle after potential cycling. The Pt weight loss associated with the formation of Pt crystallites near the cathode/membrane interface largely contributed to the measured electrochemical surface area loss, while particle growth of the normalPtxCo particles via Ostwald ripening played a lesser role.

Actualizing In Situ X-ray Absorption Spectroscopy Characterization of PEMFC-Cycled Pt-Electrodes

“…Thus, the dealloying process with potential cycling is unambiguously confirmed from both Pt and Co perspectives. This phenomenon has been previously reported by Chen et al 25 on the basis of microscopic characterizations on the PtCo/C electrocatalyst operated in MEAs, and even on Pt/C electrocatalysts without the second transition metal. 26 Large crystalline Pt and Co 2+ species were found in the ionomer phase in both the membrane and the electrode, as a consequence of the PtCo/C dealloying followed by dissolution.…”

“…As shown in Figure 5, no FT-peaks arisen from the Ni-O scattering around 1.5 Å were presented in the FT-EXAFS spectra at all three cycling stages, and the spectra can be well fitted with two Ni-Ni and Ni-Pt scattering vectors (the right side of Figure 5 and Table II). In addition, the absence of the Pt-O scattering at 0.54 V revealed by the fitting indicates that the amount of the Pt trapped in the ionomer, which is in the form of oxides, 25 is insignificant compared to the Pt in the GDL.…”

“…The lack of the long range order of the CoO x , which is evidenced by the absence of the Co-Co scattering around 2.5 Å and any peaks further featured by CoO, indicates that the CoOx in the membrane and/or ionomer is highly amorphous. In addition to Co, significant amount of Pt is also dissolved upon long-term PEMFC operation and migrates into the membrane forming large single crystalline Pt, 25,26 and its XAS signal cannot be separated from the XAS signals from the cathode electrode. The dissolved species in the membrane and the ionomer are electrically disconnected and thus electrochemically inactive.…”

“…The dissolved species in the membrane and the ionomer are electrically disconnected and thus electrochemically inactive. 25 Therefore, operando XAS characterization on MEAs is seriously limited by this issue throughout the whole potential cycling stage with the aim of providing the structural and electronic properties of the electrocatalyst under working conditions.…”

Exploring the phase space of time of flight mass selected PtxY nanoparticles

“…[32][33][34][35][36][37][38] However, in the harsh acidic and oxidising environment of a fuel cell, these alloys degrade by dealloying. [39][40][41] In our laboratory we have taken a different approach, namely to study alloys of Pt and rare earths such as Y, Gd, Ce and La. 33,[42][43][44][45][46][47][48] These alloys have a particularly negative heat of formation, which should provide them with long term-kinetic stability against dealloying at the cathode of a fuel cell.…”