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

Abstract

The family of the PtM (M represents transition metals such as Co, Ni, Pd, etc.) alloys is the most promising cathode electrocatalysts for proton exchange membrane fuel cells (PEMFCs) owing to their superior oxygen reduction reaction (ORR) activity to pure Pt. However, the activity gain fades with long-term PEMFC operation, and the degradation mechanism is not yet fully understood. To truly understand the degradation mechanism of the carbon supported PtM nanoparticles (PtM/C) in the cathode of a membrane electrode assembly (MEA) upon long-term PEMFC operation, it is essential to characterize the PEMFC-cycled electrode under working conditions. Herein, we showed that operando X-ray absorption spectroscopy (XAS) characterization of PtM/C electrocatalysts cycled in a PEMFC has inherent difficulties since Pt and especially M dissolve during PEMFC operation and migrate into the membrane; the bulk XAS spectrum is an average of the signals from the electrode and the membrane. Alternatively, we developed a method that allows for in situ XAS characterization on PEMFC-cycled PtM/C electrocatalysts. We justified the method by showing that the dissolved species in the membrane were separated from the PtM/C electrocatalyst in the cathode, and the in situ XAS signals arose exclusively from the electrocatalyst. Despite recent progress in the development of non-platinum electrocatalysts for the oxygen reduction reaction (ORR), carbon supported platinum-based nanoparticles (NPs) are still so far the only viable electrocatalysts for practical proton exchange membrane fuel cells (PEMFCs) in automotive vehicles, owing to their high activity and durability under the highly oxidative and corrosive conditions in the cathode of a PEMFC. The ORR activity of platinum can be significantly improved by alloying Pt with a wide range of transition metals (denoted as M) such as Co, Ni, Y, and the activity improvement has been attributed to the strain and/or ligand effects induced by M via optimization of the Pt-O binding energy and surface coordinate configuration. 1-5 However, the activity gain induced by M is generally not sustainable owing primarily to the dissolution of M during long-term PEMFC operation, which leads to the attenuation of strain/ligand effects, particle growth, and the loss of the favorable surface coordinate configuration. 6 Huang et al. 7 recently reported that both the activity and durability of the PtNi/C octahedral NPs can be markedly improved by doping with a transition metal such as Mo on the surface. However, neither the degradation mechanism of the PtM/C electrocatalysts during long-term PEMFC operation, nor the corresponding alleviation mechanism, is fully understood, which limits further improvements of the PtM/C electrocatalysts for PEMFCs.Elucidating the degradation mechanism of the PtM/C electrocatalysts during long-term PEMFC operation is technically challenging as it requires proper electrochemical testing methods and characterization methods, and a combination thereof. A regular way is to potentially cycle ...

Safeguarding the RuO2 phase against lattice oxygen oxidation during acidic water electrooxidation

“…S8, Table S2, ESI†). 23 Also, the intensity of Pt–Pt bond (black dashed line) in PtCoRu/C was lower than that of PtRu/C, suggesting a reduced coordination number of Pt owing to the formation of the PtCo alloy (Table S2, ESI†). 24 After the thermal annealing, the Pt–O bond in the PtCo–RuO 2 /C at near 1.5 Å was newly formed, while the Pt–RuO 2 /C had negligible peaks from Pt–O at the same region (Fig.…”

Investigating Degradation Mechanisms in PtCo Alloy Catalysts: The Role of Co Content and a Pt-Rich Shell Using Operando High-Energy Resolution Fluorescence Detection X-ray Absorption Spectroscopy

“…It has been reported that the high white line intensity of the Co K-edge (around 1.8) could be identified as the feature of Co 2+ inorganic salt. 56,57 The transformation of Co oxide or the Co 2+ salt (in this research, it was analyzed as CoSO 4 according to the experimental condition and LCA analysis) in MEA was reported to have unfavored the cell performance, 58 which was suggested as another factor that influenced the performance of the MEA. In addition, the results also indicate less Co oxide generation on CS nanostructure.…”

Highly Durable Pt-Based Core–Shell Catalysts with Metallic and Oxidized Co Species for Boosting the Oxygen Reduction Reaction

“…In their XAS studies, Hwang et al 59 suggested that the observed minor fraction of surface Co oxide species is very unstable in an acidic environment and immediately dissolves from the particle surface, resulting in a Pt-rich skin. In another in situ XAS study by Miller et al, 60 redeposited CoO was detected in the MEA membrane as a product of catalyst degradation/dissolution during electrochemical cycling. Wang et al discussed the presence of FeO x C y and CoO as an unfavored and ORR activity-reducing species for PtM 3 (M = Ni, Fe, and Co) nanoparticles characterized by the scanning transmission electron microscopy-electron energy loss spectroscopy technique.…”

“…In another in situ XAS study by Miller et al . , redeposited CoO was detected in the MEA membrane as a product of catalyst degradation/dissolution during electrochemical cycling. Wang et al discussed the presence of FeO x C y and CoO as an unfavored and ORR activity-reducing species for PtM 3 (M = Ni, Fe, and Co) nanoparticles characterized by the scanning transmission electron microscopy-electron energy loss spectroscopy technique .…”