Understanding the Stability of Manganese Chromium Antimonate Electrocatalysts through Multimodal In Situ and Operando Measurements

Improving electrocatalyst stability is critical for the
development
of electrocatalytic devices. Herein, we utilize an on-line electrochemical
flow cell coupled with an inductively coupled plasma-mass spectrometer
(ICP-MS) to characterize the impact of composition and reactant gas
on the multielement dissolution of Mn(−Cr)–Sb–O
electrocatalysts. Compared to Mn2O3 and Cr2O3 oxides, the antimonate framework stabilizes
Mn at OER potentials and Cr at both ORR and OER potentials. Furthermore,
dissolution of Mn and Cr from Mn(−Cr) −Sb–O is
driven by the ORR reaction rate, with minimal dissolution under N2. We observe preferential dissolution of Cr totaling 13% over
10 min at 0.3, 0.6, and 0.9 V vs RHE, with only 1.5% loss of Mn, indicating
an enrichment of Mn at the surface of the particles. Despite this
asymmetric dissolution, operando X-ray absorption spectroscopy (XAS)
showed no measurable changes in the Mn K-edge at comparable potentials.
This could suggest that modification to the Mn oxidation state and/or
phase in the surface layer is too small or that the layer is too thin
to be measured with the bulk XAS measurement. Lastly, on-line ICP-MS
was used to assess the effects of applied potential, scan rate, and
current on Mn–Cr–Sb–O during cyclic voltammetry
and accelerated stress tests. With this deeper understanding of the
interplay between oxygen reduction and dissolution, testing procedures
were identified to maximize both activity and stability. This work
highlights the use of multimodal in situ characterization techniques
in tandem to build a more complete model of stability and develop
protocols for optimizing catalyst performance.