Ag-based electrocatalysts
are promising candidates to catalyze
the sluggish oxygen reduction reaction (ORR) in anion exchange membrane
fuel cells (AEMFC) and oxygen evolution reaction (OER) in unitized
regenerative fuel cells. However, to be competitive with existing
technologies, the AEMFC with Ag electrocatalyst must demonstrate superior
performance and long-term durability. The latter implies that the
catalyst must be stable, withstanding harsh oxidizing conditions.
Moreover, since Ag is typically supported by carbon, the strict stability
requirements extend to the whole Ag/C catalyst. In this work, Ag supported
on Vulcan carbon (Ag/VC) and mesoporous carbon (Ag/MC) materials is
synthesized, and their electrochemical stability is studied using
a family of complementary techniques. We first employ an online scanning
flow cell combined with inductively coupled plasma mass spectrometry
(SFC-ICP-MS) to estimate the kinetic dissolution stability window
of Ag. Strong correlations between voltammetric features and the dissolution
processes are discovered. Very high silver dissolution during the
OER renders this material impractical for regenerative fuel cell applications.
To address Ag stability during AEMFC load cycles, accelerated stress
tests (ASTs) in O2-saturated solutions are carried out
in rotating disk electrode (RDE) and rotating ring-disk electrode
(RRDE) setups. Besides tracking the ORR performance evolution, an ex situ long-term Ag dissolution study is performed. Moreover,
morphological changes in the catalyst/support are tracked by identical-location
transmission electron microscopy (RDE-IL-TEM). Voltammetry analysis
before and after AST reveals a smaller change in ORR activity for
Ag/MC, confirming its higher stability. RRDE results reveal a higher
increase in the H2O2 yield for Ag/VC after the
ASTs. The RDE-IL-TEM measurements demonstrate different degradation
processes that can explain the changes in the long term performance.
The results in this work point out that the stability of carbon-supported
Ag catalysts depends strongly on the morphology of the Ag nanoparticles,
which, in turn, can be tuned depending on the chosen carbon support
and synthesis method.