18 research outputs found
Electrocatalysis of the HER in acid and alkaline media
Trends in the HER are studied on selected metals (M= Cu, Ag, Au, Pt, Ru, Ir,
Ti) in acid and alkaline environments. We found that with the exception of
Pt, Ir and Au, due to high coverage by spectator species on non-noble metal
catalysts, experimentally established positions of Cu , Ag, Ru and Ti in the
observed volcano relations are still uncertain. We also found that while in
acidic solutions the M-Hupd binding energy most likely is controlling the
activity trends, the trends in activity in alkaline solutions are controlled
by a delicate balance between two descriptors: the M-Had interaction as well
as the energetics required to dissociate water molecules. The importance of
the second descriptor is confirmed by introducing bifunctional catalysts
such as M modified by Ni(OH); e.g. while the latter serves to enhance
catalytic decomposition of water, the metal sites are required for
collecting and recombining the produced hydrogen intermediates
Origin of Anomalous Activities for Electrocatalysts in Alkaline Electrolytes
Pt extended surfaces and nanoparticle electrodes are used to understand the origin of anomalous activities for electrocatalytic reactions in alkaline electrolytes as a function of cycling/time. Scanning tunneling microscopy (STM) of the surfaces before and after cycling in alkaline electrolytes was used to understand the morphology of the impurities and their impact on the catalytic sites. The nature of the contaminant species is identified as 3d-transition metal cations, and the formation of hydr(oxy)oxides of these elements is established as the main reason for the observed behavior. We find that, while for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) the blocking of the sites by the undesired 3d-transition metal hydr(oxy)oxide species leads to deactivation of the reaction activities, the CO oxidation reaction and the hydrogen evolution reaction (HER) can have beneficial effects from the same impurities, the latter being dependent on the exact nature of the adsorbing species. These results show the significance of impurities present in real electrolytes and their impact on electrocatalysis.Office of Science, Office of Basic Energy Sciences, Division of Materials Science, U.S. Department of Energy [DE-AC02-06CH11357]Office of Science, Office of Basic Energy Sciences, Division of Materials Science, U.S. Department of EnergyChemical Sciences and Engineering Division at Argonne National LaboratoryChemical Sciences and Engineering Division at Argonne National LaboratoryCAPESCAPESFAPESPFAPES
Origin of Anomalous Activities for Electrocatalysts in Alkaline Electrolytes
Pt extended surfaces and nanoparticle electrodes are
used to understand
the origin of anomalous activities for electrocatalytic reactions
in alkaline electrolytes as a function of cycling/time. Scanning tunneling
microscopy (STM) of the surfaces before and after cycling in alkaline
electrolytes was used to understand the morphology of the impurities
and their impact on the catalytic sites. The nature of the contaminant
species is identified as <i>3d</i>-transition metal cations,
and the formation of hydrÂ(oxy)Âoxides of these elements is established
as the main reason for the observed behavior. We find that, while
for the oxygen reduction reaction (ORR) and the hydrogen oxidation
reaction (HOR) the blocking of the sites by the undesired <i>3d</i>-transition metal hydrÂ(oxy)Âoxide species leads to deactivation
of the reaction activities, the CO oxidation reaction and the hydrogen
evolution reaction (HER) can have beneficial effects from the same
impurities, the latter being dependent on the exact nature of the
adsorbing species. These results show the significance of impurities
present in real electrolytes and their impact on electrocatalysis
In Situ Anomalous Small-Angle X‑ray Scattering Studies of Platinum Nanoparticle Fuel Cell Electrocatalyst Degradation
Polymer electrolyte fuel cells (PEFCs) are a promising
high-efficiency
energy conversion technology, but their cost-effective implementation,
especially for automotive power, has been hindered by degradation
of the electrochemically active surface area (ECA) of the Pt nanoparticle
electrocatalysts. While numerous studies using ex situ post-mortem
techniques have provided insight into the effect of operating conditions
on ECA loss, the governing mechanisms and underlying processes are
not fully understood. Toward the goal of elucidating the electrocatalyst
degradation mechanisms, we have followed Pt nanoparticle growth during
potential cycling of the electrocatalyst in an aqueous acidic environment
using in situ anomalous small-angle X-ray scattering (ASAXS). ASAXS
patterns were analyzed to obtain particle size distributions (PSDs)
of the Pt nanoparticle electrocatalysts at periodic intervals during
the potential cycling. Oxide coverages reached under the applied potential
cycling protocols were both calculated and determined experimentally.
Changes in the PSD, mean diameter, and geometric surface area identify
the mechanism behind Pt nanoparticle coarsening in an aqueous environment.
Over the first 80 potential cycles, the dominant Pt surface area loss
mechanism when cycling to 1.0–1.1 V was found to be preferential
dissolution or loss of the smallest particles with varying extents
of reprecipitation of the dissolved species onto existing particles,
resulting in particle growth, depending on potential profile. Correlation
of ASAXS-determined particle growth with both calculated and voltammetrically
determined oxide coverages demonstrates that the oxide coverage is
playing a key role in the dissolution process and in the corresponding
growth of the mean Pt nanoparticle size and loss of ECA. This understanding
potentially reduces the complex changes in PSD and ECA resulting from
various voltage profiles to a response dependent on oxide coverage