Detailed
understanding of the nature of the active centers in non-precious-metal-based
electrocatalyst, and their role in oxygen reduction reaction (ORR)
mechanistic pathways will have a profound effect on successful commercialization
of emission-free energy devices such as fuel cells. Recently, using
pyrolyzed model structures of iron porphyrins, we have demonstrated
that a covalent integration of the Fe–Nx sites into π-conjugated carbon basal plane modifies
electron donating/withdrawing capability of the carbonaceous ligand,
consequently improving ORR activity. Here, we employ a combination
of in situ X-ray spectroscopy and electrochemical
methods to identify the various structural and functional forms of
the active centers in non-heme Fe/N/C catalysts. Both methods corroboratively
confirm the single site 2e– × 2e– mechanism in alkaline media on the primary Fe2+–N4 centers and the dual-site 2e– × 2e– mechanism in acid media with the significant role
of the surface bound coexisting Fe/FexOy nanoparticles (NPs) as the secondary
active sites.
Development
and optimization of non-platinum group metal (non-PGM)
electrocatalysts for oxygen reduction reaction (ORR), consisting of
transition metal–nitrogen–carbon (M–N–C)
framework, is hindered by the partial understanding of the reaction
mechanisms and precise chemistry of the active site or sites. In this
study, we have analyzed more than 45 M–N–C electrocatalysts
synthesized from three different families of precursors, such as polymer-based,
macrocycles, and small organic molecules. Catalysts were electrochemically
tested and analyzed structurally using exactly the same protocol for
deriving structure-to-property relationships. We have identified possible
active sites participating in different ORR pathways: (1) metal-free
electrocatalysts support partial reduction of O2 to H2O2; (2) pyrrolic nitrogen acts as a site for partial
O2 reduction to H2O2; (3) pyridinic
nitrogen displays catalytic activity in reducing H2O2 to H2O; (4) Fe coordinated to N (Fe–N
x
) serves as an active site for four-electron
(4e–) direct reduction of O2 to H2O. The ratio of the amount of pyridinic and Fe–N
x
to the amount of pyrrolic nitrogen serves
as a rational design metric of M–N–C electrocatalytic
activity in oxygen reduction reaction occurring through the preferred
4e– reduction to H2O.
New non‐PGM catalysts from the family of Fe‐N‐C pyrolyzed materials are reported. They are synthesized using a templating silica powder with iron nitrate and carbendazim (CBDZ) precursors (sacrificial support method). The synthesis involves high temperature pyrolysis, followed by etching of the sacrificial support (silica) and obtaining a “self‐supported” open frame morphology catalyst. Both the temperature of heat treatment and Fe to CBDZ ratio play a crucial role in the final catalytic activity in oxygen reduction reaction (ORR). Prepared materials have extremely high durability in RDE tests, ending up with more than 94% of initial activity (by E1/2 value) after 10 000 cycles in an oxygen atmosphere, which is the result we report for the first time. Evaluation of these new M‐N‐C catalysts in a single membrane electrode assembly (MEA) has shown an exceptionally high open circuit voltage (OCV) of 1 V and the world's second best performance with no IR correction. MEA tests have shown high current density of 700 mA cm‐2 at 0.6 V and 120 mA cm‐2 at 0.8 V. In‐depth structure‐to‐property correlation presents an evidence that Fe‐Nx centers are the active sites playing a key role in oxygen reduction reaction.
Activities of Cu nanoparticles supported
on carbon black (VC),
single-wall carbon nanotubes (SWNTs), and Ketjen Black (KB) toward
CO2 electroreduction to hydrocarbons (CH4, C2H2, C2H4, and C2H6) are evaluated using a sealed rotating disk electrode
(RDE) setup coupled to a gas chromatograph (GC). Thin films of supported
Cu catalysts are deposited on RDE tips following a procedure well-established
in the fuel cell community. Lead (Pb) underpotential deposition (UPD)
is used to determine the electrochemical surface area (ECSA) of thin
films of 40 wt % Cu/VC, 20 wt % Cu/SWNT, 50 wt % Cu/KB, and commercial
20 wt % Cu/VC catalysts on glassy carbon electrodes. Faradaic efficiencies
of four carbon-supported Cu catalysts toward CO2 electroreduction
to hydrocarbons are compared to that of electrodeposited smooth Cu
films. For all the catalysts studied, the only hydrocarbons detected
by GC are CH4 and C2H4. The Cu nanoparticles
are found to be more active toward C2H4 generation
versus electrodeposited smooth copper films. For the supported Cu
nanocatalysts, the ratio of C2H4/CH4 Faradaic efficiencies is believed to be a function of particle size,
as higher ratios are observed for smaller Cu nanoparticles. This is
likely due to an increase in the fraction of under-coordinated sites,
such as corners, edges, and defects, as the nanoparticles become smaller.
embedded iron particles that are not directly involved in the oxygen reduction pathway. The high ORR activity and durability of catalysts involving this second site, as demonstrated in fuel cell, are attributed to the high densityof active sites and the elimination or reduction of Fenton-type processes. The latter are initiated byhydrogen peroxide but are known to be accelerated by iron ions exposed to the surface, resulting in the formation of damaging free-radicals.
Highly active self-supported PdxBi catalysts are synthesized by the sacrificial support method. Self-supported PdxBi catalysts have a porous nanostructured morphology with high surface areas (in the range from 75 to 100 m(2) g(-1)), making PdxBi a state-of-the-art catalyst. Pd4Bi displays the highest activity toward glycerol oxidation. In situ Fourier transform infrared spectroscopy highlights the unique catalytic behavior of self-supported PdxBi materials due to their particular structure and morphology. The confinement of reactants and intermediates in pores acting as nanoreactors is responsible for the high selectivity as a function of the electrode potential: aldehyde and ketone at low potentials, hydroxypyruvate at moderate potentials, and CO2 at high potentials. Moreover, the selectivity depends on the electrode history: it is different for the positive potential scan direction than for the reverse direction, where the catalyst becomes selective toward the production of carboxylates.
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