| 1. |
- Liu, Si, et al.
(författare)
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Electrocatalytic Water Oxidation at Neutral pH–Deciphering the Rate Constraints for an Amorphous Cobalt‐Phosphate Catalyst System
- 2022
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Ingår i: Advanced Energy Materials. - : Wiley-VCH Verlagsgesellschaft. - 1614-6832 .- 1614-6840. ; 12:46
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Tidskriftsartikel (refereegranskat)abstract
- The oxygen evolution reaction (OER) is pivotal in sustainable fuel production. Neutral-pH OER reduces operational risks and enables direct coupling to electrochemical CO2 reduction, but typically is hampered by low current densities. Here, the rate limitations in neutral-pH OER are clarified. Using cobalt-based catalyst films and phosphate ions as essential electrolyte bases, current–potential curves are recorded and simulated. Operando X-ray spectroscopy shows the potential-dependent structural changes independent of the electrolyte phosphate concentration. Operando Raman spectroscopy uncovers electrolyte acidification at a micrometer distance from the catalyst surface, limiting the Tafel slope regime to low current densities. The electrolyte proton transport is facilitated by diffusion of either phosphate ions (base pathway) or H3O+ ions (water pathway). The water pathway is not associated with an absolute current limit but is energetically inefficient due to the Tafel-slope increase by 60 mV dec−1, shown by an uncomplicated mathematical model. The base pathway is a specific requirement in neutral-pH OER and can support high current densities, but only with accelerated buffer-base diffusion. Catalyst internal phosphate diffusion or other internal transport mechanisms do not limit the current densities. A proof-of-principle experiment shows that current densities exceeding 1 A cm−2 can also be achieved in neutral-pH OER.
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| 2. |
- Pasquini, Chiara, et al.
(författare)
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Operando tracking of oxidation-state changes by coupling electrochemistry with time-resolved X-ray absorption spectroscopy demonstrated for water oxidation by a cobalt-based catalyst film
- 2021
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Ingår i: Analytical and Bioanalytical Chemistry. - : Springer Nature. - 1618-2642 .- 1618-2650. ; 413:21, s. 5395-5408
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels. The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle.
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| 3. |
- Abrashev, Miroslav V., et al.
(författare)
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Origin of the heat-induced improvement of catalytic activity and stability of MnOx electrocatalysts for water oxidation
- 2019
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Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 7:28, s. 17022-17036
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Tidskriftsartikel (refereegranskat)abstract
- Catalysis of the oxygen evolution reaction (OER) by earth-abundant materials in the near-neutral pH regime is of great interest as it is the key reaction for non-fossil fuel production. To address the pertinent stability problems and insufficiently understood structure-activity relations, we investigate the influence of moderate annealing (100-300 degrees C for 20 min) for two types of electrodeposited Mn oxide films with contrasting properties. Upon annealing, the originally inactive and structurally well-ordered Oxide 1 of birnessite type became as OER active as the non-heated Oxide 2, which has a highly disordered atomic structure. Oxide 2 also improved its activity upon heating, but more important is the stability improvement: the operation time increased by about two orders of magnitude (in 0.1 M KPi at pH 7). Aiming at atomistic understanding, electrochemical methods including quantitative analysis of impedance spectra, X-ray spectroscopy (XANES and EXAFS), and adapted optical spectroscopies (infrared, UV-vis and Raman) identified structure-reactivity relations. Oxide structures featuring both di-mu-oxo bridged Mn ions and (close to) linear mono-mu-oxo Mn3+-O-Mn4+ connectivity seem to be a prerequisite for OER activity. The latter motif likely stabilizes Mn3+ ions at higher potentials and promotes electron/hole hopping, a feature related to electrical conductivity and reflected in the strongly accelerated rates of Mn oxidation and O-2 formation. Poor charge mobility, which may result from a low level of Mn3+ ions at high potentials, likely promotes inactivation after prolonged operation. Oxide structures related to the perovskite-like zeta-Mn2O3 were formed after the heating of Oxide 2 and could favour stabilization of Mn ions in oxidation states lower than +4. This rare phase was previously found only at high pressure (20 GPa) and temperature (1200 degrees C) and this is the first report where it was stable under ambient conditions.
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