Abstract
AbstractDiscovering precious metal‐free electrocatalysts exhibiting high activity and stability toward both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Herein, a highly active OER catalyst, Fe0.3Ni0.7OX supported on oxygen‐functionalized multi‐walled carbon nanotubes, is substantially activated into a bifunctional ORR/OER catalyst by means of additional incorporation of MnOX. The carbon nanotube‐supported trimetallic (Mn‐Ni‐Fe) oxide catalyst achieves remarkably low ORR and OER overpotentials with a low reversible ORR/OER overvoltage of only 0.73 V, as well as selective reduction of O2 predominantly to OH−. It is shown by means of rotating disk electrode and rotating ring disk electrode voltammetry that the combination of earth‐abundant transition metal oxides leads to strong synergistic interactions modulating catalytic activity. The applicability of the prepared catalyst for reversible ORR/OER electrocatalysis is evaluated by means of a four‐electrode configuration cell assembly comprising an integrated two‐layer bifunctional ORR/OER electrode system with the individual layers dedicated for the ORR and the OER to prevent deactivation of the ORR activity as commonly observed in single‐layer bifunctional ORR/OER electrodes after OER polarization.
Highlights
Earth-abundant transition metal oxides evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems
Materials used for the latter are commonly known as bifunctional oxygen electrodes (BOEs), finding application in electrochemical devices which require the reversible conversion of water into oxygen and vice versa, such as rechargeable metal–air batteries and reversible fuel cells.[13]
This value is comparable to the benchmark OER-performance of iron-containing nickel oxide-based OER catalysts reported in the literature, including NiFeOX,[20] NiFeBX,[21] and NiFe layered double hydroxide (LDH).[22]
Summary
The MOX/MWCNTS-Ox catalysts were prepared following a previously described synthesis route.[14,15] This synthesis procedure offers a number of advantages, including the possibility for controlled preparation of a large number of MOX/MWCNTsOx-type samples, containing one or more earth-abundant transition metals at different metal loadings and ratios, by merely varying the composition of the precursor solutions. In the case of Mnn(Fe0.3Ni0.7)1−nOX/MWCNTs-Ox, the diversity of highly defective phases together with the narrow distribution of particle sizes achieved only upon combining the different transition metals could contribute to the observed enhancement of activity (Figure 2) and selectivity (Figure 3). These values, do not correlate with the observed ORR/OER activities, suggesting that the activity enhancement does not originate from surface effects. The observed discrepancies are all within the experimental error of the EDX technique preventing to make firm conclusions on the stability of the catalysts based exclusively on the SEM and EDX results
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