Ni–Co
oxides with varying Ni:Co ratios have been synthesized
following a facile sol–gel procedure. The influence of variation
in composition on the electrocatalytic activity of Ni–Co oxides
(Ni:Co = 1:1, 1:2, 1:3, and 1:4) for oxygen reduction reaction (ORR)
and oxygen evolution reaction (OER) is explored in high-pH media.
The lattice structure, size, morphology, and surface composition of
the synthesized nanocatalysts have been carefully verified by using
X-ray diffraction and various microscopic and spectroscopic techniques.
Among the synthesized catalysts, the Ni–Co oxide with Ni:Co
= 1:3 exhibits the best electrochemical performance with onset potentials
of 0.86 and 1.51 V for ORR and OER, respectively. Furthermore, it
demonstrates a small potential difference (0.82 V) between the potential
corresponding to 10 mA cm–2 current density for
OER and the half-wave potential for ORR. The catalyst exhibits admirable
catalytic capability, almost comparable to that of the benchmark Pt/C
catalyst and rivalling that of the IrO2/C catalyst for
OER. This work shed light on the development of a highly active non-precious
metal-based energy conversion catalyst and the effect of variation
of compositional ratios on the electro-catalytic performance, which
will further help rationally design efficient electrocatalysts for
sustainable energy applications.
One of the primary challenges in relation to phosphoric acid fuel cells is catalyst poisoning by phosphate anions that occurs at the interface between metal nanoparticles and the electrolyte. The strong adsorption of phosphate anions on the catalyst surface limits the active sites for the oxygen reduction reaction (ORR), significantly deteriorating fuel cell performance. Here, antipoisoning catalysts consisting of Pt-based nanoparticles encapsulated in an ultrathin carbon shell that can be used as a molecular sieve layer are rationally designed. The pore structure of the carbon shells is systematically regulated at the atomic level by high-temperature gas treatment, allowing O 2 molecules to selectively react on the active sites of the metal nanoparticles through the molecular sieves. Besides, the carbon shell, as a protective layer, effectively prevents metal dissolution from the catalyst during a long-term operation. Consequently, the defect-controlled carbon shell leads to outstanding ORR activity and durability of the hybrid catalyst even in phosphoric acid electrolytes.
The performance of polymer electrolyte membrane fuel cells (PEMFCs) is deteriorated by the occurrence of reverse current flow. To suppress the reverse current flow, a hydrogen oxidation reaction (HOR)‐selective catalyst based on the molecular sieve effect should be developed. Here, considering the carbon solubility in transition metals, carbon shell‐encapsulated metal alloy nanoparticles with superior HOR selectivity were fabricated using carbon source in metal precursors. In particular, we introduced carbon solubility as the important parameter that should be considered in the catalyst design. Using Pt and Co with different carbon solubility, as a model catalyst, the effect of the metal composition of the core material on the carbon shell structure was discussed in detail. Due to the formation of high‐density carbon shell and Pt‐rich surface on the alloy nanoparticle, the HOR selectivity was remarkably enhanced. Therefore, this study provides insights into the development of carbon shell‐encapsulated nanoparticles for reaction‐selective catalysis.
The development of non−Pt or carbon−based catalysts for anion exchange membrane fuel cells (AEMFCs) requires identification of the active sites of the catalyst. Since not only metals but also carbon materials exhibit oxygen reduction reaction (ORR) activity in alkaline conditions, the contribution of carbon-based materials to ORR performance should also be thoroughly analyzed. However, the conventional CN− poisoning experiments, which are mainly used to explain the main active site of M−N−C catalysts, are limited to only qualitative discussions, having the potential to make fundamental errors. Here, we report a modified electrochemical analysis to quantitatively investigate the contribution of the metal and carbon active sites to ORR currents at a fixed potential by sequentially performing chronoamperometry with two reaction inhibitors, CN− and benzyl trimethylammonium (BTMA+). As a result, we discover how to quantify the individual contributions of two active sites (Pt nanoparticles and carbon support) of carbon−supported Pt (Pt/C) nanoparticles as a model catalyst. This study is expected to provide important clues for the active site analysis of carbon-supported non−Pt catalysts, such as M−N−C catalysts composed of heterogeneous elements.
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