Iron-nitrogen-carbon (Fe-N-C) is hitherto considered as one of the most satisfactory alternatives to platinum for the oxygen reduction reaction (ORR). Major efforts currently are devoted to the identification and maximization of carbon-enclosed FeN moieties, which act as catalytically active centers. However, fine-tuning of their intrinsic ORR activity remains a huge challenge. Herein, a twofold activity improvement of pristine Fe-N-C through introducing Ti C T MXene as a support is realized. A series of spectroscopy and magnetic measurements reveal that the marriage of FeN moiety and MXene can induce remarkable Fe 3d electron delocalization and spin-state transition of Fe(II) ions. The lower local electron density and higher spin state of the Fe(II) centers greatly favor the Fe electron transfer, and lead to an easier oxygen adsorption and reduction on active FeN sites, and thus an enhanced ORR activity. The optimized catalyst shows a two- and fivefold higher specific ORR activity than those of pristine catalyst and Pt/C, respectively, even exceeding most Fe-N-C catalysts ever reported. This work opens up a new pathway in the rational design of Fe-N-C catalysts, and reflects the critical influence of Fe 3d electron states in FeN moiety supported on MXene in ORR catalysis.

Proton adsorption on metallic catalysts is a prerequisite for efficient hydrogen evolution reaction (HER). However, tuning proton adsorption without perturbing metallicity remains a challenge. A Schottky catalyst based on metal-semiconductor junction principles is presented. With metallic MoB, the introduction of n-type semiconductive g-C N induces a vigorous charge transfer across the MoB/g-C N Schottky junction, and increases the local electron density in MoB surface, confirmed by multiple spectroscopic techniques. This Schottky catalyst exhibits a superior HER activity with a low Tafel slope of 46 mV dec and a high exchange current density of 17 μA cm , which is far better than that of pristine MoB. First-principle calculations reveal that the Schottky contact dramatically lowers the kinetic barriers of both proton adsorption and reduction coordinates, therefore benefiting surface hydrogen generation.

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Fine-tuning single-atom catalysts (SACs) to surpass their activity limit remains challenging at their atomic scale. Herein, we exploit p-type semiconducting character of SACs having a metal center coordinated to nitrogen donors (MeN x ) and rectify their local charge density by an n-type semiconductor support. With iron phthalocyanine (FePc) as a model SAC, introducing an n-type gallium monosulfide that features a low work function generates a space-charged region across the junction interface, and causes distortion of the FeN 4 moiety and spin-state transition in the Fe II center. This catalyst shows an over two-fold higher specific oxygen-reduction activity than that of pristine FePc. We further employ three other n-type metal chalcogenides of varying work function as supports, and discover a linear correlation between the activities of the supported FeN 4 and the rectification degrees, which clearly indicates that SACs can be continuously tuned by this rectification strategy.

Efficient catalysis of the methanol oxidation reaction (MOR) greatly determines the widespread implementation of direct methanol fuel cells. Exploring a suitable support for noble metal catalysts with regard to decreasing the mass loading and optimizing the MOR activity remains a key challenge. Herein, we achieve an over 60% activity enhancement of a palladium (Pd) catalyst by introducing a two-dimensional Ti3C2T x MXene as the support compared to a commercial Pd/C catalyst. Not only are more catalytically active Pd sites exposed on the Pd/MXene catalyst while maintaining a low mass loading, but the introduction of the MXene support also significantly alters the surface electronic structure of Pd. Specifically, spectroscopy and density functional theory (DFT) computations indicate that sufficiently electronegative terminations of the Ti3C2T x MXene surface can induce strong metal–support interactions (SMSI) with the Pd catalyst, leading to optimal methanol adsorption. This MXene-supported Pd catalyst exhibits a much higher MOR current density (12.4 mA cm–2) than that of commercial Pd/C (7.6 mA cm–2). Our work largely optimizes the intrinsic activity of a Pd catalyst by the utilization of MXene surface terminations, and the crucial SMSI effects revealed herein open a rational avenue to the design of more efficient noble metal catalysts for MOR.