Among them, the nitrogen-coordinated transition-metal (TM) single-atoms (SAs) supported on carbon substrates have emerged as a new class of ORR electrocatalysts with enormous potentials. [3-5] These SA electrocatalysts (SAECs) anchor TM-SAs to the carbon substrates via TMnitrogen (TMN x) coordination bonds that also act as the ORR active sites. It has been commonly accepted that the ORR activity of such TMN x-coordinated SA sites can be promoted by optimizing the binding strengths of ORR intermediates (e.g., *O 2 , *OOH, *OH, *O) to the active site via the altering of their electronic structures. [6] Various approaches have been reported to alter the electronic structures of TMN x-coordinated SA sites by modulating N types and coordinating numbers, [7] partially replacing N with other nonmetal elements (e.g., O, S, and P), [8] or the chemical compositions of carbon substrates. [9] Recently, the hetero-SAs (h-SAs) involving two different TMs (e.g., Co/Zn, Fe/Co, Fe/ Zn) have been successfully anchored to the carbon substrates as ORR SAECs. [10] Such an approach takes the advantage of the coexistence of two different TM-SA sites, through the pairing The development of oxygen reduction reaction (ORR) electrocatalysts based on earth-abundant nonprecious materials is critically important for sustainable large-scale applications of fuel cells and metal-air batteries. Herein, a hetero-single-atom (h-SA) ORR electrocatalyst is presented, which has atomically dispersed Fe and Ni coanchored to a microsized nitrogen-doped graphitic carbon support with unique trimodal-porous structure configured by highly ordered macropores interconnected through mesopores. Extended X-ray absorption fine structure spectra confirm that Fe-and Ni-SAs are affixed to the carbon support via FeN 4 and NiN 4 coordination bonds. The resultant Fe/Ni h-SA electrocatalyst exhibits an outstanding ORR activity, outperforming SA electrocatalysts with only Fe-or Ni-SAs, and the benchmark Pt/C. The obtained experimental results indicate that the achieved outstanding ORR performance results from the synergetic enhancement induced by the coexisting FeN 4 and NiN 4 sites, and the superior mass-transfer capability promoted by the trimodal-porous-structured carbon support. The development of oxygen reduction reaction (ORR) electrocatalysts based on earth-abundant nonprecious materials to replace the scarce platinum-group-metal-based ones is critically important for sustainable large-scale commercial applications of fuel cells and metal-air batteries. [1] The extensive research efforts over the recent years have led to a variety of The ORCID identification number(s) for the author(s) of this article can be found under

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The review introduces the mechanisms of heterogeneous hydrogen evolution (HER) and oxygen evolution reactions (OER), summarizes in-situ characterization techniques and surveys strategies to boost the activities of metal oxide electrocatalysts.

Although various chemicals and fuels have been successfully synthesized via the electrocatalytic CO 2 reduction reaction (CO 2 RR), the selective reduction of CO 2 to CO is widely recognized as one of the most economically viable reactions due to its simplicity and needing the least electrons transfer to form the product. [2] Among the reported CO 2 RR electrocatalysts, metallic Ag ones have demonstrated high catalytic selectivity toward CO, however, requiring high cathodic potentials (e.g., ≥0.9 V vs RHE). [3] Various approaches have been attempted to improve the CO 2 RR performance of Ag electrocatalysts. For instance, Kim and co-workers immobilized small Ag NPs (≈5 nm) on carbon support to achieve a high faradic efficiency (FE CO ) of 84.4% at −0.75 V versus RHE. [4] Luo and co-workers reported the use of the dominant Ag (100) facet exposed at the active edge of the triangular Ag nanoplates to attain a high FE CO of 96.8% at −0.855 V versus RHE. [5] Recently, the same team investigated the structure sensitivity of Ag nanocubes toward CO 2 RR and unveiled that the Ag nanocubes enclosed by {100} facets with the edge lengths below 25 nm can achieve a superb FE CO of 99.0% at −0.856 V versus RHE. [6] They attributed the superb FE CO to the increased percentage of edge active sites. These approaches utilize the edge active sites of Ag nanocrystals to improve CO 2 RR performance, however, such highly active edge sites areThe electrocatalytic CO 2 RR to produce value-added chemicals and fuels has been recognized as a promising means to reduce the reliance on fossil resources; it is, however, hindered due to the lack of high-performance electrocatalysts. The effectiveness of sculpturing metal/metal oxides (MMO) heterostructures to enhance electrocatalytic performance toward CO 2 RR has been well documented, nonetheless, the precise synergistic mechanism of MMO remains elusive. Herein, an in operando electrochemically synthesized Cr 2 O 3 -Ag heterostructure electrocatalyst (Cr 2 O 3 @Ag) is reported for efficient electrocatalytic reduction of CO 2 to CO. The obtained Cr 2 O 3 @Ag can readily achieve a superb FE CO of 99.6% at −0.8 V (vs RHE) with a high J CO of 19.0 mA cm −2 . These studies also confirm that the operando synthesized Cr 2 O 3 @Ag possesses high operational stability. Notably, operando Raman spectroscopy studies reveal that the markedly enhanced performance is attributable to the synergistic Cr 2 O 3 -Ag heterostructure induced stabilization of CO 2 •−/*COOH intermediates. DFT calculations unveil that the metallic-Ag-catalyzed CO 2 reduction to CO requires a 1.45 eV energy input to proceed, which is 0.93 eV higher than that of the MMO-structured Cr 2 O 3 @Ag. The exemplified approaches in this work would be adoptable for design and development of high-performance electrocatalysts for other important reactions.

A new strategy has been innovatively proposed for wrapping the Ni-incorporated and N-doped carbon nanotube arrays (Ni-NCNTs) on porous Si with robust Ni−Si interfacial bonding to form the core−shell-structured NCNTs-Ni 2 Si@Si. The hierarchical porous silicon core was first fabricated via a novel self-templating synthesis route based on two crucial strategies: in situ thermal evaporation of crystal water from the perlite for producing porous SiO 2 and subsequent magnesiothermic reduction of porous SiO 2 into porous Si. Ni-NCNTs were subsequently constructed based on the Ni-catalyzed tip-growth mechanism and were further engineered to fully wrap the porous Si microparticles by forming the Ni 2 Si alloy at the heterojunction interface. When the prepared NCNTs-Ni 2 Si@Si was evaluated as the anode material for Li-ion batteries, the hierarchical porous system in the Si core and the rich void spaces in carbon nanotube arrays contributed to the remarkable accommodation of volume expansion of Si as well as the significant increase of Li + diffusion and Si utilization. Moreover, the Ni 2 Si alloy, which chemically linked the Ni-NCNTs and porous Si, not only provided good electronic contact between the Si core and carbon shell but also effectively prevented the CNTs' detachment from the Si core during cycling. The multifunctional structural design rendered the whole electrode highly stable and active in Li storage, and the electrochemically active NCNTs-Ni 2 Si@Si electrode delivered a high reversible capacity of 1547 mAh g −1 and excellent cycling stability (85% capacity retention after 600 discharge−charge cycles) at a current density of 358 mA g −1 (0.1 C) as well as good rate performance (778 mAh g −1 at 2 C), showing great potential as an efficient and stable anode for high energy density Li-ion batteries.