Electrocatalysts for oxygen-reduction and oxygen-evolution reactions (ORR and OER) are crucial for metal-air batteries, where more costly Pt- and Ir/Ru-based materials are the benchmark catalysts for ORR and OER, respectively. Herein, for the first time Ni is combined with MnO species, and a 3D porous graphene aerogel-supported Ni/MnO (Ni-MnO/rGO aerogel) bifunctional catalyst is prepared via a facile and scalable hydrogel route. The synthetic strategy depends on the formation of a graphene oxide (GO) crosslinked poly(vinyl alcohol) hydrogel that allows for the efficient capture of highly active Ni/MnO particles after pyrolysis. Remarkably, the resulting Ni-MnO/rGO aerogels exhibit superior bifunctional catalytic performance for both ORR and OER in an alkaline electrolyte, which can compete with the previously reported bifunctional electrocatalysts. The MnO mainly contributes to the high activity for the ORR, while metallic Ni is responsible for the excellent OER activity. Moreover, such bifunctional catalyst can endow the homemade Zn-air battery with better power density, specific capacity, and cycling stability than mixed Pt/C + RuO catalysts, demonstrating its potential feasibility in practical application of rechargeable metal-air batteries.

from electric energy into chemical energy, but also offers a promising platform for utilizing intermittently renewable energy sources (e.g., wind and solar). [1][2][3] To improve the splitting efficiency, precious Pt-based and Ir/Ru-based electrocatalysts are usually employed in the practical electro lysis to reduce the activation energy barriers of two core half-reactions involved in water splitting, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. [4,5] Unfortunately, the large-scale utilization of these noble metals has been severely blocked by their limited abundance and high cost. As such, numerous endeavors have been undertaken to exploit for an alternative to noble catalysts during the past decades. [6][7][8][9][10] Emerging as a new class of crystalline materials, metal-organic frameworks (MOFs) constructed by bridging metal ions with organic linkers have drawn considerable attention to themselves owing to their porous, high specific surface, and tailorable features. [11][12][13][14] In recent years, MOFs have also been demonstrated as an ideal precursor to fabricate functional transition metal (TM)-carbon-based nanohybrids, which hold great promise as low-cost and efficient electrocatalysts for water splitting. [15][16][17][18][19][20] Since the chemical and the physical properties of the MOFsderived TM-carbon-based nanohybrids are highly dependent on the MOF precursors, it has been widely acknowledged that building MOF precursors with favorable composition, morphology, and surface structure is a prerequisite to enable these nanohybrids with satisfactory electrocatalytic activity. [21] To this end, there has been much attention surrounding the dedicate design of MOF precursors. [22][23][24][25][26][27][28][29][30][31][32] Currently, most work basically focus on the 3D MOF precursors owing to their high structural stability, large specific surface areas, and abundant pores. [24,25] After a refined post-treatment, their derived TM-carbon-based nanohybrids can inherit well original 3D nanostructure. Furthermore, the solid feature of MOF precursor is turned to the hollow counterpart sometimes. [26,27] These merits ensure the rapid mass transfer ability and rich potential active sites, thus improving the electrocatalytic activity. For instance, Pan et al. synthesized well-inherited hollow CoP@NC nanohybrids by direct calcination of well-performed core-shell CoZn-ZIF dodecahedra under Ar at 900 °C followed by an oxidationphosphorization process, which showed remarkable OER catalytic activity with an overpotential of 310 mV to drive a current Construction of well-defined metal-organic framework precursor is vital to derive highly efficient transition metal-carbon-based electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting. Herein, a novel strategy involving an in situ transformation of ultrathin cobalt layered double hydroxide into 2D cobalt zeolitic imidazolate framework (ZIF-67) nanosheets grafted with 3D ZIF-67 p...

Developing nonprecious electrocatalysts via a cost‐effective methods to synergistically achieve high active sites exposure and optimized intrinsic activity remains a grand challenge. Here a low‐cost and scaled‐up chemical etching method is developed for transforming nickel foam (NF) into a highly active electrocatalyst for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The synthetic method involves a Na2S‐induced chemical etching of NF in the presence of Fe, leading to a growth of ultrathin Fe‐doped Ni3S2 arrays on the NF substrate (FexNi3‐xS2 @ NF). The combined experimental and theoretical investigations reveal that the incorporated Fe cations significantly modulate the morphology and the surface electron density of Ni3S2, and thus significantly boost the electrochemically active surface area, electron transfer, and optimize the hydrogen/water absorption free energy. The developed Fe0.9Ni2.1S2 @ NF requires overpotentials of only 72 mV at 10 mA cm−2 for HER and 252 mV at 100 mA cm−2 for OER in 1.0 m KOH, respectively, enabling an alkaline electrolyzer at a low cell voltage of 1.51 V to drive 10 mA cm−2 for overall water splitting. More broadly, this synthetic approach is very versatile and can be used to synthesize other ultrathin metal sulfides (e.g., Fe–Cu–S, Fe–Al–S, and Fe–Ti–S).

increasing interest since it exhibits a suppressed Fenton reactivity in comparison to Fe-N-C while maintains a remarkable catalytic activity. [18-20] To rationally design CoN -C catalyst for ORR, downsizing active species to single-atom scale and intentionally incorporating specific N into carbon matrix have been proposed to facilitate the catalytic process. [21-27] The former strategy can achieve a maximum atom-utilization efficiency and full exposure of active sites while the latter strategy usually involves pyridinic-N construction to optimize the charge distribution and improve the density of states at the Fermi level of the adjacent C atoms, facilitating the oxygen adsorption and reduction reaction. [28,29] For example, Yin et al. synthesized singleatom CoN x-C electrocatalyst through the pyrolysis of cobalt-coordinated framework porphyrin with graphene and found that it exhibited a high half-wave potential of 0.83 V, much better than Co nanoparticles-N-C electrocatalyst (0.73 V). [30] Han et al. investigated the size effect on the electrocatalytic activity of Co catalysts from nanometer to singleatom scale, demonstrating that cobalt single atoms on N-doped carbon could achieve a higher half-wave potential (0.82 V) and a larger limiting diffusion current density (4.96 mA cm −2) than atomic Co clusters (0.81 V, 4.44 mA cm −2) and Co nanoparticles counterpart (0.80 V, 3.86 mA cm −2). [31] Wang et al. developed a laser irradiation strategy to modulate the relative contents of pyridinic and pyrrolic nitrogen dopants in the electrocatalyst and reported that pyridinic-NCo bonding instead of pyrrolic-N bonding could optimize the adsorption energy of reaction intermediates in ORR process. [32] Despite prominent achievements that have been made recently, most studies on CoN -C catalysts focused on only one of the above-proposed strategies, and thus their catalytic performance is still unsatisfied to meet the practical application. Therefore, developing an effective synthetic strategy for the integration of generating atomically dispersed active sites and achieving pyridinic-N-optimized electronic structure to increase the catalytic activity of CoN -C catalyst is highly demanded but remains significant challenging. Herein, we have innovatively developed a highly effective lysozyme (Lys)-assisted metal-organic framework (MOF) approach to prepare single-atom Co implanted pyridinic-N doped porous carbon catalysts. During the pyrolysis process, the attached Lys on the surrounding of Co-ZIF-8 (zeolitic imidazolate frameworks) not only can effectively trap metal atoms Engineering transition metal-nitrogen-carbon (TM-N-C) catalysts with highdensity accessible active sites and optimized electronic structure holds great promise in the context of the electrochemical oxygen reduction reaction (ORR). Herein, a novel modification of a lysozyme-modified zeolitic imidazolate framework with isolated Co atoms anchored on dominated pyridinic-N doped carbon (Co-pyridinic N-C) is reported. The atomically dispersed Co allows the maximum ...

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