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Zinc-air batteries (ZABs) are promising energy storage systems because of high theoretical energy density, safety, low cost, and abundance of zinc. However, the slow multi-step reaction of oxygen and heavy reliance on noble-metal catalysts hinder the practical applications of ZABs. Therefore, feasible and advanced non-noble-metal electrocatalysts for air cathodes need to be identified to promote the oxygen catalytic reaction. In this review, we initially introduced the advancement of ZABs in the past two decades and provided an overview of key developments in this field. Then, we discussed the working mechanism and the design of bifunctional electrocatalysts from the perspective of morphology design, crystal structure tuning, interface strategy, and atomic engineering. We also included theoretical studies, machine learning, and advanced characterization technologies to provide a comprehensive understanding of the structure-performance relationship of electrocatalysts and the reaction pathways of the oxygen redox reactions. Finally, we discussed the challenges and prospects related to designing advanced non-noble-metal bifunctional electrocatalysts for ZABs.

Transition-metal sulfide (TMS) has been regarded as themost promisingalternative to construct non-noble metal catalysts in the electrochemicalhydrogen evolution reaction (HER). The excellent edge activation ofMoS(2) and abundant catalytic active sites of Ce2S3 can be utilized as the premise of constructing heterogeneousmaterials for electrochemical applications. Afterward, the heteroatoms(C, N, O, and Na) have been introduced to increase the original numberof active sites (the number of the S-Mo-S on the edgemolecular layer of MoS2 and exposed metal active centerof Ce2S3). Additionally, the heterostructureof HAs@MoS2/Ce2S3 and ordered/amorphouscarbon can enhance the activity by creating defects and localizeddisorder fields and accelerating the charge separation/transfer fromcerium (Ce) sites to molybdenum (Mo) sites. Combining the above advantages,HAs@MoS2/Ce2S3 shows promising propertiestoward HER not only in acidic environments with a low overpotential(147.0 mV) but also in alkaline (160.5 mV) environments.

Cobalt-based Layered double hydroxides (Co-LDHs) through designing composite nanostructures and heterogenous junctions, has absorbed an increased concern for dual-function applications including electrochemical sensing and energy devices. Herein, the g-C 3 N 4 and GO are applied as two-dimensional (2D) scaffolds for one-pot synthesis of 2D α/β-Co(OH) 2 phase junction (α/β-Co(OH) 2 /g–C 3 N 4 /rGO, denoted as CCG) in dopamine sensing and oxygen evolution reaction (OER). The coexist of hetero-phase structure (α and β-Co(OH) 2 ) toward crystalline phase has been confirmed by various techniques. Dependent on hetero-phase structural and compositional characteristics of CCG, CCG has revealed excellent electrochemical performance: a ranking sensing response to dopamine with ppb-level sensitivity and wide detection range; and high electrocatalytic ability (38.02 mV dec −1 ) with more stable catalytic capacity toward oxygen evolution reaction (OER). The above considerable potentials for CCG ascribes to the synergistic effect of heterostructure, phase structure along with defect of α/β-Co(OH) 2 .

A hetero-phase structure of cobalt hydroxide [alpha/beta-Co(OH)(2)] in situ formed with the partly reduced graphene oxide [Co(OH)(2)/PRGO] has been fabricated by a coprecipitation method. Depending on the interactions (nucleation, bonding with oxygen-rich groups, and partial conversion of GO into rGO) provided by GO, cobalt ions utilized the interactions under alkaline conditions provided by 2-methylimidazole to form hetero-phase alpha/beta-Co(OH)(2) and maintain the unstable-structure [alpha-Co(OH)(2)]. Beneficial to the large electrochemical active surface, rich redox active sites, and electronic transfer capability derived from hetero-phase structure [alpha/beta-Co(OH)(2)] and the partly reduced graphene oxide, the Co(OH)(2)/PRGO has an extraordinary detection performance of DA: a multi-section continuous detection range (0.1 nM to 450 mu M), a ultra-low limit of detection (0.078 nM, S/N = 3), and a strong anti-interference ability, coupled with an application potential in biological foods (pork) and in the body environment (serum).

Detection of hydrazine originated from electrochemical media has recently gained considerable attention in the sensing field.Herein,to improve electron transfer capacity,a polyoxometalate metal-organic framework (POMOF,NENU-3) is in-situ nucleated onto the carboxyl functionalized reduced graphene oxide (CFG) (NENU-3/CFG,abbreviation N3/CFG).The N3/CFG supported onto carbon cloth electrodes (CCEs) has been investigated for hydrazine detection.The amperometric results display that the POMOF/CFG to hydrazine has a broad linear range (0.09-362.5 μmol/L) and low detection limit (24 nmol/L).In addition,the POMOF/CFG-based sensors pos-sess good anti-interference capability,boosted stability and feasibility.Furthermore,when applied to the detection of practical sam-ples,acceptable relative recoveries of 96.26％-107.30％ are obtained.

A dense  zeolitic imidazolate framework (ZIF) nanosheet is for the first time molded by reduced graphite oxide (RGO) and graphitic carbon nitride (g-C3N4) to fabricate an original 2D/2D/2D heterojunction (ZIF/g-C3N4/RGO nanohybrid), which is pipetted onto carbon cloth electrode (CCE) (ZIF/g-C3N4/RGO/CCE) as an electrochemical sensor. Profiting from the renowned synergistic and coupling effects, the resulting nanohybrid endows excellent electrocatalytic activity towards hydrazine. Amperometric detection reveals that the hybrid sensor possesses a low detection limit of 32 nM (S/N = 3) in a monitoring range of 0.0001 to 1.0386 mM, along with a high sensitivity 93.71 μA mM−1 cm−2. Importantly, the minimum detection concentration of hydrazine in the actual sample is lower than the maximum allowable limit of the World Health Organization (WHO) and has high reproducibility (RSD = 4.82%). As expected, the high sensing capability  of ZIF/g-C3N4/RGO combines the advantages of abundant surface-active sites and high conductivity along with 2D interfaces between ZIF, g-C3N4, and RGO nanosheets. This study provides a promising to expand 2D-based ternary nanojunction as a bridge for promoting sensing performance. Graphical abstract