The elementary steps of HER in alkaline media are as follows: T o t a l :   2 H 2 O + 2 e − → H 2 + 2 O H − (1) V o l m e r   S t e p : M + H 2 O + e − → M − H ∗ + O H − (2) H e t r o v s k y   s t e p : M − H ∗ + H 2 O + e − →   H 2 + M + O H − (3) T a f e l   s t e p :   2 M − H ∗ → H 2 + 2 M (4)

M is the active site on the catalyst, and M–H* is the hydrogen adsorption on the active site above the catalyst.

The HER process involves the Volmer, Heyrovsky, and Tafel steps. In the Volmer step, water is dissociated to form a reactive H_ad intermediate. In the Heyrovsky step, another H₂O is adsorbed to M–H^* and then reacts with the second electron to form H₂ and OH⁻. In the Tafel step, two adjacent M–H^* are combined to generate H₂. A whole HER process will follow two pathways—Volmer–Tafel or Volmer–Heyrovsky reactions (Chen et al., 2019)—consisting of three consecutive stages—water dissociation, water adsorption, and hydrogen production.

Generally, the adsorption-free energy of M–H* (ΔG_H*) is widely accepted as a key description in evaluating the kinetics of the HER catalyst. Optimum ΔG_H* to zero is desirable for achieving excellent HER performance, which is regarded as a guide for developing highly efficient HER electrocatalysts. In addition, water adsorption energy ( E a d w a t e r ) and the activation energy of water dissociation ( E a c w a t e r ) reflect the catalytic behaviors of HER. The lower value of E a d w a t e r will result in better affinity between water molecules and the catalyst surface. The decrease of E a c w a t e r is proportional to the water dissociation rate because of the smaller the activation energy required. Therefore, computational energies are convenient descriptions when designing high-performance catalysts.

The elementary steps of HOR in alkaline solution are shown as follows: T o t a l : H 2 + 2 O H − → 2 H 2 O + 2 e − (5) T a f e l   S t e p :   H 2 + 2 M → 2 M − H ∗ (6) H e t r o v s k y   s t e p : H 2 + M + O H − → M − H * + H 2 O + e − (7) V o l m e r   s t e p :   M − H * + O H − → M + H 2 O + e − (8)

HOR proceeds in the reverse sequences to HER; in alkaline solution, HOR proceeds through pathways of Tafel–Volmer or Heyrovsky–Volmer. M–H^* is formed through the Tafel or Heyrovsky step, and then follows the Volmer step to form H₂O. In the Tafel step, H₂ is dissociated in M (active site) without electron transfer to generate M–H^* and is adsorbed on the surface of M. During the Heyrovsky step, H₂ transfers electrons to M on the surface of the catalyst and is dissociated to generate M–H^* and H₂O with the coordination of adjacent OH⁻. Finally, M–H^* binds with OH⁻ to form H₂O in the Volmer step.

Recently, Markovic et al. proposed a bifunctional mechanism (Okubo et al., 2019). They took the hydroxyl binding energy as another active descriptor and suggested that OH_ad is an important factor for efficient HOR under alkaline conditions. There is much discussion comparing the hydrogen binding energy (HBE) theory and the bifunctional mechanism. Although these two contradictory theories coexist, they succeed in guiding the synthesis of HOR catalysts and reflect the complexity of the HOR mechanisms in alkaline solutions.

Synergistic metal-support interaction (SMSI) can be observed between metal catalysts and solid supports (Song et al., 2015; Song et al., 2020). SMSI provides possibilities for designing catalysts, modulating their d-band of catalytic sites and optimizing their binding energy of intermediates, thus resulting in accelerated reaction kinetics. Indeed, solid support is important for determining SMSI (Song et al., 2015; Song et al., 2018). An ideal solid support not only improves SMSI and re-modifies the electronic structure of metal sites but also restrains the agglomeration of metal species.

Carbon-based materials are excellent solid supports due to their large surface area, tailorable porous structure, high resistance, and convenient surface functionalization. Vulcan-XC 72R, a commercial carbon, possesses suitable surface area, pore size, and low-cost. Ni₃N nanoparticles can be well-dispersed on a Vulcan-XC 72R surface and exhibit a robust HOR performance (Ni et al., 2019). In addition, a series of novel carbon-based materials have been developed. Yuan et al. (2020) designed cobalt nitrides embedded in nitrogen-doped carbon (Co₄N@NC). NC acted as a conductive network to supply abundant active sites and efficiently retained the surface oxidation of Co₄N. SMSI between Co₄N and NC optimized DOS near the Fermi level, resulting in an enhancement of HER kinetics. Moreover, such improved HER performance has been observed in CoN/N-doped carbon nanotubes because one-dimensional nanotubes can provide ordered channels for rapid mass-diffusion and electron transfer. Porous metal–organic frameworks (MOFs) have emerged as precursors to obtaining TMN nitrides under controlled nitridation. Feng et al. (2019) synthesized MOF-74-derived NiCoN catalysts with porous rodlike structures. The abundant synergistic active sites obtained have accelerated electron transfer and modulated the chemical environment, resulting in all-pH HER performance (Feng et al., 2019). A series of nickel nitrides and cobalt nitrides derived from MOFs were reported (Hu et al., 2020; Wang et al., 2022). Jia et al. (2022) reported the construction of Ni₃N/MOF-74, which shows a remarkably HER performance with an overpotential of 73 mV to afford 10 mA cm⁻² in a 1.0 M KOH solution (Jia et al., 2022).

Transition metal carbides/nitrides (MXenes) as new two-dimensional materials are considered promising candidates for solid supports (Zhang et al., 2021). MXenes discovered by Gogotsi and colleagues in 2011 possess a larger specific surface area, high conductivity, good hydrophilicity, robust stability, and various functional groups (Naguib et al., 2011). Very recently, the effect of MXenes’ support on HER performance has been explored. CoN_x/MXenes have been reported by Li et al. (Li et al., 2021): the construction of CoN_x and MXenes can expose rich active sites, optimize electronic structure, and promote intermediate adsorption and desorption, thus improving HER kinetics. The DFT results prove furthermore that more electrons accumulate at the surface of the Co atom, arising from the strong charge transfer caused by the interface coupling between CoN_x and MXenes. These improve mass/charge transport and promote water adsorption/activation in the Volmer step in HER. These studies have proven that MXenes are a promising support for exploring and developing cost-effective catalysts with high activities.

Hetero-interface engineering is another promising strategy for developing high-performance PGM-free electrocatalysts. The interface effects will reconstruct active sites by modifying the electrochemical environment between two different active components. The reconstructed active sites at the heterogeneous interfaces can show higher activity than individual components. Up to now, the modifications of TMN-based interface catalysts, such as transition metal nitrides/transition metal (TMNs/TM), transition metal nitrides/transition metal nitrides (TMNs/TMNs), and transition metal nitrides/metal oxides (TMNs/MOs), have been reported.

It is well-known that the construction of TMNs/TM can efficiently improve conductivity and optimize the binding behaviors of catalytic sites toward reactants, intermediates, and products. In 2018, Ni₃N/Ni interfacial catalysts were synthesized by electrodeposition combined with controlled nitridation, which is a pioneering report on a Ni₃N-based electrocatalyst with bifunctional HER/HOR activity (Song et al., 2018). The construction of a Ni₃N/Ni heterogeneous interface benefited the electron transfer at the interfacial sites and improved electrophilic properties. Strong electron redistribution occurred at the interface between Ni₃N and Ni, which significantly promoted initial water adsorption and reduced the energy barrier of the subsequent water dissociation. Moreover, the H* species were achieved at the interfacial region. The ΔG_H* of the interfacial site was as low as 0.01 eV, close to the ideal value of zero—resulting in an exceptional HER/HOR catalytic property. This group further developed a Co₂N/Co interface with better robustness and Co tolerance (Song et al., 2019). Other TMNs/TM systems have been explored by Fan group (Fan et al., 2018) and Sun group (Sun et al., 2020). The former designed an atomic in-growth Co–Ni₃N interface (Zhu et al., 2018). The epitaxial Co–Ni₃N interfaces in nanoscale are responsible for robust stability due to small interfacial energy. In addition, the nanoconfinement effect can accelerate the electron transfer between Co and Ni₃N domains at the epitaxial interface, resulting in excellent electrocatalyzing hydrogen production. Wang group further explored the chemical anchoring and electronic regulation at Ni₃N/Ni. The formation of the Ni−N bond made the heterogeneous interface produce unique electronic states by changing the electrons’ state. Strong electron redistribution occurred at the interface between Ni₃N and Ni, where the water formation reaction barrier could be significantly reduced, resulting in enhanced HOR performance (Wang et al., 2021).

The TMNs/TMN heterogeneous interface has attracted much attention due to its faster charge transfer and compatibility. CoN/Ni₃N catalysts with a grass-like structure were synthesized via a controlled pyrolyzation (Ray et al., 2018). This generated abundant active centers with expeditious charge and mass transportation, inducing an enhanced HER property in 1.0 M KOH solution. A similar phenomenon was observed in Ni₃N/Co₃N (Huang et al., 2021) and Ni₃N/Mo₂N (Dai et al., 2022). In 2022, Song et al. reported a novel Ni₃N/Co₂N epitaxial interface, which exhibited superior bifunctional HER/HOR performance with respect to the state-of-the-art Pt catalyst due to an accelerating electron transfer at interface region. The bifunctional Ni₃N/Co₂N electrocatalysts exhibited excellent performance in a Swagelok type Ni-H₂ battery with robust stability, demonstrating excellent rechargeability over 5000 cycles (Song et al., 2022).

Heterogeneous TMNs/MOs have proven to be efficient interfacial catalysts. Although MOs are not active for hydrogen electrochemistry, the addition of MOs benefits the modulation of the d-band and electronic structure of MOs, accelerating the kinetics of hydrogen electrochemistry. Sun et al. reported a Co₄N/CeO₂ interfacial system, where the in situ growth of CeO₂ can efficiently modify the electronic structure of the nearby Co₄N surface, favoring H₂O dissociation and intermediated H^* adsorption (Sun et al., 2020). Zhou group prepared Ni₃N/V₂O₃ by nitriding a NiV–LDH (layered double hydroxide) precursor (Zhou et al., 2021). The introduction of V₂O₃ made the interface couple between Ni₃N and V₂O₃; the interfacial interaction resulted in charge redistribution and generated more unoccupied electrons on the N and O in the interface region, which efficiently optimized the H₂O adsorption/desorption ability. This hypothesis was further confirmed by DFT calculation, where Ni₃N/V₂O₃ exhibits lower E a d w a t e r , E a c w a t e r , and ΔG_H* than Ni₃N.

Other TMN-based interface catalysts have also been reported. Sun et al. (2018) explored the abundant defects on a CoN/Ni₂P surface (Sun et al., 2018). Hu et al. (2019)developed a series of CoN/Co₂P-based interfacial catalytic sites (Guo et al., 2018; Hu et al., 2019). Ren et al designed Ni₃N/Ni(OH)₂ with excellent HER/HOR performance (Ren et al., 2022). The introduction of nickel hydroxide reduced the center of the Ni d-band center and thus reduced the adsorption strength. The strong electronic coupling made Ni₃N negatively charged by interface engineering with seamless heterojunctions. The negatively charged Ni sites improved the bond interaction between the Ni sites and surface H* groups. Ni₃N/Ni(OH)₂ surface engineering promoted water dissociation and hydroxyl adsorption in the Volmer step toward HER and HOR.

As shown previously, interface engineering has become an effective strategy for improving catalyst activity by generating electron redistribution at the interface and realizing synergistic effects.

Heteroatom doping can be utilized to modify the charge redistribution of host materials. In contrast to interfacial engineering, heteroatom doping can retain the original structure and morphology of host materials. Due to different electronegativities, heteroatoms adjust the electronic structures of host catalytic sites and gear up the catalytic performance of electrocatalysts, killing “two birds with one stone” (Zhang et al., 2021).

Recently, Zhu group prepared Co doping for a Ni₃N heterostructure catalyst (Zhu et al., 2018). The XRD results confirmed that the Ni₃N phase did not change with the addition of Co. Epitaxial in-growth Co promotes electrons transferal from Co to Ni₃N, where these added electrons can stabilize the H atom, thus reducing ΔG_H* to improve HER performance. Ni doping of CoN is further demonstrated by Yu et al. (Yu et al., 2019). The enhanced performance was mainly due to CoN catalytic sites being modified by nearby Ni, leading to more active sites, improved conductivity, and fast charge transfer. In addition, Zhang et al. doped V into Ni₃N/Ni to fabricate V doping Ni₃N/Ni (Zhang et al., 2021). The charge redistribution is mainly confined to the N and Ni sites nearest to the doping V. The coupling of Ni and V dopant can promote water adsorption and optimize the value of ΔG_H*. Zhang et al. reported a V-doped Ni₃N nanosheet, denoted as V doping Ni₃N NS(Zhang et al., 2021). V doping can effectively adjust the electronic structure of Ni₃N, reduce the energy barrier of RDS, and promote dehydrogenation kinetics and H* adsorption/desorption. Wu et al. further proved that Cr doping can downshift the high unoccupied d-orbital of Ni₃N (Wu et al., 2021). Cr doping exists as a Cr-n₆ state, which can redistribute the surface electronic state of Ni₃N to reduce the average d-band energy of Ni₃N. The downshift of d-band energy can strengthen the orbital coupling between the unpaired electrons of O 2p and the unoccupied states of Ni 3d—beneficial to the adsorption and dissociation of water. Cr doping Ni₃N can also promote the subsequent solution of H through the synergistic effect. Therefore, the doping of Cr enhanced the electron coupling with water molecules and promoted the dissociation kinetics of water molecules, thereby improving the HER performance, which is very close to the performance of Pt/C.

Heteroatom dopant can adjust electron density, modulate the orbit of catalytic sites, improve the conductivity of host materials, and retain the intrinsic component and structure of the host catalyst, thus improving the catalytic performance of TMNs. These results unambiguously show the advantage of the heteroatom dopant in improving catalytic performances.