Electrochemical hydrogen production from water electrolysis is currently considered a sustainable strategy to address the energy crisis and fossil fuel-related environmental problems due to its benefits such as abundant supplies and zero carbon emissions [[1], [2], [3], [4]]. Due to the sluggish kinetics from the high equilibrium potential (1.23 VRHE) of the water splitting under standard conditions, materials with unique behaviors, including high intrinsic activity, abundant surface-active sites, and great electrochemical durability in harsh operating conditions, are essential to accelerate the HER and OER processes [[5], [6], [7], [8], [9]]. Although noble metal-related materials are benchmark catalysts for the electrochemical water-splitting reaction, their scarcity and high cost are major limitations; therefore, designing cost-efficiency electrocatalysts with suitable adsorption/desorption strength is vital for boosting the water-splitting performance [3,[10], [11], [12]]. Recently, 2D MXenes with the general structure Mn+1XnTx are of interest as potential materials for electrochemical reactions due to their diverse properties [6,[13], [14], [15], [16]]. Unlike other 2D materials, MXenes have fascinating characteristics toward electrochemical reactions such as (i) two-dimensional structures generating abundant surface active sites and short mass-charge transfer pathways [[17], [18], [19]]; (ii) high electrical conductivity (6000–1000 S cm−1) accelerating the electron transfer [17,20,21]; (iii) abundant tunable surface terminating groups with the favorable hydrophilicity facilitating the infiltration of electrolyte and thus contributing to the adsorption of reactive species in water electrolysis process [17,[22], [23], [24]]; (iv) diverse surface functional groups making MXenes easily coupled with other compounds to generate new synergistic surface-active sites [17,[25], [26], [27]]; (v) low work function and electronegative surface making MXenes be potential carrier materials, which can regulate the electronic structure of active centers [17,28]. For the electrochemical hydrogen production, Wang et al. [29] used a density functional theory (DFT)-based screening method to construct a volcano plot between the electron number (Ne) of surface oxygen atoms and hydrogen adsorption free energy (ΔGH⁎), indicating that the Ti2CO2 and W2CO2 MXenes were highly active catalysts for the HER among 10 mono-metal carbides due to their lowest ΔGH⁎ value of 0.12 eV, which close to 0.09 eV of Pt(111) surface [29], suggesting that MXenes were potential alternatives to platinum-based materials for the HER. Despite the achievements, aside from limitations (e.g., low flexibility, large contact resistance, and poor stability in oxygen-containing atmospheres), the restacking by the van-der-Waals forces and hydrogen linkage between adjacent layers limits surface-active sites and hinders the transport pathways of electrolyte ions, drastically reducing the electrochemical activity and stability [17,[30], [31], [32], [33], [34]]. One efficient strategy to attenuate the MXene restacking and improve their catalytic performance is to introduce interlayer spacers [[35], [36], [37], [38], [39]]. For instance, Li et al. [36] designed a novel Ni3Se4-NiSe2-Co3O4 triple-interface heterostructure on Ti3C2Tx MXenes having the significant work function difference as a bifunctional HER/OER catalyst with low HER/OER overpotentials of 36 mVRHE/218 mVRHE at 10 mA cm−2, respectively. Notably, a Ni3Se4-NiSe2-Co3O4/MXene-based water electrolyzer achieved a low-cell voltage of 1.64 V to drive 100 mA cm−2 and an impressive stability of 100 h. Similarly, Feng et al. [40] reported the trifunctional electrocatalytic performance (i.e., ORR, OER, HER) in the alkaline electrolyte of Fe-C-N/Nb4C3Tx material with the ORR half-wave potential and OER/HER overpotentials of 0.911 VRHE, 290/91 mVRHE at 10 mA cm−2, respectively. Theoretical outcomes attributed the catalytic efficiency of MXene-based hybrid to the optimized electronic structure at the interface between components in overall catalyst systems, thereby modifying adsorption and desorption energies of reactants and intermediates during electrochemical reactions [36,37,40]. However, accurately exploring the effect of each component on overall electrocatalyst systems and rational synthetic routes to achieve advanced 2D MXene-based composites with the optimal constituent and abundant surface-active sites is still a big challenge. Moreover, the construction of 3D MXene structures is of interest to prevent the restacking and thus enhance the surface utilization efficiency and ion/mass transport pathways, but current 3D MXene architectures have been fabricated through complex synthetic routes (e.g., spray drying, template, 3D printing, etc.) with the limited productivity, toxic chemicals, or difficult controllable porous size resulting in the poor scalability [[41], [42], [43]].

Another effective strategy to increase the electrocatalytic efficiency of pristine 2D MXenes is to incorporate non-metals (e.g., N, P, S, B, etc.) into 2D MXene matrix, significantly increasing interlayer spacing, electrical conductivity, and surface adsorbability of electrochemically active species [5,33,44]. The introduction of heteroatoms in MXene hosts can generate more defects and active π electrons of MXenes, optimizing the electronic structures of elements in the MXene matrix to increase the intrinsic catalytic activity of electrocatalysts [45,46]. Nitrogen is the most preferred dopant among heteroatoms available because of its beneficial effects for electron transfer during electrochemical reactions, boosting the activity of carbon- and transition-earth metal-based materials [[47], [48], [49], [50], [51]]. One of the first studies of N-doped 2D MXenes was demonstrated by Wang's group in 2017 [52], which applied a high-temperature solid-state route under ammonia gas flow to design N-doped Ti3C2Tx MXenes as a cost-effective electrode for supercapacitors. The formation of the N-doped Ti3C2Tx electrode was attributable to the substitution of carbon by nitrogen, increasing the layer distance and electron concentration of Ti3C2Tx MXenes and thus boosting the gravimetric capacitance up to 460 %. The great efficiency of N-doped MXenes for electrochemical energy-related storage systems has been demonstrated in recent works [32,33,53], indicating the potential of nitrogen doping for accelerating the electrochemical performance of pristine MXenes. Inspired by this, An et al. [54] used a nitridation strategy to design nitrogen-doped 2D Ti2CTx MXenes with the enlarged MXene distance and formation of Ti-Nx species, supplying abundant surface-active sites and facilitating the electron transfer between N-Ti2CTx and electrolyte for accelerating the acidic HER activity. Instead of considering Ti–Nx motifs, Lee et al. [55] reported that all nitrogen-based bonds (i.e., Ti–N, NH, and O–Ti–N) modified the electronic configuration of MXenes, thereby increasing the electronic conductivity and decreasing the ΔGH⁎ value for the HER. In short, the introduction of nitrogen can drastically improve electrochemical properties of pristine MXenes that are attributed to several aspects: (i) expanded MXene interlayer distance and formed defective sites, generating abundant surface-active sites and facilitating the ion diffusion and charge transfer pathways [[55], [56], [57], [58], [59], [60]]; (ii) modified Fermi energy and regulated adsorption and desorption strength of reactant/intermediates because of changing the electronic structure of MXenes, increasing the electrochemical efficiency of nitrogen-doped MXenes [54,55,61,62]; (iii) efficient removal of adverse terminating groups on the MXene surface by the formation of N-related groups (e.g., Ti-Nx, NH, and O-Ti-N), facilitating the electrochemical performance [55,59,63,64]; (iv) effective inhibition of the MXene oxidation by the presence of nitrogen-containing sources as protective agents, enhancing the activity and stability of MXenes [52,[65], [66], [67], [68], [69]].

Fig. 1a shows an overview of publications and citations related to 2D MXenes for the HER and OER process, indicating that studies on N-doped MXenes for the HER and OER are still in their infancy. Therefore, a comprehensive overview of the doping mechanism and properties-efficiency relationship of N-doped MXene-related materials is necessary to guide the further development of N-doped 2D MXene-related materials toward hydrogen production and other electrochemical applications. We herein systematically summarize the progress in fabrication methods, theoretical simulation predictions and identifications of physical and chemical behaviors, and applications of nitrogen-doped MXene-related catalysts toward the HER and OER, as shown in Fig. 1b. To start this review, the doping mechanism and typical physicochemical behaviors of pristine 2D MXenes with the nitrogen dopant are briefly introduced. Also, available theoretical calculations and identification methods of nitrogen-doped MXene-based materials regarding the different doping/substituting positions for the HER and OER are described in Section 2. Then, current fabrication strategies and their advantages/disadvantages of N-doped MXenes, and the role of synthetic conditions in forming nitrogen-related species are discussed in Section 3. Recent applications of nitrogen-doped MXene-related electrocatalysts for hydrogen production are briefly summarized in Section 4. Prospects and challenges of nitrogen-doped MXenes-related electrocatalysts for electrochemical hydrogen generation are concisely proposed in Section 5. Finally, this work can provide detailed guidelines for developing next-generation N-doped MXene-based materials for electrochemical hydrogen production and other fields.