Progress in the Study of Transition Metal-Based Carbon Nanotube Composites for Electrochemical Hydrogen Evolution

TMs refer to a series of metallic elements in the d region of the periodic table, including precious metals (Au, Ag, Pt, Rh, Ir, Ru and Os) and common metals such as manganese, Fe, Co, Ni, Cu and Zn. Due to their partially filled d orbitals, TMs frequently possess unpaired electrons, enabling facile complexation with various metallic or nonmetallic elements to attain electronic stability. The quantum size effect and small size effect of nanomaterials can significantly improve the electrical and magnetic properties of TMs-based nanomaterials. TMs-based nanomaterials include TMs elemental nanomaterials and TMs sulfides [60,61,62], TMs selenides [63,64,65], TMs phosphide [66,67,68], TMs borides [69,70] etc. Because of its unique physical, optical properties and electronic structure, TMs-based nanomaterials have emerged as promising alternatives to Pt-group materials for HER applications. The hybrid composite material of TMs and CNT can not only effectively improve conductivity, but also optimize its activity through synergistic effects, solving the problem of corrosion of metal active sites in electrocatalytic processes, thereby improving the stability of hybrid composite catalysts [71,72]. 3.1. Noble Metal/CNTs Composite Catalysts In light of the limited literature on experimental studies concerning Pd, Ir, and Os carbon nanotube composite catalysts, this section will focus exclusively on Pt and Ru. The analysis of the remaining metals will be systematically addressed in Section 3.3. 3.1.1. Pt /CNTs Composite Catalysts Substantial efforts have been devoted to developing advanced catalysts for η reduction and HER efficiency enhancement. To date, Pt-based nanomaterials are recognized as the most effective electrocatalysts for HER, primarily due to their low absolute value of ΔG_H^∗ (−0.09 eV), which is close to zero [63]. Pt exhibits both high electrocatalytic activity and exceptional stability, establishing it as the benchmark for commercial electrocatalysts [73,74]. Usually, in the electrocatalytic electrolytic reaction, an electrocatalyst is coated on the electrode surface to reduce the η and accelerate the kinetic reaction [75]. However, the scarcity of Pt in nature and its inadequate long-term stability under operational conditions hinder large-scale production, thereby limiting the widespread adoption of hydrogen technologies [76]. Therefore, it is crucial to minimize the Pt content in electrocatalysts while maintaining high catalytic activity. Zhang et al. [77] dispersed Pt NPs on multi-walled Ni (Pt/CNT-H) as an effective electrocatalyst for HER. The catalyst demonstrated outstanding electrocatalytic activity in acidic media. At a current density of 10 mA cm⁻², the η of Pt/CNT-H was 19 mV, significantly lower than the η of the Pt/C catalyst (38 mV). The enhanced HER activity of Pt/CNT-H arises from the combined effects of Pt-Ni electronic synergy and a cross-linked 3D conductive network. Tavakkoli et al. [78] introduced a novel electrochemical method for effectively dispersing Pt atoms on the side walls of single-walled CNTs. DFT calculation shows that the SWNT sidewall can effectively fix Pt atoms. The findings highlight SWNTs' promise as substrate materials for single-atom Pt catalyst stabilization. Theoretical calculations further confirm that single-atom Pt sites anchored on SWNTs exhibit HER electrocatalytic activity comparable to conventional bulk Pt catalysts. The present work pioneers an innovative strategy for developing electrochemically active catalysts containing minimal Pt loading. The application of CNTs allows for the drastic reduction of precious metal usage, a critical advantage for scalable commercialization. To date, Pt maintains its status as the most effective HER catalyst, demonstrating optimal balance between reaction kinetics, durability, and practical applicability. This makes it suitable for industrial applications, such as proton exchange membrane (PEM) electrolyzers in acidic environments [79,80]. Nevertheless, the acidic operational environment in PEM electrolyzers induces significant cathode degradation. On the traditional carbon black substrate, Pt nanoclusters are easy to aggregate during the on/off electrochemical cycle of electrolytic cell connected with renewable energy. So far, this has led to the confrontation between ultra-low Pt load (<1000 ng Pt cm⁻²) and the high durability requirements of PEM electrolyzers. Rajala et al. [81] established a facile and scalable approach to fabricate Pt nanowires (PtNWs) on single-walled CNTs (SWCNTs) (Figure 5a), effectively reducing the reliance on platinum group metals. The Pt/SWNT-O₃ catalyst incorporates high aspect ratio PtNWs onto single-SWNTs with an ultra-low Pt content (340 ng_Pt cm⁻²) (Figure 5c) for the HER. In acidic aqueous solutions, its activity (10 mA cm⁻² at −18 mV vs. RHE) is comparable to that of state-of-the-art Pt/C (Figure 5b) and it also exhibits excellent stability (Figure 5d). Tang et al. [82] achieved ultralow overpotential water splitting capability in alkaline solutions by incorporating platinum nanoparticles bonded to carbon nanotubes (Pt-NPs-bonded@CNT) with strong Pt-C bonds (Figure 5e). the Pt-NPs-bonded@CNT also had outstanding hydrogen evolution performance in 1.0 M KOH, as presented in Figure 5f. This enhancement arises from the synergy between PtNWs and SWNTs-PtNW edge sites simultaneously facilitate hydrogen adsorption and reduce H-H repulsion, leading to superior HER activity. Pt-based materials remain the performance benchmark for hydrogen evolution catalysis in acidic media [83,84]. However, Pt-based catalysts exhibit significantly restricted HER activity in alkaline media. Researchers have fabricated diverse Pt-derived HER electrocatalysts using varied design approaches [85,86,87,88]. Support materials optimize Pt-based HER catalysts by both anchoring ultrafine NPs and facilitating interfacial electronic interactions between Pt and the substrate [87]. Modulating the chemical state of Pt represents an effective strategy to enhance catalytic performance through electronic structure optimization [86]. It is widely reported in the literature that the active site of HER in Pt-based catalysts is metal Pt [86,88]. However, by changing the chemical state of Pt, some recent reports show that the high valence state of Pt with Pt-O bond (such as PtII) may actually be the real catalytic center, which shows better HER performance than metal Pt [89,90]. Engineering Pt-based catalysts with high-valence Pt centers and optimized support materials could enable efficient alkaline HER. In summary, HER electrocatalyst was prepared by transforming Pt metal into platinum compounds (such as oxides, tellurides and alloys), which had good electrocatalytic activity and stability in both alkaline and acidic electrolytes [91,92,93,94]. These materials can be combined with CNTs, which will provide more choices for industrial HER electrocatalyst. 3.1.2. Ru/CNTs Composite Catalysts Pt exhibits optimal hydrogen adsorption energy, minimal η, superior exchange current density and favorable Tafel slope characteristics, establishing it as the benchmark HER electrocatalyst [95]. Beyond the prohibitive cost and limited availability, Pt exhibits unsatisfactory electrochemical stability due to dissolution in corrosive electrolytes and irreversible nanoparticle aggregation through Ostwald ripening mechanisms [96,97]. With its competitive price (one-third of Pt), operational durability, and high hydrogen evolution efficiency, Ru is extensively employed in metallic catalyst systems [98,99]. Despite numerous studies on Ru-based electrocatalysts, few demonstrate Pt-comparable HER activity. However, compared with Pt NPs, Ru NPS shows greater binding energy which makes it easy to reunite under turnover conditions [100]. Consequently, anchoring and dispersing catalytic sites on conductive supports is a vital strategy for developing durable electrocatalysts. This approach not only prevents metal aggregation but also enhances electron transfer between the catalyst and the electrode [101]. The unique combination of CNTs’ large surface area, metal-comparable conductivity, robust stability, and moderate hydrophilicity enables efficient electrocatalytic applications in aqueous environments [102]. Recent studies have demonstrated that CNTs effectively stabilize Ru NPs on HER mixed electrode. Liu et al. [103] developed a novel Ru catalyst (Ru@CNT) supported on nitrogen-doped CNTs, which exhibited excellent catalytic performance in 1 M KOH. A current density of 10 mA cm⁻² can be achieved with a η of 36.69 mV, and its Tafel slope is 28.82 mV dec⁻¹. The exceptional catalytic performance primarily originates from atomically dispersed Ru species stabilized through chelation on N-doped CNTs substrates. The 1D porous architecture of N-CNTs further enhances active site accessibility by providing abundant anchoring opportunities. Wang et al. [104] oxidized commercial CNTs with nitric acid to introduce oxygen-containing functional groups, particularly carboxylic –COOH, onto their surfaces, resulting in oxidized carbon nanotubes (OCNTs) Ruthenium nanoparticle catalysts were prepared on the support of (Ru@OCNT). Due to its excellent HER performance, Ru@OCNT electrocatalyst has wide applicability in large-scale hydrogen production. Romero et al. [105] combined the versatility of organic synthesis methods for metal-based nanostructures with thermal oxidation treatments to create Ru-containing nanomaterials supported on CNTs. This strategy allows a series of hybrid nanomaterials with different Ru/RuO₂ compositions and different structural sequences to be obtained. The electrode of NPs containing RuO₂ core and surface metal Ru is better than that containing pure RuO₂ NPs or Ru NPs. The initial η of Ru@RuO₂/CNT/GC electrode is 200 mV, and the current density is 10 mA cm⁻² when the η is 272 mV. The calculation results show that this is due to two factors: (1) The amorphization of external Ru atoms increases the number of hydrogen atoms that can be adsorbed; (2) The Ru center on the surface is partially oxidized by RuO₂, which overall weakens the adsorption of H. Meng et al. [20] proposed a coaxial wrapping method of defective amorphous CNTs sponge network loaded with Ru NPs. The presence of amorphous carbon facilitates both uniform dispersion and size confinement of Ru NPs. The incorporation of oxygen vacancies enables electronic structure modulation of metal sites, enhancing charge transfer kinetics and consequently boosting catalytic efficiency. To elucidate the underlying catalytic mechanism, a theoretical model was developed (Figure 6a). The defect-induced electron transfer in Ru effectively downshifts its d-band center, thereby optimizing the adsorption energy of water molecules and significantly promoting their dissociation and subsequent dehydrogenation in the HER. Three typical hydrogen adsorption sites were selected (Figure 6b), with site 3 representing the equatorial region. The ∆G_H* value is closer to 0 eV (Figure 6c), indicating higher catalytic efficiency. In summary, computational results demonstrate that amorphous carbon-to-Ru charge transfer modulates Ru-H binding energy to thermodynamically optimal levels, thereby maximizing HER catalytic efficiency. Electrocatalysts with Ru NPs anchored on multi-walled carbon nanotubes (Ru@MWCNT) can catalyze HER with excellent activity. Uniform and small Ru NPs can be formed by introducing carboxylic acid groups (–COOH) into MWCNT to form Ru carboxylic acid complexes. Kweon et al. [106] reported that Ru NPs are uniformly deposited on MWCNTs as an efficient HER catalyst. HER efficiency of Ru@MWCNT and Pt/C catalyst was evaluated in 1.0 M KOH solution (Figure 6d). The smaller the slope of Tafel, the faster the catalytic reaction of Ru@MWCNT is than that of Pt/C (Figure 6e). Ru@MWCNT exhibits a superior exchange current density (2.4 mA cm⁻²) compared to Pt/C (1.4 mA cm⁻²), demonstrating enhanced HER activity in alkaline electrolytes (Figure 6f). The catalyst's superior HER activity stems from its enlarged ECSA and improved charge transport properties [107,108,109]. However, CNTs’ strong hydrophobic character often causes metal NP coalescence into oversized particles with insufficient Ru-CNT interfacial interactions [110,111]. Heteroatom doping (S/N/O) enhances CNTs hydrophilicity, improving noble metal-CNT affinity and facilitating interfacial electron transfer. Lin et al. [112] demonstrated effective structural coupling between ruthenium nanoclusters and phosphorus- and oxygen–doped CNTs (Ru-POCA). The performance comparison of HER electrocatalyst of noble metal–based carbon nanotube composites is shown in Table 1. 3.2. Non-Noble Metal/CNTs Composite Catalysts Some TMs (Ni, Co, W, Mo, Fe, etc.) can provide adsorption/desorption sites for reaction intermediates, change the adsorption/desorption ability of adjacent metal atoms H, and promote the electrocatalytic process. The catalytic performance of non-noble metals follow the following order: Ni > Co > Fe, Co > W > Cu > V > Zr [115]. For sustainable energy applications, catalysts composed of earth–abundant, low-cost elements are crucial to ensure economic feasibility. This chapter only focuses on the electrochemical hydrogen evolution catalysts made of Fe, Co and Ni [116]. 3.2.1. Fe/CNTs Composite Catalysts Fe is abundant in soil and possesses good electrical conductivity, showing great potential for electrocatalytic hydrogen evolution across the entire pH range [117]. However, in aqueous environments, especially under acidic conditions, iron is prone to corrosion, leading to material and performance degradation. These shortcomings urge researchers to look for cheap, available and stable support materials to achieve efficient catalytic reduction of protons to produce hydrogen. Jin et al. [118] prepared and characterized three new iron phosphine complexes FeP^R (R = F, H, Me) and their covalently bonded CNT hybrids CNT-F-FeP^R. Therefore, it may be a feasible strategy to covalently connect CNTs with catalytic metal complexes such as FeP^R (R is defined as ligand substituent) to improve the HER performance in aqueous medium. CNT-f-FeP^F exhibits superior electrochemical performance in aqueous media. TMs oxides are good candidates as electrocatalysts in HER reaction [119]. Shi et al. [120] successfully synthesized a new type of Fe₃O₄ NPs by a simple method, and wrapped it in a dendritic carbon layer (DCL) structure wrapped with hollow carbon nanotubes (HCNTs). The composite material demonstrated superior HER catalytic activity owing to the synergistic interplay between N dopants, metal oxide NPs, and hollow CNTs architectures. This distinctive architectural configuration provides novel insights for designing advanced electrocatalytic materials. Chen et al. [121] used ionic liquid (IL) as Shuang Yuan of Fe and P to prepare iron phosphate by microwave irradiation. Moreover, CNTs play a dual role: they facilitate the formation of iron phosphate NPs (Figure 7a) and enhance the conductivity of the catalyst, thereby improving its catalytic performance (Figure 7b,c). Iron-based catalysts are prone to oxidation–induced deactivation, which can be mitigated through CNTs encapsulation. Yu et al. [122] used iron NPs encapsulated by N-CNTs as the integrated electrode of HER on foamed iron, and achieved an η of 525 mV at a current density of 10 mA cm⁻² (Figure 7d–f). This electrocatalyst also exhibits excellent HER performance across different pH levels (Figure 7g). 3.2.2. Co/CNTs Composite Catalysts The electrocatalytic HER performance of Co has garnered considerable research interest in the field of transition metal-based materials in recent years. While conventional volcano plot relationships indicate Co possesses strong hydrogen intermediate (H_ad) binding energy, its HER activity remains substantially lower than PGMs positioned at the volcano peak. By adjusting the electronic structure and geometric structure of the active center, the performance of Co-based catalyst was optimized to make it have catalytic activity and stability similar to Pt. In alkaline electrolyte, the strong oxygen affinity of Co is beneficial to accelerate the dissociation of water before H_ad is adsorbed by Volmer step, because water molecules are proton donors. Accordingly, the last decade has seen significant advances in synthesizing Co-based catalysts spanning alloys, oxides, and chalcogenides [123,124,125,126]. The catalytic efficiency can be further enhanced by systematically tuning key parameters: electronic states, band structures, adsorption energetics, conductivity, surface area, and active site density [127,128]. However, its poor conductivity and insufficient exposure of active sites are still the only factors restricting its practical application. CNTs have clear channel structure and large specific surface area, which is beneficial to the transfer of electrons and mass [129]. Combining with CNTs can not only obtain larger electrochemical active area and improve conductivity, but also anchor metal compounds. Zou et al. [130] reported the synthesis of cobalt-rich nitrogen-doped carbon nanotubes (Co-NRCNTs), which can effectively catalyze the HER with activity close to that of Pt. Due to the coating of CNTs on cobalt, the corrosion of electrolyte on the catalyst is greatly reduced. CNTs have nitrogen doping and defect-rich structures, which have good restraint and protection effects. N-rich doping enhances CNTs’ electrical conductivity while simultaneously generating abundant active sites for HER electrocatalysis [131]. Unlike doping CNTs, modifying Co is also an important method to enhance hydrogen evolution activity. Co phosphide exhibits near-optimal H_ad binding energy at proton-accepting sites, making it a highly promising HER catalyst [132,133]. Cai et al. [134] synthesized Co₂P/Co₄N/CNTs composite using zeolite pyrazole acid skeleton -67 (ZIF-67) as self-sacrifice template. The composite catalyst demonstrates exceptional HER activity due to the cooperative effects between Co₄N and Co₂P phases, abundant exposed active sites, and the superior charge transport capability of CNTs (η₁₀₀ = 228 mV). Yan et al. [135] proposed an integrated coordination-polymerization strategy to prepare Co@N-CNT@g-C₃N₄, and carried out an effective HER at all pH values. Importantly, Co particles, CNTs and g-C₃N₄ are skillfully assembled at the same time, and a tightly integrated interface is constructed, thus improving the electron transfer efficiency. Yang et al. [136] successfully prepared a HER electrocatalyst coupling Co nanoparticles with N–doped carbon nanotubes/graphitic nanosheets (Co@NCNTs/NG) through a high-temperature pyrolysis method. In Co@NCNTs/NG-1, the NCNTs are interspersed in a manner that promotes thorough contact among different phases (Figure 8a,b). This arrangement not only accelerates the transfer of electrons and reaction species throughout the reaction but also prevents the aggregation of Co nanoparticles, thereby enhancing stability. Co@NCNTs/NG–1 exhibits outstanding HER performance in 1 M KOH solution (Figure 8c–e). Co@NCNTs/NG-1 demonstrates a stable potential for 50 hours at a constant current density of 50 mA cm⁻² (Figure 8f), indicating exceptional stability under alkaline HER conditions. Indeed, the interaction between Co nanoparticles and NCNTs plays a crucial role in enhancing HER electrocatalytic activity. The tight anchoring of cobalt nanoparticles on NCNTs effectively suppresses their aggregation during pyrolysis. Moreover, the strong interaction between Co nanoparticles and NCNTs contributes to the improved durability of Co@NCNTs/NG-1 in HER applications. 3.2.3. Ni/CNTs Composite Catalysts Ni is one of the non-precious metals in group 10, with atomic number 28 and silvery white appearance. Its valence is 0, 1, 2 or 3, and it is one of the most promising Pt substitutes. It is the fifth most abundant element on the earth, and it is cheaper. For more than ten years, Ni has been widely used as HER electrocatalyst, especially in alkaline medium, because of its good catalytic activity, good corrosion resistance in hot concentrated alkaline solution and more stability than Fe and Co [137]. However, due to its high η and Tafel slope, pure Ni is not an effective electrocatalyst, but its electrocatalytic activity can be improved through nanostructures, which include making nano-sized Ni particles to increase its surface area. Other methods include preparing Ni alloys such as NiAl₃, Ni₃Al, NiMo, NiCo and NiFe, and doping heteroatoms such as N and P, can reduce adsorption energy, accelerate hydrolysis, provide sufficient protons and improve hydrogen production. The doping of carbon nanostructures such as CNTs improves the dispersibility and conductivity of the catalyst [138]. Wang et al. [139] synthesized Ni NPs encapsulated by N-doped bamboo CNTs by one-pot pyrolysis at low temperature. The incorporation of metallic Ni facilitated the conversion of the catalyst carrier from PCN to N-doped carbon, thereby markedly enhancing charge transfer efficiency. The coexistence of multiphase Ni, N-rich doping, and compartmentalized CNTs morphology creates a high density of active centers, enabling efficient alkaline HER catalysis. Ni/NC-0.35 shows the best electrocatalytic stability and activity with η₁₀ of 133 mV. Yan et al. [140] demonstrated a simple template participation strategy, which was used to directly grow Ni NPs and N-doped CNTs on carbon nanorod substrates in situ to form hierarchical branching structure (Ni@N-CNT/NRs). The meticulous design of this unique layered structure not only increases the surface area but also reduces the charge transfer resistance and enhances mechanical robustness. The list of HER electrocatalysts for Fe, Co and Ni-based CNTs composites is shown in Table 2. 3.3. Polymetallic and Alloy/CNTs Composite Catalysts Recently, a lot of work has been devoted to the manufacture of TMs and their derivatives for HER electrocatalysis. These studies demonstrate that alloy-type TMs (e.g., NiCo, FeCo and FeNi) exhibit substantially improved catalytic activity relative to monometallic systems, attributable to electronic structure modulation, reduced activation barriers, and accelerated reaction kinetics through synergistic effects. The FeNi alloy system is particularly remarkable as Fe doping cooperatively tailors the electronic structure of Ni sites and fine-tunes reactant adsorption energetics, thereby significantly enhancing electrocatalytic performance [151]. However, the intrinsic ferromagnetism and chemical instability of bare FeNi alloys under harsh conditions lead to severe nanoparticle aggregation and consequent active site loss. Consequently, a precisely controlled synthetic strategy is crucial to ensure robust immobilization of metal species while preventing FeNi nanoparticle aggregation. NCNTs exhibit ideal properties for electrocatalysis: large accessible surface areas, strong charge-transfer capability, minimal diffusion resistance, and remarkable mechanical durability. This strategy significantly enhances the alloy's electrical conductivity while simultaneously protecting active particles from corrosion and detachment through encapsulation effects. The coupling of NCNTs with HER-active FeNi modulates the electronic configuration and lowers the energy barrier for favorable adsorbate binding, thereby achieving concurrent high activity and stability. Therefore, it is a promising method to embed NiFe alloy particles in NCNTs, which can meet the application requirements of electrode catalysts in electrolytic cells. To sum up, Zhang et al. [152] developed a simple and efficient method to prepare N-doped CNTs encapsulated FeNi alloy nanoparticles (FeNi@NCNTs). The experimental results show that the Fe₁Ni₄@NCNTs material has the highest activity and durability in HER. NCNTs' self-catalyzing hierarchical design optimally exposes active sites while minimizing electron transport distances, thereby amplifying catalytic synergy. Fe doping simultaneously enhances the intrinsic catalytic activity of metallic Ni through electronic modulation. This work establishes a foundational strategy for developing alloy-based catalysts with exceptional activity and stability for overall water splitting applications. Optimizing the performance of monoatomic catalysts has been a hot topic in recent years, but achieving the synergistic effect of diatomic catalysts remains a significant challenge. Recent years have witnessed substantial advancements in non-precious metal HER catalysts through extensive investigations. Shakir et al. [153] synthesized a ternary composite of CNTs-coated CoFe₂O₄/CoFe LDH (CoF/CoFe LDH@CNTs) using hydrothermal and ultrasonic methods. The SEM image in Figure 9a reveals a stacked plate/sheet-like morphology along with nanoparticles of CoF/CoFe LDH. Following the formation of the CoF/CoFe LDH@CNTs composite (Figure 9b), the CoF/CoFe LDH appears uniformly wrapped by thin, thread-like CNTs, suggesting strong interfacial bonding between the components due to the ultrasonic-assisted synthesis. CoF/CoFe LDH@CNTs exhibits the lowest η and Tafel slope values, indicating superior HER performance compared to other electrocatalysts (Figure 9c,d). Among these, transition metal/nitrogen-doped carbon-based materials (TMs/NC), particularly cobalt/nitrogen-doped carbon (Co/NC) materials [154], have shown excellent catalytic activity. They are considered one of the most promising candidates to replace current advanced precious metal-based catalysts for renewable energy technologies. Hu et al. [155] synthesized RuM (M = Cu, Rh, and Pd) alloy NPs via nanosecond laser ultrafast confined alloying (LUCA). In summary, the sufficient incorporation of non-precious metals in the alloy significantly reduces catalyst costs while maintaining or even enhancing the intrinsic activity of the alloyed catalyst. The integration of CNTs with Cu NPs, Ru NPs, and RuCu alloy NPs significantly enhanced the catalytic activity, following the order: RuCu > Ru > Cu NPs, demonstrating the synergistic effect of RuCu alloy in boosting HER performance (Figure 9e–g). Figure 9h,i demonstrate that the incorporation of Cu not only accelerates water dissociation to provide more protons for subsequent reactions, but also facilitates H adsorption/desorption. Figure 9j reveals a negative shift in the Hupd peak potential for Ru₉₅Cu₅/CNTs (0.061 V) compared to Ru/CNTs (0.076 V), suggesting that the improved HER performance stems from moderated hydrogen chemisorption strength, which promotes more efficient hydrogen desorption. These studies have promoted the development of TMs-based CNTs composites in hydrogen evolution. The HER electrocatalysts for polymetallic and alloy/CNTs composites are shown in Table 3. 3.4. The Environmental Impact of HER Results The HER reaction process includes two steps: the Volmer step and either the Tafel step or the Heyrovsky step [169]. In the case of an acidic electrolyte, the first step of the Volmer reaction involves the electrochemical discharge to form an adsorbed hydrogen intermediate (H^∗) on the active site. Subsequently, adjacent adsorbed H^∗ intermediates combine to generate H₂ (Tafel reaction). Alternatively, the adsorbed H^∗ can combine with a proton, accompanied by an electron transfer reaction to produce H₂ (Heyrovsky reaction) [11]. The HER process in alkaline environments also involves the dissociation of water, which leads to the need for more energy. In practical applications, it is highly desirable for ideal electrocatalysts to operate under harsh conditions. Therefore, the design and development of highly efficient pH-universal HER electrocatalysts is essential. Recently, TMs-based catalysts have been reported, but these catalysts are susceptible to acid and alkali corrosion [99]. Chen et al. [170] designed Co@CNTs|Ru catalysts demonstrate exceptional pH-universal activity, delivering 10 mA cm⁻² at ultralow overpotentials (10 mV alkaline, 32 mV acidic, 63 mV neutral) with 50-h durability, where CNTs effectively prevent catalyst corrosion. In summary, CNTs not only protect TMs from acid/alkali corrosion but also provide spatial nanoconfinement to enhance electron/charge transfer, making them an ideal choice for developing pH-universal HER electrocatalysts.