Acid-stable bimetallic phosphide-silver core-shell nanowires with a seamlessly conductive network for enhanced hydrogen evolution reaction

Abstract

Developing low-cost and high-performance acid-resistant electrocatalysts is essential for the industrialization of hydrogen production via proton exchange membrane water electrolysis. Herein, an acid-stable bimetal phosphide (NiCoP) catalyst wrapped around silver nanowires (Ag NWs), forming a seamless conductive core-shell structure (NiCoP@Ag NWs), is reported to enhance the hydrogen evolution reaction (HER). The incorporation of Ag NWs creates an uninterrupted conductive network that facilitates efficient electron transfer and provides a large electrolyte-accessible surface area for mass transport. The synergistic interaction among Ni, Co, and P further optimizes electronic structure and decreases the energy barrier of NiCoP@Ag NWs for H* adsorption and desorption. More importantly, the distinctive core-shell structure imparts outstanding acid resistance to the catalyst. Notably, NiCoP@Ag NWs displays remarkable HER performance, with a low overpotential of 109 mV (significantly lower than Ni₂P@Ag NWs at 144 mV and Co₂P@Ag NWs at 174 mV) at a current density of 10 mA/cm², along with excellent durability exceeding 100 h in acidic media. These features surpass most reported non-noble metal catalysts, demonstrating extraordinary potential for practical hydrogen production via acidic water electrolysis.

Graphical abstract

Keywords

bimetal phosphide / Ag nanowires (Ag NWs) / core-shell structure / hydrogen evolution reaction (HER)

1 Introduction

Hydrogen is regarded as a prospective energy carrier due to its exceptional properties and high energy density, exceeding that of oil and coal [1,2]. Renewable energy-driven electrocatalytic water splitting provides a sustainable and cost-effective approach to producing high-purity green hydrogen [3,4]. Among available technologies, proton exchange membrane water electrolysis (PEMWE) demonstrates significant advantages over conventional alkaline water electrolysis (AWE), including higher current density, reduced ohmic resistance, elevated operating pressure, and superior hydrogen gas purity [5,6]. Currently, precious metal-based materials (e.g., Pt and Ru) remain the most efficient cathode electrocatalysts for the hydrogen evolution reaction (HER) in PEMWE, owing to their rapid kinetics and robust stability [7,8]. However, their high cost and limited availability pose significant challenges to their large-scale application [9,10]. This underscores the urgent need to develop alternative HER catalysts to be capable of delivering high-efficiency hydrogen evolution under acidic conditions [11].

In recent decades, non-noble metal electrocatalysts such as transition metal sulfides [12,13], nitrides [14,15], borides [16], carbides [17–19], selenides [20,21] have demonstrated outstanding HER activity. Among these, transition metal phosphides (TMPs) have attracted substantial attention as promising catalysts, owing to their exceptional physicochemical properties and remarkable catalytic performance [22]. The negatively charged phosphorus species present on TMP surfaces, generated via electron transfer, serve as active sites for protons adsorption while simultaneously enabling hydrogen transfer from phosphorus to transition metal atoms (M) [23,24]. Various TMPs have been extensively studied, with catalysts based on Ni, Co, Fe, and Mo showing especially promising performance [25]. Recent strategies have focused on further enhancing HER activity in both alkaline and acidic environments by doping additional metals to modulate electronic structures, thereby leveraging synergistic effects among multiple metal centers [26–28]. This is accomplished through the doping of additional metals, which modulates the electronic structure of metallic phosphides, thereby fully exploiting the synergistic effects among distinct metal centers. For instance, Duan et al. [29] fabricated an NCP@NCH/CC catalyst featuring three-dimensional self-supporting nanoarchitectures, where core-shell NiCoP nanowires and NiCo LDH nanosheets synergistically promote water dissociation and improve catalytic activity, achieving low overpotentials (141–182 mV) in both alkaline and acidic media.

Silver nanowires (Ag NWs) are widely recognized for their exceptional electrical conductivity (σ ≈ 6.3×10⁷ S/m) and mechanical flexibility. Zhou et al. [30], for instance, developed a Ni-Co@Ag NWs heterostructure that achieved highly efficient HER with an overpotential of only 33 mV at 10 mA/cm² in alkaline media. In this system, Ag NWs significantly reduced charge transfer resistance, leveraging their intrinsic conductivity, and facilitated electron transport to the active Ni–Co active centers. However, limited acid-resistance remains a critical obstacle to broader applicability.

In the present study, an acid-stable bimetallic phosphide (NiCoP) catalyst is introduced, ingeniously engineered to wrap around a network of Ag NWs. Notably, the Ag NWs function as a high-conductivity substrate, remarkably increasing the specific surface area of the catalytic system while forming a continuous electron-conducting pathway to the active sites. Upon uniform coating, the NiCoP catalyst forms a cohesive conductive core-shell structure on the Ag NWs framework, when uniformly coated on the Ag NWs framework. This hierarchical architecture significantly enhances HER kinetics and operational stability in acidic conditions.

Theoretical analysis reveals that NiCoP@Ag NWs exhibit a lower energy barrier for H* adsorption (ΔG_H* = −0.38 eV) and desorption compared to Ni₂P@Ag NWs and Co₂P@Ag NWs, owing to the synergistic interactions among Ni, Co, and P. Consequently, the NiCoP@Ag NWs electrode exhibits remarkable HER performance in acidic media, achieving current densities of 10 and 100 mA/cm², at overpotentials of only 109 and 177 mV, respectively. In addition to high activity, the catalyst demonstrates excellent operational durability, maintaining stable performance for over 100 h of continuous operation.

These characteristics exceed those of most reported non-noble metal HER catalysts, highlighting the significant technological potential of this system for industrial hydrogen production. The synergistic integration of favorable kinetics and robust structural stability establishes the NiCoP@Ag NWs catalyst as a viable alternative to conventional noble metal catalysts in proton exchange membrane electrolyzers.

2 Experimental section

2.1 Synthesis of Ag NWs

Ag NWs were synthesized via a characteristic hydrothermal protocol, where an ethylene glycol-based precursor solution containing 0.012 mol/L polyvinylpyridone, 0.051 mol/L AgNO₃, and 7.19 μmol/L FeCl₃ was thermally treated at 110 °C for 12 h [30]. Following synthesis, the nanowires were purified through successive washings with acetone and ethanol to remove residual byproducts. The purified nanowires were then homogeneously dispersed in ethanol to form a stable colloidal suspension. This well-dispersed suspension was subsequently applied onto the surface of flexible cloth fabric via a controlled coating process, resulting in a highly conductive substrate with uniform nanowire distribution.

2.2 Synthesis of NiCoP@ Ag NWs

Ag NWs cloth was infiltrated into a mixed solution containing 4 mmol NiCl₂·6H₂O, 2 mmol CoCl₂·6H₂O, 9 mmol NaH₂PO₂·H₂O, 4 mmol NaCl, and 50 mL deionized water. NiCoP was synthesized via a facile and rapid electrodeposition process, employing chronoamperometry at a constant potential of −1.0 V (versus saturated calomel electrode (SCE)) for 400 s. After synthesis, the samples were thoroughly rinsed with deionized water and air-dried at ambient temperature for further characterization.

2.3 Characterizations

Morphological features and elemental distribution of the catalyst were investigated using a JEOL JEM2100 transmission electron microscope equipped with EDS. Crystalline structure analysis was conducted through XRD measurements on a Bruker D8 Advance with Cu Kα radiation (λ = 1.5406 Å), and data were processed using Jade 9.0 software. Surface chemical states were examined by XPS on a Thermo Fisher Scientific instrument with Al Kα excitation (hν = 1486.6 eV), and the spectra were analyzed using Casa XPS software.

2.4 Electrochemical measurements

Electrochemical measurements were performed using a CHI 660E electrochemical workstation (CH Instruments) using a standard three-electrode configuration at room temperature. The prepared electrode served as the working electrode, an SCE as the reference electrode, and a platinum sheet as the counter electrode. The electrolyte was 0.5 mol/L H₂SO₄ aqueous solution. LSV with a scan rate of 5.0 mV/s and 95% iR compensation was employed to evaluate HER activity. EIS was conducted across a frequency range of 0.1 Hz to 100 kHz to characterize charge transfer behavior. The ECSA was quantified by determining the double-layer capacitance (C_dl) through CV. CV curves were recorded within an appropriate potential window at varying scan rates, extracting current difference (Δj) in non-Faradaic regions. C_dl values were derived from the linear slope of Δj versus scan rate (v), with C_dl directly proportional to the effective ECSA. Catalyst stability was assessed via chronopotentiometry by monitoring potential variations during prolonged operation under constant current density. Potential values were standardized against the reversible hydrogen electrode (RHE) using

(1)\mathit{E}(\mathrm{R}\mathrm{H}\mathrm{E})=\mathit{E}(\mathrm{S}\mathrm{C}\mathrm{E})+0.059\mathrm{p}\mathrm{H}+\mathrm{0.241.}

2.5 Theoretical computation

All quantum mechanical calculations were performed using spin-polarized DFT as implemented in the Vienna Ab initio Simulation Package (VASP) [31–33]. The projector-augmented wave (PAW) method [34,35] was employed to describe electron-ion interactions, with exchange-correlation effects treated through the Perdew-Burke-Ernzerhof (PBE) parameterization of the generalized gradient approximation (GGA) [36]. Systematic parameter optimization established a plane-wave basis set with a 500-eV kinetic energy cutoff and a Monkhorst-Pack k-point mesh of 4 × 4 × 1 for precise Brillouin zone sampling. The conjugate-gradient (CG) algorithm was utilized to achieve electronic convergence in self-consistent field calculations, with iterative processes terminating when energy differences between consecutive steps fell below 10⁻⁵ eV. Structural optimizations employing quasi-Newton dynamics maintained rigorous convergence criteria, requiring atomic force to be below 0.01 eV/Å on relaxed atoms [37,38].

A NiCoP model with a (111) surface was constructed using a 2 × 2 × 1 supercell, followed by structural optimization. A vacuum layer of 15 Å was introduced along the z-axis to prevent artificial interactions between periodic images. For comparison, Ni₂P and Co₂P models were also created. Hydrogen adsorption characteristics on NiCoP, Ni₂P, and Co₂P were evaluated thermodynamically. The adsorption enthalpy (∆E_H*) for atomic hydrogen was calculated as

(2)\mathrm{\Delta }{E}_{{\mathrm{H}}^{\ast }}=E_{\mathrm{s}\mathrm{l}\mathrm{a}\mathrm{b}+\mathrm{H}}-E_{\mathrm{s}\mathrm{l}\mathrm{a}\mathrm{b}}-E_{{\mathrm{H}}_2}.

This methodology extended to multiple hydrogen atoms with consistent computational parameters across systems. The free energy (ΔG) during transformation was evaluated using

(3)\mathrm{\Delta }G=\mathrm{\Delta }E+\mathrm{\Delta }\mathrm{Z}\mathrm{P}\mathrm{E}–T\mathrm{\Delta }S.

where ΔE is the change in total energy, ΔZPE represents the zero-point energy correction, and ΔS accounts for the entropy change between initial and final states.

3 Results and discussion

3.1 Synthesis and structure characterization of catalysts

As shown in Fig. 1(a), a facile two-step preparation method is employed to synthesize NiCoP on the surface of Ag NWs. First, the Ag NWs are uniformly deposited onto a cloth substrate. Next, NiCoP is grown in situ on the Ag NWs-coated cloth via an electrodeposition process, using NiCl₂·6H₂O, CoCl₂·6H₂O, and NaH₂PO₂·H₂O as Ni, Co, and P sources, respectively. The preparation parameters, including electrodeposition time, deposition potential, and precursor concentrations have been systematically optimized through extensive experimental screening, as presented in Fig. S1 (Electronic Supplementary Material). To demonstrate the excellent properties of the dual-transition metal phosphide, mono-metal phosphide samples (Ni₂P@Ag NWs and Co₂P@Ag NWs) were synthesized under the same conditions. Figure S2 shows the macroscopic morphology of these electrocatalysts.

The surface morphology of NiCoP@Ag NWs, observed via scanning electron microscopy (SEM) in Fig. 1(b), confirms the uniform coating of NiCoP over the Ag NWs. The resultant nanoscale protrusions formed on the NiCoP layer enhance the electrochemically active surface area, thereby facilitating efficient adsorption and subsequent reduction of protons (H⁺) at catalytic sites. Upon exposure to air, partial oxidation of surface phosphide to phosphate may occur, potentially leading to the formation of the amorphous layer, consistent with previous findings [39].

High-resolution transmission electron microscopy (HRTEM) was employed to examine the crystalline structure of NiCoP@Ag NWs, (Figs. 1(c)–1(d) and S3). The analysis reveals distinct lattice fringes with interplanar spacings of 0.206 and 0.221 nm, corresponding to the (201) and (111) crystallographic planes of NiCoP, respectively. Fourier Transform diffraction pattern demonstrates the presence of (111), (211), and (221) crystallographic faces of the NiCoP. Moreover, energy-dispersive X-ray spectroscopy (EDS) elemental mapping images (Figs. 1(e)–1(i)) demonstrate that P, Co, and Ni atoms are uniformly distributed along the Ag NWs surface.

These results confirm the successful construction of NiCoP on Ag NWs, forming a seamless conductive core-shell structure, in which the rough surface morphology of NiCoP further contributes to an increased catalytic active area, enhancing the overall HER performance.

The phase composition of the synthesized samples was analyzed using X-ray diffraction (XRD). To reduce substrate-related interference, the phosphide materials were deposited on fluorine-doped tin oxide (FTO) substrates for XRD measurements, as shown in Fig. 2(a). Notably, all diffraction patterns were carefully analyzed after excluding the strong signals originating from the FTO substrate. The XRD pattern of NiCoP@FTO displays a distinct peak at 44.6°, corresponding to the (201) crystal plane of NiCoP (PDF#04-001-6153). In comparison, Ni₂P@FTO exhibits characteristic peaks at 31.8° and 55.0°, which are ascribed to the (011) and (211) crystal planes of Ni₂P (PDF#03-065-3544), respectively. For Co₂P@FTO, a diffraction peak at 44.8° is attributed to the (201) crystalline plane of Co₂P (PDF#00-054-0413). These findings are consistent with the previous structural characterizations, which confirms the successful formation of NiCoP bimetallic phosphide on the Ag NWs surface.

Further, X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical states of NiCoP, Ni₂P, and Co₂P on the Ag NWs surface. As illustrated in Fig. S4, the survey spectrum confirms the presence of Ni, Co, P, Ag, and O elements in NiCoP@Ag NWs, as well as in the control samples. The Ni 2p spectra (Fig. 2(b)) of NiCoP@Ag NWs and Ni₂P@Ag NWs exhibit four pairs of peaks. A 2p_3/2 binding energy peak at 852.4 eV, along with a peak at 869.4 eV, suggests the presence of partially charged Ni⁰ species associated with Ni–P bonding, consistent with metallic nickel characteristics [40,41]. The relative Ni–P peak intensity in NiCoP@Ag NWs is lower than that in Ni₂P@Ag NWs, suggesting that Co incorporation reduces the Ni–P coordination. Peaks corresponding to oxidized Ni^2+/3+ states are attributed to surface oxidation from air and water exposure during preparation [42,43].

The Co 2p spectrum (Fig. 2(c)) displays three pairs of distinct peaks, including prominent signals at 776.8 eV (2p_3/2, assigned to Co–P) and 782.2 eV (2p_3/2, attributed to Co²⁺ species) [44,45]. Among them, the 776.8 eV (2p_3/2) peak, close to the binding energy of metallic cobalt, indicates partially charged Co⁰ species in a Co–P configuration, corroborated by a corresponding 792.1 eV feature. Compared to their mono-metal counterparts, the binding energy of Ni^2+/3+ and Co²⁺ in NiCoP@Ag NWs shift positively, indicating that bimetallic doping induces electron redistribution, which is favorable to optimize H* adsorption energy.

In the P 2p spectrum (Fig. 2(d)), a peak at 129.3 eV is assigned to the P 2p_1/2 orbital, corresponding M-P bonding within the phosphide framework [46]. A dominant peak at 133.3 eV is assigned to P–O species in phosphate or phosphite environments [47]. The relatively weak P 2p_1/2 peak in NiCoP suggests that surface phosphide is largely encapsulated by phosphide layers [48]. Notably, the incorporation of Co promotes increased phosphate formation, as indicated by the stronger M–P signal in Ni₂P [49,50]. Additionally, the binding energy of phosphorus in NiCoP (128.8 eV) lies between that of Ni₂P (129.4 eV) and Co₂P (127.8 eV), suggesting an electronic rearrangement around the P atom in the bimetallic phosphide.

Collectively, these XPS results confirm the formation of a strong electronic interaction within the synthesized NiCoP@Ag NWs nanohybrid architecture, supporting the enhanced electrocatalytic activity observed.

3.2 Density function theory (DFT) calculations

The electronic properties of NiCoP@Ag NWs were further examined through theoretical analysis. Specifically, to elucidate the electronic interactions among Ni, Co, and P in NiCoP, density functional theory (DFT) calculations were performed to analyze charge density distributions and density of states (DOS). The results revealed a significant electron redistribution in NiCoP, characterized by reduced electron density around Ni and Co atoms and increased density surrounding the P atoms, consistent with the XPS findings (Figs. 3(a)–3(c)). Furthermore, the DOS analysis demonstrated that NiCoP exhibits the largest separation between the d-band center and Fermi level (EF) compared to Ni₂P and Co₂P (Fig. 3(d)), indicating improved hydrogen adsorption (H*) and desorption (H₂) properties. Moreover, the calculated Gibbs free energies for H* adsorption (ΔG_H*) on NiCoP, Ni₂P, and Co₂P are −0.38, −0.42, and −0.67 eV, respectively (Figs. 3(e) and S5). These values suggest that the synergistic effect among Ni, Co, and P effectively modulates the electronic structure and lowers the energy barrier for H* adsorption and desorption in NiCoP@Ag NWs. This optimized configuration contributes to enhanced catalytic activity, positioning NiCoP@Ag NWs as a promising candidate for efficient HER under acidic conditions.

3.3 Electrocatalytic hydrogen evolution performance

To comprehensively evaluate the HER performance of NiCoP@Ag NWs, several control catalysts with similar compositions but different architectures and conductive substrates were investigated, including Ni₂P@Ag NWs, Co₂P@Ag NWs, and NiCoP on Ni foam (denoted as NiCoP@NF). All electrochemical measurements were conducted in 0.5 mol/L H₂SO₄ using a standard three-electrode configuration. The NiCoP@NF was synthesized using the same procedure as NiCoP@Ag NWs, with Ni foam serving as the substrate in place of Ag NWs.

Figures 4(a) and 4(b) shows the electrochemical polarization curves of NiCoP @Ag NWs catalyst as well as control samples for HER. NiCoP @Ag NWs exhibits a remarkably low overpotential of 109 mV to achieve a current density of 10 mA/cm², outperforming NiCoP@NF (170 mV), Ni₂P@Ag NWs (144 mV), and Co₂P@Ag NWs (174 mV). At a higher current density of 100 mA/cm², NiCoP@Ag NWs still maintains a competitive overpotential of 177 mV. To assess the electrocatalytic HER kinetics, Tafel slopes were derived from the polarization curves (Fig. 4(c)). NiCoP@Ag NWs displays the lowest Tafel slope of 62.6 mV/dec, compared to NiCoP@NF (68.7mV/dec), Ni₂P@Ag NWs (72.1 mV/dec), and Co₂P @Ag NWs (102.1 mV/dec), indicating faster HER kinetics. Although Pt/C still outperforms all tested systems (Fig. S6), NiCoP@Ag NWs significantly narrows the performance gap between noble and non-noble metal-based catalysts in acidic environments. The Tafel slope of NiCoP@Ag NWs is close to 40 mV/dec, which suggests that the electrocatalytic HER kinetics of NiCoP@Ag NWs is determined by the Heyrovsky mechanism [51].

The electrochemical impedance spectroscopy (EIS) (Fig. 4(d)) further supports this finding, as NiCoP@Ag NWs exhibits a significantly reduced charge transfer resistance (R_ct = 0.74 Ω), outperforming NiCoP@NF (0.81 Ω), Ni₂P@Ag NWs (1.19 Ω), and Co₂P@Ag NWs (1.91 Ω). This reduced interfacial resistance underscores the critical role of the Ag NWs conductive network in enhancing charge transport kinetics and overall conductivity. Moreover, the electrochemical surface area (ECSA) was estimated using double-layer capacitance (C_dl) values derived from cyclic voltammetry (CV) measurements (Figs. S7(a)–S7(d)). As shown in Fig. 4(e), the calculated C_dl values confirm the enhanced interfacial activity of NiCoP@Ag NWs, consistent with the EIS observations. NiCoP@Ag NWs displays the highest C_dl (38.39 mF/cm²), significantly greater than those of NiCoP@NF (10.52 mF/cm²), Ni₂P@Ag NWs (13.52 mF/cm²), and Co₂P@Ag NWs (2.02 mF/cm²), confirming a larger active surface area due to the conductive core-shell structure.

Experiments reveal that bimetallic phosphide catalysts (NiCoP@Ag NWs) have higher HER activity concerning monometallic phosphide (Ni₂P/Co₂P @Ag NWs), and the catalytic performance of the catalytic system with Ag NWs substrate is also higher than that with Ni foam substrate. These results are consistent with DFT predictions and confirm the optimized electronic structure and decrease the energy barrier of NiCoP@Ag NWs for H* adsorption and desorption by the synergism of Ni, Co, and P in bimetal phosphide.

Acid-resistant stability is a crucial performance indicator for evaluating the practical feasibility of NiCoP@Ag NWs catalysts in industrial-scale PEMWE for hydrogen production. Operating under harsh acidic conditions (pH ≈ 0) requires the catalyst to maintain exceptional electrocatalytic activity alongside long-term structural and chemical stability. To assess this, the catalytic durability of NiCoP@Ag NWs was tested via chronopotentiometry at a constant current density of 10 mA/cm². As illustrated in Fig. 4(f), the linear sweep voltammetry (LSV) curve after 100 h of continuous operation nearly overlaps with that recorded before the test. The overpotential exhibits only a slight increase from 108 to 114 mV, highlighting the catalyst’s outstanding electrochemical stability.

The minor fluctuations observed during prolonged operation are likely attributable to transient hydrogen bubble formation at the electrode-electrolyte interface. Post-test SEM (Fig. S8) confirms that the core-shell nanowire architecture remains structurally intact without significant degradation, indicating robust mechanical stability under acidic HER conditions, which can be attributed to the synergistic effect of Ni, Co, and P within the bimetal phosphide framework and the protective effect of the core-shell structure of Ag-NWs.

To further assess the potential for industrial application, additional constant-current durability testing was conducted at a high current density of 100 mA/cm². As demonstrated in Fig. S9, NiCoP@Ag NWs exhibited excellent operational stability after 50 h of continuous testing, showing minimal potential fluctuation (< 10 mV), underscoring the catalyst’s robust stability under high-current conditions, an essential requirement for practical electrolyzer designs.

Compared with the majority of documented non-noble catalysts operating in acidic conditions, NiCoP@Ag NWs catalytic system also exhibited outstanding electrocatalytic performance for HER (Fig. 4(g)), indicating that NiCoP@Ag NWs hold its viability for practical applications in the PEMWE-related fields.

4 Conclusions

A non-noble metal NiCoP@Ag NWs catalyst with an integrated core-shell nanostructure was successfully fabricated through a cost-effective and facile electrochemical deposition approach. The incorporation of Ag NWs creates a seamlessly conductive network, serving as an effective electron transport pathway while simultaneously offering an extensive electrolyte-accessible surface area to enhance mass diffusion. The unique core-shell structure of NiCoP@Ag NWs endows the catalyst with exceptional acid resistance. The synergism of Ni, Co, and P further optimizes the electronic structure and decreases the energy barrier of NiCoP@Ag NWs for H* adsorption and desorption. Consequently, the fabricated NiCoP@Ag NWs catalyst exhibits low overpotential and long-term operational stability in an acidic medium. This work demonstrates the effectiveness of an innovative strategy in fabricating acid-stable catalysts by incorporating Ag NWs into transition metal phosphides.

5 Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 52301257).

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