Decoupling bulk composition and alloying effects in Ni-Mo materials for electrocatalytic hydrogen evolution at near-neutral pH

Ni-Mo electrocatalysts are extensively studied for the hydrogen evolution reaction (HER). This includes HER at near-neutral pH conditions relevant to, e.g., microbial electrosynthesis (MES). So far, the role of the Ni:Mo ratio in bulk for determining HER performance remains unclear, especially for commercially available powdery electrocatalysts. Here, we examined commercial Ni-Mo powders with different nominal Ni:Mo ratios to assess how composition and mass loading influence HER performance. Despite differences in bulk Ni:Mo ratios, no significant variation in HER performance was observed at equal mass loadings. In contrast, increasing the electrocatalyst mass loading from 0.35 ± 0.04 to 1.30 ± 0.07 mg cm −2 consistently improved HER performance, decreasing the overpotential at −1 mA cm −2 from 463 ± 50 to 210 ± 9 mV, with an intermediate value of 367 ± 33 mV at a loading of 0.61 ± 0.02 mg cm −2. Comprehensive characterization by XRD, SEM, EDX, and XPS shows that the Ni-Mo powders are not true Ni-Mo alloys but rather physically mixed phases with notable Ni- and Mo-rich segregation. These results demonstrate that the bulk Ni:Mo ratio does not accurately reflect the catalytic structure and hence cannot be used alone to assess structure-function relationships for HER performance. This study underscores the importance of thorough characterization, even of commercial electrocatalysts, and highlights the significance of surface structure over nominal bulk composition for Ni-Mo-based HER electrocatalysts.

The electrochemical hydrogen evolution reaction (HER) is of paramount importance in emerging technologies. This includes microbial electrosynthesis (MES), where HER is operated at near-neutral pH (Angenent et al., 2024). Compared with acidic and alkaline pH, HER kinetics at near-neutral pH are much slower (Zhou et al., 2020) leading to lower current densities. This is creating a need for efficient, low-cost, earth-abundant electrocatalysts that operate effectively under near-neutral pH, low ionic strength, and ambient temperature. Carbonaceous materials, especially carbon felt (Baek et al., 2022; Bian et al., 2024), graphite plates (Nevin et al., 2010), and graphite rods (Kracke et al., 2020), have been used as cathodes for HER in MES in most studies due to their cost-effectiveness and biocompatibility. Bed electrodes provide a highly promising reactor platform that uses HER for MES (Rodrigo Quejigo et al., 2019; Pelaz et al., 2026). Here, among a plethora of materials used for creating beds, granular biochar (GB) (Pouresmaeil et al., 2025) or spherical GB (Pouresmaeil et al., 2026) enables scalable bed electrode configurations and provides a versatile platform for electrocatalyst incorporation.

Nickel-based electrocatalysts have attracted considerable attention for HER due to their favorable performance, stability, and cost-effectiveness (Zhang X.-Y. et al., 2022). Among these, bimetal Ni-Mo materials are widely recognized as promising electrocatalysts (Zhang X.-Y. et al., 2022; Luo et al., 2023) being also used for HER in MES (Kracke et al., 2019; Kracke et al., 2020; Kracke et al., 2021; Jayathilake et al., 2022; Deutzmann et al., 2023; Bian et al., 2024; Bian et al., 2025). Several studies have reported improved HER performance by incorporation of Mo into Ni on different electrode backbones, often attributing this improvement to synergistic electronic interactions, modified hydrogen adsorption energetics, and optimized surface properties (Zhang et al., 2017; Schalenbach et al., 2018; Bau et al., 2020; Zhang X.-Y. et al., 2022; Luo et al., 2023). In particular, higher HER performance of the Ni-Mo electrocatalysts is frequently attributed to different bulk Ni:Mo ratios, ranging from near-equimolar (Meng et al., 2024) to Ni-rich (Navarro-Flores et al., 2005; Mitov et al., 2017; Kracke et al., 2019; Kracke et al., 2020) or Mo-rich compositions (Chen and Hu, 2020; Rossi et al., 2023; Bian et al., 2024; Bian et al., 2025). However, in many cases, variations in bulk Ni:Mo ratios are inherently accompanied by changes in phase structure (Schalenbach et al., 2018), alloy formation (Schalenbach et al., 2018), and surface composition (Meng et al., 2024). This coupling complicates the interpretation of reported trends and highlights the need for studies that more explicitly separate compositional effects from phase- or even surface-related features. More broadly, recent studies on multicomponent HER electrocatalysts have shown that the arrangement of the elements and their atomic interactions are crucial for electrocatalytic performance, already strongly indicating that elemental composition alone is often insufficient to explain HER performance (Lin et al., 2024; Shu et al., 2024; Wang et al., 2025). Consequently, the extent to which the nominal bulk composition, which is often the only parameter being reported for commercially available materials, alone governs HER performance remains an open question.

In this study, three commercially available Ni-Mo powders with different nominal bulk Ni:Mo ratios were selected to address whether nominal bulk composition alone is a reliable predictor for HER performance under near-neutral pH conditions. Spray coating was selected, as it enables uniform deposition of the catalyst ink on spherical GB surface while providing sufficient control over catalyst loading and reproducible electrode preparation (Saei et al., 2025). Structural and surface characterizations revealed that these materials consist of physically separated Ni- and Mo-rich phases rather than true Ni-Mo alloys. However, this provides a useful model system to assess the role of nominal bulk composition in the absence of alloying effects. The HER performance of these materials was evaluated under near-neutral pH conditions, using spray-coated spherical GB cathodes. The main findings of this study emphasize the importance of thorough structural verification of commercially sourced electrocatalyst materials and stress the clear need to distinguish the effects of nominal bulk composition from those arising from phase composition and alloy formation when analyzing bimetal Ni-Mo electrocatalysts for HER.

Granular biochar and support electrode preparation

Biochar was produced from beechwood spheres via pyrolysis at 850 °C, as described in Supplementary Material. The resulting GB, is a three-dimensional (3-D) sphere-like body (Pouresmaeil et al., 2026). A titanium wire was used as a contactor to make the cathodes from GB. For detailed methods of cathode preparation and geometric surface area determination, please see the Supplementary Material.

Spray coating of Ni-Mo electrocatalyst on granular biochar (GB) electrodes

Three inks were prepared from commercial Ni-Mo powders (American Elements, The Materials Science Manufacturer ®, USA), designated as Ni-Mo (20:80), (50:50), and (80:20), as described in the Supplementary Material. The Ni-Mo-loaded GB (GB-Ni-Mo) were prepared by spray coating with a Smart-Airbrush (pressure 25 Psi, airflow 8 L min -1, Redoxme AB, Sweden). The airbrush was adjusted manually using the preset handle to control layer-by-layer coating at room temperature with air as carrier gas. The GB electrodes were briefly rinsed with deionized water immediately prior to spray coating to improve surface wettability. The distance between the nozzle and the electrode was fixed at 6 cm. Three different Ni-Mo mass loadings were applied to the GB by filling the gravity ink container of the airbrush with 100, 200, or 400 µL of ink. The Ni-Mo mass loading was confirmed by weighing the final, dried mass of each GB-Ni-Mo, as described in the Supplementary Material. Three independent GB were coated with each ink and each volume. Table 1 lists all GB-Ni-Mo cathodes, including detailed information on the Ni-Mo type, mass loading, and electrode ID.

Electrochemical measurements

The electrochemical tests were performed in the two-chamber reactors equipped with a three-electrode setup, as described previously (Pouresmaeil et al., 2025; Pouresmaeil et al., 2026). The details of the setup, electrolyte composition, and operating conditions are provided in the Supplementary Material. Cyclic voltammetry (CV) was performed on all GB (n = 3) and GB-Ni-Mo cathodes (n = 3) at a scan rate of 1 mV s -1 for 25 cycles and 10 cycles, respectively. The potential window was set to −0.4 to −1.5 V and −0.4 to −1.3 V vs. Ag/AgCl (sat. KCl), for GB and GB-Ni-Mo cathodes, respectively. The formal potential (E f) for the HER redox couple (H 2 O/H 2) at 35 °C and pH = 7.0 is −502 mV vs. Ag/AgCl (sat. KCl), equivalent to −305 mV vs. standard hydrogen electrode (SHE) (Pouresmaeil et al., 2025). The data from the last cycle of each CV was used for the determination of overpotential for HER at a current density per geometric surface area (j geom.) of −1 mA cm -2 (η − 1 m A c m − 2 by using the tangent method as described in previous studies (Wirth et al., 2012; Pouresmaeil et al., 2025; Pouresmaeil et al., 2026).

Structural characterization

The commercial Ni-Mo powders and their corresponding inks were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), and Energy-dispersive X-ray spectroscopy (EDX/EDS) with details of all techniques being provided in the Supplementary Material.

Statistical analysis

The CV profiles were measured on fully individually prepared electrodes that are defined as independent replicate. The standard deviation of all datasets from three independent replicates was calculated to quantify measurement uncertainties. To assess HER performance in terms of changes in overpotential for HER (η − 1 m A c m − 2) one-way ANOVA at the 0.05 level was performed. Data were processed using Origin software Version 10.1.5.132 (Origin 2024, OriginLab Corporation, Northampton, MA, USA).

Hydrogen evolution reaction (HER) performance of Ni-Mo coated granular biochar cathodes (GB-Ni-Mo)

The HER overpotential at a geometric current density (j geom.) of −1 mA cm -2 (η − 1 m A c m − 2) was determined from CV measurements to evaluate HER performance of the Ni-Mo electrocatalyst. The CV curves in Figures 1A–F show the average of forward and backward scans of the last cycle of each CV with standard deviations calculated from three independent replicates for each ink and Ni-Mo mass loading. The η − 1 m A c m − 2 was obtained from the CV data and presented as bar charts in Figures 1B,D,F, illustrating the effect of increased mass loading across three nominally different Ni-Mo powders. The blank GB cathodes required η − 1 m A c m − 2 = 708 ± 14 mV for HER, whereas GB-Ni-Mo cathodes with varying Ni-Mo mass loadings exhibited notably lower η − 1 m A c m − 2. For a given Ni-Mo mass loading all GB-Ni-Mo cathodes exhibited similar j geom. values regardless of the bulk Ni:Mo ratios. Increasing the Ni-Mo mass loading consistently reduced HER overpotential, decreasing η − 1 m A c m − 2 to 463 ± 50, 367 ± 33, and 210 ± 9 mV at loadings of 0.35 ± 0.04, 0.61 ± 0.02, and 1.30 ± 0.07 mg cm -2, respectively.

Structural characterization

Phase composition analysis

The commercial Ni-Mo powders were analyzed by XRD, and the corresponding diffraction patterns are shown in Figure 2 as green for nominal (20:80), blue (50:50), and red (80:20) curves, respectively. Pure Ni exhibits a face-centered cubic (fcc) crystal structure, while pure Mo crystallizes in a body-centered cubic (bcc) structure (Schalenbach et al., 2018). For all here investigated Ni-Mo powders, distinct Ni (fcc) diffraction peaks were observed at 44.5°, 51.8°, and 76.4°, representing the three characteristic peaks of pure Ni (fcc) (Davar et al., 2009). Additionally, peaks at 40.5°, 58.6°, and 73.7° were identified as characteristic peaks of metallic Mo (bcc) (Zhang L. W. et al., 2022). The insets in Figures 2A,B highlight representative Mo and Ni peaks for all commercial Ni-Mo powders, revealing that the peak positions remain unchanged regardless of the bulk Ni:Mo ratio. Furthermore, the phase composition of each Ni-Mo powder (wt%) was determined using Rietveld refinement method, as summarized in the inset table in Figure 2. Overall, XRD results indicate that the commercial Ni-Mo powders consist of a physical mixture of elemental Ni and Mo phases rather than a Ni-Mo alloy.

Morphology and chemical composition analyses

SEM images of the Ni-Mo powders are shown in Supplementary Figure S3. The micrographs reveal distinct practical features in Ni-rich and Mo-rich regions (Section 2.2 in the Supplementary Material).

EDX maps of Ni-Mo powders further support the heterogeneous distribution of Ni and Mo, Figures 3A–I. In all samples, Ni and Mo signals are distributed non-uniformly. In the Mo-rich sample (Figures 3A–C), Mo-dense regions are distributed across the surface, and the Ni signals occupy the Mo-free spots. Conversely, the Ni-rich sample (Figures 3G–I) shows Ni-rich regions, with Mo distributed in complementary regions. The Ni-Mo (50:50) sample (Figures 3D–F) also shows distinct separation between Ni- and Mo-rich areas despite having a balanced overall mass composition. The bulk Ni:Mo ratios in the Ni-Mo powders were derived from the overall EDX-SEM elemental map and summarized in Supplementary Table S2. In addition to the powder characterization, EDX mapping was performed on the spray-coated Ni-Mo ink deposited on a silicon wafer to evaluate the distribution of electrocatalyst within the carbon-based ink. EDX maps and elemental composition of the inks are shown in Supplementary Figure S5 and Supplementary Table S3.

Surface chemistry

XPS was performed to analyze the surface composition of Ni-Mo powders. The XPS surveys confirmed the presence of Ni, Mo, and O in all Ni-Mo powders, as summarized in detail in Supplementary Table S4. The high-resolution Ni 2p and Mo 3d signals of all Ni-Mo powders were deconvoluted into the typical components (Chen et al., 2017; Vernickaitė et al., 2020), corresponding to Ni 0, Ni 3+, Mo 0, and Mo 6+, respectively.

The HER performance of the commercial Ni-Mo electrocatalysts clearly depends on electrocatalyst mass loading (see Figure 1). Surprisingly, no statistically significant differences are observed between samples with different nominal bulk Ni:Mo ratios (20:80, 50:50, and 80:20) at the same mass loadings. The η − 1 m A c m − 2 decreases substantially with increasing mass loading, indicating that catalyst loading has a strong influence on the measured HER performance However, as electrochemical active surface area (ECSA) has not been determined, the observed improvement could not be only attributed to enhanced intrinsic electrocatalytic activity and may instead result from an increased ECSA associated with higher catalyst loading. Statistical analysis (one-way ANOVA, p > 0.05) further confirms that the nominal bulk Ni:Mo ratio does not significantly affect HER performance under the tested conditions. This finding contradicts with previous studies reporting composition-dependent HER performance of Ni-Mo electrocatalyst (Zhang X.-Y. et al., 2022; Rossi et al., 2023; Meng et al., 2024). However, the influence of Ni:Mo composition reported in the literature is often linked to the structure of the Ni-Mo, since changes in composition can be accompanied by differences in phase structure, alloy formation, and elemental distribution. Therefore, the apparent discrepancy between the here gained results and previous reports may stem from differences in the actual structure of the investigated Ni-Mo materials rather than from nominal bulk composition alone. To explore this possibility, the structural and surface characteristics of the three commercial Ni-Mo powders were examined.

XRD analysis of the commercial Ni-Mo powders and corresponding catalyst inks offers valuable insights into the structural basis of this behavior. Unlike reported Ni-Mo alloy phases, no peak shifts or unique diffraction features were observed for the commercially available samples. Previous studies have demonstrated that incorporating Mo into the Ni lattice shifts the Ni (fcc) peaks toward lower diffraction angles due to lattice distortion (Schalenbach et al., 2018; Zhang L. W. et al., 2022). Additionally, diffraction peaks associated with Ni-Mo alloys have been reported around 43° (Schalenbach et al., 2018; Zhang L. W. et al., 2022; Meng et al., 2024). However, neither peak shifts nor additional alloy peaks are observed in the current samples. Instead, the diffraction patterns show only the characteristic peaks of elemental Ni (fcc) and elemental Mo (bcc), with consistent peak positions across all compositions. These results indicate a lack of significant atomic-level incorporation and confirm that the studied commercial Ni-Mo powders mainly consist of a physical mixture of Ni and Mo phases rather than a true alloy.

This interpretation is further supported by SEM and EDX analyses. SEM images of the Ni-Mo powders used here show distinct particle features corresponding to Ni-rich and Mo-rich regions, indicating limited interdiffusion during synthesis. EDX elemental mapping reveals a heterogeneous distribution of Ni and Mo, with clearly separate compositional domains forming islands enriched in either element. This segregation is consistently observed across all compositions, including the Ni-Mo (50:50) sample. The absence of nanoscale co-localization or uniform overlap between Ni and Mo signals suggests limited nanoscale mixing and the presence of compositionally segregated Ni-rich and Mo-rich regions, rather than a homogeneous Ni-Mo alloy, as would be expected for successfully alloyed systems (Sahasrabudhe et al., 2018; Ding et al., 2025; Lee et al., 2026). All in all, these findings confirm that the materials are composed of physically mixed phases rather than Ni-Mo alloys. In addition, EDX elemental mapping of Ni-Mo inks showed a homogeneous distribution of Ni and Mo throughout the coating, indicating that ink preparation and spray coating process did not result in noticeable segregation of the electrocatalyst. The measured Ni:Mo ratio remained comparable to that of the corresponding Ni-Mo powder, indicating that the relative composition of the electrocatalyst is preserved during ink preparation and spray coating.

XPS analysis provides complementary surface-sensitive insights into valence states and surface composition of an electrocatalyst (Chen et al., 2017). High-resolution Ni 2p and Mo 3d spectra revealed the presence of both metallic and oxidized Ni and Mo species, corresponding to Ni 0 /Ni 3+ and Mo 0 /Mo 6+, respectively. According to Supplementary Table S4, the identified Ni and Mo components showed comparable binding energies across all Ni-Mo samples, with no shifts in binding energy despite their different nominal bulk Ni:Mo ratios. Moreover, Supplementary Table S4 presents the XPS-derived surface atomic concentrations, surface Ni:Mo ratios, and the relative fractions of the metallic (Ni 0 and Mo 0) and oxidized Ni and Mo species (Ni 3+ and Mo 6+) derived from peak fitting. Since XPS probes only the outermost few nanometers of a material (<10 nm), these values mainly represent the surface composition rather than the bulk. Therefore, differences between the XPS-derived surface atomic concentrations and the EDX-derived bulk compositions are expected due to the different information-depth sensitivities of the two techniques. The oxidized Ni and Mo species (Ni 3+ and Mo 6+) are most likely attributed to the formation of oxide layers following exposure of the powders to ambient air during sample handling and transfer prior to XPS analysis (Biesinger et al., 2011). Although the Ni-rich sample (Ni-Mo (80:20)) exhibits a higher relative fraction of Ni 0 and Mo 0 than the other two samples (Ni-Mo (20:80) and (50:50)), no statistically significant differences in HER performance were observed among the three commercial Ni-Mo powders. Therefore, the XPS results do not reveal a quantitative correlation between the surface composition or oxidation-state distribution and the determined HER performance. Furthermore, the absence of a systematic shift in binding energy among the three Ni-Mo samples is consistent with the EDX and XRD results, which support that the material consists of physically mixed Ni- and Mo-rich phases rather than a homogeneous Ni-Mo alloy.

Overall, the combined electrochemical and physicochemical analyses allow a consistent interpretation of the observed HER performance. The characterization results show that the studied commercial Ni-Mo powders do not form a homogeneous Ni-Mo alloy structure but instead consist of the physically separated Ni- and Mo-rich phases. Unlike homogeneous Ni-Mo alloys, where variations in alloy composition influence the local atomic environment and thus HER performance, changing the nominal bulk Ni:Mo ratio in a phase-separated Ni-Mo mainly affects the relative abundance of the individual Ni- and Mo-phases without creating the characteristics of atomic-scale interactions seen in true Ni-Mo alloys. Therefore, the compositional (synergistic) effects commonly reported for homogeneous Ni-Mo alloys are unlikely to be fully developed in such commercial phase-separated Ni-Mo materials. This interpretation is consistent with the lack of statistically significant differences in HER performance among the three nominal bulk Ni:Mo ratios under the tested near-neutral pH conditions. Although the supplier reported that the powders were mechanically alloyed, the combined XRD, EDX-SEM, and XPS analyses found no evidence of a homogeneous alloy structure. Therefore, the nominal bulk Ni:Mo ratio alone cannot be assumed to represent the catalyst structure or reliably predict its electrocatalytic performance. From a practical perspective, commercially available Ni-Mo powders intended for HER should be structurally verified before use, as phase composition and alloy formation may have a greater influence on electrocatalytic HER performance than the nominal bulk composition itself.

This study shows that the HER performance of the investigated commercial Ni-Mo electrocatalysts is influenced more strongly by electrocatalyst loading than by their nominal bulk Ni:Mo ratio under the tested conditions. No statistically significant differences in HER performance were observed among GB cathodes spray-coated with commercial Ni-Mo having different nominal bulk Ni:Mo ratios of 20:80, 50:50, and 80:20 at the same mass loadings. Combined XRD, EDX-SEM, and XPS characterization revealed that these commercial powders do not form homogeneous Ni-Mo alloys but instead consist of physically separate Ni- and Mo-rich phases. This structural difference provides a possible explanation for the absence of composition-dependent HER performance observed in this study. In homogeneous Ni-Mo alloys, changes in composition can modify the local atomic environment and associated interactions, whereas changing the nominal bulk Ni:Mo ratio in the investigated phase-separated Ni-Mo primarily changes the relative proportion of the individual phases without creating the atomic-scale interactions characteristic of a true Ni-Mo alloy. The apparent difference between the present findings and previous reports of composition-dependent HER performance, therefore, highlights that the nominal bulk Ni:Mo ratio alone is not a reliable predictor of HER performance and should be interpreted together with the phase structure and extent of alloy formation. Consequently, detailed structural verification of commercially available Ni-Mo electrocatalysts before electrochemical evaluation is important for establishing meaningful composition-performance relationships. Future studies should complement such structural characterization with ECSA, electrochemical impedance spectroscopy, and kinetic analyses, as well as theoretical approaches such as density functional theory (DFT) calculations, to gain further insight into intrinsic electrocatalytic activity, charge-transfer properties, and HER reaction pathways. Long-term electrochemical stability, including corrosion assessment, should also be examined when evaluating the suitability of GB electrodes spray-coated with commercial Ni-Mo for extended MES operation.