Carbon‑Coated Molybdenum Phosphide Nanoparticles: A Low‑Cost Catalyst with 131 mV Overpotential for Alkaline Hydrogen Evolution

Abstract

Molybdenum phosphide (MoP) is a promising non‑precious catalyst for the hydrogen evolution reaction (HER). Yet, its limited stability and conductivity in alkaline media have hindered practical deployment. We report a simple, two‑step synthesis that introduces N‑ and C‑co‑doped MoP (MoP‑NC) nanoparticles using urea as both carbon and nitrogen precursors. The resulting material delivers an overpotential of only 131 mV to reach a current density of 10 mA cm⁻² in 1‑M KOH (pH 14) and retains performance after 1,000 cyclic voltammetry cycles.

Background

Global energy demand and the depletion of fossil fuels have accelerated the search for sustainable alternatives. Hydrogen, produced via water splitting, stands out as a clean energy vector. Conventional photolysis and electrolysis, however, suffer from low efficiency unless paired with advanced electrocatalysts that exhibit low HER overpotential. Noble metals such as platinum achieve this benchmark but remain cost‑prohibitive and scarce. Consequently, the scientific community has focused on low‑cost, earth‑abundant catalysts with competitive HER activity.

Transition metal phosphides (TMPs), especially molybdenum phosphide (MoP), have emerged as attractive candidates due to their favorable electronic structure and catalytic properties. Previous studies have demonstrated MoP’s efficacy in acidic media, but its performance in alkaline solutions has been limited by intrinsic conductivity and durability. Incorporating carbon materials can enhance electron transport, while nitrogen doping can further improve catalytic kinetics. In this work, we exploit urea—an inexpensive, dual carbon/nitrogen source—to simultaneously coat MoP nanoparticles with carbon and dope them with nitrogen, achieving superior HER activity in alkaline electrolyte.

Presentation of the Hypothesis

We hypothesize that coating MoP nanoparticles with a carbon matrix will improve conductivity and stability, while nitrogen incorporation will enhance hydrogen evolution kinetics in alkaline environments.

Testing the Hypothesis

Materials

Urea (CH₄N₂O), glucose (C₆H₁₂O₆), ammonium dihydrogen phosphate (NH₄H₂PO₄), ammonium heptamolybdate ((NH₄)₆Mo₇O₂₄·4H₂O), KOH, and ultrapure water were used without further purification.

Sample Preparation

For MoP‑NC synthesis, 0.240 g of ammonium heptamolybdate, 0.167 g of ammonium dihydrogen phosphate, and 2.000 g of urea were dissolved in 50 mL of deionized water and sonicated for 15 min. The solution was then heated to 80 °C, stirred for 90 min, and freeze‑dried to yield a white precursor. This precursor was calcined from room temperature to 900 °C at 5 °C min⁻¹ under N₂ for 120 min. Control samples—MoP‑C (using glucose), Bulk‑MoP (no carbon), and Mo‑NC (no phosphorus)—were prepared following identical protocols.

Characterizations

X‑ray diffraction (XRD, Bruker D8‑Advance, Cu Kα, λ = 1.54056 Å) confirmed the orthorhombic MoP phase. Field‑emission scanning electron microscopy (FE‑SEM, Hitachi S‑4800) and transmission electron microscopy (TEM, JEOL JEM‑2100) revealed uniformly dispersed, amorphous nanoparticles clustered into dense aggregates, indicating effective carbon encapsulation. Energy‑dispersive X‑ray spectroscopy (EDS) mapping showed homogeneous distribution of Mo, P, C, and N. X‑ray photoelectron spectroscopy (XPS, Mg Kα) indicated Mo in +3 and +6 states, P in P³⁻ and PO₄³⁻ forms, and the presence of C–N/C=N bonds confirming nitrogen doping.

Electrochemical Test

Electrochemical measurements employed a standard three‑electrode cell (Pt wire counter, SCE reference, glassy carbon working electrode). The catalyst ink (5 mg catalyst, 350 µL isopropanol, 650 µL water, 50 µL 5 wt% Nafion) was sonicated for 30 min, then 10 µL was dropped on a 5 mm diameter glassy carbon electrode, yielding an areal loading of 0.485 mg cm⁻². All tests were conducted in 1 M KOH at 25 °C. Linear sweep voltammetry (LSV, 10 mV s⁻¹) assessed HER activity, while Tafel analysis derived kinetic parameters. Stability was evaluated by 1,000 CV cycles at 100 mV s⁻¹. Double‑layer capacitance (C_dl) was extracted from CVs (0.847–0.947 V vs RHE) across 20–200 mV s⁻¹, and electrochemical impedance spectroscopy (EIS, 10 mV, 1–10⁵ Hz) measured charge‑transfer resistance.

Implications of the Hypothesis

XRD patterns (Figure 1a) display characteristic MoP reflections at 27.95°, 32.17°, 43.15°, 57.48°, 57.95°, 64.93°, 67.03°, 67.86°, and 74.33°, confirming phase purity. SEM images (Figure 1b) show small, densely packed clusters separated by narrow gaps, a morphology that balances high surface area with structural integrity. TEM and HRTEM (Figures 1d‑e) reveal clear lattice fringes (0.28 nm) corresponding to the (100) plane, surrounded by amorphous carbon layers. EDS mapping (Figures 1f‑i) confirms uniform elemental distribution.

XPS spectra further elucidate electronic states: Mo 3d peaks at 231.5/228.2 eV (Mo³⁺) and 235.5/232.4 eV (Mo⁶⁺), P 2p peaks at 130.7/129.4 eV (P³⁻) and 133.9 eV (PO₄³⁻), C 1s peaks at 228.7/284.8/286.3 eV (O‑C=O, C–N/C=C, C–C), and N 1s peaks at 398.4/402.1/394.5 eV (pyridinium, quaternary N, N–Mo). These results confirm successful nitrogen incorporation into the carbon matrix.

Electrochemical performance (Figure 3a) shows MoP‑NC achieving an overpotential of 131 mV at 10 mA cm⁻², outperforming Mo‑NC, Bulk‑MoP, and MoP‑C. The Tafel slope (Figure 3b) of 66 mV dec⁻¹ for MoP‑NC indicates rapid HER kinetics, better than Pt/C (58 mV dec⁻¹) and other control samples. Stability tests (Figure 3c) reveal negligible loss after 1,000 CV cycles, confirming robust durability. C_dl values (Figure 3e) demonstrate a markedly larger electrochemically active surface area for MoP‑NC (10.9 mF cm⁻²) compared to controls. EIS spectra (Figure 3f) indicate the lowest charge‑transfer resistance for MoP‑NC, attributable to the conductive carbon coating and nitrogen doping.

Conclusions

We have developed an efficient, two‑step route to synthesize N‑ and C‑co‑doped MoP nanoparticles encapsulated in a conductive carbon matrix. This structure delivers exceptional HER activity—131 mV overpotential at 10 mA cm⁻²—in alkaline electrolyte, surpassing previously reported single MoP materials. The catalyst also exhibits remarkable stability, with negligible degradation after 1,000 CV cycles. These findings establish carbon‑coated MoP as a promising, low‑cost alternative for large‑scale alkaline hydrogen production.