Unlocking the synergistic promoter role of phosphorus in evolving NiFe phosphides for enhanced water oxidation
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- Phosphorus drives NiFeP’s reconstruction into active NiFe (oxy)hydroxide, suppresses Fe dissolution, and modulates Ni’s electronic structure via residual PO43−.
- PO43− and Fe synergistically act as a redox buffer, preventing Ni over-oxidation, narrowing the bandgap, and stabilizing key oxygen evolution reaction intermediates to lower the energy barrier.
- The restructured catalyst achieves a low overpotential of 225 mV at 10 mA cm−2 and maintains stable operation in alkaline media for over 100 h at current densities up to 500 mA cm−2.
Credit: Ningning Shi, Mingcheng Gao, M. Maneesha, C. S. Praveen*, Panpan Liu, Shengnan Yue, Wangjing Xie, Dechao Chen, Yu Tang, Yuanqing Wang*, Hua Fan, Xing Huang*.
As the global economy's thirst for energy depletes traditional fossil fuels and intensifies environmental crises, the transition to clean, dispatchable hydrogen via electrochemical water splitting has become an urgent imperative. Yet the anodic oxygen evolution reaction (OER)—a sluggish, four-electron process—remains the primary bottleneck throttling overall efficiency. While noble-metal oxides such as RuO2 and IrO2 deliver acceptable activity, their scarcity and exorbitant cost render large-scale deployment economically untenable. Earth-abundant NiFe-based catalysts have long been pursued as alternatives, but the atomistic mechanisms by which anionic species like phosphorus direct structural evolution and catalytic enhancement have remained elusive. Now, researchers from Fuzhou University, Shanghai University, and Cochin University of Science and Technology, led by Professor Xing Huang, Professor Yuanqing Wang, and Professor C.S. Praveen, have presented a breakthrough study that fundamentally redefines the role of phosphorus in NiFe phosphide electrocatalysts.
Why This Catalyst Matters
Conventional wisdom has largely treated phosphorus in metal phosphides as a sacrificial template—leachable during anodic polarization to leave behind active metal (oxy)hydroxides—or as a passive electronic modifier. This work breaks that paradigm by demonstrating that phosphorus is far more than a disposable scaffolding. Through systematic identical-location transmission electron microscopy (IL-TEM), spectroscopy, and electrochemical analysis, the team reveals that phosphorus actively orchestrates a triple function: it accelerates the reconstruction of NiFeP into defect-rich NiFe (oxy)hydroxide nanosheets, suppresses the dissolution of Fe ions that plagues conventional NiFe precursors, and—most critically—leaves behind residual phosphate oxyanions (PO43-) that synergize with Fe to serve as an intrinsic redox buffer. This dual-anion synergy modulates the electronic structure of Ni, preventing over-oxidization while simultaneously stabilizing key reaction intermediates—a mechanism previously undocumented in the literature.
Innovative Design and Mechanism
The catalyst is synthesized through a multi-step engineering strategy: nickel precursor prisms are first fabricated via PVP-assisted reflux, then chemically transformed into hollow NiFe cyanide frameworks through coordination with [Fe(CN)6]3-, and finally phosphidated at 350 °C under argon to yield hollow NiFeP prisms composed of crystalline NixFe2-xP domains enveloped by amorphous phases containing Ni, Fe, O, P, and K. Under anodic OER conditions, IL-TEM captures the dynamic reconstruction in real time: the hollow prisms progressively evolve into ultrathin nanosheets over 120 minutes, with phosphorus largely dissolving while the Ni/Fe ratio remains preserved.
Density functional theory calculations unveil the synergistic promoter role of residual PO43-. In the reconstructed NiFe (oxy)hydroxide lattice, Fe initially lowers the average oxidation state of Ni, while intercalated PO43- acts as an effective redox buffer—maintaining a higher proton concentration across the operational potential window and preventing Ni from becoming over-oxidized. Bader charge analysis confirms that PO43- withdraws electron density from the Ni–O lattice, buffering Ni oxidation states while stabilizing oxygenated adsorbates. Hybrid functional calculations further reveal that PO43- and Fe cooperatively narrow the bandgap from ~0.85 eV to ~0.15 eV, dramatically enhancing electronic conductivity and charge delocalization. Free-energy profiles demonstrate that this synergy compresses the energy span among *OH, *O, and *OOH intermediates, reducing the theoretical overpotential by 0.38 V relative to pristine NiOOH.
Outstanding Performance
The reconstructed NiFeP catalyst (NiFeP-A) delivers exceptional OER activity in alkaline media, achieving a low overpotential of merely 225 mV at 10 mA cm-2—outperforming pre-NiFe (344 mV), NiFeO (300 mV), Ni2P (296 mV), and even commercial RuO2 (276 mV). The Tafel slope of 31 mV dec-1 indicates superior reaction kinetics, while the charge-transfer resistance of 1.36 Ω confirms remarkably fast electron transfer. The catalyst exhibits the highest intrinsic activity among all samples, with a specific activity of 23.63 mA cm-2, a mass activity of 314.2 A g-1, and a turnover frequency of 0.14 s-1 at 1.53 V vs. RHE. Durability tests demonstrate robust stability over 100 h at current densities of 10, 100, and even 500 mA cm-2 without significant degradation. When paired with commercial Pt/C in a two-electrode configuration for overall water splitting, the system requires only 1.51 V to reach 10 mA cm-2—superior to the commercial Pt/C || RuO2 benchmark (1.56 V)—and maintains stable operation for over 100 h.
Applications and Future Outlook
This work transcends the conventional pre-catalyst narrative and establishes phosphorus as an active, synergistic component in electrocatalytic water oxidation. By unlocking the triple role of phosphorus—structural director, dissolution suppressor, and electronic modulator—the study provides a rational blueprint for designing anion-engineered, earth-abundant electrocatalysts. The insights into redox-buffering mechanisms and dynamic structure–performance relationships pave promising avenues for next-generation alkaline electrolyzers, offering a viable pathway toward cost-effective, high-efficiency green hydrogen production at industrial scales.
Stay tuned for more groundbreaking research from this collaborative team at Fuzhou University, Shanghai University, and Cochin University of Science and Technology!
Journal
Nano-Micro Letters
Method of Research
News article
Article Title
Unlocking the Synergistic Promoter Role of Phosphorus in Evolving NiFe Phosphides for Enhanced Water Oxidation
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