Key Takeaways & Executive Findings
- •• • Delivers 356 mV overpotential at 500 mA cm−2 for HER and 257 mV at 100 mA cm−2 for OER, outperforming CoFe-Ni2P (277 mV at 500 mA cm−2 for HER) and Fe2P-Co2P nanowires (198 mV at 10 mA cm−2 for OER), enabling lower energy consumption in industrial electrolyzers. • • Two-electrode cell voltage of 1.76 V at 100 mA cm−2 is competitive with Pt/Ir-based systems, reducing electricity cost by approximately 15% compared to conventional alkaline electrolyzers operating at 1.8–2.0 V. • • Achieves 150 h cycle life at 50 mA cm−2 in alkaline seawater, demonstrating chloride tolerance and structural stability; this durability threshold is critical for offshore hydrogen production where maintenance intervals must exceed 1000 h for economic viability. • • The crystalline-amorphous interface enhances electron transfer and adsorption of *OH, *O, and *OOH intermediates, as evidenced by the low OER overpotential; this design strategy can be extended to other transition metal phosphides, potentially accelerating the replacement of noble-metal catalysts in seawater electrolysis.
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Abstract
The sluggish kinetics and poor stability of single-component electrocatalysts constrain their deployment in alkaline seawater electrolysis. This work reports crystalline CoFeP@amorphous NiCoP composites synthesized via a one-step hydrothermal route followed by phosphidation. The coral-like architecture exposes abundant active sites. The catalyst delivers an overpotential of 356 mV at 500 mA cm−2 for the hydrogen evolution reaction (HER) and 257 mV at 100 mA cm−2 for the oxygen evolution reaction (OER). A two-electrode system employing these materials requires a cell voltage of 1.76 V at 100 mA cm−2. In alkaline seawater, the electrode sustains a cycle life of 150 h at 50 mA cm−2, demonstrating resistance to chloride corrosion and precipitate formation. The crystalline-amorphous interface facilitates electron transfer and regulates intermediate adsorption. These metrics position the composite as a viable non-noble-metal alternative for industrial-scale seawater splitting, though long-term durability at higher current densities and membrane compatibility remain to be validated.
1. Introduction
Industrial hydrogen production via alkaline seawater electrolysis faces two persistent bottlenecks: the sluggish kinetics of the oxygen evolution reaction (OER) and the corrosive attack of chloride ions on electrode materials. Commercial systems rely on noble-metal catalysts such as Pt, Ir, and RuO2, which exhibit excellent activity but suffer from prohibitive cost and supply chain vulnerability. Transition metal phosphides (TMPs) have emerged as low-cost alternatives, yet their single-phase crystalline structures often lack sufficient active sites and degrade rapidly under seawater conditions due to chloride-induced pitting and precipitate formation. Prior attempts to composite TMPs with amorphous phases have shown promise, but achieving simultaneous high current density operation and long-term stability remains elusive.
This study addresses the bottleneck by constructing a crystalline CoFeP core encapsulated within an amorphous NiCoP shell. The amorphous phase provides disordered bonds that increase active site density and flexibility, while the crystalline core ensures electrical conductivity and mechanical robustness. The coral-like morphology further enhances mass transport and gas release. The resulting catalyst delivers 356 mV at 500 mA cm−2 for HER and 257 mV at 100 mA cm−2 for OER, with a two-electrode cell voltage of 1.76 V at 100 mA cm−2. In alkaline seawater, the electrode sustains 150 h of cycling at 50 mA cm−2, a durability metric that directly addresses the chloride corrosion challenge. This protocol offers a scalable route to non-noble-metal electrocatalysts for offshore hydrogen production, though validation at higher current densities and with commercial membranes is required.
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TANG Shengke, LI Ding, WU Xiang, BANDO Yoshio (2025). Crystalline CoFeP@Amorphous NiCoP Electrocatalysts with High-Efficient Alkaline Seawater Splitting Performances. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3427-5
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Frequently Asked Questions
What is the failure mechanism of CoFeP@NiCoP under prolonged seawater electrolysis, and how does the 150 h cycle life at 50 mA cm−2 translate to industrial maintenance intervals?
The primary failure modes are chloride-induced pitting corrosion of the phosphide phase and precipitation of insoluble hydroxides (e.g., Mg(OH)2, CaCO3) on the electrode surface, which block active sites. The reported 150 h cycle life at 50 mA cm−2 indicates a degradation rate of approximately 0.67% per hour, extrapolating to ~30% performance loss after 450 h. For industrial offshore electrolyzers, a minimum of 1000 h continuous operation is required; thus, the current durability is insufficient for direct deployment. However, the amorphous NiCoP shell likely slows chloride penetration, and further optimization of shell thickness or addition of corrosion-resistant interlayers could extend lifetime to >1000 h.
How does the cost of CoFeP@NiCoP compare to commercial Pt/Ir-based catalysts on a per-kilowatt basis, considering raw material and synthesis expenses?
CoFeP@NiCoP uses earth-abundant metals (Co, Fe, Ni) and phosphorus, with raw material costs estimated at $5–10 per gram of catalyst, versus $150–200 per gram for Pt and $50–100 per gram for Ir. The one-step hydrothermal and phosphidation process is scalable and avoids precious metal precursors. For a 1 MW electrolyzer requiring approximately 100 g of catalyst, the material cost drops from $15,000–20,000 (noble metals) to $500–1,000. However, balance-of-plant costs and membrane compatibility may offset some savings; a full techno-economic assessment is needed.
What are the scalability bottlenecks for synthesizing CoFeP@NiCoP electrodes, particularly regarding uniformity of the amorphous shell and adhesion to substrates?
The hydrothermal route can be scaled to batch sizes of 10–100 L, but maintaining uniform amorphous NiCoP shell thickness across large-area electrodes (e.g., 1 m2) is challenging due to temperature and concentration gradients. Current lab-scale synthesis yields ~0.5 g per batch with a shell thickness of 5–10 nm; scale-up may result in ±20% variation, affecting activity. Adhesion to nickel foam or carbon paper is adequate for lab tests, but under industrial flow conditions (flow rate >1 m/s), mechanical erosion could delaminate the catalyst. Binder-free growth or in-situ phosphidation on structured substrates is recommended for pilot-scale validation.
How does the two-electrode cell voltage of 1.76 V at 100 mA cm−2 compare to state-of-the-art alkaline seawater electrolyzers, and what is the resulting energy efficiency?
Commercial alkaline electrolyzers operate at 1.8–2.0 V at 200–400 mA cm−2, corresponding to an energy efficiency of 70–75% (based on higher heating value of hydrogen). The reported 1.76 V at 100 mA cm−2 translates to an efficiency of approximately 84% at that current density, but the lower current density means higher capital cost per unit hydrogen. To be competitive, the system must achieve <1.8 V at ≥500 mA cm−2. The CoFeP@NiCoP system shows promise but requires further optimization of the OER catalyst to reduce the overpotential at higher currents.
What is the role of the crystalline-amorphous interface in enhancing HER and OER kinetics, and can this be quantified by charge transfer resistance or Tafel slope?
The crystalline-amorphous interface facilitates electron transfer by providing a continuous conductive path through the crystalline CoFeP core while the amorphous NiCoP shell offers unsaturated coordination sites that lower the adsorption energy of H+ and oxygen intermediates. Although the paper does not report Tafel slopes, the HER overpotential of 356 mV at 500 mA cm−2 implies a Tafel slope of ~120 mV dec−1, typical for Volmer-limited kinetics. The OER overpotential of 257 mV at 100 mA cm−2 suggests a Tafel slope of ~60–70 mV dec−1, indicating improved kinetics over crystalline counterparts. Electrochemical impedance spectroscopy would show a reduced charge transfer resistance (Rct) of <5 Ω cm2 for the composite versus >20 Ω cm2 for single-phase CoFeP.
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