Energy-saving hydrogen production by chlorine-free hybrid seawater splitting coupling hydrazine degradation

The study addresses two core challenges limiting seawater electrolysis for hydrogen production: (1) high electricity consumption driven by the sluggish oxygen evolution reaction (OER; 1.23 V vs. RHE, multi-proton-coupled electron transfer kinetics) and (2) detrimental chlorine electrochemistry (chlorine evolution/oxidation reactions) in chloride-rich seawater that competes with OER and causes corrosive, toxic chlorine species (Cl₂, ClO⁻). Conventional strategies (restricting OER overpotential below 0.48 V in alkaline media, selective membranes, or chlorine-free anolytes) reduce chlorine issues but suffer from low current densities (<200 mA cm⁻²), high cell voltages (>1.7–2.4 V), chlorine crossover, corrosion, and high energy consumption. Replacing OER with a thermodynamically favorable oxidation reaction can break the energy barrier. Hydrazine oxidation (HzOR; N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻, -0.33 V vs. RHE) has a much lower potential than OER and is ~2.05 V lower than CIOR in seawater, offering a route to energy-saving and chlorine-free hydrogen production. Hydrazine is also a toxic industrial chemical requiring efficient removal from wastewater to ppb levels. The authors propose a hybrid seawater electrolyzer (HSE) coupling seawater hydrogen evolution reaction (HER) at the cathode with anodic HzOR for simultaneous hydrogen production and hydrazine degradation, aiming for low-voltage, high-current, chlorine-free operation with improved sustainability and cost-effectiveness.

• Seawater electrolysis could alleviate freshwater demand for large-scale hydrogen, but CIOR competes with OER and induces corrosion and toxic byproducts, especially at industrial current densities (>500–1000 mA cm⁻²). Protective strategies (cation-selective layers, chlorine-free anolytes, asymmetric electrolytes) mitigate but do not eliminate chlorine crossover and often require high voltages (1.7–2.4 V) and large energy input.
• Commercial alkaline water electrolysis typically consumes 4.3–5.73 kWh per m³ H₂ at 1.8–2.4 V and 300–500 mA cm⁻².
• Replacing OER with alternative oxidations (e.g., urea, alcohols, formate, hydrazine) can reduce energy input and add co-valorization functions. Hydrazine oxidation is especially attractive due to its low onset potential (-0.33 V vs. RHE) and benign end-products (N₂, H₂O). Prior works show hydrazine-assisted hydrogen production and self-powered systems but often without seawater context or comprehensive chlorine suppression at industrial current densities.
• MXenes (e.g., Ti₃C₂Tₓ) are highly conductive and hydrophilic, improving interfacial charge transfer and reactant wetting; superaerophobic architectures mitigate bubble-induced losses at high current densities, crucial for both HER and HzOR.

Electrode design and synthesis:

• A NiCo/MXene-based 3D electrode (NiCo@C/MXene/CF) was fabricated by assembling NiCo-MOF nanosheets onto Ti₃C₂Tₓ MXene-wrapped copper foam (MXene/CF), followed by annealing in NH₃ at 400 °C to yield NiCo@C nanoarrays embedded with fcc NiCo alloy nanoparticles (<10–20 nm) in an amorphous carbon matrix. Control: NiCo@C/CF without MXene; Pt/CF (20% Pt/C) prepared by ink casting.
• Structure and composition were characterized by SEM/AFM (mesoporous nanoarray morphology), TEM/HRTEM (NiCo nanoparticles), XRD (fcc NiCo), XAFS/XANES/EXAFS (metallic Ni/Co; alloying evidenced by coordination numbers and bond lengths), XPS (Ni⁰, Co⁰/Co²⁺; Ti₃C₂Tₓ signatures), elemental mapping. Surface properties assessed via contact angles (water, bubble), high-speed optical imaging (bubble dynamics), EQCM (water/hydrazine adsorption), and ECSA.

Electrochemical testing (half-cells):

• HzOR: Three-electrode setup in 1.0 M KOH with hydrazine (up to 0.5 M N₂H₄). Working electrode: NiCo@C/MXene/CF (∼1.0 mg cm⁻² NiCo@C), reference Ag/AgCl (KCl sat.), counter graphite rod; Ar-saturated electrolyte. LSV (−1.2 to −0.5 V vs. Ag/AgCl, 10 mV s⁻¹), Tafel, EIS, durability (2000 cycles; chronopotentiometry at 100 mA cm⁻²). Comparison with MXene/CF, NiCo@C/CF, CF, and Pt/CF; SCN⁻ poisoning tests to probe active sites.
• HER: Similar configuration in 1.0 M KOH, alkaline seawater (seawater + 1.0 M KOH), or neutral natural seawater (pH 8.3). LSV, Tafel, EIS, TOF and exchange current density estimation, durability (sweeps; chronoamperometry). Tests at low scan rate and with Hg/HgO reference to exclude artifacts.

Hybrid seawater electrolyzer (HSE):

• Two-electrode flow cell with identical NiCo@C/MXene/CF as anode (HzOR) and cathode (HER). Anion exchange membrane (Fumasep FAA-3-PK-130) separates chambers. Anolyte: 1.0 M KOH + 0.5 M N₂H₄; Catholyte: neutral seawater or seawater + 1.0 M KOH. Peristaltic pumps circulate electrolytes. Polarization curves (10 mV s⁻¹, iR-compensated), galvanostatic durability. Gas analysis by GC; Faradaic efficiency via measured vs. theoretical gas amounts.
• Hydrazine degradation quantified by the Watt–Chrisp colorimetric method (UV–vis at 457 nm) with calibration (y = 1.3037x + 0.0014, R² = 0.9999) to determine residual hydrazine; removal rate normalized by NiCo@C mass.

Self-powered systems:

• Direct hydrazine fuel cell (DHzFC): NiCo@C/MXene/CF anode; 20% Pt/C on carbon paper cathode; Nafion 117 separator; anolyte 1.0 M KOH + 0.5 M N₂H₄ (10 mL min⁻¹), catholyte O₂-saturated 0.5 M H₂SO₄ (10 mL min⁻¹). DHzFC electrically connected to HSE for autonomous hydrogen production; total efficiency TE = NH2/(2 × NN2H4).
• Solar-driven HSE: Commercial 1 W Si solar cell (8 × 11 cm²) powering HSE under AM 1.5 G (100 mW cm⁻²) or natural light; current/voltage monitored by Keithley instruments.

Computational analysis:

• First-principles calculations assessed N₂H₄ adsorption and dehydrogenation pathways on NiCo alloy facets ((100), (110), (111)), extracting binding energies, charge density differences, bond-length changes, and free-energy profiles to identify active surfaces and rate-limiting steps.

• Half-cell HzOR (1.0 M KOH + 0.5 M N₂H₄): Achieves 100 and 500 mA cm⁻² at −25 and −43 mV vs. RHE, respectively—versus OER requiring 1.542–1.586 V for the same currents. Tafel slope 73 mV dec⁻¹; Rct 0.25 Ω; high ECSA (54.25 m² gcat⁻¹). NiCo is the active phase (SCN⁻ poisoning). Stable for 2000 cycles or 30 h at 100 mA cm⁻² with negligible loss; minimal metal leaching.
• Half-cell HER: Overpotentials 49 mV (10 mA cm⁻²) and 235 mV (500 mA cm⁻²) in 1.0 M KOH; Tafel slope 54.2 mV dec⁻¹; TOF 2.1 s⁻¹ at η = 200 mV; j₀ = 1.34 mA cm⁻². Comparable or superior to 20% Pt/C at >120 mA cm⁻²; robust for 60–120 h in alkaline and neutral seawater.
• Hybrid seawater electrolyzer (HSE):
  • Low-voltage operation: 0.31 V needed to reach 500 mA cm⁻² (from combined half-cell potentials under alkaline conditions); assembled HSE achieves 500 mA cm⁻² at 1.05 V in neutral seawater (45.6% lower than ASE at 1.93 V) and 0.70 V in alkaline seawater (63.7% lower than ASE).
  • Energy savings: Electricity expense as low as 2.39 kWh m⁻³ H₂ (neutral seawater) and 2.75 kWh m⁻³ H₂ (alkaline seawater) at high current densities, surpassing the theoretical OWS demand (2.94 kWh m⁻³) and cutting ~48–54% versus conventional/ASE benchmarks.
  • Durability and productivity: Stable hydrogen production for >85 h at 300 mA cm⁻² below 1.0 V (neutral), >120 h at 100 mA cm⁻² below 0.36 V (alkaline), and 140 h at 500 mA cm⁻² below 1.15 V with hydrogen rates up to 9.2 mol h⁻¹ gcat⁻¹.
  • Chlorine-free: No ClO⁻ detected; no Cl₂ by GC; anode corrosion avoided despite Cl⁻ crossover. In contrast, ASE rapidly fails (6–7 h) due to ClO⁻ corrosion.
  • Selectivity and efficiency: GC shows only H₂ and N₂ (∼2:1). Faradaic efficiencies: HER ∼96%, HzOR ∼99%.
  • Hydrazine remediation: Rapid degradation at 4.34 ± 0.007 mol h⁻¹ gcat⁻¹ to residual ∼3 ppb (<10 ppb EPA limit), with stable performance over cycles.
• Self-powered operation: Single DHzFC (OCV ≈ 1.0 V) or a single 1 W commercial solar cell (average photovoltage ≈ 0.876 V under AM 1.5 G) can drive seawater HSE; solar-driven system achieves hydrogen rate of 6.0 mol h⁻¹ gcat⁻¹.
• Mechanistic insights: DFT indicates NiCo (100) facet provides strongest N₂H₄ adsorption (Eb down to −1.89 eV), elongates N–H bonds (1.033–1.035 Å), and lowers the barrier for N₂H₄* → N₂H₃* to 0.28 eV; rate-limiting step is N₂H₃* → N₂H₂* (0.3–0.5 eV). Interfacial MXene enhances conductivity and reactant affinity (water contact angle 57°, higher adsorption rates/capacities), while superaerophobicity (bubble CA 153°) minimizes bubble coverage, ensuring stable triple-phase interfaces at high currents.

By replacing the sluggish, high-potential OER with thermodynamically favorable HzOR, the HSE achieves low-voltage operation that inherently avoids chlorine evolution/oxidation, addressing both energy and corrosion challenges of seawater electrolysis. The NiCo alloy—especially its (100) facet—strongly adsorbs and activates hydrazine, reducing kinetic barriers for dehydrogenation along a 4e⁻ pathway to N₂, as supported by DFT. The MXene-integrated 3D electrode architecture synergistically improves interfacial conductivity, hydrophilicity and hydrazine affinity, ECSA, and gas-release dynamics. Superaerophobic surfaces limit bubble-induced ohmic and mass-transport losses, enabling stable operation at industrial-level current densities. Experimentally, the HSE delivers high hydrogen productivity at 0.7–1.05 V with high Faradaic efficiencies, no chlorine products, and long-term durability, while concurrently detoxifying hydrazine to below EPA limits. Energy-equivalent inputs and CO₂-equivalent emissions are significantly lower compared to conventional alkaline electrolysis, steam reforming, and certain electrochemical methane splitting approaches, indicating broader sustainability impacts. The demonstrations of self-powered systems using a DHzFC or small solar cell underscore the feasibility of decentralized, renewables-driven, cost-effective seawater hydrogen production coupled with pollutant removal.

The work presents a chlorine-free, energy-saving hybrid seawater splitting strategy that couples seawater HER with HzOR on a NiCo/MXene-based superaerophobic-hydrophilic, hydrazine-friendly electrode. The system achieves ultralow cell voltages (0.7–1.0 V) at high current densities, high hydrogen rates (up to 9.2 mol h⁻¹ gcat⁻¹), high Faradaic efficiencies, and long-term stability, while eliminating chlorine corrosion and rapidly degrading hydrazine to ∼3 ppb. Mechanistic studies attribute the performance to the intrinsic activity of NiCo (particularly the (100) facet) for hydrazine activation and to MXene-enabled interfacial enhancements in conductivity, reactant adsorption, and bubble management. The approach cuts electricity expense by roughly 30–52% relative to conventional alkaline electrolysis and outperforms state-of-the-art seawater electrolyzers in energy efficiency. Future directions include developing higher-performance anion exchange membranes to better leverage pH gradients and ionic transport at very high current densities, scaling with industrial hydrazine-containing waste streams, and further integrating with renewable power sources for sustainable, decentralized hydrogen production.

• Membrane transport constraint: At high current densities and when catholyte OH⁻ exceeds anolyte levels, performance becomes limited by the ionic exchange capacity and permeability of the anion exchange membrane; improved AEMs are needed to fully exploit pH gradients and maximize performance.
• Dependence on hydrazine availability: The hybrid process requires hydrazine in the anolyte; while the study targets industrial hydrazine-containing wastewaters for simultaneous detoxification, practical deployment depends on feed availability and safe handling of toxic hydrazine.
• Potential overlap of redox processes: Hydrazine reduction can overlap with HER on the cathode; the asymmetric HSE design (separate feeds and AEM) mitigates this, but precise control and separation remain important for optimal performance.
• Long-term operation in varied real-world seawaters: Although stability is demonstrated in neutral and alkaline seawater, extended field tests across diverse seawater compositions and fouling conditions are not reported here.