Efficient overall water splitting in acid with anisotropic metal nanosheets

The study addresses the urgent need for efficient and durable electrocatalysts for water electrolysis in acidic media, where the oxygen evolution reaction (OER) is the primary bottleneck due to high overpotentials and rapid corrosion/dissolution of most metals. While HER is facile in acid and PEM technology is advancing, state-of-the-art acidic OER catalysts (Ir oxides) still require >300 mV overpotential and are costly; Ru is more active and cheaper but suffers severe degradation. The research aims to develop a low-Ir, Ru-based catalyst that can deliver high OER activity at low overpotential with long-term stability in acid, enabling efficient overall water splitting.

Prior work shows: (i) HER in acids can reach 10 mA cm−2 at low overpotentials with Pt; (ii) acid-compatible OER catalysts are limited, with Ir oxides offering moderate stability but high overpotentials; (iii) Ru is highly active for OER but degrades quickly; (iv) strategies to stabilize Ru (heavy Ir doping ≥30 at.%, calcination, strong support interactions) typically reduce activity. Design principles are better established for alkaline OER than for acid. Recent advanced catalysts (e.g., IrO2/SrIrO3, IrNiOx) improved activity/stability but still face high potentials and cost. Literature also highlights facet-dependent oxidation behavior of Ru surfaces and the challenge of preventing oxide formation/dissolution under OER conditions.

Synthesis: RuIr-NC was synthesized by hot-injection: an aqueous mixture of RuCl3·nH2O (or RuCl2H2O) and H2IrCl6 was added dropwise to triethylene glycol (TEG) containing polyvinylpyrrolidone (PVP) at 220–230 °C, yielding coral-like Ru-Ir nanosheets with ~3 nm thickness and ~57 ± 7 nm lateral particle size, comprising ~6 at.% Ir (solid solution). Reference materials included Ru NPs, Ir NPs, and RuIr nanospherical particles (RuIr-NS) synthesized via a heat-up method, and an isotropic Ru-Ir sample (RuIr-L) prepared by heat-up followed by vacuum annealing. Composition verified by EDS/XRF (~94–96 at.% Ru, 4–6 at.% Ir). Characterization: TEM and aberration-corrected HAADF-STEM (with EDS) to resolve morphology and atomic distribution; 3D STEM tomography to confirm coral-like 2D sheet assembly; synchrotron XRD with Rietveld refinement to assess anisotropic growth and crystallite sizes (15.2 nm for (100), 13.4 nm for (110), 3.1 nm for (0002)); HAXPES and lab XPS for bulk/surface electronic states and composition; XAFS (XANES/EXAFS) at Ru K-edge and Ir L3-edge for local structure; PCA/target transformation and linear combination fitting to quantify Ru metal vs RuO2 under potential; ICP-MS to quantify metal dissolution during electrochemical cycling. Electrochemistry: RDE measurements in 0.05 M H2SO4 at room temperature; background and 85% iR-corrected LSVs (5 mV s−1, 1600 rpm), using cathodic scans to avoid overestimation from oxidation; Ag/AgCl reference and Pt (OER) or graphite (HER) counter electrode. Chronopotentiometry (CP) at 1 and 10 mA cm−2geo for stability. Faradaic efficiency for OER measured by GC in a separated H-cell. Electrochemically active surface area (EASA) determined by Cu underpotential deposition (UPD) in 0.5 M H2SO4 + 5 mM CuSO4. Operando XANES conducted in a custom cell at SPring-8 during potential steps from OCP to 1.8 V (increment 0.05 V) with slow scan to track oxidation states. Ex situ HAADF-STEM at selected potentials (1.25, 1.40, 1.80 V) to assess structural evolution and facet-dependent oxide growth. Overall water-splitting cell: Two-electrode acidic cell (0.05 M H2SO4) with RuIr-NC as both anode and cathode (loading ~0.15 mg cm−2 on carbon paper, ~1×2 cm2 electrodes). Benchmark cell used commercial IrO2 (Premetek) anode and Pt/C cathode. Performance assessed by polarization curves and CP at 10 mA cm−2geo.

- OER activity (acid, 0.05 M H2SO4, RDE): RuIr-NC reaches 10 mA cm−2geo at 165 mV overpotential, outperforming RuIr-NS (242 mV), Ir NPs (371 mV), Ru NPs (550 mV), and literature benchmarks. - Mass activity: At 1.45 V, RuIr-NC achieves 796 A g−metal, 2–4× higher than highly active reported catalysts and two orders higher than IrO2/RuO2 NPs. - Specific activity: At 1.45 V, js = 4.4 mA cm−2 (from Cu UPD-normalized EASA), about an order of magnitude higher than stable high-performance references (e.g., IrOx/SrIrO3 ≈ 0.67 A cm−2). - Stability (OER): Under CP at 1 mA cm−2geo, RuIr-NC shows no noticeable potential increase over 122 h; sustains 10 mA cm−2geo for 40 h. RuIr-NS degrades within ~1 h; Ir NPs degrade within ~12 h. - Dissolution: After five polarization scans, RuIr-NS loses ~95% Ru and ~85% Ir; RuIr-NC loses only ~25% Ru and ~15% Ir. - Operando XANES: With increasing potential, both catalysts oxidize, but at 1.8 V RuIr-NC retains ~60% metallic Ru component vs ~25% for RuIr-NS. Ir in RuIr-NC is also more resistant to oxidation. - Ex situ microscopy under potential: RuIr-NS largely becomes amorphous (hydro)oxides with particle loss; RuIr-NC maintains coral-like nanosheets with extended hcp (0001) lattice up to 1.8 V. Amorphous oxide thickness is facet-dependent (~1 nm along [0001] vs ~5 nm along [10-10]). - HER (acid): Overpotential at 10 mA cm−2geo is 46 mV for RuIr-NC (vs RuIr-NS 60 mV, Ir NPs 61 mV, Ru NPs 98 mV, Pt/C 42 mV). Tafel slope 32.0 mV/dec (close to Pt/C 30.5 mV/dec), indicating efficient Tafel step. - Overall water splitting (acidic two-electrode cell): RuIr-NC||RuIr-NC achieves 10 mA cm−2geo at 1.485 V and operates >120 h with <5% overpotential increase, outperforming IrO2||Pt/C benchmarks. - Mechanism: Superior performance linked to anisotropic nanosheet morphology exposing extended {0001} facets that resist oxidation and dissolution; electronic structure and simple size effects ruled out as primary causes (HAXPES/XAFS show similar electronic/local structures between RuIr-NC and RuIr-NS; isotropic RuIr-L with similar crystallite size/EASA performs worse).

The findings demonstrate that structural anisotropy—specifically, coral-like assemblies of ~3 nm thick Ru-Ir nanosheets with extended {0001} facets—critically enhances both activity and durability for acidic OER, while also enabling high HER performance. Operando XANES and ex situ HAADF-STEM show that RuIr-NC resists oxidation and dissolution to a much greater extent than spherical or isotropic counterparts, maintaining a higher fraction of metallic Ru under high potentials and preserving nanosheet morphology. The facet-dependent oxide growth suggests that the low-surface-energy (0001) planes impede formation of thick amorphous oxides and suppress dissolution pathways that plague Ru-rich catalysts. Control studies (HAXPES/XAFS) indicate similar electronic structures and local coordination in RuIr-NC and RuIr-NS; thus, the nanosheet architecture, not composition or electronic differences, predominantly drives performance. This structure stabilizes active states under OER, reduces overpotentials, and enables a bifunctional catalyst that delivers record-low overall cell voltages and long-term operation in acid, addressing a key barrier to PEM-compatible water electrolysis.

A Ru-Ir solid-solution nanocatalyst with only 6 at.% Ir, engineered as coral-like 2D nanosheets exposing extended {0001} facets, achieves best-in-class acidic OER activity (165 mV at 10 mA cm−2geo), exceptional mass/specific activities, and unprecedented durability (≥122 h at 1 mA cm−2geo). It also exhibits near-Pt HER performance and enables an acidic two-electrode electrolyzer operating at 10 mA cm−2geo with 1.485 V for >120 h. Mechanistic studies attribute the performance to the facet-controlled resistance to oxidation/dissolution rather than electronic or size effects. These results highlight structural optimization of metal nanomaterials as a powerful route to durable, efficient acidic OER/HER catalysts. Future research can leverage established nanocrystal shape-control databases to rapidly screen anisotropic morphologies, explore facet engineering across bimetallic systems, and translate these designs to membrane-electrode assemblies for practical PEM electrolyzers.

- The catalyst still relies on noble metals (Ru and Ir), albeit with low Ir content (6 at.%). - Electrochemical evaluations were performed in 0.05 M H2SO4 with RDE and a lab-scale two-electrode cell; performance under industrial PEM electrolyzer conditions and at higher current densities remains to be validated. - Despite improved stability, some dissolution/oxidation persists (e.g., ~25% Ru loss after five polarization scans; amorphous oxide formation under high potentials). - Long-term durability beyond ~120 h and under dynamic operational profiles was not assessed. - The hot-injection synthesis and nanosheet architecture may pose scalability and process-control challenges for large-scale production.