Photocatalytic water splitting with cobalt and nickel catalysts
Photocatalytic water splitting converts sunlight into chemical fuel by driving the formation of hydrogen and oxygen from water. The process brings together semiconductor physics, molecular catalysis, surface chemistry and energy engineering, making it a natural subject for applied bioinorganic chemistry.
Cobalt and nickel are especially valuable because they are more abundant and generally less costly than platinum, iridium and ruthenium. Their coordination chemistry allows researchers to tune oxidation states, ligand environments and reaction pathways for hydrogen evolution and oxygen evolution, while their earth-abundant supply supports larger-scale development.
For Australian researchers, this field connects strongly with solar resources, renewable hydrogen policy and regional energy needs. Work presented through the ISABC 2023 symposium reflects the international interest in metal-based catalysts, photochemistry and practical routes towards cleaner fuel production.
How sunlight drives water splitting
A photocatalytic system absorbs photons with enough energy to promote electrons from a valence band into a conduction band. The resulting electrons can reduce protons to hydrogen, while holes oxidise water to oxygen. Efficient operation depends on suitable band positions, rapid charge separation and catalytic surfaces that lower the activation barriers for both half-reactions.
Cobalt and nickel can participate in several ways. Nickel phosphides, sulphides and layered hydroxides are often investigated as hydrogen-evolution or oxygen-evolution materials, while cobalt oxides, phosphates and molecular complexes can mediate oxidation chemistry. In some designs, these metals act as co-catalysts on a light-absorbing semiconductor rather than absorbing most of the sunlight themselves.
The complete reaction is demanding because oxygen evolution requires four electrons and four protons for every oxygen molecule formed. Catalysts must therefore manage multiple proton-coupled electron-transfer steps without encouraging corrosion, recombination or competing reactions such as photocatalyst decomposition.
Designing cobalt and nickel active sites
Molecular cobalt complexes offer precise control over geometry and electronic structure. Ligands can stabilise cobalt in different oxidation states, position proton relays near the metal centre and influence whether hydrogen forms through metal hydrides or alternative intermediates. This control is useful for mechanistic studies, although molecular catalysts may require anchoring or encapsulation to remain stable in aqueous systems.
Nickel-based materials can provide high surface areas and adaptable compositions. Nickel hydroxide and nickel oxyhydroxide are particularly relevant to alkaline oxygen evolution, where changes between Ni(II), Ni(III) and higher-valent states may be linked to catalytic activity. Cobalt-containing layered double hydroxides can alter conductivity, defect density and the availability of reactive surface sites.
Researchers should distinguish intrinsic catalytic activity from apparent performance caused by loading, light intensity or surface area. Techniques such as X-ray absorption spectroscopy, operando Raman spectroscopy, isotope labelling and gas chromatography can help identify the active species and verify that hydrogen and oxygen are produced in the expected two-to-one ratio.
Comparing catalyst platforms
The best catalyst depends on the reaction half-cycle, electrolyte, light absorber and intended operating conditions. A material that performs well for hydrogen evolution may be unsuitable for oxygen evolution, while a robust powder suspension may be difficult to integrate into a panel or membrane reactor.
| Catalyst platform | Typical role | Strengths | Key limitations |
|---|---|---|---|
| Nickel phosphide or sulphide | Hydrogen evolution | Good conductivity and adaptable surface chemistry | Possible oxidation, sulphur or phosphorus leaching |
| Cobalt oxide or phosphate | Oxygen evolution | Access to several oxidation states and water-oxidation pathways | Charge recombination and relatively modest conductivity |
| NiFe or CoFe layered hydroxide | Oxygen evolution | Strong alkaline activity and tunable composition | Usually requires alkaline electrolyte and careful electrode integration |
| Molecular cobalt complex | Mechanistic hydrogen or oxygen chemistry | Precise ligand and redox control | Stability, immobilisation and costly synthesis |
| Cobalt or nickel co-catalyst on a semiconductor | Charge extraction and surface catalysis | Can improve separation and reaction kinetics | Interface defects and long-term photostability |
Australian laboratories often assess these materials against conditions that resemble local deployment. Strong sunlight in Perth, Adelaide and inland New South Wales can support solar-driven testing, yet high temperatures, dust and intermittent cloud affect catalyst durability and reactor design. Seawater is attractive for coastal hydrogen projects, but chloride can trigger corrosion or unwanted oxidation products.
Moving from laboratory tests to useful systems
A catalyst suspension may show impressive hydrogen evolution under a high-intensity lamp while offering limited value outdoors. Meaningful comparisons should report the light spectrum, photon flux, catalyst loading, reactor geometry, sacrificial reagents, pH, gas-analysis method and solar-to-hydrogen efficiency. Apparent quantum yield is helpful, but it should be paired with long-duration testing and independent confirmation of product gases.
For Australia’s renewable hydrogen market, systems must also address water treatment, land use, electrical integration and safety. The Australian Renewable Energy Agency has supported hydrogen innovation, while state-based approvals and environmental requirements can influence projects differently in Queensland, Victoria and Western Australia. Using treated wastewater or desalinated water may be feasible, but the energy and infrastructure costs need to be included in any lifecycle assessment.
A practical development pathway may combine a visible-light semiconductor with a nickel hydrogen-evolution co-catalyst and a cobalt- or iron-containing oxygen-evolution layer. Protective coatings, conductive supports and selective membranes can reduce photocorrosion and limit gas crossover. Pilot designs should measure performance through hot summers and after repeated wet-dry cycles rather than relying on short laboratory runs.
Connecting research with collaboration
Applied bioinorganic chemistry benefits from collaboration between synthetic chemists, materials scientists, electrochemists and engineers. Researchers studying cobalt and nickel catalysts can contribute through ligand design, nanoscale characterisation, reactor development, computational modelling and environmental assessment. This breadth is particularly important when replacing precious metals with materials that may have complex stability or toxicity profiles.
Conference programmes are useful for identifying complementary expertise and emerging methods. The scientific programme for the Ioannina symposium provides a relevant example of how bioinorganic chemistry spans catalytic systems, spectroscopy, biological interfaces and applied materials research. Australian groups can use similar networks to build partnerships across universities, industry and public research organisations.
Commercial translation also depends on communicating results beyond specialist journals. A clear catalyst specification, reproducible testing protocol and realistic cost model can help connect academic discoveries with clean-energy developers. Innovation directories and technology networks, including this project marketplace, may offer additional routes for presenting concepts to potential partners, provided technical claims are supported by independent evidence.
The strongest future systems will be designed around durability, resource availability and complete energy accounting. Cobalt and nickel catalysts are promising because their coordination and solid-state chemistry can be tuned extensively, but performance must be demonstrated under realistic illumination, water quality and operating conditions. Researchers developing the next generation of solar hydrogen technology should share validated data, pursue safer materials and connect fundamental mechanisms with Australian deployment needs.