Introduction Climate change and rising global energy demands have prompted an urgent search for new renewable energy solutions. While great technological advances in accessing sustainable forms of energy such as wind and solar power have been made, the storage of these energies for on-demand usage and transport remains a major challenge. Molecular fuels offer an attractive option for resolving this issue owing to the high energy density that can be concentrated within chemical bonds.1–3 In particular, the splitting of water into hydrogen and oxygen in separate half reactions is a promising path forward as this process is completely carbon neutral as shown in :the following equations Ideally, water splitting can be driven by sustainable energy sources such as solar and wind power, and the sole combustion product from burning hydrogen is water. With the eventual goal of incorporating both half reactions in an integrated artificial device driven by a sustainable energy input, an essential scientific challenge to address is the design and implementation of efficient catalyst systems for water reduction .and oxidation Focusing on the reductive side, the catalytic conversion of protons to hydrogen (reaction (1)) is the key fuel-generating transformation for water-splitting cycles. Nature provides ,exquisite examples of catalysts in the form of hydrogenases which are capable of using earth-abundant iron and/or nickel metal centers to reversibly interconvert protons to hydrogen at -low thermodynamic potentials with high efficiencies and activ ities (rates up to 100–10 000 moles of hydrogen per mole of -catalyst per second). 4–6 Notable advances in utilizing hydro genases for water splitting applications have been reported and their catalytic mechanisms can be studied in molecular detail. 4,7–11 However, challenges remain in utilizing these complex macromolecules, including the low density of metal active sites compared to their overall large size and their relative long-term instability under ambient conditions. 1 Metal complexes play a crucial role in the development of clean hydrogen production technologies. Here's how: Electrocatalysis: Metal complexes can act as electrocatalysts in a process called the hydrogen evolution reaction (HER). During HER, an electrical current is used to split water molecules into hydrogen gas and oxygen. The right metal complex can lower the energy barrier for this reaction, making it more efficient and requiring less electrical input [1]. This is attractive for using renewable electricity sources like solar or wind power to produce clean hydrogen fuel. Photocatalysis: Some metal complexes can also be photocatalysts. These absorb light and use the energy to drive reactions, including hydrogen production from water. Researchers are developing photocatalytic systems that mimic natural photosynthesis to generate hydrogen using sunlight and water [2]. Formic Acid Decomposition: Metal complexes can be efficient catalysts for hydrogen production from formic acid. Formic acid can be a liquid carrier of hydrogen, meaning it can store hydrogen in a chemical form and release it later on demand. Metal complexes can break down formic acid into hydrogen gas, offering an alternative method for hydrogen generation [3]. Lowering the Energy Barrier: During the hydrogen evolution reaction (HER), water molecules are split into hydrogen gas (H₂) and oxygen (O₂) through the following simplified reaction: 2H₂O + 2e⁻ -> H₂ + 2OH⁻ This process naturally has an energy barrier, requiring some energy input to get it going. Metal complexes act as electrocatalysts by lowering this energy barrier. The metal center in the complex interacts with water molecules, weakening the O-H bonds and facilitating their cleavage. The complex also helps transfer electrons between the electrode and the water molecules, accelerating the overall HER process. Factors Affecting Efficiency: The efficiency of a metal complex electrocatalyst depends on several factors: Binding Affinity: The complex needs to bind water molecules strongly enough to activate them, but not so strongly that releasing hydrogen becomes difficult. Electron Transfer Rates: The complex must efficiently transfer electrons between the electrode and the water molecules. Stability: The complex should be stable under the reaction conditions to avoid degradation and maintain its catalytic activity over time. Types of Metal Complexes in Electrocatalysis: Transition metal complexes are the most common type used for HER electrocatalysis. These metals, like iron, nickel, cobalt, and molybdenum, have partially filled dorbitals that can readily accept and donate electrons, making them ideal for electron transfer processes. Metalloenzymes: Nature provides inspiration with enzymes like hydrogenase, which use complex metal centers to efficiently produce hydrogen. Researchers are developing biomimetic complexes that mimic these natural catalysts for improved efficiency and selectivity Mechanism: Cobalt complexes often activate water molecules via a stepwise process. The cobalt center, in its +2 or +3 oxidation state, can bind to an OH group from a water molecule. Following electron transfer, the O-H bond weakens, and a proton is released. This cycle continues, leading to hydrogen gas evolution [1]. Image:(This is an example of a possible structure, many variations exist) Advantages: Some cobalt complexes show high activity for HER. Disadvantages: Stability can be an issue, with some complexes degrading over time Mechanism: Iron-based catalysts, particularly iron-sulfur clusters, mimic the active site in the hydrogenase enzyme. These clusters facilitate the dissociation of water molecules through a complex interplay of electron transfer and proton shuttling [2]. Image: Advantages: Iron is abundant and inexpensive, making iron complexes attractive for scalable production. Disadvantages: Activity can be lower compared to some other transition Mechanism: Nickel complexes, especially nickel-iron layered double hydroxides, are another promising class of HER catalysts. These materials provide active sites with strong water binding and efficient electron transfer pathways [3]. Image: Advantages: Nickel complexes often demonstrate good activity and stability. Disadvantages: The presence of nickel can make the catalyst susceptible to deactivation by Mechanism: Molybdenum-based catalysts often involve molybdenum centers coordinated with various ligands. These complexes can activate water molecules through a combination of Lewis acid-base interactions and electron transfer processes [4]. Image: (This is an example, many variations exist) Advantages: Molybdenum complexes can be highly active and selective for HER. Disadvantages: Designing molybdenum catalysts with optimal performance can be more complex compared to some other transition metals.
