With the advent of the energy crisis, hydrogen is one of the ideal alternatives to traditional fossil fuels because of its good combustion performance, non-toxic, and reduced greenhouse effect. Nowadays, with the development of science and technology, the use of hydrogen energy has become more and more common. Fuel cells using hydrogen energy as raw materials have been widely developed and utilized. There are many ways to obtain hydrogen, such as electrocatalytic hydrogen production, photocatalytic hydrogen production, biological hydrogen production, and hydrogen production from fossil fuels containing hydrocarbons. Among all the ways, electrocatalytic and photocatalytic methods seem to be the most ideal ways. The earth is rich in water storage, and pure hydrogen can be directly obtained by electrolysis of water. However, electrolysis requires large energy and high-power consumption. Therefore, it is necessary to find suitable electro-catalytic hydrogen production catalysts. It is well known that high performance catalysts for hydrogen production from water are mainly based on noble metals. For example, platinum-based complexes. However, from a cost perspective, the development of simple and efficient non-precious metal complexes electrocatalysts is the best choice.
Electrocatalytic hydrogen production by coordination complexes is a semi-reaction on the cathode during water decomposition, which contains multi-step elementary reactions. Under acidic conditions, the source of proton is H3O+. Electrocatalytic hydrogen production by coordination complexes generally undergoes three processes:
(1) Volmer step: H3O+ + e- → Hads + H2O (electronics transfer)
(2) Heyrovsky step: H3O+ + e- + Hads → H2↑ + H2O (electrochemical desorption)
(3) Tafel step: 2Hads → H2↑ (complex desorption)
Throughout the hydrogen production process, the adsorption and desorption processes of hydrogen atoms on the coordination complex surface are a pair of competitive reactions. If the adsorption process is stronger than the desorption process, it is easy to form hydrogen, but it is not conducive to the discharge of hydrogen. Conversely, it is not conducive to the formation of hydrogen. Only when they reach a good balance can they show good hydrogen production activity. Usually, Gibbs free energy of hydrogen adsorption is used to represent the catalytic ability of the coordination complex. The closer the value is to 0, the easier the hydrogen production reaction will be.
An increasing number of coordination complexes have been found to have electrocatalytic hydrogen production activity. These coordination complexes include ruthenium, copper, iridium, etc. Besides, researchers are also developing novel and efficient coordination complexes for electrocatalytic hydrogen production. For example, a novel nickel(II) complex [Ni(L)2Cl]Cl with a bidentate phosphinopyridyl ligand 6-((diphenylphosphino)methyl)pyridine-2-amine (L) was synthesized as a metal‐complex catalyst for hydrogen production from protons. The turnover frequency (TOF) value evaluated on the basis of the electrochemical reduction is 8400 s-1 at an overpotential of circa 590 mV, indicating that the NiII complex has a high proton-reduction ability even under weak acid conditions. This result will provide the foundation for the development of more similar transition metal complexes as hydrogen-generation catalysts.
Figure 1. Mechanism for catalytic H2 generation
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- Tatematsu, R.; et al. Electrocatalytic hydrogen production by a nickel (II) complex with a phosphinopyridyl ligand. Angew. Chem. Int. Ed. 2016, 55: 5247-5250.