Pigment-Acceptor-Catalyst Triads for Photochemical Hydrogen Evolution

Designing Zn-doped nickel sulfide catalysts with an optimized electronic structure for enhanced hydrogen evolution reaction

Designing non-noble-metal electrocatalysts with excellent performance and economic benefits toward the hydrogen evolution reaction (HER) is extremely crucial for future energy development. In particular, the rational cationic-doped strategy can effectively tailor the electronic structure of the catalysts and improve the free energy of the adsorbed intermediate, thus enhancing HER performance. Herein we reported Zn-doped Ni₃S₂ nanosheet arrays supported on Ni foam (Zn-Ni₃S₂/NF) that were synthesized by a two-step hydrothermal process for improving HER catalysis under alkaline conditions. Remarkably, the obtained Zn-Ni₃S₂/NF displays excellent HER catalytic performance with an overpotential of 78 mV to reach a current density of 10 mA cm^-2 and dramatic long-term stability for 18 h in 1 M KOH. In addition, the results based on the density functional theory calculations reveal that Zn dopants can modulate the electronic structure of Ni₃S₂ and optimize the hydrogen adsorption free energy (ΔG_H*). Thus cationic-doping engineering provides an efficient method to enhance the intrinsic activities of transition-metal sulfides, which may contribute to the development of nonprecious electrocatalysts for HER.

Is electron ping-pong limiting the catalytic hydrogen evolution activity in covalent photosensitizer-polyoxometalate dyads?

Understanding the limitations of catalytic processes enables the design of optimized catalysts. Here, femtosecond transient absorption spectroelectrochemistry is used to explore the photophysics of polyoxometalate-based covalent photosensitizer-hydrogen evolution catalyst dyads. The study shows that the presence of light-driven forward and backward electron transfer, i.e. "electron ping-pong", is a limiting factor for charge accumulation on the polyoxometalate. Based on this insight, chemical means of optimizing catalyst performance are proposed.

A Nickel Dithiolate Water Reduction Catalyst Providing Ligand-Based Proton-Coupled Electron-Transfer Pathways

A nickel pyrazinedithiolate ([Ni(dcpdt)₂ ]^2- ; dcpdt=5,6-dicyanopyrazine-2,3-dithiolate), bearing a NiS₄ core similar to the active center of [NiFe] hydrogenase, is shown to serve as an efficient molecular catalyst for the hydrogen evolution reaction (HER). This catalyst shows effectively low overpotentials for HER (330-400 mV at pH 4-6). Moreover, the turnover number of catalysis reaches 20 000 over the 24 h electrolysis with a high Faradaic efficiency, 92-100 %. The electrochemical and DFT studies reveal that diprotonated one-electron-reduced species (i.e., [Ni^II (dcpdt)(dcpdtH₂ )]^- or [Ni^II (dcpdtH)₂ ]^- ) forms at pH<6.4 via ligand-based proton-coupled electron-transfer (PCET) pathways, leading to electrocatalytic HER without applying the highly negative potential required to generate low-valent nickel intermediates. This is the first example of catalysts exhibiting such behavior.

Rational Design of an Electron-Reservoir Pt(II) Complex for Efficient Photocatalytic Hydrogen Production from Water

Herein we report a Pt(II) complex containing a 4,4'-bis[4-(triphenylsilyl)phenyl]-2,2'-bipyridine ligand as a molecular catalyst for water splitting. Systematic studies of the electrochemical and electronic properties of this catalyst, in comparison with two control complexes, reveal electron-reservoir characteristics upon two-electron reduction. A turnover number of 510,000 was recorded by employing this complex as a water reduction catalyst in combination with a state-of-the-art photosensitizer and N,N-dimethylaniline as a sacrificial electron donor, which represents a large improvement over the control complexes that do not contain the tetraphenylsilyl ligand substitution.