Molecular Engineering of Photocathodes based on Polythiophene Organic Semiconductors for Photoelectrochemical Hydrogen Generation

Bio-inspired Catalyst-Modified Photocathode for Bias-Free Photoelectrochemical NADH Regeneration

Cofactors such as nicotinamide adenine dinucleotide (NADH) and its phosphorylated form (NADPH) play a crucial role in natural enzyme-catalyzed reactions for the synthesis of chemicals. However, the stoichiometric supply of NADH for artificial synthetic processes is uneconomical. Here, inspired by the process of cofactor NADPH regeneration in photosystem I (PSI), catalyst-modified photocathodes are constructed on the surface of polythiophene-based semiconductors (PTTH) via self-assembly for photoelectrochemical catalytic NADH regeneration. With the assistance of viologen (vi2+) electron transfer mediators (similar function as Ferredoxin in PSI) linked to the [Rh(Cp*)(bpy)] catalyst, the Rh-vi2+@PTTH photocathode exhibits higher photocurrent density (-665 µA cm-2) with a high apparent turnover frequency (TOF, 168.4 h-1) under a relatively positive potential (0.0 V vs RHE). In addition, through holistic functional mimics of the photosystem, a tandem photoelectrochemical cell is constructed by assembling a CoPi@BiVO4 photoanode (artificial photosystem II, PSII) with the Rh-vi2+@PTTH photocathode. This system achieves a production rate of 42.5 µm h-1 cm-2 and a TOF of 179.3 h-1 without an externally applied bias for NADH regeneration. The photo-generated NADH is directly employed to assist glutamate dehydrogenase (GDH) in the catalytic conversion of α-ketoglutarate to L-glutamate. This study presents a novel strategic approach for constructing bias-free photoelectrochemical NADH regeneration systems.

Molecular and Heterojunction Device Engineering of Solution-Processed Conjugated Reticular Oligomers: Enhanced Photoelectrochemical Hydrogen Evolution through High-Effective Exciton Separation

Covalent organic frameworks (COFs) face limited processability challenges as photoelectrodes in photoelectrochemical water reduction. Herein, sub-10 nm benzothiazole-based colloidal conjugated reticular oligomers (CROs) are synthesized using an aqueous nanoreactor approach, and the end-capping molecular strategy to engineer electron-deficient units onto the periphery of a CRO nanocrystalline lattices (named CROs-Cg). This results in stable and processable "electronic inks" for flexible photoelectrodes. CRO-BtzTp-Cg and CRO-TtzTp-Cg expand the absorption spectrum into the infrared region and improve fluorescence lifetimes. Heterojunction device engineering is used to develop interlayer heterojunction and bulk heterojunction (BHJ) photoelectrodes with a hole transport layer, electron transport layer, and the main active layers, using a CROs/CROs-Cg or one-dimensional (1D) electron-donating polymer HP18 mixed solution via spinning coating. The ITO/CuI/CRO-TtzTp-Cg-HP18/SnO2/Pt photoelectrode shows a photocurrent of 94.9 µA cm‒2 at 0.4 V versus reversible hydrogen electrode (RHE), which is 47.5 times higher than that of ITO/Bulk-TtzTp. Density functional theory calculations show reduced energy barriers for generating adsorbed H* intermediates and increased electron affinity in CROs-Cg. Mott-Schottky and charge density difference analyses indicate enhanced charge carrier densities and accelerated charge transfer kinetics in BHJ devices. This study lays the groundwork for large-scale production of COF nanomembranes and heterojunction structures, offering the potential for cost-effective, printable energy systems.

Polymeric viologen-based electron transfer mediator for improving the photoelectrochemical water splitting on Sb2Se3 photocathode

The photogenerated charge carrier separation and transportation of inside photocathodes can greatly influence the performance of photoelectrochemical (PEC) H2 production devices. Coupling TiO2 with p-type semiconductors to construct heterojunction structures is one of the most widely used strategies to facilitate charge separation and transportation. However, the band position of TiO2 could not perfectly match with all p-type semiconductors. Here, taking antimony selenide (Sb2Se3) as an example, a rational strategy was developed by introducing a viologen electron transfer mediator (ETM) containing polymeric film (poly-1,1'-dially-[4,4'-bipyridine]-1,1'-diium, denoted as PV2+) at the interface between Sb2Se3 and TiO2 to regulate the energy band alignment, which could inhibit the recombination of photogenerated charge carriers of interfaces. With Pt as a catalyst, the constructed Sb2Se3/PV2+/TiO2/Pt photocathode showed a superior PEC hydrogen generation activity with a photocurrent density of -18.6 mA cm-2 vs. a reversible hydrogen electrode (RHE) and a half-cell solar-to-hydrogen efficiency (HC-STH) of 1.54% at 0.17 V vs. RHE, which was much better than that of the related Sb2Se3/TiO2/Pt photocathode without PV2+ (-9.8 mA cm-2, 0.51% at 0.10 V vs. RHE).

Efficient urea electrosynthesis from carbon dioxide and nitrate via alternating Cu-W bimetallic C-N coupling sites

Electrocatalytic urea synthesis is an emerging alternative technology to the traditional energy-intensive industrial urea synthesis protocol. Novel strategies are urgently needed to promote the electrocatalytic C-N coupling process and inhibit the side reactions. Here, we report a CuWO₄ catalyst with native bimetallic sites that achieves a high urea production rate (98.5 ± 3.2 μg h^-1 mg^-1_cat) for the co-reduction of CO₂ and NO₃^- with a high Faradaic efficiency (70.1 ± 2.4%) at -0.2 V versus the reversible hydrogen electrode. Mechanistic studies demonstrated that the combination of stable intermediates of *NO₂ and *CO increases the probability of C-N coupling and reduces the potential barrier, resulting in high Faradaic efficiency and low overpotential. This study provides a new perspective on achieving efficient urea electrosynthesis by stabilizing the key reaction intermediates, which may guide the design of other electrochemical systems for high-value C-N bond-containing chemicals.

Molecular-Modified Photocathodes for Applications in Artificial Photosynthesis and Solar-to-Fuel Technologies

Nature offers inspiration for developing technologies that integrate the capture, conversion, and storage of solar energy. In this review article, we highlight principles of natural photosynthesis and artificial photosynthesis, drawing comparisons between solar energy transduction in biology and emerging solar-to-fuel technologies. Key features of the biological approach include use of earth-abundant elements and molecular interfaces for driving photoinduced charge separation reactions that power chemical transformations at global scales. For the artificial systems described in this review, emphasis is placed on advancements involving hybrid photocathodes that power fuel-forming reactions using molecular catalysts interfaced with visible-light-absorbing semiconductors.

NiCo2O4 thin film prepared by electrochemical deposition as a hole-transport layer for efficient inverted perovskite solar cells

Spinel NiCo₂O₄ is a promising p-type semiconductor for optoelectronic devices; however, it is difficult to prepare uniform and large-area NiCo₂O₄ films, which hinders its application as a hole transport material for perovskite solar cells (PSCs). In this study, a novel, mild, and low-cost KCl-assisted electrochemical deposition (ECD) approach was developed to directly prepare a uniform NiCo₂O₄ film on a fluorine-doped tin oxide (FTO) substrate. A uniform NiCo₂O₄ film prepared through an ECD approach was used as a hole-transport layer (HTL) in inverted PSCs. The resulting NiCo₂O₄ HTL-based device achieved a power conversion efficiency (PCE) of 19.24% with negligible hysteresis and excellent reproducibility. Additionally, it outperformed a NiO_x-based device (PCE = 18.68%). The unsealed devices retained 90.7% of their initial efficiency when subjected to stability measurements for 360 h in an ambient atmosphere. This study shows the great potential of ECD-prepared NiCo₂O₄ HTLs for large-area PSCs in the future.

An electrochemical deposition approach was developed to prepare a NiCo₂O₄ hole transport layer for inverted perovskite solar cells.

Bulk Heterojunction Organic Semiconductor Photoanodes: Tuning Energy Levels to Optimize Electron Injection

The use of a bulk heterojunction of organic semiconductors to drive photoelectrochemical water splitting is an emerging trend; however, the optimum energy levels of the donor and acceptor have not been established for photoanode operation with respect to electrolyte pH. Herein, we prepare a set of donor polymers and non-fullerene acceptors with varying energy levels to probe the effect of photogenerated electron injection into a SnO₂-based substrate under sacrificial photo-oxidation conditions. Photocurrent density (for sacrificial oxidation) up to 4.1 mA cm^-2 was observed at 1.23 V vs reversible hydrogen electrode in optimized photoanodes. Moreover, we establish that a lower-lying donor polymer leads to improved performance due to both improved exciton separation and better charge collection. Similarly, lower-lying acceptors also give photoanodes with higher photocurrent density but with a later photocurrent onset potential and a narrower range of pH for good operation due to the Nernstian behavior of the SnO₂, which leads to a smaller driving force for electron injection at high pH.