Recent advances and challenges of photoelectrochemical cells for hydrogen production

Semiconductor-Based Photoelectrocatalysts in Water Splitting: From the Basics to Mechanistic Insights-A Brief Review

Hydrogen and oxygen serve as energy carriers that can ease the transition of energy due to their high energy densities. Nonetheless, their production processes entail the development of efficient and low-cost storage and conversion technologies. In this regard, photoelectrocatalysts are materials based on the photoelectronic effect where electrons and holes interact with H2O, producing H2 and O2, and in some cases, this is achieved with acceptable efficiency. Although there are several reviews on this topic, most of them focus on traditional semiconductors, such as TiO2 and ZnO, neglecting others, such as those based on non-noble metals and organic ones. Herein, semiconductors like CdSe, NiWO4, Fe2O3, and others have been investigated and compared in terms of photocurrent density, band gap, and charge transfer resistance. In addition, this brief review aims to discuss the mechanisms of overall water-splitting reactions from a photonic point of view and subsequently discusses the engineering of material synthesis. Advanced composites are also addressed, such as WO3/BiVO4/Cu2O and CN-FeNiOOH-CoOOH, which demonstrate high efficiency by delivering photocurrent densities of 5 mAcm-2 and 3.5 mA cm-2 at 1.23 vs. RHE, respectively. Finally, the authors offer their perspectives and list the main challenges based on their experience in developing semiconductor-based materials applied in several fields. In this manner, this brief review provides the main advances in these topics, used as references for new directions in designing active materials for photoelectrocatalytic water splitting.

Layered double hydroxide modified bismuth vanadate as an efficient photoanode for enhancing photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting has attracted significant interest as a promising approach for producing clean and sustainable hydrogen fuel. An efficient photoanode is critical for enhancing PEC water splitting. Bismuth vanadate (BiVO4) is a widely recognized photoanode for PEC applications due to its visible light absorption, suitable valence band position for water oxidation, and outstanding potential for modifications. Nevertheless, sluggish water oxidation rates, severe charge recombination, limited hole diffusion length, and inadequate electron transport properties restrict the PEC performance of BiVO4. To surmount these constraints, incorporating layered double hydroxides (LDHs) onto BiVO4 photoanodes has emerged as a promising approach for enhancing the performance. Herein, the latest advancements in employing LDHs to decorate BiVO4 photoanodes for enhancing PEC water splitting have been thoroughly studied and outlined. Initially, the fundamental principles of PEC water splitting and the roles of LDHs are summarized. Secondly, it covers the development of different composite structures, including BiVO4 combined with bimetallic and trimetallic LDHs, as well as other BiVO4-based composites such as BiVO4/metal oxide, metal sulfide, and various charge transport layers integrated with LDHs. Additionally, LDH composites incorporating materials like graphene, carbon dots, quantum dots, single-atom catalysts, and techniques for surface engineering and LDH exfoliation with BiVO4 are discussed. The research analyzes the design principles of these composites, with a specific focus on how LDHs enhance the performance of BiVO4 by increasing the efficiency and stability through synergistic effects. Finally, challenges and perspectives in future research toward developing efficient and stable BiVO4/LDHs photoelectrodes for PEC water splitting are described.

Origin of Enhanced Photoelectrochemical Activity of the n-CdS/p-type Semiconductor Interface: Homojunction or Heterojunction?

Surface engineering of photoelectrodes is considered critical for achieving efficient photoelectrochemical (PEC) cells, and various p-type materials have been investigated for use as photoelectrodes. Among these, the p-type semiconductor/n-type CdS heterojunction is the most successful photocathode structure because of its enhanced onset potential and photocurrent. However, it is determined that the main contributor to the enhanced activity is the Cd-doped layer and not the CdS layer. In this study, a Cd-doped n+p-buried homojunction of a CuInS2 photocathode is first demonstrated without a CdS layer. The homojunction exhibited a more active and stable PEC performance than the CdS/CuInS2 heterojunction. Moreover, it is confirmed that Cd doping is effective for other p-type materials. These results strongly suggest that the effects of Cd doping on photocathodes should be carefully investigated when designing CdS/p-semiconductor heterojunction photoelectrodes. They also indicate that the Cd-doped layer has great potential to replace the CdS layer in future photoelectrode designs.

Advances and challenges in the modification of photoelectrode materials for photoelectrocatalytic water splitting

With the increasing consumption of fossil fuels, the development of clean and renewable alternative fuels has become a top priority. Hydrogen (H2) is an ideal primary clean energy source for its extremely high gravimetric energy density, carbon-free combustion, and abundant natural resources. Photoelectrocatalytic (PEC) water splitting is among the most promising approaches for converting sunlight and water into H2. However, the cost-effectiveness and the overall solar to hydrogen conversion efficiency of PEC water splitting are still big challenges. In the past few decades, several studies have been devoted to this technology, and it is essential to develop economical photoelectrocatalysts with high conversion efficiency. Therefore, there is an urgent need for a comprehensive and updated review of recent advances in the design, manufacture, and modification of efficient PEC water splitting systems. This review first starts with the basic mechanism of photoelectrochemical water splitting. Then the problems in PEC water splitting are discussed, and the methods of photoelectrode modulation such as nanostructure fabrication, doping engineering, surface modification, and heterojunction construction are introduced. Finally, the critical challenges and future trends/perspectives in the PEC water splitting are discussed.

Enhancing the PEC Efficiency in the Perspective of Crystal Facet Engineering and Modulation of Surfaces

Generation of hydrogen is one of the most promising routes to harvest solar energy for its sustainable utilization. Among different routes, the photoelectrochemical (PEC) process to split water using solar light to produce hydrogen is the green method to generate hydrogen. The sluggish kinetics through complicated pathways makes the oxygen evolution reaction the rate limiting step of the overall water splitting process. Therefore, development of an efficient photoanode for the sustainable oxidation of water is most challenging in an efficient overall PEC water splitting process. The low solar to hydrogen conversion efficiency arises from the slow surface kinetics, poor hole diffusion, and fast charge recombination processes. There have been strategies to improve catalytic performances through the removal of such detrimental effects. The generation of engineered surfaces is one of the important strategies recently adopted for the enhancement of the catalytic efficiencies. The present review has been focused on the discussion of engineered surfaces using crystal facet engineering, protective surface layer, passivation using the atomic layer deposition (ALD) technique, and cocatalyst modified surfaces to enhance the catalytic efficiency. Some of the important parameters defining catalyst performance are also discussed at the beginning of the review.