A review of 2D-based counter electrodes applied in solar-assisted devices

• It is critical to substitute costly CEs with efficient and low-cost replacements.
• We focus on 2D materials used as CEs, mainly GO, rGO, and TMDC-based nanoflakes.
• We discuss the chief factors involving the material and performance improvement.

The alarming energy and environmental crisis of our modern era necessitates scientists to propose alternatives to replace the scarce and harmful fossil fuels. The Sun, as a free, and abundant clean energy resource, presents a motivation for researchers to convert the sunlight to electrical power and store the H2 as an energy carrier by developing solar cells as well as water splitting devices, respectively. One of the fundamental parts of a solar energy convertor (solar cell/photoelectrochemical device), where the reduction reaction occurs, is the counter electrode (CE); the CE is commonly based on rare Pt. It is critical to substitute costly CEs with efficient, low-cost replacements, which would draw widespread attention to two-dimensional (2D) materials. Over the past few years, graphene and graphene-like materials, especially transition metal dichalcogenides (TMDC), and also their hybrid systems have been suggested to be new path to electronic/optoelectronic devices for their unusual properties. The growing number of studies and research articles published on 2D counter electrodes call for a comprehensive outlook on new approaches to electrode fabrications and optimization methods, along with a clear classification of 2D material-based counter electrodes. Essentially, reviewing the most accredited papers, much light is shed on a global energy research roadmap accelerating scientists’ efforts in this area. In this review, we have focused on 2D materials used as CEs; mainly graphene sheets in pure, doped, or composite forms; as well as TMDC-based CEs. In each section, the most common syntheses and characterization approaches are reviewed as well as introducing the most efficient systems.

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