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

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 H-2 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. (C) 2016 Elsevier B.V. All rights reserved.