Review on Metals and Metal Oxides in Sustainable Energy Production: Progress and Perspectives

The exponential population growth on earth has put an enormous strain on energy resources, which come in various forms like fossil fuels, geothermal energy, and so on. Currently, fossil fuels are fulfilling most of our energy requirements. However, their nonrenewable nature and the production of toxic and greenhouse gases have forced the research community to explore different renewable and nontoxic energy resources. Among different renewable energy resources, hydrogen is considered an important alternative energy source. However, the production of hydrogen (H2) from water is a nonspontaneous process. Therefore, different processes like photocatalysis and photoelectrocatalysis are employed to carry out the water splitting. For these processes, there is a dire need to develop different substances that can act as catalysts. For energy applications, different metals (particularly noble metals) and metal oxides (mostly transition metal oxides) have shown promising catalytic applications in the past decade. Herein, we discuss the progress made in the synthesis of metal and metal oxide nanostructures, followed by a discussion regarding the advances made in the application of these nanostructures as photocatalysts, electrocatalysts, and photoelectrocatalysts. From an energy application standpoint, it is found that doping and heterostructure development are the most advantageous methods used to date to improve the efficiency of metal and metal oxide nanostructures. Also, the development of dye-sensitized metal oxide catalysts for energy applications is considered a powerful method to develop highly efficient photocatalysts and electro-/photoelectrocatalysts. Finally, the limitations and challenges facing the practical application of these nanostructures are also discussed.