Metal-free electrocatalyst for HER, OER and CO2RR: Towards green & sustainable energy solutions

Abstract

Noble metals are frequently used in the quest for a practical and efficient way to address the world's energy needs. Unfortunately, the high cost and limited availability of noble metal electrocatalysts have prevented the broad application of renewable energy. In the quest for a sustainable energy source, metal-free systems have become a strong contender for the three essential electrocatalytic reactions: hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and carbon dioxide reduction reaction (CO₂RR). For the economical production of clean, renewable energy, metal-free electrocatalysis is essential. Over the past ten years, significant advancements in metal-free electrocatalysis have been made. This paper outlines the emergence of metal-free systems and illustrates their possible use in the renewable energy sector. Particularly, we started by providing an overview of the basic principles of electrochemical reactions. Later on, a study of metal-free electrocatalyst classification and recent advancements is performed; these include materials based on carbon (e.g., CNT, g-C3N4, graphene, etc.), covalent organic frameworks (e.g., triazine, porphyrin, thiadiazole, etc.), and others (biopolymer types, molecular types, etc.). Additionally, the application and synthesis process for the specific electrocatalytic reaction are described simultaneously. Overall, the essence of the key challenges and future outlooks to enhance the potential in metal-free electrocatalysis is provided. This review emphasizes the crucial role of metal-free electrocatalysts (MFEs) for reducing our dependency on fossil fuels and reaching carbon neutrality.