General definition of hydrogen energy and related electrochemical technologies

Abstract

Electrochemical energy conversion, based on the transformation between electrical energy and chemical energy, is crucial for the storage and utilization of renewable electrical energy, as well as the eco-friendly production of valuable fuels and chemicals. Here we put forward a general definition of hydrogen energy that covers the energy storage/release in any hydrogen-involved chemical bonds, such as H–H, C–H, and N–H bonds. Given that H2 gas, hydrocarbons, and hydronitrogens are not only high-energy-density fuels but also important industrial feedstock s, the successful implementation of efficient energy storage/release in the H–H (hydrogen cycle), C–H (carbon cycle), and N–H (nitrogen cycle) bonds are of great significance in establishing sustainable energy systems and reducing dependence on fossil fuels, which contributes to address global energy crisis and environmental problems. We provide a concise overview of the current development status of some key low-temperature (< 100 °C) electrochemical technologies relevant to general hydrogen energy, including fuel cells, water electrolysis, CO2 electrolysis, and N2 electrolysis. This discussion aims to elucidate the major challenges and future trends in this field.