Research progress in green synthesis of ammonia as hydrogen-storage carrier under 'hydrogen 2.0 economy'

Abstract

Hydrogen energy is characterized by its environmental friendliness, high efficiency, lack of carbon emissions and wide range of applications. However, its transportation and storage are challenges that limit further development of the hydrogen-energy industry. Ammonia is a carbon-free hydrogen-rich carrier. The storage of hydrogen in ammonia has unique advantages of high energy density, easy storage and transportation, reliable safety, a mature industrial foundation and no tail-end carbon emissions. However, industrial ammonia synthesis still heavily relies on the Haber-Bosch process, which accounts for significant energy consumption and greenhouse gas emissions. Therefore, the development of green and sustainable ammonia-synthesis methods is extremely important and urgent. Recently, ammonia-synthesis technologies such as electrocatalysis, photocatalysis, photoelectrocatalysis and biocatalysis have successfully produced ammonia from nitrogen and water, resulting in lower costs. The nitrogen-reduction-reaction conditions of these methods are mild and can be carried out under ambient temperatures and atmospheric pressure with low energy consumptions. Meanwhile, these methods bypass the traditional hydrogen-production section and their routes are simpler. Therefore, these technologies can be used to flexibly integrate renewable energy, including intermittent renewable energy, to achieve distributed ammonia synthesis. These benefits contribute to both global energy and environmental sustainability goals. In this study, the mechanisms of ammonia synthesis under ambient conditions are reviewed and the technical difficulties of various catalysts for ammonia synthesis are summarized. Based on the optimization strategies reported for various catalysts, the high-performing catalysts reported for ammonia synthesis are reviewed and the developmental trend of this field has been forecasted.