In the last two decades, the world including China has made a significant progress in hydrogen energy, including the construction of hydrogen infrastructure, key materials and components, the development of fuel cell vehicle industries, etc. However, up to now, the development of the hydrogen energy industry worldwide still faces significant bottlenecks and difficulties related to large-scale storage, long distance transportation, and safety in distribution network. Therefore, a large-scale hydrogen energy application poses a significant challenge for the development of hydrogen energy industries. This strategic study puts forward the interdisciplinary science frontiers of innovative combination of ammonia and hydrogen as new energy or energy carriers, i.e. ammonia-hydrogen new energy, and its associated key research directions. Ammonia-hydrogen new energy refers to a new energy system with ammonia and hydrogen as direct energy or energy carriers. Both hydrogen and ammonia (NH3) are carbon-free fuels. They can be obtained through renewable energy, and they can be converted into each other. Ammonia can be synthesized through green hydrogen, and hydrogen can be efficiently prepared by ammonia cracking, and the two can be used separately in different application scenarios, or they can be mixed and utilized synergistically. Ammonia is an efficient hydrogen storage medium, with a hydrogen mass fraction of 17.6% and can be liquified easily at -33 degrees C at atmospheric pressure or 1 MPa at room temperature. The large-scale storage and long distance transportation infrastructure of liquid ammonia are well developed. More importantly, both NH3 and H2 are zero-carbon fuels. The successful implementation of the ammonia-hydrogen new energy strategy will provide a complete solution to the large-scale application and true industrialization of hydrogen energy. Ammonia-hydrogen new energy has become a forward-looking and strategic development direction in the field of clean energy worldwide. In 2019, Ammonia Energy Association (AEA) put forward the new concept of "Ammonia = Hydrogen 2.0", aiming at promoting hydrogen economy via ammonia, specifically "Building an energy export industry using Green Ammonia". Countries around the world are making plans to develop ammonia-hydrogen new energy, including Japan, Republic of Korea, the Netherlands, Norway, Australia, etc. China also puts forward a plan to include hydrogen and ammonia as energy storage vectors. Developing high-temperature industrial kiln ammonia-hydrogen zero-carbon combustion technology, ammonia-hydrogen zero-carbon transportation equipment technology, ammonia high-temperature combustion nitrogen oxide emission control technology and other disruptive technologies can open up new major application scenarios of hydrogen energy, and provide innovative and disruptive technologies for high-temperature manufacturing, transportation, power generation and other industries to achieve carbon peaking and carbon neutrality targets. Ammonia-hydrogen new energy science and technology is a major frontier in multidisciplinary interdisciplinary fields, including energy science and technology, material science and engineering, chemistry and chemical engineering, power engineering and engineering thermophysics, transportation engineering, etc. In this paper, four key research and development areas are discussed, including low-cost, large-scale green ammonia production technologies; ammonia-hydrogen zero-carbon combustion technologies for high-temperature manufacturing industry; ammonia-hydrogen zero-carbon technologies for vehicles and marine applications; ammonia-hydrogen zero-carbon combustion technologies for aircraft engines and gas turbines. Critical science and technology challenges in these areas are also discussed. This paper also puts forward deliberated policy proposals to strengthen the researches of interdisciplinary frontiers of scientific and technological issues, so as to provide strategic guidelines to solve the major difficulties facing hydrogen energy storage and transportation technologies and to address challenges in expanding and implementing application scenarios of hydrogen energy.
