Hydrogen and ammonia are gaining attention as alternative fuels due to their carbon-free combustion feature, and offer a potential solution for decarbonization. However, they also face challenges to overcome. Currently, most hydrogen is produced from natural gas through steam methane reforming (SMR), which produces significant GHGs, called as “gray” hydrogen. In contrast, “green” hydrogen, produced by water electrolysis using renewable energy, generate little GHGs but not yet economically feasible due to high production costs. Liquefied hydrogen (LH2) has logistical challenges, such as low energy density and high liquefaction costs, as well as the risk of GHG emissions from the energy-intensive liquefaction process. Ammonia is easier to transport due to its milder liquefaction conditions, and can be used directly as a fuel once the ammonia-fueled engines currently under development are commercialized. However, like hydrogen, the environmental impact of ammonia depends on how it is produced. “Gray” ammonia, synthesized from fossil fuels, has high GHG emissions, while green ammonia, produced from green hydrogen, is more sustainable but still requires careful consideration of its energy-intensive synthesis process. Additionally, ammonia is a toxic substance that poses health risks and requires stringent safety measures.