Ammonia is emerging as a viable alternative to fossil fuels in combustion systems, aiding in the reduction of carbon emissions. However, its use faces challenges, including NOx emissions and low flame speed. Innovative approaches and technologies have significantly advanced the development and implementation of ammonia as a zero-carbon fuel. This review explores current advancements in using ammonia as a fuel substitute, highlighting the complexities that various systems need to overcome before reaching full commercial maturity in support of practical decarbonising global strategies. Different from other reviews, this article incorporates insights of various industrial partners currently working towards green ammonia technologies. The work further addresses fundamental complexities of ammonia combustion, crucial for its practical and industrial implementation in various types of equipment.

This article reviews the critical role of material selection and design in ensuring efficient performance and safe operation of gas turbine engines fuelled by ammonia–hydrogen. As these energy fuels present unique combustion characteristics in turbine combustors, the identification of suitable materials becomes imperative. Detailed material characterisation is indispensable for discerning defects and degradation routes in turbine components, thereby illuminating avenues for improvement. With elevated turbine inlet temperatures, there is an augmented susceptibility to thermal degradation and mechanical shortcomings, especially in the high-pressure turbine blade—a critical life-determining component. This review highlights challenges in turbine design for ammonia–hydrogen fuels, addressing concerns like ammonia corrosion, hydrogen embrittlement, and stress corrosion cracking. To ensure engine safety and efficacy, this article advocates for leveraging advanced analytical techniques in both material development and risk evaluation, emphasising the interplay among technological progress, equipment specifications, operational criteria, and analysis methods.

Polymer electrolyte membrane fuel cells (PEMFCs) are gaining attention as viable energy carriers for future aerospace propulsion systems due to their high-power density, lightweight and compact design, zero emissions, scalability, quiet operation, and relatively reliable performance. However, maintaining optimal performance and durability under transient thermal conditions remains a critical challenge, particularly in aerospace environments. Despite extensive research on PEMFCs, the transient thermal effects remain underexplored. This study employs a validated numerical simulation model to investigate the transient responses of a PEMFC subjected to thermal shock cycles, where the bipolar plate walls experience abrupt temperature drops to 10 °C for durations of 3 to 19 s. The simulation model was benchmarked against experimental data from the literature, demonstrating deviations of less than 10% in the polarization curves, confirming its reliability for predicting transient behaviors. Results reveal that during these thermal shocks, the current density decreases by approximately 15%, from 9263 A/m² at 50 °C to 7709 A/m² at 10 °C, with recovery times exceeding 4 s. Significant deviations were observed in oxygen concentration, particularly at the cathode catalyst layer, where minimum levels decreased by over 20%. Similarly, the water content in the membrane showed an overshoot above steady-state levels postrecovery, remaining elevated for extended periods. Liquid water saturation in the gas diffusion layers (GDLs) increased significantly near the hydrogen inlets during cold conditions, obstructing reactant flow and further impacting performance. This study provides detailed predictions of the steady-state and transient responses of PEMFCs to temperature reduction cycles. The findings contribute to advancing thermal management strategies and improving system resilience under transient conditions, thereby addressing a key challenge in sustainable aviation.