## 1 Introduction
### Review of Alternative Fuels for the Marine Sector
#### 1.2.3 Zero-Carbon Fuels  
Zero-carbon fuels are fuels whose use does not result in CO\({}_{2}\) emissions. These fuels represent promising measures for ship decarbonisation and reaching the IMO's 2050 goal [42].  
The electrification of ships represents a game changer for the decarbonisation of the shipping industry. There are three types of electrified ships, i.e., plug-in hybrid ships, hybrid ships, and all-electric ships. Both plug-in hybrid ships and hybrid ships include diesel engines and batteries, while all-electric ships refer to the sole use of batteries for ship power [43]. The main drawbacks of full electrification are limitations regarding battery capacity, degradation and weight, investment costs, charging infrastructure at the docks, and sailing distance [44; 45; 46]. Different types of batteries are available for maritime purposes. [PERSON] et al. [47] investigated three batteries (lithium-ion (Li-ion), nickel-metal hydride, and lead batteries) for use in ferries. Li-ion batteries were highlighted as the most environmentally friendly and cost-effective option. With further development of battery technology, i.e., metal-air batteries [48], the full electrification of ships that operate in the open sea could be feasible.  
Hydrogen use onboard ships also achieves zero-emission shipping. Based on its cleanliness, i.e., the sources used for its production, hydrogen can be classified by different colours (grey, brown, blue, yellow, pink, green, etc.). However, hydrogen is still primarily produced from natural gas by steam reforming (known as grey hydrogen) [49]. Due to its low volumetric energy density, hydrogen is difficult to store. Often stored in its liquid form, hydrogen evaporates due to heat leakage into the cryogenic tank, known as boil-off gas, which represents a drawback of liquid hydrogen storage [50]. Due to the fast kinetics of electrochemical reactions and its only by-product being water, hydrogen represents the most appropriate fuel for fuel cells. There are different types of fuel cells that are classified based on their operating temperature: low-temperature fuel cells (\(\sim\)80 \({}^{\circ}\)C), intermediate-temperature fuel cells (\(\sim\)200 \({}^{\circ}\)C), and high-temperature fuel cells (650-1000 \({}^{\circ}\)C) [51]. The application of fuel cells onboard usually refers to satisfying auxiliary power needs [52; 53]. However, their use for propulsion is entering a new phase, starting with the first ferry fully powered by fuel cells fuelled with liquid hydrogen which has been in operation in Norway since March 2023 [54].  
Hydrogen can also be used in an Internal Combustion Engine (ICE), which is less expensive to produce, has a longer lifetime, and does not require fuel purification before use (which is required for low-temperature fuel cells) [55]. However, its use in ICEs encounters several challenges, e.g., potentially high combustion temperatures, which lead to high NO\({}_{\text{X}}\) emissions [56].  
Ammonia is a hydrogen-rich fuel whose storage onboard ships is easier than that of hydrogen. It is the second most produced chemical in the world, used mainly as a fertiliser.  
Its use on board (in ICEs or fuel cells) does not result in CO\({}_{2}\) and SO\({}_{\rm X}\) emissions, while NOx emissions can be eliminated with the proper catalyst. Its main drawbacks are toxicity (for humans and marine life) and corrosiveness, low energy density, and infrastructure, which should be expanded to cover the maritime sector [42].