1. Introduction

The June 2020 national strategic plan for hydrogen energy in Germany set a target of reaching a domestic electrolyzer capacity of 5 million kW by 2030 and 10 million kW by 2040 [112]. However, the coalition agreement signed by the German government in December 2021 revised the 2030 target to 10 million kW of electrolyzer capacity, which doubled the original goal [113]. Hydrogen demand in Germany is expected to be 64 to 110 billion kWh in 2030 and 392 to 657 billion kWh in 2045 [114]. It is estimated that German domestic capacity will only be able to meet approximately 15% of the demand by 2030. By 2050, the production of hydrogen is predicted to consume about a quarter of the EU’s renewable electricity. As an essential component of an integrated energy system, hydrogen and P2X will continue to gain importance.

To efficiently develop the hydrogen economy globally, renewable hydrogen, which is more expensive, should mainly be utilized in industries that cannot be directly electrified. Beyond electrolysis, alternative methods of producing green hydrogen are also being extensively explored [115]. For instance, recent research has discussed the potential of the aqueous phase reforming process in treating and valorizing carbon-laden industrial wastewater. This process, capable of converting oxygenated molecules into hydrogen under relatively mild conditions, is seen as a promising avenue for the development of a circular, low-waste economy, although challenges remain in fully realizing this process at an industrial scale [116]. Xie presented a novel method for direct seawater electrolysis for hydrogen production, effectively addressing the issues of electrode side reactions and corrosion triggered by seawater’s complex components. The authors successfully operated this system, demonstrating its high potential for practical applications, and enabling efficient and flexible seawater electrolysis with prospective applications in one-step water-based effluent treatment, resource recovery, and hydrogen production [117]. Despite years of research by public stakeholders in industry, science, and other relevant sectors, there are only five application areas currently considered safe for the use of hydrogen energy and P2X: the chemical industry, steel production, vehicles, maritime transport, and air transport.

Hydrogen serves as a crucial feedstock rather than a source of energy in the chemical industry, where it is utilized for the production of basic materials like industrial gases, fertilizers, and petrochemicals, and their derivatives [118]. In addition, as described in the previous sections, renewable hydrogen can be converted into various products via diverse processes, such as methanol and Fischer–Tropsch production followed by refining, yielding conventional transportation fuels. It is noteworthy that methanol is an essential precursor for the production of synthetic resins, with a worldwide production that represents 25% of total methanol production [119]. Although most basic chemicals and petrochemicals rely on carbon, ammonia production relies on nitrogen, where hydrogen energy serves as the input. Recent research explores its potential as a zero-carbon fuel, particularly in the shipping industry and stationary power generation [120,121,122,123]. In addition, the advantage that ammonia does not require carbon dioxide in the production process offsets its high toxicity and the danger it poses to the aquatic environment [124]. Germany is set to construct its first ammonia import terminal at Brunsbüttel, with RWE planning to import 300,000 tons of green ammonia annually to produce nitrogen fertilizers and mineral oil products at the River Elbe port [125].

In 2022, German produced 32.1 million tons of crude steel, of which 70% were produced using blast furnaces and converters and the remaining 30% using electric arc furnaces [126]. As a result, direct CO₂ emissions from the steel industry were approximately 43.7 million tons in the same year. The steel industry’s potential for reducing carbon emissions can be realized through three different approaches [127,128,129,130]:

• Substituting fossil fuels like coal and natural gas with hydrogen in the deep processing of crude steel.
• Increasing the share of electric arc furnaces that use renewable energy for steel production, which requires the use of climate-neutral hydrogen in the natural gas burners of electric arc furnaces in the long term.
• Direct reduction utilizing green hydrogen and transitional hydrogen-rich fuels (such as natural gas), coupled with a move to primary steel production with carbon capture, utilization, and storage (CCUS) technology. Compared to the blast furnace pathway, the all-green hydrogen model has a 95% greater potential for CO₂ reduction.

As per recent estimations, the European steel industry may experience a significant upsurge in hydrogen demand, with projections indicating an increase to 45 billion kWh by 2030 and to 123 billion kWh by 2050 [131]. However, in order to encourage the steel industry to invest in low carbon steel production, there is a need for targeted market projections, which can help to develop sales markets for green steel. The establishment of standards and product labeling are also critical prerequisites in the development of a sales market for green steel [132].

4. Hydrogen in Vehicles

A conventional internal combustion engine can lead to bulky and inefficient vehicles. In contrast, fuel cell electric vehicles (FCEVs) are powered by hydrogen, which convert energy into electricity more efficiently, and have only water as a byproduct. Hydrogen fuel cells are two to three times more efficient than internal combustion engines, making them a promising technology for the future of the transportation sector. FCEVs have the potential to reduce greenhouse gas emissions by 46.6% compared to conventional vehicles (CVs) [133]. However, a major obstacle to the large-scale use of hydrogen in the transportation sector is how to store it more efficiently. Due to its low density, hydrogen needs to be compressed and cooled for storage and cannot be stored as easily as conventional fossil fuels. Physically sealed storage methods, such as compression tanks, are the most favorable for storing hydrogen, primarily using all composite materials (Type IV) and sometimes metal-lined composites (Type III) [134]. The Toyota Mirai, as one of the first commercially sold FCEVs, distinguished itself in August 2021 when its second-generation model achieved a world record by travelling 1,360 km on a single fill of 5.65 kg of hydrogen [135]. FCEVs augmented by hydrogen refueling stations are expected to break the mileage limits of conventional battery-driven vehicles due to their high thermal efficiency in practical applications [136].

5. Hydrogen in Maritime Transportation

Maritime transportation is the linchpin of global trade, facilitating more than 80% of global freight transportation [137]. To reduce shipping emissions by 50% by 2050 compared to 2008, the International Maritime Organization (IMO) has set a target under its 2019 greenhouse gas strategy [138]. The majority of maritime vessels currently rely on single-fuel diesel engines, burning low-sulfur fuel oil and gas oil, and this technology is also employed in most new ship orders. Although liquefied natural gas (LNG) is the only commercially available alternative fuel and can potentially reduce CO₂ emissions by up to 25%, it is viewed as a transition fuel due to its limited potential to lower greenhouse gas emissions and the high risk of incomplete methane combustion emissions [139]. High energy fuels (HEFs) are being considered as the most viable option for decarbonizing shipping in this century. Presently, synthetic methane or liquid Fischer–Tropsch fuels can be used as alternative fuels in dual-fuel marine engines, and methanol-fueled engines are already available on the market, with new ammonia-fueled engines under development, and fuel cells offer the possibility of using hydrogen energy directly. However, HEFs are not yet commercially available and cannot compete with fossil fuels. The selection of a suitable winner from among the various alternative fuels remains challenging, given the factors to consider, including fuel availability, infrastructure and storage, technology maturity, energy density, price, and environmental friendliness. The future of marine fuel is highly uncertain due to the typical service life of vessels of twenty to thirty years and the high cost of retrofitting ship fuel systems. This uncertainty inhibits investment because of the risk of stranded assets [140].

6. Hydrogen in Air Transportation

Civil aviation is actively exploring alternative fuels, with hydrogen and e-Kerosene emerging as two promising options [141]. E-Kerosene, a ready-to-use fuel produced by reacting green hydrogen with CO₂ from Direct Air Capture (DAC) technologies, produces 90% fewer greenhouse gas emissions over its life cycle than fossil jet fuel A/A-1. Additionally, e-Kerosene’s lack of sulfur and lower NOx emissions from combustion compared to fossil jet fuels help to mitigate the non-CO₂ effect [142]. Nevertheless, when accounting for non-CO₂ effects, e-Kerosene’s greenhouse effect reduction compared to fossil jet fuel A/A-1 is approximately 50% [142]. The utilization of green hydrogen as a direct final energy carrier could potentially achieve 100% reduction in CO₂ emissions from fuel combustion. Despite uncertainties about the feasibility of direct hydrogen use as an end-use energy carrier in commercial aviation, Airbus has announced plans to launch a long-range hydrogen-powered aircraft in 2035 [143]. However, differing opinions on the role of hydrogen in aviation have been voiced, with the CEO of Boeing stating that it will not play a major role until 2050 [144]. The commercial viability of these alternative fuels depends on factors such as infrastructure, technology maturity, cost, and environmental impact, and further research and development efforts are required to address these challenges.