1. Introduction一、简介

The June 德国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年6月发布的国家氢能战略规划设定了到2030年国内电解槽产能达到500万千瓦、2040年达到1000万千瓦的目标[112 ^[1]. However, the coalition agreement signed by the German government in December ]。然而，德国政府2021 revised the 年12月签署的联盟协议将2030 target to 10年目标修改为1000万千瓦电解槽容量，是原目标的两倍[ million113 kW]。预计 of electrolyzer capacity, which doubled the original goal ^[2]. Hydrogen demand in Germany is expected to be 2030 年德国的氢需求量将达到 64 to至 110 billion kWh in 2030 and 392 to 657 billion0 亿千瓦时，2045 年将达到 392 至 6570 亿千瓦时[ kWh in 2045 ^[3]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 ^[4][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 ^[5][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 ^[6][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 ^[7][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 ^[8][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 ^{[9][10][11][12]}[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 ^[13][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 ^[14][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 ^[15][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 ^{[16][17][18][19]}[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.提高使用可再生能源进行钢铁生产的电弧炉的比例，这需要在电弧炉的天然气燃烧器中长期使用气候中性的氢气。
• 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 (利用绿色氢和过渡性富氢燃料（例如天然气）直接还原，并利用碳捕获、利用和储存（CCUS) technology. Compared to the blast furnace）技术转向初级钢铁生产。与高炉路径相比，全绿色氢模型的CO pathway, the all-green hydrogen model has a 95% greater potential for CO₂ reduction.减排潜力高出95%。

As根据最近的估计，欧洲钢铁行业的氢需求可能会大幅增加，预计到 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 2052030 年将增加到 450 亿千瓦时，到 2050 年将增加到 1230 ^[20].亿千瓦时[131 However,]。然而，为了鼓励钢铁行业投资低碳钢生产，需要进行有针对性的市场预测，这有助于开发绿色钢铁的销售市场。标准和产品标签的制定也是绿色钢铁销售市场发展的关键先决条件[ in132 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 ^[21].]。

传统的内燃机会导致车辆体积庞大且效率低下。相比之下，燃料电池电动汽车（FCEV）由氢提供动力，可以更有效地将能量转化为电能，并且副产品只有水。氢燃料电池的效率是内燃机的两到三倍，使其成为交通运输领域未来有前途的技术。与传统车辆 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 CV) 相比，FCEV 有潜力将温室气体排放量减少 46.6% compared[ to conventional vehicles (CVs) ^[22]. 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 133]。然而，氢在交通运输领域大规模使用的一个主要障碍是如何更有效地储存氢。由于密度低，氢需要压缩和冷却才能储存，不像传统化石燃料那样容易储存。物理密封存储方法，例如压缩罐，最有利于存储氢气，主要使用所有复合材料（IV) and sometimes metal-lined composites (Type III)型），有时使用金属衬里复合材料（III ^[23].型）[134 The]。丰田 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 作为首批商业销售的 FCEV 之一，于 2021 年 8 月脱颖而出，其第二代车型在一次加注 5.65 公斤氢气的情况下行驶 1,360 km on公里，创下了世界纪录 [ a135]。加氢站增强的燃料电池汽车由于其在实际应用中的高热效率而有望突破传统电池驱动汽车的里程限制[ single136 fill of 5.65 kg of hydrogen ^[24]. 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 ^[25]. ]。

海上运输是全球贸易的关键，为全球80%以上的货运提供了便利[ of137 global]。为了到 freight transportation ^[26]. To reduce shipping emissions by 50% by 2050 compared to年将航运排放量比 2008, the International Maritime Organization 年减少 50%，国际海事组织 (IMO) has set a target under its在其 2019 greenhouse gas年温室气体战略中设定了目标 [ strategy138 ^[27].]。目前大多数海船依靠单一燃料柴油发动机，燃烧低硫燃油和瓦斯油，大多数新船订单也采用了这项技术。尽管液化天然气 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 是唯一可商用的替代燃料并且可以潜在地减少 CO ₂ emissions by up排放量减少高达 to 25%,25%，由于其降低温室气体排放的潜力有限且不完全甲烷燃烧排放的高风险，它被视为过渡燃料[ 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 ^[28]. 139]。高能燃料（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）被认为是本世纪航运脱碳最可行的选择。目前，合成甲烷或液体费托燃料可用作双燃料船用发动机的替代燃料，甲醇燃料发动机已经上市，新型氨燃料发动机正在开发中，燃料电池提供了可能性直接利用氢能。然而，HEF 尚未商业化，无法与化石燃料竞争。考虑到燃料可用性、基础设施和储存、技术成熟度、能量密度、价格和环境友好性等因素，从各种替代燃料中选择合适的获胜者仍然具有挑战性。由于船舶的典型使用寿命为二十至三十年以及改装船舶燃料系统的高昂成本，船用燃料的未来具有高度不确定性。由于搁浅资产的风险，这种不确定性抑制了投资[140 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 ^[29]. ]。

民航正在积极探索替代燃料，其中氢和电子煤油成为两种有前景的选择[ aviation141 is actively exploring alternative fuels, with hydrogen and e-Kerosene emerging as two promising options ^[30]. E-Kerosene, a ready-to-use fuel produced by reacting green hydrogen with]。电子煤油是一种即用型燃料，由绿色氢与直接空气捕获 CO₂ from Direct Air Capture (DAC) technologies,技术中的 producesCO 90% fewer greenhouse gas emissions over₂反应生成，在其生命周期内产生的温室气体排放量比化石喷气燃料 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- 少 90%。此外，与化石喷气燃料相比，电子煤油不含硫且燃烧产生的 NOx 排放量较低，有助于减轻非 CO ₂效应[ effect142 ^[31].]。然而，在考虑非 Nevertheless, when accounting for non-CO ₂ effects, e-Kerosene’s影响时，与化石航空燃料 greenhouse effect reduction compared to fossil jet fuel A/A-1 is approximately 相比，电子煤油的温室效应减少约为 50% ^[31].[ The utilization of green hydrogen as a direct final energy carrier could potentially achieve 100% reduction in 142]。利用绿色氢作为直接的最终能源载体有可能实现燃料燃烧产生的CO _{2排放量减少100%。}尽管直接使用氢气作为商业航空最终用途能源载体的可行性存在不确定性，但空中客车公司已宣布计划在 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 ^[32].年推出远程氢动力飞机[143 However, differing opinions on the]。然而，对于氢在航空中的作用存在不同意见，波音公司首席执行官表示，氢要到 role of hydrogen in aviation have been voiced, with the CEO of Boeing stating that it will not play a major role until 2050 ^[33].年才会发挥主要作用[ The144 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. ]。这些替代燃料的商业可行性取决于基础设施、技术成熟度、成本和环境影响等因素，需要进一步的研究和开发工作来应对这些挑战。