Annual global carbon dioxide (CO₂) emissions reached 34.2 gigatonnes (Gt) in 2019 as a result of extensive and unrestricted use of fossil fuels to fulfill ∼80% of society’s energy needs at the current level of ∼585 exajoules (EJ)/year. (1,2) Transportation that provides mobility to passengers and freight is responsible for approximately 25% of the overall CO₂ emission. (3,4) Considering the current rate of population growth and associated increases in energy consumption, it has been projected that the corresponding global energy demand will be increased by at least 50% before 2050. (1,2,5) To meet such needs while minimizing the environmental impacts by curtailing anthropogenic CO₂ emissions, large-scale deployment of low-carbon renewable energy (RE) is necessary. (6−8) Despite a moderate increase in the overall share of RE in the current energy landscape, recent studies indeed indicated that a full transition to 100% RE is attainable within the next 3 decades or so with a cost-efficient vision of deep electrification of heat and transportation sectors around the globe. (9−11) Thus, this energy transition is no longer a matter of technical feasibility or economic viability, but political will. (12) One of the most critical enabling factors for the RE future is a means of storing and transporting RE at a multi-EJ scale to manage the energy system by peak shaving and valley filling, to overcome the issues of intermittency of solar and wind energies, etc., and to serve regions where RE is difficult to produce due to economic and environmental constraints. (13) The choice of the storage system is highly application dependent, based on the type (stationary or mobile), scale, time, cost, safety, etc. Storing RE in the form of chemical fuels has been considered logical for both short- and long-term storage, particularly for use in the transportation sector. (14,15) In this regard, hydrogen (H₂) is a promising energy vector for its efficient utilization in fuel cells (FCs). (16−18) Although liquid H₂ (LH₂) offers advantages for easy conversion into gaseous H₂, the liquefaction step consumes almost 30–40% of the energy content of H₂ in addition to boil-off losses during transportation. (19,20) The necessary storage and shipping infrastructures for long distances are yet to be developed. (18) To circumvent the inherent issues with LH₂ storage and transport, several other carbon-based fuels such as methanol (CH₃OH), methylcyclohexane (MCH), etc. as well as ammonia (NH₃) have been recommended as more technically viable options, certainly because of their desired properties (Table 1). (21,22) Compared to LH₂, NH₃ can be easily liquefied either by increasing the pressure to ∼10 bar at room temperature or by cooling down to −33 °C under 1 atm. Moreover, NH₃ is safe and easy to store and transport because of its low vapor pressure and high boiling point. The H₂ content in NH₃ (17.65%) is higher than those of methanol (MeOH, 12.5%) and methylcyclohexane (MCH, 6.1%) (Table 1), and its volumetric energy density (12.92–14.4 MJ/L) is comparable to that of MeOH (11.88 MJ/L) but significantly higher than those of MCH (5.66 MJ/L) and lithium-ion batteries (0.9–2.63 MJ/L) when the lower heating value (LHV) of H₂ is considered. With the mature NH₃ production, storage, and transportation infrastructures, NH₃ has been recognized as a sustainable H₂ and energy carrier for the future for both mobile and stationary applications. At 20 °C and 10 bar. At −33 °C. ^, Values are the corresponding hydrogen energy densities, calculated based on the LHV of a stoichiometric amount of hydrogen (LHV_H2 = 120 MJ/kg). For steam reforming of methanol, one must evaporate both methanol and water. In a real case, a stoichiometric excess of 50% water is typically used. NH₃ is currently the second most highly produced chemical in the world, with a global manufacturing capacity of ∼230 million tonnes (Mt) per year. (26−28) Currently, ∼180 Mt of NH₃ is produced annually, predominantly through the steam methane reforming (SMR) process to generate desired H₂, followed by industrial NH₃ synthesis (vide infra). (29,30) NH₃ has an annual market value of ∼US$70 billion. (31) NH₃ is primarily used in the production of fertilizers (80%), in addition to immense applications in the manufacture of explosives (5%) and other materials and chemical commodities (15%). (32) Its production has been one of the most energy-intensive industrial chemical processes, demanding around 200–400 GJ/ton_NH3 for Birkeland–Eyde and cyanamide processes in the early 1900s, followed by the Haber–Bosch (H-B) process which consumed around 100 GJ/ton_NH3 at early stages. (33) While the efficiency of the H-B process has been improved considerably (30–60 GJ/ton_NH3) by the integration of efficient SMR process instead of coal gasification, the process still demands 1–2% of global energy. (29,31,32,34−36) As a consequence, it is one of the biggest emitters of industrial CO₂ (1–1.5% of anthropogenic CO₂ emissions). (37) Since H₂ production by SMR accounts for 80% of energy during the NH₃ production, numerous efforts have been focused on the development of the low-temperature electrochemical synthesis of NH₃. However, the state-of-the-art production rate needs to be increased at least by 1–2 orders of magnitude for practical applications. (38) With the development of various efficient electrolyzers, such as polymer electrolyte membrane (PEM), solid oxide electrolyzer cells (SOECs), and alkaline electrolyzer cells (AECs), electrochemical H₂ production and its subsequent use in the H-B process were considered as an attractive low-carbon pathway for the large-scale production of so-called “green NH₃”. (39) Although alkaline electrolysis in electrolyzers is known to be one of the easiest methods for on-site high-purity H₂ generation with well-established technology, its low current density makes this technology impractical for continuous H₂ generation in large quantities, as would be required to drive a vehicle. (40) In practice, PEM electrolyzers are employed for H₂ production due to their high efficiency and better lifetime under ambient conditions. In general, electrolyzers demand a theoretical minimum of 21.18 GJ/ton_NH3 (based on the LHV of the H₂ content in NH₃). However, at the industrial scale, the electrolyzers operate at an efficiency of 60–70%, requiring at least 30.3–35.3 GJ/ton_NH3. Furthermore, the production of nitrogen (N₂) by an air-separation unit (ASU) and H-B loop compressors using current electrified technology consumes around 2.7 GJ/ton_NH3, representing a significant saving compared to the methane-fed H-B process (6.9 GJ/ton_NH3). (32) Therefore, on average, the ideal NH₃ production by this pathway costs 33.0–38.0 GJ/ton_NH3, for an overall power-to-fuel (PTF) efficiency of 55.7–64.3% (Table 2). By contrast, the PTF efficiency of 49.3–57.9% for LH₂ is lower than that of NH₃ due to the high energy demand of 36.0–48.0 GJ/ton_H2 for compression and liquefaction. (41) The transportation efficiency (TE) of H₂ carriers has been evaluated for a distance of 12 000 km by considering the propulsion engine and dead weight tons of liquefied natural gas tankers (Table 2). (42) In both cases, the fuel and energy demands of the ship are supplied by the combustion of hydrogen energy carriers being transported. The TE of LH₂ (84%) is lower than that of NH₃ (90%) due to the boil-off loss and high energy demand for compressed storage. Transport efficiency. H₂ generation from carrier. Power-to-fuel cell. Fuel cell-to-power. Power-to-fuel-to-power. In order to generate electrical power from NH₃ by feeding it into FCs, two pathways are envisioned. One is catalytic cracking of NH₃ to generate H₂ for fuel cell applications, viz., PEM fuel cells (PEMFCs) and alkaline fuel cells (AFCs). This process has been proposed as a major RE distribution mechanism to supply green NH₃ worldwide for H₂ refilling stations, as the direct on-board cracking is deemed impractical (Figure 1). (43) The cracking process requires high temperatures of >500 °C for the production of high-purity H₂ (>99.97%, particularly for vehicle applications), which demands thermal energy (GJ_T) of 4.2 GJ_T/ton_NH3 (including H₂ loss). (44) Since PEMFCs are highly vulnerable to the trace amounts of NH₃ (<0.1 ppm) in H₂, NH₃ conversion to H₂ must be conducted with a highly efficient purification and separation system, which consumes an additional 0.5 GJ_T/ton_NH3. AFCs, on the other hand, are also sensitive to traces of CO₂ (present in the air), affecting cell operation, thus demanding pure O₂, which increases the cost of operation. As a result, these separation and purification processes would inevitably incur substantial cost. (45) The energy-intensive nature of these systems can result in an overall thermal loss of 1.7 GJ_T/ton_NH3. More importantly, additional electrical energy of 2.0–4.3 GJ_e/ton_NH3 is required for the compression of H₂ to 880 bar for refilling fuel cell electric vehicles (FCEVs) at 700 bar. (21,46,47) Thus, the heat and electricity requirements for the cracking and compressing processes per unit of NH₃ demand ∼0.3 and 0.16–0.34 (assuming a fuel cell efficiency of 60%) units of NH₃, respectively, resulting in an overall conversion efficiency of 61.0–68.5% at the point of use. Moreover, the integration of an energy-intensive cracking reactor with a H₂ compression system may complicate the fueling and refilling processes on the consumer side. These limitations can further escalate due to the intricacy of the cracking system and the performance and lifetime of catalysts in the presence of impurities. (43) Figure 1. Power-to-power (PTP) energy consumption of NH₃ as a hydrogen energy carrier. The other pathway of direct utilization of NH₃ in fuel cells appears to be advantageous. While direct ammonia fuel cells (DAFCs) are still at low technology readiness levels (TRLs), (48−50) solid oxide fuel cells (SOFCs) can be deployed in the near future, as the NH₃ cracking occurs internally within the SOFC; thus, the requirement for a H₂ separation system can be evaded. (29,23,51) However, the high operation temperatures (550–900 °C) (23) suggest that SOFCs may only be suitable for continuous stationary applications without frequent on-and-off cycles. (21) Hence, SOFCs may be applied in heavy-duty vehicles like those used in aviation, shipping, trucking, etc., where frequent on–off cycles or fast starts are not mandatory, as they are for light-duty vehicles (cars, motorcycles, taxi, and buses). (52) Furthermore, the anode materials, responsible for the catalytic decomposition of NH₃ to H₂, should be stable, durable, and tolerant to high temperatures during FC operations, because anode degradation still represents a major hurdle for the commercialization of SOFCs. (53) Of late, Minutillo et al. proposed a novel plant configuration, based on the NH₃-fueled SOFC technology, for the simultaneous on-site co-generation of H₂, and electricity in refueling stations. (54) However, further improvement in the SOFC and NH₃ cracking technology is imperative for materialization of such concepts. The use of NH₃ in combustion engines is left out of the current discussion because this technology does not require H₂ production from NH₃ and can lead to significant problems such as difficult ignition, low flame speed, higher compression, etc. in addition to NO_x emissions from combustion of pure NH₃ or ammonia-fuel blends. (55) While based on these ideal fuel production efficiencies (note: these numbers are higher than those of the state-of-the-art technologies) and actual transportation analysis, and NH₃ indeed has a great potential as a viable energy storage option, the large-scale decarbonization of the transportation sector by employing NH₃ as a H₂ carrier does not offer clear advantages compared to those of LH₂ in terms of overall power-to-fuel-to-power (PFP) efficiency (Table 2). The requirement of a significant amount of energy for the cracking process restricts its use for H₂ release. Moreover, both the financial and energy costs for purifying and compressing the released H₂ to fill the tank of a FCEV are significant, where the invested electrical and thermal energy is difficult to recover during on-boarding applications. In addition to its technical challenges, the toxicity (OSHA exposure limit of 50 ppm), hydrophilicity, and corrosive nature of NH₃ call for a leak-proof infrastructure to avoid accidental release and equipment corrosion and to encourage social approval. In our opinion, NH₃ may serve as an e-fuel for stationary electricity generation with SOFCs to supply power in regions where RE is difficult to produce and grid extensions cannot reach, but it has a limited role as a H₂ carrier due to the large energy requirement for cracking and compressing at the customer end. Importantly, to make a meaningful contribution toward climate change mitigation, it is more effective to prioritize the use of green NH₃ production to decarbonize the current fossil-fuel based NH₃ industry at the scale of 180 Mt per year. (29,32) S.C. and R.K.P. contributed equally. Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. Financial support is provided by King Abdullah University of Science and Technology (KAUST). We thank Professors Zhiping Lai and Yu Han for useful discussions. This article references 55 other publications.

中文翻译：

---

氨作为运输部门氢能源载体的局限性

2019 年全球二氧化碳 (CO ₂ ) 年排放量达到 34.2 吉吨 (Gt)，原因是广泛且不受限制地使用化石燃料以满足目前约 585 艾焦 (EJ)/年的社会能源需求的约 80% . (1,2) 为乘客和货物提供机动性的交通运输约占 CO ₂总排放量的25% 。(3,4) 考虑到当前的人口增长率和相关的能源消耗增长，预计到 2050 年，相应的全球能源需求将至少增加 50%。 (1,2,5) 为满足这样的需求需求，同时通过减少人为 CO ₂最大限度地减少对环境的影响排放，大规模部署低碳可再生能源 (RE) 是必要的。(6-8) 尽管可再生能源在当前能源格局中的总体份额略有增加，但最近的研究确实表明，在未来 30 年左右的时间里，通过具有成本效益的深度发展愿景，可以完全过渡到 100% 可再生能源。全球供热和运输部门的电气化。(9-11) 因此，这种能源转型不再是技术可行性或经济可行性问题，而是政治意愿问题。(12) 可再生能源未来最关键的促成因素之一是一种以多 EJ 规模存储和运输可再生能源的手段，以通过削峰填谷来管理能源系统，以克服太阳能和风能的间歇性问题能量等，并服务于因经济和环境限制而难以生产可再生能源的地区。(13) 存储系统的选择高度依赖于应用，基于类型（固定或移动）、规模、时间、成本、安全性等。以化学燃料的形式存储可再生能源对于短期和和长期储存，特别是用于运输部门。(14,15) 在这方面，氢 (H₂ ) 是一种很有前途的能量载体，因为它在燃料电池 (FC) 中的有效利用。(16-18) 尽管液态 H ₂ (LH ₂ ) 具有易于转化为气态 H _{2 的}优势，但液化步骤除了在运输过程中蒸发损失外，还消耗了几乎 30-40% 的 H ₂能量含量。(19,20) 远距离运输所需的仓储和运输基础设施尚待开发。(18) 为了规避 LH ₂储存和运输的固有问题，其他几种碳基燃料，如甲醇 (CH ₃ OH)、甲基环己烷 (MCH) 等以及氨 (NH ₃) 已被推荐为技术上更可行的选项，当然是因为它们所需的属性（表 1）。(21,22) 与 LH ₂相比，NH ₃可以通过在室温下将压力增加到约 10 bar 或在 1 个大气压下冷却到 -33 °C 来轻松液化。此外，NH ₃因其低蒸气压和高沸点而安全且易于储存和运输。NH _{3 中}的H ₂含量（17.65%）高于甲醇（MeOH，12.5%）和甲基环己烷（MCH，6.1%）（表1），其体积能量密度（12.92-14.4 MJ/L）与MeOH（11.88 MJ/L)，但在考虑H _{2 的}较低热值 (LHV) 时，明显高于 MCH (5.66 MJ/L) 和锂离子电池 (0.9-2.63 MJ/L) 。凭借成熟的 NH ₃生产、储存和运输基础设施，NH ₃已被公认为未来移动和固定应用的可持续 H ₂和能源载体。在 20 °C 和 10 bar 下。在-33°C。^,值是相应的氢能量密度，根据化学计量的氢 (LHV) 的 LHV 计算_{H 2} = 120 MJ/kg)。对于甲醇的蒸汽重整，必须同时蒸发甲醇和水。在实际情况下，通常使用化学计量过量的 50% 的水。NH ₃目前是世界上产量第二高的化学品，全球制造能力约为每年 2.3 亿吨 (Mt)。(26-28) 目前，每年生产约 180 Mt NH ₃，主要通过蒸汽甲烷重整 (SMR) 工艺生成所需的 H ₂，然后是工业 NH ₃合成（见下文）。(29,30) NH ₃的年市场价值约为 700 亿美元。(31) NH ₃主要用于生产化肥 (80%)，此外还广泛用于制造炸药 (5%) 和其他材料和化学商品 (15%)。(32) 其生产一直是能源密集度最高的工业化学工艺之一，在 1900 年代初期，Birkeland-Eyde 和氰胺工艺需要约 200-400 GJ/吨_{NH 3}，其次是 Haber-Bosch (HB) 工艺早期消耗约 100 GJ/吨_{NH 3}。(33) 虽然 HB 工艺的效率得到了显着提高（30–60 GJ/ton _{NH 3}) 通过整合高效的 SMR 工艺而不是煤气化，该工艺仍需要全球 1-2% 的能源。(29,31,32,34-36) 因此，它是最大的工业 CO ₂排放国之一（占人为 CO ₂排放量的1-1.5% ）。(37) 由于SMR 产生的H ₂占 NH ₃生产过程中能量的 80% ，因此许多努力都集中在低温电化学合成 NH _{3 的开发上。}. 然而，为了实际应用，最先进的生产率至少需要提高 1-2 个数量级。(38) 随着各种高效电解槽的发展，如聚合物电解质膜（PEM）、固体氧化物电解槽（SOECs）和碱性电解槽（AECs），电化学H ₂生产及其后续在HB工艺中的应用被考虑作为大规模生产所谓“绿色 NH ₃ ”的有吸引力的低碳途径。(39) 虽然电解槽中的碱性电解是采用成熟技术现场生成高纯度 H ₂的最简单方法之一，但其低电流密度使得该技术不适用于连续 H ₂大量发电，如驾驶车辆所需。(40) 在实践中，PEM 电解槽因其在环境条件下的高效率和更长的寿命而被用于 H ₂生产。一般情况下，电解槽需求21.18 GJ /吨理论最小_{NH 3}（基于H的LHV ₂在NH内容₃）。然而，在工业规模上，电解槽的运行效率为 60-70%，至少需要 30.3-35.3 GJ/吨_{NH 3}。此外，使用当前电气化技术的空气分离装置 (ASU) 和 HB 回路压缩机生产氮气 (N ₂ ) 消耗约 2.7 GJ/吨_{NH 3}，与甲烷供给的 HB 工艺（6.9 GJ/吨_{NH 3}）相比显着节省。(32) 因此，平均而言，该途径的理想 NH ₃生产成本为 33.0–38.0 GJ/吨_{NH 3}，整体动力燃料 (PTF) 效率为 55.7–64.3%（表 2）。相比之下，由于压缩和液化需要 36.0–48.0 GJ/ton _{H 2}的高能量需求，LH ₂的 PTF 效率为 49.3–57.9%低于 NH ₃。(41) H ₂的运输效率（TE）通过考虑推进发动机和液化天然气油轮的自重吨（表 2），已经评估了 12 000 公里的航程。(42) 在这两种情况下，船舶的燃料和能源需求都是由运输的氢能载体燃烧提供的。由于蒸发损失和压缩存储的高能量需求，LH ₂ (84%)的 TE低于 NH ₃ (90%)。运输效率。来自载体的H ₂代。动力燃料电池。燃料电池供电。动力到燃料到动力。为了通过将NH ₃馈入 FC 来从 NH ₃产生电能，设想了两种途径。一是催化裂解NH ₃生成H ₂用于燃料电池应用，即 PEM 燃料电池 (PEMFC) 和碱性燃料电池 (AFC)。该过程已被提议作为主要的可再生能源分配机制，以在全球范围内为 H ₂加气站提供绿色 NH ₃，因为直接车载裂解被认为是不切实际的（图 1）。(43) 裂解过程需要 >500 °C 的高温以生产高纯度 H ₂ (>99.97%，特别是用于车辆应用)，需要4.2 GJ _T /ton _{NH 3 的}热能 (GJ _T ) （包括H ₂损失）。(44) 由于 PEMFC 极易受到H _{2 中}痕量 NH ₃ (<0.1 ppm) 的影响，NH₃转化为H ₂必须通过高效的纯化和分离系统进行，这额外消耗0.5 GJ _T /吨_{NH 3}。另一方面，AFC 也对痕量 CO ₂（存在于空气中）敏感，影响电池运行，因此需要纯 O ₂，这增加了运行成本。因此，这些分离和纯化过程将不可避免地产生大量成本。(45) 这些系统的能源密集型特性可能导致 1.7 GJ _T /ton _{NH 3}的总热损失。更重要的是，额外的电能为 2.0–4.3 GJ _e /ton _NH3是将 H ₂压缩到 880 bar 以在 700 bar 下重新填充燃料电池电动汽车 (FCEV) 所需的条件。(21,46,47) 因此，每单位 NH ₃裂解和压缩过程的热量和电力需求分别需要~0.3 和 0.16-0.34（假设燃料电池效率为 60%）单位的 NH ₃，从而导致在使用时的整体转换效率为 61.0-68.5%。此外，能源密集型裂解反应器与 H ₂压缩系统可能会使消费者侧的加油和再填充过程复杂化。由于裂化系统的复杂性以及在杂质存在下催化剂的性能和寿命，这些限制会进一步升级。(43) 图1. NH ₃作为氢能载体的功率转换(PTP) 能耗。在燃料电池中直接利用NH ₃的另一途径似乎是有利的。虽然直接氨燃料电池 (DAFC) 仍处于低技术准备水平 (TRL)，但在不久的将来可以部署 (48-50) 固体氧化物燃料电池 (SOFC)，因为 NH ₃裂解发生在 SOFC 内部；因此，对于 H ₂的要求可以避开分离系统。(29,23,51) 然而，高操作温度 (550–900 °C) (23) 表明 SOFC 可能仅适用于没有频繁开关循环的连续固定应用。(21) 因此，SOFC 可应用于重型车辆，例如用于航空、航运、卡车运输等的重型车辆，在这些车辆中，频繁开关循环或快速启动不是强制性的，因为它们适用于轻型车辆（汽车、摩托车、出租车和公共汽车）。(52) 此外，负责将 NH ₃催化分解为 H ₂的负极材料, 应该是稳定的、耐用的，并且在 FC 操作期间能够耐受高温，因为阳极退化仍然是 SOFC 商业化的主要障碍。(53) 最近，Minutillo 等人。提出了一种基于 NH ₃燃料 SOFC 技术的新型工厂配置，用于在加氢站同时现场热电联产 H ₂和电力。(54) 然而，SOFC 和 NH ₃裂解技术的进一步改进对于实现这些概念势在必行。NH ₃在内燃机中的使用被排除在当前讨论之外，因为该技术不需要从 NH ₃生产H ₂除了纯 NH ₃或氨燃料混合物的燃烧产生的NO _x排放之外，还会导致诸如难以点火、低火焰速度、更高压缩率等重大问题。(55) 虽然基于这些理想的燃料生产效率（注意：这些数字高于最先进技术的数字）和实际运输分析，但 NH ₃作为可行的储能选项确实具有巨大潜力，与LH ₂相比，通过使用 NH ₃作为 H ₂载体的运输部门的大规模脱碳并没有提供明显的优势在整体功率-燃料-功率 (PFP​​) 效率方面（表 2）。裂解过程需要大量能量限制了其用于释放H ₂。此外，净化和压缩释放的 H ₂以填充 FCEV 的罐的财务和能源成本都很高，在车载应用期间难以回收所投资的电能和热能。除了技术挑战之外，NH ₃的毒性（OSHA 暴露限值为 50 ppm）、亲水性和腐蚀性要求防漏基础设施，以避免意外释放和设备腐蚀，并鼓励社会认可。我们认为，NH ₃可以用作电子燃料为静止的发电用的SOFC在区域供应电源，其中RE是难以制造和网格扩展不能达到，但它具有作为h的作用有限₂载体由于用于裂化和大能量需求在客户端压缩。重要的是，为了对减缓气候变化做出有意义的贡献，优先使用绿色 NH ₃生产来使当前基于化石燃料的 NH ₃脱碳更为有效工业规模为每年 180 吨。(29,32) SC 和 RKP 贡献相同。本观点中表达的观点是作者的观点，不一定是 ACS 的观点。资金支持由阿卜杜拉国王科技大学 (KAUST) 提供。感谢赖志平教授和韩宇教授的有益讨论。本文引用了 55 篇其他出版物。