Ammonia is an industrial large-volume chemical. It is used in fertilizers and many chemical products and materials, and it pops up as a candidate green energy vector. Today, the industrial production of ammonia is dominated by the Haber–Bosch process departing from natural gas or other fossil fuel. This process is responsible for about 1.6% of the global CO₂ emissions. Electrochemical ammonia production from water and nitrogen gas using renewable electricity is a potential solution to reduce the CO₂ footprint of ammonia production. Electrocatalysts with steadily increasing Faradaic efficiency are being reported, but there seems to be a trade-off between ammonia selectivity and catalytic activity. Hydrogen gas is the main byproduct. Here we show low ammonia selectivity of electrocatalysts not to be an obstacle to energy-efficient ammonia production. The SECAM process (Solar ElectroChemical AMmonia synthesis) integrates nitrogen gas production from air, electrocatalytic ammonia synthesis, reaction product separation, and hydrogen recycling with an overall energy efficiency similar to that of the Haber–Bosch process. The electrochemical ammonia synthesis process can be powered with photovoltaics and take advantage of the day–night cycle for converting the excess hydrogen byproduct produced during the day to make additional ammonia at night. The process can be operated using electrocatalysts with Faradaic efficiencies of ammonia synthesis as low as 10%. The activity of electrocatalysts is the critical property to be improved for energy-efficient production of green ammonia. The Haber–Bosch process for ammonia production is one of the oldest industrial catalytic processes.(1) The first ammonia plant went on stream in 1913.(2) In the Haber–Bosch process, N₂ gas is reduced to NH₃ using H₂ gas (eq 1):(1)Elevated temperatures are needed to activate the iron-based catalyst. The reaction kinetics are peculiar in the sense that the chemisorption of nitrogen gas molecules on the catalyst surface limits the reaction rate.(3,4) The H₂ for the Haber–Bosch process is typically produced by methane steam reforming. CO₂ emission of the Haber–Bosch process amounts to up to 1.9 ton per ton of ammonia produced.(5) In 2017, ammonia production worldwide was responsible for ca. 420 Mt of CO₂.(6) The use of water and electricity instead of hydrogen out of methane is an alternative pathway for ammonia synthesis (eq 2).(2)Different types of electrocatalysts performing this reaction have been reported, but they exhibit two shortcomings: low activity and low ammonia selectivity. Currently, the highest reported Faradaic efficiencies of electrochemical ammonia synthesis from water and air are in the range of 60%.(6−8) One exception, which uses a fundamentally different process with Li-cycling, achieves a Faradaic efficiency of 88.5% at a temperature of 450 °C.(9) Common electrochemical ammonia synthesis has hydrogen gas as the main byproduct, consuming a significant part of the invested electric energy. The SECAM process presented here uses this hydrogen gas for two purposes: (i) reaction with oxygen out of the air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor and (ii) performing electrocatalytic ammonia synthesis using H₂. The SECAM process has two modes of operation: energy-intensive production of ammonia out of nitrogen gas and water according to eq 1 (mode A, Figure 1) and an energy-extensive production of ammonia out of an N₂/H₂ gas mixture according to eq 2 (mode B, Figure 1). Figure 1. Overview of the half reactions occurring in the electrochemical cell, for mode A and mode B. In mode A (Figure 2A), air is used as a source of nitrogen. To make air suitable for ammonia production, O₂ is removed by reaction with H₂. This can be done in a fuel cell, generating electricity, or in a burner, generating heat to recover the energy. Next, the obtained gas containing already some water from the reaction of O₂ with H₂, is sent through a humidifier where additional water vapor is added. After these two steps, the hydrated nitrogen gas is fed to the electrochemical cell, where ammonia is formed on the cathode. The hydrogen evolution reaction (HER) competes with ammonia synthesis. When mode A is run such as to produce exactly the amount of hydrogen needed to eliminate the O₂ from the intake air, the electrocatalyst should have a Faradaic efficiency for ammonia production of 85%. State-of-the-art electrocatalysts have lower Faradaic efficiencies,(10) and excess H₂ gas is produced. The resulting outlet gas of the cathode compartment is composed of NH₃, H₂, and unreacted N₂. NH₃ is condensed out of the gas stream, resulting in a residual stream of N₂ and H₂. Part of this stream serves the O₂ removal out of inlet air; part is stored in a tank as feed for mode B. Water is the source of H atoms and air is the source of N atoms. The molar ratio of the excess N₂/H₂, produced in mode A, is fixed at 1/3 by tuning the air and water intake of the process. Figure 2. SECAM process for electrochemical ammonia production. (A) Energy-intensive operation mode of the electrochemical reactor, producing ammonia from water and air. (B) Energy-extensive operation mode of the electrochemical reactor, producing ammonia from an N₂/H₂ gas mixture. In mode B (Figure 2B), the H₂/N₂ gas mixture from the storage vessel is sent to the anode of the electrochemical cell, where the hydrogen oxidation reaction (HOR) takes place. Next, the remaining gas is sent to the cathode, where ammonia and hydrogen gas are formed. In mode B ammonia is produced until all N₂ and H₂ gas is converted by recycling. Modes A and B make use of the same electrochemical cell. Ammonia production in mode B consumes less than 20% of the electric power required for mode A. Operation of SECAM according to modes A and B is dependent on the availability of electric energy. When a large amount of energy is available, for example using photovoltaics during daytime, mode A is executed. When the energy supply is limited, for example with photovoltaics at night or on cloudy days, mode B is executed. Several SECAM reactors can run in parallel and be operated either in mode A or B to optimize the ammonia productivity according to the availability of renewable energy. To facilitate the condensation of ammonia, the SECAM processes are operated at a pressure of 0.8 MPa. At this pressure, ammonia condenses at 20 °C.(2) An additional benefit of the increased pressure is a positive effect on the reaction rate by the first order kinetics.(11) The downside of the increased pressure is that it entails additional energy consumption and materials cost for making the reactor pressure resistant. Alternatively, the process can be run at atmospheric pressure if the produced ammonia is recovered by an extraction with water. The energy needed for ammonia production and the share of operation modes A and B of SECAM processes with different Faradaic efficiencies at the two locations are presented in Table 1. The specifications of the process parameters are given in the Supporting Information. The overpotentials used for the different half reactions are according to the state of the art.(8,12−15) Average energy consumption and average daily operation time of modes A and B are reported. In Leuven, Belgium, a solar panel with an efficiency of 15% produces on average 430 Wh/m²·day(16) with a maximal power delivery of 150 W/m². This means mode A needs 150 W peak capacity (electrochemical reactor + compressor) for every square meter of solar panel. The produced solar electricity is sufficient to operate in mode A for 2.9 h/day on average. During these 2.9 h, a reactor with a Faradaic efficiency of 10% produces enough N₂/H₂ gas mixture for mode B to run for 21.1 h. Together, this completes a day cycle of 24 h. Therefore, the minimal required Faradaic efficiency is 10% in Belgium. The highest average solar irradiation is encountered in the Atacama desert in Chile. There, a solar panel with 15% efficiency produces 967 Wh/m²·day.(17) This energy is sufficient for mode A to run 6.4 h/day on average. During these 6.4 h, a reactor with a Faradaic efficiency of 22.5% produces enough N₂/H₂ gas mixture for mode B to run for 17.6 h. Therefore, the minimal Faradaic efficiency needed in Chile is higher than in Belgium, viz. 22.5%. The energy consumption of SECAM ammonia synthesis is plotted against the Faradaic efficiency of the electrocatalysts in Figure 3. The energy consumption is highest at low Faradaic efficiency and drops rapidly when improving the Faradaic efficiency from 10% to 20%. At Faradaic efficiencies above 20%, the energy needs flatten. Enhancing the Faradaic efficiency from 30% to 85% causes a gain in energy of 37%. Electrocatalyst development has been focused on enhancing Faradaic efficiency. The ARPA-E (Advanced Research Projects Agency-Energy) determined a minimal FE of 90% for the process to be economically feasible.(18) However, with SECAM, ammonia synthesis could already be viable at Faradaic efficiencies of 20–30%. Therefore, the main focus of future research should be on how to increase the current density, which on state-of-the-art electrodes is still far too low for commercial ammonia production and should be raised to at least 5–10 mA/cm². Figure 3. Average energy consumption of SECAM per mole of ammonia produced, against Faradaic efficiency of the electrocatalyst with an overpotential of 250 mV, compared to the energy consumption of the natural gas-based Haber–Bosch process.(5,20,21) Despite the high pressures and temperatures required for the Haber–Bosch process, it is surprisingly efficient at the very large scale at which it is operated.(19) The traditional natural gas-based Haber–Bosch process is reported to have an energy consumption ranging from 0.58 MJ/mol to 0.81 MJ/mol, depending on the source.(5,20,21) SECAM is estimated to have similar energy requirements, ranging from 0.56 MJ/mol at 85% Faradaic efficiency to 0.92 MJ/mol at 20% Faradaic efficiency. As an additional advantage, SECAM allows for small-scale decentralized production, which is of interest to economically less developed parts of the world.(22) With an efficient delocalized ammonia production at farms, transport costs for fertilizers can also almost be eliminated.(23) The process discussed in this Viewpoint has limited CO₂ footprint and can be operated with a fluctuating power source. The SECAM process performs energy-efficient electrochemical ammonia production that is competitive with the Haber–Bosch process in terms of energy consumption. In contrast to the Haber–Bosch process, the discussed process is efficient at a small scale and allows a delocalized ammonia production. The process can be operated at a Faradaic efficiency as low as 20–30%. Practical implementation of SECAM processes is awaiting the development of electrocatalysts with enhanced current density even at modest Faradaic efficiency. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.0c00455.

• Detailed information on the executed calculations and assumptions made; a graphical representation of the process streams under varying operating conditions (PDF)

Detailed information on the executed calculations and assumptions made; a graphical representation of the process streams under varying operating conditions (PDF) Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. Electronic Supporting Information files are available without a subscription to ACS Web Editions. The American Chemical Society holds a copyright ownership interest in any copyrightable Supporting Information. Files available from the ACS website may be downloaded for personal use only. Users are not otherwise permitted to reproduce, republish, redistribute, or sell any Supporting Information from the ACS website, either in whole or in part, in either machine-readable form or any other form without permission from the American Chemical Society. For permission to reproduce, republish and redistribute this material, requesters must process their own requests via the RightsLink permission system. Information about how to use the RightsLink permission system can be found at http://pubs.acs.org/page/copyright/permissions.html. We gratefully acknowledge the financial support of the Flemish Government through the Moonshot cSBO project P2C (HBC.2019.0108) and through long-term structural funding (Methusalem). This article references 23 other publications.

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使用法拉第效率受限的电催化剂从空气和水中生产高能效氨

氨是一种工业化的大批量化学品。它被用于化肥以及许多化学产品和材料，并作为绿色能源的候选载体而出现。如今，氨的工业生产主要由哈伯-博世（Haber-Bosch）工艺取代天然气或其他化石燃料而形成。此过程约占全球CO ₂排放量的1.6％。使用可再生电力从水和氮气中生产化学氨是减少CO ₂的潜在解决方案氨生产的足迹。据报道，具有不断提高的法拉第效率的电催化剂，但似乎在氨选择性和催化活性之间进行了权衡。氢气是主要副产物。在这里，我们显示出低的电催化剂氨选择性不会成为高能效氨生产的障碍。SECAM工艺（太阳能氨化学合成）将空气中的氮气生产，氨气的电催化合成，反应产物的分离以及氢气的循环集成在一起，其总能效类似于Haber–Bosch工艺。电化学氨合成过程可以使用光电驱动，并利用昼夜循环将白天产生的过量氢副产物转化为夜间额外的氨。该工艺可以使用氨合成法拉第效率低至10％的电催化剂进行。电催化剂的活性是要提高能源效率来生产绿氨的关键性能。用于生产氨的哈伯-博世工艺是最古老的工业催化工艺之一。（1）1913年第一家氨厂投产。（2）在北哈伯-博世工艺中使用H ₂气体（等式1）将₂气体还原为NH ₃：（1）需要升高温度以活化铁基催化剂。在一定意义上说，反应动力学是特殊的，因为氮气分子在催化剂表面的化学吸附会限制反应速率。（3,4）Haber-Bosch过程的H ₂通常是由甲烷蒸汽重整产生的。哈伯-博世过程中每生产一吨氨，CO ₂排放量高达1.9吨。（5）2017年，全球氨生产量约占总排放量的一半。420 Mt的CO _2。（6）使用水和电代替甲烷中的氢是氨合成的替代途径（等式2）。（2）已经报道了进行该反应的不同类型的电催化剂，但是它们表现出两个缺点：低活性和低氨选择性。目前，据报道，从水和空气中合成氨的法拉第效率最高，约为60％。（6-8）一个例外，它使用与锂循环基本不同的工艺，在室温下法拉第效率达到88.5％。温度为450°C。（9）常见的电化学氨合成以氢气为主要副产物，消耗了大量的投资电能。此处介绍的SECAM工艺将氢气用于两个目的：（i）与空气中的氧气反应以制备氮气和水以供入氨合成反应器中；（ii）使用H进行电催化氨合成₂。SECAM工艺有两种运行模式：根据等式1（图A，模式A，从氮气和水中大量生产氨）和从N ₂ / H ₂气体混合物中大量生产氨根据式2（模式B，图1）。图1.模式A和模式B在电化学电池中发生的半反应概述。在模式A（图2A）中，空气被用作氮源。为了使空气适于生产氨，O- ₂是通过用H反应除去₂。这可以在燃料电池中发电或在燃烧器中产生热量以回收能量。接下来，从O ₂的反应获得的已经包含一些水的气体H ₂与H _2一起通过加湿器，在其中添加了额外的水蒸气。在这两个步骤之后，将水合氮气送入电化学电池，在此在阴极上形成氨。析氢反应（HER）与氨合成竞争。当运行模式A以便精确产生从进气中消除O ₂所需的氢气量时，电催化剂的法拉第效率应为85％。最先进的电催化剂具有较低的法拉第效率，（10）并产生过量的H ₂气体。阴极室的所得出口气体由NH ₃，H ₂和未反应的N _2组成。NH ₃气体从气流中冷凝出来，形成残留的N ₂和H ₂流。该物流的一部分用于从进口空气中去除O ₂。一部分存储为模式B的进料罐。水是H原子的来源，而空气是N原子的来源。在模式A中产生的过量N ₂ / H ₂的摩尔比通过调节过程的空气和水的摄入量而固定为1/3。图2. SECAM工艺生产电化学氨。（A）电化学反应器的高能耗操作模式，由水和空气产生氨。（B）电化学反应器的耗能操作模式，由N ₂ / H ₂产生氨混合气体。在模式B（图2B）中，来自存储容器的H ₂ / N ₂气体混合物被送到电化学电池的阳极，在此发生氢氧化反应（HOR）。接下来，剩余的气体被送到阴极，在阴极形成氨气和氢气。在模式B下，直到所有N ₂和H ₂都产生氨气体通过回收转化。模式A和B使用相同的电化学电池。模式B中的氨生产消耗的能量不到模式A所需的电能的20％。SECAM根据模式A和B的运行取决于电能的可用性。当有大量能量可用时，例如在白天使用光伏电池，则执行模式A。当能量供应受到限制时，例如在夜晚或阴天使用光伏电池，则执行模式B。多个SECAM反应器可以并行运行，并根据可再生能源的可用性以A或B模式运行，以优化氨的生产率。为了促进氨的冷凝，SECAM工艺在0.8 MPa的压力下运行。在此压力下，氨在20°C冷凝。（2）升高压力的另一个好处是通过一级动力学对反应速率产生积极影响。（11）升高压力的不利方面是，这会带来额外的能耗和材料成本，从而使反应堆耐压。或者，如果通过用水萃取回收产生的氨，则该方法可以在大气压下进行。表1列出了氨生产所需的能量以及两个位置具有不同法拉第效率的SECAM工艺的操作模式A和B的份额。工艺参数的规格在支持信息中给出。用于不同半反应的过电势根据现有技术而定。（8，12-15）报告了模式A和B的平均能耗和平均每日运行时间。在比利时鲁汶，效率为15％的太阳能电池板平均产生430 Wh / m² ·day（16），最大功率输出为150 W / m ²。这意味着模式A对于每平方米的太阳能电池板需要150 W的峰值容量（电化学反应器+压缩机）。产生的太阳能足以在模式A下平均每天运行2.9小时。在这2.9小时内，法拉第效率为10％的反应器产生的N ₂ / H ₂气体混合物足以使模式B运行21.1小时。一起完成一个24小时的一天周期。因此，在比利时，所需的最低法拉第效率为10％。在智利的阿塔卡马沙漠遇到的平均日照量最高。在那里，效率为15％的太阳能电池板产生967 Wh / m ²·天。（17）该能量足以使模式A平均每天运行6.4小时。在这6.4小时内，法拉第效率为22.5％的反应堆产生了足够的N ₂ / H ₂模式B的混合气运行17.6小时。因此，智利所需的最低法拉第效率高于比利时，即比利时。22.5％。图3中绘制了SECAM氨合成的能耗与电催化剂的法拉第效率的关系。法拉第效率低时的能耗最高，当将法拉第效率从10％提高到20％时能耗迅速下降。法拉第效率超过20％时，能源需求将趋于平坦。将法拉第效率从30％提高到85％可使能源增加37％。电催化剂的开发一直集中在提高法拉第效率上。ARPA-E（美国能源部高级研究计划局）确定该工艺在经济上可行的最低FE为90％。（18）但是，对于SECAM，法拉第效率为20％到30％时，合成氨已经可行。因此，未来研究的主要重点应该放在如何增加电流密度上，这在最先进的电极上对于工业化氨生产来说仍然太低了，应该提高到至少5-10 mA / cm²。图3.与基于天然气的Haber-Bosch工艺的能耗相比，每摩尔生产的氨中SECAM的平均能耗，相对于电势为250 mV的电催化剂的法拉第效率。（5,20,21）尽管Haber–Bosch工艺需要很高的压力和温度，但在非常大的操作规模下仍具有惊人的效率。（19）据报道，基于天然气的传统Haber–Bosch工艺的能耗范围为从0.58 MJ / mol到0.81 MJ / mol，取决于来源。（5,20,21）SECAM估计具有相似的能量需求，范围从85％法拉第效率的0.56 MJ / mol到20时的0.92 MJ / mol法拉第效率百分比。SECAM的另一个优势是可以进行小规模的分散生产，₂占用空间，可以用波动的电源进行操作。SECAM工艺可实现高能效的电化学氨生产，在能耗方面可与Haber–Bosch工艺竞争。与Haber-Bosch工艺相反，所讨论的工艺在小规模范围内是有效的，并且可以产生离域的氨气。该过程可以以低至20％至30％的法拉第效率运行。SECAM工艺的实际实施正等待开发出即使电流强度适中的法拉第效率也能提高电流密度的电催化剂。可从https://pubs.acs.org/doi/10.1021/acsenergylett.0c00455免费获得支持信息。

• 有关执行的计算和假设的详细信息；在不同的操作条件下流程流的图形表示（PDF）

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