Designing a scalable system to capture CO₂ from the air and convert it into valuable chemicals, fuels, and materials could be transformational for mitigating climate change.(1−3) Climate models predict that negative greenhouse gas emissions will be required by the year 2050 in order to stay below a 2 °C change in global temperature.(4) The processes of CO₂ capture, CO₂ conversion, and finally product separation all require significant energy inputs; devising a system that simultaneously minimizes the energy required for all steps is an important challenge. To date, a variety of prototype or pilot-level CO₂ capture and/or conversion systems have been designed and built targeting the individual objectives of either capture or conversion. One approach has focused on CO₂ removal from the atmosphere and storage of pure pressurized CO₂.(5,6) Other efforts have concentrated on CO₂ conversion processes, such as electrochemical reduction(7−10) or fermentation.(10,11) Only a few concepts or analyses have been developed for complete end-to-end processes that perform both CO₂ capture and transformation.(12,13) Here we explore an approach for the design of a CO₂ capture and conversion system: (i) formation of bicarbonate or carbonate ((bi)carbonate) through dissolution of CO₂ into basic solution; followed by (ii) electrochemical reduction to syngas or formate; and finally (iii) transformation into useful chemicals, fuels, and materials. Unlike traditional electrochemical systems, in which gaseous CO₂ is the primary chemical feedstock that is converted into products, our analysis focuses on the transformation of CO₂ carried by (bi)carbonate solutions (Figure 1). This approach offers several advantages for coupling CO₂ capture with conversion. First, capturing CO₂ from the atmosphere does not require an energy-intensive heating step to recover gaseous CO₂ from a capture medium for later conversion. Second, by transforming the CO₂ carried by (bi)carbonate ions, the process avoids the energy-intensive compression of gaseous CO₂ and allows for significantly higher conversion efficiency per mole of captured CO₂. We outline the energy requirements for various steps in this (bi)carbonate feedstock route and compare it to systems with conventional CO₂ capture and conversion processes. Figure 1. Schematic representation of the steps for various prototype systems designed to capture CO₂ and/or convert it to either concentrated and pressurized CO₂ or to a value-added product. The blue arrows represent prototype processes that capture and convert CO₂; gray arrows represent prototype processes that focus only on CO₂ conversion, and the pink arrow represents the process that we propose. The numbers in brackets correspond to references for the various processes (refs (5−13,15,and18)). There are many possible configurations for CO₂ capture and conversion systems, but five steps are integral to every conventional design (Figure 2): (i) a CO₂ source; (ii) a capture medium; (iii) a process to release CO₂ from the capture medium; (iv) CO₂ compression into a concentrated gas stream; and (v) conversion of CO₂ into fuels, chemicals, and/or materials (e.g., hydrocarbons). The design of an energy-efficient integrated system for capture and conversion requires careful consideration of the CO₂ source. Whether it is a point source with a mixed-process gas stream (e.g., flue gas from a coal-fired power plant) or a relatively dilute source (e.g., atmosphere) will dictate the CO₂ concentration in the feedstock and therefore the type of capture media that is most appropriate to use. There are a variety of options for CO₂ capture media, ranging from polymer membranes to organic and inorganic liquid adsorbers.(14) After collection by the capture media, the release of CO₂ is usually accomplished by heating the capture media itself. The collected CO₂ must then be compressed to a suitable pressure for flow and injection into a conversion system. As will be shown below, these two processes related to CO₂ capture and compression potentially constitute a major energy input that must be considered when designing a complete system for mitigating global atmospheric CO₂. Figure 2. Synopsis of the various steps involved in capturing CO₂ and transforming it into valuable chemicals. To better facilitate an end-to-end comparison of complete systems, we analyze CO₂ capture and conversion systems that have been realized to date at a pilot-plant level. We divide them into three categories: systems that (i) perform direct air capture, (ii) use a concentrated CO₂ source for conversion, and (iii) focus on only the conversion of CO₂. The steps for each pilot-plant and prototype are shown in Figure 1, and a more detailed schematic is given in the Supporting Information, Figures S1 and S2. We compare the pilot processes using the metric of energy required to remove/transform one mole of CO₂ from the atmosphere because this metric directly affects the operational expenses of the plant. However, it should be noted that this metric does not account for other expenses (labor, maintenance, materials, etc.) needed to operate the plant or the construction capital expense and amortization period. Because of these considerations, we have therefore chosen to not compare fermentation processes to electrochemical or thermocatalytic processes. Fermentation is an interesting CO₂ conversion technique that shows significant promise because of its high selectivity for products like ethanol and other multiple carbon products.(10−12) Fermenters also operate near room temperature, making the energy requirements per mole of CO₂ transformed low (Supporting Information, section 1G,H).(10,11) However, fermentation processes require labor to operate and maintain the system, reaction times can be long, and fermenters are typically operated as a batch reactor rather than in a continuous chemical process. These considerations make it difficult to directly compare electrochemical and fermentation processes using only energy required per mole of CO₂ captured/transformed as a metric. In this Viewpoint, we are proposing an electrochemical process and will therefore focus our attention on comparing it to similar systems. Pilot-plants that remove CO₂ from the atmosphere and produce concentrated and pressurized CO₂ streams have been designed using a capture medium of potassium hydroxide (Carbon Engineering)(5) or polyamines (ClimeWorks).(6) The potassium hydroxide process requires ∼0.28 MJ per mole of CO₂ captured (kJ/mol CO₂) (Supporting Information, section 1A)(5) while the polyamine process requires 0.58 MJ/mol CO₂.(6) For both of these atmospheric CO₂ capture processes, the enthalpy required to release the CO₂ from the capture medium and subsequent pressurization of the gaseous CO₂ dominates the total energy requirements of the system. Other pilot-scale processes have been developed that capture CO₂ from concentrated sources. For instance, CO₂ emitted from a geothermal power plant is dissolved into a potassium hydroxide solution, which is then pumped underground to react with naturally occurring basalt rock (CarbFix).(15) By taking advantage of location-specific geology for CO₂ transformation, the system requires only 0.13 MJ/mol CO₂ (Supporting Information, section 1B).(16) Another process captures CO₂ from a geothermal power plant, then pressurizes and heats the CO₂ in the presence of H₂ generated via water electrolysis to create methanol and water (Carbon Recycling International). This process requires 1.53 MJ/mol CO₂ (Supporting Information, section 1C), but it also produces a chemical fuel in the form of methanol.(13) As expected, this process requires more energy than the other systems described because it incorporates a CO₂ conversion process. Pilot-scale CO₂ conversion systems have also received extensive attention.(7−11,17) A pilot-scale CO₂ electrolyzer was shown to create CO and O₂ using an energy input of 0.81 MJ/mol CO₂ (Supporting Information, section 1D).(9) Other pilot-scale CO₂ electrolyzers have been demonstrated to make CO(7) or formic acid,(8) requiring 0.61 MJ/mol CO₂ or 0.75 MJ/mol CO₂, respectively (Supporting Information, section 1E). Finally, an approach for carbon sequestration uses a process that injects CO₂ into cement during production, allowing cement producers to use less binder and offset their CO₂ emissions in the process; using this method, 25 lbs of CO₂ can be stored per cubic yard of cement (Supporting Information, section 1H).(17) Notably, none of the these transformation processes account for the energy required to capture the CO₂ from the atmosphere or a concentrated point source. As mentioned above, procuring a pressurized CO₂ source in suitable purity does not represent a trivial amount of energy. In order to construct an alternative process with a modest energy consumption alternative, we seek to evaluate the energy-intensive steps of each process. Energy consumption varies greatly depending on the source of CO₂ and the desired products. While the process of CO₂ conversion requires significantly greater energy than CO₂ capture, conversion schemes discussed above have yet to report their conversion efficiency. On the basis of our calculations, all reported devices have CO₂ conversion efficiencies less than 40% (Supporting Information, section 3A–H) with the majority having conversion efficiencies less than 10%.(18) For many electrolysis processes the energy required per mole of CO₂ would at minimum double unless the unused CO₂ is recycled through the system. To decrease the overall energy requirements for CO₂ capture and conversion, it is interesting to consider a system that converts nearly 100% of the captured CO₂. Electrochemically reducing CO₂ carried by (bi)carbonate instead of pressurized CO₂ bypasses energy-intensive processes necessary for concentration and compression of gaseous CO₂ (see Figures 2 and 3 for schematics of the overall process designs). This change in the carbon-bearing feedstock molecule allows us to avoid energy-intensive processes, such as heating or vacuum cycling of a sorbent material to release the captured CO₂, along with the subsequent compression of the captured CO₂ into a sufficient pressure that can adequately supply a CO₂ conversion system, such as a gas-diffusion electrolyzer. Furthermore, transforming CO₂ carried by a (bi)carbonate ion allows for high conversion efficiency per mole of captured CO₂, unlike many other electrolysis devices operating with gaseous CO₂ as the carbon input (Supporting Information, section 3A–H). Figure 3. Schematic representation of processes and energy requirements for various proposed schemes that capture CO₂ and transform it to value-added products. The bottom rows show the commercial syngas synthesis process, with a feed stock of either coal or natural gas. The energy needed to produce 1 mole of CO while related to the cost needed to operate the plant does not encompass the full picture of expenses such as materials, maintenance, and labor. An example schematic system could combine (bi)carbonate formation with syngas production (Figure 3), which can then be flexibly converted to valuable chemicals, fuels, and materials. Assuming the first step of CO₂ capture uses an already demonstrated air contactor design,(5) ∼0.01 MJ/mol CO₂ would be required (Supporting Information, section 2A), depending on the alkalinity of the solution. The solution of (bi)carbonate could then be fed into an electrolyzer where it is converted to syngas (CO and H₂). Recent reports of gas-diffusion electrodes using a silver catalyst at the cathode and a bipolar membrane have shown that CO₂ can be generated locally near the catalyst from a (bi)carbonate electrolyte, which can then be converted to syngas.(19,20) Importantly, no CO₂ was observed in the output stream of chemical products,(19) thus eliminating the need to separate the syngas from the unconverted CO₂ stream. These reported prototype devices(19,20) required between ∼0.7 MJ/mol CO₂ and achieved a desirable syngas ratio of between 3:1 and 2:1 (H₂:CO) (Supporting Information, section 2B,C). In all of the electrolysis processes discussed above, the oxygen evolution reaction (OER) occurs at the counter electrode. It should also be noted that the base does not need to be replenished as long as the rate of CO₂ capture is matched to the rate of CO production. To create a plant that could capture and transform 1 t of CO₂ per day would require a capture system of approximately 16.6 m², an electrode area of 0.4 m², and an array of solar panels covering an area of 4571 m² (Supporting Information, section 2D). Additional space for solar energy storage and syngas processing would also be required but would be negligible compared to the area needed for the photovoltaic system generating the electricity. If this system were to be used to capture all CO₂ emitted globally daily, approximately 100 million tons of CO₂, the system would need to be around 460 000 km². This is slightly larger than the area of California (∼424 000 km²) and only around 0.09% of the area of the earth. Notably, the total energy requirement of the (bi)carbonate conversion system is 0.7 MJ/mol CO to make syngas, while commercial processes to make syngas from coal require 1.5−3.7 MJ/mol CO (Supporting Information, section 2E).(21) To make syngas from natural gas requires 0.8 MJ/mol CO (Supporting Information, section 2F,G).(22) It should be noted, however, that the energies listed for these conventional industrial processes do not account for the energy required to extract the coal or natural gas feedstocks. The (bi)carbonate process we have outlined compares favorably on an energetic basis to current commercial processes, and unlike typical gasification systems, it also produces a syngas stream without any contaminants. As is well-known, syngas is a versatile feedstock for further generation of a variety of useful chemicals and fuels. For example, butanol, hexanol, acetate, and ethanol can be synthesized from syngas by using a fermenter.(10,12) Another option is to synthesize methanol by heating and pressurizing syngas in the presence of a CuZnOxAlOx catalyst.(23) A third option would be to use Fischer–Tropsch processes to make longer-chain hydrocarbons, such as diesel fuel, which can be synthesized at 240 °C and 25 bar in the presence of a cobalt catalyst.(24) Syngas is not the only chemical product that can be electrochemically synthesized from CO₂-bearing bicarbonate solutions. Several studies have shown that bicarbonate solutions can also be electrochemically reduced to formate at low overpotentials (Supporting Information, section 2H).(25−28) The global market for formic acid is relatively small (currently ∼570 million USD) but expected to grow significantly in the coming years because of the increased use of formic acid in the rubber industry.(29) The energy required to electrochemically synthesize formate from bicarbonate via atmospheric CO₂ capture is 0.8 MJ/mol CO₂ (Figure 4). Formic acid not only is a useful chemical in and of itself but also can be transformed into valuable chemicals such as methanol by molecular catalysts,(30−33) thermocatalysis,(34−36) or fermentation processes.(37) Although the heat of combustion is modest (0.25 MJ/mol), formic acid could conceivably be used to generate power either directly in a formic acid fuel cell(38−41) or as a hydrogen carrier(42−45) for a hydrogen fuel cell. Figure 4. Schematic representation of processes and energy requirements for various proposed schemes that capture CO₂ and transform into formic acid. This system-level design approach for CO₂ capture and conversion highlights the favorable characteristics of processes that use a (bi)carbonate solution as the carbon-bearing feedstock. Instead of reducing CO₂ directly from a pressurized gas stream, CO₂ molecules contained in (bi)carbonate solutions are electrochemically reduced to form value-added chemicals, thus removing energy-intensive steps, and with a potential for a near-unity conversion rate of the captured CO₂ molecules. While we have focused on syngas production in our analyses, the main advantage of the proposed process involves changing the starting feedstock from gaseous CO₂ to aqueous CO₂-carrying (bi)carbonate solutions, whose advantage is enhanced when coupled with a direct-air capture system to remove atmospheric CO₂. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.0c00234.

• Figures S1 and S2; energy input calculations for CO₂ capture and conversion processes (section S1), energy input calculations for bicarbonate or carbonate feedstock systems (section S2), and CO₂ electrolyzer conversion efficiencies (section S3) (PDF)

Figures S1 and S2; energy input calculations for CO₂ capture and conversion processes (section S1), energy input calculations for bicarbonate or carbonate feedstock systems (section S2), and CO₂ electrolyzer conversion efficiencies (section S3) (PDF) A.J.W, J.S.D, and H.A.A. conceived the idea and examined the feasibility of such a process. A.J.W. and E.D. performed all of the calculations and research required to complete the energy analysis of various systems. J.S.D. advised throughout the process and provided valuable input for determining the viability of various proposed processes. A.J.W., J.S.D, and H.A.A. wrote the Viewpoint, and all authors commented on the manuscript. Any opinions, findings, and conclusions expressed in this material are those of the authors and do not necessary reflect those of DOE or NSF. 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. This work is done within the Joint Center for Artificial Photosynthesis, a Department of Energy (DOE) Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number De-SC0004993. A.J.W. acknowledges support from the Resnick Sustainability Institute at Caltech for fellowship support and from the National Science Foundation (NSF) Graduate Research Fellowship Program under Base Award No. 174530. This article references 45 other publications.

中文翻译：

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碳酸氢盐或碳酸盐工艺，用于二氧化碳捕集和电化学转化的耦合

设计一个可扩展的系统以从空气中捕获CO ₂并将其转化为有价值的化学物质，燃料和材料，对于缓解气候变化可能具有变革性意义。（1-3）气候模型预测，到2050年，温室气体排放量将为负值为了使全球温度保持在2°C以下。（4）CO ₂捕获，CO ₂转化和最终产品分离的过程都需要大量的能量输入；设计一个同时使所有步骤所需的能量最小化的系统是一项重要的挑战。迄今为止，各种原型或中试水平的CO ₂已经针对捕获或转换的各个目标设计并构建了捕获和/或转换系统。一种方法集中在CO ₂从大气和纯CO加压的存储除去₂。（5,6）的其他努力集中于CO ₂转化工艺，如电化学还原（7-10）或发酵。（10,11 ）对于完成同时进行CO ₂捕获和转化的完整的端到端过程，仅开发了一些概念或分析。（12,13）在这里，我们探索了一种设计CO ₂捕获和转化系统的方法：（ i）通过溶解CO ₂形成碳酸氢盐或碳酸盐（碳酸氢根）基本解决方案；（ii）电化学还原为合成气或甲酸盐；最后（iii）转变为有用的化学品，燃料和材料。与传统的电化学系统不同，在传统的电化学系统中，气态CO ₂是转化为产品的主要化学原料，我们的分析着重于碳酸氢盐溶液携带的CO ₂的转化（图1）。该方法提供了将CO ₂捕集与转化耦合的几个优点。首先，从大气中捕获CO ₂不需要耗费大量能量的加热步骤即可从捕获介质中回收气态CO ₂以便以后转化。其次，通过转化CO ₂该方法避免了由碳酸氢根离子携带的能量密集型压缩气态CO ₂并允许每摩尔捕获的CO ₂明显提高转化效率。我们概述了该碳酸氢盐原料路线中各个步骤的能源需求，并将其与具有常规CO ₂捕集和转化过程的系统进行比较。图1.各种原型系统的步骤示意图，这些原型系统旨在捕获CO ₂和/或将其转换为浓缩和加压的CO ₂或增值产品。蓝色箭头表示捕获和转化CO _2的原型过程; 灰色箭头表示仅关注CO的原型过程₂转换，粉红色箭头表示我们建议的过程。括号中的数字对应于各种过程的参考（参考（5-13、15和18））。CO ₂捕集和转化系统有许多可能的配置，但每个常规设计都需要五个步骤（图2）：（i）CO ₂排放源；（ii）捕获介质；（iii）从捕获介质释放CO ₂的过程；（iv）将CO ₂压缩成浓气流；（v）将CO ₂转化为燃料，化学物质和/或材料（例如碳氢化合物）。用于捕获和转化的节能集成系统的设计需要仔细考虑CO ₂资源。无论是混合工艺气流的点源（例如，来自燃煤电厂的烟道气）还是相对稀薄的源（例如，大气）都将决定原料中的CO ₂浓度，因此决定了原料的类型。捕获最适合使用的媒体。CO ₂捕集介质有多种选择，从聚合物膜到有机和无机液体吸附剂。（14）在被捕集介质收集后，CO ₂的释放通常是通过加热捕集介质本身来完成的。然后必须将收集到的CO ₂压缩到合适的压力，以便流动并注入转化系统。如下所示，这两个过程与CO _2有关捕获和压缩可能构成主要的能源输入，在设计用于缓解全球大气CO ₂的完整系统时必须考虑这一因素。图2.捕获CO ₂并将其转化为有价值的化学品所涉及的各个步骤的概要。为了更好地促进完整系统的端到端比较，我们分析了迄今为止在中试工厂水平实现的CO ₂捕集和转化系统。我们将其分为三类：（i）执行直接空气捕获，（ii）使用浓缩的CO ₂源进行转化，以及（iii）仅关注CO ₂转化的系统。每个试验工厂和原型的步骤如图1所示，在支持信息中的图S1和S2给出了更详细的示意图。我们使用从大气中去除/转化一摩尔CO ₂所需能量的指标来比较试验过程，因为该指标直接影响工厂的运营费用。但是，应该注意的是，该指标并未考虑运营工厂所需的其他费用（人工，维护，材料等）或建设资本费用和摊销期限。由于这些考虑，因此我们选择不将发酵过程与电化学或热催化过程进行比较。发酵是有趣的CO ₂转化技术因对乙醇和其他多种碳产品等产品具有高选择性而显示出巨大的希望。（10-12）发酵罐也可在室温附近运行，从而使每摩尔CO ₂转化所需的能量较低（支持信息，第1G节） （10,11）但是，发酵过程需要人工来操作和维护系统，反应时间可能会很长，而且发酵罐通常以间歇式反应器运行，而不是以连续化学过程运行。这些考虑因素使得仅使用每摩尔CO ₂所需的能量就难以直接比较电化学和发酵过程捕获/转化为指标。在这种观点下，我们提出了一种电化学过程，因此将注意力集中在将其与类似系统进行比较上。使用氢氧化钾（Carbon Engineering）（5）或多胺（ClimeWorks）的捕获介质设计了从大气中去除CO ₂并产生浓缩和加压的CO ₂物流的中试装置。（6）氢氧化钾工艺需要〜每捕获一摩尔CO ₂ 0.28 MJ （kJ / mol CO ₂）（支持信息，第1A节）（5），而多胺工艺则需要0.58 MJ / mol CO _2。（6）对于这两种大气CO ₂捕获工艺，释放CO ₂所需的焓来自捕获介质的气体和随后的气态CO ₂加压控制了系统的总能量需求。还开发了其他中试规模的过程，这些过程可以从浓缩源中捕获CO ₂。例如，将地热发电厂排放的CO ₂溶解在氢氧化钾溶液中，然后将其泵入地下与天然玄武岩岩石（CarbFix）反应。（15）通过利用特定位置的地质进行CO ₂转化，该系统仅需要0.13 MJ / mol CO ₂（支持信息，第1B节）。（16）另一个过程是从地热发电厂捕获CO ₂，然后加压并加热CO ₂在通过水电解产生的H ₂的存在下生成甲醇和水（Carbon Recycling International）。此过程需要1.53 MJ / mol CO ₂（支持信息，第1C节），但它还会产生甲醇形式的化学燃料。（13）如所预期的，该过程比上述其他系统需要更多的能量，因为它结合了CO ₂转化过程。中试规模的CO ₂转化系统也受到广泛关注。（7-11,17）的中试规模CO ₂电解槽显示出CO和O来创建₂使用0.81 MJ /摩尔CO的能量输入₂（支持信息， （1D）节。（9）其他中试规模的CO ₂电解槽已被证明可以生产分别需要0.61 MJ / mol CO ₂或0.75 MJ / mol CO _2的CO（7）或甲酸（8）（支持信息，第1E节）。最后，一种固碳方法是使用一种在生产过程中将CO ₂注入水泥的过程，从而使水泥生产商在该过程中使用更少的粘结剂并抵消其CO ₂排放。使用这种方法，每立方码水泥可以存储25磅的CO ₂（支持信息，第1H节）。（17）值得注意的是，这些转换过程都没有考虑从大气或大气中捕获CO ₂所需的能量。集中点源。如上所述，采购加压的一氧化碳_2种纯度合适的能源并不代表微不足道的能量。为了构建具有适度能耗替代方案的替代过程，我们寻求评估每个过程的能源密集型步骤。能量消耗根据CO ₂的来源和所需产品的不同而有很大差异。尽管CO ₂转化过程需要比CO ₂捕获多得多的能量，但上述转化方案尚未报告其转化效率。根据我们的计算，所有报告的设备均具有CO ₂转换效率低于40％（支持信息，第3A–H节），大多数转换效率低于10％。（18）对于许多电解过程，每摩尔CO ₂所需的能量至少会翻倍，除非未使用的CO ₂通过系统回收。为了降低捕获和转化CO ₂的总体能量需求，考虑将近100％的捕获CO ₂转化的系统是很有趣的。电化学还原CO ₂由碳酸（氢）进行加压CO代替₂旁路能源密集型必要气态CO的浓度和压缩过程₂（有关整个过程设计的示意图，请参见图2和3）。含碳原料分子的这种变化使我们避免了能量密集型过程，例如吸附剂材料的加热或真空循环以释放捕获的CO ₂，以及随后将捕获的CO ₂压缩到足够的压力以致可以充分提供CO ₂转化系统，例如气体扩散电解槽。此外，与许多其他使用气态CO _2进行操作的电解装置不同，由（碳酸氢根）离子携带的转化CO ₂可实现每摩尔捕获的CO ₂较高的转化效率。作为碳输入（支持信息，第3A–H节）。图3.捕获CO ₂并将其转化为增值产品的各种拟议方案的过程和能源需求的示意图。最下面的行显示了商业合成气的合成方法，其原料为煤或天然气。产生1摩尔CO所需的能源，尽管与运行该工厂所需的成本有关，却不包括材料，维护和人工等费用的全部信息。一个示例性的原理图系统可以将碳酸氢盐的生成与合成气的生产相结合（图3），然后可以灵活地将其转化为有价值的化学物质，燃料和材料。假设CO _2的第一步捕集使用已经证明的空气接触器设计，（5）根据溶液的碱度，将需要〜0.01 MJ / mol CO ₂（支持信息，第2A节）。然后可以将碳酸氢盐的溶液进料到电解槽中，在电解槽中将其转化为合成气（CO和H ₂）。在阴极和双极膜上使用银催化剂的气体扩散电极的最新报告显示，CO ₂可以从（双）碳酸盐电解质中在催化剂附近局部生成，然后可以转化为合成气。（19,20 ）重要的是，在化工产品的输出流中未观察到CO ₂（19），因此无需将合成气与未转化的CO _2分离流。之间所需的这些报告的原型装置（19,20）~0.7 MJ /摩尔CO ₂，并取得的3之间的期望的合成气比例：1和2：1（H ₂：CO）（支持信息，第2B，C）。在上面讨论的所有电解过程中，析氧反应（OER）发生在对电极上。还应注意，只要将CO ₂捕获速率与CO生成速率匹配，就不需要补充碱。要创建一个每天可捕获和转化1吨CO ₂的工厂，将需要约16.6 m ²的捕获系统，0.4 m ²的电极面积以及覆盖4571 m ²的太阳能电池板阵列（支持信息，第2D节）。还需要用于太阳能存储和合成气处理的额外空间，但是与光伏系统发电所需的面积相比，该空间可以忽略。如果要使用该系统捕获全球每天排放的所有CO ₂，大约1亿吨CO ₂，则该系统将需要大约460 000 km ²。这比加利福尼亚州的面积稍大（〜424 000 km ²），仅占地球面积的0.09％。值得注意的是，碳酸氢盐转化系统的总能量需求为合成气为0.7 MJ / mol CO，而由煤制取合成气的商业流程则需要1.5-3.7 MJ / mol CO（支持信息，第2E节）。（21 ）从天然气制取合成气需要0.8 MJ / mol CO（支持信息，第2F，G节）。（22）但是，应注意的是，这些常规工业过程中列出的能量并未考虑提取煤炭或天然气原料。我们概述的碳酸氢盐工艺在能量基础上与当前的商业工艺相比具有优势，并且与典型的气化系统不同，它还可以产生不含任何污染物的合成气流。众所周知，合成气是一种通用原料，可用于进一步生成各种有用的化学物质和燃料。例如，可以使用发酵罐从合成气中合成丁醇，己醇，乙酸盐和乙醇。（10,12）另一个选择是在CuZnOxAlOx催化剂存在下通过加热和加压合成气来合成甲醇。（23）第三种一种选择是使用费-托工艺生产更长链的碳氢化合物，例如柴油，可以在钴催化剂存在下于240°C和25 bar的压力下合成。（24）合成气不是唯一的化学产品可以由一氧化碳电化学合成₂轴承碳酸氢盐溶液。几项研究表明，碳酸氢盐溶液也可以在低超电势下被电化学还原成甲酸盐（支持信息，第2H节）。（25-28）甲酸的全球市场相对较小（目前约5.7亿美元），但有望增长（29）通过大气CO ₂捕集从碳酸氢盐电化学合成甲酸的能量为0.8 MJ / mol CO _2。（图4）。甲酸不仅本身是一种有用的化学物质，而且还可以通过分子催化剂，（30-33）热催化，（34-36）或发酵过程转化为有价值的化学物质，例如甲醇。（37）燃烧是适度的（0.25 MJ / mol），可以考虑使用甲酸直接在甲酸燃料电池（38-41）中或作为氢燃料电池的氢载体（42-45）来发电。图4.捕获CO ₂并转化成甲酸的各种拟议方案的过程和能量需求的示意图。这种用于CO _2的系统级设计方法捕获和转化突出了使用碳酸氢盐溶液作为含碳原料的工艺的有利特性。代替直接从加压气流中还原CO ₂，将（碳酸氢盐）溶液中包含的CO ₂分子进行电化学还原以形成增值化学品，从而消除了能源密集型步骤，并具有接近统一转化率的潜力被捕获的CO ₂分子。尽管我们在分析中着重于合成气的生产，但拟议方法的主要优势在于将原料从气态CO ₂变为含水CO ₂载碳酸氢盐溶液，当与直接空气捕获系统结合以去除大气中的CO ₂时，其优势会得到增强。可从https://pubs.acs.org/doi/10.1021/acsenergylett.0c00234免费获得支持信息。

• 图S1和S2；CO ₂捕集和转化过程的能量输入计算（第S1节），碳酸氢盐或碳酸盐原料系统的能量输入计算（第S2节）和CO ₂电解槽转化效率（第S3节）（PDF）

图S1和S2；CO ₂捕集和转化过程的能量输入计算（S1节），碳酸氢盐或碳酸盐原料系统的能量输入计算（S2节）和CO ₂电解器转换效率（第S3节）（PDF）AJW，JSD和HAA构思了此想法，并研究了这种方法的可行性。AJW和ED执行了完成各种系统的能量分析所需的所有计算和研究。JSD在整个过程中提供建议，并为确定各种拟议过程的可行性提供了宝贵的意见。AJW，JSD和HAA撰写了《观点》，所有作者都对该手稿进行了评论。本材料中表达的任何观点，发现和结论均为作者的观点，不一定反映DOE或NSF的观点，发现和结论。本观点中表达的观点是作者的观点，不一定是ACS的观点。作者宣称没有竞争性的经济利益。无需订阅ACS Web版本即可获得电子支持信息文件。美国化学学会在任何可版权保护的支持信息中拥有版权权益。ACS网站上提供的文件只能下载供个人使用。未经美国化学学会许可，不得以其他方式允许用户以机器可读形式或任何其他形式全部或部分复制，重新发布，重新分发或出售ACS网站上的任何支持信息。为了获得复制，重新发布和重新分发此材料的许可，请求者必须通过RightsLink许可系统处理自己的请求。有关如何使用RightsLink权限系统的信息，请访问http://pubs.acs.org/page/copyright/permissions.html。这项工作是在能源部（DOE）能源创新中心的人工光合作用联合中心内完成的，该中心得到了美国能源部科学办公室的支持，编号为De-SC0004993。AJW感谢Caltech的Resnick Sustainability Institute提供的研究金支持以及美国国家科学基金会（NSF）研究生研究奖学金计划（基本奖为174530）的支持。本文引用了其他45种出版物。