Prospects of sunlight driven Air-to-Methanol synthesis via CO2 electrolysis
The race to save planet earth has led to significant advancement in technologies for harvesting renewable energy, carbon capture and conversion. Futures scenarios are being envisioned where CO2 is captured from air and converted to valuable fuels and chemicals, with methanol (MeOH) being the most coveted product. Here we assess two potential air-to-MeOH pathways that harvest solar power via concentrated photovoltaic (CPV) cells for direct air capture (DAC) of CO2 and subsequent conversion to MeOH by exploiting CO2 electrolysis. Specifically, we perform technoeconomic and life-cycle analysis on single-step (direct CO2-to-MeOH electrolysis) and three-step (integration of H2O electrolysis, CO2-to-CO electrolysis, and hydrogenation reactor) air-to-MeOH routes. Our results indicate that in current scenario, the envisioned air-to-MeOH routes are not economically and environmentally compelling with high levelized costs of MeOH ~1180-1730 $/tonMeOH and CO2 emissions of ~2.29-2.69 ton CO 2 /tonMeOH. Using sensitivity analysis, we reveal targets for CPV capital cost ($290/kW), DAC capital cost ($375/(ton-CO2/year)), and electricity emission intensity (<275 kg-CO2/MWh) which will make the three-step route commercially and environmentally viable as a near-term technology. In contrast, direct CO2-to-MeOH electrolysis will need drastic performance improvement to be economically competitive, with required current densities >300 mA/cm 2 , energy efficiency >45% and stack stability >2 years. We hope this study will garner the key stakeholders to advance discussions about the cost and potential of this envisioned air-to-fuel technology.
Introduction<!>Process description<!>st generation: Three-step air-to-MeOH pathway<!>nd generation: Single-step air-to-MeOH pathway<!>Economic Analysis<!>Sensitivity analysis<!>Carbon footprint of air-to-methanol pathways<!>Conclusion and perspective
<p>Fossil fuels have traditionally been our preferred energy source as they are cheap to extract, abundant, and have a high calorific value. They are responsible for the tremendous economic growth we have experienced in the last 50 years, but that has come at a cost. The use of fossil fuels has led to an increase in greenhouse gases in the atmosphere. 1,2 This has damaged the environment and led to climate change, with the last five years (2015-2019) recorded as the hottest ever on planet earth. 3 In order to save our planet, we continue our transition towards renewable energy use and develop technologies for carbon capture, conversion, use, and storage. 2 A clean and sustainable future can be envisioned as one where all these technologies will be working together for our energy needs and producing valuable fuels and chemicals.</p><p>Fortunately, worldwide race on the development of low-cost and sustainable energy technologies has triggered significant progress over the last couple of decades. Solar photovoltaic (PV) power now represents the cheapest and most widely deployed renewable energy source with over 580 GW of cumulative installed capacity worldwide as of 2020. 4 The levelized cost of electricity (LCOE) from solar PV now competes with fossil fuels and in many geographical locations, it is available at less than 5 cents/kWh. 5 This has spurred the research, development, and commercialization of electrochemical technologies, including water electrolysis, 6 and CO2 electrolysis. 7 We have witnessed tremendous progress in the field of electrochemical CO2 reduction to high-value fuels and chemicals such as CO, C2H4, HCOOH, which can now be produced at high current densities (>500 mA/cm 2 ) and faradaic efficiencies (>80%). [7][8][9][10][11] In parallel, there has been a significant advancement in the technology for CO2 capture via the direct air capture (DAC) process. DAC is technically and economically challenging as the concentration of CO2 in the air is quite low (~400 ppm). 12 Several research groups around the world are developing energy-efficient DAC technology 13 with a focus on materials and process optimization. [14][15][16] Carbon</p><p>Engineering and Climeworks are two of the few companies pursuing DAC technology, with their pilot-scale DAC plant running since 2015 and 2017 respectively, with a capacity to extract ~1-3 ton CO2/day. 17,18 In this regard, an economic study on Carbon Engineering's pilot-plant estimated the cost of CO2 capture between $94 and $232/ton-CO2. 17 On the other hand, Climeworks reported a capture cost of $600/ton-CO2 at its Swiss plant and expected to bring costs down to as low as $100/ton-CO2. 19 While these emerging technologies for harvesting solar energy, capturing, and converting CO2 have received attention, a comprehensive system-level analysis of a process that combines these emerging technologies and provides a glimpse of the future energy landscape is yet to be reported. In this regard, we present a thorough assessment of a process that captures energy from sunlight and CO2 from the air to convert it into MeOH in what can be called an "Air-to-MeOH" process. MeOH production is attractive due to its large market size (37 billion USD in 2020), ease of storage, transportation, and its versatility to be converted into olefins or use as a fuel. [20][21][22] In order to harvest solar power, we consider harvesting sunlight via concentrated PV (CPV) cells.</p><p>While silicon-based PV cells dominate the market with ~ 95% share of current production, they are close to reaching their theoretical efficiencies of ~ 29%, leaving only economy of scale as the main driver to reduce the cost of solar electricity in the future. 23 Unlike Si cells, CPV cells based on III-V group elements can theoretically reach over 80% solar cell efficiency. 24 CPV cells have reached ~ 47% efficiency and on a modular level, 43.4% efficient CPV modules have been demonstrated. 25 Moreover, they can operate at high light concentrations (>1000 suns) largely compensating the higher cost associated with the manufacturing of III-V based cells. Lastly, we use an aqueous solution-based DAC system to provide CO2, while two different routes for MeOH synthesis are considered based on their relative technology readiness level (TRL). The 1 st generation air-to-MeOH pathway integrates a H2O electrolyzer, CO2 electrolyzer, and conventional hydrogenation reactor, while the 2 nd generation involves direct CO2 electrolysis process. Our results indicate that the economic feasibility of both the proposed MeOH synthesis routes is significantly influenced by the capital costs of different emerging technologies that is CPV, DAC unit, and electrolyzers. A decrease in capital costs of CPV and DAC could make the 1 st generation air-to-MeOH route economically competitive as compared to the current fossil-fuel based MeOH production route. In contrast, the 2 nd generation MeOH route will not only require a reduction of capital costs but also require significant improvement in electrolyzer performance i.e. current densities and energy efficiency.</p><!><p>In the proposed pathways, about 1100 metric tons per day (tpd) of MeOH is produced using CO2 as feedstock, based on the typical capacity of a MeOH plant of 550-5000 tpd. 26 The CO2 feed is received from the DAC unit, while the electric power for the entire plant is generated in a hybrid fashion whereby a CPV solar farm provides electricity during the day and grid electricity is utilized at night. This approach allows for avoiding expensive renewable electricity storage options such as batteries or compressed H2. We considered two pathways for air-to-MeOH conversion in this study based on their relative TRL (Figure 1). In the first pathway what we can consider as 1 st generation, the CO2-to-CO electrolyzer used is relatively mature with TRL of 6-7 and is commercially available from several companies such as Haldor Topsoe, Siemens, Sunfire, Avantiun, Opus 12, Dioxide Materials. 7 Similarly, the H2O electrolyzer and hydrogenation reactor have a TRL of 7-9 as they are widely available for commercial application. 27,28 In the second route, a direct CO2-to-MeOH electrolyzer (2 nd generation) is used which exhibits a TRL of 1-3</p><p>given its operation is still in the laboratory-scale. 7 A schematic of the proposed pathways is shown in Figure 1, with key process parameters presented in Error! Reference source not found. and a detailed description is discussed hereafter.</p><!><p>In the 1 st generation route, a DAC unit captures and concentrates CO2 from the air which is fed to the CO2-to-CO electrolyzer for producing CO via reaction R1. 29 CO2 + H2O → CO + H2O+ 0.5 O2</p><p>The O2 produced at the anode is directly sent to the compression unit before storage. The CO, H2O produced at cathode along with the unconverted CO2 are directed to the gas-liquid separation unit to remove CO2 and CO from H2O. The H2O is recycled to the electrolyzer, while the CO2 and CO is directed to the hydrogenation reaction for MeOH production via reaction R2 to R4. The required H2 for hydrogenation reactions is supplied by H2O electrolyzer via reaction R5. 6 H2O + → H2 + 0.5 O2 Veq = 1.229 V (R5)</p><p>The described process contrasts with the conventional methanol synthesis process where the CO and H2 for hydrogenation are derived from fossil fuels (mainly from natural gas) via steam reforming. The hydrogenation products are sent for gas-liquid separation to remove the gaseous (unconverted CO2 and CO) and liquid (i.e., MeOH and water) products. The gaseous products are recycled to the hydrogenation reaction, while the liquid products are sent to the distillation unit for MeOH purification to 99 wt.%.</p><!><p>The source of CO2 and electricity is the same as in 1 st generation pathway, but the CO2 is directly fed to a CO2-to-MeOH electrolyzer, to facilitate the electrochemical reaction (R6). 31 CO2 + 2H2O → CH3OH + 1.5O2 Veq = 1.199 V (R6)</p><p>In the CO2 electrolyzer, MeOH and H2 are produced at the cathode, while O2 produced at the anode is sent to the three-stage compression unit to achieve market specifications (200 bar). The cathode products are sent to the gas-liquid separation unit to separate the gaseous (unconverted CO2 and H2) and liquid (electrolyte and MeOH) components. The gaseous components are then directed to the pressure swing adsorption (PSA) unit to remove CO2 from H2 which is then recycled back to the CO2 electrolyzer, while the H2 is compressed and stored. The electrolyte properties are adapted from literature, 32 which considers water as the main component of the electrolyte and we assume that the concentration of MeOH has an insignificant effect on the electrolyzer performance. Therefore, the liquid product of the CO2 electrolyzer can be continuously recirculated so that a higher concentration (10-60 wt.%) of MeOH is obtained prior to entering the distillation unit. The distillation unit produces high purity MeOH (99 wt.%) and the electrolyte is recycled to the CO2 electrolyzer. An alkaline electrolyzer was selected to facilitate CO2 electrolysis (1 st and 2 nd generation).</p><p>A Proton Exchange Membrane (PEM) electrolyzer was used to facilitate H2O electrolysis (1 st generation) due to its high efficiency, high current density, and high-pressure operation. Aspen</p><p>Plus is used to model the hydrogenation reactions, gas-liquid separation, compression, and distillation. The capital and operating costs of the Aspen Plus-simulated processes is estimated using Aspen Process Economic Analyzer. Further details regarding the process simulation are provided in the SI. The financial parameters which are constant throughout the analysis are summarized in Table 1. The techno-economic parameters used in base and optimistic scenarios are summarized in Table 2, where the current scenario is represented as the base scenario, and the future scenario is represented as the optimistic scenario. Natural gas requirement (GJ/ton CO2) 5. 25  8</p><p>The plant is assumed to be erected in an area with high direct normal irradiance (> 7 kWh/m 2 /day) with average daylight of 9 hours such as those available in the United States of America 44,45 and Saudi Arabia. 46,47 In the base scenario, the stack cost of the CO2 electrolyzer is $280/kW ($941/m 2 ) which drops to $180/kW ($605/m 2 ) in the optimistic scenario, due to the improvements in performance and economy of scale (Please refer to section 5A of SI). 48 Based on the literature reports, we assumed CO2-to-CO electrolyzer is operated at a CO2 single-pass conversion of 50% for both the base and optimistic scenarios. 49 The stack cost of the PEM H2O electrolyzer in the base scenario is $342/kW ($12,996/m 2 ) 48 which decreases to $143/kW ($7,722/m 2 ) in the optimistic scenario and are taken from DOE Report 2020. 48 The stack replacement schedule is shown in Table 2, at a cost of ~15% of the total uninstalled capital cost for PEM electrolyzer 50 and ~50% of the stack cost for alkaline electrolyzer. 50 The cost of the CPV module and dual dual-axis tracker in the base scenario is $920/kW based on current module efficiency of 32% and cumulative installed capacity of ~ 370 MW. 51 In the optimistic scenario the CPV capital cost decreases to $150/kW, based on a global installed capacity of 189 GW and using a learning rate of 18% (Please refer to section 5B of SI). 52 The capital cost of DAC is adapted from a pilot-scale unit which costs $694/ton CO2 per year. 17 In the optimistic scenario, we assumed a 75% cost reduction due to the increase of cumulative installed capacity by the year 2030. 53 A natural gas price of $6/GJ is selected in the base scenario, dropping to $4/GJ in the optimistic scenario. A current grid electricity price of $0.04/kWh is used in the base scenario and considering the rapid drop in the cost of renewable electricity, the electricity price in the optimistic scenario is expected to decrease to $0.03/kWh. 5 In this study, the selling price of O2 and H2 was also varied according to the market situation as listed in Table 2.</p><!><p>Figure 2a shows the calculated levelized cost of MeOH (LCM). Under the base scenario, the LCM is ~1180 $/ton and ~1730 $/ton for 1 st and 2 nd generation routes, respectively, which is much higher than the current market price of MeOH ranging between 300 to 500 $/ton. 65 The low market price of MeOH is due to the mature hydrogenation technology used which converts syngas (CO and H2) into MeOH at high temperature (~523.15 K) and high pressure (50-100 bar). 66 For both air-to-MeOH routes, the operating cost is the dominant (~50%) contributor followed by capital costs (~30%), but these costs are much higher for 2 nd generation as compared to the 1 st generation leading to higher LCM. This is due to the mature technologies used in 1 st generation route, such as the CO2-to-CO electrolyzer, H2O electrolyzer, and hydrogenation reactor which operate at much higher current densities and energy efficiencies. On the other hand, in 2 nd generation route we used a direct CO2-to-MeOH electrolyzer, which is still operated on a laboratory scale at moderate current densities and energy efficiency. This results in a larger electrolyzer area, CPV solar farm area, and electricity requirement, thereby resulting in higher capital and operating costs. Another major difference is the contribution of the stack replacement cost in 2 nd generation, which is significantly higher than 1 st generation, due to the poor stability assumed for catalysts used for direct CO2-to-MeOH electrolysis. Under the optimistic scenario, the LCM drops drastically for both to ~445 $/ton and ~470 $/ton for 1 st and 2 nd generation routes, respectively. In such a scenario, the LCM of both routes (single-step and three-step) is comparable with the current market price of MeOH. The synergetic effect between the decrease of equipment cost and the increase of catalyst performance results in the decrease of capital and operating costs as shown in Figure 2b and 2c, respectively.</p><p>While capital cost of CPV is the dominant contributor to total capital costs, the grid electricity cost is a dominant contributor to annual operating costs. This expected finding can be attributed to the high electricity consumption for MeOH production (1 rst Gen: 11.4 MWhe/ton; 2 nd Gen: 15.4 kWhe/ton) which also dictates both the size and cost of the CPV farm. In the optimistic scenario, improvements in catalyst performance reduce the electricity consumption from 11.4 to 10.2 kWhe/kg-MeOH and 15.4 to 12.0 kWhe/kg-MeOH for 1 rst and 2 nd generation routes, respectively.</p><p>The combination of the reduced energy requirement along with a decrease in capital costs, electricity price, and natural gas prices suppress the total capital and operating costs in optimistic scenario. As summarized in Table 2, we forecast significant improvements in the performance of the direct CO2-to-MeOH electrolyzer versus the more mature CO2-to-CO and H2O electrolyzers.</p><p>This leads to a drastic decrease in capital and operating costs for 2 nd generation which results in similar LCM for both routes in the optimistic scenario. We observed that the economic feasibility of air-to-MeOH routes is influenced by multiple economic and technical parameters. In the proceeding section, we carried out a detailed sensitivity analysis to understand the key drivers of air-to-MeOH route.    3 while keeping the remaining parameters constant at the base scenario. It should be noted that the base scenario used for sensitivity analysis is the same as the optimistic scenario used in Table 2.</p><!><p>Figure 3a and 3b suggest that the LCM is very sensitive to the capital cost of CPV for both routes. As discussed earlier, given the significant energy requirement for the air-to-MeOH process (11.4-15.4 kWhe/kg MeOH), and the fact that CPV provides power to all the units i.e., DAC, electrolyzer, PSA, and distillation unit makes it important to have capital cost for CPV less than $292/kW and $216/kW for 1 st generation and 2 nd generation pathway, respectively. This huge energy requirement for the air-to-MeOH process is also the reason for the significant influence of electricity prices on the LCM. Similarly, the capital cost of DAC also has a substantial influence on the LCM as CO2 is the main feedstock for MeOH production. A DAC capital cost of below $298/(ton-CO2/year) is needed to lower the LCM to a competitive range (compared with the market price).</p><p>An interesting observation for 1 st generation pathway (Error! Reference source not found.a) is that only the parameters associated with power generation (CPV and grid) and CO2 capture (DAC) exhibit significant influence on the LCM, while the performance of electrolyzers has minor effect on the LCM. This is due to the maturity of the electrolysis technology used in 1 st generation, whereby the different units operate at high performance and a variation within the range taken in this analysis does not affect the LCM. A CPV capital cost of lower than $291/kW, DAC capital cost less than $376 (ton-CO2/year) and a grid electricity price of 3.9 cents/kWh is required to reduce the LCM below the current market price. In contrast, in the single-step MeOH route of 2 nd generation (Error! Reference source not found.b), the current density and energy efficiency (EE) also have a significant influence on the LCM. The current density shows a considerable effect, and one would reap the economic benefit when the electrolyzer operates at a current density of >220 mA/cm 2 . An similar finding is reported in the literature, 12 showing the significant effect of current density particularly at low current densities. Regarding EE, the LCM becomes comparable with the market price when the CO2-to-MeOH electrolyzer operates at EE higher than 47%. From the market aspect, only the O2 selling price has a moderate influence on the LCM, while the selling price of H2 and the natural gas price has a minor effect. This can be attributed to the significant amount of O2 production, while the H2 production and natural gas consumption are much lower.    To get a clear picture about performance targets for 2 nd generation route that is via direct CO2-to-MeOH electrolysis, we carried out a sensitivity analysis by changing parameters which define the performance of an electrolyzer i.e., current density, EE and stability. The EE of an electrolyzer is calculated via the product of the faradaic efficiency (FE) and the applied cell voltage. The applied cell voltage is the sum of the thermodynamic voltage, reaction overpotentials, and cell losses.</p><p>Figure 4a illustrates the effect of EE and grid electricity price on the LCM. The adverse effect of low EE on the LCM can be compensated by the low grid electricity price. For example, when EE is ~ 40%, we can afford maximum grid electricity price of ~ 0.025 $/kWh. However, when the EE increases to 60%, the grid electricity price can be ~ 0.043 $/kWh. The catalyst stability is also an important factor which effects the operating costs and resulting LCM.</p><p>Figure 4b shows the interplay between current density and stability. When operating at high current density of ~ 400 mA/cm 2 , one can even afford to replace the stack on a yearly basis.</p><p>On the other hand, when operating current density is low (<200 mA/cm 2 ), it will require very stable (> 5 years) system. The analysis reveals the challenges ahead for direct-CO2-to-MeOH electrolysis given the fact that till date, demonstrated performance has been low with EE (<20%), current density of <50 mA/cm 2 and stability of <50 hours.   3.</p><!><p>To this end, we conducted a cradle-to-gate life cycle analysis (LCA) on one and three-step air-to-MeOH routes. In this analysis, we focus on the primary energy requirement including natural gas extraction (4 kg-CO2/GJ 67 ) and natural gas combustion (50 kg-CO2/GJ 68 ). We ignored any 2 nd Gen 2 nd Gen emissions associated with construction and decommissioning and water treatment as these contributions are minimal as compared to the operating phase of the plant. 69 We assumed an emission intensity of electricity from CPV as 19 kg-CO2/MWh, based on literature reports. 70 The LCA analysis revealed that the cradle-to-gate CO2 emissions of proposed route heavily depend on emission intensity of the grid as shown in Figure 5. For example, considering the emission intensity of grid electricity in US (453 kg-CO2/MWh (Table S.7)), we estimated net CO2 emissions of ~2.29 kg-CO2/kg MeOH for 1 st generation, which was lower than that of 2 nd generation (~2.69 kg-CO2/kg MeOH). The higher process efficiency of 1 rst generation (49%) as compared to the one in 2 nd generation (43%) resulted in lower CO2 emissions. These calculated emissions will be two times higher than the conventional MeOH synthesis route using natural gas (0.68 -1.08 kg CO2e/kg MeOH). 71 The LCA analysis also helps set targets for grid electricity emission intensity, where we observe that an emission intensity of <275 kg-CO2/MWh and <245 kg-CO2/MWh is needed for 1 st and 2 nd generation, for net CO2 emissions to be within the range of conventional MeOH synthesis route. Based on current projections for the US grid electricity emission intensity, 72 such a situation would be possible by the year 2050.</p><p>The proposed air-to-MeOH routes could also lead to zero or net negative CO2 emissions if the grid emission is below 97 kg-CO2/MWh. In this regard, one could consider the example of Iceland where 100% renewable electricity on grid emits only ~8 kg-CO2/MWh. 73 In a such a scenario, net negative CO2 emissions can be obtained for both 1 st generation (-0.</p><!><p>We are in a race against time to address the ever-rising CO2 emissions and resulting climate change. We are under immense pressure to quickly adapt renewable energy sources, develop technologies for energy storage, carbon capture, conversion, and use. A clean and sustainable future can be envisioned as one where all these technologies are working together for our energy needs to produce valuable fuels and chemicals. The importance of MeOH in this context cannot be overstated. Because of its high energy density, ease of storage and transportation, it can potentially act as an excellent energy vector. In this study we have presented a comprehensive At the same time, our analysis also revealed that the three-step route presented in 1 st generation could possibly be feasible for renewable MeOH production in near-future when certain any breakthrough technology; it can only be brought to life with strong incentives. As an example, the silicon PV industry experienced a massive price drop of more than 80% coming from subsidized industries that could sell products with little to no profit. On the other hand, CPV has suffered from a lack of dedicated supply chain, which has forced every company to develop and produce most of the components (trackers modules, etc.) themselves. Similar cost reductions can be expected with DAC, where a capital cost below $380/(ton-CO2/year) is required to reap economic benefits. These numbers have already been proven on a smaller pilot scale such as the one by Carbon Engineering who estimated the cost of CO2 capture between $94/ton-CO2 and $232/ton-CO2. 17 The DAC capital cost is predicted to plummet to ~$174/(ton-CO2/year) in 2030 as the installed cumulative capacity reach ~4,000 Mton-CO2/year. 54 This capacity would require autonomous and mass production manufacturing process. From the performance point of view, increasing DAC performance is much more challenging as the CO2 concentration in the atmosphere is small (~400 ppm). Last but not the least, the CO2 emissions from the proposed airto-MeOH routes will decrease as the share of renewable electricity on the grid increases across the world.</p><p>On the other hand, direct CO2 electrolysis route (2 nd generation) is an attractive technology but will need drastic improvements in performance to be economically competitive, with required current densities >300 mA/cm 2 , EE >45% and considerable improvements in catalyst stability (> 2 years). Considering the rapid development of electrochemical technologies, in an optimistic scenario this route would likely be compelling in the future. The emergence of artificial intelligence and machine learning would accelerate the development of catalysts and electrolyzer design. To conclude, the commercialization of air-to-MeOH processes in the near-future will depend on continuous innovation and at the same time needs productive discussions and decisions among the key stakeholders.</p>