Hydroxide exchange membrane carbon capture using a nickel hydroxide symmetric battery cell

Hydroxide exchange membrane carbon capture using a nickel hydroxide symmetric battery cell | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Hydroxide exchange membrane carbon capture using a nickel hydroxide symmetric battery cell Yushan Yan, James Buchen, Teng Wang, Ben Achrai, Jean-Philippe Hiegel, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5627423/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Electrochemical carbon capture devices can be a low energy cost solution for direct air capture (DAC) using renewable electricity. Historically electrochemical carbon-capture has targeted a range of concentrations from atmospheric (400 ppm CO2 ) (DAC), 1 to life-support (5000 ppm CO2 ), 2 to point-source capture (10% CO 2 ). 3 The hydroxide exchange membrane nickel hydroxide symmetric battery cell with two identical electrodes has low voltage requirements making it more suitable for DAC than other electrochemical approaches. A 25 cm 2 laboratory cell shows an average energy cost of 1.15 MWh·ton CO2 −1 and a CO 2 flux of 78 kg CO2 ·m 2 ·yr − 1 at 2 mA·cm − 2 . A manufacturable 25 cm 2 cell is durability tested for 5000 hours and achieves an energy of 0.46 MWh·ton CO2 −1 and a flux of 62 kg CO2 ·m 2 ·yr − 1 at the end of the test. A DAC pilot system with a stack of 9 scaled-up 300 cm 2 cells demonstrates an energy of 0.83 MWh·ton CO2 −1 and a flux of 75 kg CO2 ·m 2 ·yr − 1 and meets the 300 Pa pressure drop required for DAC. 4 Modular design projects improvements in cost from manufacturing and economies of scale, and a pathway to below 100 $ ·ton CO2 −1 from learning rates of past energy technologies. 5 Physical sciences/Energy science and technology/Carbon capture and storage Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation Physical sciences/Engineering/Chemical engineering Physical sciences/Chemistry/Electrochemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Direct air capture (DAC) has been identified as one of the key net negative carbon technologies to achieve a net zero future. 6 , 7 DAC is required to offset continued emissions from dilute CO 2 sources e.g., agriculture and construction. 8 The majority of current DAC technologies at scale (> 1 kton CO2 ∙yr − 1 ) are sorbent based with significant energy cost. 9 The energy cost is the major operating expense and is primarily driven by the temperature swing required to regenerate the sorbent. 10 , 11 The United States Department of Energy (DOE) has set forth a cost target of 100 $ ·ton CO2 −1 which is not possible to meet with current industrial electricity costs near 70 $ ·MWh − 1 and current temperature swing adsorption devices energy costs above 1.5 MWh·ton CO2 −1 . 12 , 13 A combination of lower electricity costs and lower DAC energy costs are required to meet the target DAC cost. Electrochemical pH gradient devices are a growing research area for low energy cost carbon capture. 1 , 14 – 19 The pH gradient is built by generating hydroxide (OH − ) at the cathode and consuming OH − at the anode. An acid-base equilibrium with CO 2 allows for the capture of CO 2 at the cathode and release at the anode. 20 This compliments other electrochemical CO 2 capture devices based on p K a gradients of an electrochemically active species allowing for the capture and release of CO 2 . 2 , 21 , 22 This work explores a hydroxide exchange membrane carbon capture (HEMCC) device with symmetric Ni(OH) 2 electrodes to produce the pH gradient for CO 2 capture and release (Fig. 1 a). At the cathode NiOOH is reduced to Ni(OH) 2 while at the anode Ni(OH) 2 is oxidized to NiOOH. This redox chemistry is known for its reversibility and cycle durability and has been commercially used in the cathode of alkaline Ni-MH battery technologies. 23 – 25 This symmetric cell has a thermodynamic equilibrium voltage of zero. Most of the voltage observed is to produce the pH gradient with the remainder driving the polarization of the electrodes. There is a resistance component as well, but this is small due to the low current densities used in the device, typically < 5 mA·cm − 2 . Scaleup is necessary for DAC technologies with several orders of magnitude growth required to meet net zero targets. 26 This HEMCC device was first developed at a 25 cm 2 laboratory scale able to capture 200 g CO2 ·yr − 1 (Fig. 1 b). It was built with electrodes formed by electrochemical precipitation of nickel hydroxide on nickel foam. A manufacturable version of the 25 cm 2 electrode was developed and durability tested for 5000 hours. This durable electrode was scaled to 300 cm 2 and integrated into a 9-cell stack for a pilot DAC system (Fig. 1 c). The first commercial modular DAC system is under construction for operation in 2026 with a capacity target of 1 kton CO2 ·yr − 1 (Fig. 1 d). Results and Discussion Figure 2 shows the data of a typical laboratory 25 cm 2 cell. Figures 2 a,b are time series of the operating cell voltage and cathode outlet CO 2 concentration with an initial conditioning cycle followed by ten regular cycles, all at 2 mA·cm − 2 . Within each regular cycle there are two steps, one for the charge/discharge and the other for the regeneration. CO 2 is captured in both steps, but at different voltages. The cell voltage and the difference between the 400 ppm cathode inlet and cathode outlet CO 2 allow for calculation of key process parameters of capacity, electron efficiency, energy cost, and flux. Figure 2 c-f presents these parameters excluding the regeneration steps. Capacity tracks long-term changes in the device. Electron efficiency is defined here as the ratio of CO 2 molecules captured per electron passed in the cell with a maximum stoichiometric ratio of 1 CO 2 per electron. Energy cost is the largest operating cost of the device while the flux determines the size and thus capital cost of the system. An average of 1.15 MWh·ton CO2 −1 energy cost including the regeneration step was achieved in this single cell dataset with a flux of 78 kg CO2 ·m − 2 ·yr − 1 . Each cycle was integrated using the definitions in SI Discussion 5 and the mean was taken excluding the conditioning cycles and assuming all CO 2 that is captured on the cathode is released in the anode to a product stream. The HEMCC has a lower energy cost than the incumbent technology presented by Carbon Engineering of 1.47 MWh·ton CO2 −1 for their device when excluding the blower and compressor. 10 Figure 2 | Laboratory scale 25 cm 2 HEMCC and its behavior. The HEMCC is comprised of two identical electrochemically precipitated Ni(OH)2 electrodes and an 80µm Piperion® membrane. The cathode air flow was 1 L∙min -1 with 400 ppm CO 2 and 90% RH. The anode received a sweep gas of 90% RH N 2 at 1 L∙min -1 . a. The cell voltage (V) and b. the cathode outlet CO 2 concentration (ppm) are used to calculate the key process variables c-f during the charge/discharge step c. capacity, d. electron efficiency, e. energy, and f. flux, all excluding the regeneration step. Regeneration steps are excluded to highlight the low energy cost of the charge-discharge step. Capture still occurs during regeneration but at higher energy cost. The first cycle on each side is a 1.5-hour regeneration hold for break-in followed by 10–75 minute cycles on each electrode. The cycle length is chosen based on an ex-situ capacity test to minimize regeneration (SI Discussion 7 ). c. Capacity achieved an average of 1.89 mAh·cm - 2 which was 85% of the ex-situ capacity tests. d. Electron efficiency achieved 0.25. e. Energy cost averaged 0.76 MWh∙ton CO2 -1 while f. flux averaged to 72 kg CO2 ∙m 2 ∙yr -1 . g. and h. show a single cycle to highlight the change in voltage and capture in the charge/discharge and regeneration steps. i., j., and k. are graphic aids to describe the build ΔpH, maldistributed ΔpH, and rebuild ΔpH sections in g. and h. The ΔpH between the peak pH at the cathode and the CO 2 release pH at the anode drives the CO 2 capture rate in the cell, see SI Discussion 2. The CO 2 release pH is the pH that achieves equilibrium between CO 2 in the anode and the concentrated CO 2 gas product stream. The dotted red primary reaction coordinate lines are rough position where OH - is produced in the cathode and consumed in the anode. i. The build ΔpH section is directly after the gas lines switch in the cell. Redox reactions take place close to the membrane with the cathode near 100% state of charge and the anode near 0% state of charge. The pH at the back of the cathode electrode is close to the pH of the anode in the previous cycle. j. The maldistributed ΔpH section is at the end of a charge/discharge step. This occurs because of the uneven mass loading of Ni(OH) 2 /NiOOH when making the electrode and uneven distribution of resistance through the electrode. Near the end of the charge/discharge step, low loading and low resistance volumes of the cathode will have fully been consumed, no longer producing OH - . Surrounding higher loading and higher resistance volumes continue producing OH - but the average pH across the electrode drops lowering the average driving force for capturing CO 2 . The anode in this section is still charging and has not reached 100% state of charge because of competing OER. k. The rebuild ΔpH section is the beginning of the regeneration after the cathode has reached 0% state of charge. The cathode reaction coordinate is brought to the front towards the membrane where catalytic ORR reactions are predominant. The full area of the electrode is used again, and the average pH increases to have a higher rate of CO 2 capture. The anode continues to charge towards 100% for the next cycle. Relatively low energy cost is the main benefit of this electrochemical carbon capture approach. Figure 2 e shows several cycles of CO 2 capture at an average of 1.15 MWh·ton CO2 −1 ; when broken down by the steps the charge/discharge and regeneration steps averaged 0.76 MWh·ton CO2 −1 and 2.15 MWh·ton CO2 1 respectively. Both steps can be improved by improving the electron efficiency of the device. 14 Electron efficiency of 1 indicates ionic transport of only HCO 3 − in the cell where 0.5 would be predominantly CO 3 2− , below 0.5 as observed in this cell has either unreacted OH − being transported or back diffusion of HCO 3 − or CO 3 2− from the anode. Future work can improve electron efficiency by adjusting membrane parameters such as ionic exchange capacity (IEC), conductivity, and water uptake. Improved electron efficiency at the same current density directly increases flux of CO 2 which decreases energy and capital costs. The CO 2 capture rate changes with time and the state of charge of the electrode. Figure 2 h shows the single half cycle behavior of CO 2 capture. At the beginning of the charge/discharge step there is time lag as the pH gradient is reversed from the previous cycle. A steady increase in capture rate is seen as the pH gradient is formed, which takes time proportional to the total IEC of the membrane and ionomer. Piperion® is used for the membrane and ionomer based on PAP-TP-85 with an IEC of 2.37 meq.·g − 1 . 29 The state of charge determines the CO 2 capture behavior for the remainder of the charge/discharge step. The surface of the electrode near the membrane discharges first due to the difference in conductivity between the metal, metal hydroxide, and ionomer phases of the electrode. 30 , 31 The primary reaction coordinate, and thus the peak pH, moves from the membrane to the back of the electrode as the state of charge changes in each electrode. At a moderate state of charge, higher capture rates are expected because CO 2 diffuses into and reacts with a larger volume of high pH ionomer. A similar behavior was seen in a hydrogen HEMCC previously studied by using an interlayer volume. 15 Near the end of the cycle volumes of anode with lower mass loading of NiOOH/Ni(OH) 2 or lower local resistance will run out of NiOOH earlier than the surrounding volumes and no longer produce OH − ; these volumes no longer contribute to the pH gradient and lower the average pH at the primary reaction coordinate. This manifests in a dip in CO 2 capture near the end of the charge/discharge step. During the charge/discharge step the flux averages 72 kg CO2 ·m −− 2 ·yr −− 1 , while the average peak flux is 97 kg CO2 ·m − 2 ·yr − 1 . The capture rates during regeneration do not depend on the state of charge of the cathode which has been depleted. The pH gradient is reestablished by a relatively thin layer of the electrode near the membrane performing catalytic ORR. 30 , 31 There is a short time dependent growth of capture at the beginning of regeneration to reestablish the pH gradient across the area of the electrode after the decrease observed in the charge/discharge step. After that a steady state constant capture rate is achievable. During the regeneration step the flux averages 93 kg CO2 ·m −−2 ·yr −−1 , while the average peak flux is 97 kg CO2 ·m − 2 ·yr − 1 . Technical pathways to improve the device include improving mass transport (diffusion) from the flow field channels into and within the porous electrodes, decreasing the effect of time lag for CO 2 and decreasing the requirement for regeneration. Decreasing the effect of time lag can improve both the flux and energy cost of the cycle. There are two ways to reduce the time lag, one by decreasing the total ion capacity of membrane and electrode or the other by increasing the electrode capacity. Either option has a trade-off to optimize. Decreasing the total ion capacity by decreasing membrane thickness and ionomer amount decreases the number of ions required to displace at the beginning of each cycle. This cannot be decreased to zero because there needs to be sufficient ionic conductivity and mechanical stability. Additionally, thinner membranes can lead to increased bicarbonate back diffusion offsetting gains in time lag. 1 , 15 Increasing the capacity of an electrode lengthens the time for a full cycle and thus a higher percentage of time is spent in a high flux region of a cycle. Increasing the electrode capacity can increase the mass transport resistance for diffusion of CO 2 into the electrode by decreasing the porosity of the electrode or increasing the diffusive path length. The laboratory prepared electrodes nominally have a porosity of 30–40%. Optimization of this porosity can lead to flux and energy cost gains. Decreasing the amount of time required for regeneration can also decrease the overall energy cost of the system. In Ni-MH batteries the amount of OER that occurs depends on the C-rate used and the dopants present. 27 , 32 The C-rate is defined as a ratio of the current density in mA to the capacity of the electrode in mAh. The electrodes in Fig. 2 ran at a C-rate of 0.85. Lower C-rates allow for lower voltage operation of the battery limiting the catalytic OER which has a high kinetic overpotential. Dopants can either improve the kinetics of the Ni(OH) 2 oxidation or hinder the kinetics of the OER reaction. Dopants such as cobalt have shown the ability to limit OER and can be added to limit need for regeneration. 33 , 34 Leaning on nickel hydride battery research to limit regeneration requirement to 10% of a cycle length could decrease the average full cycle energy cost to 0.90 MWh·ton CO2 −1 with this laboratory scale system. The electrodes shown in Fig. 2 were electrochemically precipitated; this method was chosen for tunability of the Ni(OH) 2 loading early in research. There are several other methods to produce Ni(OH) 2 including electrodeposition and bulk precipitation. 24 Improvements were made to the laboratory scale electrode to produce large manufacturable electrodes for pilot scale operation. Such 25 cm 2 electrodes were tested for 5000 hours with the cell supplied with ambient air held at 32°C and 82–87% RH. Three segments of the corresponding data are shown in Fig. 3 . The end of the 5000-hour run highlighted a low energy cost near 0.46 MWh·ton CO2 −1 with a flux of 62 kg CO2 ·m − 2 ·yr − 1 . The more manufacturable 25 cm 2 cell shows an increase in cycle length or capacity over time (Fig. 3 ). This is likely the result of a self-limiting corrosion process in the electrode. The nickel foam used in the electrode as a scaffold for Ni(OH) 2 can be oxidized to Ni(OH) 2 at potentials above 0.11 V vs reversible hydrogen electrode (RHE). 23 Any exposed bare nickel can go through the oxidation process. The corrosion stops when accessible nickel metal is fully converted. An increase in capacity is seen between the first and second segments, but not between the second and third segments suggesting the complete conversion of the available nickel sites. This phenomenon was also observed at the laboratory scale and is shown in SI Discussion 4. An increase of the average flux is also seen, contributing to a lower energy cost. This corroborates the concept that reducing the effect of time lag with a larger capacity electrode will lead to higher average flux and lower average energy cost. This manufacturable electrode was scaled up from 25 cm 2 to 300 cm 2 and used in a 9 x 300-cm 2 stack (Fig. 3 d,f). This stack averaged an energy cost of 0.83 MWh·ton CO2 −1 with a flux of 75 kg CO2 ·m − 2 ·yr − 1 . At the system level a blower is required to push air through the DAC device and 300 Pa is considered an upper limit for DAC. 4 The laboratory scale pressure drop suffers from triple serpentine flow fields adapted from bench top fuel cells with a pressure drop of 2800 Pa for a 1 L·min − 1 flow rate as discussed in SI Discussion 3. The scaled system exhibits a total pressure drop of less than 300 Pa (10x lower in comparison to the lab cells) while maintaining similar performance metrics. Such low pressure drops range is obtained by an innovative design of the stack hardware (inlets/outlets and flow field morphology) - needed to allow energy efficient Air Streaming Unit (ASU, e.g., axial fan) and an overall energy-efficient DAC process. Figure 4 shows a levelized cost reduction pathway to 92 $ ·ton CO2 −1 in line with the United States DOE targets. Durability at the 25 cm 2 size and performance at the 9-cell stack size lays the groundwork for the 1000 ton CO2 ·yr − 1 Gen-0 pilot scale. The Gen-1 system will follow research pathways discussed to decrease time lag and regeneration as well as increasing CO 2 flux. Gen-1 will additionally focus on the full system requirements including a blower and compressor. Improved airflow design can lower pressure drop to decrease blower costs. 4 , 35 Electrochemical compression is to be explored as shown in proton exchange membrane electrolyzers (PEMEL) for hydrogen production with a cost benefit compared with conventional compression alone. 36 – 38 Module standardization for assembly line production and expected improvement in energy consumption are the main drivers for the Gen-2 system. 39 Further improvement of the CO 2 flux and economies of scale will enable Gen-3 First-Of-A-Kind (FOAK) megaton systems. 40 – 42 At this scale pricing improvements can be achieved for membrane polymers and peripheral components. 43 Finally, centralized control rooms, automation, digital twin machine learning, and AI-driven maintenance strategies are expected to drive final costs towards the Nth-Of-A-Kind (NOAK) Gen-3 systems. Conclusion The nickel hydroxide HEMCC technology shown here has a low energy cost compared to conventional temperature swing adsorption devices. Producing the pH gradient is the primary energy cost for the device, followed by the polarization of the electrodes, and resistance. At the laboratory scale the concept was developed to produce a device that has an average energy cost of 1.15 MWh·ton CO2 −1 at an average flux of 78 kg CO2 ·m − 2 ·yr − 1 . The energy cost and flux can be improved by decreasing the time lag for pH gradient formation and minimizing parasitic OER reactions. The time lag can be decreased by increasing electrode capacity which aids in increasing the average flux while decreasing the average energy cost. Anodic OER reactions can be limited by employing traditional Ni-MH battery dopants. By limiting OER, the energy cost can be reduced by reducing the required amount of regeneration. Scale up of the nickel hydroxide HEMCC has started where the original concept was expanded from a 25 cm 2 cell to a 9-cell stack of 300 cm 2 cells. A pilot module containing these stacks is in operation capturing CO 2 from ambient air. A 5000-hour durability test has shown stability of the electrode design with an end-of-test energy cost of 0.46 MWh·ton CO2 −1 at a flux of 62 kg CO2 ·m − 2 ·yr − 1 . This electrode design was shown to be able to scale into a stack with a low pressure drop of 300 Pa meeting technical requirements for DAC. The prototype is being iterated to drive down cost along learning curves like those seen in solar photo voltaic cells and lithium-ion batteries. Device level research is expected to achieve many of the early-stage improvements, while manufacturability and economies of scale will drive costs down below the 100 $ ·ton CO2 target set forth for DAC. Methods Electrode preparation Lab scale electrode preparation was done by electrochemical precipitation of nickel hydroxide onto nickel foam.The nickel foam was from MTI corporation (product name EQ-bcnf-16m) with 99.9% purity, 95% porosity, 80–110 pores per inch and thickness of 1.6mm. Three 25cm 2 nickel foam substrates were cut retaining a 1 cm 2 tab for connecting with an alligator clip. The three substrates were compressed in a hydraulic press to 512 µm by using a 512 µm thick steel plate as a guide barrier. These were weighed and then placed in prepared 1 M sulfuric acid solution for 1 hour to remove the surface oxide layer. The oxide layer removed is assumed to be negligible in mass for purposes of measuring amount of mass added to the working electrode. These substrates were set parallel in a 0.05 M nickel chloride solution using 98% anhydrous nickel chloride salt from Millipore Sigma (CAS No: 7718-54-9). NiCl 2 hexahydrate (Sigma-Aldrich, ReagentPlus) was also tested without any noticeable performance difference. The substrates were suspended in place with copper electrode clips as shown in SI Fig. 9 . The outer electrodes were connected via wire to be at the same voltage as the counter electrode, the middle electrode was the working electrode for Ni(OH) 2 to be precipitated onto. Two outer electrodes were used to evenly grow Ni(OH) 2 on both sides of the central working electrode. The distance between electrodes were approximately controlled to 1 cm. A reference Ag/AgCl electrode from Pine Research (RREF0021) tracked voltage in the cell during electrochemical precipitation. A current density of -5 mA·cm 2 for 4 hours was applied to the working electrode. Afterwards, the electrodes were rinsed with DI water and set out to dry overnight. The next day the weight of the working electrode was measured to determine the mass of deposition. As a control, the counter electrodes were also weighed to measure their loss of Ni. The loss of Ni of the counter electrodes closely aligns with the oxidation to Ni 2+ in the solution as expected this balances the loss of Ni 2+ during precipitation and maintains a relatively stable bulk Ni 2+ concentration. The pair of counter electrodes lost 0.57 g Ni (27% of initial mass) during the process while the working electrode gained 0.83 g during the process. After electrochemical precipitation the electrodes were spray coated with Piperion® ionomer. A solution of 1.3333g of 5% ionomer in ethanol solution was added to 6.2222g IPA and sonicated for 30 min. This solution was sprayed on one side of the electrode and then set out to dry for 1 day. Weighing the next days showed that 25% of the initial ionomer mass was transferred to the electrode during the spraying. Ex-situ three electrode cycling The electrode with ionomer was then placed in a 1 M KOH bath in the same set up as the corrosion test in SI Fig. 7,8 . Three series of current densities were used to cycle each electrode, 1-cycle at 2 mA·cm 2 , 30-cycles at 5 mA·cm 2 , and 5-cycles at 2 mA·cm 2 . A break in is conducted at 2 mA·cm − 2 . First the electrode is charged from Ni(OH) 2 to NiOOH for three hours finishing with a plateau at OER voltage. Then the electrode is discharged back to Ni(OH) 2 to completion with a cutoff voltage of 1.1 V vs RHE. This break in charge is longer than the final 5-cycles at 2 mA·cm − 2 to allow for impurities in the deposited Ni(OH) 2 structure to escape without physically damaging the deposited layer. Next, 30 cycles at 5 mA·cm 2 are conducted to stabilized the Ni(OH) 2 layer. It was found that 30 cycles were required to reach a plateau in capacity for an electrode; the current density 5 mA·cm 2 was chosen to speed up the process. The final 5 cycles were conducted at 2 mA·cm 2 to compare with the 2 mA·cm 2 used during the direct air capture tests. Membrane Electrode Assembly The final assembly of the laboratory scale 25cm 2 cell uses a Scribner 25 cm 2 cell with triple serpentine flow fields. (see SI Fig. 11 ) A 2.5”x3” piece of 80µm Piperion® membrane was used between two electrodes. Two electrochemically precipitated nickel hydroxide electrodes were cut to fit precisely to their respective gaskets. A total of 336µm in FEP gaskets were used on each side for the 512µm thick electrodes. This leads to a 0.39 compression ratio once the cell bolts are torqued to 100 PSI. Declarations Acknowledgements This work is supported by the University of Delaware, U.S. Department of Energy National Energy Technology Laboratory (DE-FE0031955), and U.S. Department of Defense Army Research Laboratory (W911NF-23-2-0028). Versogen provided Piperion® membrane and ionomer for this research. Authors and Affiliations These authors contributed equally: R. James Buchen & Teng Wang Department of Chemical and Biomolecular Engineering at University of Delaware R. James Buchen, Teng Wang, K. Braden Geiger, P. Brian Setzler, & Yushan Yan RepAir Ben Achrai, Y. Naama Gluz, Maurice Artoul, & Jean-Phillippe Hiegel Contributions Y.Y. and P.B.S. supervised the project. P.B.S. provided the initial concept and guidance on fundamentals of electrochemical pH driven CO 2 capture. R.J.B. designed the test station and laboratory scale electrodes. R.J.B. and T.W. developed testing protocol and evaluation of the direct air capture system. R.B.G. conducted electrochemical tests and related data processing. T.W. carried out materials characterization. B.A. led the development and scale up efforts of the main technological aspects including durable electrodes, stack hardware design and the accommodating system for operating the DAC process. 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The case for high-pressure PEM water electrolysis. Energy Conversion and Management 261 (2022). https://doi.org/10.1016/j.enconman.2022.115642 Hancke, R., Bujlo, P., Holm, T. & Ulleberg, Ø. High-pressure PEM water electrolyser performance up to 180 bar differential pressure. Journal of Power Sources 601 (2024). https://doi.org/10.1016/j.jpowsour.2024.234271 Abbasi, R. et al. A Roadmap to Low-Cost Hydrogen with Hydroxide Exchange Membrane Electrolyzers. Adv Mater 31, e1805876 (2019). https://doi.org/10.1002/adma.201805876 Rubin, E. S., Azevedo, I. M. L., Jaramillo, P. & Yeh, S. A review of learning rates for electricity supply technologies. Energy Policy 86, 198–218 (2015). https://doi.org/10.1016/j.enpol.2015.06.011 Gerke, B. F. N., Allison T.; Fisseha, Kibret S.. Recent price trends and learning curves for household LED lamps from a regression analysis of internet retail data. (Lawrence Berkeley National Laboratory, 2015). Schmidt, O., Hawkes, A., Gambhir, A. & Staffell, I. The future cost of electrical energy storage based on experience rates. Nature Energy 2 (2017). https://doi.org/10.1038/nenergy.2017.110 Orangi, S. et al. Historical and prospective lithium-ion battery cost trajectories from a bottom-up production modeling perspective. Journal of Energy Storage 76 (2024). https://doi.org/10.1016/j.est.2023.109800 Mathias, M. F. et al. Two Fuel Cell Cars In Every Garage? The Electrochemical Society Interface 14, 24–35 (2005). https://doi.org/10.1149/2.F05053if Additional Declarations Yes there is potential Competing Interest. Y.Y. is the Founder and CEO and P.B.S. is a Co-Founder for Versogen, the producer of Piperion®. Y.Y. and B.A. are Co-Founders for RepAir the scale-up company. Supplementary Files SIHydroxideExchangeMembraneCarbnCaptureHEMCCusingsymmetricalnickelhydroxidebatteries.docx SI Hydroxide Exchange Membrane Carbn Capture (HEMCC) using symmetrical nickel hydroxide batteries Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5627423","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Physical Sciences - Article","associatedPublications":[],"authors":[{"id":391454256,"identity":"8766adab-277d-48af-9958-29e6233cc615","order_by":0,"name":"Yushan Yan","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAvElEQVRIiWNgGAWjYNCCCgbGBnYgzUO8ljNALcwkaWFsI0WLwfGzxyR/zjss28/MwPjgbRsxWs7kpUnzbjtsPLOZgdlwLlFaDuSYSTNuO5y44TADmzQvUVrOvzGT/DnncOL+wwzsv4nTciPHTIK3AWgLMwMbM1FaJG+8MbbmOZZuPOMwY7PknHNEaOE7n2N480eNtWx/e/PBD2/KiNCicICBRQLCZGwgQj0QyDcwMH8gTukoGAWjYBSMWAAA8wg3BQU7yXgAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-6616-4575","institution":"University of Delaware","correspondingAuthor":true,"prefix":"","firstName":"Yushan","middleName":"","lastName":"Yan","suffix":""},{"id":391454257,"identity":"144999e5-0f74-4f73-9efd-7515ab9aa930","order_by":1,"name":"James Buchen","email":"","orcid":"","institution":"University of Delaware","correspondingAuthor":false,"prefix":"","firstName":"James","middleName":"","lastName":"Buchen","suffix":""},{"id":391454258,"identity":"1b4bd723-0c2d-46a7-b030-f5f9a7666bf0","order_by":2,"name":"Teng Wang","email":"","orcid":"","institution":"University of Delaware","correspondingAuthor":false,"prefix":"","firstName":"Teng","middleName":"","lastName":"Wang","suffix":""},{"id":391454259,"identity":"5df82464-0dd9-458e-90e7-6cea673aea10","order_by":3,"name":"Ben Achrai","email":"","orcid":"","institution":"RepAir","correspondingAuthor":false,"prefix":"","firstName":"Ben","middleName":"","lastName":"Achrai","suffix":""},{"id":391454260,"identity":"6b074887-c61b-4667-ba15-0e8095c28405","order_by":4,"name":"Jean-Philippe Hiegel","email":"","orcid":"","institution":"RepAir","correspondingAuthor":false,"prefix":"","firstName":"Jean-Philippe","middleName":"","lastName":"Hiegel","suffix":""},{"id":391454261,"identity":"7a841c35-5119-4864-86f7-95734cdb5748","order_by":5,"name":"Naama Gluz","email":"","orcid":"","institution":"RepAir","correspondingAuthor":false,"prefix":"","firstName":"Naama","middleName":"","lastName":"Gluz","suffix":""},{"id":391454262,"identity":"f1694fc7-0e8e-4142-9eda-0f72a8658369","order_by":6,"name":"Maurice Artoul","email":"","orcid":"","institution":"RepAir","correspondingAuthor":false,"prefix":"","firstName":"Maurice","middleName":"","lastName":"Artoul","suffix":""},{"id":391454263,"identity":"ff3457ab-e776-4362-bdff-bcd671b38782","order_by":7,"name":"Braden Geiger","email":"","orcid":"","institution":"University of Delaware","correspondingAuthor":false,"prefix":"","firstName":"Braden","middleName":"","lastName":"Geiger","suffix":""},{"id":391454264,"identity":"35f06692-a6cc-42a8-8e02-95185396be7c","order_by":8,"name":"Brian Setzler","email":"","orcid":"","institution":"University of Delaware","correspondingAuthor":false,"prefix":"","firstName":"Brian","middleName":"","lastName":"Setzler","suffix":""}],"badges":[],"createdAt":"2024-12-12 01:30:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5627423/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5627423/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":73463383,"identity":"437ab7ec-4717-417a-96ee-c807f1360282","added_by":"auto","created_at":"2025-01-10 08:20:18","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":668107,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHEMCC device schematic and scales from 200 g\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eCO2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e·yr\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e-1 \u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eto 200 ton\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eCO2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e·yr\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e-1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e a.\u003c/strong\u003e A laboratory scale MEA schematic shown with two porous Ni(OH)\u003csub\u003e2\u003c/sub\u003e electrodes separated by a hydroxide exchange membrane. Flow fields provide a path for air and a purified CO\u003csub\u003e2 \u003c/sub\u003eoutlet stream.\u0026nbsp; During cycling the cathode produces OH\u003csup\u003e-\u003c/sup\u003e and the anode consumes OH\u003csup\u003e-\u003c/sup\u003e generating a pH gradient. A cycle stops when the cathode NiOOH is consumed. During the cycling step a competing oxygen evolution reaction (OER) occurs at the anode which prevents fully charging the anode. A regeneration step is performed after the cathode NiOOH is fully consumed, where the cathode performs oxygen reduction reaction (ORR) to fully charge the anode. The capture and release of CO\u003csub\u003e2\u003c/sub\u003e is based on an acid-base equilibrium driven by the pH gradient. \u003cstrong\u003eb. \u003c/strong\u003eA laboratory scale 25 cm\u003csup\u003e2 \u003c/sup\u003emembrane electrode assembly (MEA). \u003cstrong\u003ec.\u003c/strong\u003e A stack of 9-300 cm\u003csup\u003e2\u003c/sup\u003e cells in pilot operation. \u003cstrong\u003ed.\u003c/strong\u003e Commercial RepAir® module under construction.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-5627423/v1/1246d8afdd87bae08cef0df8.png"},{"id":73463382,"identity":"dd5f7007-e5e7-4cfc-a642-0365ed026515","added_by":"auto","created_at":"2025-01-10 08:20:18","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":157775,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eLaboratory scale 25 cm\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e HEMCC and its behavior. \u003c/strong\u003eThe HEMCC is comprised of two identical electrochemically precipitated Ni(OH)2 electrodes and an 80μm Piperion® membrane. The cathode air flow was 1 L∙min\u003csup\u003e-1\u003c/sup\u003e with 400 ppm CO\u003csub\u003e2\u003c/sub\u003e and 90% RH. The anode received a sweep gas of 90%\u0026nbsp;RH N\u003csub\u003e2\u003c/sub\u003e at 1 L∙min\u003csup\u003e-1\u003c/sup\u003e. \u003cstrong\u003ea.\u003c/strong\u003e The cell voltage (V) and \u003cstrong\u003eb.\u003c/strong\u003e the cathode outlet CO\u003csub\u003e2\u003c/sub\u003e concentration (ppm) are used to calculate the key process variables \u003cstrong\u003ec-f \u003c/strong\u003eduring the charge/discharge step: \u003cstrong\u003ec.\u003c/strong\u003e capacity, \u003cstrong\u003ed.\u003c/strong\u003e electron efficiency, \u003cstrong\u003ee.\u003c/strong\u003e energy, and \u003cstrong\u003ef.\u003c/strong\u003e flux, all excluding the regeneration step. Regeneration steps are excluded to highlight the low energy cost of the charge-discharge step. Capture still occurs during regeneration but at higher energy cost. The first cycle on each side is a 1.5-hour regeneration hold for break-in followed by 10-75 minute cycles on each electrode. The cycle length is chosen based on an ex-situ\u003cstrong\u003e \u003c/strong\u003ecapacity test\u003cstrong\u003e \u003c/strong\u003eto minimize regeneration \u003cstrong\u003e(SI Discussion 7\u003c/strong\u003e)\u003cstrong\u003e.\u003c/strong\u003e \u003cstrong\u003ec.\u003c/strong\u003e Capacity achieved an average of 1.89 mAh·cm\u003csup\u003e-2 \u003c/sup\u003ewhich was 85% of the ex-situ capacity tests. \u003cstrong\u003ed.\u003c/strong\u003e Electron efficiency achieved 0.25. \u003cstrong\u003ee.\u003c/strong\u003e Energy cost averaged 0.76 MWh∙ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e while \u003cstrong\u003ef.\u003c/strong\u003e flux averaged to 72\u0026nbsp;kg\u003csub\u003eCO2\u003c/sub\u003e∙m\u003csup\u003e2\u003c/sup\u003e∙yr\u003csup\u003e-1\u003c/sup\u003e. \u003cstrong\u003eg.\u003c/strong\u003e and \u003cstrong\u003eh.\u003c/strong\u003e show a single cycle to highlight the change in voltage and capture in the charge/discharge and regeneration steps. \u003cstrong\u003ei., j., and k. \u003c/strong\u003eare graphic aids to describe the build ΔpH, maldistributed ΔpH, and rebuild ΔpH sections in \u003cstrong\u003eg. \u003c/strong\u003eand\u003cstrong\u003e h.\u003c/strong\u003e The ΔpH between the peak pH at the cathode and the CO\u003csub\u003e2\u003c/sub\u003e release pH at the anode drives the CO\u003csub\u003e2\u003c/sub\u003e capture rate in the cell, see \u003cstrong\u003eSI Discussion 2.\u003c/strong\u003e The CO\u003csub\u003e2\u003c/sub\u003e release pH is the pH that achieves equilibrium between CO\u003csub\u003e2\u003c/sub\u003e in the anode and the concentrated CO\u003csub\u003e2\u003c/sub\u003e gas product stream. The dotted red primary reaction coordinate lines are rough position where OH\u003csup\u003e- \u003c/sup\u003eis produced in the cathode and consumed in the anode. \u003cstrong\u003ei.\u003c/strong\u003e The build ΔpH section is directly after the gas lines switch in the cell. Redox reactions take place close to the membrane with the cathode near 100% state of charge and the anode near 0% state of charge. The pH at the back of the cathode electrode is close to the pH of the anode in the previous cycle. \u003cstrong\u003ej.\u003c/strong\u003e The maldistributed ΔpH section is at the end of a charge/discharge step. This occurs because of the uneven mass loading of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/NiOOH when making the electrode and uneven distribution of resistance through the electrode. Near the end of the charge/discharge step, low loading and low resistance volumes of the cathode will have fully been consumed, no longer producing OH\u003csup\u003e-\u003c/sup\u003e. Surrounding higher loading and higher resistance volumes continue producing OH\u003csup\u003e-\u003c/sup\u003e but the average pH across the electrode drops lowering the average driving force for capturing CO\u003csub\u003e2\u003c/sub\u003e. The anode in this section is still charging and has not reached 100% state of charge because of competing OER. \u003cstrong\u003ek.\u003c/strong\u003e The rebuild ΔpH section is the beginning of the regeneration after the cathode has reached 0% state of charge. The cathode reaction coordinate is brought to the front towards the membrane where catalytic ORR reactions are predominant. The full area of the electrode is used again, and the average pH increases to have a higher rate of CO\u003csub\u003e2\u003c/sub\u003e capture. The anode continues to charge towards 100% for the next cycle.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-5627423/v1/ed7fc69fa8d4087f6ac0154c.png"},{"id":73463244,"identity":"41ed36a9-72f7-4a25-9b53-fcd4e49106d6","added_by":"auto","created_at":"2025-01-10 08:12:18","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":177234,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDurability on manufacturable 25cm\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e electrodes and performance on pilot stack. a-c. \u003c/strong\u003eThree segments of a \u0026gt;5000 hour durability test using a manufacturable 25cm\u003csup\u003e2\u003c/sup\u003e cell, operated using ambient air held at 32 °C and 82-87 % RH. Missing cycles in segment 2 and 3 contained software errors and were omitted. \u003cstrong\u003ea.\u003c/strong\u003e Energy cost across three segments averaged 0.67, 0.53, 0.46 MWh·ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e for the respective segments. \u003cstrong\u003eb.\u003c/strong\u003e Flux averaged 48, 60, 62 kg\u003csub\u003eCO2\u003c/sub\u003e·m\u003csup\u003e-2\u003c/sup\u003e·yr\u003csup\u003e-1 \u003c/sup\u003efor the respective segments. \u003cstrong\u003ec.\u003c/strong\u003e Cycle duration averaged 1.4, 2.3, 2.4 hours\u003csup\u003e \u003c/sup\u003efor the respective segments. Increased capacity is associated with corrosion process between the first and second segments. Flux and energy cost improved over time as the capacity increased. \u003cstrong\u003ed and e.\u003c/strong\u003e Performance data of a 9 x 300-cm\u003csup\u003e2\u003c/sup\u003e cell stack operated in ambient conditions between 20-25 °C and 75-90% RH with \u003cstrong\u003ed.\u003c/strong\u003e average energy cost of 0.83 MWh·ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e and \u003cstrong\u003ee.\u003c/strong\u003e average flux of 75 kg\u003csub\u003eCO2\u003c/sub\u003e·m\u003csup\u003e-2\u003c/sup\u003e·yr\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-5627423/v1/82fe07eb9fcac28e64db1de7.png"},{"id":73463246,"identity":"90ef76bc-d739-4e9f-9c40-b6df35d6669c","added_by":"auto","created_at":"2025-01-10 08:12:18","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":35709,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eLevelized cost projection for Ni(OH)2 based HEMCC\u003c/strong\u003e The Gen-0 pilot scale is in production with a calculated cost of 566 $·ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e. To get from Gen-0 to Gen-1, research developments in the HEMCC device are expected to follow learning curves like solar and battery industries to reduce cost. Advancements from Gen-1 to Gen-2 are expected to mostly derive from improvements in manufacturability and energy consumption. From Gen-2 to Gen-3 First-Of-A-Kind (FOAK) economies of scale will provide the greatest benefit. To achieve Gen-3 Nth-Of-A-Kind systems, centralized control rooms, digital twins, and AI-assisted maintenance are expected to drive the final costs.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-5627423/v1/8264b8b5e864d10229270d64.png"},{"id":73464267,"identity":"157b40b0-4553-498a-9d8b-db923d0dda28","added_by":"auto","created_at":"2025-01-10 08:28:19","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2202644,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5627423/v1/af69cbd2-5f4b-4392-a02f-ae76a6b3dd43.pdf"},{"id":73463248,"identity":"deb9195d-2b43-4d7f-95be-e34005be31c1","added_by":"auto","created_at":"2025-01-10 08:12:19","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3702058,"visible":true,"origin":"","legend":"SI Hydroxide Exchange Membrane Carbn Capture (HEMCC) using symmetrical nickel hydroxide batteries","description":"","filename":"SIHydroxideExchangeMembraneCarbnCaptureHEMCCusingsymmetricalnickelhydroxidebatteries.docx","url":"https://assets-eu.researchsquare.com/files/rs-5627423/v1/5c17ba661108d6d5b91d68a6.docx"}],"financialInterests":"\u003cb\u003eYes\u003c/b\u003e there is potential Competing Interest.\nY.Y. is the Founder and CEO and P.B.S. is a Co-Founder for Versogen, the producer of Piperion®. Y.Y. and B.A. are Co-Founders for RepAir the scale-up company.","formattedTitle":"Hydroxide exchange membrane carbon capture using a nickel hydroxide symmetric battery cell","fulltext":[{"header":"Introduction","content":"\u003cp\u003eDirect air capture (DAC) has been identified as one of the key net negative carbon technologies to achieve a net zero future. \u003csup\u003e \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e \u003c/sup\u003e DAC is required to offset continued emissions from dilute CO\u003csub\u003e2\u003c/sub\u003e sources e.g., agriculture and construction. \u003csup\u003e \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e \u003c/sup\u003e The majority of current DAC technologies at scale (\u0026gt;\u0026thinsp;1 kton\u003csub\u003eCO2\u003c/sub\u003e∙yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) are sorbent based with significant energy cost. \u003csup\u003e \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e \u003c/sup\u003e The energy cost is the major operating expense and is primarily driven by the temperature swing required to regenerate the sorbent. \u003csup\u003e \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e \u003c/sup\u003e The United States Department of Energy (DOE) has set forth a cost target of 100 \u003cspan\u003e$\u003c/span\u003e\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e \u003csup\u003e\u0026minus;1\u003c/sup\u003e which is not possible to meet with current industrial electricity costs near 70 \u003cspan\u003e$\u003c/span\u003e\u0026middot;MWh\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and current temperature swing adsorption devices energy costs above 1.5 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e \u003csup\u003e\u0026minus;1\u003c/sup\u003e. \u003csup\u003e \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e \u003c/sup\u003e A combination of lower electricity costs and lower DAC energy costs are required to meet the target DAC cost.\u003c/p\u003e \u003cp\u003eElectrochemical pH gradient devices are a growing research area for low energy cost carbon capture. \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan additionalcitationids=\"CR15 CR16 CR17 CR18\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e The pH gradient is built by generating hydroxide (OH\u003csup\u003e\u0026minus;\u003c/sup\u003e) at the cathode and consuming OH\u003csup\u003e\u0026minus;\u003c/sup\u003e at the anode. An acid-base equilibrium with CO\u003csub\u003e2\u003c/sub\u003e allows for the capture of CO\u003csub\u003e2\u003c/sub\u003e at the cathode and release at the anode. \u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e This compliments other electrochemical CO\u003csub\u003e2\u003c/sub\u003e capture devices based on p\u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e gradients of an electrochemically active species allowing for the capture and release of CO\u003csub\u003e2\u003c/sub\u003e. \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThis work explores a hydroxide exchange membrane carbon capture (HEMCC) device with symmetric Ni(OH)\u003csub\u003e2\u003c/sub\u003e electrodes to produce the pH gradient for CO\u003csub\u003e2\u003c/sub\u003e capture and release (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). At the cathode NiOOH is reduced to Ni(OH)\u003csub\u003e2\u003c/sub\u003e while at the anode Ni(OH)\u003csub\u003e2\u003c/sub\u003e is oxidized to NiOOH. This redox chemistry is known for its reversibility and cycle durability and has been commercially used in the cathode of alkaline Ni-MH battery technologies. \u003csup\u003e\u003cspan additionalcitationids=\"CR24\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e This symmetric cell has a thermodynamic equilibrium voltage of zero. Most of the voltage observed is to produce the pH gradient with the remainder driving the polarization of the electrodes. There is a resistance component as well, but this is small due to the low current densities used in the device, typically\u0026thinsp;\u0026lt;\u0026thinsp;5 mA\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eScaleup is necessary for DAC technologies with several orders of magnitude growth required to meet net zero targets. \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e This HEMCC device was first developed at a 25 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e laboratory scale able to capture 200 g\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). It was built with electrodes formed by electrochemical precipitation of nickel hydroxide on nickel foam. A manufacturable version of the 25 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e electrode was developed and durability tested for 5000 hours. This durable electrode was scaled to 300 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and integrated into a 9-cell stack for a pilot DAC system (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). The first commercial modular DAC system is under construction for operation in 2026 with a capacity target of 1 kton\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the data of a typical laboratory 25 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cell. Figures\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea,b are time series of the operating cell voltage and cathode outlet CO\u003csub\u003e2\u003c/sub\u003e concentration with an initial conditioning cycle followed by ten regular cycles, all at 2 mA\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. Within each regular cycle there are two steps, one for the charge/discharge and the other for the regeneration. CO\u003csub\u003e2\u003c/sub\u003e is captured in both steps, but at different voltages. The cell voltage and the difference between the 400 ppm cathode inlet and cathode outlet CO\u003csub\u003e2\u003c/sub\u003e allow for calculation of key process parameters of capacity, electron efficiency, energy cost, and flux. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec-f presents these parameters excluding the regeneration steps. Capacity tracks long-term changes in the device. Electron efficiency is defined here as the ratio of CO\u003csub\u003e2\u003c/sub\u003e molecules captured per electron passed in the cell with a maximum stoichiometric ratio of 1 CO\u003csub\u003e2\u003c/sub\u003e per electron. Energy cost is the largest operating cost of the device while the flux determines the size and thus capital cost of the system. An average of 1.15 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e energy cost including the regeneration step was achieved in this single cell dataset with a flux of 78 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Each cycle was integrated using the definitions in SI Discussion 5 and the mean was taken excluding the conditioning cycles and assuming all CO\u003csub\u003e2\u003c/sub\u003e that is captured on the cathode is released in the anode to a product stream. The HEMCC has a lower energy cost than the incumbent technology presented by Carbon Engineering of 1.47 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e for their device when excluding the blower and compressor. \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e \u003cb\u003e| Laboratory scale 25 cm\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/b\u003e\u003c/sup\u003e \u003cb\u003eHEMCC and its behavior.\u003c/b\u003e The HEMCC is comprised of two identical electrochemically precipitated Ni(OH)2 electrodes and an 80\u0026micro;m Piperion\u0026reg; membrane. The cathode air flow was 1 L∙min\u003csup\u003e-1\u003c/sup\u003e with 400 ppm CO\u003csub\u003e2\u003c/sub\u003e and 90% RH. The anode received a sweep gas of 90% RH N\u003csub\u003e2\u003c/sub\u003e at 1 L∙min\u003csup\u003e-1\u003c/sup\u003e. \u003cb\u003ea.\u003c/b\u003e The cell voltage (V) and \u003cb\u003eb.\u003c/b\u003e the cathode outlet CO\u003csub\u003e2\u003c/sub\u003e concentration (ppm) are used to calculate the key process variables \u003cb\u003ec-f\u003c/b\u003e during the charge/discharge step\u003c/strong\u003e \u003cp\u003e \u003cb\u003ec.\u003c/b\u003e capacity, \u003cb\u003ed.\u003c/b\u003e electron efficiency, \u003cb\u003ee.\u003c/b\u003e energy, and \u003cb\u003ef.\u003c/b\u003e flux, all excluding the regeneration step. Regeneration steps are excluded to highlight the low energy cost of the charge-discharge step. Capture still occurs during regeneration but at higher energy cost. The first cycle on each side is a 1.5-hour regeneration hold for break-in followed by 10\u0026ndash;75 minute cycles on each electrode. The cycle length is chosen based on an ex-situ capacity test to minimize regeneration \u003cb\u003e(SI Discussion 7\u003c/b\u003e). \u003cb\u003ec.\u003c/b\u003e Capacity achieved an average of 1.89 mAh\u0026middot;cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e which was 85% of the ex-situ capacity tests. \u003cb\u003ed.\u003c/b\u003e Electron efficiency achieved 0.25. \u003cb\u003ee.\u003c/b\u003e Energy cost averaged 0.76 MWh∙ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e-1\u003c/sup\u003e while \u003cb\u003ef.\u003c/b\u003e flux averaged to 72 kg\u003csub\u003eCO2\u003c/sub\u003e∙m\u003csup\u003e2\u003c/sup\u003e∙yr\u003csup\u003e-1\u003c/sup\u003e. \u003cb\u003eg.\u003c/b\u003e and \u003cb\u003eh.\u003c/b\u003e show a single cycle to highlight the change in voltage and capture in the charge/discharge and regeneration steps. \u003cb\u003ei., j., and k.\u003c/b\u003e are graphic aids to describe the build ΔpH, maldistributed ΔpH, and rebuild ΔpH sections in \u003cb\u003eg.\u003c/b\u003e and \u003cb\u003eh.\u003c/b\u003e The ΔpH between the peak pH at the cathode and the CO\u003csub\u003e2\u003c/sub\u003e release pH at the anode drives the CO\u003csub\u003e2\u003c/sub\u003e capture rate in the cell, see \u003cb\u003eSI Discussion 2.\u003c/b\u003e The CO\u003csub\u003e2\u003c/sub\u003e release pH is the pH that achieves equilibrium between CO\u003csub\u003e2\u003c/sub\u003e in the anode and the concentrated CO\u003csub\u003e2\u003c/sub\u003e gas product stream. The dotted red primary reaction coordinate lines are rough position where OH\u003csup\u003e-\u003c/sup\u003e is produced in the cathode and consumed in the anode. \u003cb\u003ei.\u003c/b\u003e The build ΔpH section is directly after the gas lines switch in the cell. Redox reactions take place close to the membrane with the cathode near 100% state of charge and the anode near 0% state of charge. The pH at the back of the cathode electrode is close to the pH of the anode in the previous cycle. \u003cb\u003ej.\u003c/b\u003e The maldistributed ΔpH section is at the end of a charge/discharge step. This occurs because of the uneven mass loading of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/NiOOH when making the electrode and uneven distribution of resistance through the electrode. Near the end of the charge/discharge step, low loading and low resistance volumes of the cathode will have fully been consumed, no longer producing OH\u003csup\u003e-\u003c/sup\u003e. Surrounding higher loading and higher resistance volumes continue producing OH\u003csup\u003e-\u003c/sup\u003e but the average pH across the electrode drops lowering the average driving force for capturing CO\u003csub\u003e2\u003c/sub\u003e. The anode in this section is still charging and has not reached 100% state of charge because of competing OER. \u003cb\u003ek.\u003c/b\u003e The rebuild ΔpH section is the beginning of the regeneration after the cathode has reached 0% state of charge. The cathode reaction coordinate is brought to the front towards the membrane where catalytic ORR reactions are predominant. The full area of the electrode is used again, and the average pH increases to have a higher rate of CO\u003csub\u003e2\u003c/sub\u003e capture. The anode continues to charge towards 100% for the next cycle.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRelatively low energy cost is the main benefit of this electrochemical carbon capture approach. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee shows several cycles of CO\u003csub\u003e2\u003c/sub\u003e capture at an average of 1.15 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e; when broken down by the steps the charge/discharge and regeneration steps averaged 0.76 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e and 2.15 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e1\u003c/sup\u003e respectively. Both steps can be improved by improving the electron efficiency of the device. \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e Electron efficiency of 1 indicates ionic transport of only HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in the cell where 0.5 would be predominantly CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, below 0.5 as observed in this cell has either unreacted OH\u003csup\u003e\u0026minus;\u003c/sup\u003e being transported or back diffusion of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e or CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e from the anode. Future work can improve electron efficiency by adjusting membrane parameters such as ionic exchange capacity (IEC), conductivity, and water uptake. Improved electron efficiency at the same current density directly increases flux of CO\u003csub\u003e2\u003c/sub\u003e which decreases energy and capital costs.\u003c/p\u003e \u003cp\u003eThe CO\u003csub\u003e2\u003c/sub\u003e capture rate changes with time and the state of charge of the electrode. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh shows the single half cycle behavior of CO\u003csub\u003e2\u003c/sub\u003e capture. At the beginning of the charge/discharge step there is time lag as the pH gradient is reversed from the previous cycle. A steady increase in capture rate is seen as the pH gradient is formed, which takes time proportional to the total IEC of the membrane and ionomer. Piperion\u0026reg; is used for the membrane and ionomer based on PAP-TP-85 with an IEC of 2.37 meq.\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e The state of charge determines the CO\u003csub\u003e2\u003c/sub\u003e capture behavior for the remainder of the charge/discharge step. The surface of the electrode near the membrane discharges first due to the difference in conductivity between the metal, metal hydroxide, and ionomer phases of the electrode. \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e The primary reaction coordinate, and thus the peak pH, moves from the membrane to the back of the electrode as the state of charge changes in each electrode. At a moderate state of charge, higher capture rates are expected because CO\u003csub\u003e2\u003c/sub\u003e diffuses into and reacts with a larger volume of high pH ionomer. A similar behavior was seen in a hydrogen HEMCC previously studied by using an interlayer volume. \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e Near the end of the cycle volumes of anode with lower mass loading of NiOOH/Ni(OH)\u003csub\u003e2\u003c/sub\u003e or lower local resistance will run out of NiOOH earlier than the surrounding volumes and no longer produce OH\u003csup\u003e\u0026minus;\u003c/sup\u003e; these volumes no longer contribute to the pH gradient and lower the average pH at the primary reaction coordinate. This manifests in a dip in CO\u003csub\u003e2\u003c/sub\u003e capture near the end of the charge/discharge step. During the charge/discharge step the flux averages 72 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026minus;\u0026thinsp;1\u003c/sup\u003e, while the average peak flux is 97 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe capture rates during regeneration do not depend on the state of charge of the cathode which has been depleted. The pH gradient is reestablished by a relatively thin layer of the electrode near the membrane performing catalytic ORR. \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e There is a short time dependent growth of capture at the beginning of regeneration to reestablish the pH gradient across the area of the electrode after the decrease observed in the charge/discharge step. After that a steady state constant capture rate is achievable. During the regeneration step the flux averages 93 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026minus;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026minus;1\u003c/sup\u003e, while the average peak flux is 97 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTechnical pathways to improve the device include improving mass transport (diffusion) from the flow field channels into and within the porous electrodes, decreasing the effect of time lag for CO\u003csub\u003e2\u003c/sub\u003e and decreasing the requirement for regeneration. Decreasing the effect of time lag can improve both the flux and energy cost of the cycle. There are two ways to reduce the time lag, one by decreasing the total ion capacity of membrane and electrode or the other by increasing the electrode capacity. Either option has a trade-off to optimize. Decreasing the total ion capacity by decreasing membrane thickness and ionomer amount decreases the number of ions required to displace at the beginning of each cycle. This cannot be decreased to zero because there needs to be sufficient ionic conductivity and mechanical stability. Additionally, thinner membranes can lead to increased bicarbonate back diffusion offsetting gains in time lag. \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e Increasing the capacity of an electrode lengthens the time for a full cycle and thus a higher percentage of time is spent in a high flux region of a cycle. Increasing the electrode capacity can increase the mass transport resistance for diffusion of CO\u003csub\u003e2\u003c/sub\u003e into the electrode by decreasing the porosity of the electrode or increasing the diffusive path length. The laboratory prepared electrodes nominally have a porosity of 30\u0026ndash;40%. Optimization of this porosity can lead to flux and energy cost gains.\u003c/p\u003e \u003cp\u003eDecreasing the amount of time required for regeneration can also decrease the overall energy cost of the system. In Ni-MH batteries the amount of OER that occurs depends on the C-rate used and the dopants present. \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e The C-rate is defined as a ratio of the current density in mA to the capacity of the electrode in mAh. The electrodes in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e ran at a C-rate of 0.85. Lower C-rates allow for lower voltage operation of the battery limiting the catalytic OER which has a high kinetic overpotential. Dopants can either improve the kinetics of the Ni(OH)\u003csub\u003e2\u003c/sub\u003e oxidation or hinder the kinetics of the OER reaction. Dopants such as cobalt have shown the ability to limit OER and can be added to limit need for regeneration. \u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e Leaning on nickel hydride battery research to limit regeneration requirement to 10% of a cycle length could decrease the average full cycle energy cost to 0.90 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e with this laboratory scale system.\u003c/p\u003e \u003cp\u003eThe electrodes shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e were electrochemically precipitated; this method was chosen for tunability of the Ni(OH)\u003csub\u003e2\u003c/sub\u003e loading early in research. There are several other methods to produce Ni(OH)\u003csub\u003e2\u003c/sub\u003e including electrodeposition and bulk precipitation. \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e Improvements were made to the laboratory scale electrode to produce large manufacturable electrodes for pilot scale operation. Such 25 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e electrodes were tested for 5000 hours with the cell supplied with ambient air held at 32\u0026deg;C and 82\u0026ndash;87% RH. Three segments of the corresponding data are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The end of the 5000-hour run highlighted a low energy cost near 0.46 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e with a flux of 62 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe more manufacturable 25 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cell shows an increase in cycle length or capacity over time (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This is likely the result of a self-limiting corrosion process in the electrode. The nickel foam used in the electrode as a scaffold for Ni(OH)\u003csub\u003e2\u003c/sub\u003e can be oxidized to Ni(OH)\u003csub\u003e2\u003c/sub\u003e at potentials above 0.11 V vs reversible hydrogen electrode (RHE). \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e Any exposed bare nickel can go through the oxidation process. The corrosion stops when accessible nickel metal is fully converted. An increase in capacity is seen between the first and second segments, but not between the second and third segments suggesting the complete conversion of the available nickel sites. This phenomenon was also observed at the laboratory scale and is shown in SI Discussion 4. An increase of the average flux is also seen, contributing to a lower energy cost. This corroborates the concept that reducing the effect of time lag with a larger capacity electrode will lead to higher average flux and lower average energy cost.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis manufacturable electrode was scaled up from 25 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e to 300 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and used in a 9 x 300-cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e stack (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed,f). This stack averaged an energy cost of 0.83 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e with a flux of 75 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. At the system level a blower is required to push air through the DAC device and 300 Pa is considered an upper limit for DAC. \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e The laboratory scale pressure drop suffers from triple serpentine flow fields adapted from bench top fuel cells with a pressure drop of 2800 Pa for a 1 L\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e flow rate as discussed in SI Discussion 3. The scaled system exhibits a total pressure drop of less than 300 Pa (10x lower in comparison to the lab cells) while maintaining similar performance metrics. Such low pressure drops range is obtained by an innovative design of the stack hardware (inlets/outlets and flow field morphology) - needed to allow energy efficient Air Streaming Unit (ASU, e.g., axial fan) and an overall energy-efficient DAC process.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows a levelized cost reduction pathway to 92 \u003cspan\u003e$\u003c/span\u003e\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e \u003csup\u003e\u0026minus;1\u003c/sup\u003e in line with the United States DOE targets. Durability at the 25 cm\u003csup\u003e \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e \u003c/sup\u003e size and performance at the 9-cell stack size lays the groundwork for the 1000 ton\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;yr \u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e Gen-0 pilot scale. The Gen-1 system will follow research pathways discussed to decrease time lag and regeneration as well as increasing CO\u003csub\u003e2\u003c/sub\u003e flux. Gen-1 will additionally focus on the full system requirements including a blower and compressor. Improved airflow design can lower pressure drop to decrease blower costs. \u003csup\u003e \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e \u003c/sup\u003e Electrochemical compression is to be explored as shown in proton exchange membrane electrolyzers (PEMEL) for hydrogen production with a cost benefit compared with conventional compression alone. \u003csup\u003e \u003cspan additionalcitationids=\"CR37\" citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e \u003c/sup\u003e Module standardization for assembly line production and expected improvement in energy consumption are the main drivers for the Gen-2 system. \u003csup\u003e \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e \u003c/sup\u003e Further improvement of the CO\u003csub\u003e2\u003c/sub\u003e flux and economies of scale will enable Gen-3 First-Of-A-Kind (FOAK) megaton systems. \u003csup\u003e \u003cspan additionalcitationids=\"CR41\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e \u003c/sup\u003e At this scale pricing improvements can be achieved for membrane polymers and peripheral components. \u003csup\u003e \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e \u003c/sup\u003e Finally, centralized control rooms, automation, digital twin machine learning, and AI-driven maintenance strategies are expected to drive final costs towards the Nth-Of-A-Kind (NOAK) Gen-3 systems.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe nickel hydroxide HEMCC technology shown here has a low energy cost compared to conventional temperature swing adsorption devices. Producing the pH gradient is the primary energy cost for the device, followed by the polarization of the electrodes, and resistance.\u003c/p\u003e \u003cp\u003eAt the laboratory scale the concept was developed to produce a device that has an average energy cost of 1.15 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e at an average flux of 78 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The energy cost and flux can be improved by decreasing the time lag for pH gradient formation and minimizing parasitic OER reactions. The time lag can be decreased by increasing electrode capacity which aids in increasing the average flux while decreasing the average energy cost. Anodic OER reactions can be limited by employing traditional Ni-MH battery dopants. By limiting OER, the energy cost can be reduced by reducing the required amount of regeneration.\u003c/p\u003e \u003cp\u003eScale up of the nickel hydroxide HEMCC has started where the original concept was expanded from a 25 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cell to a 9-cell stack of 300 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cells. A pilot module containing these stacks is in operation capturing CO\u003csub\u003e2\u003c/sub\u003e from ambient air. A 5000-hour durability test has shown stability of the electrode design with an end-of-test energy cost of 0.46 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e at a flux of 62 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This electrode design was shown to be able to scale into a stack with a low pressure drop of 300 Pa meeting technical requirements for DAC.\u003c/p\u003e \u003cp\u003eThe prototype is being iterated to drive down cost along learning curves like those seen in solar photo voltaic cells and lithium-ion batteries. Device level research is expected to achieve many of the early-stage improvements, while manufacturability and economies of scale will drive costs down below the 100 \u003cspan\u003e$\u003c/span\u003e\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e target set forth for DAC.\u003c/p\u003e "},{"header":"Methods","content":"\u003ch3\u003eElectrode preparation\u003c/h3\u003e\n\u003cp\u003eLab scale electrode preparation was done by electrochemical precipitation of nickel hydroxide onto nickel foam.The nickel foam was from MTI corporation (product name EQ-bcnf-16m) with 99.9% purity, 95% porosity, 80\u0026ndash;110 pores per inch and thickness of 1.6mm. Three 25cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e nickel foam substrates were cut retaining a 1 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e tab for connecting with an alligator clip. The three substrates were compressed in a hydraulic press to 512 \u0026micro;m by using a 512 \u0026micro;m thick steel plate as a guide barrier. These were weighed and then placed in prepared 1 M sulfuric acid solution for 1 hour to remove the surface oxide layer. The oxide layer removed is assumed to be negligible in mass for purposes of measuring amount of mass added to the working electrode.\u003c/p\u003e \u003cp\u003eThese substrates were set parallel in a 0.05 M nickel chloride solution using 98% anhydrous nickel chloride salt from Millipore Sigma (CAS No: 7718-54-9). NiCl\u003csub\u003e2\u003c/sub\u003e hexahydrate (Sigma-Aldrich, ReagentPlus) was also tested without any noticeable performance difference. The substrates were suspended in place with copper electrode clips as shown in \u003cb\u003eSI Fig.\u0026nbsp;9\u003c/b\u003e. The outer electrodes were connected via wire to be at the same voltage as the counter electrode, the middle electrode was the working electrode for Ni(OH)\u003csub\u003e2\u003c/sub\u003e to be precipitated onto. Two outer electrodes were used to evenly grow Ni(OH)\u003csub\u003e2\u003c/sub\u003e on both sides of the central working electrode. The distance between electrodes were approximately controlled to 1 cm.\u003c/p\u003e \u003cp\u003eA reference Ag/AgCl electrode from Pine Research (RREF0021) tracked voltage in the cell during electrochemical precipitation. A current density of -5 mA\u0026middot;cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e for 4 hours was applied to the working electrode. Afterwards, the electrodes were rinsed with DI water and set out to dry overnight. The next day the weight of the working electrode was measured to determine the mass of deposition. As a control, the counter electrodes were also weighed to measure their loss of Ni. The loss of Ni of the counter electrodes closely aligns with the oxidation to Ni\u003csup\u003e2+\u003c/sup\u003e in the solution as expected this balances the loss of Ni\u003csup\u003e2+\u003c/sup\u003e during precipitation and maintains a relatively stable bulk Ni\u003csup\u003e2+\u003c/sup\u003e concentration. The pair of counter electrodes lost 0.57 g Ni (27% of initial mass) during the process while the working electrode gained 0.83 g during the process.\u003c/p\u003e \u003cp\u003eAfter electrochemical precipitation the electrodes were spray coated with Piperion\u0026reg; ionomer. A solution of 1.3333g of 5% ionomer in ethanol solution was added to 6.2222g IPA and sonicated for 30 min. This solution was sprayed on one side of the electrode and then set out to dry for 1 day. Weighing the next days showed that 25% of the initial ionomer mass was transferred to the electrode during the spraying.\u003c/p\u003e\n\u003ch3\u003eEx-situ three electrode cycling\u003c/h3\u003e\n\u003cp\u003eThe electrode with ionomer was then placed in a 1 M KOH bath in the same set up as the corrosion test in \u003cb\u003eSI Fig.\u0026nbsp;7,8\u003c/b\u003e. Three series of current densities were used to cycle each electrode, 1-cycle at 2 mA\u0026middot;cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, 30-cycles at 5 mA\u0026middot;cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, and 5-cycles at 2 mA\u0026middot;cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. A break in is conducted at 2 mA\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. First the electrode is charged from Ni(OH)\u003csub\u003e2\u003c/sub\u003e to NiOOH for three hours finishing with a plateau at OER voltage. Then the electrode is discharged back to Ni(OH)\u003csub\u003e2\u003c/sub\u003e to completion with a cutoff voltage of 1.1 V vs RHE. This break in charge is longer than the final 5-cycles at 2 mA\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e to allow for impurities in the deposited Ni(OH)\u003csub\u003e2\u003c/sub\u003e structure to escape without physically damaging the deposited layer.\u003c/p\u003e \u003cp\u003eNext, 30 cycles at 5 mA\u0026middot;cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e are conducted to stabilized the Ni(OH)\u003csub\u003e2\u003c/sub\u003e layer. It was found that 30 cycles were required to reach a plateau in capacity for an electrode; the current density 5 mA\u0026middot;cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e was chosen to speed up the process. The final 5 cycles were conducted at 2 mA\u0026middot;cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e to compare with the 2 mA\u0026middot;cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e used during the direct air capture tests.\u003c/p\u003e\n\u003ch3\u003eMembrane Electrode Assembly\u003c/h3\u003e\n\u003cp\u003eThe final assembly of the laboratory scale 25cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cell uses a Scribner 25 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cell with triple serpentine flow fields. (see \u003cb\u003eSI Fig.\u0026nbsp;11\u003c/b\u003e) A 2.5\u0026rdquo;x3\u0026rdquo; piece of 80\u0026micro;m Piperion\u0026reg; membrane was used between two electrodes. Two electrochemically precipitated nickel hydroxide electrodes were cut to fit precisely to their respective gaskets. A total of 336\u0026micro;m in FEP gaskets were used on each side for the 512\u0026micro;m thick electrodes. This leads to a 0.39 compression ratio once the cell bolts are torqued to 100 PSI.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work is supported by the University of Delaware, U.S. Department of Energy National Energy Technology Laboratory (DE-FE0031955), and U.S. Department of Defense Army Research Laboratory (W911NF-23-2-0028). Versogen provided Piperion\u0026reg; membrane and ionomer for this research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors and Affiliations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThese authors contributed equally: R. James Buchen \u0026amp; Teng Wang\u003c/p\u003e\n\u003cp\u003eDepartment of Chemical and Biomolecular Engineering at University of Delaware\u003c/p\u003e\n\u003cp\u003eR. James Buchen, Teng Wang, K. Braden Geiger, P. Brian Setzler, \u0026amp; Yushan Yan\u003c/p\u003e\n\u003cp\u003eRepAir\u003c/p\u003e\n\u003cp\u003eBen Achrai, Y. Naama Gluz, Maurice Artoul, \u0026amp; Jean-Phillippe Hiegel\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eY.Y. and P.B.S. supervised the project. P.B.S. provided the initial concept and guidance on fundamentals of electrochemical pH driven CO\u003csub\u003e2\u003c/sub\u003e capture. R.J.B. designed the test station and laboratory scale electrodes. R.J.B. and T.W. developed testing protocol and evaluation of the direct air capture system. R.B.G. conducted electrochemical tests and related data processing. T.W. carried out materials characterization. B.A. led the development and scale up efforts of the main technological aspects including durable electrodes, stack hardware design and the accommodating system for operating the DAC process. Y.N.G led the MEA optimization efforts and M.A conducted the DAC testing J-P.H. developed the TEA analysis. R.J.B. wrote the manuscript, with input from all authors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding author\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCorrespondence to Yushan Yan [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eY.Y. is the Founder and CEO and P.B.S. is a Co-Founder for Versogen, the producer of Piperion\u0026reg;. Y.Y. and B.A. are Co-Founders for RepAir the scale-up company.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eShi, L. \u003cem\u003eet al.\u003c/em\u003e A shorted membrane electrochemical cell powered by hydrogen to remove CO2 from the air feed of hydroxide exchange membrane fuel cells. Nature Energy 7, 238\u0026ndash;247 (2022). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s41560-021-00969-5\u003c/span\u003e\u003cspan address=\"10.1038/s41560-021-00969-5\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBell, W. L. Synthesis and Evaluation of Electroactive CO₂ Carriers. SAE International 97, 544\u0026ndash;552 (1988).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRheinhardt, J. H., Singh, P., Tarakeshwar, P. \u0026amp; Buttry, D. A. 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The Electrochemical Society Interface 14, 24\u0026ndash;35 (2005). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1149/2.F05053if\u003c/span\u003e\u003cspan address=\"10.1149/2.F05053if\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5627423/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5627423/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eElectrochemical carbon capture devices can be a low energy cost solution for direct air capture (DAC) using renewable electricity. Historically electrochemical carbon-capture has targeted a range of concentrations from atmospheric (400 ppm\u003csub\u003eCO2\u003c/sub\u003e) (DAC), \u003csup\u003e \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e \u003c/sup\u003e to life-support (5000 ppm\u003csub\u003eCO2\u003c/sub\u003e), \u003csup\u003e \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e \u003c/sup\u003e to point-source capture (10% CO\u003csub\u003e2\u003c/sub\u003e). \u003csup\u003e \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e \u003c/sup\u003e The hydroxide exchange membrane nickel hydroxide symmetric battery cell with two identical electrodes has low voltage requirements making it more suitable for DAC than other electrochemical approaches. A 25 cm\u003csup\u003e \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e \u003c/sup\u003e laboratory cell shows an average energy cost of 1.15 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e \u003csup\u003e\u0026minus;1\u003c/sup\u003e and a CO\u003csub\u003e2\u003c/sub\u003e flux of 78 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 2 mA\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. A manufacturable 25 cm\u003csup\u003e \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e \u003c/sup\u003e cell is durability tested for 5000 hours and achieves an energy of 0.46 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e \u003csup\u003e\u0026minus;1\u003c/sup\u003e and a flux of 62 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the end of the test. A DAC pilot system with a stack of 9 scaled-up 300 cm\u003csup\u003e \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e \u003c/sup\u003e cells demonstrates an energy of 0.83 MWh\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e \u003csup\u003e\u0026minus;1\u003c/sup\u003e and a flux of 75 kg\u003csub\u003eCO2\u003c/sub\u003e\u0026middot;m\u003csup\u003e2\u003c/sup\u003e\u0026middot;yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and meets the 300 Pa pressure drop required for DAC. \u003csup\u003e \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e \u003c/sup\u003e Modular design projects improvements in cost from manufacturing and economies of scale, and a pathway to below 100 \u003cspan\u003e$\u003c/span\u003e\u0026middot;ton\u003csub\u003eCO2\u003c/sub\u003e \u003csup\u003e\u0026minus;1\u003c/sup\u003e from learning rates of past energy technologies. \u003csup\u003e \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e \u003c/sup\u003e \u003c/p\u003e","manuscriptTitle":"Hydroxide exchange membrane carbon capture using a nickel hydroxide symmetric battery cell","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-01-10 08:12:14","doi":"10.21203/rs.3.rs-5627423/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-energy","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nenergy","sideBox":"Learn more about [Nature Energy](http://www.nature.com/nenergy/)","snPcode":"","submissionUrl":"","title":"Nature Energy","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"26cb20aa-7a78-4ec2-b2ca-c2d23a203341","owner":[],"postedDate":"January 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":41723488,"name":"Physical sciences/Energy science and technology/Carbon capture and storage"},{"id":41723489,"name":"Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation"},{"id":41723490,"name":"Physical sciences/Engineering/Chemical engineering"},{"id":41723491,"name":"Physical sciences/Chemistry/Electrochemistry"}],"tags":[],"updatedAt":"2026-03-19T21:35:21+00:00","versionOfRecord":[],"versionCreatedAt":"2025-01-10 08:12:14","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5627423","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5627423","identity":"rs-5627423","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}