Hollow-fiber gas penetration electrodes efficiently produce renewable synthetic fuels

Graphical abstract

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Increasing CO₂ levels in the atmosphere due to the consumption of large amounts of fossil fuels is exacerbating global climate change, which seriously threatens sustainable human development. As a responsible and respectable country, China has announced remarkable goals of reaching a carbon emission peak and achieving carbon neutrality, which are challenging missions with enormous contributions to the world. As a strategic support, disruptive and transformative energy technologies are urgently required to develop new green and low-carbon energy industries. Renewable energy sources such as wind and sun, which can generate zero-carbon electric power, have been rapidly developed in recent years. In China, the cumulatively installed capacity of renewable clean energy power generation had reached 1.06 billion kW by the end of 2021, accounting for nearly 44.8% of the total installed capacity [1]. However, owing to the intrinsic inferiorities of instability and anti-peak-load regulation, the power from these renewable energy sources cannot be integrated well into the electric grid, resulting in substantial waste and further development restrictions [2].

Employing intermittent renewable electricity to power the reduction of CO₂ to fuels is a new transformative energy and negative carbon technology that cannot only reduce CO₂ emissions through the utilization of CO₂ as feedstock but can also store renewable energy as fuel. Powered by renewable electricity, CO₂ can be reduced to various synthetic fuels such as CO, HCOOH, CH₃OH, C₂H₅OH, and C₂H₄. These renewable synthetic fuels can be consumed for power transportation directly or to produce other fuels via the Fischer-Tropsch synthesis process to reduce our dependency on fossil fuels, and the released CO₂ can be recycled to form a carbon-neutrality cycle (Fig.1).

In contrast to thermal catalysis under harsh conditions, electrocatalytic CO₂ reduction can be performed at room temperature and atmospheric pressure with a low energy demand and facility requirement, which makes it economically viable for practical applications. More importantly, the reaction of CO₂ electroreduction under ambient conditions is highly controllable with a well-defined electroactive component, and the products vary and depend on distinct active sites with different potentials. By employing CO₂ electroreduction technology, renewable synthetic fuels, such as CO [3–5] and HCOOH [6–8], have been selectively synthesized with Faradaic efficiencies usually above 90%, and the current densities related to the conversion rate need improvement. The production of multi-carbon renewable fuels, such as ethylene, ethanol, and acetate, is much inferior in terms of selectivity and activity and is currently being improved by regulating catalysts and electrolytic systems.

Although impressive progress has been made, the low CO₂ solubility in aqueous electrolytes (34 mmol/L at 25°C and 1 atm) [9] and long mass transfer distance hinder CO₂ mass transport, resulting in a very limited current density of tens of mA/cm² [10–13], which is far below the requirement for industrial applications. One strategy for addressing this issue is to construct gas-diffusion electrodes (GDEs) by coating highly active catalysts on microporous layers decorated with superhydrophobic polytetrafluoroethylene and conductive carbon particles [14–17]. Such GDEs integrated with flow cells or membrane electrode assemblies could promote the rapid diffusion of CO₂ to the active sites, thus enabling the reaction to operate at an industrial-scale current density (≥ 200 mA/cm²) [18–22]. However, in this system, CO₂ inevitably reacts with OH⁻ to generate carbonate/bicarbonate species that can block the mass transfer channels, and GDEs must also provide sufficient hydrophobicity to prevent frequent flooding [23,24]. Furthermore, the integrated polymer binder may age and loosen during long-term electrolysis, which deteriorates the stability and performance. All of these imperfections limit the potential of GDE systems for industrial applications. Therefore, designing and constructing a new type of functional electrode configuration is essential for achieving excellent selectivity and stability for CO₂ electroreduction at an industrial-scale current density.

A three-dimensional hollow fiber porous material with a compact structure can serve as a self-supported electrode without any additives and exhibits promising potential for efficient and high-rate CO₂ electroreduction due to enhanced mass transport. The self-supported hollow fiber electrode fabricated by a facile and scalable method (Fig.2) consisted of only a single active component with an abundant surface area and tunable pore structure. The rich hierarchical porous structure of the hollow fibers provided convenient mass transfer channels for CO₂ electroreduction reactions (Fig.3(a)). With the end of the hollow fiber sealed, gaseous CO₂ can be forced to penetrate the porous channels to make good contact with the catalyst and electrolyte, promoting CO₂ electroreduction at the gas-catalyst-liquid three-phase boundary. This type of electrode configuration is termed “gas penetration electrode (GPE)” and with its greatly enhanced mass transfer ability, it displays substantial potential in CO₂ electroreduction for industrial applications.

Research on hollow-fiber GPEs for synthesizing renewable fuels has been increasing, and the huge advantages of hollow-fiber GPEs have already been reported. Kas et al. obtained CO yields at least one order of magnitude higher using Cu hollow fibers for CO₂ electroreduction compared to nanocrystalline Cu electrodes [25]. Subsequently, Chen et al. constructed freestanding Cu-Sn alloy hollow fibers for selective formate formation from CO₂ electroreduction with a Faradaic efficiency of 91% by suppressing hydrogen evolution [26]. Additionally, Yuan et al. adopted an electrodeposition method to decorate Cu hollow fibers with controlled Sn [27,28] or Bi [29] surfaces. The highly rough surfaces provide more catalytically active sites for CO₂ electroreduction, resulting in a higher formate partial current density, outperforming other Cu-based GDEs. However, these hollow fiber electrodes have not been working to their full penetrating effect and thus deliver very limited current densities (≤ 200 mA/cm²), which is insufficient to afford economically viable CO₂ electrochemical conversion.

Recently, our group fabricated Cu hollow fiber GPEs using a phase-inversion/sintering method (Fig.2), which can stably synthesize formate from CO₂ electroreduction at a high current density of 210 mA/cm² with yields of approximately 16 and 30 times those of Cu foam and Cu foil, respectively [30]. We also fabricated a hierarchical-micro/nanostructured hollow fiber electrode composed of only metallic Ag for CO₂ electroreduction, which achieved the efficient formation of CO at ampere-level current density (> 1 A/cm²) [31]. Surprisingly, the CO₂ conversion rate on this Ag hollow fiber GPE exceeded 50% at a high space velocity of 31000 mL/(g_cat∙h) under ambient conditions. The large current densities (~1.26 A/cm²) and high CO Faradaic efficiencies (~93%) remained quite stable in a continuous test for a long lifespan. Hollow fiber GPEs with an enhanced and oriented mass transfer function enable the continuous transport of CO₂ to the catalyst surface and promote the generation of a three-phase interface between gaseous CO₂, the electrode, and the electrolyte, thus achieving an ultrahigh current density, outperforming state-of-the-art hollow-fiber electrodes (Tab.1). These insightful studies confirmed that hollow-fiber GPEs exhibit compelling performance for stable CO₂ electroreduction at industrial-scale current densities.

Hollow fibers are generally fabricated using the coextrusion/coaxial spinning method with a phase inversion and sintering process [26–33] (Fig.2). The spinneret used in this method consisted of at least two coaxial tubes. The core solution, normally water, is injected into the central tube to form the hollow fiber. Meanwhile, the sample solution, including the target material, polymer, and organic solvent mixed by ball milling, is excluded from the other tube to generate the shell of the hollow fiber. The shell of the hollow fiber is solidified by inverting the organic solvent with an inorganic solution. The removal of the polymer and final sintering gives rise to a mechanically strong hollow fiber with a finger-like porous structure on the shell. Such a method can be used to fabricate hollow fibers continuously with high flux and uniform structures, making it facile and compatible with existing large-scale production processes. More importantly, the inner and outer diameters of the hollow fibers can be easily adjusted by tuning the flow rate of the core and shell solutions or the diameter of the spinneret, respectively, to optimize the three-phase interface reaction kinetics of CO₂ electroreduction. Moreover, the hollow fiber electrode configuration can be expanded from one-layered to multilayered hollow fibers and from one-component to multicomponent hollow fibers to achieve synergistic electrocatalytic CO₂ reduction, which may significantly promote the formation of multicarbon renewable synthetic fuels.

The hollow fiber configuration is suitable for large-scale applications because of its high surface/volume-specific ratio, controllable fabrication process, low cost, and suitability for industrial modules. In particular, the higher surface/volume ratio enables hollow fibers to be easily assembled and scaled up. That is, the single hollow fiber on the working electrode can be expanded to an array of multiple well-arranged hollow fibers (Fig.3(b)), making it viable to scale up CO₂ electroreduction to renewable synthetic fuels. Therefore, the electrode area can be enlarged from a few square centimeters at the laboratory level to thousands of square centimeters at the industrial level. Accordingly, the electrode arrangement and layout of the electrolytic cell must be optimized to meet the amplification requirements of the electronic, gas, and electrolyte flow paths. That is, the hollow fiber array arrangement, including the spacing, length, amount, and size of the current conductor and the electrolytic cell, must be compatible with each other, which is a great challenge. The rules of electrode amplification and electrolysis cell design are underway to construct a perfectly suited electrolyzer to overcome the most critical bottleneck in engineering amplification of electrocatalytic CO₂ reduction. In addition, metal selection for producing target renewable fuels, construction of hierarchical micro/nanostructures on hollow fibers for enhancing mass transfer, and a thorough investigation of the reaction mechanism and the kinetic process of CO₂ electroreduction should be focused upon in future research.

The electrocatalytic CO₂ reduction route for producing renewable synthetic fuels using hollow-fiber gas penetration electrodes has huge market potential. In the future, an energy system will be established using renewable energy as the main body. Hollow-fiber integrated electrolytic units powered by zero-carbon electricity are feasible for producing renewable synthetic fuels from CO₂ and water. Large-scale synthesized renewable fuels, such as syngas with controllable ratios, can be combined with the Fischer-Tropsch synthesis process to produce long-chain hydrocarbons and other fuels [34]. The released CO₂ from the consumption of these fuels can be recycled to form a negative carbon cycle. The advantages of high power output, low fabrication costs, and easy engineering amplification of hollow-fiber gas penetration electrodes make them potentially promising for industrial applications. In addition to the electrocatalytic CO₂ reduction route, hollow-fiber gas penetration electrodes that efficiently produce renewable synthetic fuels can also be achieved through the routes of photoelectrocatalytic CO₂ reduction, electrocatalytic CH₄ oxidation, photoelectrocatalytic CH₄ oxidation, and electrocatalytic N₂ reduction. These are also promising for industrial applications, even though they are still in the early stages.

References

[1] NationalEnergy Administration of China. The development of renewable energy reached a new level in 2021. 2022–01–28, available at National Energy Administration website

[2] NationalEnergy Administration of China. New energy consumption capacity. 2022–06–10, available at National Energy Administration website

[3] Won D H, Shin H, Koh J. . Highly efficient, selective, and stable CO₂ electroreduction on a hexagonal Zn catalyst. Angewandte Chemie International Edition, 2016, 55(32): 9297–9300

[4] Ma M, Trzesniewski B J, Xie J. . Selective and efficient reduction of carbon dioxide to carbon monoxide on oxide-derived nanostructured silver electrocatalysts. Angewandte Chemie International Edition, 2016, 55(33): 9748–9752

[5] Liu M, Pang Y, Zhang B. . Enhanced electrocatalytic CO₂ reduction via field-induced reagent concentration. Nature, 2016, 537(7620): 382–386

[6] Gao S, Jiao X, Sun Z. . Ultrathin Co₃O₄ layers realizing optimized CO₂ electroreduction to formate. Angewandte Chemie International Edition, 2016, 55(2): 698–702

[7] Wang Y, Zhou J, Lv W. . Electrochemical reduction of CO₂ to formate catalyzed by electroplated tin coating on copper foam. Applied Surface Science, 2016, 362: 394–398

[8] Zheng X, De Luna P, García de Arquer F P. . Sulfur-modulated tin sites enable highly selective electrochemical reduction of CO₂ to formate. Joule, 2017, 1(4): 794–805

[9] Ross M B, De Luna P, Li Y. . Designing materials for electrochemical carbon dioxide recycling. Nature Catalysis, 2019, 2(8): 648–658

[10] Zhao C, Wang J. Electrochemical reduction of CO₂ to formate in aqueous solution using electro-deposited Sn catalysts. Chemical Engineering Journal, 2016, 293: 161–170

[11] Ma M, Liu K, Shen J. . In situ fabrication and reactivation of highly selective and stable Ag catalysts for electrochemical CO₂ conversion. ACS Energy Letters, 2018, 3(6): 1301–1306

[12] Zhao C, Dai X, Yao T. . Ionic exchange of metal-organic frameworks to access single nickel sites for efficient electroreduction of CO₂. Journal of the American Chemical Society, 2017, 139(24): 8078–8081

[13] Choi J, Kim J, Wagner P. . Energy efficient electrochemical reduction of CO₂ to CO using a three-dimensional porphyrin/graphene hydrogel. Energy & Environmental Science, 2019, 12(2): 747–755

[14] Lv J J, Jouny M, Luc W. . A highly porous copper electrocatalyst for carbon dioxide reduction. Advanced Materials, 2018, 30(49): 1803111

[15] Dinh C T, Burdyny T, Kibria M G. . CO₂ electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface. Science, 2018, 360(6390): 783–787

[16] Ma W, Xie S, Liu T. . Electrocatalytic reduction of CO₂ to ethylene and ethanol through hydrogen-assisted C–C coupling over fluorine-modified copper. Nature Catalysis, 2020, 3(6): 478–487

[17] Dinh C T, García de Arquer F P, Sinton D. . High rate, selective, and stable electroreduction of CO₂ to CO in basic and neutral media. ACS Energy Letters, 2018, 3(11): 2835–2840

[18] Huang J E, Li F, Ozden A. . CO₂ electrolysis to multicarbon products in strong acid. Science, 2021, 372(6546): 1074–1078

[19] Chen C, Li Y, Yu S. . Cu-Ag tandem catalysts for high-rate CO₂ electrolysis toward multicarbons. Joule, 2020, 4(8): 1688–1699

[20] Möller T, Ngo Thanh T, Wang X. . The product selectivity zones in gas diffusion electrodes during the electrocatalytic reduction of CO₂. Energy & Environmental Science, 2021, 14(11): 5995–6006

[21] Peng C, Luo G, Xu Z. . Lithiation-enabled high-eensity nitrogen vacancies electrocatalyze CO₂ to C₂ products. Advanced Materials, 2021, 33(40): 2103150

[22] Yang P P, Zhang X L, Gao F Y. . Protecting copper oxidation state via intermediate confinement for selective CO₂ electroreduction to C₂₊ fuels. Journal of the American Chemical Society, 2020, 142(13): 6400–6408

[23] Li H, Liu T, Wei P. . High-rate CO₂ electroreduction to C₂₊ products over a copper-copper iodide catalyst. Angewandte Chemie International Edition, 2021, 60(26): 14329–14333

[24] Zhang X, Li J, Li Y Y. . Selective and high current CO₂ electro-reduction to multicarbon products in near-neutral KCl electrolytes. Journal of the American Chemical Society, 2021, 143(8): 3245–3255

[25] Kas R, Hummadi K K, Kortlever R. . Three-dimensional porous hollow fibre copper electrodes for efficient and high-rate electrochemical carbon dioxide reduction. Nature Communications, 2016, 7(1): 10748

[26] Chen B, Xu J, Zou J. . Formate-selective CO₂ electrochemical reduction with a hydrogen-reduction-suppressing bronze alloy hollow fiber electrode. ChemSusChem, 2020, 13(24): 6594–6601

[27] Rabiee H, Zhang X, Ge L. . Tuning the product selectivity of the Cu hollow fiber gas diffusion electrode for efficient CO₂ reduction to formate by controlled surface Sn electrodeposition. ACS Applied Materials & Interfaces, 2020, 12(19): 21670–21681

[28] Rabiee H, Ge L, Zhang X. . Stand-alone asymmetric hollow fiber gas-diffusion electrodes with distinguished bronze phases for high-efficiency CO₂ electrochemical reduction. Applied Catalysis B: Environmental, 2021, 298: 120538

[29] Rabiee H, Ge L, Zhang X. . Shape-tuned electrodeposition of bismuth-based nanosheets on flow-through hollow fiber gas diffusion electrode for high-efficiency CO₂ reduction to formate. Applied Catalysis B: Environmental, 2021, 286: 119945

[30] Zhu C, Shen G, Chen W. . Copper hollow fiber electrode for efficient CO₂ electroreduction. Journal of Power Sources, 2021, 495: 229814

[31] Li S, Chen W, Dong X. . Hierarchical micro/nanostructured silver hollow fiber boosts electroreduction of carbon dioxide. Nature Communications, 2022, 13(1): 3080

[32] Chen W, Chen S, Liang T. . High-flux water desalination with interfacial salt sieving effect in nanoporous carbon composite membranes. Nature Nanotechnology, 2018, 13(4): 345–350

[33] Lei L, Pan F, Lindbrathen A. . Carbon hollow fiber membranes for a molecular sieve with precise-cutoff ultramicropores for superior hydrogen separation. Nature Communications, 2021, 12(1): 268

[34] Huang Z, Zhu L, Li A. . Renewable synthetic fuel: turning carbon dioxide back into fuel. Frontiers in Energy, 2022, 16(2): 145–149

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