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/(gcat∙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.
Tab.1 Performance comparison of CO2 electroreduction on the current hollow-fiber electrodes |
| Catalysts | Electrolyte | Potential/(V vs. RHE) | Products | Jtotal/(mA∙cm−2) | FE/% | Ref. |
|---|---|---|---|---|---|---|
| Cu hollow fiber | 0.5 mol/L KHCO3 | −1.1 | Formate | 210 | 80 | [30] |
| Activated Ag hollow fiber | 0.5 mol/L KHCO3 | −0.83 | CO | 1262 | 93 | [31] |
| Cu hollow fiber | 0.3 mol/L KHCO3 | −0.4 | CO | ~10 | 72 | [25] |
| Cu-Sn45% hollow fiber | 0.5 mol/L KHCO3 | −0.75 | Formate | 66 | 91 | [26] |
| Cu-Sn hollow fiber | 0.5 mol/L KHCO3 | −1.2 | Formate | 88 | 78 | [27] |
| Cu6Sn5 hollow fiber | 0.5 mol/L KHCO3 | −1.1 | Formate | 136 | 89 | [28] |
| CuBi2O3 hollow fiber | 0.5 mol/L KHCO3 | −1.0 | Formate | 141 | 85 | [29] |
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.