CHEMICAL SCIENCE

The first industrial-grade current density pure water CO2 co-electrolysis of ethylene


On August 18, 2022, Professor Zhuang Lin’s team from the School of Chemistry and Molecular Sciences of Wuhan University published an article entitled “Bifunctional ionomers for efficient co-electrolysis of CO2 and pure water toward ethylene production at industrial-scale current.” densities” research results.

The study reported the first industrial-grade current density pure water CO2 co-electrolyticated alkaline membrane electrolyzer to ethylene. Selective preparation of ethylene by introducing bifunctional ionomers (ionomers) that can both conduct ions and cooperate with CO2 activation into the cathode promotes CO2 reduction. In pure water systems without the addition of any additional liquid electrolyte, the net current density generated by ethylene reaches 420 mA cm-2 and the tank pressure is only 3.54 V, which is the highest performance report of CO2 electrolyzed ethylene in the current non-expendable electrolyte system.

The co-first authors of this article are Li Wenzheng and Yin Zhenglei; The corresponding author of this article is Professor Zhuang Lin, associate researcher of Wang Gongwei. The School of Chemistry and Molecular Sciences of Wuhan University is a communication unit.

Background

Electrocatalytic CO2 reduction can use the electrical energy generated by renewable energy sources to convert greenhouse gas CO2 into high value-added fuels or chemical products such as CO, ethylene, ethanol, etc., to achieve artificial carbon cycling. To achieve industrial-grade current density, either a flow Cell or a polymer membrane electrolyzer (MEA) based on a gas diffusion electrode (GDE) is required. Among them, the anode spacing and internal resistance in the flow electrolyzer are large, resulting in a large loss of electrolytic cell pressure and very low energy conversion efficiency; The anode in the membrane electrolyzer is tightly pressed on the polymer electrolyte diaphragm with a thickness of only microns, and this zero gap structure design can greatly reduce the internal resistance and improve the energy conversion efficiency of electrolysis. At the same time, the membrane electrolyzer design can also use pure water as the working medium, so as to avoid the use of liquid electrolytes caused by electrolyte consumption, electrode inactivation, device maintenance difficulties and other adverse effects. Similar electrochemical technologies (including fuel cells, water electrolyzers) have been developed over the years and have completed technological innovations from liquid electrolytes to polymer electrolyte/pure water systems. However, CO2 electrolysis technology based on polymer electrolyte/pure water systems is still rarely reported.

In 2019, Professor Zhuang Lin’s research team reported the first case of pure water CO2 membrane electrolyzer preparation of CO, reaching an industrial-grade current density of 500 mA cm-2 (Energy Environ. Sci., 2019, 12, 2455), received a lot of attention. Based on this progress, the research team further attempted to reduce the CO2 depth to more economically valuable ethylene. By introducing a bifunctional basic polymer polyquaternium polyquaternium polyetherethereketone (QAPEEK) into the Cu gas diffusion electrode, the pure water CO2 membrane electrolyzer achieves low cell pressure, industrial-grade current density, and high selective preparation of ethylene. The 1 A cm-2 current density has a slot voltage of only 3.73 V, and the net current density of ethylene at a 3.54 V slot pressure reaches 420 mA cm-2.

Graphic and text analysis

In the co2 membrane electrolyzers that have been reported, it is often necessary to add liquid electrolytes to the anode side. It is found that the liquid electrolyte can reach the cathode through the polymer membrane, and with the evaporation and electrochemical consumption of water in the gas diffusion electrode, the phenomenon of “salt analysis” will gradually appear, thereby hindering the mass transfer of CO2 gas and the occurrence of serious hydrogen evolution side reactions.

Fig. 1: Schematic diagram of the structure of the membrane electrolyzer (a, b), electrolytic properties of the liquid electrolyte on the anode side (c) and the phenomenon of “salting” (d)

The use of pure water as the working medium can avoid the harm of “salt precipitation”, but directly operating with pure water medium, due to the low utilization rate of the cathode catalytic layer, it is difficult to achieve high current density. Ionomers need to be added to the cathodic catalytic layer for ion conduction to ensure that the device operates properly at high current densities.

Fig. 2: Electrolytic performance of pure water on the anode side of CO2 membrane electrolyzer (a), electrochemical area of cathodic catalytic layer under different electrolyte conditions (b, c, d)

Many studies in the literature have shown that surface modification can significantly affect the performance of electrocatalytic CO2 reduction, and the ideal ionate needs not only to be able to conduct ions, but also to synergistically catalyze CO2 reduction. Testing in H-type electrolytic cells has found that QAPEEK modified Cu electrodes can effectively promote ethylene production compared with a variety of commercially available polymers.

Fig. 3: Comparison of the selectivity of catalytic CO2 reduction of QAPEEK(a, b) with different contents of the Cu electrode and the modification of different polymers (d), (c) QAPEEK polymer structure

Combined with in situ attenuation total reflective surface enhanced infrared spectroscopy (ATR-SEIRAS) and non-aqueous system cyclic voltammetry scanning, it was found that the carbonyl group in QAPEEK can promote CO2 activation after electrochemical reduction, thereby accelerating the conversion of CO2 into *CO intermediates, and the high coverage of *CO intermediates is conducive to C-C coupling to produce ethylene.

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Figure 4: In situ attenuation total reflective surface enhanced infrared spectroscopy (ATR-SEIRAS) to study surface species changes before and after Cu electrode modification QAPEEK

QAPEEK was further modified into the Cu gas diffusion electrode and assembly testing of pure water CO2 membrane electrolyzers was carried out. By regulating the amount of QAPEEK ionospolymers to optimize the three-phase reaction interface, the CO2 electrolysis performance was effectively improved. At an optimized dosage of 50 μg QAPEEK, the maximum ethylene selectivity reaches 50% and it can run stably for 5 h. QAPEEK modified Cu gas diffusion electrodes yield a higher net ethylene current density compared to a variety of commercially available ionostomers.

Figure 5: Pure water CO2 membrane electrolyzer performance comparison: (a-d) performance of cu gas diffusion electrodes with different dosages of QAPEEK, (e) stability test results, ethylene net current density (f) and electrochemical area (g) of different amounts of QAPEEK modified electrodes, (h) comparison of ethylene net current density of different ionomers modified Cu gas diffusion electrodes with optimized dosages

In order to further improve the current density, the strategy of preparing copper-aluminum alloys by magnetron co-sputtering and dissolving aluminum in situ was adopted to improve the roughness of the cathode catalytic layer. Finally, the current density under the pressure of the 3.73 V electrolytic cell reaches 1 A cm-2, and the net current density generated by the ethylene under the pressure of the 3.54 V cell reaches 420 mA cm-2, which is the highest performance report of CO2 electrolytic ethylene in the current non-consumable electrolyte system.

Figure 6: Pure water CO2 membrane electrolyzer performance (a-c) using porous Cu gas diffusion electrodes, and comparison with ethylene net current density/cell pressure in the literature (d)

Summary and outlook

The study reported for the first time the industrial-grade current density pure water CO2 co-electrolysis of ethylene, which produced a net current density of 420 mA cm-2 and a slot pressure of only 3.54 V. In order to promote the conversion of CO2 to ethylene, it breaks through the design ideas of traditional regulatory catalysts and performs catalytic surface modification by introducing bifunctional ionates. While conducting ions, it can co-catalyze the reduction and conversion of CO2, so as to prepare ethylene with high selectivity. This study not only provides a new idea for the design of functionalized polymer electrolytes, but also strongly promotes the practical application of CO2 electrolytic conversion technology.

Related paper information:https://doi.org/10.1038/s41560-022-01092-9



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