Brief introduction of the results
Under acidic conditions, the electrochemical reduction of carbon dioxide (CO2) can achieve a high single-pass carbon efficiency. However, the competitive hydrogen evolution reaction (HER) reduces the selectivity of the electrochemical reduction of CO2, and both the formation of CO and its subsequent coupling during the electrochemical reduction are essential for the formation of multi-carbon (C2+) products. These two reactions depend on different catalyst properties and are difficult to achieve in a single catalyst.
Based on this,Edward H., University of Toronto, CanadaAcademician Sargent (corresponding author) et alThe decoupling of the CO2 to C2+ reaction into two steps, i.e., CO2 to CO and CO to C2+, is reported to achieve the desired transformation by deploying two different catalyst layers in tandem. The first catalyst is atomically dispersed phthalocyanine cobalt (COPC), which reduces CO2 to CO with high selectivity. This process increases the availability of local CO, thereby enhancing the C-C coupling step achieved on the second layer catalyst, which is a Cu nanocatalyst with a Cu-ion interface. The optimized tandem electrode achieves 61% C2H4 Faraday efficiency (Fe) and 82% C2+ Fe at 25 °C at a current density of 800 mA cm-2. At a current density of 800 mA cm-2 and a CO2 flow rate of 2 mL min-1, the single carbon efficiency of the system is 90 3%, C2H4 Fe is 55 3%, and total C2+ Fe is 76 2%.
Background:
Alkaline and neutral electrolytes inhibit hydrogen evolution (HER) and promote C-C coupling in electrochemical CO2 reduction (CO2RR). However, in this case, more than 75% of the input CO2 is chemically lost by reacting with hydroxyl ions (OH-) to form (bi)carbonates, resulting in the local high alkalinity of the cathode, reducing the single conversion efficiency (SPCE) of CO2 and applying energy to recover the lost CO2 reactants. The use of acidic electrolytes in CO2RR can improve CO2 utilization by reducing (BI) carbonate formation and CO2 crossover, but in acidic media, HER is more likely to compete with CO2RR kinetically, resulting in poor CO2RR selectivity.
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Using density functional theory (DFT) calculations, the authors investigated the reaction energetics of CO2RR to Cu, noting the surface concentration of the reaction intermediate (i.e., CO) and the adsorbed h(h*, * denoting the surface site). The authors found that greater CO coverage on Cu reduced the Gibbs free energy (δGh*) adsorbed by H, resulting in an increase in HER overpotential. The greater the CO coverage on Cu, the lower the energy barrier of the C-C coupling reaction. In the experiment, the input gas was converted from CO2 to CO, and it was found that the production of C2H4 and C2+ increased, and the Faraday efficiency (Fe) of H2 decreased. The authors employed a spatial decoupling strategy, the tandem approach, using an optimized catalyst for the first chemical conversion of CO2 to CO, and a second catalyst for a different CO to C2+.
Figure 1Spatial decoupling strategy for tandem catalytic acidic CO2RR
The authors dispersed phthalocyanine cobalt (CoPC) atoms on a hollow carbon (HC) support (copc@hc) in which a single CoPC molecule was anchored to the support and pyrolysis was obtained to obtain a zeolite imidazolic acid backbone (ZIF-8). copc@hc catalyst has a hollow morphology with no COPC agglomeration. X-ray absorption spectra showed that the copc@hc Co K side shifted to a higher energy position, while the white line peak intensity increased, demonstrating a strong catalyst-support interaction to reduce Co agglomeration. The results showed that there was an electronic interaction between COPC and N species on HC support. copc@hc catalyst at 300 mA cm-2 was 94% CoFe and less than 3% H2Fe.
Figure 2Synthesis and structural analysis of copc@hc
The authors designed a tandem electrode consisting of a C-C coupling catalyst with a CO2 conversion catalyst on top of a layer. A copc@hc catalyst was added to the top of the sputtered Cu (SCU) layer to construct a copc@hc SCU tandem electrode. Then, a 3D Cu-ionomer interface catalyst layer was introduced between the copc@hc and sputtered Cu, consisting of Cu NPs coated with ionopolymers, and a copc@hc Cu tandem electrode was constructed. In the containing 05 m h3po.5 m kh2po4 and 2In a buffered acidic electrolyte of 5 M Kcl, the optimized copc@hc Cu tandem electrode was coated with perfluorosulfonic acid (PFSA) ions at 25 nm Cu NPS, increasing the Fe of C2H4 from 30% to 54% and C2+ from 36% to 80%, while the total current density was increased by a factor of four to 800 mA cm-2.
The Fe values of the Cu electrode without series structure for C2H4 (27%) and C2+ (41%) were much lower. Under the same conditions, the two-layer tandem electrode increases the SPCE from 52% to 87%, while the copc@hc Cu tandem electrode shows higher C2H4 and C2+ current densities at lower potentials. Techno-economic analyses have shown that the energy intensity of C2H4 production is 300 GJ tonne-1, which is 50% lower compared to the previous efficient conversion of CO2 to C2H4 in acidic systems. The authors constructed copc@hc (Cu+copc@hc) tandem electrodes with 61% C2H4 Fe and 82% C2+ Fe at 800 mA cm-2. When optimizing single-channel carbon utilization at a 2 mL min-1 CO2 flow rate, the SPCE was 90 3%;At 800 mA cm-2, C2H4 Fe was 55 3% and C2+ Fe was 76 2%.
Figure 3Acidic CO2RR performance
Using density functional theory (DFT) calculations, the authors investigated the CO2RR pathway on periodic Cu plates covered by a Con4-C layer, and the results showed that the Con4-C Cu model reduced the reaction energy required for C-C coupling by about 011 EV and changed the reaction pathway in favor of *CCO hydrogenation, favoring the formation of C2H4. The comprehensive DFT and empirical results show that the synergy between COPC and CU promotes C-C coupling and also affects the selectivity of C2H4. Using in-situ Raman spectroscopy, the authors investigated the adsorption of CO on the Cu surface. The Raman spectrum of Cu is in -1The peak at 361 cm-1 at 2 V is attributed to Cu-Co stretching, indicating that CO2 is converted to Co at the Cu surface. In the Raman spectrum of the COPC Cu tandem electrode, the peak blue shift at 361 cm-1 to 389 cm-1 indicates that the CoP is more strongly bound at the Cu site due to the CoPc being in the vicinity of the adsorbed Co. Control experiments showed that the higher CoPC coverage on the Cu surface was mainly related to the Co-Co peak at 532 cm-1. When CoPC forms aggregates, it cannot effectively catalyze the conversion of CO2 to Co, and only the Cu-Co peak at 365 cm-1 is observed.
Figure 4DFT calculations and in-situ Raman measurements
Bibliographic information
efficient multicarbon formation in acidic co2 reduction via tandem electrocatalysis. nature nanotechnology,, doi: 10.1038/s41565-023-01543-8.