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In recent years, climate change caused by excessive CO2 emissions from fossil fuels has been one of the major environmental problems of the modern world. Global fossil fuels emit nearly 3.6 billion tonnes of CO2 each year, while green plants and autotrophs consume only 6% of total CO2 emissions each year. With the implementation of the "carbon peak, carbon neutral" policy, the conversion of CO2 into high value-added fuels or chemical raw materials is becoming an important way to achieve energy and environmental sustainability. Among the many CO2 conversion technologies, electrocatalytic CO2 reduction is a promising strategy. However, as a typical greenhouse gas, the C=O bond in CO2 has a high bond energy (750 kJ mol-1), so it is necessary to develop an efficient and stable electrocatalyst to activate it and convert it into a high value-added product.
It is worth noting that the products of electrocatalytic CO2 reduction are various, not only CO, HCOOH, CH3OH, CH4 and other C1 compounds, but also C2H4, C2H5OH, (CH2OH) 2, CH3COOH, C2H2O4 and other C2 compounds, and even C3H6, C3H7OH, C3H6O and other C3 compounds. In 2018's Joule, Canada's E. Academician H. Sargent pointed out that CO, HCOO-, C2H4, C2H5OH are the most promising target products. However, in the actual electrocatalytic CO2 reduction process, not only there is a competitive hydrogen evolution side reaction, but also the selectivity of the target product is also adversely affected.
Today's study of two recently published top-of-the-top papers, the two types of catalysts reported by them are very representative, and the ideas are worth learning. The first was published in the international journal Adv. Mater., the authors designed a two-atom catalyst with atomic-scale dispersed Ni–Zn bilocus that exhibited up to 99% Faraday efficiency during the electrocatalytic reduction of CO2 reduction; the second article was published in the international journal Nat. On Commun., the authors achieved a FARaday efficiency of up to 95% of CO2 reduction to formate by forming an active surface of Sn-Bi/SnO2 in situ on the Bi0.1Sn alloy, and the system can run continuously and stably for 100 days. Here's a good look at the two articles.
【Atomic-level dispersion Ni–Zn synergistic interaction enhances CO2 electroretrivation】
First author: Youzhi Li, Bo Wei
Corresponding authors: Lu Yingying, Li Zhongjian, Zhang Ruifeng
Communication units: Zhejiang University, Beihang University
DOI: 10.1002/adma.202102212
Full text at a glance
Bimetallic atomic catalysts have different coordination environments, quantum size effects, and interactions with supports compared to single-atom catalysts. Its tunable electronic structure has a significant effect on reactant adsorption, intermediate bonding, energy barrier, and steady-state dynamics in CO2RR, resulting in superior performance over single-atom catalysts. However, the current lack of understanding of the mechanisms by which the activity of diatomic catalysts is enhanced, hindering the rational design of high-performance catalysts. In addition, in addition to studying thermodynamic pathways such as Gibbs's free energy, interactions and dynamic pathways on the microscopic interface layer are equally indispensable in mechanism exploration.
Figure 1. Structural characterization of Ni–Zn–N–C.
In this paper, the authors successfully constructed a diatomic bimetallic site with chitosan as the C and N sources and zinc chloride and nickel chloride as metal precursors, in which the loads of Ni and Zn were 0.84 wt% and 1.32 wt%, respectively.
Figure 2. Electron interactions in Ni–Zn–N–C.
By combining theory and experimentation, it can be found that there is a significant electron interaction between the Ni and Zn atoms in the constructed NiZn–N6–C catalyst, which can change the electronic structure of the catalyst, resulting in unobserved synergistic effects in the single-atom catalyst.
Figure 3. Density functional theory calculations.
Studies have shown that heteronuclear coordination causes specific changes in the d-electron state of metal atoms in diatomic sites. Thus, the Ni–Zn bimetallic sites show a lower free energy barrier (ΔG), a band gap between the smaller Ni(3d) orbital d-band center (εd) and the Fermi energy level (EF), and the thermodynamically promote the adsorption of intermediates.
Figure 4. In situ characterization of the prepared catalyst in 0.5 M KHCO3 solution and three-dimensional VCDD analysis.
In situ FTIR showed that NiZn–N6 had stronger *COOH adsorption than Ni–N4 and Zn–N4. In addition, the Zn site can not only adjust the d-band position of the Ni site, but also affect the adsorption of the intermediate.
Figure 5. The electrochemical CO2 reduction performance of the prepared catalyst.
Experimental tests have shown that the Ni–Zn–N–C catalyst can achieve cofaladi efficiency of up to >90% at a wide potential window of −0.5 to −1.0 V, and a maximum of 99% at −0.8 V. This performance is superior to Ni/Zn monometallic site catalysts.
Figure 6. CI–NEB calculation kinetic reaction process and AIMD calculation.
Kineticly, the Ea required for the hydrogenation reaction of CO2 on NiZnN6 is lower than the values on Ni–N4–C and Zn–N4–C, corresponding to the rate determination step (RDS) in all primary reactions.
Literature source
Youzhi Li, Bo Wei, Minghui Zhu, Jiacheng Chen, Qike Jiang, Bin Yang, Yang Hou, Lecheng Lei, Zhongjian Li, Ruifeng Zhang, Yingying Lu. Synergistic Effect of Atomically Dispersed Ni–Zn Pair Sites for Enhanced CO2 Electroreduction. Adv. Mater. 2021. DOI: 10.1002/adma.202102212.
Literature links: https://doi.org/10.1002/adma.202102212
【Regulating stable active sites by redox for CO2 reduction to formate】
First author: Le Li, Adnan Ozden
Corresponding authors: Zhong Miao, Dong Hao, Edward H. Sargent
Communication units: Nanjing University, University of Toronto
DOI: 10.1038/s41467-021-25573-9
Of all carbon dioxide reduction (CO2R) products, formic acid (HCOOH) or formate (HCOO–) are commonly used in pharmaceutical, electrometallurgical, leather, and fuel cell applications. However, so far, the catalysts and systems reported to have produced HCOO– through CO2R have not met the requirements of high selectivity (Faraday efficiency (FE)), high reaction rate (current density), high energy efficiency (EE), and long-term stability.
In the catalysts currently reported, Sn has a strong binding energy to *OCHO, so it is conducive to the first step of CO2 hydrogenation process of CO2 conversion to formate. However, the binding energy of Sn to *COOH and *H is at a moderate level, making it difficult to completely inhibit the generation of CO and H2, thus limiting the FE of HCOO– to 80–85%.
Figure 1. Structure and elemental analysis of Bi0.1Sn electrocatalysts.
In this paper, the authors successfully prepared BixSn, Bi, and Sn precatalysts on a polytetrafluoroethylene (PTFE) gas diffusion substrate by thermal evaporation strategy. All BixSn, Bi and Sn form a dense layer of granular membranes, thus ensuring the conductivity of CO2R.
Figure 2. DFT calculation result.
The study found that Bi4Sn64 can improve the reaction energy of CO and H2, thereby inhibiting the formation of CO, H2 and C2+ hydrocarbons. Therefore, alloying Sn with Bi can be used as a strategy to increase HCOO– yield.
Figure 3. CO2 reduction performance of Bi0.1Sn, Bi and Sn catalysts.
Experimental tests showed that in the 1 M potassium bicarbonate (KHCO3) and potassium hydroxide (KOH) electrolyte with a pH of 11, the prepared Bi0.1Sn catalyst can stably reduce CO2 to formate during continuous operation of more than 2400 hours (100 days). At a current density of 100 mA cm-2, faraday efficiencies are as high as 95% and cathode energy efficiencies can exceed 70%.
Figure 4. Structure and elemental analysis of Bi and Bi0.1Sn electrocatalysts before and after CO2 reduction at pH 11 and 100 mA cm-2 current densities.
The results show that after a wide range (1.5–12.5%) of Bi is incorporated into Sn, the crystal plane of Sn-Bi alloy and Sn-Bi/SnO2 composites provides a site close to the optimal binding energy for *OCHO, thereby reducing the reaction energy of CO2 to formate. Moreover, the by-products CO and H2 are also inhibited due to the increased reaction energy on the Sn-Bi alloy. Critically, the redox-regulated Sn-Bi/SnO2 surface remains active at all times and protects the Bi0.1Sn catalyst from corrosion or reconstitution during long CO2R runs.
Figure 5. CO2 reduction performance in anion exchange membrane (AEM)-based MEA system.
Subsequently, we further evaluated the CO2R performance of the Bi, Sn and Bi0.1Sn catalysts in anion exchange membrane (AEM)-based MEA systems. In the current density range of 30 to 180 mA cm-2, the FE of Bi0.1Sn catalyzed CO2R to formate has been maintained at 90%. And when the current density is 120 mA cm-2 and the full battery potential is –3.6 V, the FE peak of the formate reaches 97.8%, and the full battery energy efficiency reaches 36%.
Le Li, Adnan Ozden, Shuyi Guo, F. Pelayo Garcıa de Arquer, Chuanhao Wang, Mingzhe Zhang, Jin Zhang, Haoyang Jiang, Wei Wang, Hao Dong, David Sinton, Edward H. Sargent, Miao Zhong. Stable, active CO2 reduction to formate via redox-modulated stabilization of active sites. Nature Communications. 2021. DOI: 10.1038/s41467-021-25573-9.
Literature links: https://doi.org/10.1038/s41467-021-25573-9
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