Xiong Yujie team of USTC, the latest Angew! CO2 electrolysis to produce concentrated formic acid

Mondo Science Updated on 2024-02-19

Brief introduction of the results

The preparation of formic acid by electrocatalytic CO2 reduction may be a key link in the carbon cycle, but its practical feasibility is largely limited by the number and concentration of the product. Based on this,Prof. Xiong Yujie and Associate Prof. Liu Jingxiang of the University of Science and Technology of China, and Associate Professor Cai Weiwei of the China University of Geosciences(Corresponding author) et al. demonstrated the preparation of formic acid by continuous CO2 electrocatalytic reduction (for 300 hours) at an industrial-grade current density (i.e., 200 mAcm2) using a scalable lattice distorted bismuth catalyst with a product concentration of 2 m. The optimized catalyst produces formatic acid Faraday with an efficiency of 942% and at 1At 16 a·cm2, the yield reached 217 mmol·cm−2·h−1。In order to evaluate the practicability of the system, the authors conducted a comprehensive techno-economic analysis and life cycle assessment, and the results showed that the method proposed by the authors has great potential to replace the traditional methyl formate hydrolysis method for industrial formic acid production. In addition, the formic acid prepared by the above method can be directly used as fuel for air-breathing formic acid fuel cells, which has a power density of 55 MW cm2 and 201% thermal efficiency.

Background:

Due to the dependence on fossil fuels and the threat of global environmental problems, there is an urgent need to find alternative energy sources to reduce the concentration of carbon dioxide. Electrochemical CO2 reduction reaction (ECO2RR) has attracted widespread attention as a renewable energy medium, powered by renewable electricity, for the production of fuels. Liquid Eco2RR products, especially formic acid (FA), are considered a promising energy medium due to their high energy density, ease of storage and transport, and compatibility with existing fossil fuel infrastructure. Recently, the concept of formic acid economy has attracted great attention from the public and the scientific community, and the use of formic acid in ECO2RR as an energy vector in the global economy has attracted great attention from the public and scientific community.

**Reading guide

Figure 1Preparation process and structure standards of catalysts

The authors first prepared amorphous BioX (denoted BioX-LiL) by the LiL method using Bi powder as a precursor, which can be easily stored and converted to metal Bi by electroreduction when used (Figure 1A). As shown in Figure 1B, the size of the prepared BioX-LiL (about 50 nm) is smaller than that of the bulk BI, indicating that the bulk BI is crushed during the LIL process. Interestingly, no significant lattice fringes and diffraction were observed in the HRTEM images, and the Fast Fourier Transform (FFT) and XRD plots of BioX-Lil also showed its amorphous structure (Figure 1C). With electroreduction, these defects can be preserved during the rapid conversion of BioX-Lil to metal BI, which is beneficial for catalytic applications. Specifically, BioX-Lil has an amorphous structure with abundant unsaturated sites and lattice distortion, which makes it an ideal precursor for the preparation of Eco2RR-deficient metal Bi catalysts.

RD-BI was prepared by in-situ electrochemical conversion using amorphous BioX-Lil as raw material. As shown in Figure 1D, the XRD spectra of RD-BI show a higher degree of crystallinity that can point to rhomboidal BI (PDF 85-1331), and its peaks are shifted to a higher angle compared to bulk BI and the BI sample (LD-BI) converted from BI2O3, indicating that RD-BI has a smaller plane spacing. These results indicate the presence of compressive strain in RD-Bi due to lattice distortion in its structure. In addition, transmission electron microscopy (TEM) images (Figure 1E) showed that RD-Bi exhibited a spherical topography with an average diameter of 50 nm when mixed with carbon black. The high-angle annular darkfield scanning TEM (HAADF-STEM) image shows that the lattice spacing of RD-Bi is about 032 nm (Figure 1f), which is attributed to the diamond-shaped BI (012) plane. A large amount of lattice distortion can be clearly observed on RD-Bi (Fig. 1f), which further confirms its defect-rich characteristics. In addition, the lattice strain distribution of RD-Bi was investigated by geometric phase analysis (GPA) (Figure 1G).

Figure 2CO2 electrocatalytic performance test

The authors then used an H-type cell in argon (Ar-) or CO2-saturated with 0The Eco2RR performance of all prepared samples was evaluated in 5 M KhCO3. As can be seen from the LSV curve in Figure 2A, RD-Bi has the highest current density of all the prepared electrocatalysts. In addition, formic acid is the main product of ECO2RR against RD-Bi over the entire potential range (Figure 2B). The Faraday efficiency (Fe) of formic acid against RD-Bi is 07 to 1The wide potential range of 2 V is maintained above 80%. In contrast, bulk BI and LD-BI showed less than 80% Fes formate over the entire potential range. As shown in Figure 2C, the generation rate of RD-Bi is significantly higher than that of bulk BI and LD-BI over the entire potential range, with a maximum generation rate of 441 mAcm2 with a generation rate of 8235 µmol h−1 cm−2。RD-BI is better than LD-BI (23.).8 mA2) and block bi (105 ma cm2) respectively 42x and 19 times. Over the entire study potential range, the EE of RD-Bi was more than 50%, at 0The maximum EE of 9 V is 63% relative to RHE, which is much higher than the maximum value of body BI (50.).6%) and the maximum value of LD-BI (56%). This excellent Eco2RR performance can also be reproduced using commercial BI powder as a precursor for BioX-LiL, further confirming the utility of the lattice twisting catalyst developed by the authors.

In addition, RD-Bi has a high degree of stability (Figure 2D) with negligible current density attenuation in 100 hours of continuous electrocatalytic testing, with an average Fe of about 90% formate. Considering the CO2 mass transmission limitations of conventional H-type batteries, it is difficult for this system to achieve high current densities. Therefore, in order to further study the Eco2RR performance of RD-BI, the authors set up a flow cell that facilitates the diffusion of CO2 to the catalyst surface. As shown in Figure 2e, the LSV curve for RD-BI is in the -0 range1The high current density of 3 A cm2 is significantly higher than that of bulk BI and LD-BI. Subsequently, at 02 and 1A time-potential assay test was performed between 4 A cm-2 to study the products of ECO2RR on all prepared samples. rd-bi at 1The Fe value of formic acid at 2 A cm-2 is as high as 942% at 02 ~ 1.Formic acid Fe values are more than 90% over a wide range of current densities of 2 A cm-2 (Figure 2F). Under this formic acid Fe, RD-Bi also reached 1. in all prepared samplesThe highest formic acid of 16 a cm2 (Figure 2g) corresponds to a formic acid yield of 217 mM cm2H1, which is one of the highest values in the recently reported ECO2RR (Figure 2H).

Figure 3MEA testing and economic feasibility analysis

Although a high current density is obtained in the flow cell of RD-BI, formic acid is the main product and further purification is required to obtain formic acid. In addition, the poor stability of the flow cell also limits the generation of large amounts (100 ml) of high concentrations (1 m) of Fa. Therefore, MEA electrolyzers are employed, which contain a solid electrolyte of conductive protons to avoid the production of formic acid (Figure 3A). In addition, the authors established a humid heat ventilated collection system to collect the Fa produced by Eco2RR, regulating the concentration of Fa by simply changing the humidity of the system. Excitingly, the current density of the device can be 4Reach more than 200 mA cm-2 at a battery potential of 1 V and provide 86 at 150 mA cm-25% FA peaks Fe (Figures 3B and C), which is comparable to the recently reported catalysts. More importantly, the authors' system achieved continuous production of a 2 mFa solution at an operating current density of 200 mA for 300 hours without significant performance degradation (Figure 3D).

The practical application of this technology depends on its economic and environmental viability. Therefore, the authors compared the FA yield of the ECO2RR system with the traditional methyl formate hydrolysis system by TEA and LCA. The authors' findings suggest that this system has several advantages over traditional methods (Fig. 3e, f). It shows lower capital expenditures and a shorter static investment** period. Notably, electricity prices dominate the overall cost of production, accounting for about 60% of total costs (Figure 3e). Recognizing the importance of electricity prices, the authors conducted sensitivity analyses to understand their impact on the marginal and static investment periods of the system's contribution (Figure 3f). The results show that the system has a high sensitivity to electricity price, and with the increase of electricity price, the contribution margin increases, and the static investment period increases. Interestingly, when the price of electricity is below 0$03 kWh, the contribution margin of this system is higher than that of traditional methods. Nonetheless, the static investment period of the system is always shorter than that of the traditional method, highlighting the economic advantages of the system in terms of FA production. In terms of LCA, the system shows high potential in terms of reducing carbon emissions by using renewable energy as electricity**, which is significantly superior to traditional methods (Figure 3g).

In addition, when comparing the environmental impact of hydropower-driven ECO2RR with conventional methyl formate hydrolysis, the authors found that the electrochemical process had a much smaller impact on 18 environmental issues (Figure 3H). The above results show that the system is not only an economically superior option, but also an environmentally friendly technology compared to traditional FA production methods.

Figure 4Direct formic acid fuel cell test

The ultimate goal of this work is to demonstrate the feasibility and practicability of ECO2RR to replace traditional methyl formate hydrolysis. To further demonstrate the superiority of this system, the authors use the produced FA directly to drive direct FA fuel cells (DFAFCs). The authors then assembled a passively aspirated DFAFC with PDPT C as the anode catalyst and PT C as the cathode catalyst (Figure 4A). As shown in Figure 4B, the FA solution produced by ECO2RR not only drives the aspirated DFAFCs, but also produces a power density of up to 55 MW cm-2 at 25°C and ambient pressure, confirming the utility of the FA produced by the system developed by the authors.

In addition, as can be seen from the constant current discharge curve in Figure 4C, the voltage of DFAFCc is about 0At 55 V, no significant voltage drop was observed over 2 h, indicating that the 2 M FA-driven aspirated DFAFCs produced by the Eco2RR have excellent durability. Excitingly, the FA-driven DFAFCs generated by the Eco2RR system have such high discharge power density and stability that they can be used to power light-emitting diodes (Figure 4D) and small automotive systems (Figure 4E). More interestingly, the thermal efficiency of DFAFCc is about 45 according to the constant current discharge curve4%。That is, the system developed by the authors can achieve an overall thermal efficiency of 124-20.1%, which is very close to the thermal efficiency of a conventional gasoline engine (i.e. 20-40%). The satisfactory energy efficiency of converting carbon dioxide into formic acid and then into electricity is considered a key link in the formic acid economy, which paves the way for a complete cycle of the formic acid economy.

Bibliographic information

chao zhang, xiaobin hao, jiatang wang, xiayu ding, yuan zhong, yawen jiang, ming-chung wu, ran long, wanbing gong, changhao liang, weiwei cai, jingxiang low, yujie xiong, angew. chem. int. ed., e202317628.

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