Hydrogen energy has the advantages of high energy density and zero carbon emissions, and is one of the important components of future energy strategy. IRO2, RuO2 and PT are commonly used as catalysts for OER and HER in industrial electrochemical water splitting, but their development is still limited by high cost and low stability. The alloying method can effectively adjust the adsorption free energy of the electronic structure and intermediates, thereby accelerating the catalytic process. At the same time, high-entropy alloys (HEAs) have unique high-entropy effects and cocktail effects, which can significantly enhance the catalytic activity.
Professor Fang Feng of Southeast UniversitywithHuo Wenyi, Nanjing Forestry UniversityThe team and others used magnetron sputtering to deposit a Feconicupd thin film electrocatalyst with a face-centered cubic structure on carbon fiber cloth (CFC), and explained the research significance of HEA in the field of electrocatalysis through the method of experimental and theoretical calculations.
Calculation method:
In this paper, the Castep quantum mechanics module in Materials Studio software is used for theoretical calculations. Based on the Peb functional under the Generalized Gradient Approximation (GGA), the Kohn-Sham wave function in the plane wave is used to describe the valence electron interaction relationship. The Broyden Fletcher Goldfarb Shanno (ultra-soft pseudopotential) algorithm was selected for the correlation ground state geometry optimization.
Results & Discussion
The authors successfully synthesized a single-phase face-centered cubic HEA thin film material on carbon fiber cloth by magnetron sputtering, and the valence states of the five transition metals were all dominated by 0 valence, and the presence of +2 valence in Fe had high chemical activity and relatively low redox potential, indicating that each element was mainly bonded by metal bonds.
Figure 1Phase and microstructure of feconicupd thin-film electrocatalysts
Figures 2 and 3 show the relationship between the properties of the catalyst in terms of basic HER and OER. On the HER side, the Feconicupd overpotential and Tafel slopes are 29., respectively7 mv and 472 mV·dec 1, comparable to PT C catalysts for industrial applications, conforming to the Volmer-Heyrovsky reaction mechanism. And the charge transfer ability and stability performance are good.
In terms of OER, Feconicupd also has excellent overpotential, Tafel slope and current density, which is much lower than that of commercial RuO2 catalysts. And in the long-term stability test, there was no obvious attenuation of activity. The performance of the catalyst remained stable after the continuous catalytic electrolysis of water at a high current density of 800 mA·cm-2 for more than 100 hours. Combined with the morphology analysis of the samples after the test, it was shown that there was no interfacial delamination of the HEA film and the stability was good.
Figure 2Activity relationship of feconicupd thin-film electrocatalysts in alkaline hydrogen evolution reactions
Figure 3Activity relationship of feconicupd thin film electrocatalyst in alkaline oxygen evolution reaction
Subsequently, the authors explained the reaction mechanism, and constructed a computational model of Feconicupd thin-film electrocatalyst based on the element ratio in EDS and the single-phase face-centered cubic results. Figure 4 shows the HER activity**, which first confirms that the catalyst has a high conductivity and a high electron content in the vicinity of the Cu and Pd atoms (Figure S14).
The D orbitals of Ni and Co are close to the Fermi level, which is an important contributor to water splitting, and helps to stabilize the adsorption of intermediates and maintain the initial electrochemical activity. Only 0The hydrolysis reaction can be completed with an energy barrier of 18 EV. In addition, in the process of H* dissociation, Pd plays a more significant role in reducing the energy barrier, and the electron transfer of the Co-Ni-PD site is rapid, the center of the D band is shifted down compared with other sites, and the antibond electronic state is filled, which can reduce the adsorption and desorption Hyrovsky reaction, and promote hydrogen production.
Figure 4Calculation results of Feconicupd thin film electrocatalyst in HER
In terms of OER mechanism**, Fourier transform alternating current voltammetry (FTACV) and in-situ Raman characterization show that the catalyst surface is reconstituted in situ at the elevated external voltage, and Feconicupd exhibits the highest current in the II region (Fig. 5A), which is due to the full redox and inherent synergistic effects of various active metal sites. The HRTEM results and XPS tests further confirmed that the materials that appeared in the surface reconstruction were grown outside the inner part of HEA, with a small thickness, and the highly reactive Ni(FeCo)OOH species generated on the surface had selective growth characteristics, which played a role in stabilizing the structure at the Cu and PD sites, and did not obviously participate in the in-situ reconstruction.
Figure 5Map and morphology evolution of Feconicupd thin-film electrocatalysts in OER testing
DFT calculations show that FeConicupd thin film electrocatalysts have higher EF energy and stronger charge transport capacity in TDOS. In addition, the generation of Ni(Co)OOH can effectively reduce the reaction barrier (O* OOH*) of the rate-determining step of the OER process, and the lowest potential barrier at the Ni site, optimizing the oxygen evolution kinetics process (Fig. 6fg). At the same time, the adjustment of Fe Co can provide the optimal position of the D-band to move towards the Fermi level, which in turn affects the affinity for OH* (the first step). The Cu PD site can promote the transfer of electrons and synergistically accelerate the OER process, which shows significantly better OER activity than commercial RuO2 catalysts.
Figure 6Calculation results of Feconicupd thin-film electrocatalyst in terms of OER
Conclusions and prospects
In this paper, a feconicupd thin-film electrocatalyst with high activity and stability was designed by magnetron sputtering. Combined with theoretical calculations, it is shown that the abundant active sites and the cocktail effect of high-entropy materials can enhance the basic HER activity, and the growth of multiple transition metal hydroxides by the in-situ reconstruction of the reaction process is the internal reason for reducing the reaction rate-determining step. This study not only provides a basis for the design of HEA, but also provides a basis for the theoretical research of multiple transition metals in the field of electrochemical water splitting.
Bibliographic information
wang, s., xu, b., huo, w., feng, h., zhou, x., fang, f., jiang, j. (2022). efficient feconicupd thin-film electrocatalyst for alkaline oxygen and hydrogen evolution reactions. applied catalysis b: environmental, 313, 121472.