First-principles Study of Pt-supported MXene as Single-atom Catalysts for Oxygen Reduction Reaction and Oxygen Evolution Reaction

第一性原理研究 MXene 負載 Pt 作為 ORR/OER 單原子催化劑

Student thesis: Doctoral Thesis

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Award date6 Oct 2023

Abstract

MXenes, transition metal carbides or nitrides, are a typical two-dimensional layered material, which with rich chemical composition and tunable physical and chemical properties. Owing to their excellent electrical conductivity, good mechanical stability, and tunable physicochemical properties, MXenes have significant application prospects in the fields of energy storage and energy conversion. On the other hand, due to the diversity of MXenes, studies on their structures and electronic properties are still unclear, and highly active MXene-based catalysts still need to be explored.

In this thesis, based on density functional theory, we systematically explored Pt-doped MXenes as single-atom catalysts (SACs) for the oxygen reduction reaction/oxygen evolution reaction (ORR/OER) reactions. The elemental composition and structural parameters of MXenes are closely related to their electrochemical properties. Based on the structure-activity relationship, we systematically discuss the applications of Pt-doped MXenes in catalysis.

Firstly, we explored the effect of biaxial strain on the catalytic activity of Pt-VF-Ti2CF2 SACs. We found that strain can significantly enhance the activity of the catalyst, which is due to the strain altering the electronic properties of the catalyst, thereby adjusting the adsorption strength of catalytic intermediates on the catalyst. Furthermore, the adsorption of OH on Pt-VF-Ti2CF2 exhibited an abnormal d-band center model, which was attributed to the repulsion between the adsorbed state and the metal d electrons. Moreover, the functionalized F atoms in Pt-VF-Ti2CF2 also lead to an abnormal phenomenon in which the d-band center of Pt atoms shifts to the Fermi level with the increase of compressive strain.

Secondly, we explored the effect of biaxial strain on Pt-VO-Ti2CO2 to determine whether O-functionalized Ti2C satisfied the anomalous d-band center. The calculation results show that Pt-VO-Ti2CO2 obeys the d-band center, and with the increase of tensile strain, the catalytic activity of the catalyst is significantly improved. Under the solvation effect, the ηORR/OER of Pt-VO-Ti2CO2 with 6% tensile strain decreased to 0.41/0.31 V. By rearranging the Nernst equation, the theoretical polarization curve of our calculated can be directly compared with the experimental results.

Thirdly, based on the above research, we further discuss the applicability of the d-band center model in MXenes, which are functionalized by two elements (Pt-VN(L)-Ti2CNL). We found that functionalized atoms can regulate the electronic properties of catalysts to enhance their catalytic activity. But the d-band center, electron transfer, and work function of a catalyst are not the only unique factors or descriptors of catalytic performance. In addition, some suggestions for improving the catalytic performance of catalysts have been proposed, such as 1), doping Pt atom on the side of N-functionalized Janus-MXenes; 2), the small electronegativity difference between N and L in Ti2CNL; 3), not using functional atoms with excessive electronegativity such as F atoms. Moreover, all our discussed SACs have good selectivity for the ORR rather than the hydrogen evolution reaction (HER).

Finally, considering the high cost of DFT calculations and the complexity of the factors affecting the catalytic activity of MXene-based SCAs, we employed machine learning (ML) to accelerate the screening of efficient catalysts and reveal the intrinsic factors affecting catalytic activity. We studied the catalytic performance of Pt-doped dual transition metal (DTM) Janus-MXenes and built an ML database. It was used to establish machine learning (ML) models, then to explore the physical and chemical properties affecting the catalytic overpotential. Thus, some excellent catalysts, including Pt-VO-MnTiCO2 (ηORR/OER=0.24/0.38 V) and Pt-VO-PdTiCO2 (ηORR/OER=0.33/0.36 V) were found, and the EOH, Eb, d-band center of the Pt atom, the Hxf and Hof and other factors that affect catalytic performance of SACs have been reported.

In conclusion, we systematically discussed the various influencing factors of Pt-doped MXene-based SACs on the ORR/OER and proposed some suggestions for improving the catalytic activity. This thesis is expected to be instructive for the design and development of high-efficiency catalysts.