Background: Platinum (PT)-based multi-component alloys are widely recognized as the best catalysts for oxygen reduction reactions (ORRs) in proton exchange membrane fuel cells (PEMFCS). The alloying process can effectively adjust the microgeometry and electronic structure of the active metal atoms and promote the catalytic kinetics. In addition, the addition of non-*** can effectively reduce the use of platinum, thereby significantly reducing the cost of PEMFC. Alloying can also change the work function of the center and surface of the d-band of the platinum atom, affecting the catalytic activity. The medium entropy alloy is an alloy of 3 4 elements, and the configuration entropy is 1r 15R (R stands for gas constant), which maximizes the benefits of high-entropy alloys while providing a clearer catalytic active site.
Professor Liu Jingjun of Beijing University of Chemical TechnologyTheoretically, they designed a set of PT-based medium-entropy alloys with precise and controllable composition and structure, and revealed the intrinsic activity and structural stability from the perspectives of geometric and electronic structure, work function change and reaction free energy.
Figure 1Design ideas of entropy alloy in PTModels and calculation methodsAdoptionDMOL3 module of the Materials Studio packageThe electrocatalytic hydrogen evolution performance and electronic properties were studied theoretically, and the electron exchange correlation was calculated based on the PBE functional under the Generalized Gradient Approximation (GGA) to optimize the alloy configuration.
The DFT Half-Nuclear Pseudopotential (DSPP) method is used to introduce the relativistic effect to process the core electrons, and the selection of the core electron processing base set is double numerical plus polarization (DNP). For platinum and platinum-based alloys (111), the surface is built with a four-layer structure, each layer has 16 atoms, and a vacuum of 15 is established in the z-direction to reduce the interaction of adjacent layers.
Results & Discussion
In the stable PTCU(111) model, part of the PT atoms are replaced by guest SN atoms to form an atomically accurate entropy alloy in PTCUSN (PT:Cu:SN 11:4:1), as shown in Figure 2(a). In PTCUSN, there is a PT-skin structure (PTCUSN-skin) or Cu atom segregation (PTCUSN-cuseg) in the subsurface layer of the entropy alloy, as shown in Fig. 2(b, c). In addition, the obtained PTCUSN alloy has a PT-SKIN structure, and the exogenous SN atoms tend to replace PT (PTCUSN-SNSEG) in the surface layer, as shown in Fig. 2(D). A similar treatment was applied to the PTCUW (PT:Cu:W 11:4:1) alloy.
Figure 2PTCUSN structure diagram
Firstly, the cohesion energy of the entropy alloy in PTCUSN and PTCUW is calculated, where the cohesion energy of PTCUSN-SNSEG is 521 EV compared to 5 for PTCUW-WSEG52 EV, with pure PT (5.).46 EV) is very similar, indicating that Sn or W can easily form solid-solution alloys with PT, and the system formation energy of both is also the lowest, with good thermodynamic stability. The ORR properties of the above-mentioned medium-entropy alloys such as PTCUSN and PTCUW in acidic solution are analyzed, and Fig. 3(A) shows the ORR kinetic linear scanning voltammetry (LSV) curves of DFT using the thermodynamically stable PT, PTCU, PTCUW-WSEG and PTCUSN-SNSEG model structures.
The half-wave potential of the ORR on PTCUSN-SNSEG is 086 V (vs RHE), which is much higher than the pure PT of 080 V or 0 ptcu83 V, as shown in Figure 4(b). The half-wave potential of PTCUW-WSEG is 085 V, which is only 10 mV lower than PTCUSN-SNSEG and also higher than PT or PTCU. Therefore, according to the kinetic LSV simulation results, the designed medium-entropy PTCUSN and PTCUW alloys exhibit good ORR activity, which can be attributed to the unique microgeometry and electronic structure of the medium-entropy alloys.
Figure 3Catalyst theory LSV and half-wave potential
The stability of entropy PTCUSN and PTCUW alloys in the study was calculated by DFT. The Pt atomic vacancy formation energy (ΔEVAC) is a viable indicator of the stability of Pt-based alloys, as a larger vacancy formation energy indicates stronger metallic bonding and less dissociation tendency. The PT vacancies formed at different positions on the surface of the medium-entropy alloy and their corresponding δevac values are shown in Figure 4 and Table S2.
The results show that the vacancy formation energy on the surface of pure PT(111) is 089 ev。In contrast, the δevac of PTCU-skin (144 EV) higher than PTCU (134 ev)。It is shown that the presence of Cu atoms can improve the stability of pt atoms, and in all cases, the δevac of PTCUSN-SNSEG is as high as 199 EV, as shown in Figure 4 (f), is higher than PT (0.).89 ev)、ptcu (1.34 EV) or PTCUW alloy (138 ev)。
These results show that Sn or Cu atoms have a significant effect on the stability of enhanced medium-entropy alloys. For PTCUW, PTCUW-WSEG has a maximum vacancy formation energy of 180 EV, which is lower than the 1 of the PTCUSN-SNSEG99 ev。Therefore, we can observe that PTCUSN-SNSEG has better structural stability than PTCUW-WSEG, and it effectively hinders the dissolution of PT atoms during electrochemical processes.
Figure 4Vacancies forming energy of PT alloys
The D-band center of the active metal is a descriptor for determining the ORR activity of PT-based alloys due to its ability to directly correlate ORR kinetics. The model structures used in the D-band center calculations are the thermodynamically stable PTCUW-WSEG and PTCUSN-SNSEG structures. Figure 5 shows the PDOS of PT atoms on the surface of different samples. The center of the D-band of PTCUSN-SNSEG is 266 EV, 2 more than PT30EV and PTCU 243 EV is more negative.
As a comparison, the center of the D-band of PTCUW-WSEG is calculated to be 260 EV, slightly higher than PTCUSN-SNSEG. The negative shift of the center of the D-band contributes to the weakening of oxygen adsorption on the alloy surface, thereby promoting the kinetic process of ORR. Δeo* of PTCUSN-SNSEG (2.)53 EV) was higher than that of other control samples, indicating that the adsorption of O* by PTCUSN alloy was weaker, and the ORR process was more active. It can be seen that the ORR activity of PTCUSN-SNSEG is higher than that of PTCUW-WSEG.
Figure 5Catalyst PDOS and D-band center
In order to understand the influence of the microstrain effect of the medium-entropy alloy on the catalytic activity from the atomic perspective, the effect of doped Sn or W atoms on the PT-PT spacing was calculated, as shown in Figure 6. The two dielectric entropy alloys have a compressed (111) surface, and the average pt-pt bond length is less than that of pt(111) (2.)868a), which is caused by lattice mismatch between surface and substrate atoms. The compressive strain of PTCUSN-SNSEG is 366%, while the compressive strain of PTCU is 311%, as shown in Figure 6. The compressive strain of PTCUW-WSEG is 419%。
Sn or W can further enhance the strain effect of PTCU alloys, but the strain effect caused by W is greater than that of Sn. This is because the atomic radius of w is less than that of sn. Compared with other control samples, the optimized electronic and geometrical structure of PTCUSN-SNSEG gives it higher ORR catalytic activity.
Figure 6Strain effects of PT-based alloys
The relationship between the center of the D band and the compressive strain of the platinum-series entropy alloy in PTCUW-WSEG and PTCUSN-SNSEG is shown in Figure 8. It has been observed that there is a volcanic relationship between the center of the D-zone and the compressive strain. This correlation may arise from the compression distortion between the D-bands that causes the orbits to overlap, resulting in D-band broadening or deviation from the center of the D-band.
Figure 7Volcanic curve of the center of the d-zone with compressive strain
Finally, the ORR free energy of the catalyst is calculated, and the free energy diagram is shown in Figure 9. Due to the scaled properties of the binding energy of ORR intermediates, the total ORR free energy of an ideal catalyst is 4The 92 EV should be evenly distributed over the four steps of proton-electron transfer. When the impressed potential U= 0V, the free energy order of the ORR process of PT, PTCU and the two medium-entropy alloys is downhill, indicating that the ORR occurs spontaneously. However, compared with other compounds, PTCUSN-SNSEG has similar reaction energies to each step of the ideal curve. In order to further demonstrate that the medium-entropy PTCUSN-SNSEG has high ORR activity, the starting potential of the ORR (UONSET) was calculated.
The calculated uonset value of ptcusn-snseg is 109 V, higher than PT (0..)82 V) or PTCU (085 v)。This result is consistent with the highest activity of PTCUSN-SNSEG shown in Figure 3(a). This increase in activity is attributed to the negative shift of the center of the d-band and the compressive strain of the medium-entropy alloy. In addition, PTCUSN-SNSEG (493 EV) and PTCUW-WSEG (498 EV), indicating that the activation barrier of electrons to the reactant on the surface of the electrocatalyst is low, which can increase the ORR rate. The results show that the medium-entropy alloy can improve the ORR catalytic performance more favorably than the other PT-based alloys we calculated.
Figure 8ORR free energy of PT alloys
Conclusions and prospects
In this paper, the excellent ORR properties of PT medium-entropy alloys are theoretically calculated, and compared with PT and PTCU, medium-entropy alloys have good stability. In addition, the unique geometry and electronic structure provide theoretical support for the application of entropy alloys in Proton Exchange Membrane Fuel Cells (PEMFCS) in PTCUSN, and also provide a good calculation case for the research of next-generation high-efficiency catalysts for ORR.
Bibliographic information
chen, z., jin, c., ji, x., liu, j. (2022). atomistic understanding of pt-based medium entropy alloys for oxygen reduction electrocatalysis based on first principles. international journal of hydrogen energy.