The use of traditional fossil energy is closely related to the rapid development of modern society, however, the subsequent energy consumption and environmental pollution will in turn restrict the further development of society. Therefore, there is an urgent need for environmentally friendly and renewable alternative energy sources to replace traditional fossil fuels. Storage and electrochemical energy conversion technologies such as fuel cells and rechargeable metal-air batteries, which have the advantages of safety, green, pollution-free and high power density, can effectively solve the above problems.
However, the development of efficient electrocatalysis is required because the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the oxygen electrode severely hinder the overall reaction efficiency of the battery. Traditionally, platinum-based catalysts have been effective catalysts for ORR, and oxides of ruthenium (Ru) and iridium (IR) have been excellent OER catalysts. However, the scarcity, high cost, low stability, susceptibility to co-poisoning, and single catalytic function of these catalysts hinder the further commercial application of this technology. Therefore, it is of great significance to develop green, efficient, safe, durable, low-pollution, and low-cost ORR and OER dual-function electrocatalysis.
To this end,Liu Chao, Jiangxi University of Science and TechnologyDensity functional theory (DFT) was used to study graphene-supported lanthanum oxide clusters (LA2O3-GRA) as bifunctional electrocatalysts.
Models and calculation methods
Figure 1 la2o3-gra.Schematic diagram
According to Br**is's law, the actual crystal plane is parallel to the crystal surface with the highest atomic density. Therefore, by calculating the atomic density of the cubic lanthanum oxide crystal plane, the authors found that (1 1 0), (1 1 0), (2 0) have the highest atomic density, indicating that it is the actual surface of the crystal. As shown in Figure 1, based on the actual outer surface, the authors extracted the smallest cage-like clusters from the cubic lanthanum oxide crystals, and in addition, the reduction in the size of lanthanum oxide atoms produced unsaturated coordination atoms, thereby adjusting their electronic structure and regulating their catalytic performance. The authors used a (5 5 1) graphene sheet supercell as the carrier of the La2O3 cluster, and set up a vacuum layer of 30 to shield the interaction between adjacent layers.
For all DFT calculations of the LA2O3-GRA system, the authors employed:DMOL3 module on Material Studio software, and a generalized gradient approximation (GGA) with the Perdew Burke Ernzerhof (PBE) function was selected to optimize the La2O3-GRA model and commutative association.
In addition, the authors used Grimme's DFT-D method to deal with van der Waals interactions, and a conductor-like shielding model (COSMO) to simulate the H2O solvent environment, with a corresponding dielectric constant of 7854。Since LA is a heavy element with many (57) electrons, the authors used the density functional heminuclear pseudopotential (DSPP) to describe its electrons, and the convergence criterion for the structure was set as follows: the energy is 10 10 5 ha with a maximum displacement of 005 with a maximum force of 002 ha· -1, and in order to ensure the accuracy of the calculation results, the author sets the monkhost-pack k point grid to 5 5 1.
Results & Discussion
Figure 2Geometry of O-Gra-1, La-Gra-2, and Face-Gra-1
As shown in Figure 2, among the optimal O-Gra, La Gra, and Face Gra structures, O-Gra-1 has the highest stability, followed by Face-Gra-1, and La-Gra-2 has relatively low stability. This means that O-Gra-1 is the most readily available configuration in experimental synthesis, while La-Gra-2 is less likely to be obtained.
Therefore, the activity of O-Gra-1 essentially determines the performance of the entire system. The authors performed a simple O2 adsorption test and found that the configuration of LA-GRA-2 changed after oxygen adsorption, thus excluding LA-GRA-2. As can be seen in Figure 2, there are six active sites on the La2O3 cluster, and the sites in the same layer are equivalent, so they can be classified into different types;O-Gra-1 can be divided into upper and lower sites, Face-Gra-1 can be divided into upper, middle and lower sites, while La-Gra-2 is only divided into upper and middle sites and no lower sites because the lower sites are used to attach graphene substrates.
Figure 3Adsorption of oxygen on O-Gra-1, Face-Gra-1, and La-Gra-2
Since the adsorption of O2 is critical to the occurrence of ORR, and its desorption is also an integral part of the OER cycle, the authors first calculated the oxygen adsorption at the active site. As shown in Figure 3, O2 can be adsorbed on a single active site, with both side and end adsorption, or with both bridge-edge adsorption. For O-Gra-1 and Face-Gra-1, due to the strong adsorption of O2 on the catalyst, the optimized back-end edge adsorption mode is converted into side adsorption, and the adsorption intensity of the bridge edge is always smaller than the corresponding side adsorption mode due to space constraints, all of which indicate that the main mode of O2 on the catalyst is side adsorption.
However, after the optimization of the O2 adsorption model on LA-GRA-2, the loading form of LA2O3 changed severely, indicating that LA-GRA-2 was unstable. Therefore, the authors excluded the la-gra-2 configuration.
Figure 4Diagram of free energy at different positions on O-Gra-1
As shown in Figure 4, for the upstream sites, the ORR potential control step (PDS) for P1 and P2 is the last step *OH*+OH, and the corresponding ORR is 139v。As a reverse reaction of the ORR, the PDS of OER on P1 is *OH*O and needs to be overcome by 134 EV energy, while P2 PDS is *ooh O2 only needs to overcome 0The energy of 99 EV corresponds to an oer of 058v。
As with the upper site, the PDS of the lower sites P1 and P2 are also the *oh oh of the last step, and the corresponding ORR is 1.41 v。The oer of p1 is smoother, and the pds is *o *ooh, and the oer is 070v；This is because the synergistic effect of *O on the lower site with the substrate enhances its adsorption strength and smooths out the energy change from *OH to *OOH.
Figure 5Diagram of free energy at different positions on face-gra-1
As shown in Figure 5, the PDS at the upper, middle, and lower sites are the last step for either P1 or P2: *OH OH with corresponding ORRs of 1., respectively69v、1.69v and 156v。As for OER, the episite P2 has an optimal route, i.e., PDS is *2OH *OOH and OER is 079v；The activity of P1 and P2 in the middle position is similar, and the corresponding oer is 0., respectively96v and 097V, while for the lower site P1, OER is 060v。However, in general, the bifunctional electrocatalytic performance of face-gra-1 is poor.
Figure 6Free energy plots of different positions on OH modified O-Gra-1
As shown in Figure 6, for PDS at upper and lower 1 and upper and lower 2 sites, either P1 or P2 is the last step: *OH OH has an ORR of 051 V and 048 v。It has good activity at the upper site P2, and the corresponding ORR is 044 V, while the PDS of P1 is *ooh o, and the corresponding orr is 075 v。For OER, although the adsorption intensity of OH is similar, the closer the reaction site is to the substrate, the easier it is to accept electrons, which means that the catalytic rate is faster. The optimal site for the upper and lower 1 and upper and lower 2 sites is p1, and the corresponding oer is 052v and 059V, the best upper episite is P2, and the OER is 063v。Since the lower site is very close to the substrate, the synergistic effect enhances the adsorption of O atoms, so the main occurrence of ORR and OER on the lower lower site is P1, and the corresponding ORR is 046V, OER is 034v。
Figure 7Free energy plots at different positions on OH modified face-gra-1
As shown in Figure 7, in both cases, the optimal ORR path for the upper and lower 1 and upper and lower 2 sites is ORR 0., respectively66v and 0For P1 at 48V, the optimal OER path is 0., respectively61v and 039v p1. In the upper and upper middle positions, the best activity was P2 with an ORR of 067v and 059V, OER is 067v and 059v。For the median site, the optimal site is P2 with an ORR of 080V, OER is 075v。P1 is the best of the lower middle loci, ORR 064v，ηoer/0.56v。Compared with the above sites, P1 in the lower and lower sites has the best bifunctional catalytic activity, i.e., the ORR is 033V, OER is 037v。
Figure 8Scaled relationship between adsorption free energies of reaction intermediates. ((a)δg*oh vs.δg*ooh，(b)δg*oh vs.△g*o，(c) g*2oh vs.δg*2OH) and volcano diagram ((D) orr vsδg*oh，(e)ηoer vs.G*O-ΔG*O(P1) and (F) Oer ΔG*2OH-ΔG*OH(P2)).
As shown in Figure 8(ac), ΔG*OOH can be expressed as a function of ΔG*OH, i.e., ΔG*OOH=081δg*oh+3.30（r2=0.93）。ΔG*O needs to be divided into two types: ordinary *O (ΔG*O') and synergistic *O (G*O"), G*O' can be expressed as ΔG*O' = 139δg*oh+2.34, r2 is 099, while "ΔG*O" can be expressed as ΔG*O" = 105δg*oh+1.56，r2=0.99。For *2oh, δg*2oh can be expressed as δg*2oh = 155δg*oh+1.54，r2=0.98。
Based on the proportional relationship between the δg*ads of OOH, O, 2OH and OH described above, the authors constructed volcanic diagrams of ORR and OER. The PDS of the best path in P1 and P2 is always *H+O2 OOH or *OH OH, and the corresponding volcano plots of ORR and δG*OH are shown in Figure 8(D). The volcano map of OER should be divided into two paths: P1 and P2. The oer of p1 can be expressed by δg*o-δg*oh;In addition, due to the presence of two types of *O, two volcano maps can be obtained. As can be seen in Figure 8(e), the peak overpotential of synergistic oxygen is smaller, which means that synergistic oxygen is more favorable for OER. The oer of P2 can be described as ΔG*2OH-ΔG*OH, as shown in Figure 8(f). The presence or absence of OH ligands in different loading modes, different active sites, and OH ligands will result in different distributions in the volcano map, suggesting that they have a large influence on catalytic activity.
Conclusions and prospects
The optimal catalytic site (Up-down1) of the most stable configuration modified by the OH group (O-Gra-1) exhibited excellent bifunctional catalytic activity, i.e., the ORR was 051V, OER is 052v。In addition, the optimal active site (Up-DOWN2) of OH-modified FACE-Gra-1 showed excellent performance, i.e., ORR of 048V, OER is 039v。Finally, based on the dependence between the adsorption free energy (δgads) of reaction intermediates, the formed volcanic diagram between the catalytic activity and the δgads can be used for the catalytic performance of structurally similar catalysts**.
Bibliographic information
wang d, jin l, liu m, et al. the graphene-supported lanthanum oxide cluster as efficient bifunctional electrocatalyst for oxygen reaction[j]. molecular catalysis, 2023, 535: 112879.