Brief introduction of the results
Oxygen reduction reaction (ORR) is a critical reaction in various renewable energy technologies, and platinum is the best catalyst for ORR. However, the high cost of platinum limits its commercial application. Therefore, researchers have been working on finding cost-effective alternative catalysts, among which fe-n-c is one of the most promising catalyst candidates. However, the rate-limiting steps of the ORR on Fe-N-C remain unclear, which severely limits the development of catalysts. Therefore,DucUniversity of X. Austin, Liu Yuanyue et alThe activation energy of the ORR step on Fe-N-C was calculated by the potentiostatic mixing solvation kinetic model (CCP-HS-DM). By incorporating explicit solvation and electrode potential into the model, the model effectively simulates the electrochemical kinetics of the solid-water interface, revealing a different reaction mechanism from what is commonly believed: the rate-determining step is not a hydrogenation step, but an oxygen molecule on a Fe atom that replaces the pre-adsorbed water molecule. The work in this paper provides new insights into the catalytic mechanism of ORR and Fe-N-C, and emphasizes that the current research on multiphase electrocatalysis should provide as much kinetic information as possible.
Calculation method:
CP-HS-DM is a de novo molecular dynamics (CP-AIMD) that can achieve a constant potential with explicit water. The unit cell has a thin film of water molecules on top of the catalyst, while the remaining space is filled with a implicit solution that is modeled as a continuous dielectric with a point ion charge. This implicit solution is used to balance the extra charge in the explicit region and introduces a region with a smooth electrostatic potential distribution that can be used to obtain the counter electrode potential.
In this paper, the PAW pseudopotential of the VASP quantum computing software package is used to calculate the first-principles theory of the regular ensemble, and the exchange-related effect is described by using the generalized gradient approximation (GGA)-PBE functional. In all calculations, van der Waals correction was performed using the DFT-D3 method, with a truncation energy of 400 EV. The unit cell is composed of 6 6 graphene (with a FeN4 complex instead of 6 carbon) and 44 explicit H2O molecules (one more H to simulate acidic conditions), and the k-point grid for sampling in the Brillouin zone is 3 3 1.
**Reading guide
It is generally accepted that the ORR reaction pathway on Fe-N-C has the following steps (Figure 1A:*): O2OO, *OO+H++E-OOH, *OOH+H++E-O+, *O+H++E-OH, *OH++E-OH, *OH++E-H2O. The experimental determination of the rate-determining step is extremely challenging, and density functional theory (DFT) provides a way to calculate the reaction energy barrier of the step, so that the question about the rate-determining step can be answered in principle. However, due to the complexity of the system and the need to deal with the effects of solvation and electrode potential, it is difficult to directly calculate the activation energy of heterogeneous electrochemistry. Therefore, most computational studies are based on the assumption that the thermodynamics of each step are calculated and thus the kinetics are inferred, so the step uphill (or downhill with the least resistance) with the greatest resistance is called the velocity-determining step, which has the highest activation energy.
Most ** indicate that the *oo + H+ + e-OOH or *OH + H+ + e-H2O steps are thermodynamically confined steps. However, thermodynamics is not necessarily related to kinetics and is essential for determining the information on the rate-determining step and activation energy, so the authors studied the activation energy of the reaction. In this paper, it was found that the adsorption of O2 was consistent with the desorption of H2O. As shown in Figure 1c, when the O2 in the solvent approaches Fe to form a Fe-OO bond, it displaces the adsorbed H2O, breaks the Fe-OH2 bond, and eventually, O2 replaces H2O and is adsorbed on Fe. In this process, Fe atoms remain bonded to the adsorbate (H2O, O2, or both) with a reaction energy barrier of 045 EV, as shown in Figure 1b. In the transition state, both H2O and O2 bind to the Fe site, exhibiting a different reaction mechanism: Step *H2O H2O and O2 + Oo steps are not separated;Instead, they occur at the same time, so they should be written together as *H2O + O2 O2 + H2O (Figure 1A).
Figure 1 (a) Schematic diagram of the ORR mechanism;(b) *h2o + o2 *o2 + h2o at 0Free energy diagram at 5 v vs she;(c) Representative structures selected from the reaction coordinate values marked with arrows. The brown, blue, gray, red, and white spheres represent the atoms of iron, nitrogen, carbon, oxygen, and hydrogen, respectively.
Here, the next steps, i.e., *oo + H+ +e-OOH, *ooH + H+ +E-O + H2O, *O + H+ +E-oh, and *OH + H+ +E-OH2 electrochemical steps, are further investigated. Figure 2 shows *OOH + H+ +E-O + H2O, in which the resulting OH is formed by proton attack as the *OH bond is broken. In this process, the system gradually acquires an O with an excess electron concentration, which attracts protons more easily and promotes the formation of H2O (the barrier is only 0.).15 ev）。This change in the number of electrons cannot be observed in traditional charge simulations, so the potentiostatic method used in this paper is required.
Figure 2 (a) u = 05 V vs SHE*ooH + + E *O + H2O free energy and net charge change plot;(b) Representative structures selected from the reaction coordinate values marked with arrows. Protons transferred to *OOH are highlighted in green.
Figure 3 illustrates the reaction energy barriers for each step of the ORR, because in the simulation in this paper (without geometric constraints), the protons spontaneously diffuse to the adsorbate, completing the reaction, and the solid*O + H+ +E-OH and *OH + H+ +E-OH2 steps have no barriers. The results showed that the *H2O + O2 O2 + H2O step had the highest activation energy and was the rate-limiting step of the reaction. To test whether this conclusion still holds true at lower potentials, u = 0The ORR of 1 V vs She, as shown in Figure 3, found in this paper that all electrochemical steps become barrier-free, while the *H2O + O2 O2 + H2O step still has 026 EV barrier. Therefore, the rate limiting step is still *H2O + O2 O2 + H2O.
Fig.3 Energy barriers of the ORR process at different potentials.
In order to understand the principle, the adsorption energies of O2 and H2O on Fe-N-C at different potentials are calculated. As shown in Figure 4a, the reduction of the potential enhances O2 adsorption while attenuating H2O adsorption. Thus, the lower barrier at lower potentials can be explained by the stronger driving force provided by the enhanced binding to O2 and the weakened binding to H2O. The change in binding strength due to potential can be attributed to the different surface charges at different potentials. Calculations show that when the electric potential is from 05 V down to 0At 1 V, pure Fe-N-C gains about 1 electron, and this large change in surface charge results in a significant change in the electronic state occupation, which changes the chemical reactivity of Fe-N-C. On adsorption, O2 gains electrons from Fe-N-C, while H2O donates electrons (see Figure 4B), and increasing the electron charge on Fe-N-C will enhance the transfer of electrons to the adsorbate while inhibiting the reverse transfer. Thus, lowering the potential strengthens the binding to O2 and weakens the binding to H2O.
Fig. 4 (a) Adsorption energy of Fe Nc to H2O and O2 at different potentials(b) Isosurface changes in differential charge density due to adsorption. Yellow represents electron accumulation and cyan represents electron consumption (equivalent = 0.).015 e å−3)。The black arrows indicate the direction of electron transfer, and the numbers indicate the magnitude of charge transfer using the Bader.
Conclusions and prospects
In this paper, a de novo molecular dynamics model of a potential was used to reveal the rate-determining step of ORR on Fe-N-C, and a new reaction mechanism was discovered, that is, O2 can replace the pre-adsorbed H2O on Fe without exposing the Fe site. This step has the greatest activation energy in the ORR step, and its barrier counter-decreases as the electrode potential decreases, which is due to the stronger adsorption for O2 and weaker adsorption for H2O when the surface carries more charge. This phenomenon can be further demonstrated by O2 adsorption to gain electrons from the surface and H2O to the surface. These results suggest that sites with more negative charges are more active to ORRs and demonstrate the importance of kinetic information in understanding and designing heterogeneous electrocatalysts.
Bibliographic information
yu, s., levell, z., jiang, z., zhao, x., liu, y. (2023). what is the rate-limiting step of oxygen reduction reaction on fe–n–c catalysts?. journal of the american chemical society.
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