Elsevier

Carbon

Volume 139, November 2018, Pages 129-136
Carbon

Breaking the scaling relations for oxygen reduction reaction on nitrogen-doped graphene by tensile strain

https://doi.org/10.1016/j.carbon.2018.06.026Get rights and content

Abstract

The scaling relations between adsorption energies of different reaction intermediates on a variety of surfaces simplify the catalyst design, but meanwhile they make it difficult to achieve the optimal catalyst. The oxygen reduction reaction (ORR) at the cathode of fuel cells, for instance, cannot attain the ideal efficiency even with the nowadays best-performed catalysts, due to the correlation between the adsorption energies of O, OH and OOH. In this work, based on the nitrogen doped graphene, we compare the ORR reaction on active sites with different deformation while in similar structural and chemical environments. Since both the tensile strain and the adsorption of O tend to stretch and even break the N-C bond, tensile strain enhances the adsorption strength of O atom while leaving that of OH and OOH approximately unchanged. In contrast, the local curvature cannot uncouple the correlation between the adsorption strength of O, OH and OOH. The results suggest that it is possible to improve the catalytic performance by tuning the structure of catalyst to be in selective resonance with adsorption of a specific intermediate, which provides a new way to design optimal catalysts.

Introduction

A heterogeneous catalyst stimulates a reaction by tuning the intermediates to their different desirable energy levels [[1], [2], [3], [4]]. However, scaling relations between the adsorption energies of different intermediates of reactions have been found across various surfaces [[5], [6], [7], [8], [9], [10], [11]]. Although enabling fast catalyst selection to guide experiments, the scaling relations nevertheless set a fundamental limitation to the performance of the catalysts. For the oxygen reduction reaction (ORR) at the cathode of fuel cells, for instance, due to the scaling relations between the adsorption energies of the intermediates, namely O, OH and OOH, even the most desirable Pt-based catalyst displays a large overpotential [6,12,13]. Together with the high cost of Pt, it hinders the commercial viability of fuel cells [[14], [15], [16]]. Therefore, designing more active electrocatalysts of low price by breaking the scaling relations is of great importance [[17], [18], [19]].

Carbon alloy catalysts (CACs), which are multi-component materials mainly composed of carbon atom, have shown promising applications in catalyzing a variety of reactions [[20], [21], [22], [23]]. Among which, graphene-based catalysts have attracted a lot of attention since graphene was successfully isolated from graphite [24]. It has been found that the graphene, with modification of heteroatoms, defects and edges, possesses surprisingly high catalytic activity [[25], [26], [27]]. Besides, graphene is ready to stretch or form corrugations under tensile or compressive stress [[28], [29], [30]]. Theoretically, it has been shown that the electronic structure of graphene can be tuned by strain and curvature [[28], [29], [30], [31], [32], [33]]. It is thus worthy to check whether these additional tunable factors can provide opportunities to uncouple the correlation between the binding strength of intermediates in ORR [8,34].

It has been reported that the scaling relation between the adsorption energies of O and OH is not so strictly satisfied as that between the adsorption energies of OH and OOH. A quite general scaling relationship is established between the adsorption energy of OH and that of OOH almost for all ORR catalyst, i.e., ΔGO*OH=ΔGO*H+3.2eV [6,[8], [9], [10]]. Instead, although the relations ΔGO*=kΔGO*H+a are still present, a is sensitive to the surface structures of catalysts [10,11,13]. Especially in the graphene-based catalysts, both the parameters k and a depend on the local chemical environments of the active sites [34,35]. The uncertainty in the scaling relations has already indicated a possibility of tuning the adsorption energy of O independent of those of OH and OOH in graphene-based catalysts. However, the diverse chemical environments of the different active sites in the previous work prevent us from clarifying the physical nature of uncoupling the correlation between the adsorption strength of O and that of OH and OOH [34].

In the current work, based on the nitrogen doped graphene, we compare the ORR reaction on active sites with different deformation while in similar structural and chemical environments. We find that the applied tensile strain can regulate the adsorption strength of O separately while leaving that of OH and OOH approximately unchanged. In contrast, the local curvature originated from compressive stress cannot uncouple the correlation between the adsorption strength of O, OH and OOH. The physical origination lies in that both the tensile strain and the adsorption of O coincidentally tend to increase the local bond length between N and the active C. The results suggest that it is possible to improve the catalytic performance by tuning the catalyst to be selectively in resonance with the one of intermediates.

Section snippets

Methods

The ideal ORR includes four electron transfer steps involving different intermediates. According to whether O2 molecule dissociates or not before it is hydrogenated, the ORR mechanism can be categorized into the dissociative one and the associative one [[36], [37], [38]]. The ORR on N-doped graphene prefers the associative mechanism since O2 takes an end-on configuration. Moreover, unlike metal-based catalysts, the OOH intermediate does not tend to decompose [34]. Therefore we just consider the

Results and discussion

The typical structures before and after adsorption of the intermediates are shown in Fig. 1(c). As expected, the main features change little upon deformation, except that the local bond length or curvature varies. Therefore the active sites with different deformation are indeed in similar structures and chemical environments.

Firstly, we investigate the planar models and study the influence of the tensile strain on the N-C bond length before adsorption dNC* and the corresponding ΔG. As seen in

Conclusion

The influences of the tensile strain and the rippling deformation on the ORR catalytic efficiency of N-doped graphene have been theoretically studied by using first-principles calculations. Since both the tensile strain and the adsorption of O tend to break the N-C bond, the adsorption strength of O atom is resonantly enhanced by the tensile strain, while those of OH and OOH remain unchanged. The underlying mechanism for tensile deformation is consistently confirmed from different aspects,

Acknowledgments

The numerical calculations have been carried out at the High Performance Computing Center of Nanjing University. This work was supported by the National Natural Science Foundation of China (Grant No. 11174123), the Basic Research Program of Jiangsu Province (Grant No. BK20161390) and the Fundamental Research Funds for the Central Universities (Grant No. 020414380100).

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