Evaluation of electrocatalytic dinitrogen reduction performance on diamond carbon via density functional theory
Graphical abstract
Introduction
The nitrogen circulation, as one of the most important circles on Earth, exerts great impact on human beings and other organisms [1]. In air, dinitrogen gas accounts for 78.1% of air composition [2], however, the huge utilization of elemental nitrogen from air, i.e. nitrogen fixation, is not easy. Naturally, some microorganisms can convert N2 to ammonia using nitrogenases [3]; some N-containing compounds can also be formed in electrical storms [1,4], which can be used by plants. But these two routes for nitrogen fixation are not enough for meeting the demands of social development. Currently, artificial nitrogen fixation heavily relies on the Haber-Bosch (H-B) process, which occurs under high temperature and pressure with the massive consumption of global energy and significant emission of greenhouse gases [5]. Facing the world growing energy crisis and changes in global climate, searching for a sustainable substitution for the transitional H-B process is highly desirable.
Electrochemical nitrogen fixation has been regarded as the most promising route for the synthesis of ammonia in that it can convert N2 and water into ammonia on active sites of electrocatalysts using renewable electricity, such as from wind and solar sources, without the added release of CO2 in coal-fired power stations [6,7]. However, the performance of electrocatalysts to date is still far from pragmatic industrial applications due to the large challenges of low FE and production rate of ammonia; areas which have already attracted tremendous attention from scientists [[8], [9], [10], [11], [12], [13], [14], [15], [16]].
Defect engineering has been adopted in recent years as a universal strategy to tailor catalyst materials for specific and demanding reactions [[17], [18], [19], [20], [21], [22]]. Moreover, reviewing defect electrocatalytic mechanisms further deepens the understanding of the defect mechanism concept - staring from the common concept of heteroatom doping to a new concept of topological defects and then facilitates the subsequent rational design of advanced electrocatalysts [20]. Of special interest are the intrinsic defects on the edge of graphene or basal plane holes that are active for oxygen reduction, evidenced by the subtle measurement device of a micro-electrochemical testing system, which, for the first time, suggests that LCCs are active due to the different charge distribution of the basal-plane hole or edge with that of the basal plane [23]. More recently, intrinsic defects (vacancies and dislocation) within LCCs on graphene were investigated to show promising activity towards eNRR [24]. Inspired by those works, designing electrocatalysts intentionally with such defects/active sites in the low-coordination environment should be highly effective.
Carbon-based electrocatalysts have been intensively investigated for electrochemical nitrogen reduction due to their catalytic potentials, stabilities and abundant storages on Earth compared with metal-based ones [[25], [26], [27], [28], [29], [30], [31]]. For example, the pioneering work of B-doped graphene for electrocatalytic nitrogen reduction reaction (eNRR) by Yu et al. [30] demonstrated an efficient metal-free carbon-based electrocatalyst for NRR; Liu et al. [31] reported a N-doped porous carbon with the pyrrolic and pyridinic N as active sites for the cleavage of N ≡ N; Moreover, graphyne-based electrocatalysts have increasingly drawn attention for designing effective eNRR catalysts due to unique physical properties and large surface area [28,[32], [33], [34], [35]]. To now, large efforts have been already devoted into designing various efficient, robust and cost-effective carbon-based electrocatalysts for NRR, but are mainly limited to sp-/sp2-hybridized carbon-based materials - the sp3-hybridized carbon materials are rarely reported. Recently, Wanninayake et al. [36] demonstrated the effect of the carbon hybridization structure (sp2/sp3 carbon) on the electrochemical CO2 reduction, experimentally and theoretically, which suggested the vital role of the host carbon-structure. Herein, two commonly exposed surfaces of C(111) and C(110) of diamond carbon with the sp3-hybridized configuration for the first time were tested for eNRR by DFT, which are highly expected to show the reactivity due to LCC sites caused by dangling bonds on surface, based on the before-mentioned discussion. The theoretical results evidence that the LCC surface of sp3-configurated diamond carbon can act as an effective metal-free electrocatalyst for eNRR with the maximum free-energy change of 0.73 eV along the associative distal pathway on C(111). We show that diamond carbon electrocatalysts display their advantages of suppressing the competing side reaction of HER, which was also observed in boron-doped diamond carbon (BDD) electrodes for the electrocatalytic CO2 reduction [[37], [38], [39]]. In addition, diamond carbon is favourably endowed with outstanding physical and chemical properties, such as high electron and hole mobilities [40], chemical stability and wide electrochemical potential window [41], making it a suitable material for pragmatic electrocatalysis applications. This research demonstrates the effect of LCCs, which can be from intrinsic and extrinsic defects on carbon-based materials, on dinitrogen reduction, and boost a brand-new direction of designing diamond carbon-composited NRR electrocatalysts.
Section snippets
Computational details
Spin-polarized calculations were performed on (111) and (110) surfaces of () and () supercells (catalysts models are shown in Fig. 1) of diamond carbon, with lattice parameters of a = b = 8.75 Å, α = β = 90°, γ = 60°, and a = b = 8.75 Å, α = β = 90°, γ = 70.5°, respectively, using DMol3 code available in the Materials Studio package [42,43]. The thickness of vacuum is set to 20 Å to avoid possible interaction in the z-direction. The two surface models investigated all have five layers,
Nitrogen chemisorption on C(111) and C(110)
Nitrogen capture and activation plays a critical role in the eNRR process [50], which can directly affect subsequent hydrogenation of the NN bond and thus determine the activity of catalysts. Fig. 2 displays three adsorption configurations of dinitrogen on C(111) and C(110) and corresponding NN bond length after adsorption. For more clarity on N2 adsorption configurations, the bare surfaces and corresponding structures are displayed in Fig. S1. According to the adsorption energy (Eads)
Conclusion
In summary, two commonly exposed C(111) and C(110) surfaces of sp3-hybridized diamond carbon were investigated for eNRR by DFT method, and calculation results indicate that the sp3-configurated low-coordinated carbon is active for dinitrogen capture and reduction with moderate free-energy changes. Especially, together with the sluggish HER due to larger free-energy changes for the release of absorbed hydrogen, the C(111) surface of diamond carbon is expected to be highly promising for eNRR
CRediT authorship contribution statement
Zhongyuan Guo, Siyao Qiu, Huan Li, Yongjun Xu, Steven J. Langford and Chenghua Sun conceived and designed the project and Zhongyuan Guo performed the DFT calculations. Zhongyuan Guo, Siyao Qiu, Huan Li, Yongjun Xu, Steven J. Langford and Chenghua Sun discussed the DFT results. Zhongyuan Guo wrote the original draft and Siyao Qiu, Huan Li, Yongjun Xu, Steven J. Langford and Chenghua Sun reviewed/edited it. Siyao Qiu, Steven J. Langford and Chenghua Sun supervised this project.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the financial support by Guangdong Innovation Research Team for Higher Education (2017KCXTD030) and High-level Talents Project of Dongguan University of Technology (KCYKYQD2017017) and Engineering Research Center of None-food Biomass Efficient Pyrolysis and Utilization Technology of Guangdong Higher Education Institutes (2016GCZX009) and Research Center of New Energy Materials (KCYCXPT2017005). The authors also thank the National Computational Infrastructure (NCI), which
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