Elsevier

Journal of Catalysis

Volume 348, April 2017, Pages 151-159
Journal of Catalysis

Active sites and mechanism on nitrogen-doped carbon catalyst for hydrogen evolution reaction

https://doi.org/10.1016/j.jcat.2017.02.021Get rights and content

Highlights

  • Nitrogen-doped carbon catalyst with decent activity for hydrogen evolution reaction.

  • Dopant nitrogen atoms act as the active site for hydrogen evolution reaction.

  • Electrocatalytic activity is superior in acid vs. alkaline media.

  • Hydrogen evolution reaction proceeds by Volmer-Heyrovsky mechanism.

  • Hydrogen underpotential deposition is possible on the nitrogen-doped carbon.

Abstract

The nature of active sites and mechanism of hydrogen evolution reaction (HER) on the nitrogen-doped carbon catalyst is extensively investigated, by combining physicochemical and electrochemical methods. Two carbon catalysts, with the same chemical nature but different nitrogen content, are employed in this investigation. Electrochemical methods are applied to investigate the electrochemical behavior at different pH values (1.0–2.0, 12.0–13.0). It is found that increasing nitrogen content has a positive effect on the electrocatalytic activity, and therefore, the doped nitrogen atoms should be the active sites. The kinetic current, normalized by the surface nitrogen content, is found to be the same for the two catalysts, confirming the above claim. As such, HER is proposed to proceed on these active sites by the Volmer-Heyrovsky mechanism. The electrochemical tests reveal that the electrocatalytic activity closely relies on the solution pH, which is due to the chemical evolution of the active site in different solutions. In acid media, the electrocatalytic activity increases with the concentration of proton, and the Tafel slope is ca. 120 mV dec−1. It is proposed that the electrochemical desorption of proton on the doped nitrogen atoms is the rate determining step (r.d.s.). In alkaline media, the electrocatalytic activity increases with pH, because the increase in pH dramatically enhances the basicity or the surface charge density, thereby facilitating charge transfer and improving activity. In alkaline media, Tafel analysis shows that Heyrovsky step is the rate determining.

Introduction

Hydrogen, as a clean energy carrier, can address the concern on energy sustainability and environmental emissions [1]. Water electrolysis is one of the most effective approaches to generate hydrogen at room temperature [2]. Among a wide variety of available catalysts, Pt and its alloys show the best electrocatalytic activity with an extremely high exchange current density [3], [4], [5], [6]. However, its high cost and source scarcity raise great challenges to the large-scale production of hydrogen [7], [8], [9]. In response, extensive efforts have been undertaken to develop non-Pt electrocatalysts, including metal alloys [10], carbides [9], [11], phosphides [12], [13], borides [11], sulfides [14], [15], and nitrides [7], [16], [17].

Very recently, metal (Co, Mo, Ni)-encapsulated [18], [19], [20], [21], [22], [23] and heteroatom (N, S, P)-doped carbon [24], [25], [26], [27], [28], [29], [30], [31], [32] represents one new class of non-Pt catalysts for the hydrogen evolution reaction (HER). Qiao et al. [24] found that the nitrogen-doping can dramatically improve the electrocatalytic activity of carbon, and the co-doping with phosphor can further activate the adjacent carbon atom and enhance the reactivity. Moreover, Qiao et al. [25], [26] developed interfacial heterogeneous electrocatalysts by integrating C3N4 nanolayers with nitrogen-doped graphene sheets. These catalysts exhibited an unbeatable HER performance with a very positive onset potential and remarkable durability. In addition, the electrocatalytic activity could be further improved by doping with multi-heteroatoms (N, S, P) [27], [33], [34], and the co-doping effect is generally attributed to the so-called synergistic effect.

It should be pointed out that most of the published work is devoted to the material preparation and characterization. Fundamental problems, such as the intrinsic nature of the active site and HER mechanism, have not been well clarified yet. The density functional theory (DFT) calculation is generally used to study the active site and the mechanism [24], [25], [28], [35]; however, it is noted that the results are sometimes inconsistent. The active sites for the hydrogen adsorption are proposed to be either the edge-type nitrogen atoms [25] or the dopant-activated carbon atoms [24].

The phenomenological method is still a realistic way to clarify the active site and mechanism due to the lack of reliable in-operando characterization methods. Proton can be used as an electrochemical probe as its concentration is found to yield a paramount effect on the activity. On Pt, the HER proceeds fast in acid media, but it is two orders of magnitude slower in alkaline media [36]. Elbert [37] attributed this finding to the switch of the rate determining step (r.d.s.) from the Tafel step to the Volmer step. In line with this understanding, the core-shell Ru@Pt catalyst, which showed a weaker hydrogen binding energy, behaved much better than did the pure Pt catalyst.

To our knowledge, the pH effect on the nitrogen-doped carbon catalyst for HER has been rarely investigated so far. In our previous work [38], the mechanism of the oxygen reduction reaction was extensively investigated in both acid and alkaline media. In this work, two carbon catalysts, which have basically the same chemical nature except the content of the dopant nitrogen, are used to study the nature of the active sites and the mechanism of the HER. Extensive methods are used to study the evolution in the interfacial structure and the electrochemistry of the catalysts at different pH values. Based on the above results, the nature of active sites and mechanism is proposed in a wide pH window. The findings can both deepen the fundamental understanding on the interfacial electrochemistry of the nitrogen-doped carbon catalyst and guide the development of novel high-performance electrode materials for the HER.

Section snippets

Synthesis of NOMC and NOMC-NH3

The nitrogen-doped ordered mesoporous carbon (NOMC) material was synthesized by the aid of the nanocasting method. The details were previously reported [39], [40], [41], [42], [43]. Briefly, SBA-15 was initially synthesized in a hydrothermal method using Pluronic P123 as the structure-directing agent. Then, the carbon precursor (pyrrole) was impregnated into SBA-15, which was then polymerized and carbonized at 900 °C in argon. Finally, the SBA-15 template was dissolved out and the NOMC catalyst

Texture features

In Fig. 1, the N2 adsorption/desorption isotherms and the pore size distribution of the two nitrogen-doped carbon catalysts, are depicted. As can be seen, the two isotherms are similar in shape and belong to type-IV curve. The pore size distribution curves are overlapping, and the pore diameter sits in the range of 2–10 nm. The quantitative results of the surface area and pore diameter are listed in Table S1. The specific surface area of NOMC and NOMC-NH3 is respectively to be 654 and 794 m2 g−1;

Conclusions

In the present work, the effect of pH on the nitrogen-doped carbon catalysts for the hydrogen evolution reaction was thoroughly investigated. It was found that the pH value has a considerable effect on the surface properties of the catalyst and thereby the electrochemical performance. Salient findings are as follows:

First, the doped nitrogen atoms are the active sites of the nitrogen-doped carbon catalysts for the HER, and the increase of the nitrogen content can effectively improve the

Acknowledgments

The work described in this paper was jointly supported by the National Natural Science Foundation of China (Nos. 21476087, 21576101, 21676106), National Key Research and Development Program of China (No. 2016YFB0101200 (2016YFB0101204)), and the Fundamental Research Funds for the Central Universities. Prof. Tsiakaras is also grateful to Ministry of Education and Science of the Russian Federation (Mega-Grant, contract no. 14.Z50.31.0001) for funding.

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