Materials Today Energy
Recent progress, developing strategies, theoretical insights, and perspectives towards high-performance copper single atom electrocatalysts
Graphical abstract
Introduction
The increasingly exhausted traditional energy sources and pollution problems generated an ongoing challenge facing us now. One of the most promising ways is to use sustainable and renewable energy such as water, solar, and wind [[1], [2], [3], [4]]. However, utilizing these energy needs to develop the affordable and efficient energy conversion devices (e.g. fuel cell, electrolytic cell, and metal-air battery), realizing the conversion from chemical energy to electric energy [[5], [6], [7], [8]]. The conversion efficiency of those devices closely correlates with the performance of catalysts. To date, the commercial electrocatalysts are predominantly based on noble metal, such as Pt, Au, IrO2, and RuO2. Unfortunately, their limited reserves on earth and poor long-term stability limit their large-scale industrial applications [[9], [10], [11], [12], [13]].
To solve the bottleneck regarding the precious metal catalysis, some efforts are devoted to reduce the usage of precious metal, and great achievements have been obtained. For instance, by alloying Pt with other non-precious metals (i.e. Ni, Co), the resultant binary catalysts not only show the improved catalytic activity but also possess desirable durability [[14], [15], [16], [17], [18], [19], [20]]. Another strategy is to seek low-cost alternatives. In this contribution, transition metal–based catalysts emerge as a rising star wherein a well-adjusted d-band center would give rise to promising catalytic performance [[21], [22], [23], [24]].
To maximize the atomic utility, single-atom catalyst (SAC) was proposed [25] and as one of the typical SACs, recent years have witnessed the upsurge of the MNC catalysts, wherein the reaction center of MNx motif (M are usually transition metals such as Fe, Co, Ni, and x is the coordination number) is supported by carbon matrix [[26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. For one thing, the interaction between the support and individual metal atoms is enhanced by N and the stability of the catalyst is improved. For another thing, the manipulation of N coordination is able to change the electronic structure of metal atoms, achieving excellent activity and selectivity. As shown in Table 1, MNC (M = Fe, Co, Ni, etc.) materials have demonstrated excellent catalytic performance in various electrocatalytic applications [[42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83]]. Particularly, FeNC catalysts have been extensively studied to replace Pt-based catalysts in oxygen reduction reaction (ORR), and they exhibit comparable or even better onset potential (Eonset) or/and half-wave potential (E1/2) [44,45]. However, the Fe element is also active in Fenton reaction (Fe2+ + H2O2 → Fe3+ + OH− + OH·), and the generated radical is detrimental to fuel cells’ durability.
Tremendous efforts have been paid to make balance between activity and durability. For instance, because the performance of MNC catalysts is sensitive to the adjacent nitrogen, coordination engineering has been implemented as a facile strategy to boost catalytic activity. For instance, FeN3, in contrast to the saturated FeN4, can effectively transform N2 into NH3 [82]. Similarly, CoN5 catalyst designed by Pan et al. exhibits Faradaic efficiency (FE) of more than 99% at −0.73 and −0.79 V to produce CO in CO2RR, much higher than those of CoN4 and CoN3 counterparts [69]. Recently, high-purity pyrrole-like FeN4 site has been revealed by Zhang et al. [84], which is in contrast to the traditional pyridine-N in most MN4 motifs and exhibits outstanding ORR activity with an ultrahigh active area current density of 6.89 mA/m2 in an acid medium.
Another strategy is to seek non-Fe-based SACs. Specifically, early or late 3d as well as 4d/5d transition metal–based MNC materials have received more and more attention to date, including Cr, Cu, Zn, Ru, Ir, etc. [1,42,[54], [55], [56], [57],62,63,66,72,[85], [86], [87], [88], [89]]. Among them, the CuNC family has been widely studied in a range of electrocatalytic reactions (e.g. ORR, OER, CO2RR, NRR, and even bifunction of them) and different catalytic activity and selectivity have been demonstrated with different coordination condition, providing a desirable model to study the correlation between intrinsic properties and geometry configurations. Furthermore, in contrast to Fe and Co, CuNC is inactive in Fenton reaction, which endows them promising stability in hash environment [90]. The inspiration of CuNC was from copper metalloenzymes which is able to catalyze the ORR efficiently with low overpotentials, where the coordinatively unsaturated CuN centers are considered as the active sites. To mimic such configurations, sacrificial agents are applied to convert CuN4 into CuN2 site and the latter shows better ORR performance. In the unsaturated configuration, the Cu is in low valence state. Moreover, the catalytic selectivity is found to be related to the distance from the reaction center. More specifically, adjacent CuN2 sites can selectively reduce CO2 to C2H4, a desirable C2 product, while larger distance from the two reaction center produce C1 product [71]. On the other hand, Wagh et al. synthesized a catalyst with CuN3 sites, which exhibits excellent bifunctional ORR and OER activity [62]. However, to the best of our knowledge, there are few studies that systematically summarize these achievements and comments on the origin of the coordination-related performance.
Herein, this review provides an overview of the recent endeavor on CuNC promptly from both experimental and theoretical viewpoint (Fig. 1). We first provide a detailed discussion about the environment configuration–related catalytic performance, and then the experimental synthesis and characterization technologies for distinct reaction moieties are summarized and compared. We also provide theoretical insights into the electronic structure and free energy diagram analysis which is beneficial to understand the relationship between the properties and local atomic structure, revealing reaction mechanism and guiding experiments. Finally, the challenges and opportunities are presented, providing an outlook for future research.
Section snippets
Configuration-classified application of CuNx catalysts
The coordination configurations of copper atoms in CuNC catalysts include CuN2, CuN3, and CuN4. Here, we take coordination configuration as the standard classification. In each of coordination configuration, the catalytic performance and application in a wide range of electrocatalytic reactions, including HER, ORR, OER, CO2RR, NRR, and HCOOH oxidation, are provided. Moreover, the stability of CuNx reaction center is also included.
Preparation for various CuNx configurations
Because the coordination environment has a substantial influence on the catalytic activity, it is important to control the coordination number of CuNC catalysts. In this contribution, researchers have made good progress in designing and preparing CuNC catalysts, and various approaches have been developed, as summarized below (Fig. 6a–d).
Pyrolysis synthesis is one of the most effective strategies to prepare SACs with high stability and catalytic activity [[74], [75], [76]]. The outstanding
Typical characterization of CuNC atomic structure
Several characterization techniques have been used to gain insight into the atomic-level structures of SACs. Valuable information about the atomic-level configuration and the electron structure can be obtained by analyzing the X-ray absorption structure (XAS) [[95], [96], [97], [98], [99], [100]]. XAS can usually be divided into X-ray near-edge absorption spectrum (XANES) and extended edge X-ray absorption spectrum (EXAFS). XANES can determine the types of elements and the valence states of the
Theoretical simulations
Density functional theory (DFT) calculations have received more and more attention in electrocatalytic community. On one hand, in view of the limiting by the current testing measurements, theoretical calculation is a useful assistance in distinguishing the local atomic structure. On the other hand, DFT study can provide vivid pictures of electronic structure and charge population, which are vital to understand the reaction mechanism and beneficial to better design of targeted catalyst [70,[101]
Conclusion and outlook
This review highlights the effect of the coordination environment of copper atoms on the catalytic activity of CuNC catalyst in a number of electrochemical reactions, including ORR, CO2RR, NRR, and OER. The coordination environment and catalytic performance of copper are outlined as follows: (I) CuN2 sites are mainly used as active sites in ORR catalysis, which also promote the formation of C2 products in the CO2RR. On the other hand, CuN2 sites exhibit excellent NRR catalytic activity. (II) CuN
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.
Acknowledgments
The authors thank the financial supports from National Natural Science Foundation of China (No. 21573037, 51774251) and the Hebei Science Foundation for Distinguished Young Scholars (B2017203313).
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