DOI:10.30919/es.1804232

Received: 20 Mar 2018
Accepted: 24 Apr 2018
Published online: 24 Apr 2018

Nitrogen Coordinated Single Atomic Metals Supported on Nanocarbons: A New Frontier in Electrocatalytic CO2 Reduction

Fuping Pan, Xianmei Xiang, Ying Li*

Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, USA

* Corresponding Author(E-mail) : [email protected]


Abstract 

The rising level of atmospheric CO2 caused by increased fossil energy consumption is linked to global warming effects. Electrochemical CO2 reduction (CO2RR) can convert CO2 into value-added chemical fuels by using renewable energy-generated electricity as the energy input, providing a promising solution to mitigate the CO2 emissions. Compared to conventional precious metal catalysts for CO2RR, carbon-based catalysts are made of earth-abundant elements and less expensive, with great potential for large-scale applications. In this Review, recent advances of designing and synthesizing nitrogen coordinated single atomic transition metals supported on nanocarbons (M−NX-C; M=Fe, Ni, Co) as electrocatalysts for CO2RR are reviewed from both experimental and computational aspects. The catalytic mechanisms and design principles are highlighted, and the correlations of catalyst synthesis-structure-performance relationships are discussed. The disparities in catalytic activity between noble metal catalysts (Ag and Au) and M−NX-C catalysts suggest that there is still much room to develop more advanced M−NX-C catalysts. Therefore, several strategies, including mechanism exploration, M−NX sites and carbon supports engineering, and large-scale fabrication of atomically dispersed catalysts and reactor systems design are proposed to propel the use of M−NX-C for achieving high-efficient and cost-effective CO2-to-fuels conversion.


Table of Content

High level of CO2 in atmosphere causes global warming. Recent designing and synthesizing nitrogen coordinated single atomic transition metals supported on nanocarbons as electrocatalysts for CO2 reduction into chemical fuels has been summarized.

 

 

 

 

 


Keywords: Atomic catalysts; metal-nitrogen coordination; nanocarbons; CO2 reduction, electrocatalysis


1. Background and motivation

The heavy dependency on fossil fuels for the production of energy and the increasing human activities lead to the massive emissions of greenhouse gases, mainly CO2, into the atmosphere. The atmospheric concentration of CO2 has rapidly increased from 322 ppm in 1967 to 407 ppm in 2017, which is estimated to reach 600 ppm by 2100.1 The ever-increasing CO2 emissions have disastrous consequences on global warming, climate change, human’s health, and extinction of species. The ideal approach to mitigate these changes caused by CO2 emissions is to convert CO2 into harmless or useful products, which requires the development of green and sustainable strategies for efficient and massive CO2 conversion.

Various CO2 conversion approaches have been developed and pursued intensively, including thermochemical reforming,2-4 biochemical conversion,5,6 photocatalysis,7-10 and electrocatalytic reduction.11-14 Among these techniques, the electrocatalytic CO2 reduction in aqueous media is believed to be a more promising route because of the following advantages: easy manipulation at ambient conditions, low-cost process only using abundant water and CO2 as feedstock, and high viability of large-scale applications.12,15,16 More attractively, the CO2 electrolysis process can be powered by clean electricity generated from solar, wind, geothermal, and tidal energy, and thus this technology can not only mitigate atmospheric CO2 concentrations but also store intermittent renewable energy in the form of transportable fuels simultaneously. In this case, the carbon-neutral society could be achieved, which will be beneficial to the sustainable development.17

 However, this desired CO2-to-electrofuels solution imposes great technological challenges because CO2 is a fully oxidized and thermodynamically stable molecule because of its linear and centrosymmetric molecular structure, which makes CO2 reduction reaction (CO2RR) very difficult to occur and requires large overpotentials.14,18,19 Combined with challenges of possible multiple products and competition over hydrogen evolution reaction (HER), a suitable catalyst is thus essential to achieve a sustainable CO2 reduction process with high efficiency, selectivity, and stability. Through screening catalysts with different topography, compositions, and structures by experimental and theoretical routes, to date, a variety of electrocatalysts have been designed and fabricated, including noble metals,20-22 transition metal oxides/chalcogenides,12,23,24 metal-free carbons,1,25 and single atomic nitrogen coordinated metals supported on carbons (M−NX-C).26-29 Up to now, there have been several excellent reviews that systematically summarized and commented the advancements and progresses achieved on these heterogonous catalysts except M−NX-C because M−NX-C was very recently discovered for CO2 reduction.1,12,30 Taking advantages of low-cost carbon as supports,31-33 maximum atom efficiency of isolated M,27,34 and optimal binding strength between M−NX and CO intermediate,35 high-efficient and selective reduction of CO2 to CO has been demonstrated on M−NX-C,26,27,36 making M−NX-C promising electrocatalysts for future large-scale CO2 electrolysis. Therefore, it is important and necessary to summarize and discuss the current achievements in the design, preparation, catalytic activity and mechanism of M−NX-C for CO2 reduction, which will provide insights on the fabrication of next-generation M−NX-C catalysts for realizing an efficient and cost-effective electrochemical CO2-to-fuels process.

In this review, we present the recent progresses of CO2RR on single-atomic M−NX-C and perspectives on the rational design of advanced catalysts. We first start with an introduction of the fundamental theory regarding atomic configurations, catalytic mechanisms, and design principles of M−NX-C for CO2RR. We then discuss three types of M−NX-C classified by metal centers including Fe, Ni, and Co, and correlate their structure and compositions with activity and selectivity to unveil the underlying catalytic role of various metal centers. Finally, the challenges and opportunities on the design of advanced M−NX-C in this emerging area are discussed.

2. Identification of the atomic structure of M−NX sites

The M−NX sites are comprised of the electronic coordination between M centers and N atoms inserted into carbon lattices.37 As for N structures, it has been widely accepted that N doping can occur either via substituting carbon atoms in the graphitic structure or reacting between gaseous N-based sources with oxygen-containing functionalities of carbons, which commonly leads to the formation of edge-like pyridinic N, five-membered heterocyclic rings-like pyrollic N, and bulk-like graphitic N.38-43 Among them, it was demonstrated that metals ions preferentially coordinate with pyridinic N rather than graphitic N due to the existence of lone-pair electron on pyridinic N.26,44 When a chemical bond between M and N is formed, the binding energy of pyridinic N shifts to a higher value owing to the introduction of an ion withdrawing effect caused by the replacement of a proton on pyridinic N by a metal ion. This change in the electronic environment of N atoms has been experimentally confirmed by X-ray photoelectron spectroscopy (XPS) analyses.26,45,46

It has been demonstrated that M−NX complexes can be formed by the electronic interaction between M and N atoms during the annealing of M,N,C-containing precursors at high temperatures.47-49 However, since the single M atoms tend to aggregate during the high-temperature process due to their high mobility, the metallic phases can be commonly generated simultaneously.50-53 Thus, the accurate recognition of single-atomic M−NX is relatively difficult caused by the co-presence of multiple heterogeneous components, which commonly requires the use of advanced electron microscopy and spectroscopy to distinguish the M−N coordination from metallic clusters and particles. High-angle angular dark-field scanning transmission electron microscopy (HAADF-STEM) is a powerful technique to image isolated M atoms. By using HAADF-STEM, Chung et al.54 realized the direct identification of the atomic structure of Fe−NX. The STEM image in Figure 1a shows single atoms dispersed across the carbon surface with dots exhibiting bright contrast, which were confirmed to be primarily Fe by electron energy loss spectroscopy (EELS) (Figure 1b). However, the EELS spectrum was recorded from a very limited area of ~1 to 2 Å combined with the instability of the individual Fe atom, and the weak signal makes it impossible to determine the valence states of the individual Fe from EEL fine-structure analysis.

Figure 1. (a) HAADF-STEM image of individual Fe atoms (labeled 1, 2, and 3) in a few-layer graphene sheet. (b) EEL spectra of the N K-edge (NK) and Fe L-edge (FeL) acquired from single atoms (1 and 2) and few-layer graphene (3), demonstrating the presence of N around the Fe atoms. Reproduced with permission from ref.54 Copyright 2017, Science. (c) Ni K-edge EXAFS analysis of an atomic M−N4-C in R spaces. Curves from top to bottom are the Ni−N, Ni−O and Ni−C two-body backscattering signals χ2 included in the fit and the total signal (red line) superimposed on the experimental signal (black line). The measured and calculated spectra show excellent agreement. The inset in d shows the structure of a NiN4 moiety derived from the EXAFS result, where the teal, red, blue and grey spheres represent Ni, O, N and C, respectively. Reproduced with permission from ref.55 Copyright 2018, Nature. The proposed structures of bulk-hosted M−N4 (d) and edge-hosted M−N2+2 (e). Reproduced with permission from ref.56 Copyright 2018, ACS. In the figure, the gray, blue, yellow, red, and white balls represent C, N, M, O, and H atoms, respectively.

To further identify the chemical state of M centers and their local coordination with N, X-ray absorption spectroscopy (XAS) is commonly performed using synchrotron radiation sources, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES).27,55,57 These methods are very powerful for discriminating M−N, M−C, and M−M bonds. Fei et al.55 conducted thorough analyses of various M−NX moieties (M=Fe, Co, Ni)  by comparing experimental and simulated EXAFS spectra on M−NX-C catalysts. From their results (Figure 1c), the single M centers with four N atoms and one adsorbed O atom in the first coordination sphere and four C atoms in the second coordination sphere were confirmed. This directly reveals the formation of M−N4 moiety, where the M is bonded with four N atoms embedded in the carbon lattices. On the other hand, Mossbauer spectroscopy has also been used to discriminate the geometric structure of Fe−NX. Recent findings manifested that two kinds of Fe−N4 could be generally formed in Fe−N4-C synthesized by thermal treatment of Fe, N, and C-containing precursors.58,59 One is the bulk-hosted Fe−N4 moieties (Figure 1d), which embeds in the bulk of a graphitic layer and is fully encapsulated by carbon atoms. Another is the edge-hosted Fe−N2+2 (Figure 1e), where two N-doped graphitic layers are connected by a Fe atom bonded with two N atoms at the edges of each graphitic layer. The formation of Fe−N2+2 also leads to a distortion of carbon plane environment that causes the out-of-plane position of Fe, while the Fe−N4 maintains the in-plane structure.58,59 In addition, many kinds of Fe−NX (x=2, 5, 6)60-62 and Co−NX (x= 2, 3, 4)36,48 structures have also been proposed as possible configurations in M−NX-C. Although these advancements on identifying the structure of M−NX have been achieved, there is still a debate for the structural configurations of M−NX because various synthetic routes and parameters will give a broad possibility on the formation of different M−NX moieties. Future works are suggested to combine above-mentioned characterization techniques together to determine the detailed structure of M−NX in a specific catalyst.

3. Fundamentals of CO2RR on M−NX-C

3.1 Mechanism

Electrochemical CO2 reduction is commonly conducted in a two-chamber electrolyzer consisting of an anode and a cathode separated with an ion conducting membrane to protect the re-oxidization of reduced products. At the anode, water is oxidized to oxygen and produce proton (H+), whereas CO2 is reduced to carbon-containing pieces at the cathode. The overall process involves the formation of H+ at the anode, migration of H+ through the electrolyte, and reaction of H+ with CO2 at the catalytic sites of the cathode to form products.12 For CO2RR on M−NX-C, three major steps are commonly involved:  i) chemical adsorption of  CO2 on M sites; ii) CO2 activation, electron transfer and combination with H+ to cleave C−O bonds and/or form C−H bonds; and iii) configuration rearrangement of intermediates and products desorption from M−NX-C surface to electrolyte.12,56 The molecular-level insight into the activation and reduction of CO2 on Ni−N4 under operando conditions has been investigated by Yang et al.27 It was found that the delocalization of the unpaired electron in the 3dx2−y2 orbital and spontaneous charge transfer from monovalent Ni(ɪ) to the carbon 2p orbital in CO2 to form a CO2δ− species took place after adsorbing CO2 on Ni−N4 via bonding between Ni atoms of Ni−N4 and C atom of CO2. DFT calculations further indicated that the increase in the density of states around 3–6 eV below the Fermi level results from the 1π orbital of the bent CO2 molecules adsorbed on Ni(i) and the delocalization of the Ni 3d orbital. During electrochemical CO2 reduction, the recovery of low-oxidation-state Ni after one cycle of CO2 reduction took place, in which the CO was produced through (Ni(ɪ)*+ CO2 + 2H+ + 2e® Ni(ɪ)*+ CO+ H2O), as shown in Figure 2.