Electrocatalytic reduction of CO₂ to CO over iron phthalocyanine-modified graphene nanocomposites

Highlights

• FePc is dispersed on graphene as the single-atom iron catalyst for the CO₂RR to CO.

• FePc-graphene samples with different FePc amount and iron valence state are gotten.

• Our results prove the positive role and synergy of Fe(Ⅱ)Pc with Fe(Ⅲ)Pc and graphene.

• DFT reveals that the catalysis for CO₂RR is Fe(Ⅱ)Pc/FePc(Ⅲ) dimer > Fe(Ⅱ)Pc > Fe(Ⅲ)Pc.

Abstract

The non-precious materials with iron-pyrrolic nitrogen-carbon (Fe–N–C) moiety have attracted an increasing attention on the CO₂ reduction reaction (CO₂RR). However, the influence of iron valence state and environmental synergy is not still clear. Here, iron phthalocyanine (FePc) molecule with an intrinsic Fe–N₄–C moiety is enough dispersed on graphene as the single-atom iron catalyst through a facile chemical method. The FePc-graphene composites with different FePc content and iron valence state are synthesized to investigate their catalysis for the CO₂RR to CO. The onset overpotential of 190 mV and Faradaic efficiency of >90% may be achieved in the optimal composites. Experimental and calculational results prove the positive role and synergy of Fe(II)Pc with Fe(III)Pc and graphene. The formation of ∗COOH intermediate is confirmed to be the rate-limiting step. The theoretical calculation reveals that Fe(II)Pc should have higher activity than Fe(III)Pc, and Fe(II)Pc/FePc(III) dimer may be better than individual one. This work clearly exhibits the effect of Fe^2+/3+- pyrrolic N₄–C on the CO₂RR.

Introduction

Since the industrial revolution began, the combustion of fossil fuels has resulted in the rapid increase of atmospheric CO₂ concentration from 200 to 400 ppm, subsequently a series of environmental problems such as the ocean hypercapnia and global warming are becoming worse [1,2]. Now, the electrocatalytic reduction of CO₂ to the valuable chemicals is regarded as a promising route to maintain the carbon neutrality with using intermittent renewable energy like solar and/or wind energy [3,4]. Due to the high cost and scarcity of noble metals with the excellent electrocatalytic performance, such as Au, Ag and their alloys [[5], [6], [7]], the non-precious electrocatalysts with high activity and selectivity have attracted more and more attentions [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. It has been widely reported that the materials containing metal-N-C (M-N-C) moiety may have high catalytic performance for the CO₂RR to CO [[7], [8], [9], [10], [11], [12], [13], [14], [15]]. Among them, the Fe–N–C functional groups are particularly focused on wing to their possible higher activity than precious metals [[8], [9], [10], [11], [12], [13], [14]].

At present, the M-N-C electrocatalysts are usually prepared through two kinds of ways. One is through the high-temperature pyrolysis of precursors containing Metal, N and C groups [[7], [8], [9], [10], [11], [12], [13], [14], [15]]. It is also found that the as-obtained materials with Fe-pyrrolic/pyridinic-N₄ moiety have high catalytic activity [[8], [9], [10], [11], [12], [13], [14]]. This way is efficient but inevitably generates complicated structures, such as M-N_x (X = 1–4), M-C and defects in the carbon matrix [[7], [8], [9], [10], [11], [12], [13], [14], [15]]. Therefore, the environment of active sites is too complicated to understand the real catalytic mechanism. For example, it was not easy to prepare Fe²⁺-pyrrolic N₄–C and Fe³⁺-pyrrolic N₄–C for the CO₂RR under a same environment, and then to judge which is best [8,9]. Another way is to use the conjugated molecules with M-N-C central group, e.g. metal phthalocyanine (MPc) like FePc, CoPc and NiPc [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]. Because of their accessibility, low price and stable clear structure, their catalysis for the CO₂RR have been widely explored [17,18]. However, their disadvantages of easy aggregation and poor conductivity seriously limit their activities and study on the catalytic mechanism [19,20]. Actually, whatever the preparation methods is used to synthesize the M-N-C catalysts for the CO₂RR, it is known that a feasible route is to fully disperse M-N-C moiety into a conductive substance through Metal-N bond [7,[9], [10], [11], [12], [13], [14], [15]], π-π interaction [[20], [21], [22], [23], [24]], and/or axial coordination connection [8,[25], [26], [27], [28]]. Nevertheless, this is only the prerequisite for the study of M-N-C catalysis for CO₂RR, despite huge progress has been made in the field.

As we known, the heterocatalysis of gas and/or liquid on solid catalyst involves the electron and mass transfers during the adsorption of reactants, the multiple-step reaction and desorption of products [4]. It is difficult to balance the three processes with a single-component catalyst. For metal active site, a single valence state may only benefit to either reactant/intermediate absorption or intermediate/product desorption. The more excellent catalysis was usually found through the synergy of different metal valence state [19,24,25,[29], [30], [31]]. Therefore, the mass and electron transfers will have to be fully considered before we design catalyst, besides the enough exposure of active sites. Theoretically, the synergistic role of each component in M-N-C composites in the three processes should be paid more attention. For example, Fe²⁺ has a higher bind energy for the CO₂ adsorption although it may have a serious disadvantage on the CO desorption [13,17]. During the electrocatalytic CO₂RR, it has been proved that the CO₂ adsorption and subsequent addition of proton to ∗COOH are usually the rate-limiting step [11,24,32,33]. Hence, the existence of Fe(II) may promote the CO₂RR under the suitable circumstances. Meanwhile, in most case, the use or formation of carbon substrate as matrix is usually very helpful for the electron transfer and the dispersion of active sites [4,[7], [8], [9], [10], [11], [12], [13], [14], [15],[18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29],34,35]. For the Fe–N–C materials obtained through high-temperature pyrolysis, it can be reasonably speculated that their excellent catalytic activity should be closely associated with the synergistic effect of so-called active sites and other environmental factors. Recently, Shao’s group firstly reported that Fe²⁺-pyridinic N–C moieties exhibited the best catalytic performance in all reported similar materials when it was embedded in a defective graphitic layer rather than complete one [13]. The work from Chen’s group revealed that Fe³⁺(III)- pyrrolic N–C should have more catalytic activity than Fe²⁺(II)- pyridinic N–C, and was comparable with noble metals [9]. Obviously, the environment of both Fe–N–C moiety and central Fe site should play an important role in the CO₂RR. Then, what is the difference between Fe(II)-pyrrolic N–C and Fe(III)-pyrrolic N–C groups in the CO₂RR? This inspires us to further explore the catalytic mechanism of Fe-pyrrolic N–C materials for the CO₂RR to CO with FePc molecules as the single-atom Fe active sites. Here, the key and challenge are how to homogeneously disperse the abundant FePc molecules on carbon substrate and to adjust the Fe valence state without the obvious change of environment around active sites.

In this work, the FePc-graphene composites were synthesized through a facile graphene-assisted wet chemical reaction basing on the previous study [35]. The scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and element mapping were used to analyze the formation process of the single-atom Fe catalysts through the ratio change of FePc precursor to graphene oxide (GO). After the preparation of different FePc-graphene composites, the effect of the graphene amount, Fe valence state and oxygen content on the electrocatalytic CO₂RR to CO were systematically investigated. The Fourier transform infrared spectroscopy (FT-IR), Raman and X-ray photoelectron spectroscopy (XPS) were measured to carefully analyze the catalytic mechanism. The positive role of Fe(II)Pc and synergistic effect of Fe(II), Fe(III) and graphene were found in FePc-graphene composites due to the existence of strong interaction between FePc and graphene. The optimal FePc-graphene composites may convert CO₂ to CO with Faradaic efficiency (FE) of >90% between −0.5 V and −0.6 V vs. reversible hydrogen electrode (RHE). The Density Functional Theory (DFT) calculation revealed that Fe(II)Pc should have better catalytic activity than Fe(III)Pc, while the activity of Fe(II)Pc/Fe(III)Pc dimer may be higher than pure one. This work gained an insight into the catalysis of Fe²⁺/³⁺-pyrrolic N₄–C for the CO₂RR to CO.