Next Article in Journal
Modeling of the Anaerobic Digestion of Organic Wastes: Integration of Heat Transfer and Biochemical Aspects
Next Article in Special Issue
High Hydrogen Evolution Reaction (HER) and Hydrogen Oxidation Reaction (HOR) Activity RhxSy Catalyst Synthesized with Na2S for Hydrogen-Bromine Fuel Cell
Previous Article in Journal
Investigation of Solar Photovoltaic-Thermal (PVT) and Solar Photovoltaic (PV) Performance: A Case Study in Ghana
Previous Article in Special Issue
Platinum and Platinum Group Metal-Free Catalysts for Anion Exchange Membrane Fuel Cells
Open AccessArticle

CO2 Electrochemical Reduction by Exohedral N-Pyridine Decorated Metal-Free Carbon Nanotubes

1
Institute of Chemistry of OrganoMetallic Compounds (ICCOM-CNR) and Consorzio INSTM, Via Madonna del Piano, 10-50019 Sesto F.no, Florence, Italy
2
Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES), UMR 7515 CNRS-University of Strasbourg (UdS), 25, rue Becquerel, CEDEX 02, 67087 Strasbourg, France
3
Alexander Butlerov Institute of Chemistry, Kazan Federal University, 420008 Kazan, Russia
*
Authors to whom correspondence should be addressed.
Energies 2020, 13(11), 2703; https://doi.org/10.3390/en13112703
Received: 1 May 2020 / Revised: 18 May 2020 / Accepted: 21 May 2020 / Published: 28 May 2020
(This article belongs to the Special Issue Electrocatalysts for Fuel Cells and Hydrogen Production)

Abstract

Electrochemical CO2 reduction reaction (CO2RR) to fuels and chemicals represents nowadays one of the most challenging solutions for renewable energy storage and utilization. Among the possible reaction pathways, CO2-to-CO conversion is the first (2e) reduction step towards the production of a key-feedstock that holds great relevance for chemical industry. In this report we describe the electrocatalytic CO2-to-CO reduction by a series of tailored N-decorated carbon nanotubes to be employed as chemoselective metal-free electrocatalysts. The choice of an exohedral functionalization tool for the introduction of defined N-groups at the outer surface of carbon nanomaterials warrants a unique control on N-configuration and electronic charge density distribution at the dangling heterocycles. A comparative electrochemical screening of variably N-substituted carbon nanomaterials in CO2RR together with an analysis of the electronic charge density distribution at each heterocycle have suggested the existence of a coherent descriptor for the catalyst’s CO faradaic efficiency (FECO). Evidence allows to infer that N-configuration (N-pyridinic vs. N-pyrrolic) of exohedral dopants and electronic charge density distribution at the N-neighboring carbon atoms of each heterocycle are directly engaged in the activation and stabilization of CO2 and its reduction intermediates.
Keywords: CO2 reduction reaction (CO2RR); metal-free electrocatalysts; pyridine nuclei; exohedral chemical grafting; multi-walled carbon nanotubes CO2 reduction reaction (CO2RR); metal-free electrocatalysts; pyridine nuclei; exohedral chemical grafting; multi-walled carbon nanotubes

1. Introduction

The steady state increase of CO2 concentration in the atmosphere caused by all main anthropic activities (including the massive employ of fossil fuels), is largely responsible of global warming effects and associated environmental issues such as climate changes, sea level rise and ocean acidification; phenomena that are seriously affecting our lifestyle. Accordingly, the development of new (electro)chemical technologies for CO2 conversion into products or energy-vectors of added value represents an effective “two-birds one-stone” approach for mitigating climate effects and supplying the growing energy demand to our modern society [1,2]. One promising approach to reduce CO2 emissions while fostering the transition towards a carbon neutral economy is given by the electrochemical CO2 reduction reaction (CO2RR) into chemicals to be stored, transported and used on demand [3,4]. CO2RR holds great expectations because of its generally mild operating conditions together with the possibility to control the reduction products (i.e., CO, formic acid, alcohols, acetic acid and small hydrocarbons) through the judicious tuning of the potentials applied to the electrochemical cell. Moreover, electrochemical devices powered by electricity generated from renewable sources such as wind, solar and hydro power contribute to make the transformation even more sustainable [5]. Despite the high energetic and environmental significance of CO2 electroreduction, the chemical inertness of CO2 generally requires high applied overpotentials to proceed with its electrochemical conversion. At the same time, the main competing process at work in aqueous electrolytes [hydrogen evolution reaction–HER] easily occurs under the thermodynamic potentials at which CO2 electroreduction occurs. This side-reaction deeply affects the overall process efficiency [6]. In addition, the occurrence of multiple electron transfers under CO2RR conditions and their close thermodynamic redox potentials [7,8] are at the origin of a generally poor process selectivity. As a result, the development of highly efficient and selective electrocatalysts for the process remains a challenging matter, partially addressed only for selected electroreduction products. Selective CO2 electroreduction to carbon monoxide (CO) is a strategic process [9] to produce a syngas component that is exploited for the synthesis of a series of useful organic chemical products and intermediates [10]. In contrast with highly endothermic and energy demanding transformations such as the reverse water-gas-shift (rWGS) reaction [11], the selective CO2-to-CO reduction can be achieved electrochemically already at ambient conditions with renewable energy sources. The latter reduction is a 2-protons/2-electrons process [12] where CO2 is firstly reductively adsorbed at the catalyst surface to give an intermediate that undergoes a second proton-coupled electron transfer to give CO (and H2O) that desorbs from the catalyst. Accordingly, an ideal electrocatalyst for the process should facilitate CO2 surface adsorption/activation while favoring the rapid CO desorption from its surface.
Over the last decades, great efforts in this area have been devoted mainly to the design, synthesis and use of metal-containing catalysts [13,14,15,16,17,18]. However, their production costs and their sensitivity to poisoning/deactivation, together with often moderate performance in terms of process activity and selectivity, have limited their widespread application in electrocatalysis. New electrocatalytic materials including single-phase nitrogen doped carbons have recently emerged as valuable alternative to metal-based systems [19] with comparable performance in CO2 reduction along with markedly reduced material costs [20]. Studies in the field of metal-free electrocatalysts have shown how the inclusion (doping) of light elements (i.e., N, B, S) in structure of graphitic nanocarbons improves their CO2RR performance compared to pristine pure-carbon counterparts. Indeed, doping alters the material electronic properties and changes the carbon network morphology, thus creating high-energy surface sites that facilitate CO2 interaction and foster its subsequent electrochemical conversion [21]. To date, various N-doped nanocarbons have been successfully applied as electrocatalysts for the CO2 conversion to get target products such as CO [22,23,24,25,26,27,28,29], formic acid (HCOOH) [30,31,32] or acetic acid (CH3CO2H) [33]. Despite these achievements, the nature of the active sites directly engaged in CO2 activation/conversion remains rather elusive and the generally complex nature of all synthesized N-doped carbon nanomaterials hampers a definitive comprehension of the structure-composition-reactivity relationships at the origin of their catalytic performance. The co-existence of different N-configurations (i.e., pyridinic, quaternary, pyrrolic) in classically chemical-vapor-deposition (CVD)-prepared N-doped nanocarbons together with the presence of metal traces coming from the experimental conditions applied to their production have raised serious debates within the scientific community on the nature of the active species for the process, leading to often conflicting experimental and theoretical results [19,21,22,23,24,26,27,28,34,35,36] (For classically N-doped nanocarbons where pyridinic-N sites are claimed to be responsible for CO2RR performance to CO, see: [26,27,36]. For classically N-doped nanocarbons where graphitic-N atoms are claimed to be responsible for CO2RR performance to CO, see: [21,28]. For classically N-doped nanocarbons where pyridinic and graphitic-N atoms are claimed to be responsible for CO2RR performance to CO, see: [19,23]. For classically N-doped nanocarbons where neightbouring C-atoms to N-sites or C-N polarized bonds are claimed to be responsible for CO2RR performance to CO, see: [22].). We have recently reported on the role of aziridine-decorated carbon nanotubes as highly efficient and chemoselective electrocatalysts for the CO2-to-CO conversion [37]. The use of an exohedral functionalization strategy for the ad-hoc introduction of surface exposed N-containing functionalities [38,39,40,41,42,43] has contributed to clarify the role of selected N-containing groups towards CO2 activation and its electrochemical conversion. Tailored N-functionalized catalysts provide a useful tool for the comprehension of the CO2RR mechanism at work on these metal-free systems and offer an ideal platform to rule out any conflicting or synergistic contribution raising from the presence of residual metal traces.
Since pyridine nuclei are commonly recognized among the most active sites for CO2RR in more conventionally prepared N-doped nanocarbons [19,26,27,28], in this paper we focused on a series of N-pyridinic heterocycles as dangling groups covalently linked at the surface of multi-walled carbon nanotubes to be employed as metal-free electrocatalysts. The comparative study unveils the role of pyridinic nuclei for the process, suggesting the existence of a coherent descriptor for the catalyst selectivity towards CO (faradaic efficiency, FECO) based on the electronic charge density distribution at the carbon atoms neighboring N-sites of the variably substituted heterocycles. In line with related studies from the literature [31,44], our conclusions support the key role played by N-neighboring carbon atoms on the CO2 activation mechanism operative with these metal-free electrocatalysts.

2. Materials and Methods

Multi-walled carbon nanotubes (MWCNTs-98% in C) were obtained from Sigma-Aldrich (Lot# MKBH5814V) and used as received. Unless otherwise stated, all other chemicals were purchased from commercial suppliers and used as received without further purification. Samples sonication was conducted on an Elma S15 Elmasonic sonicator bath (37 kHz) while cooling samples in a water/ice mixture throughout the treatment. Samples [email protected]1–4 and [email protected]6 (Scheme 1) used in the study have been prepared according to literature procedures [38,41,43] and characterized in terms of N-loading (elemental analysis and acid-base titration) and N-configuration (X-ray photoelectron spectroscopy—XPS) as to confirm their ultimate identity. The new sample [email protected]5 (Scheme 1) was prepared following a general procedure for the exohedral MW-functionalization via aryldiazonium salt chemistry. The functionalization protocol was similar to that already used for the other samples and described hereafter in brief for the sake of completeness. MWCNTs (80 mg) were weighed into a two-necked 150 mL flask and suspended in 65 mL of dry and degassed oDCB (ortho-dichlorobenzene). After sonication for 30 min in an ultrasonic bath, a solution of 4-amino-2,6-dimethylpyridine (1.72 mmol) in dry and degassed acetonitrile (30 mL) and isopentylnitrite (0.35 mL, 2.58 mmol) were added in sequence and the suspension was additionally sonicated for 10 min. Afterwards, the stirred mixture was heated up to 80 °C for 14 h before being cooled down to room temperature, diluted with ethylacetate (20 mL) and sonicated for 20 min. The solid residue was then recovered by centrifugation/washing/decantation procedure using ethylacetate (20 mL) and dichloromethane (twice 15 mL) as solvent. The solid residue was finally suspended in dichloromethane, sonicated for 15 min and filtered through a 0.2 µm-pore PTFE filter. [email protected]N5 was finally dried at 50 °C under vacuum to constant weight and stored on air at room temperature. X-ray Photoelectron Spectroscopy (XPS) measurements were conducted on a Thermo-VG scientific spectrometer, operating in ultrahigh vacuum (UHV) and equipped with a CLAM4 (Multiplier Channel Detector—MCD) hemispherical electron analyzer. The Al Kα line (1486.6 eV) of a dual anode X-ray source was used as incident radiation. Charge correction of the spectra was performed by taking the sp2 graphitic component of the C 1s spectrum as internal reference (Binding Energy, BE = 284.6 eV). Survey and high-resolution spectra were recorded in constant pass energy mode (100 and 20 eV, respectively). High resolution spectra of detected elements were acquired for semi-quantitative and detailed BE chemical shift analysis. The high-resolution spectra were fitted with mixed Gaussian-Lorentzian peaks after a Shirley background subtraction. The determined standard deviation in the peak position was ±0.2 eV. Atomic percentages were calculated using the corresponding core level peaks properly normalized to the photoemission cross section and assuming a homogeneous distribution arrangement model. Elemental analyses were carried out on a Thermo FlashEA 1112 Series CHNS-O elemental analyzer and elemental average values were calculated over three independent runs. Acid-base titration was conducted as follows: 5 mg of [email protected]1–5 were suspended in 7 mL of a HCl standard solution (2.8 × 10−3 M, standardized with Na2CO3 as primary standard), sonicated for 30 min and stirred at room temperature for 48 h. Afterwards, the suspension was centrifuged and three aliquots of the supernatant solution were titrated with a standardized solution of NaOH (2 × 10−3 M). The N% was calculated as the average value over the three independent titrations.

2.1. Electrochemical Measurements

2.1.1. Working Electrode Preparation

A catalyst ink was prepared by dispersing 30 mg of each [email protected]1–6 sample in 2.5 g of isopropanol, 1.5 g of pure Milli-Q® water and 70 mg of a 5 wt.% Nafion solution in lower aliphatic alcohols and water. The resulting suspension was then sonicated for 60 min until a homogeneous ink was obtained. The latter was then deposited on carbon cloth (12–16 cm2, cleaned, dried and weighted before deposition) by brush painting through seven–nine successive deposition/drying cycles. Teflonized Carbon cloth (Graphitestore©, 410 μm thickness) was cleaned before use by sonication in a 1:1 water:isopropanol solution and finally dried at 70 °C to constant weight. The catalyst loading on the as-prepared electrode was determined by weight difference between the cathode and the bare carbon cloth and fixed to 1.2 ± 0.1 mg/cm2 for each sample.

2.1.2. Cell Assembly

Electrochemical measurements were conducted in a home-made vertically oriented three-dimensionally (3D)-printed ABS [45] cell designed with a gas collector on the top (Figure S1) and divided into two halves hosting the cathodic and anodic compartments separated by a Nafion® 117 membrane. The cathode (working electrode) was secured by a screw-terminal connected to an insulated copper wire. The Ag/AgCl/KClsat. reference electrode was placed in close proximity to the working electrode while a Pt wire, employed as counter-electrode, was set in the anodic part. Both compartments were filled with a 0.1 M KHCO3 solution and sealed with rubber gaslock® plugs. Prior to each measurement, the cathodic part was saturated by bubbling CO2 (99.99% purity) for 30 min and the collector cone gas headspace was completely evacuated and filled with the electrolyte.

2.1.3. Electrolysis Experiments and Products Analysis

The electrolysis experiments were conducted at different potential values (ranging from −1.0 to −1.5 V vs. Ag|AgCl|KClsat.) at electrolysis times ranging from 10 min to 3 h depending on the applied potential and resulting currents. After each electrolysis run, 100 μL of gases collected within the cone were withdrawn using a Hamilton gastight syringe and analyzed by gas chromatography (Carboxen Column, settled up with a heating ramp designed to separate H2, O2, N2, CO, CO2 and C1–C3 gases). The cell was then saturated again with CO2 and the experiment repeated changing the applied potential. The cathodic liquid was collected at the end of each experimental cycle and analyzed by HPLC chromatography in order to detect any trace of liquid reduction products. Measurements were run on a Shimadzu-UFLC apparatus under isothermal conditions (65 °C), equipped with a refractive index detector (RID) and an Alltech OA-1000 Organic Acid Column of 300 mm (length) and 6.5 mm (i.d.). A 0.01 N H2SO4 solution was used as eluent at a flow rate of 0.4 mL min−1. The KHCO3 buffer solution is suppressed by passing the sample in cation exchange resin, prior to the injection. The same procedure was performed for the standard HCOOH solutions. The Faradaic efficiency (FE%) of the products were calculated from GC analysis data and CO productivities (expressed as NLCO gN−1 h−1) at fixed potential values have been calculated according to the relative current density values.

2.2. Computational Methods

The structures of surface N-containing groups (as zero-charge, H-saturated heterocycles) at the CNTs were optimized with a B3LYP functional [46] using a 6–31G** basis set on all atoms. A Grimme D3 dispersion correction was also included in the optimization, through the Gaussian IOp(3/124 = 30) keyword [47]. Bader charge partitioning was made through the free algorithm developed at Texas University available at http://theory.cm.utexas.edu/bader/. The Gaussian *.cube file from the optimizations was taken as the starting point for the Bader analysis on a charge density grid [48].

3. Results and Discussion

The study sheds light on the role played by carbon atoms neighboring N-sites of variably substituted pyridine nuclei, employed as metal-free active sites for CO2RR. To this aim, five N-decorated multi-walled carbon nanotubes containing variably substituted pyridine nuclei together with a model N-pyrrolic functionalized sample have been prepared according to literature procedures (Scheme 1, [email protected]1 [40,41]; [email protected]2 [37,41]; [email protected]3 [43]; [email protected]4 [43]; [email protected]5 and [email protected]6 [41]). We took advantage of well consolidated aryldiazonium salt functionalization protocols as efficient synthetic tools for the exohedral covalent grafting of MWCNTs with pyridine- and pyrrole-containing dangling moieties (Scheme 1). All N-decorated samples have been thoroughly characterized in terms of N-content (elemental analysis and acid-base titration) and N-configuration of dangling heterocycles available at the outer material surface (XPS) before being fabricated into carbon cloth (Cc) electrodes (see details provided in the Experimental Section) and tested as metal-free systems for CO2RR under comparable conditions (vide infra).
Sample [email protected]6 containing carbazole units was prepared as a model of a non-basic N-containing heterocycle embedded in a conjugated sp2 framework similar to [email protected]2. Although the role of N-configuration of heterodopants (N-pyridinic, N-pyrrolic, N-aminic and N-quaternary) in complex carbon nanomaterials remains controversial, it is accepted that inclusion of nitrogen(s) contributes to break the electroneutrality of Csp2 networks and generates carbon sites proximal to the heteroelement that take part (somehow) to CO2 activation and its subsequent electrochemical reduction. There is an interesting parallelism between the supposed mechanism at work in N-doped nanocarbons applied to CO2RR and their action as metal-free electrocatalysts within another highly challenging electroreduction process: the oxygen reduction reaction (ORR) [49,50]. [email protected]1 and [email protected]2 were previously reported by us as benchmark metal-free ORR electrocatalysts containing well defined edge-type N-defects exposed in carbon nanostructures as effective sites for the process to occur [41,43]. Similar to ORR, a redistribution of the charge density around the nitrogen atoms in these metal-free systems creates high-energy carbon sites (typically proximal to the heteroelement) that improve the material adsorption properties towards CO2 or its reduction intermediates, hence facilitating its overall electrochemical conversion [31,44]. The electronic charge distribution on the selected N-heterocycles can be invoked as a descriptor for the catalyst/substrate interaction once a methodology for the fine tuning of N-dopants in C-nanostructures is properly set-up. In addition, N-basic heterocycles (with pyridinic basicity > pyrrolic basicity) naturally foster the interaction of the metal-free electrocatalyst with acidic molecules (i.e., CO2) thus favoring the generation of local gradients of CO2 concentration close to the catalyst active sites. Our approach to the covalent grafting of tailored N-containing functionalities at the surface of C-based nanomaterials allows to play on the electronic properties of the heterocycles through an ad-hoc selection of electron donating (ED) or electron withdrawing (EW) groups attached to the rings. By this way, a fine tuning of the electronic charge density distribution at the N-site as well as at its neighboring carbon atoms [22] can be achieved for each dangling functional group (N1-N6).
All prepared N-decorated samples have been fully characterized by different techniques before being handled to prepare the corresponding electrodes for CO2RR (see experimental section). To this aim, X-ray photoelectron spectroscopy (XPS), elemental analysis and acid-base titration (whenever applicable) were accomplished and data are outlined in Figure 1 and Table 1.
Figure 1A–E show typical N 1s profiles for samples [email protected]1–5 featured by a unique component in the 398.8–398.6 eV range, consistent with the presence of pyridine-type nuclei [51]. One component at higher binding energy values (400.3 eV) was measured in [email protected]6 (Figure 1F) in accordance with the presence of N-pyrrolic heterocycles [52]. Finally, minor N 1s shoulders at higher binding energy values and present in selected samples from this series (Figure 1B–D) were attributed to commonly observed and not-catalytically relevant nanomaterial contaminations [41,42,53,54] raising from solvents and/or reagents residues retained by the material porosity and not entirely removed throughout the work-up procedure.
Previous studies have confirmed that pristine MWCNTs as such did not show any CO2 electroreduction performance, with HER being the only electrocatalytic process at work under more reducing overpotentials [37]. On the other hand, qualitative cyclic voltammetric (CV) tests conducted under N2 and CO2 saturated conditions with functionalized MWCNTs containing dangling N-groups at their outer surface resulted into electroactive catalysts featuring with variable CO2 conversion/reduction properties depending on the nature of the N-containing group engaged in the electroreduction process [20,37]. Accordingly, all new electrocatalytic materials prepared in this contribution ([email protected]1–6) and MWCNTs were fabricated into carbon cloth (Cc) working electrodes ([email protected]1–6/Cc and MWCNTs/Cc) and directly scrutinized as CO2RR electrocatalysts within a home-made 3D-printed ABS [45] three-electrode cell operating in a 0.1 M KHCO3 solution, equipped with an Ag|AgCl|KClsat reference electrode, a Pt counter electrode in the anodic compartment separated by a Nafion® 117 membrane and a gas collector cone on the top for the regular sampling of all produced volatiles (Figure S1). All gaseous products (H2 and CO only) generated throughout the electrocatalytic runs conducted at variable potentials have been systematically sampled using a gas-tight syringe and injected in a gas chromatograph (GC) for analysis. Finally, the possible formation of liquid reduction products has been verified through HPLC analysis of all solutions collected at the end of each electrochemical run.
One main advantage of our metal-free catalyst technology is the given possibility to rule out any contribution to CO2 electrocatalytic reduction raising from the presence of metal impurities potentially present in pristine carbon nanomaterials (MWCNTs). Indeed, it has been well recognized that such ubiquitous impurities (often catalytically active in electrocatalysis) cannot be completely removed from classically prepared N-doped nanocarbons even after thorough purification procedures [55]. Our approach to electrocatalysts preparation allows to test pristine MWCNTs as a blank sample in the process (vide infra). Accordingly, their comparison with ad-hoc N-functionalized materials unambiguously explains the role played by exohedral N-groups only in CO2RR.
The total current density values and composition of reaction products have been determined for each electrocatalyst in the −1.0 ÷ −1.5 V range at electrolysis time lengths varying from 10 min to 3 h as a function of the applied potential. Longer electrolysis times at lower overpotentials have been applied in order to collect a reasonable amount of gas for an accurate GC analysis of products and their relative ratio. Irrespective of the electrocatalyst at work, only gaseous products have been detected with [email protected]1–6, with CO being the unique component from CO2 electroreduction along with H2 as a side-product from water electrolysis (HER). No sample from this series led to the formation of liquid products, whatever the electroreduction potential applied to the cell. According to our previous evidences with pristine MWCNTs, only H2 was detected after electrolysis with MWCNT/Cc even under CO2 saturated atmosphere [37]. As mentioned above, this blank trial confirmed the role played by exohedral and discrete N-functionalities in [email protected]1–6 in CO2RR and it rules out any potential contribution raising from the metallic residues in C-nanocarriers.
Figure 2A summarizes the CO faradaic efficiency (FECO) values measured on each N-decorated electrocatalyst as a function of the applied potential, while Figure 2B refers to the CO current density (JCO) values given for each electrocatalyst ([email protected]1–6/Cc) at −1.1 V. The latter potential was selected as the mean V value in the range where all electrocatalysts offer their highest FECO (Jtot values registered at V where each electrocatalyst shows its highest FECO are outlined on Figure S2 for the sake of completeness). Figure S3 reports the H2 faradaic efficiency (FEH2) plots for [email protected]1–6/Cc in the −1.0 ÷ −1.5 V range, while Table S1 details JCO and Jtot values given for each electrocatalyst at −1.1 V along with productivity values calculated for each [email protected]1–6/Cc electrocatalyst and expressed as NLCOgN−1h−1.
As a first trial, pristine MWCNTs were fabricated into a Cc electrode and tested as a pure carbon-based electrocatalyst (blank sample) for the process. As Figure 2A and Table S1 show, MWCNTs revealed null or negligible activity in CO2RR whatever the applied potential in the −1.0 ÷ −1.5 V range vs. reference electrode. On the other hand, the acridine and 2,6(Me)2Py-functionalized electrocatalysts [email protected]2/Cc and [email protected]5/Cc were the most performing CO2RR systems from this series, showing FECO values up to 52 and 44% at −1.2 V and −1.15 V vs. Ag|AgCl|KClsat, respectively (Figure 2A and Table S1, entries 7 and 16). Notably, the plain Pyridine-functionalized electrocatalyst ([email protected]1) showed only moderate CO2RR performance with an FECO of 21% at −1.1 V vs. the reference electrode. As expected, at more reducing potentials (up to −1.5 V), HER was the main side-process at work with all these metal-free electrocatalysts (Table S1, entries 4, 7, 16 vs. 5, 8, 17, and Figure S3). In spite of an identical N-configuration for the three heterocycles (only N-pyridinic sites in [email protected]2, [email protected]5 and [email protected]1) and a very close N-loading of dangling groups in each sample (Table 1), their different electrocatalytic behavior prompted us to postulate the existence of a key-action played by the electronic charge distribution in each N-pyridinic heterocycle. Such an electronic distribution, driven by the nature and position of ring-substituents, was supposed to modulate the inherent ability of N-containing groups to interact more or less efficiently with CO2 and/or its reduction intermediates. To this aim, the o-methoxy pyridine ([email protected]3) and naphthyridine-functionalized ([email protected]4) samples were fabricated into Cc electrodes and studied as CO2RR electrocatalysts under identical conditions. Once again, both samples showed only moderate performance with FECO as high as 23% and 15% at −1.05 V and −1.15 V vs. [Ag][AgCl][KClsat] for [email protected]3/Cc and [email protected]4/Cc, respectively (Figure 2A and Table S1, entries 9 and 13). As an additional trial, carbazole-functionalized MWCNTs (N-pyrrolic heterocycle) were scrutinized as CO2RR electrocatalysts in the same potential range. As Figure 2 clearly shows, [email protected]6/Cc was the worst electrocatalyst for the process with a maximum of 7% FECO at −1.20 V vs. [Ag][AgCl][KClsat], irrespective to its relatively high N-loading [56].
Figure 2B finally refers to JCO values vs. time as measured for each electrocatalyst at −1.1 V as potential (see also Table S1). Current densities (mA/cm2) are constant throughout the whole electrochemical runs thus confirming the excellent stability of each functionalized material (produced in the form of Cc electrocatalyst), under CO2RR conditions. In addition, the XPS N 1s core region profile of the most representative sample from this series ([email protected]2) recorded on the recovered electrocatalyst (after 90 min CO2RR) did not show any qualitative profile alterations (Figure S4).
This comparative study among tailored N-decorated carbon-based electrocatalysts suggests the existence of coherent descriptors for the CO faradaic efficiency (FECO): the N-configuration (N-pyridinic vs. N-pyrrolic) of the exohedral dopant(s) and the electronic charge density distribution at the N-neighboring carbon atoms of each dangling heterocycle. To better rationalize this descriptor, we have carried out a DFT pre-optimization on each dangling substituent (N1–N6) at the B3LYP//6–31G** level of theory and a Bader charge analysis has been performed to deduce the Bader atomic charges present on both N and Cα atoms, whose values are outlined in Table 2. In general, the numerical values obtained through our calculations are comparable to those appearing in other literature studies performed on similar metal-free hetero-doped carbon nanomaterials [57,58,59]; this makes us confident about the accuracy of our calculations. The N atomic charge measured on each heterocycle is roughly the same on all N-pyridinic nuclei (N1–5), with minor differences ascribable to the delocalization of the nitrogen electronic lone pair as a function of the ring substituents. Such electronic charge values are a measure of the Lewis-basic properties of N-sites in N1–5, and hence, a measure of the interaction strength with a Lewis-acid reagent such as CO2. The higher the electronic charge density at the N-site, the higher the affinity towards CO2. As a matter of fact, the poorly basic carbazole N6 (featuring a lower N electronic charge density) will show a reduced affinity for CO2. Such an acid-base interaction is supposed to be at the origin of the electrocatalytic activity of pyridine-based metal-free electrocatalysts and it reduces the free energy barriers associated to the first (and kinetically sluggish) electron transfer process [60]. A more pronounced difference in qCα values measured on all variably substituted heterocycles prompted us to look for a coherent correlation between qCα and the different process selectivity towards CO (FECO) measured on each electrocatalyst. Indeed, absorption and stabilization of CO2 and its reduction intermediates at the carbon atoms adjacent to N sites are also claimed as a key driving force for the process to occur [31,44]. Figure 3 was obtained by plotting the FECO value recorded on each N-decorated sample ([email protected]1–6) at −1.1 V vs. the qCα value calculated at the respective heterocycles. A mean qC value between Cα and Cα′ was arbitrarily considered for both dissymmetrical N3 and N4 samples without altering the general trend outlined in Figure 3.
At first glance, data on Figure 3 follow a typical volcano plot with the acridine-decorated [email protected]2 sample being the best performing CO2RR system among the scrutinized materials. It can be inferred that both the basic character of the N-containing heterocycles and the electronic charge distribution at Cα/Cα′ play a role on the ultimate electrocatalytic performance of these metal-free systems. While the former is expected to foster CO2 interaction with N-dangling groups (reducing the energy barriers for the first electron transfer), the latter is expected to stabilize (more or less efficiently) the resulting radical anion CO2•−* before its further reduction to CO [23] (see Scheme 2 for an hypothetical CO2 and CO2•− interaction mode on acridine decorated MWCNTs as a model N-decorated/Cc example). The lower the qCα value (N1 and N6), the weaker the stabilization of the radical anion CO2•−* and the driving-force to its generation. On the other hand, the higher the positive Cα charge, the strongest the resulting CO2•−* stabilization that hurdles its further reduction to CO. Because of relatively low (N1 and N6) or relatively high (N3 and N4) CO2•−* adsorption energies at Cα, hydrogen evolution largely dominates over CO2 reduction with these electrocatalysts.
2,6-(Me)2Py and acridine frameworks (in [email protected]5 and [email protected]2, respectively) create an ideal balance between N-basicity for CO2 activation/conversion to give the corresponding CO2•−* radical ion (first e transfer) and its subsequent stabilization by neighboring C-sites.
These conclusions are in line with evidences by Su et al. based on density functional theory (DFT) calculations carried out on N-doped carbon nanotubes containing variable fractions of pyrrolic, pyridinic and quaternary N-species. These authors demonstrated that improved activity of doped nanocarbons as electrocatalysts for CO2RR can be achieved via stabilization of the key intermediate CO2•−* [28]. The same authors also pointed out that another important factor that influences the selectivity of N-doped carbon materials in the CO2-to-CO conversion is the ability to release the reaction product CO at the end of the catalytic cycle. The different thermodynamic affinity for carbon monoxide has been estimated through the calculation of the corresponding Gibbs energy values (ΔG) of the adduct formation (CO + N-heterocycle → adduct). The results showed that pyridinic N-sites are featured by a very low interaction strength with CO compared to other N-configurations. This finding is fully consistent with our experimental evidences, where even for the most performing electrocatalyst from this series ([email protected]2), CO was the unique reduction product detected both in gas and liquid-phase.

4. Conclusions

This contribution describes a series of exohedrally N-decorated carbon nanomaterials with variably substituted N-pyridine nuclei as model metal-free electrocatalysts for the selective CO2-to-CO electroreduction. The functionalization approach, used to introduce tailored N-functionalities at the nanomaterial surface of electrically conductive C-nanocarriers, offers a useful tool to shed light on the role played by these heterocycles in CO2RR. It allows to finely tune the electronic properties of all covalently linked heterocycles, with a consequent control of the electronic charge density distribution at the N-site and its neighboring Cα/Cα′atoms. A comparative CO2RR electrochemical screening of all N-decorated samples has suggested the existence of a coherent descriptor (a volcano-like trend) between the catalyst’s CO faradaic efficiency (FECO) and the electronic charge density distribution on each heterocycle engaged in the process. It can be inferred that both the basic character of each heterocycle as well as the electronic charge distribution at Cα/Cα’ play a role on the ultimate electrocatalytic performance of these metal-free systems. While the former is supposed to foster CO2 interaction with N-dangling groups (reducing the energy barriers for the first electron transfer), the latter is expected to stabilize the resulting radical anion CO2•−* before its further reduction to CO. Accordingly, the ideal balance between N-heterocycle basicity and CO2•− radical ion stabilization ability by neighboring C-sites is at the origin of the catalyst selectivity towards CO (FECO).
These findings allow to shed light on the role of pyridinic nuclei in CO2RR by N-doped nanocarbons; they provide a privileged viewpoint on the reaction mechanism at work and they offer a useful tool for the preparation of more catalytically active systems for the process.

Supplementary Materials

The following are available online at https://www.mdpi.com/1996-1073/13/11/2703/s1, Figure S1: Scheme of the home-made 3D-printed ABS three-electrode cell used in the study; Figure S2: Total current density values (Jtot) and their stability vs. time, measured for each electrocatalytic material [email protected]1–6 at the potential value where they present the highest FECO; Figure S3: H2 faradaic efficiency as a function of the applied potential (−1.0 ÷ −1.5 V range); Figure S4: N 1s core regions and relative fits for the high resolution XPS spectrum of [email protected]2 before (A) and after (B) CO2RR. Sample B has been recovered from [email protected]2/Cc after 90 min of electrocatalytic run; Table S1: Jtot, JCO, CO and H2 faradaic efficiency (FE) and productivity obtained with [email protected]1–6/Cc and MWCNT/Cc.

Author Contributions

Conceptualization, F.V. and G.G.; Data curation, A.R., L.L., C.P.-H., F.V. and G.G.; Formal analysis, G.T., J.F., A.R., L.L. and D.Y.; Funding acquisition, F.V. and G.G.; Investigation, G.T., J.F., D.Y. and G.G.; Methodology, A.R., L.L. and G.G.; Resources, G.G.; Validation, G.T. and C.P.-H.; Writing—original draft, G.G.; Writing—review and editing, F.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the TRAINER project (Catalysts for Transition to Renewable Energy Future) of the “Make our Planet Great Again” program (Ref. ANR-17-MPGA-0017), by the Italian MIUR through the PRIN 2017 Projects Multi-e (20179337R7) “Multielectron transfer for the conversion of small molecules: an enabling technology for the chemical use of renewable energy” and (2017YH9MRK) “Novel Multilayered and Micro-Machined Electrode Nano-Architectures for Electrocatalytic Applications (Fuel Cells and Electrolyzers) by the Ente Cassa di Risparmio di Firenze through the “EnergyLab” project.

Acknowledgments

This work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University.

Conflicts of Interest

The authors declare no competing interests.

References

  1. Martens, J.A.; Bogaerts, A.; De Kimpe, N.; Jacobs, P.A.; Marin, G.B.; Rabaey, K.; Saeys, M.; Verhelst, S. The Chemical Route to a Carbon Dioxide Neutral World. ChemSusChem 2010, 10, 1039–1055. [Google Scholar] [CrossRef] [PubMed]
  2. Roy, S.; Cherevotan, A.; Peter, S.C. Thermochemical CO2 Hydrogenation to Single Carbon Products: Scientific and Technological Challenges. ACS Energy Lett. 2018, 3, 1938–1966. [Google Scholar] [CrossRef]
  3. Appel, A.M.; Bercaw, J.E.; Bocarsly, A.B.; Dobbek, H.; DuBois, D.L.; Dupuis, M.; Ferry, J.G.; Fujita, E.; Hille, R.; Kenis, P.J.A.; et al. Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation. Chem. Rev. 2013, 113, 6621–6658. [Google Scholar] [CrossRef] [PubMed]
  4. Lu, Q.; Jiao, F. Electrochemical CO2 reduction: Electrocatalyst, reaction mechanism, and process engineering. Nano Energy 2016, 29, 439–456. [Google Scholar] [CrossRef]
  5. Mohammadi, A.; Mehrpooya, M. A comprehensive review on coupling different types of electrolyzer to renewable energy sources. Energy 2018, 158, 632–655. [Google Scholar] [CrossRef]
  6. Hori, Y. Modern Aspects of Electrochemistry; Vayenas, G.C., White, E.R., Eds.; Springer: Patras, Greece, 2008; p. 89. [Google Scholar]
  7. Kortlever, R.; Shen, J.; Schouten, K.J.P.; Calle-Vallejo, F.; Koper, M.T.M. Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6, 4073–4082. [Google Scholar] [CrossRef]
  8. Kuhl, K.P.; Cave, E.R.; Abram, D.N.; Jaramillo, T.F. New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 2012, 5, 7050–7059. [Google Scholar] [CrossRef]
  9. Zheng, T.; Jiang, K.; Wang, H. Recent advances in electrochemical CO2-to-CO conversion on heterogeneous catalysts. Adv. Mater. 2018, 30, 1802066. [Google Scholar] [CrossRef]
  10. Khodakov, A.Y.; Chu, W.; Fongarland, P. Advances in the Development of Novel Cobalt Fischer−Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels. Chem. Rev. 2007, 107, 1692–1744. [Google Scholar] [CrossRef]
  11. Centi, G.; Perathoner, S. Opportunities and Prospects in the Chemical Recycling of Carbon Dioxide to Fuels. Catal. Today 2009, 148, 191–205. [Google Scholar] [CrossRef]
  12. Jiang, K.; Siahrostami, S.; Akey, A.J.; Li, Y.; Lu, Z.; Lattimer, J.; Hu, Y.; Stokes, C.; Gangishetty, M.; Chen, G.; et al. Transition-Metal Single Atoms in a Graphene Shell as Active Centers for Highly Efficient Artificial Photosynthesis. Chem 2017, 3, 950–960. [Google Scholar] [CrossRef]
  13. Feng, D.-M.; Zhu, Y.-P.; Chen, P.; Ma, T.-Y. Recent advances in transition-metal-mediated electrocatalytic CO2 reduction: From homogeneous to heterogeneous systems. Catalysts 2017, 7, 373. [Google Scholar] [CrossRef]
  14. Francke, R.; Schille, B.; Roemelt, M. Homogeneously Catalyzed Electroreduction of Carbon Dioxide-Methods, Mechanisms, and Catalysts. Chem. Rev. 2018, 118, 4631–4701. [Google Scholar] [CrossRef] [PubMed]
  15. Yang, Y.; Ajmal, S.; Zheng, X.; Zhang, L. Efficient nanomaterials for harvesting clean fuels from electrochemical and photoelectrochemical CO2 reduction. Sustain. Energy Fuels 2018, 2, 510–537. [Google Scholar] [CrossRef]
  16. Zhang, L.; Zhao, Z.-J.; Gong, J. Nanostructured materials for heterogeneous electrocatalytic CO2 reduction and their related reaction mechanisms. Angew. Chem. Int. Ed. 2017, 56, 11326–11353. [Google Scholar] [CrossRef]
  17. Zhang, W.; Hu, Y.; Ma, L.; Zhu, G.; Wang, Y.; Xue, X.; Chen, R.; Yang, S.; Jin, Z. Progress and perspective of electrocatalytic CO2 reduction for renewable carbonaceous fuels and chemicals. Adv. Sci. 2018, 5, 1700275. [Google Scholar] [CrossRef]
  18. Bevilacqua, M.; Filippi, J.; Miller, H.A.; Vizza, F. Recent technological progress in CO2 electroreduction to fuels and energy carriers in aqueous environments. Energy Technol. 2015, 3, 197–210. [Google Scholar] [CrossRef]
  19. Sharma, P.P.; Wu, J.; Yadav, R.M.; Liu, M.; Wright, C.J.; Tiwary, C.S.; Yakobson, B.I.; Lou, J.; Ajayan, P.M.; Zhou, X.-D. Nitrogen-doped carbon nanotube arrays for high-efficiency electrochemical reduction of CO2: On the understanding of defects, defect density, and selectivity. Angew. Chem. Int. Ed. 2015, 54, 13701–13705. [Google Scholar] [CrossRef]
  20. Duan, X.; Xu, J.; Wei, Z.; Ma, J.; Guo, S.; Wang, S.; Liu, H.; Dou, S. Metal-Free Carbon Materials for CO2 Electrochemical Reduction. Adv. Mater. 2017, 29, 1701784. [Google Scholar] [CrossRef]
  21. Chai, G.-L.; Guo, Z.-X. Highly effective sites and selectivity of nitrogendoped graphene/CNT catalysts for CO2 electrochemical reduction. Chem. Sci. 2016, 7, 1268–1275. [Google Scholar] [CrossRef]
  22. Kumar, B.; Asadi, M.; Pisasale, D.; Sinha-Ray, S.; Rosen, B.A.; Haasch, R.; Abiade, J.; Yarin, A.L.; Salehi-Khojin, A. Renewable and metal-free carbon nanofibre catalysts for carbon dioxide reduction. Nat. Commun. 2013, 4, 2819. [Google Scholar] [CrossRef]
  23. Li, W.; Seredych, M.; Rodriguez-Castellon, E.; Bandosz, T.J. Metal-free nanoporous carbon as a catalyst for electrochemical reduction of CO2 to CO and CH4. ChemSusChem 2016, 9, 606–616. [Google Scholar] [CrossRef] [PubMed]
  24. Lu, X.; Tan, T.H.; Ng, Y.H.; Amal, R. Highly selective and stable reduction of CO2 to CO by a graphitic carbon nitride/carbon nanotube composite electrocatalyst. Chem. Eur. J. 2016, 22, 11991–11996. [Google Scholar] [CrossRef]
  25. Molly Jhong, H.-R.; Tornow, C.E.; Smid, B.; Gewirth, A.A.; Lyth, S.M.; Kenis, P.J.A. A nitrogen-doped carbon catalyst for electrochemical CO2 conversion to CO with high selectivity and current density. ChemSusChem 2017, 10, 1094–1099. [Google Scholar] [CrossRef] [PubMed]
  26. Wu, J.; Liu, M.; Sharma, P.P.; Yadav, R.M.; Ma, L.; Yang, Y.; Zou, X.; Zhou, X.-D.; Vajtai, R.; Yakobson, B.I.; et al. Incorporation of nitrogen defects for efficient reduction of CO2 via two-electron pathway on three-dimensional graphene foam. Nano Lett. 2016, 16, 466–470. [Google Scholar] [CrossRef]
  27. Wu, J.; Yadav, R.M.; Liu, M.; Sharma, P.P.; Sekhar Tiwary, C.; Ma, L.; Zou, X.; Zhou, X.-D.; Yakobson, B.I.; Lou, J.; et al. Achieving highly efficient, selective, and stable CO2 reduction on nitrogen-doped carbon nanotubes. ACS Nano 2015, 9, 5364–5371. [Google Scholar] [CrossRef]
  28. Xu, J.; Kan, Y.; Huang, R.; Zhang, B.; Wang, B.; Wu, K.-H.; Lin, Y.; Sun, X.; Li, Q.; Centi, G.; et al. Revealing the Origin of Activity in Nitrogen-Doped Nanocarbons towards Electrocatalytic Reduction of Carbon Dioxide. ChemSusChem 2016, 9, 1085–1089. [Google Scholar] [CrossRef]
  29. Zhang, B.; Zhao, T.-J.; Feng, W.-J.; Liu, Y.-X.; Wang, H.-H.; Su, H.; Lv, L.-B.; Li, X.-H.; Chen, J.-S. Polarized few-layer g-C3N4 as metal-free electrocatalyst for highly efficient reduction of CO2. Nano Res. 2018, 11, 2450–2459. [Google Scholar] [CrossRef]
  30. Liu, Y.; Zhao, J.; Cai, Q. Pyrrolic-nitrogen doped graphene: A metal-free electrocatalyst with high efficiency and selectivity for the reduction of carbon dioxide to formic acid: A computational study. Phys. Chem. Chem. Phys. 2016, 18, 5491–5498. [Google Scholar] [CrossRef]
  31. Wang, H.; Chen, Y.; Hou, X.; Ma, C.; Tan, T. Nitrogen-doped graphenes as efficient electrocatalysts for the selective reduction of carbon dioxide to formate in aqueous solution. Green Chem. 2016, 18, 3250–3256. [Google Scholar] [CrossRef]
  32. Zhang, S.; Kang, P.; Ubnoske, S.; Brennaman, M.K.; Song, N.; House, R.L.; Glass, J.T.; Meyer, T.J. Polyethylenimine-enhanced electrocatalytic reduction of CO2 to formate at nitrogen-doped carbon nanomaterials. J. Am. Chem. Soc. 2014, 136, 7845–7848. [Google Scholar] [CrossRef] [PubMed]
  33. Liu, Y.; Chen, S.; Quan, X.; Yu, H. Efficient Electrochemical Reduction of Carbon Dioxide to Acetate on Nitrogen-Doped Nanodiamond. J. Am. Chem. Soc. 2015, 137, 11631–11636. [Google Scholar] [CrossRef] [PubMed]
  34. Lum, Y.; Kwon, Y.; Lobaccaro, P.; Chen, L.; Clark, E.L.; Bell, A.T.; Ager, J.W. Trace levels of copper in carbon materials show significant electrochemical CO2 reduction activity. ACS Catal. 2016, 6, 202–209. [Google Scholar] [CrossRef]
  35. Siahrostami, S.; Jiang, K.; Karamad, M.; Chan, K.; Wang, H.; Nørskov, J. Theoretical investigations into defected graphene for electrochemical reduction of CO2. ACS Sustain. Chem. Eng. 2017, 5, 11080–11085. [Google Scholar] [CrossRef]
  36. Wu, J.; Ma, S.; Sun, J.; Gold, J.I.; Tiwary, C.S.; Kim, B.; Zhu, L.; Chopra, N.; Odeh, I.N.; Vajtai, R.; et al. A metal-free electrocatalyst for carbon dioxide reduction to multi-carbon hydrocarbons and oxygenates. Nat. Commun. 2016, 7, 13869. [Google Scholar] [CrossRef] [PubMed]
  37. Tuci, G.; Filippi, J.; Ba, H.; Rossin, A.; Luconi, L.; Pham-Huu, C.; Vizza, F.; Giambastiani, G. How to Teach an Old Dog New (Electrochemical) Tricks: Aziridine-Functionalized CNTs as Efficient Electrocatalysts for the Selective CO2 Reduction to CO. J. Mater. Chem. A 2018, 6, 16383–16389. [Google Scholar] [CrossRef]
  38. Tuci, G.; Luconi, L.; Rossin, A.; Berretti, E.; Ba, H.; Innocenti, M.; Yakhvarov, D.; Caporali, S.; Pham-Huu, C.; Giambastiani, G. Aziridine-functionalized multiwalled carbon nanotubes: Robust and versatile catalysts for the oxygen reduction reaction and Knoevenagel condensation. ACS Appl. Mater. Interfaces 2016, 8, 30099–30106. [Google Scholar] [CrossRef]
  39. Tuci, G.; Luconi, L.; Rossin, A.; Giambastiani, G. Chemical Functionalization of Carbon Nanomaterials: Bridging the Gap between Simple Carriers and Smart (Metalfree) Catalysts. Chimia 2017, 71, 568–572. [Google Scholar] [CrossRef]
  40. Tuci, G.; Rossin, A.; Luconi, L.; Pham-Huu, C.; Cicchi, S.; Ba, H.; Giambastiani, G. Pyridine-decorated carbon nanotubes as a metal-free heterogeneous catalyst for mild CO2 reduction to methanol with hydroboranes. Catal. Sci. Technol. 2017, 7, 5833–5837. [Google Scholar] [CrossRef]
  41. Tuci, G.; Zafferoni, C.; D’Ambrosio, P.; Caporali, S.; Ceppatelli, M.; Rossin, A.; Tsoufis, T.; Innocenti, M.; Giambastiani, G. Tailoring Carbon Nanotube N-Dopants while Designing Metal-Free Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Medium. ACS Catal. 2013, 3, 2108–2111. [Google Scholar] [CrossRef]
  42. Tuci, G.; Zafferoni, C.; Rossin, A.; Luconi, L.; Milella, A.; Ceppatelli, M.; Innocenti, M.; Liu, Y.; Pham-Huu, C.; Giambastiani, G. Chemical functionalization of N-doped carbon nanotubes: A powerful approach to cast light on the electrochemical role of specific N-functionalities in the Oxygen Reduction Reaction. Catal. Sci. Technol. 2016, 6, 6226–6236. [Google Scholar] [CrossRef]
  43. Tuci, G.; Zafferoni, C.; Rossin, A.; Milella, A.; Luconi, L.; Innocenti, M.; Truong Phuoc, L.; Duong-Viet, C.; Pham-Huu, C.; Giambastiani, G. Chemically functionalized carbon nanotubes with pyridine groups as easily tunable N-decorated nanomaterials for the oxygen reduction reaction in alkaline medium. Chem. Mater. 2014, 26, 3460–3470. [Google Scholar] [CrossRef]
  44. Wang, H.; Jia, J.; Song, P.; Wang, Q.; Li, D.; Min, S.; Qian, C.; Wang, L.; Li, Y.F.; Ma, C.; et al. Efficient electrocatalytic reduction of CO2 by nitrogen-doped nanoporous carbon/carbon nanotube membranes: A step towards the electrochemical CO2 refinery. Angew. Chem. Int. Ed. 2017, 56, 7847–7852. [Google Scholar] [CrossRef]
  45. ABS (Acrylonitrile-Butadiene-Styrene) is a common base-resistant thermoplastic polymer for 3D-printing.
  46. Becke, A.D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648–5652. [Google Scholar] [CrossRef]
  47. Grimme, S.A.J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. [Google Scholar] [CrossRef] [PubMed]
  48. Tang, W.; Sanville, E.; Henkelman, G. A grid-based Bader analysis algorithm without lattice bias. J. Phys. Condens. Matter 2009, 21, 084204. [Google Scholar] [CrossRef]
  49. Singh, S.K.; Takeyasu, K.; Nakamura, J. Active Sites and Mechanism of Oxygen Reduction Reaction Electrocatalysis on Nitrogen-Doped Carbon Materials. Adv. Mater. 2019, 31, 1804297. [Google Scholar] [CrossRef]
  50. Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823–4892. [Google Scholar] [CrossRef]
  51. Chizari, K.; Vena, A.; Laurentius, L.; Sundararaj, U. The effect of temperature on the morphology and chemical surface properties of nitrogen-doped carbon nanotubes. Carbon 2014, 68, 369–379. [Google Scholar] [CrossRef]
  52. Louette, P.; Bodino, F.; Pireaux, J.-J. Poly(pyrrole) (PPY) XPS Reference Core Level and Energy Loss Spectra. Surf. Sci. Spectra 2005, 12, 84–89. [Google Scholar] [CrossRef]
  53. Dyke, C.A.; Stewart, M.P.; Maya, F.; Tour, J.M. Diazonium-based functionalization of carbon nanotubes: XPS and GC-MS analysis and mechanistic implications. Synlett 2004, 2004, 155–160. [Google Scholar] [CrossRef]
  54. Price, B.K.; Tour, J.M. Functionalization of Single-Walled Carbon Nanotubes “On Water”. J. Am. Chem. Soc. 2006, 128, 12899–12904. [Google Scholar] [CrossRef] [PubMed]
  55. Wong, C.H.A.; Chua, C.K.; Khezri, B.; Webster, R.D.; Pumera, M. Graphene Oxide Nanoribbons from the Oxidative Opening of Carbon Nanotubes Retain Electrochemically Active Metallic Impurities. Angew. Chem. Int. Ed. 2013, 52, 8685–8688. [Google Scholar] [CrossRef]
  56. The higher N-loading obtained for [email protected]6 is in line with the electron rich character of carbazole heterocycles and the consequent higher reactivity of their postulated aryl radical intermediates obtained during the Tour functionalization protocol.
  57. Gouse Peera, S.; Sahu, A.K.; Arunchander, A.; Bhat, S.D.; Karthikeyan, J.; Murugan, P. Nitrogen and fluorine co-doped graphite nanofibers as high durable oxygen reduction catalyst in acidic media for polymer electrolyte fuel cells. Carbon 2015, 93, 130–142. [Google Scholar] [CrossRef]
  58. Lin, I.-H.; Lu, Y.-H.; Chen, H.-T. Nitrogen-doped carbon nanotube as a potential metal-free catalyst for CO oxidation. Phys. Chem. Chem. Phys. 2016, 18, 12093–12100. [Google Scholar] [CrossRef] [PubMed]
  59. Liu, T.F.; Ali, S.; Li, B.; Su, D.S. Revealing the role of sp2@sp3 structure of nanodiamond in direct dehydrogenation: Insight from DFT study. ACS Catal. 2017, 7, 3779–3785. [Google Scholar] [CrossRef]
  60. Zhu, Y.; Lv, K.; Wang, X.; Yang, H.; Xiao, G.; Zhu, Y. 1D/2D nitrogen-doped carbon nanorod arrays/ultrathin carbon nanosheets: Outstanding catalysts for the highly efficient electroreduction of CO2 to CO. J. Mater. Chem. A 2019, 7, 14895–14903. [Google Scholar] [CrossRef]
Scheme 1. Exohedrally N-decorated MWCNTs ([email protected]1–6) as metal free-catalysts for the CO2 reduction reaction (CO2RR).
Scheme 1. Exohedrally N-decorated MWCNTs ([email protected]1–6) as metal free-catalysts for the CO2 reduction reaction (CO2RR).
Energies 13 02703 sch001
Figure 1. High resolution X-ray photoelectron spectroscopy (XPS) N 1s core level regions for samples [email protected]1 (A); [email protected]2 (B); [email protected]3 (C); [email protected]4 (D); [email protected]5 (E); [email protected]6 (F) along with their respective peak fittings.
Figure 1. High resolution X-ray photoelectron spectroscopy (XPS) N 1s core level regions for samples [email protected]1 (A); [email protected]2 (B); [email protected]3 (C); [email protected]4 (D); [email protected]5 (E); [email protected]6 (F) along with their respective peak fittings.
Energies 13 02703 g001
Figure 2. CO2 Electroreduction performance with [email protected]1–6 and MWCNTs fabricated as carbon cloth-based electrodes and used in 0.1 M KHCO3 solution as electrolyte. (A) CO faradaic efficiency as a function of the applied potentials. (B) CO current density values (JCO) and their stability vs. time, measured for each electrocatalytic material [email protected]1–6 at −1.1 V as potential.
Figure 2. CO2 Electroreduction performance with [email protected]1–6 and MWCNTs fabricated as carbon cloth-based electrodes and used in 0.1 M KHCO3 solution as electrolyte. (A) CO faradaic efficiency as a function of the applied potentials. (B) CO current density values (JCO) and their stability vs. time, measured for each electrocatalytic material [email protected]1–6 at −1.1 V as potential.
Energies 13 02703 g002
Figure 3. Volcano plot for CO2RR on N-decorated samples [email protected]1–6. FECO was measured for each electrocatalyst at −1.1 V.
Figure 3. Volcano plot for CO2RR on N-decorated samples [email protected]1–6. FECO was measured for each electrocatalyst at −1.1 V.
Energies 13 02703 g003
Scheme 2. CO2 electroreduction process on model N-acridine decorated MWCNTs ([email protected]2/Cc) via CO2(aq) adsorption/activation (CO2#) at the basic pyridinic nitrogen followed by a first 1e reduction step to give the CO2•− radical ion stabilized (*) at Cα/Cα′ of the heterocycle. A further 1e reduction produces (CO + H2O) and regenerates CO2# that closes the catalytic cycle.
Scheme 2. CO2 electroreduction process on model N-acridine decorated MWCNTs ([email protected]2/Cc) via CO2(aq) adsorption/activation (CO2#) at the basic pyridinic nitrogen followed by a first 1e reduction step to give the CO2•− radical ion stabilized (*) at Cα/Cα′ of the heterocycle. A further 1e reduction produces (CO + H2O) and regenerates CO2# that closes the catalytic cycle.
Energies 13 02703 sch002
Table 1. Calculation of the N-loading on each functionalized sample [email protected]N1–6.
Table 1. Calculation of the N-loading on each functionalized sample [email protected]N1–6.
SamplesElemental AnalysisaAcid-base titrationbXPS analysis
C (%)N (%)N (wt.%)c
[mmol g−1]d
NPy (wt.%)
[mmol g−1]d
N (at.%)
[mmol g−1]f
MWCNTse
[email protected]1
94.08
90.18
0.20
1.76
1.56; [1.11]1.42; [1.01]2.0; [1.01]
MWCNTse,g
[email protected]2
92.34
91.35
0.29
1.57
1.28; [0.91]1.27; [0.91]1.6; [1.33]
MWCNTse
[email protected]3
94.56
90.47
0.07
1.24
1.17; [0.84]1.00; [0.71]1.2; [1.00]
MWCNTse
[email protected]4
94.54
91.35
0.27
2.14
1.87; [1.33]1.63; [1.16]2.0; [1.66]
MWCNTse
[email protected]5
93.67
91.87
0.09
1.39
1.30; [0.93]1.17; [0.84]1.5; [1.24]
MWCNTse
[email protected]6
94.30
90.89
0.14
2.83
2.69; [1.92]-2.9; [2.40]
a Average C% and N% values calculated over three independent runs. b Average N% values calculated over three independent titration tests. c The N% content calculated as difference between the N-content (%) of each functionalized sample ([email protected]1–6) and the respective “blank sample” (MWCNTs). d Relative functional group loading. e “Blank samples” prepared using identical reaction conditions, reagents and work-up procedure applied to the synthesis of samples [email protected]1–6, except for the use of isopentylnitrite as reagent. f Relative functional group loading calculated from the extrapolated N (wt.%). g Used in CO2RR (see Figure 2A,B).
Table 2. Net Bader charges (q) calculated at N and Cα atoms on N1–6 heterocycles. Bader analysis of the atomic charge density on previously reported N1–2 and N6 systems [41] has also been listed here for completeness.
Table 2. Net Bader charges (q) calculated at N and Cα atoms on N1–6 heterocycles. Bader analysis of the atomic charge density on previously reported N1–2 and N6 systems [41] has also been listed here for completeness.
EntryN-heter.nqN Energies 13 02703 i001
qCαqCα′
1N11−2.6701.1611.161
2N21−2.6961.4481.448
3N31−2.8452.4671.208
4N41−2.7452.6491.293
5N51−2.7601.2911.291
6N60−2.3560.8280.828
Back to TopTop