Water-Mediated ElectroHydrogenation of CO2 at Near-Equilibrium Potential by Carbon Nanotubes/Cerium Dioxide Nanohybrids

  • Giovanni Valenti
    Giovanni Valenti
    Department of Chemistry “Giacomo Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italy
  • Michele Melchionna
    Michele Melchionna
    Department of Chemical and Pharmaceutical Sciences, University of Trieste and Consortium INSTM, Via L. Giorgieri 1, 34127 Trieste, Italy
  • Tiziano Montini
    Tiziano Montini
    Department of Chemical and Pharmaceutical Sciences, University of Trieste and Consortium INSTM, Via L. Giorgieri 1, 34127 Trieste, Italy
  • Alessandro Boni
    Alessandro Boni
    Department of Chemistry “Giacomo Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italy
  • Lucia Nasi
    Lucia Nasi
    CNR-IMEM Institute, Parco area delle Scienze 37/A, 43124 Parma, Italy
    More by Lucia Nasi
  • Emiliano Fonda
    Emiliano Fonda
    Synchrotron SOLEIL, L’Orme des Merisiers, BP48 Saint Aubin, 91192 Gif-sur-Yvette, France
  • Alejandro Criado
    Alejandro Criado
    Carbon Nanobiotechnology Laboratory, Center for Cooperative Research in Biomaterials (CIC biomaGUNE), Basque Research and Technology Alliance (BRTA), Paseo de Miramón 182, 20014 Donostia San Sebastián, Spain
  • Andrea Zitolo
    Andrea Zitolo
    Synchrotron SOLEIL, L’Orme des Merisiers, BP48 Saint Aubin, 91192 Gif-sur-Yvette, France
  • Silvia Voci
    Silvia Voci
    Department of Chemistry “Giacomo Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italy
    More by Silvia Voci
  • Giovanni Bertoni
    Giovanni Bertoni
    CNR-IMEM Institute, Parco area delle Scienze 37/A, 43124 Parma, Italy
    CNR − Istituto Nanoscienze, Via Campi 213/A, 41125 Modena, Italy
  • Marcella Bonchio*
    Marcella Bonchio
    ITM-CNR, Department of Chemical Sciences, University of Padova and Consortium INSTM, Via F. Marzolo 1, 35131 Padova, Italy
    *Email: [email protected]
  • Paolo Fornasiero*
    Paolo Fornasiero
    Department of Chemical and Pharmaceutical Sciences, University of Trieste and Consortium INSTM, Via L. Giorgieri 1, 34127 Trieste, Italy
    ICCOM-CNR, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
    *Email: [email protected]
  • Francesco Paolucci*
    Francesco Paolucci
    Department of Chemistry “Giacomo Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italy
    ICMATE-CNR Bologna Associate Unit, University of Bologna, via Selmi 2, 40126 Bologna, Italy
    *Email: [email protected]
  • , and 
  • Maurizio Prato*
    Maurizio Prato
    Department of Chemical and Pharmaceutical Sciences, University of Trieste and Consortium INSTM, Via L. Giorgieri 1, 34127 Trieste, Italy
    Carbon Nanobiotechnology Laboratory, Center for Cooperative Research in Biomaterials (CIC biomaGUNE), Basque Research and Technology Alliance (BRTA), Paseo de Miramón 182, 20014 Donostia San Sebastián, Spain
    Ikerbasque, Basque Foundation for Science, Bilbao 48013, Spain
    *Email: [email protected]
Cite this: ACS Appl. Energy Mater. 2020, 3, 9, 8509–8518
Publication Date (Web):August 19, 2020
https://doi.org/10.1021/acsaem.0c01145
Copyright © 2020 American Chemical Society
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Abstract

The combination of multiwalled carbon nanotubes (MWCNTs) with undoped CeO2 nanoparticles (NPs) is effective for the direct electrocatalytic reduction of CO2 to formic acid (FA) at acidic pH (0.1 M HNO3), at overpotential as low as η = −0.02 V (vs RHE) with Faradic efficiency (FE) up to 65%. Exsitu and operando evidence identifies nonstoichiometric Ce4+/3+O2–x reduced sites as essential for the selective CO2 reduction reaction (CO2RR). The MWCNT-mediated electrochemical reduction of the CeO2 NPs offers a definite advantage with respect to the generally adopted thermochemical cycles (800–1500 °C) or deep hydrogenation pretreatments, thus presenting an interesting perspective for the engineering of CeO2 electrocatalysts.

Introduction

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The CO2 reduction reaction (CO2RR) is being considered a two-way strategy for both environmental remediation and a carbon-zero circular economy. (1) Among all reduction products, (2) electrocatalytic CO2 conversion into formic acid (FA) is particularly attractive, considering the application for direct formic acid fuel cells (DFAFCs) and the high volumetric hydrogen density of FA (4.4 wt %) and its low toxicity and liquid aggregation state, which make it a valuable hydrogen storage vector. (3) Indeed, FA is industrially produced by methanol carbonylation, using strong basic conditions and workup/extraction protocols. (4) Alternatively, selective hydrogenation of CO2 is performed in one step but generally requires high temperature and high H2/CO2 pressure conditions or supercritical CO2 (scCO2) conditions in combination with noble-metal catalysts (Ir, Ru, Rh, Au). (5) Therefore, the direct electrohydrogenation of CO2 to FA using water as the primary source of reactive hydrides offers a definite advantage for sustainable CO2 processing. However, CO2RR in protic environments faces two major problems: the high overpotential (6) required for CO2 activation and the current selectivity loss, due to the parallel hydrogen evolution reaction (HER) that generally occurs as a side reaction. Both issues can be addressed by a tailored design of the CO2 reduction mechanism, which depends on the active sites of the electrocatalyst and on its surface properties. (7,8) A recent work has shown that Pd-based electrocatalysts can be tuned for CO2 electrohydrogenation by introducing carbon-rich supports, conductive polymers, and Pt heterojunctions. (9) CO2 electrohydrogenation is a mechanism whereby CO2 is reduced by an electrochemically generated hydride surface on the catalyst surface. This strategy also allows the overstepping of possible CO formation as a byproduct, which for several transition metals is a serious problem, causing poisoning of the active species. (10) Following this approach, we have shown that the engineering of [email protected]2 heterostructures on high-surface-area carbon nanostructures can leverage a selective CO2 electrohydrogenation mechanism, at applied potential as low as −0.2 V (vs RHE), with Faradic efficiency (FE) up to 95%. (11)
The lowering of CO2RR overpotential is fundamentally related to the stabilization of CO2 reduction intermediates. Among redox-active oxides, ceria (CeO2) has been used to support noble metals for key catalytic applications including CO oxidation, CO2 hydrogenation, water-gas shift reactions, and alcohol electro-oxidation in fuel cell applications. (12−14) The catalytic appeal of CeO2 stems (i) from its relatively high natural abundance: Ce is among the most abundant metals on earth’s crust, reaching 66.5 ppm, even more abundant than copper, 60 ppm (15) and (ii) from the reversible Ce4+/3+ redox manifold, notably associated with the release or storage of oxygen atoms. The formation of oxygen vacancies in reduced CeO2, as also seen for other metal oxides, (16) is known to promote CO2 binding and activation by reduced Ce3+ sites with favorable electronic structures and surface and charge-transport properties. (17) In this regard, the impact of surface area/nanodimensions on the CeO2 redox properties and on the oxygen defect formation is expected to be of central importance for tuning the energetics and the selectivity of the CO2RR process. (17) Finally, the use of more available elements in the construction of catalysts meets the current requirements for the implementation of sustainable schemes for chemical production. In particular, the replacement of precious metals is a modern endeavor in heterogeneous catalysis. (18,19)
Building on this concept, we report herein a novel CeO2 nanocomposite (Figure 1) based on the combination of CeO2 nanoparticles (NPs) with multiwalled carbon nanotubes (MWCNTs). The resulting [email protected]2 electrocatalyst is effective for CO2RR to FA in acidic pH (0.1 M HNO3), at overpotential as low as η = −0.02 V and with the overall FE > 65%, based on the simultaneous production of FA and H2, because all of the hydrogen generated at the low-overpotential regime comes from a <10% reversible FA dehydrogenation. While other metals including Pb, Cd, and Sn are known to produce FA with very good efficiencies, our composite’s competitiveness capitalizes on the very low required overpotential (much lower than that of the mentioned metals) associated with excellent FE and on its low toxicity (Pb and Cd have considerable toxicity). (20−22) Notably, the state-of-the-art CO2RR with the comparably small overpotential is based on precious metals such as Pt, Au, Ag, and Pd, which, however, are associated with the tendency to yield different products such as CO. (23,24) On the other hand, cost-effective Cu catalysts required large overpotentials and typically lead to the formation of hydrocarbons. (1,25)

Figure 1

Figure 1. (A) Schematic of [email protected]2 synthesis involving a first oxidation step of the MWCNT scaffolds, followed by decoration with CeO2-NPs, grown on the MWCNT surface by controlled hydrolysis of Ce4+ tetrakis(decyloxide), Ce(ODe)4, and calcination at 250 °C. (B) Scanning transmission electron microscopy (STEM) tomographic reconstruction of [email protected]2 (the region of high density corresponding to CeO2 is rendered with a violet mesh) and a sketch of the possible mechanism of CO2 hydrogenation to formic acid. Scale bar: 20 nm.

Experimental Section

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Materials

All glassware was dried in an oven set to a temperature of 80 °C for 24 h before use. All reagents were purchased from Sigma-Aldrich and used without further purification. Phosphate buffer (PB) electrolytes were prepared readily before experiments from reagent-grade chemicals, mixing an appropriate amount of salts.

Synthesis

The pristine MWCNTs were first oxidized by a mixture of H2SO4 and HNO3. In more detail, 150 mg of pristine MWCNTs (NanoAmor, 20–30 nm diameter, 0.5–2 μm length) was dispersed in a concentrated H2SO4/concentrated HNO3 mixture (3:1 v/v, 100 mL) by sonication (6 h at 30–50 °C) and magnetic stirring (12 h at 50 °C). The suspension was washed six times by filtration [twice with water (250 mL), twice with NaOH (0.1 M, 250 mL), once with dimethylformamide (DMF) (250 mL), and once with tetrahydrofuran (THF) (250 mL)] to afford Ox-MWCNTs.
The [email protected]2 structures were prepared by a sol–gel method. The appropriate amount of Ce4+tetrakis(decyloxide) was dissolved in 25 mL of tetrahydrofuran (THF). The solution was then transferred to a sonicator, and an EtOH dispersion of Ox-MWCNTs was added. The mixture was kept under sonication for 30 min. A mixture of H2O (1 mL) and THF (10 mL) was added to ensure the complete hydrolysis of the alkoxide precursor, and the mixture was sonicated for an additional 15 min. The solid was then recovered by filtration and washed with THF three times and then dried at 120 °C overnight. The solid was finally calcined at 250 °C under static air. The reference material CeO2 was prepared similarly but in the absence of Ox-MWCNTs.

Electrochemistry

Linear sweep voltammetry (LSV), cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy experiments were carried out in a three-electrode electrochemical cell using glassy carbon electrodes (CH Instruments, diameter 3 mm) as working electrodes. Electrochemical experiments were carried out in a home-made cell having a 5 mm diameter opening in front of the substrate electrodes. An O-ring ensures a perfect tightening of the assembly, thanks to connecting screws that fit directly into the body of the cell. A Biologic SP300 potentiostat was used as the workstation for all of the electrochemical experiments. All of the experiments were carried out in 0.5 M KHCO3 or 0.1 M HNO3 buffered solution freshly prepared readily before each experiment. The electrochemical cell was equipped with a platinum spiral counter electrode and a Ag/AgCl/KCl (3 M) reference electrode. All applied potentials were measured against a Ag/AgCl reference electrode (3.0 M KCl) and converted into the RHE reference scale [using E (vs RHE) = Eapp (vs Ag/AgCl) + 0.210 V + 0.0591 V × pH, where Eapp (vs Ag/AgCl) is the applied potential corrected for the Ohmic drop] or into overpotential [using η = E (vs RHE) – E°[CO2/FA] (vs RHE), where E°[CO2/FA] is the thermodynamic potential for the CO2/FA reaction (E°[CO2/FA] = −0.2 V vs RHE)]. All of the solutions were thoroughly degassed with CO2 at least 25 min before each experiment. The products formed during the electrochemical reduction of CO2 were analyzed by online gas chromatography (GC) and ion chromatography (IC).
Thin films of the nanocomposites were prepared on the substrate working electrodes by deposition of the respective catalyst inks (methanol suspensions, 1.6 mg mL–1). The best performing films (Figure S1) combine good conductivity and an optimum catalyst loading (170 μg cm–2). Thin compact films were obtained with 5 μL aliquots subsequently added until a total deposited volume of 100 μL was reached. Those were then allowed to dry in the dark for at least 12 h, and the film thickness on the electrode surface, measured with a Tencor AlphaStep profilometer, was 8 ± 3 μm with a roughness of 3 ± 1 μm. The amount of catalysts deposited on the electrode surface was estimated to be 40 μg, with a mass contribution of CeO2 of 65 wt %. In a similar way, compact films of nanocrystalline CeO2 were prepared from analogous inks, by deposition of the same nominal amount of cerium dioxide onto the substrate electrodes.

Faradic Efficiency (FE)

The Faradic efficiency for FA production at different n overpotentials is obtained by comparing the amount of the detected product with the charges exchanged by the nanocomposite electrode [email protected]2 at the end of the electrolysis. The Faradic efficiency is defined as follows(1)where xHCO2H,n denotes the moles of FA produced determined by IC, F is the Faraday constant, m is the number of electrons needed to produce one molecule of FA, and Qtot,n is the total charge integrated from chronoamperometry. Below are reported some chronoamperometries for [email protected]2, whose integration gives Qtot,n.
As pointed out in the main text, the so-obtained FE would never reach 100% value, due to the contribution of the Ce4+/3+ reduction reaction in the overall charge, Qtot,n. One possibility arises from increasing the time scale of electrolysis or by subtracting the charge of Ce4+/3+ reduction. The latter is not a straightforward approach because at this time is not clear if Ce3+, in addition to enhancing the local CO2 concentration, also participates in the catalytic cycle. If Ce3+ donates electrons to absorbed CO2, it will become available again for further reduction, generating a positive feedback of charge that at the end would reduce the FE. Further investigation is thus needed to clarify the role of Ce3+ and to determine the precise contribution of CeO2 reduction to the overall charge and then to accurately determine the FE. Higher Faradic efficiencies may also be obtained, reducing surface inhomogeneities and maximizing the activity of CeO2 to absorb and convert CO2.

Turnover Frequency (TOF)

We estimate the TOF of the heterogeneous catalyst [email protected]2 from the charge exchanged during electrolysis in a CO2-saturated electrolyte at a different overpotential, which is proportional to the amount of generated products: normalization for the quantity of the catalyst present is taken into account by dividing for the amount of electroactive material. The formula used to calculate the TOF for FA production with [email protected]2 is the following(2)where Qn,redCO2 is the charge integrated from chronoamperometry experiments at the potential n; QCeO2 is the normalization factor for the amount of electroactive [email protected]2; time is the length of each electrolysis, usually 1 h long; and (FE)n is the Faradic efficiency for FA production at each potential n. The obtained TOFn values reflect the opposite trends of FE, which increase, and integrated charges, which decrease, when reducing the overpotential. At low overpotentials, the absolute value of the charge is the lowest, but that charge is predominantly related to FA production, rather than hydrogen evolution at high overpotential. This opposite trend results in TOF values around 100–200 h–1 in the entire potential window.

Results and Discussion

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The MWCNT scaffold boosts the electrocatalytic behavior of anchored CeO2-NPs for CO2 electrohydrogenation by enhancing the surface area and electrical conductivity for charge transport (Figure 1B). Both features are instrumental to power the redox-active domains of CeO2-NPs, generating highly accessible, reduced Ce3+-hydridic sites and proximal oxygen defects that can bind and activate CO2 while evolving to formate intermediates (HCO2, Figure 1B). (26) The morphology and electronic properties of carbon-based nanomaterials (27) are compelling features in the field of electrocatalysis. (28,29) In the present case, MWCNTs have a fundamental role in to counteracting the insulating barrier of the oxide shell and leading to an efficient generation of nonstoichiometric (Ce4+/3+O2–x) with defected active sites on the NP surface that are essential for the selective CO2RR. (17) Noteworthily, the MWCNT-mediated electrochemical activation of the CeO2 nanoislands stems as a breakthrough alternative to generally adopted high-temperature thermochemical cycles (800–1500 °C) (30) or deep hydrogenation pretreatments. (31)
It has been reported that carbon nanostructures (including MWCNTs) contain levels of transition-metal impurities such as Fe, Ni, Co, Mn, and Cu derived from the catalysts (or precursor) used in nanomaterial synthesis and that such metals could have a catalytic role. (32,33) For this reason, prior to the nanohybrid synthesis, the MWCNTs were washed several times with dilute HCl and HNO3 to remove such impurities. To confirm the exclusion of trace metals, inductively coupled Plasma optical emission spectrometry (ICP-OES) was performed on the washed MWCNTs. None of the typical metals were found above the limit of detection of the instrument (10 ppb), with the exception of Ni, whose levels were, however, extremely low (<0.1 ppm) and likely due to carbon-shelled trace impurities. Based on the absence of CO2 activation when ceria-free MWCNT are used as catalysts, the contribution of such low levels of Ni is expectedly negligible.
[email protected]2 was synthesized by adapting a previously reported procedure (Figure 1A and the Experimental Section for synthetic details), (34) whereby oxidized MWCNTs are reacted in the presence of Ce4+tetrakis(decyloxide), Ce(ODe)4, as an inorganic precursor. Oxidation of MWCNTs with HNO3/H2SO4 provides a surface distribution of oxygenated groups (COOH, C–OH, and carbonyls), which are instrumental in securing the anchoring of Ce(ODe)4. Controlled hydrolysis of the Ce4+ alkoxide precursor readily forms amorphous CeO2 islands on the MWCNT surface. Finally, calcination at 250 °C leads to the crystallization of the CeO2-NPs (Figure 1A). The nominal weight percentage of MWCNT was chosen according to previous studies on [email protected]2 structures to have the MWCNT not completely covered by CeO2, which would otherwise prevent an efficient contact between the conductive MWCNT and the supporting electrode, with the MWCNT insulated by the ceria shell. (35)
The relatively mild calcination temperature is expected not to introduce any significant amount of defects within the MWCNT surface, which may give a possible contribution to the catalysis. Raman analysis is in agreement with such a hypothesis. Thermogravimetric analysis (TGA) of the final [email protected]2 material indicates that the CNT component accounts for ∼35 wt % (Figure S2), with the remaining 65% being ascribed to the CeO2 NPs. This CeO2 NP loading results in the best compromise between good conductivity and stability of the cathode. It must also be noted that the intimate contact with the CeO2-NPs leads to a remarkable decrease of the combustion temperature onset observed for the [email protected]2 at ∼350 °C in comparison with what is generally observed for the pristine CNTs (∼600 °C). This behavior is consistent with the expected catalytic effect of CeO2-NPs, providing an excellent O buffer during oxidation. (12)
High-resolution and scanning transmission electron microscopy (HRTEM, STEM), in combination with high-angle annular dark-field imaging (STEM-HAADF), energy-dispersive X-ray spectroscopy (EDX), element mapping, and STEM tomography reconstruction, provides direct evidence for the material morphology and carbon-inorganic phase boundaries (Figures 1B, 2, S3 and S4). The inspection of the microscopy images confirms (i) the one-dimensional (1D) morphology resulting from the MWCNT templates with the diameter in the range 20–30 nm (Figure 2A–C), (ii) adhesion of ultrasmall CeO2 particles, showing a mean size of 2.8 ± 0.5 nm (Figure 2D), and (iii) the crystal lattice of the CeO2 phase (face-centered cubic structure, Figure 2G), which is obtained after material calcination.

Figure 2

Figure 2. (A) STEM-HAADF of a typical [email protected]2 and (B) corresponding EDX map highlighting the elemental distribution of C (blue) and Ce (green). Scale bar, 20 nm. (C) HRTEM of [email protected]2. Scale bar, 5 nm. The ring pattern in the fast Fourier transform (FFT) (F) can be indexed as the face-centered cubic structure of the crystalline CeO2 nanoparticles. An enlarged image of a CeO2 nanoparticle viewed along the (011̅) zone axis is shown in (E) with the corresponding FFT (G). (D) Particle size analysis of the [email protected]2.

Raman spectra recorded for [email protected]2 (Figure S5) show the typical pattern of graphitic materials, with D and G bands, respectively, at ∼1330 and ∼1580 cm–1 and an intensity ratio of these bands of ID/IG ∼ 1.27. The D band in carbon materials is associated with the presence of defects, which disrupt the graphitic network (which on the contrary gives rise to the G band). Hence, their ratio is typically used as a measure of the extent of defects on the CNT surface. (36) The low value of ID/IG (compared to that of the pristine MWCNT, 1.19) is therefore consistent with a mild modification of the nanocarbon scaffold properties with respect to the pristine MWCNTs, indicating the absence of additional defects generated during the calcination step. As expected, the appearance of a characteristic peak at 455 cm–1 confirms the formation of a CeO2 crystalline phase upon calcination (Figure S5, red and blue traces). (37)
The MWCNT interface facilitates the Ce4+ reduction, thus increasing the Ce3+ content and the oxygen vacancy population in the CeO2 phase. (12) It is worth noting that the process of vacancy formation in ceria is very complex, with the coexistence of different situations of single and multiple vacancies. (38) However, important indications are gathered here via X-ray spectroscopy. To probe the initial Ce3+ fraction of the diverse materials, X-ray photoelectron spectroscopy (XPS) analysis was performed on bulk CeO2, CeO2 NPs, and the nanocomposite [email protected]2 (Figure 3 and Tables 1 and S1 and Figure S6). The relative Ce3+ content, which is associated with the formation of oxygen vacancies, was found to increase on downsizing the cerium oxide structure to the nanoscale and significantly enhanced upon the integration of the MWCNT component, with results in the series [Ce3+]% = 11, 25, and 33 (Figure 3). (39) This observation suggests that the MWCNT contact favors electron transport/injection and delocalization in the CeO2 layer, resulting in the accumulation of Ce3+ sites and oxygen vacancies, which turns out to be crucial for both CO2 binding and activation, and the formation of the reactive hydrides. (40−42)

Figure 3

Figure 3. Fitted XPS spectrum of the Ce 3d core level of (a) bulk CeO2, (b) CeO2 NPs, and (c) [email protected]2. XPS binding energies of individual components of the Ce 3d spectra for the different Ce-based samples.

Table 1. Binding Energies of Individual Components of the Ce 3d Spectra of the Different Ce-Based Samples
 Ce 3d5/2 (eV)Ce 3d3/2 (eV) 
samplev0vv′v″v‴u0uu′u″u‴[Ce3+]
bulk-CeO2879.72882.49884.89888.32898.35898.4901.07903.19907.46916.70.11
CeO2 NPs880.37882.19884.13888.4898.2898.5900.71902.28907.44916.70.25
[email protected]2880.19882.06883.88888.0898.15899.0900.65902.1907.1916.70.33
Transmission electron microscopy and XPS analysis were also conducted on the postcatalysis material. From TEM and STEM-HAADF micrographs, it can be seen that the morphology and MWCNT-CeO2 contact, as well as the crystallinity of the CeO2 nanoparticles, remain unaltered as confirmed in the FFT of a selected area of the micrograph (Figure S7). XPS of the used catalyst confirms that no substantial change over the CeO2 structure has occurred, with the [Ce3+] in the aged [email protected]2 catalyst remaining comparable to the fresh one (0.33 vs 0.27); the slight decrease is presumably due to the required washing of the recovered catalyst before XPS analysis (Figure S8 and Table S2).

CO2 Electrocatalytic Reduction by [email protected]2

Thin films of [email protected]2 were deposited on a glassy carbon electrode ([email protected]2|GC) and studied for the electrocatalytic reduction of CO2 to FA. The CO2RR was evaluated by LSV, CV, and 1 h chronoamperometry (CA) under CO2-saturated conditions. Products were analyzed by online GC directly connecting the headspace of the electrochemical cell to the sample loop of a GC, while FA was detected by analysis of the liquid phase by IC at the end of electrolysis (see Supporting Information for further details).
Noteworthily, LSV of [email protected]2|GC performed in 0.1 M HNO3 under a CO2 atmosphere (red line, Figure 4A) shows a twofold feature: a steady current increase at applied potential as low as −0.2 V (vs RHE), followed by a sharp enhancement with the onset at −0.65 V (vs RHE) (Figure 4A, red line). This latter is ascribed to the HER, occurring at unusually high overpotential on [email protected]2|GC. Under such conditions, the Tafel analysis reveals a slope of 140 mV dec–1, typical of the HER when the Volmer step, i.e., the formation of the reactive intermediate Had, is the rate-determining step. In acids, the HER side reaction commonly occurs with electrocatalysts, such as Cu, Pt, or Pd, and at applied potentials as low as E ≥ −0.1 V (vs RHE). The overpotential shift observed for the HER on [email protected]2|GC leaves a broad selectivity window for the CO2RR. Figure S9 shows the LSV and CV comparison with or without CO2 for the [email protected]2 catalyst, also compared with the profile for MWCNT alone. Indeed, the enhancement of the electrodic response at reduction potential >−0.4 V (vs RHE, Figure 4A red line) is definitely ascribed to CO2 saturation conditions when compared to under an a Argon atmosphere (Figure S9, red line) and to MWCNT (Figure S9, black line). The electrocatalytic reduction of CO2 by [email protected]2|GC is confirmed by the analysis of the electrolysis products that reveals the production of FA at potentials as low as −0.20 V (vs RHE) (Figure 4C,D), i.e., very close to the thermodynamic limit (−0.22 V vs RHE), for the CO2/FA conversion. A check of the electrolyte solution by 1H NMR did not show the formation of any other carbon products. The initial sudden current drop observed at all investigated fixed potentials (Figure 4C) is presumably due to the Ce4+/Ce3+ reduction, after which the current remains rather stable. The slow progressive decline at longer times is likely due to a partial physical detachment of a fraction of the catalyst powder from the electrode, given that the ink deposition has not been fully optimized yet. Control experiments show that FA production is completely suppressed in the absence of CO2 (Figure S8), while any possible MWCNT-based origin of generated FA is excluded on the basis of isotope-labeling mass spectrometry experiments, using 13CO2, which confirm the retention of the 13C-label in the produced FA (Figure S10), and on the comparison of the CO2RR activity with the MWCNT (Figure S11).

Figure 4

Figure 4. (A) LSV of the nanostructured catalyst [email protected]2 (red line), the bare GC electrode (blue line), and CeO2 NPs (green line) in a CO2-saturated solution. Scan rate: 2 mV s–1. Electrolyte: 0.1 M HNO3. (B) CVs in 0.1 M PB for [email protected]2 (black curve) and CeO2 (green curve). Scan rate = 50 mV s–1, pH = 6.8. (C) CA for [email protected]2 at different overpotentials. Electrolyte: 0.1 M HNO3. From orange to black, E = −0.22, −0.32, −0.42, −0.52 V (vs RHE). The integration of these CA gives the charge of CO2 reduction, then used for FE determination. (D) Faradic efficiency of [email protected]2 for FA (red) and H2 (blue) production as a function of the applied potentials. Electrolyte: 0.1 M HNO3.

Despite the tendency of CeO2 to slowly dissolve in a strong acidic environment, (43) we did not detect any presence of cerium in the postelectrolysis characterization under our conditions. While this could hint to a better stability of CeO2 when supported on MWCNT, a future more in-depth investigation will be required for a possible translation of our fundamental studies into a real application device. Recent density functional theory (DFT) calculations have shown the favorable binding of CO2 on partially reduced ceria (i.e., in the presence of Ce3+ sites and oxygen vacancies). (44,45) The electrochemical Ce4+/3+ reduction turns out to be efficiently mediated by the MWCNT scaffold, at E ≤ ∼0.3 V vs RHE. (46) Therefore, under CO2RR conditions, generation of the non stoichiometric ceria sites (Ce4+/3+O2–x) on [email protected]2|GC is responsible for CO2 activation and conversion into FA. Noteworthily, in the absence of MWCNTs, nanocrystalline CeO2 is not able to produce any significant Faradic current in the entire potential window explored (green line, Figure 4A,B). The possible formation of some cerium carboxylate species, favored in reduced ceria, cannot be ruled out. However, such species are not likely to take part in the catalysis due to their relatively good stability. In some cases, Ce-carbonate species have been identified during the CO2RR, but they lead to the formation of CO, which is never detected during our CO2RR experiments. (30)
Operando X-ray absorption spectroscopy (XAS) is recognized as a powerful technique for understanding the nature of CO2RR-active sites (47) and here was used to gain further insights into the structural and chemical changes of [email protected]2 under an applied cathodic potential, either in the presence or in the absence of a CO2 atmosphere. To this end, the material thin film was deposited on a highly oriented pyrolytic graphite (HOPG) window and installed within the electrochemical cell (see Supporting Information for details). The X-ray absorption near-edge structure (XANES) spectra of [email protected]2 were recorded over time at the applied potential in the range −0.2 to −0.7 V, with a cathodic scan relative to the cell open-circuit potential (OCP = 0.540 V). XANES spectra were recorded initially under an Ar atmosphere (up to 280 min at −0.4 V vs RHE) and then after saturating the electrolyte solution with CO2 by continuous CO2 bubbling (up to 410 min at −0.4 V and cycled back at 0.1 V up to 500 min) and were then compared to those of the CNT-free material (CeO2 bulk, CeO2 NPs) and the CeAlO3 reference (Figure 5A and Table 2). (48) Although the content of Ce3+ cannot be precisely addressed from XANES spectra by a direct comparison or by linear combinations of standards (Figure 5A), the extended X-ray absorption fine structure (EXAFS) analysis indicates that both CeO2 NPs and [email protected]2 show (i) an average Ce–O bond length comparable within uncertainties to the bond length of CeO2 bulk and (ii) a decrease in the coordination number with respect to bulk CeO2 from 8 to 6 (Table 2). These data are consistent with an increased population of oxygen vacancies in the nanostructured samples, in close agreement with the XPS evidence, which reports much higher values and sensitivity related to surface-only fractions. (49)

Figure 5

Figure 5. (A) Selected XANES spectra compared to those of two references (top) and Δμ spectra vs OCP (bottom) of the same spectra. The dashed vertical arrows indicate the integration limits for evaluating the Ce3+ fraction reported in figure. (B) Evolution of the Ce3+ fraction before and after introducing CO2 in the cell.

Table 2. EXAFS Data Analysis Results
Ce–OCNaR(Å)bσ2(10–3 Å2)c
CeO28.0(6)2.34(1)8(1)
nano-CeO26.4(5)2.33(1)11(2)
MWCNT CeO2   
(a) dry ink6.2(6)2.33(1)13(2)
(b) −0.7 V CO26.1(6)2.35(1)12(3)
(c) −0.2 V CO26.1(5)2.34(1)9(2)
a

CN = Coordination number.

b

R = Distance to the neighboring atom.

c

σ2 = Mean-square disorder of the neighbor distance.

To capture the structural and electronic changes of [email protected]2 induced by the applied cathodic potential, (50) difference XANES spectra (Δμ) were calculated by subtracting the initial spectrum (at OCP = 0.540 V) from the operando spectrum recorded at increasing reduction potential and over time (Figure 4). Selected Δμ-XANES spectra are reported in Figure 5, where a boosting effect for Ce3+ production is apparent under a CO2 atmosphere (Figure 5A). The evaluation of the Δμ integral at around 5726 eV (integration from 5722 to 5730 eV as indicated by dashed lines and normalization by the Δμ integral of CeAlO3, which is a Ce3+ standard) provides the time-dependent fraction of Ce3+, showing a linear increase for the progressive reduction of [email protected]2 under electrocatalytic CO2 reduction, leveling off to a plateau value when the applied potential is cycled back to less reducing potentials (Figure 5B). The direct evidence of Ce3+ accumulation monitored by operando XANES is consistent with the continuous H2 formation in the explored potential range (Figure 4D and the Results and Discussionsection). H2 is known to interact with CeO2-based materials, leading to progressive surface and bulk reduction under different conditions. (51,52) It has also been reported that the reduction is facilitated under applied voltages. (53) Here, the as-evolved H2, which originates from the dehydrogenation of the as-formed FA, can contribute to the further formation of Ce3+ atoms, which occurs in parallel with the CO2RR and likely on different sites of the material, therefore being governed by independent kinetics. As a consequence, the growth of the Ce3+ fraction linearly increases over 2 h of electrolysis, while it stops when the applied potential is increased to a regime (+0.1 V) where no CO2 reduction occurs, confirming that the Ce3+ generation is a consequence of FA formation. Because the steady Ce3+ increase is not observed under an Ar atmosphere, we can conclude that the CeO2 hydrogen-mediated reduction occurs at near-equilibrium overpotential as a result of the reversible FA dehydrogenation.
Although XPS and EXAFS results both confirm the more facile formation of oxygen vacancies within the nanohybrid, it must be stressed that the exact location of such vacancies cannot be univocally determined with only these two techniques. This is particularly true for ceria, given the high mobility of oxygen in this oxide, further enhanced by the applied electrochemical potential. Electrochemical impedance spectroscopy (EIS) and CV analysis were used to address the interaction of CO2 molecules with the [email protected]2|GC. In particular, the double-layer capacitance (Cdl) was found to increase significantly, confirming the adsorption of CO2 on the electrode surface (Figure S12). In contrast, nanocrystalline CeO2 films, in the absence of MWCNTs, yield much lower Cdl values that are not affected by the presence of CO2 (Figure S13). Such a lower capacitance was attributed to a decrease of the electrochemically accessible CeO2 surface, in turn associated with the much higher resistivity of the CeO2 nanocrystals in the absence of MWCNTs. [email protected]2 displayed similar Cdl values in a 0.5 M KHCO3 buffered electrolyte, both in the presence and in the absence of CO2. This observation is consistent with the strong adsorption of carbonate on the CeO2 surface. (30) We can thus postulate that the electrocatalytic mechanism leading to the CO2 conversion into FA may occur via the formation of carbonate intermediates formed onto the ceria surface. To support the favorable adsorption of CO2 onto the nanocomposite, we carried out CO2 chemisorption experiments on the nanocomposite and on the two reference materials CeO2 and MWCNT. As shown in Figure S14, which reports the isotherms of the CO2 chemisorption normalized by the material surface area, the quantity of CO2 adsorbed per square meter is higher for the nanocomposite (Table S3) in comparison with either MWCNT or CeO2, corroborating the higher tendency of the catalyst to adsorb CO2.

Electrochemical Analysis of the Mechanistic Features

The peculiar surface activity of the substoichiometric mixed-valence Ce4+/3+ oxide in the [email protected]2 nanocomposite is clearly evident when operating in a PB solution (Figures 4B and S15). In this electrolyte, after reduction of Ce4+ to Ce3+ (eq 3), the formation of a surface CePO4 species takes place (eq 4). (46) The latter reaction can be monitored by CV through the detection of a reversible peak with E1/2 = +1.1 V (vs RHE, Figure S16). (46)(3)(4)While the formation of CePO4 leads to surface passivation and is detrimental in relation to the CO2RR (and therefore PB electrolytes should be avoided, Figure S16), monitoring of the process centered at E = +1.1 V vs RHE provides a tool to estimate the amount of electroactive ceria, a necessary step for turnover frequency (TOF) determination (vide infra). It has to be noticed that the the CePO4 can be formed only in PB and not under the CO2RR experimental conditions. The integration of the corresponding CV peaks (Figures S17 and S15) reveals that ∼20% of the total amount of CeO2 deposited on the substrate electrode is reduced/oxidized electrochemically. Interestingly, the same redox process with E1/2 = +1.1 V (vs RHE) is almost completely suppressed on films of nanocrystalline CeO2 (Figure 4B). This difference highlights the essential role of MWCNTs in trafficking electrons to the surface of CeO2, thus promoting the electrocatalytic activity for CO2RR, otherwise inhibited in thicker and more insulating films. The long-term activity of [email protected]2|GC was assessed in 0.1 M HNO3 by 1 h potential-controlled electrolysis in the range −0.22 V < ERHE < −0.52 V. Stable current densities were obtained that increase proportionally at increasing applied potential (Figure 4C). CO2 conversion into FA occurs at the near-equilibrium working potential (based on the thermodynamic value of −0.22 V vs RHE) in close analogy with the natural formate dehydrogenase enzyme (FDH) bioelectrode. (31) In addition to FA, no other typical CO2 reduction products, including CO, CH4, or CH3OH, were detected. However, the formation of H2 is always observed in the explored potential range (Figure 4D). Interestingly, the inspection of the FA/H2 selectivity, in terms of the relative FE%, shows an inversion cross-point at E <−0.4 V (vs RHE, Figure 4D), with a quite high overpotential for the HER. This is indicative of a swapping of the electrocatalytic mechanism from the low- to high-overpotential regime. Control experiments have been performed that rule out H2 generation in the absence of CO2 at low overpotential (−0.52 V < ERHE < −0.22 V) (Figure S18). This observation ascribes H2 formation to FA dehydrogenation occurring under electrocatalytic conditions and accounting for ca. 10% FE. We might conclude that the CO2RR proceeds with an overall FE of ca. 65% at low overpotential, while the HER takes over with FE up to ca. 90% at E < −0.6 V (vs RHE). Importantly, [email protected]2|GC also displayed great chemical and mechanical stability with a linear increase of the FA generation and constant FE% in prolonged electrosynthesis (>16 h, Figure S19).
A dual electrocatalytic regime for [email protected]2|GC is also apparent, considering the Tafel plot analysis (Figure S20), where the slope of 140 mV dec–1 is found at high overpotential, related to the HER, while a higher slope of 260 mV dec–1 is found at low overpotential, related to the CO2RR mechanism. In this potential region, the occurrence of multiple and sequential chemical and electron transfer steps including the Ce4+/3+ reduction, CO2/H+ absorption and protonation equilibria, Ce3+-based hydride formation, and transfer to relevant intermediates, is expected to limit the kinetics, thus affecting the Tafel slope. (54) The high Tafel slope (>120 mV dec–1) and the selective FA generation at very low overpotential are consistent with a direct electrohydrogenation mechanism by [email protected]2. (9) The TOF for the FA production and H2 evolution can be calculated from chronoamperometric experiments (see the Experimental Section) and turns out to be around 200 moles of FA for moles of CeO2 for hours at −0.22 V (vs RHE, see eq 2 and Figure S19 and a production rate of 2 μmol h–1 cm–2).

Conclusions

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The performance of [email protected]2 for CO2/FA reversible electrohydrogenation in acidic electrolytes with FE up to 65% at near-equilibrium overpotential is rather unique, considering that it is ascribed to transition-metal-free active sites. A direct comparison with the state-of-the-art electrocatalysts for the CO2RR (Figure S21 and Table S5) ranks [email protected]2 among the most promising systems reported to date in terms of the low overpotential combined with a wide selectivity window for the CO2RR. Hence, the concept of interfacing carbon nanostructures with CeO2 holds great promise as further improvements of both the carbon nanoscaffolds and the electrode engineering, such as carbon fiber paper, (55) are expected to significantly enhance the production rate. Indeed, the use of high-surface-area carbon nanohorns (CNHs) decorated with CeO2-NPs (Figures S22 and S23) has been found to produce a 5-fold increase in the total charge after a chronoamperometric measurement under analogous electrocatalytic conditions (Figure S24). The optimization of CeO2 loading might also contribute to further enhancements. Hence, the next generations of the catalyst, where the CeO2 nanocarbon composite structure and composition will be further developed, will address limitations related to the moderate current densities, projecting ceria-based hybrid materials as a potential catalyst for industrial appeal.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.0c01145.

  • Experimental section on the product analysis; transmission electron microscopy; thermo-gravimetric analysis; Raman spectroscopy; X-ray photoelectron spectroscopy, and electrochemical characterization; TGA analysis, Raman spectrum; HRTEM; XPS; tafel slope analysis; and optimization of the material loading and stability (PDF)

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Author Information

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  • Corresponding Authors
    • Marcella Bonchio - ITM-CNR, Department of Chemical Sciences, University of Padova and Consortium INSTM, Via F. Marzolo 1, 35131 Padova, Italyhttp://orcid.org/0000-0002-7445-0296 Email: [email protected]
    • Paolo Fornasiero - Department of Chemical and Pharmaceutical Sciences, University of Trieste and Consortium INSTM, Via L. Giorgieri 1, 34127 Trieste, ItalyICCOM-CNR, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italyhttp://orcid.org/0000-0003-1082-9157 Email: [email protected]
    • Francesco Paolucci - Department of Chemistry “Giacomo Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, ItalyICMATE-CNR Bologna Associate Unit, University of Bologna, via Selmi 2, 40126 Bologna, Italyhttp://orcid.org/0000-0003-4614-8740 Email: [email protected]
    • Maurizio Prato - Department of Chemical and Pharmaceutical Sciences, University of Trieste and Consortium INSTM, Via L. Giorgieri 1, 34127 Trieste, ItalyCarbon Nanobiotechnology Laboratory, Center for Cooperative Research in Biomaterials (CIC biomaGUNE), Basque Research and Technology Alliance (BRTA), Paseo de Miramón 182, 20014 Donostia San Sebastián, SpainIkerbasque, Basque Foundation for Science, Bilbao 48013, Spainhttp://orcid.org/0000-0002-8869-8612 Email: [email protected]
  • Authors
    • Giovanni Valenti - Department of Chemistry “Giacomo Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italyhttp://orcid.org/0000-0002-6223-2072
    • Michele Melchionna - Department of Chemical and Pharmaceutical Sciences, University of Trieste and Consortium INSTM, Via L. Giorgieri 1, 34127 Trieste, Italyhttp://orcid.org/0000-0001-9813-9753
    • Tiziano Montini - Department of Chemical and Pharmaceutical Sciences, University of Trieste and Consortium INSTM, Via L. Giorgieri 1, 34127 Trieste, Italy
    • Alessandro Boni - Department of Chemistry “Giacomo Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italy
    • Lucia Nasi - CNR-IMEM Institute, Parco area delle Scienze 37/A, 43124 Parma, Italy
    • Emiliano Fonda - Synchrotron SOLEIL, L’Orme des Merisiers, BP48 Saint Aubin, 91192 Gif-sur-Yvette, France
    • Alejandro Criado - Carbon Nanobiotechnology Laboratory, Center for Cooperative Research in Biomaterials (CIC biomaGUNE), Basque Research and Technology Alliance (BRTA), Paseo de Miramón 182, 20014 Donostia San Sebastián, Spain
    • Andrea Zitolo - Synchrotron SOLEIL, L’Orme des Merisiers, BP48 Saint Aubin, 91192 Gif-sur-Yvette, Francehttp://orcid.org/0000-0002-2187-6699
    • Silvia Voci - Department of Chemistry “Giacomo Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italy
    • Giovanni Bertoni - CNR-IMEM Institute, Parco area delle Scienze 37/A, 43124 Parma, ItalyCNR − Istituto Nanoscienze, Via Campi 213/A, 41125 Modena, Italyhttp://orcid.org/0000-0001-6424-9102
  • Author Contributions

    G.V. and M.M. contributed equally.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the Italian Ministero dell’Istruzione, Università e Ricerca (PRIN prot. 2017PBXPN4), the H2020 - RIA-CE-NMBP-25 Program (Grant No. 862030), and Universities of Bologna and Trieste, INSTM; Dr. Matteo Crosera (University of Trieste) is acknowledged for the ICP-OES analysis. M.P., as the recipient of the AXA Chair, is grateful to the AXA Research Fund for financial support. This work was performed under the Maria de Maeztu Units of Excellence Program from the Spanish State Research Agency (Grant No. MDM-2017-0720).

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  • Abstract

    Figure 1

    Figure 1. (A) Schematic of [email protected]2 synthesis involving a first oxidation step of the MWCNT scaffolds, followed by decoration with CeO2-NPs, grown on the MWCNT surface by controlled hydrolysis of Ce4+ tetrakis(decyloxide), Ce(ODe)4, and calcination at 250 °C. (B) Scanning transmission electron microscopy (STEM) tomographic reconstruction of [email protected]2 (the region of high density corresponding to CeO2 is rendered with a violet mesh) and a sketch of the possible mechanism of CO2 hydrogenation to formic acid. Scale bar: 20 nm.

    Figure 2

    Figure 2. (A) STEM-HAADF of a typical [email protected]2 and (B) corresponding EDX map highlighting the elemental distribution of C (blue) and Ce (green). Scale bar, 20 nm. (C) HRTEM of [email protected]2. Scale bar, 5 nm. The ring pattern in the fast Fourier transform (FFT) (F) can be indexed as the face-centered cubic structure of the crystalline CeO2 nanoparticles. An enlarged image of a CeO2 nanoparticle viewed along the (011̅) zone axis is shown in (E) with the corresponding FFT (G). (D) Particle size analysis of the [email protected]2.

    Figure 3

    Figure 3. Fitted XPS spectrum of the Ce 3d core level of (a) bulk CeO2, (b) CeO2 NPs, and (c) [email protected]2. XPS binding energies of individual components of the Ce 3d spectra for the different Ce-based samples.

    Figure 4

    Figure 4. (A) LSV of the nanostructured catalyst [email protected]2 (red line), the bare GC electrode (blue line), and CeO2 NPs (green line) in a CO2-saturated solution. Scan rate: 2 mV s–1. Electrolyte: 0.1 M HNO3. (B) CVs in 0.1 M PB for [email protected]2 (black curve) and CeO2 (green curve). Scan rate = 50 mV s–1, pH = 6.8. (C) CA for [email protected]2 at different overpotentials. Electrolyte: 0.1 M HNO3. From orange to black, E = −0.22, −0.32, −0.42, −0.52 V (vs RHE). The integration of these CA gives the charge of CO2 reduction, then used for FE determination. (D) Faradic efficiency of [email protected]2 for FA (red) and H2 (blue) production as a function of the applied potentials. Electrolyte: 0.1 M HNO3.

    Figure 5

    Figure 5. (A) Selected XANES spectra compared to those of two references (top) and Δμ spectra vs OCP (bottom) of the same spectra. The dashed vertical arrows indicate the integration limits for evaluating the Ce3+ fraction reported in figure. (B) Evolution of the Ce3+ fraction before and after introducing CO2 in the cell.

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    • Experimental section on the product analysis; transmission electron microscopy; thermo-gravimetric analysis; Raman spectroscopy; X-ray photoelectron spectroscopy, and electrochemical characterization; TGA analysis, Raman spectrum; HRTEM; XPS; tafel slope analysis; and optimization of the material loading and stability (PDF)


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