Electrocatalytic CO2 Reduction at Lower Overpotentials Using Iron(III

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Electrocatalytic CO2 reduction at lower overpotentials using iron(III) tetra(meso-thienyl)porphyrins Josh Koenig, Janina Willkomm, Roland Roesler, Warren E. Piers, and Gregory C Welch ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00761 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Applied Energy Materials

Electrocatalytic CO2 reduction at lower overpotentials using iron(III) tetra(meso-thienyl)porphyrins Josh D. B. Koenig, Janina Willkomm, Roland Roesler, Warren E. Piers, and Gregory C. Welcha* a Department

of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, T2N 1N4, Canada. * Corresponding Author Email: [email protected] Phone Number: 1-403-210-7603

Abstract The optical and electrochemical properties, as well as the CO2 reduction capability of two different iron(III) thienyl-porphyrins, namely iron(III) tetra(meso-thien-2-yl)porphyrin (FeTThP) and iron(III) tetra(meso-5-methylthien-2-yl)porphyrin (FeTThMeP), are directly compared to iron(III) tetra(meso-phenyl)porphyrin (FeTPP). Through exploitation of mesomeric stabilization effects, FeTThP and FeTThMeP were able to catalytically reduce CO2 to CO with comparable faradaic efficiencies and TONCO relative to FeTPP, all while using an overpotential 150 mV lower than the benchmark catalyst. Keywords: CO2 reduction; electrochemical catalysis; molecular catalysis; conjugated organic molecules; porphyrins; thiophenes

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Introduction The link between increased atmospheric CO2-levels and climate change has inspired a variety of emission mitigation efforts.[1] To date, one of the more promising options for CO2 removal is chemical feed-stocking, whereby CO2 can be electrochemically converted into value-added products.[2,3] One of the major issues with CO2 conversion is the notoriously energy-intensive single electron reduction required for molecular activation, whereby CO2 undergoes a kinetically unfavourable geometric rearrangement from linear to bent.[4,5] For this single-electron reduction of CO2 to proceed, a high-energy electronic driving force is required. On the other hand, this high energy input for CO2 reduction can be dramatically decreased by performing proton-coupled multi-electron chemical reductions.[6]

Metal surfaces, nanoparticles, and alloys have all been used for proton-coupled multi-electron CO2 reductions.[7,8] However, these electrocatalysts frequently suffer from several drawbacks. Foremost, system efficiencies can be hindered by competitive surface processes such as dihydrogen (H2) evolution.[8] Furthermore, these surfaces inherently lack the ability to control electron flow and surface-proximal protons.[9] Consequently, a variety of CO2 reduction products (such as HCOO-, CO, CH3OH, and CH4) can be simultaneously generated within a narrow redox window. Thus, to enhance system efficiency and product selectivity, molecular catalysts can be employed to mediate these electron-transfer processes.

Since the first reported molecular CO2 reduction catalyst,[10] a variety of homogeneous catalyst classes have been developed to effectively enhance catalytic efficiency and product selectivity.[2,3] To date, some of the best CO2 reduction catalysts are based on the iron(III) tetra(mesophenyl)porphyrin (FeTPP) framework first reported by Savéant and co-workers.[11–17] By itself, 2 ACS Paragon Plus Environment

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FeTPP was only capable of several CO2 reduction cycles before it degraded through ringsaturation processes, such as carboxylation or hydrogenation.[11] The catalytic activity of FeTPP can be enhanced upon the addition of either weak Lewis acids [11] and/or weak Brønsted acids [12,13] to the bulk solution. Moreover, the functionalization of the porphyrin’s meso-phenyl substituents can also improve catalytic performance. For example, the incorporation of positively-charged trimethylammonium-groups was found to not only stabilize the CO2-bound intermediate, but their electron-withdrawing nature also decreased the energy level of the lowest unoccupied molecular orbital (LUMO).[14–17] By stabilizing the LUMO energy level, the molecule’s reduction potential was diminished, thus allowing CO2 reduction to be achieved at lower overpotentials.

Another common method for stabilizing the LUMO energy level of a molecule is to extend π-conjugation. Exploiting these LUMO stabilizing mesomeric effects as a technique to lower CO2 reduction overpotentials is not possible for most TPP-based catalysts because the meso-phenyl substituents lie out-of-plane with respect to the porphyrin-core.[18] As a result, attempts to further improve the performance of TPP-based catalysts has necessitated the design of increasingly complex meso-substituents.[19,20] Molecular planarity, however, has been encouraged in porphyrins by introducing smaller heterocycles (such as thiophene) at the meso-positions.[21–23] By effectively extending the π-conjugation in thienyl-porphyrins, for example, it appears as though the stabilizing mesomeric effects overcome any destabilizing inductive effects associated with thiophene being more electron-rich than the phenyl groups. To the best of our knowledge, these synthetically diverse and potentially high-performing thienyl-porphyrins have yet to be explored as CO2 reduction catalysts. Herein, we directly compare the optical and electronic properties, as well as the electrocatalytic CO2 reduction proficiencies of two simple iron(III) thienyl-porphyrins, namely iron(III) tetra(meso-thien-2-yl)porphyrin chloride (FeTThP) and iron(III) tetra(meso-53 ACS Paragon Plus Environment


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methylthien-2-yl)porphyrin chloride (FeTThMeP), relative to a benchmark compound, iron(III) tetra(meso-phenyl)porphyrin chloride (FeTPP).

Results and Discussion The

porphyrin

frameworks

of

tetra(meso-phenyl)porphyrin

(TPP),

tetra(meso-thien-2-

yl)porphyrin (TThP), and tetra(meso-5-methylthien-2-yl)porphyrin (TThMeP), were synthesized following literature procedure (see electronic supplementary information, ESI, pp. S4-S6). The identity of each porphyrin was confirmed by 1H-NMR spectroscopy and high-resolution MALDITOF mass spectrometry (Figures S1-S6 in ESI). The UV-Visible absorption spectra and cyclic voltammetry (CV) of these compounds were used to probe the optical and electrochemical properties. Regarding the optical properties of TPP (Figure S14), the Soret band wavelength of maximum absorption (λmax) and the Q-band onset of absorption (λQ1) were observed at 417 nm and 644 nm, respectively.[24] Relative to TPP, both optical absorption spectra of TThP (λmax = 426 nm, λQ1 = 656 nm) and TThMeP (λmax = 431 nm, λQ1 = 662 nm) have undergone a bathochromic shift. The origin of this red-shift may be attributed to either: i) LUMO stabilizing mesomeric effects caused by the meso-thienyl groups adopting a more co-planar arrangement; or ii) highest occupied molecular orbital (HOMO) destabilizing inductive effects caused by thiophene being more electron-rich than phenyl.[21–23] CV analyses (Figure S15) revealed that the first oxidation events of TThP and TThMeP (E1/2 = 0.41V and 0.28 V vs. Fc+/0, respectively) have cathodically shifted relative to TPP (E1/2 = 0.53 V). The oxidation event of TThMeP experienced the greatest cathodic shift, indicating that thienyl-porphyrins may be susceptible to functional-group inductive effects. With respect to the reductions, TThP and TThMeP exhibited two identical reversible redox events (at E1/2 = -1.59 V and -1.94 V), both of which have anodically shifted relative to TPP (E1/2 = -1.72 V and -2.05 V). We therefore rationalized that the mesomeric effects overcome the inductive 4 ACS Paragon Plus Environment

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effects associated with the electron-rich thienyl moiety, thereby leading to LUMO stabilization and more facile reduction of the molecules.

Figure 1. UV-Vis absorption spectra and cyclic voltammograms of FeTPP (A and D, red), FeTThP (B and E, purple), and FeTThMeP (C and F, light blue).

Next, TPP was iron-metalated following literature procedure to afford FeTPP.[25] The same literature procedure was adapted to iron-metalate TThP and TThMeP, affording the previously unreported complexes FeTThP and FeTThMeP (see ESI pp. S8-S9). All three complexes were characterized by 1H-NMR spectroscopy, high-resolution MALDI-TOF mass spectrometry, UVVis absorption spectroscopy and CV (Figures S7-S15). The 1H-NMR spectra of FeTThP and FeTThMeP (Figures S9 and S11, respectively) were convoluted by the paramagnetic FeIII metalcenter. However, both complexes displayed characteristic broad β-pyrrole-H resonances between 72 to 82 ppm, similar to those observed with FeTPP.[26] Mass spectrometry of all three iron(III) porphyrins detected the [M-Cl]+ peak, highlighting the lability of the axial chloride ligand.[14–17]



Compared to free-base TPP, the optical spectrum of FeTPP maintained its Soret band (λmax = 417 nm), while the spectral fine-structure associated with the Q-bands (>500 nm) was greatly diminished and a new high-energy ICT-band (λict) emerged at 376 nm (Figure 1).[27] These same optical property changes were also observed with both FeTThP (λmax = 425 nm, λict = 381 nm) and FeTThMeP (λmax = 431 nm, λict = 383 nm). The CV-determined electrochemical properties of FeTPP revealed three reversible redox events at E1/2 = -0.68 V, -1.52 V, and -2.18 V. Comparable reversible redox events were observed for FeTThP and FeTThMeP (E1/2 ≈ -0.61 V, -1.47 V, and -2.03 V); however, each reduction event was anodically shifted relative to FeTPP. Crucially, the third reduction process of FeTThP and FeTThMeP (characteristic of CO2 reduction onset)[11–17] has anodically shifted with respect to FeTPP by ~150 mV, meaning CO2 reduction can be performed using lower overpotentials (ƞ).

Figure 2. CVs of FeTPP (A), FeTThP (B), and FeTThMeP (C) comparing catalytic current enhancement under CO2 as a function of TFE concentration. All CV scans were recorded at 100 mV/s, with samples containing 1 mM catalyst and 0.1 M TBAPF6 in CO2-saturated DMF.

The CO2 reduction capabilities of FeTThP and FeTThMeP were examined electrochemically (see ESI pp. S2-S3 for full experimental details). Under an atmosphere of CO2, all three iron(III) porphyrins displayed only moderate current enhancement near the third reduction wave (at E1/2 = -2.18 V or -2.03 V vs. Fc+/0). To enhance the catalytic activity, weak Brønsted acids such as water, 6 ACS Paragon Plus Environment

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2,2,2-trifluoroethanol (TFE), and phenol (pKa = 31.5, 24.0, and 18.8, respectively)[20] were tested as viable proton sources (Figure S16). The proton source concentration was increased incrementally until a current plateau was observed (Figure 2). For all three catalysts, water provided only minor current enhancement, while TFE and phenol both caused significant current enhancements. Ultimately, TFE was chosen as the proton source for all subsequent tests because it induced the greatest current enhancement for all three catalysts.

The catalytic rate constants (kcat) for FeTPP, FeTThP, and FeTThMeP were extracted using the plateau peak current under CO2 with TFE-added (icat), relative to the peak current of the noncatalytic redox process under argon (ip).[15,28] The calculated kcat for FeTPP (8200 s-1) was slightly higher than those of FeTThP (6100 s-1) and FeTThMeP (2800 s-1). This undesirable decrease in kcat has been observed with other compounds that exhibit lower reduction potentials.[15,28] Determining kcat also allowed the intrinsic CO2 reduction capabilities of each catalyst to be directly compared using a catalytic Tafel plot (Figure 3A). This catalytic Tafel plot highlights these thienylporphyrins can achieve significantly higher TOFs at lower applied overpotentials, suggesting that FeTThP and FeTThMeP can perform efficient CO2 reduction with less energy input than FeTPP.

To confirm this notion, the controlled potential electrolysis (CPE) of FeTPP, FeTThP, and FeTThMeP were performed in DMF using TFE as a proton source (Figure 3B and 3C). Catalytic performance was benchmarked with replicate trials of FeTPP (at E1/2 = -2.18 V vs. Fc+/0). In the one-compartment cell using a standard three-electrode setup (Figure S20A), FeTPP achieved a maximum Faradaic efficiency (F.E.) of 95 ± 4 %. CO was the major product observed by GC over the 5-hour test period. The total turnover number of CO (TONCO) for FeTPP was 40 ± 5, which is similar to the performance previously reported by Savéant and co-workers.[12] Any performance 7 ACS Paragon Plus Environment


differences can most likely be attributed to the use of glassy carbon plate working electrode rather than a Hg-pool electrode.[29] To confirm the catalyst, TFE, and CO2 were all necessary for CO2 reduction, control experiments omitting any one component were found to produce H2 as the major product, with only baseline levels of CO detected (Table S1).

Figure 3. Catalytic Tafel Plot (A) comparing the intrinsic catalytic properties of FeTPP (red), FeTThP (purple), and FeTThMeP (blue). The average TONCO (B) and Faradaic Efficiencies (C) obtained from the replicate controlled potential electrolysis experiments of each catalyst in the one-compartment cell (dashed lines) and two-compartment cell (solid lines).

With an acceptable benchmark F.E. and TONCO in hand, the CPE of FeTThP and FeTThMeP were also performed in the one-compartment cell (both at E1/2 = -2.03 V vs. Fc+/0). Under these conditions, FeTThP was able to reduce CO2 with a similar F.E. as FeTPP (92 ± 4 %), but only achieved a TONCO of 10 ± 1. The catalyst was observed to decompose and precipitate out of the bulk solution over the course of the CPE trials. FeTThP degradation most likely resulted from the thiophene-substituents undergoing oxidative electropolymerization at the counter electrode.[30] This hypothesis was supported by the noticeable loss in reversibility of TThP’s oxidation event when the porphyrin framework was over-oxidized (Figure S17). While oxidative electropolymerization may be detrimental to solution-based CPE, it is possible to envision this process being used advantageously for electrode surface immobilization of the catalyst.[31] FeTThMeP, on the other hand, performed CO2 reduction with an F.E. (94 ± 9 %), and relative to 8 ACS Paragon Plus Environment

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FeTThP, the TONCO of FeTThMeP increased three-fold (29 ± 4). This performance enhancement was likely caused by blocking thiophene’s highly reactive 5-position with a methyl group and successfully preventing destructive oxidative electropolymerization processes.

Oxidative electropolymerization could also be avoided by performing CPE experiments in a twocompartment cell, whereby the counter electrode was placed in a compartment separated by a fine frit from the working and reference electrodes (Figure S20B). FeTPP was once again tested as the benchmark, obtaining a F.E. of 60 ± 7 % and a TONCO of 23 ± 2 after replicate 5-hour experiments (where the major gaseous product detected was CO). Now that FeTThP was no longer prone to decomposition during the 5-hour CPE tests in the two-compartment cell, it achieved a F.E. of 54 ± 1 % and a TONCO of 17 ± 1. FeTThMeP produced similar amounts of CO as FeTThP (TONCO = 17 ± 3), while reaching a higher F.E. (63 ± 8 %). Both thienyl-porphyrins, therefore, reduced CO2 to CO with a comparable F.E. and TONCO relative to FeTPP, all while using an overpotential 150 mV lower than the benchmark catalyst.

Conclusion In summary, we have introduced a pair of new catalysts for CO2 reduction based on thienylporphyrins. Compared to phenyl-derived porphyrins, incorporating the smaller thiophene heterocycle has resulted in effective extension of π-conjugation, leading to stabilization of the LUMO energy level. This allowed FeTThP and FeTThMeP to achieve CO2 reduction performances comparable to FeTPP, but at a much lower overpotential. Functionalization of the 5-postion of thiophene effectively prevented any thienyl-based oxidative electropolymerization processes, which resulted in a three-fold increase in CO2 reduction performance. The synthetic



diversity of thiophene provides facile chemistry through which redox properties can be altered to further improve CO2 reduction performance. Efforts towards derivatization of the thienylporphyrin framework, as well as the surface immobilization of FeTThP, are currently ongoing in our lab.

Acknowledgments GCW acknowledges NSERC DG (435715-2013), CFI JELF (34102), CRC, and the University of Calgary. JK acknowledges QEII Scholarship program. This research was undertaken thanks in part to funding provided by the Canada First Research Excellence Fund (CFREF). Conflict of Interest Provisional patent filed Supporting Information Complete details on materials synthesis and characterization, materials full optical and electrochemical characterization, and CO2 conversion catalysis is provided.

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***Table of Contents (TOC) Graphic***


Electrocatalytic CO2 Reduction at Lower Overpotentials Using Iron(III

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