Elucidating Reactivity and Mechanism of CO2 Electroreduction at

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Elucidating Reactivity and Mechanism of CO2 Electroreduction at Highly Dispersed Cobalt Phthalocyanine Minghui Zhu, Ruquan Ye, Kyoungsuk Jin, Nikifar Lazouski, and Karthish Manthiram ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00519 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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Elucidating Reactivity and Mechanism of CO2 Electroreduction at Highly Dispersed Cobalt Phthalocyanine Minghui Zhu§, Ruquan Ye§, Kyoungsuk Jin, Nikifar Lazouski, and Karthish Manthiram* Massachusetts Institute of Technology, Department of Chemical Engineering, 77 Massachusetts Ave, Cambridge, MA 02139 § These authors contributed equally to this work Corresponding Author * E-mail: [email protected]

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ABSTRACT

Transforming carbon dioxide to carbon monoxide with electrochemical methods allows for small scale, modular conversion of point sources of carbon dioxide. In this work, through the preparation of well-dispersed cobalt phthalocyanine model catalysts immobilized on carbon paper, we revealed high turnover frequencies for reducing carbon dioxide at low catalyst loadings, which is obscured at higher loadings due to aggregation. The low catalyst loadings have also enabled mechanistic studies that provide a detailed understanding of the molecularlevel picture of how cobalt phthalocyanine facilitates proton and electron transfers in the rate limiting step. We are able to tune the rate-limiting step from an electron transfer to a concerted proton-electron transfer, enabling higher rates of carbon dioxide reduction. Our results highlight the significance of dispersion for understanding the intrinsic catalytic performance of metal phthalocyanines for electroreduction of CO2.

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Carbon dioxide (CO2) emissions from power generation could be sharply curtailed by shifting towards renewable sources such as solar and wind coupled to energy storage technologies. Nevertheless, significant carbon dioxide emissions associated with the chemical and materials industries would remain. For instance, cement production alone leads to 5% of carbon dioxide emissions worldwide.1 CO2 that would otherwise be emitted from these processes and contribute to global warming has the potential to be recycled and turned into key chemical intermediates for a wide range of value-added products.2–4 Carbon monoxide (CO), for instance, is an attractive target as it can be further converted into diverse products, including acids, esters, and alcohols, reflecting its status as a nexus molecule in the chemical industry.5–8 Among the many possible methods for converting CO2 to CO, including thermochemical and electrochemical routes, the latter enable mild operating conditions of low temperature and pressure that are conducive to modular manufacturing locally and on-demand to match the distributed availability of carbon dioxide feedstocks.9 A wide range of catalysts have been tested for electrochemical carbon dioxide reduction to carbon monoxide, ranging from pure transition metals10–14 to molecular complexes15–18. Molecular complexes are attractive due to the clear structure-property relationships that can be tuned precisely. The major classes of molecular complexes explored for carbon dioxide reduction to date include metal centers with macrocyclic ligands,19–23 bipyridine ligands,24–26 and phosphine ligands.27–29 Among these, macrocyclic ligands containing cobalt, such as cobalt phthalocyanine (CoPc) and cobalt tetraphenylporphyrin (CoTPP), are among the most selective and active for this reaction (Scheme 1).



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Scheme 1. Schematic showing electrochemical reduction of CO2 at cobalt phthalocyanine (CoPc). CO2 is reduced at the cathode to carbon monoxide and oxygen is evolved at the anode.

Significant efforts have been devoted to promoting the CO2 reduction reaction by immobilizing those molecular catalysts onto conductive supports.30–34 For instance, CoPc and CoTPP supported on carbon nanotubes (CNT) achieved higher turnover frequencies than on other carbonaceous supports, such as carbon black, reduced graphene oxide, and glassy carbon.30,31 Another strategy for increasing activity has involved tuning the electronic structure of the macrocyclic complex via functionalization.31,35,36 For instance, introduction of cyano groups on CoPc (CoPc-CN) supported on carbon nanotubes (CNT) led to an increase of the turnover frequency for carbon monoxide (TOFCO) to 4.1 s-1 at -0.63 V vs. the Reversible Hydrogen Electrode (RHE), approximately 50% higher than the un-functionalized CoPc.31 Functionalization of the macrocyclic ligand has also been extended towards generating derivatives which react to form crystalline solids, such as metal organic frameworks (MOFs) and covalent organic frameworks (COFs); a TOFCO of 2.6 s-1 at -0.67 V vs. RHE was achieved on a 4 ACS Paragon Plus Environment

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particular COF with 1% cobalt loading known as COF-367-Co(1%).37,38 Coordination of the metal center can also tune electronic properties, as was shown through axial coordination of CoPc by poly-4-vinylpyridine (CoPc-P4VP), leading to a TOFCO of 4.8 s-1 at -0.73 V vs. RHE.33 Mechanistic understanding of the macrocyclic complexes described above for carbon dioxide reduction has remained limited. This is reflected in previously reported Tafel slopes for cobalt porphyrins and phthalocyanines, as well as their derivatives, which range from 165 to 550 mV/dec,30,35,37,38 suggesting the existence of transport limitations. This hampers the thorough understanding of CO2 reduction mechanisms which could lead to the design of metal porphyrin and phthalocyanine derivatives which are more effective for CO2 reduction. In this work, we prepared highly dispersed cobalt phthalocyanine complexes supported on oxygen-functionalized carbon paper as a model catalyst. The turnover frequency increased by three orders of magnitude upon increasing the level of dispersion. In addition, such high dispersion alleviates transport limitations, allowing for clear mechanistic understanding of the conversion of carbon dioxide to carbon monoxide. This fundamental understanding of reactivity provides insights for future studies aimed at designing highly active molecular catalysts for electrocatalytically converting CO2 to CO.



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Figure 1. (A) Linear sweep voltammetry collected at a sweep rate of 50 mV/s at various CoPc loadings. (B) Faradaic efficiency for CO and H2 production at -0.73 V vs. RHE. (C) Overall current densities at -0.73 V vs. RHE. (D) Turnover frequency for CO at -0.73 V vs. RHE. Electroreduction experiments were performed in 0.1 M NaHCO3 for 1 hour and repeated at least 3 times for each loading.

CoPc and Nafion ionomer, which is used here as a binder and proton conductor, were drop-casted from dimethylformamide on oxygen-functionalized carbon paper (OxC) to form the immobilized CoPc/OxC electrodes at CoPc loadings ranging from 5 x 10-12 to 1 x 10-7 mol/cm2. The electrodes were tested for carbon dioxide reduction in a three-compartment cell containing 0.1 M NaHCO3 electrolyte saturated with carbon dioxide (see supporting information). Catalytic 6 ACS Paragon Plus Environment

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currents for carbon dioxide reduction were evident across the full range of loadings (Fig. 1A, Fig. S1) at low overpotentials relative to the standard potential of E0= -0.11 V vs. RHE. The catalyst maintained a high selectivity towards CO with Faradaic efficiencies (FECO) that decreased from 96% at the highest loading to ~ 80% at the lowest loading (Fig. 1B, Table S1), likely due to the exposure of the underlying carbon paper which is selective for H2 production (Table S2). While the magnitude of the current density decreased sharply at loadings less than ~10-8 mol/cm2 (Fig. 1C), the turnover frequency for CO increased dramatically by orders of magnitude (Fig. 1D). The TOFs were calculated based on the total number of CoPc molecules immobilized on the carbon paper. We chose not to quantify the active sites through integrating the cyclic voltammogram peak for the CoII CoI transition due to the poor accuracy of such methods at the low loadings used in this study.33,37 At a potential of -0.73 V vs. RHE, the TOFCO gradually increased with decreasing catalyst loading and approached ~100 s-1 at loadings below 2 x 10-11 mol/cm2. At this low loading, the TOFCO for CoPc/OxC is high across a wide range of polarizations (Fig. 2, Table S3). By using 1 M NaHCO3 as electrolyte, the TOFCO could be further increased to 400 s-1 at the polarization of -0.73 V vs. RHE, corresponding to a turnover number for CO (TONCO) as high as approximately 1.4 x 106 over a 1-hour period. These high TOFs come at the expense of current densities, yet enable understanding of kinetically-limited reactivity (vide infra). All catalysts exhibited high stability with relatively stable current densities and FECO for up to 6 hours (Fig. S2).



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Figure 2. TOFCO as a function of polarization in aqueous electrolytes for reported immobilized molecular catalysts, for which values are summarized in Table S3, and CoPc/OxC at a low loading of 1 x 10-11 mol/cm2.

We conducted scanning electron microscopy (SEM) to understand how catalyst morphology may impact the observed TOFCO. SEM at the highest loading revealed CoPc crystals (Fig. 3A), which were not evident at lower loadings (Fig. 3B, Fig. S3). This observation is consistent with the known stacking and aggregation of phthalocyanine via intermolecular metalnitrogen bonding.39–42 The improved catalyst dispersion at lower loadings likely improves accessibility of CoPc active sites, leading to increased TOFs. Although we cannot exclude that small aggregates still form at the lowest loadings, the measured Tafel slopes suggest transport limitations are alleviated at the lowest loadings (vide infra). Control experiments have demonstrated that the improved dispersion associated with high TOFCO depends on both the use of an oxygen-functionalized support and use of a solvent with high CoPc solubility (see supporting information, Fig. S4-5). 8 ACS Paragon Plus Environment

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Given the high turnover frequencies at low loadings, we sought to understand how catalyst loading may influence the mechanism of carbon dioxide reduction. We measured the Tafel slopes of CoPc/OxC as a function of loading (Fig. 3C, S8), which decreased from ~230 mV/dec at the highest loadings to ~120 mV/dec at the lower loadings. The value of 230 mV/dec suggests transport limitations through the CoPc crystals, as observed in SEM (Fig. 3A). The high Tafel slopes at high loadings are similar to previously reported values for cobalt porphyrins and phthalocyanines, as well as their derivatives, which are in the range of 165 to 550 mV/dec.30,35,37,38 The decrease in Tafel slope at lower loadings is attributed to the improved dispersion of CoPc molecules, which alleviates transport limitations. At low loadings, the value of ~120 mV/dec is consistent with a rate-limiting one electron transfer step.43 In light of this, we conducted all order dependence measurements at low catalyst loading (1 x 10-11 mol/cm2) and at a polarization of -0.58 V vs. RHE, which falls in the linear Tafel region in order to minimize transport limitations (Fig. S6). The order dependence of the partial current density for CO (jCO) on bicarbonate concentration (Fig. 3D) revealed a zero-order (0.17) dependence at low bicarbonate concentrations43,44 and an approximate first-order (1.40) dependence at high bicarbonate concentrations. These trends also held true at a more reductive polarization that we tested (Figure S8). At higher bicarbonate concentrations, the pH becomes more basic, suggesting that the positive order-dependence on bicarbonate is due to bicarbonate acting as a proton donor as opposed to hydronium. Given the apparent mechanistic divergence at low and high bicarbonate concentrations, we measured the order dependence of jCO on the CO2 partial pressure in both regimes. The CO2 order dependence in 0.1 M and 1 M NaHCO3 was found to be 1.0 and 0.6, respectively (Fig. 3E). This suggests approximate first-order dependence on CO2 at low bicarbonate concentrations with deviation at higher bicarbonate concentrations. The deviation



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may originate from the fact that the high HCO3- concentration contributes to dissolved CO2 more effectively especially at low CO2 partial pressure via the HCO3-  CO2(aq) equilibrium.43 Such contribution has been experimentally confirmed by the observation of CO as a product during electrolysis in 1 M NaHCO3 with only N2 fed to the cell (Table S4). At both low and high bicarbonate concentrations, the Tafel slope in these two regimes is similar (Fig. 3F).


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Figure 3. (A) SEM image of CoPc/OxC electrode (loading: 1 x 10-7 mol/cm2), on which crystals of CoPc are evident. A region with many crystals is highlighted by the red oval. Inset: High magnification SEM image and the corresponding energy-dispersive X-Ray spectroscopy mapping of Co. (B) SEM image of CoPc/OxC (loading: 1 x 10-9 mol/cm2) in which no crystals are evident. (C) Tafel slope as a function of CoPc loading in 0.1 M NaHCO3, with the red trendline as a guide. (D) CO partial current density (jCO) as a function of bicarbonate 11 ACS Paragon Plus Environment


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concentration for CoPc/OxC (1 x 10-11 mol/cm2) at -0.58 V vs. RHE, a potential which falls in the linear region of the Tafel plot for this loading (Fig. S6h). (E) CO partial current density as a function of partial pressure of CO2 (PCO2) for CoPc/OxC (1 x 10-11 mol/cm2) at -0.58 V vs. RHE. (F) Tafel plot for CoPc/OxC (1 x 10-11 mol/cm2) in 1 M NaHCO3 electrolyte.

By integrating the Tafel slope, bicarbonate order dependence, and CO2 order dependence, different mechanisms can be proposed as a function of [HCO3-] in the electrolyte. As previous in situ characterization has confirmed that CoI is the resting state for cobalt during electroreduction of CO2, the first step is proposed to involve the irreversible reduction of CoII to CoI.37,38 At low bicarbonate concentrations (< 0.3 M), the next step is a rate-determining electron transfer (ET) step and involves adsorption of CO2 with simultaneous electron transfer from CoI, forming CoII and adsorbed COO- (Scheme 2). This mechanism leads to a theoretical Tafel slope of 118 mV/dec (see supporting information). The mechanism at low bicarbonate concentrations is similar to the proposed mechanism of CO2 reduction on cobalt porphyrins, based on density functional theory calculations and electrochemical characterization under acidic conditions.45,46 At high bicarbonate concentrations (> 0.3 M), the rate-determining step shifts to a concerted proton-electron transfer step (CPET) with HCO3- as the proton donor (Scheme 2). The shift in mechanism from ET-limited to CPET-limited upon going from low to high bicarbonate concentrations is consistent with the expectation that bicarbonate is more likely to participate in the rate-determining step when it is available at higher concentrations. This in-depth mechanistic understanding is important for rational identification of conditions which enable high turnover frequencies. The mechanism at higher bicarbonate concentrations provides substantially higher turnover numbers (400 s-1) than those achieved at lower bicarbonate concentrations (100 s-1), as the rate-determining step involving a CPET is faster than the one involving ET alone. 12 ACS Paragon Plus Environment

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Scheme 2. Proposed mechanism for CO2 electroreduction to CO catalyzed by CoPc. At low and high bicarbonate concentrations, we propose reactions limited by rate determining steps (RDS) involving either an electron transfer (ET) or a concerted proton-electron transfer (CPET), respectively. Our catalytic data provides evidence for the proposed RDS; subsequent protonation, electron transfer, and CO desorption steps allow for closing the catalytic cycle.

Several design principles emerge from our work for future design of metal macrocycle catalysts for electrochemical carbon dioxide reduction. Our work highlights the importance of achieving high catalyst dispersion, especially at low loadings, for measuring the kineticallylimited turnover frequencies of metal phthalocyanines for electrochemical carbon dioxide reduction. This is necessary for unambiguous comparison of the kinetics of metal phthalocyanines derivatives. Functionalization of molecular complexes used for carbon dioxide reduction has long been explored from the perspective of achieving ideal electronic structures which provide optimized adsorption energies for key intermediates. Our work suggests that functionalization of these complexes may also be essential for preventing aggregation and 13 ACS Paragon Plus Environment


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maintaining accessibility of active sites. Furthermore, the high turnover frequency of CoPc provides ample motivation for future studies aimed at maintaining catalyst dispersion at higher loadings.

ASSOCIATED CONTENT Supporting Information. Experimental methods; Additional electrochemical data; Mechanistic analysis AUTHOR INFORMATION The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Subodh Gupta, Alan Hatton, Klavs Jensen, and Anamika Mukherjee for insightful discussions. We gratefully acknowledge financial support from Cenovus Energy and the MIT Energy Initiative Low Carbon Energy Center on Carbon Capture, Utilization, and Storage.


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Elucidating Reactivity and Mechanism of CO2 Electroreduction at

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