Efficient Bio-Inspired Oxygen Reduction Electrocatalysis with

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Efficient Bio-Inspired Oxygen Reduction Electrocatalysis with Electropolymerized Cobalt-Corroles Ariel Friedman, Lena Landau, Shmuel Gonen, Zeev Gross, and Lior Elbaz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00876 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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ACS Catalysis

Efficient Bio-Inspired Oxygen Reduction Electrocatalysis with Electropolymerized Cobalt-Corroles Ariel Friedman1, Lena Landau2, Shmuel Gonen1, Zeev Gross2*, Lior Elbaz1* 1

2

Chemistry Department, Bar-Ilan University, Ramat-Gan 5290002, Israel Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 3200008, Israel *Corresponding authors e-mails: [email protected]; [email protected] ABSTRACT: Fuel cells are considered as the only viable solution for long-range electro-mobility, but very rare and expensive platinum is currently required for catalyzing the bottleneck reaction therein: oxygen reduction. Within the search for catalysts that are not based on precious metals, cobalt corroles were uncovered to fulfill the requirements of high selectivity and low overpotential. We now report on the electropolymerization of a specifically designed catalyst, cobalt(III) complex of tris(4-aminophenyl) corrole, upon which 3D polymeric structures were obtained. Much better catalytic activity was obtained by this approach as compared to monomeric catalyst, manifested by significantly lower overpotentials, as well as higher selectivity to the desired 4e-/4H+ pathway. The performance in alkaline environment makes it the most active molecular catalyst for the oxygen reduction reaction reported to date.

Keywords: Fuel Cells • Corrole • ORR • Electrocatalysis • non-precious metal catalyst performed with them is still flourishing. Within that class, significant progress has recently been reported for 1st row transition metal complexes of corroles (metallocorroles).6-8 Corroles are macrocyclic complexes with structural resemblance to porphyrins, which due to their unique trianionic platform offer enhanced activity and stability to metals chelated by them.5, 912 Recent work revealed that the ORR occurs with much lower overpotentials when catalyzed by metallocorroles rather than by metalloporphyrinsand comparable to the state-ofthe-art catalysts for oxygen reduction introduced so far.13-15 Recently, we showed that by adsorbing the corroles on high surface area carbon (BP2000), the catalytic activity increases significantly with the highest activity in acidic conditions obtained with cobalt corroles, making it comparable to the stateof-the-art catalysts for oxygen reduction introduced so far.16-18 Nevertheless, there is still work to be done in order to further enhance the catalytic activity, mainly in shifting the ORR to the desired 4-electron mechanism. It is well known that monomeric Co porphyrins and corroles reduce oxygen via the 2 electron mechanism, and that faceto-face orientation with appropriate metal center distance is critical for the four-electron reduction pathway.8, 19 Many studies have shown that binuclear complexes and other multi-metal centered complexes catalyze the oxygen reduction reaction at higher onset potentials and mostly 4-electron mechanism and outperform the activity of their monomeric units.20-25 This improvement was

1. Introduction The main energy supply for aerobic respiration comes from the oxygen reduction reaction (ORR), by which molecular oxygen is reduced selectively to water by the perfectly arranged tris-copper and bis-heme centers present in cytochrome C oxidase.12 In alternative energy generation devices such as fuel cells and metal-air batteries, the ORR is also most crucial. Low ORR kinetic rates, operation at high overpotentials, and the consequential energy losses must be prevented for making such devices practical. Platinum-based catalysts fulfill the above requirements best, and they also reduce oxygen predominantly via the desired 4-electrons mechanism. But platinum is one of the scarcest and most expensive metals on earth. This is the main reason for the worldwide efforts on finding ORR catalysts that are not based on precious group metal (PGM) catalysts. Considering that ORR catalysis in the natural systems relies on porphyrin-chelated iron (heme), much attention has been driven to complexes composed of 1st row transition metal ions (mainly Fe and Co) bound by synthetic porphyrins and other macrocycles.3 An interesting outcome is that the state-of-the-art ORR catalysts of this class are obtained via pyrolysis of the above catalysts as to form materials that apparently preserve the M-N4 motif (metal chelated by 4 N atoms).4 What seriously hampers their further development is that it is hard to design/control/identify structures and catalytic sites.4-5 The above aspects are however quite trivial for molecular catalysts, which is actually why research

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attributed to the vicinity of the metallic centers, which in optimum distance can bind oxygen by two metal ions.26 Accordingly, we postulated that electropolymerization of the cobalt corroles will result in dense corrole layers which will bring the catalytic centers close together to enable concerted reactions, and thus increase the activity and selectivity towards the 4-electron process by virtue of cooperative effects similar to those described above. We now present the advance to 3D polymeric structure composed of well-defined catalytic centers, obtained by controlled electropolymerization of an appropriately designed cobalt corrole on carbon electrodes, as a mean for obtaining superior ORR catalysis. In addition to increase in the catalytic performance, the compact polymeric structures are expected to have a high concentration of active sites which can compensate for the relatively low turnover frequencies usually associated with all PGM-free catalysts for ORR, when compared to Pt-based catalysts.27

Triphenylphosphine, RT. phosphine, RT.

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Co(OAc)2⋅4H2O, 2. Triphenyl-

after the CVs (Figure 1, inset) serve as another clear evidence for the deposition of polyCoTAC on the electrode. Similar phenomena was obtained for electropolymerization of phenylaniline subtituted porphyrins,28-31 which gives a preliminary indication that the aniline substituents on the corrole ring are involved in the transformation of CoTAC to polyCoTAC on the electrode surface. The CV of the polyCoTAC-modified electrode in a monomer-free solution revealed stable voltammograms (Figure S1, Supporting Information; red traces), indicating that the deposited film remains electroactive and does not passivate the electrode surface.32

2. Results and discussion The monomeric cobalt corrole CoTAC with three 4-aminophenyl intended to be used for oxidative polymerization, was prepared by the standard procedures drawn in Scheme 1 (Experimrntal procedures, Supporting Information). Electropolymerization of CoTAC was conducted by cycling the potential of an Indium Tin Oxide (ITO)-coated glass electrode between 0.00 V and 0.45 V (vs. Ag wire). The ITO electrode was dipped into an acetonitrile (AN) solution containing tetraethylammonium tetra-fluoro-borate (TEAT, 0.1 M) and CoTAC (2 mM). The rise of both cathodic and anodic peaks that were observed during that process (Figure 1) was attributed to an increase in the surface concentration as a consequence of the deposition of polymerized CoTAC on the ITOcoated electrode. Pictures of the electrode before

Figure 1. Cyclic voltammogram (CV) obtained upon the electropolymerization of CoTAC on an ITO coated glass electrode (2 mM CoTAC in 0.1M TEAT/AN solution, scan rate 100 mV/s). Inset: the ITO electrode before (left) and after (right) electropolymerization.

The electropolymerization of the CoTAC was studied by using electrochemical quartz crystal microbalance (EQCM), on gold-coated quartz crystal under similar conditions to those described before. The change in frequency as a function of the applied potential for the first CV cycle is shown in Figure 2. The resonance frequency of the quartz crystal (which is proportional to the mass of deposited material) decreases only at potentials higher than 0.3 V vs. Ag/AgCl, confirming the deposition of polyCoTAC at this potential. Whereas, an increase in frequency was observed during the backward scan, as the measured currents decreased, hinting on a mass loss. This can be attributed to reduction and de-doping of the polymer, which results in the repulsion of the electrolyte counter anion that was held by the positively charged polymer. The electropolymerization and decrease in QCM frequency continues with cycling and correlates well to the increase of the accumulated

and Scheme 1. The 3-step synthesis of 5,10,15-tris(4-aminophenyl) corrolato cobalt(III)triphenylphosphine (CoTAC, with L = PPh3). Reaction conditions: a: 1. MeOH/H2O/HCl, RT, 1 hr. 2. p-chloranil, RT, overnight; b: 1. SnCl2⋅2H2O, 2. H2O/HCl, reflux under Ar atmosphere; c: 1. Co(OAc)2⋅4H2O, 2.


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charge in the Infromation).

CV

(Figure

S2,

Supporting

by using FTIR-ATR spectroscopy (Figure 3). These measurements revealed an almost complete disappearance of the characteristic NH2 stretching vibrations at 3441 cm-1 and 3331.2 cm-1, in addition to significant reduction of the peak at 1613 cm-1, which is mainly due to NH2 scissoring vibration.3334 This data is consistent with transformation of the primary aminophenyl moieties present in CoTAC to higher N-substitution levels. The FTIR-ATR spectra also reveal that the NH2 frequencies did not fully disappear, pointing towards the presence of unreacted substituents on the corrole ring. These results confirm that in the polymerization process, the aniline groups react and lose protons to become secondary or tertiary amine. For comparison, in polyaniline there are several coupling positions for linking the aniline rings to each other (head-to-tail, tail-to-tail and head-tohead)35, whereas in the case of polyCoTAC, the available coupling positions are limited, since the tail position (para to the NH2 unit) is occupied by the corrole ring. This leaves the ortho position as the only available coupling position on the phenyl ring. On the other hand, polymerization through β position on the corrole ring also needs to be considered. Another possible cross-link to be considered is head-to-head, nitrogen to nitrogen of two different corroles, either single or double bonded, which is expected to from 1, 2Diphenylhydrazine and azobenzene linking units, respectively. Hence, the nature of the bond between the CoTAC monomers may be different from that of polyaniline. In order to study the coupling position on the CoTAC, we synthesized cobalt(III)-tri(tetrafluorop-aminophenyl) corrole (CoTFAC) (Figure S3, Supporting Infromation). CoTFAC differs from CoTAC only in the ortho- and meta- positions on the aminophenyl substituents, where fluorine atoms are replacing the hydrogen atoms. Similar conditions were applied in order to try to electropolymerize the CoTFAC, and in contrast to the CoTAC, polymerization of CoTFAC was not observed (neither electrochemically nor physically – the ITO electrode remained clear – no deposition was observed), even when cycling to much higher potential (1.3V vs. RHE). These observations suggest that the electropolymerization site of the CoTAC is indeed the ortho position on the aminophenyl ring and not through the β positions on the corrole ring or head-to-head. Based on these results, and on the wellestablished aniline electropolymerization mechanism,36-39 we propose an electropolymerization mechanism similar to what

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Figure 3. FTIR-ATR spectra of CoTAC monomer powder (black) and polyCoTAC powder scraped from the electrode surface after the electropolymerization (blue) at (a) high and (b) low wavenumbers

The new chemical bonds formed during the electropolymerization of polyCoTAC were studied


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has been suggested for polymerization of other macrocycles substituted with 4-aminophenyls.40-41 It is generally accepted that the first step of the electropolymerization of aniline is an irreversible oxidation (Electrochemical step; E step), which results in the formation of a radical cation, followed by coupling of two radical anilines to form a dimer (Chemical step; C step). On every newly formed bond, two hydrogen atoms are eliminated to obtain aromaticity (another C step). The dimer is oxidized immediately to dimer-cation (E step) followed by coupling with monomer cation (Equation S1, Supporting Infromation). For the short term, chain propagation occurs, involving repeated steps of

electropolymerization mechanism and structure of polyCoTAC in Scheme 2. It is important to note that so far, only the first reaction resulting in linking unit of o-amino-diphenylamine (a) was proven by us. The following reactions to dihydrophenazine (b) and phenazine (c) only postulated based on polymerization mechanism of aniline and other phenylaniline substituted porphyrins. This mechanism is currently under through investigation – the formation of the polymer on the electrode surface limits the ability to characterize it using conventional techniques such as NMR. Hence the complexity in gathering the molecular information.

E(CCE)n. This is also described in the proposed

Scheme 2. Top: Illustration of phenazine-bridged cobaltocorroles. Bottom: plausible electropolymerization mechanism (the corrole ring is omitted). The bridging unit may change by sequential oxidative coupling steps from o-amino-diphenylamine (a) to dihydrophenazine (b) to phenazine (c).


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Figure 4. SEM images (a,b) of polyCoTAC on a glassy carbon electrode at different magnifications.

The morphology of the newly synthesized polyCoTAC was obtained with SEM imaging, uncovering that the polyCoTAC film is composed of dense, uniform, well-dispersed, threedimensional cauliflower-like structures (Figure 4). Each particle consists of small nano-metric particles with an average size of ~30nm (Figure 4b), which renders this catalyst as a good platform for heterogeneous electrocatalysis of the oxygen reduction reaction (ORR) in terms of potential surface area and roughness.26 The electrocatalytic ORR activity was studied using rotating ring-disk electrode (RRDE) in 0.5 M H2SO4 and 0.1 M KOH aqueous solutions, for both the monomeric metallocorrole and its polymer (Figure 5). To evaluate and compare their electrocatalytic activity, the onset potential (Eonset), half-wave potential (E1/2), and the number of transferred electrons (n) were measured and calculated (Equation S2, Supporting Infromation). The results are summarized in Table 1, which clearly shows that polyCoTAC exhibits better electrocatalytic activity than its monomer, both in acidic and alkaline solutions. This is apparent by the desired positive shift of the onset/half-wave potentials: 110/180 mV in the acidic solution and 40/120 mV in the alkaline solution. Moreover, the number of electrons participating in the reaction was also significantly affected: 2.14 vs. 1.77 in 0.5 M H2SO4 and 3.10 vs. 2.5 in in 0.1 M KOH. The latter results indicate that the two-electron reduction pathway (n=2) is predominant in acidic solution for both CoTAC and polyCoTAC and in basic solution for CoTAC, while polyCoTAC in alkaline solution has a larger contribution from the 4-electron (or 2+2) reduction pathway, approaching n=4. The bare GC electrode also exhibits some catalytic activity in 0.1 M KOH, which can be attributed to quinonelike moieties on its surface.42-43 Whereas, no

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Table 1. The onset potential (V), half-wave potential (V) and the number of transferred electrons (n; calculated using the Levich equation, S8) of CoTAC and polyCoTAC in 0.5 M H2SO4 and 0.1 M KOH, on glassy carbon and BP2000. Glassy Carbon

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activity was

obtained for BP2000. The origin for this effect is still unknown and currently under investigation by our group. Several possibilities for this effect are suggested such as interaction with surface functional groups, ordered vs. disordered phases in the carbon and average pore size. As can be seen in Figure 6, both in acidic and alkaline solutions, the catalytic activity of the polyCoTAC increased significantly by all measured electrochemical parameters after its in-situ electropolymerization onto BP2000 (Figure 6a, red trace), when compared to CoTAC adsorbed from solution on BP2000 (Figure 6a, black trace). We observed an increase in the onset potential to 0.928 and 0.84 V vs. RHE, increase in half-wave potential to 0.832 and 0.638 V vs. RHE and increase in the electrons number to 3.52 and 3.13 for alkaline and acidic environments, respectively. At higher overpotentials in the acidic solution, there is a clear transition from mainly twoelectron reduction pathway to mostly four-electron reduction pathway, more specifically to the 2+2 mechanism. This may be deduced from the additional wave around 0.4 V vs. RHE, attributable to reduction of hydrogen peroxide to water. A similar, but less pronounced behavior, is seen in the alkaline solution for CoTAC@BP2000 (Figure 6b, black trace), but the pathway for polyCoTAC@BP2000 (Figure 6b, red trace) remains mostly four-electron reduction throughout the whole potential range. To put the encouraging results obtained with polyCoTAC@BP2000 on scale, it was compared to the state-of-the-art pyrolyzed (non-molecular) PGM-free commercial catalyst purchased from Pajarito Powder (Figure 6a and 6b, blue trace). In acidic solution, the performance of the latter was clearly better, with about 100 mV higher onset potential and higher limiting currents. But in alkaline solution, the onset potential difference was below 50 mV, and the limiting current was obtained faster for polyCoTAC@BP2000, which hints on faster reaction kinetics for polyCoTAC@BP2000. This makes the polyCoTAC@BP2000 the best, non-heat-treated, molecular catalyst for ORR reported to-date.

Figure 5. RRDE measurements (900 rpm, 5mV/s) for evaluating ORR activity in (a) 0.5 M H2SO4 and (b) 0.1 M KOH of a bare GC electrode (green), GC electrode onto which CoTAC monomer was either deposited (black) or electropolymerized (red).

measured in acidic conditions, as expected. Also important to note is that polyCoTAC exhibits improved kinetics and reach the limiting current faster than its monomer, hinting on faster 0.0

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Figure 6. RDE measurements (900 rpm, 5mV/s) of ORR in (a) 0.5M H2SO4 and (b) 0.1M KOH. With BP2000 carbon (green) CoTAC monomer@BP2000 (black), polyCoTAC@BP2000 (red), compared to the commercial PGM-free Pajarito Powder’s catalyst (blue).

reaction kinetics. For achieving better functionality required for devices such as fuel cells, the attention was driven towards incorporating the polyCoTAC in high surface area carbon that is commonly used as a commercial catalyst support: BP2000. In our previous work we showed that the catalytic activity of monomeric metallo corroles is influenced significantly by the nature of the electrode substrate where the highest activity

3. Conclusion In this work, successful electropolymerization of metallocorrole was demonstrated for the first time. Based on well-elucidated polyaniline electropolymerization, a mechanism for the polymerization of polyCoTAC was suggested. FTIR-ATR analysis confirms the formation of new covalent bonds between the monomers. The polyCoTAC formed a three-dimensional


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disk (surface area of 0.247 cm2) and a platinum ring (surface area of 0.186 cm2) (Pine E7R9). The potential of the ring was held at 1.2 V vs. RHE. Glassy carbon rod (3 mm diameter) was used as the counter electrode. The reference electrode was a reversible hydrogen reference electrode (RHE) composed of platinized platinum wire, operating under constant purge of pure hydrogen (at atmospheric pressure). For the measurement of background currents, the electrolyte solution was purged with gaseous Ar (99.999% purity) and for ORR studies it was purged with gaseous O2 (99.999% purity). The number of transferred electrons (n) was calculated via Levich equation (eq.1):

cauliflower like structure which consists of smaller nano-particles. The catalytic activity for oxygen reduction reaction of CoTAC improved significantly after its electropolymerization, by all parameters studied in this work. This is attributed to synergistic activity of the metallocorroles. The activity of polyCoTAC was further improved when polymerized inside high surface area carbon (BP2000). This combination exhibited high performance when compared to the state-of-the-art commercial non-precious metal catalyst in alkaline solution. Further investigation of the structure and the interaction of the polyCoTAC with the BP2000 are currently underway. 4. Experimental Section Materials: The electropolymerization was performed in acetonitrile (AN) 99.9+% extra dry solution (Acros) with 0.1 M tetraethylammonium tetrafluoroborate (TEAT) and 1-2 mM corrole monomer dissolved in the solution. The electropolymerization was carried out under inert atmosphere of Ar. Three-electrode miniature Teflon electrochemical cell (200 µL) was used with either glassy carbon or indium thin oxide (ITO) as working and counter electrodes. An Ag wire was used in this cell as pseudo-reference electrode. The obtained polymetallo-corrole was rinsed with AN and dried afterwards in open air. For testing the monomer deposited on glassy carbon electrode (GCE), 5 µL of solution of 1.26*10-4 M monomer in AN were added dropwise to the disk of the RRDE electrode and left to dry at room temperature for 20 minutes. For testing the monomer on BP2000, a slurry of 10 mg BP2000/1mL ACN/ 0.56 mM CoTAC left for 24h under rotation. Then, the AN was removed and replaced with 1mL isopropanol (IP)/0.2% Nafion. 10 µL of that slurry dried on RRDE electrode for testing. For testing the polymer on GCE, the monomer was polymerized on the RRDE disk electrode from solution of 2 mM monomer/0.1 M TEAT in AN by cycling the potential between 0.00 and 0.8 V vs. Ag pseudoreference (as described above) for 15 cycles. For testing the polymer on BP2000, 10 µL of from slurry of 10 mg BP2000/1mL IP/ 0.2% Nafion dried on RRDE electrode and then coated with polymer with the same procedure as on GCE. In both cases, after the polymerization the electrode was left to dry at room temperature overnight before the measurements.

, = −0.201/ ʋ/ / ∗ Where A is the surface area of the electrode disk, D and ∗ are the oxygen diffusion coefficient (1.5*10-5 cm2s-1 in 0.5 M H2SO2 and 1.9*10-5 cm2s-1 in 0.1 M KOH) and the bulk concentration of oxygen (1.3*10-6 mol mL-1 in 0.5 M H2SO2 and 1.1*10-6 mol mL-1 in 0.1 M KOH) respectively, ν is the kinematic viscosity (0.01 cm2 s-1) and ω is the rotation speed (rpm). Electrochemical quartz crystal microbalance (EQCM) measurements were conducted with Seiko EG&G quartz crystal analyzer (QCA922A). Static cell (092-QCA-FC) used to the EQCM measurements with Pt wire as counter electrode and Ag/AgCl as reference. The QCM electrodes (QAA9M-AU) were AT-cut quartz crystal coated with 300 nm Au (diameter of 5 mm). the calibration resonance frequency was 9 MHz. FTIR-ATR spectra were recorded using a Nicolet iS 50 spectrophotometer, Thermo scientific, with ATR diamond crystal. High resolution scanning electron microscopy (HRSEM) measurements were conducted with Magellan 400 L. The sample was coated with Iridium by sputtering (Quorum, Q 150 T ES). Supporting Information SI includes synthesis procedures, experimental information, graphical schemes, equations and parameters. Acknowledgements The authors would like to thank The Israeli Ministry of Energy, VATAT and the Fuel Choices and Smart Mobility Initiative in the Israeli Prime Minister’ Office for supporting this work. A.F. would like to thank the Israeli Ministry of Energy

Characterization: All of the electrochemical work in this paper was conducted using a BioLogic VSP potentiostat. Rotating ring-disk electrode (RRDE) measurements were conducted with a glassy carbon


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for his Scholarship. This work is done in the frame work of the Israeli Fuel Cells consortium (part of

the Israeli National Center for Electrochemical Propulsion).

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Efficient Bio-Inspired Oxygen Reduction Electrocatalysis with

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