Changing the selectivity of O2 reduction catalysis with one ligand

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Changing the selectivity of O2 reduction catalysis with one ligand heteroatom Soumalya Sinha, Moumita Ghosh, and Jeffrey J Warren ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04757 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Catalysis

Changing the selectivity of O2 reduction catalysis with one ligand heteroatom Soumalya Sinhaa, Moumita Ghoshb, and Jeffrey J. Warren*a a

Department of Chemistry Simon Fraser University 8888 University Drive

Burnaby, BC V5A 1S6, Canada b

Department of Chemical Sciences

Indian Institute of Science Education and Research Kolkata Mohanpur, Nadia, WB 741246, India

ACS Paragon Plus Environment

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ABSTRACT The development of catalytic systems that selectively reduce O2 to water is needed to continue the advancement of fuel cell technologies. As an alternative to platinum catalysts, derivatives of iron (Fe) and cobalt (Co) porphyrin molecular catalysts provide one benchmark for catalyst design, but incorporation of these catalysts into heterogeneous platforms remains a challenge. Co-porphyrins can be heterogeneous O2 reduction catalysts when immobilized on to edge plane graphite (EPG) electrodes, but their selectivity for the desired 4-electron reduction of O2 to H2O is often poor. Herein, we demonstrate substantial improvements in the O2 reduction selectivity for a Coporphyrin by incorporating a 2-pyridyl group at one of the meso-positions of a Cotetraarylporphyrin

(cobalt(II)

5-(2-pyridyl)-10,15,20-triphenylporphyrin,

CoTPPy).

The

properties of CoTPPy immobilized on EPG were investigated using cyclic voltammetry, rotating disk and rotating ring-disk electrochemistry. The presence of a single 2-pyridyl group in the CoTPPy gives rise to the 4-electron reduction of O2, as opposed to the 2-electron reduction commonly associated with cobalt porphyrins. Detailed electrochemical studies of CoTPPy and related Co and Fe porphyrins are described. Use of Co instead of Fe improves overpotentials by over 200 mV with a factor of two increase in maximum turnover frequency (TOFmax). This work demonstrates that a simple change in catalyst structure can dramatically change the selectivity for O2 reduction. KEYWORDS. Dioxygen reduction, Electrocatalysis, Selectivity, Cobalt porphyrins, Proton relay


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Introduction Harvesting of solar energy that can be stored in the H2 single bond is intimately tied to high fidelity and efficient reduction of O2 to water. At present, catalysis of those closely linked redox reactions of H2 and O2 is a hurdle to widespread adoption of renewable technologies.1,2 One method to harvest the solar energy stored in chemical bonds is via fuel cells. Here, anodic H2 oxidation is coupled to cathodic O2 reduction and the associated redox processes produce electricity.3 Due to its central importance, the O2 reduction reaction (ORR) continues to be of interest, especially in the development of non-precious catalysts that are selective for the 4electron, 4-proton reduction of O2 to water. The cathodic reactions attract much interest because those non-precious O2 reduction catalysts tend to be kinetically slow and can suffer from the production of partially reduced products (e.g., O2•– or H2O2). At present, the requirement for precious metal catalysts contribute to technical and socioeconomic challenges that stand in the way of renewable technologies. Herein, we demonstrate that a simple modification to known cobalt(II) porphyrin ORR catalysts and immobilization of them on graphite is a strategy to address some of these key technical challenges, especially those related to ORR selectivity. Metalloporphyrins have long been of interest for oxygen binding and activation chemistry. In biological systems, iron porphyrins are canonical O2 carriers and activators. With respect to biological ORR, a great amount of effort has gone into modelling and understanding the mechanism of cytochrome c oxidase (CcO).4 Pioneering work on small molecule models came from Collman and co-workers. These molecules are structural and functional models of CcO, and they illustrate the importance of the heme-iron microenvironment.5,6 Both the presence of multiple redox moieties (a phenol, Cu, and Fe), and hydrogen bond partners were crucial features. Work on model protein systems from Lu and co-workers7,8 and on model chemical systems from Karlin and co-workers9 reinforces this idea.


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Inclusion of pendant Brønsted acid/base groups (also known as proton relays) and multiple redox sites is important for ORR activity. Others have recognized the importance of these features and it was recently demonstrated that prepositioned Brønsted acid/base groups can stabilize intermediates and enhance homogeneous O2 reduction electrocatalysis. A few examples here include work from Dey,10,11 Nocera,12,13 and Mayer.14,15 Furthermore, porphyrins that bear multiple redox sites also are good ORR catalysts, especially for the 4H+/4e– reduction to water.10 The above examples of ORR catalysts have been explored in homogeneous organic solutions. For reactions in water, work on heterogeneous catalysts from Anson and co-workers show that adsorption of iron16,17 and cobalt18–20 on carbon electrodes also gives rise to ORR chemistry. Many of those Coporphyrins produce H2O2 as the primary reaction product in O2 reduction, but iron porphyrins give mixtures of H2O2 and water. Notable exceptions for adsorbed Co-porphyrins are for two very simple porphyrins (porphine21 or 5,10,15,20-tetramethylporphyrin20), which reduce O2 to water. This activity is proposed to arise from a bimetallic mechanism that is made possible by the small size of the ligands. In work from our group, the heterogenous ORR chemistry of asymmetric iron 5-aryl,10-15,20triphenylporphyrins, where aryl = 2-pyridyl, 2-hydroxyphenyl, or 2-carboxyphenyl, was explored.22 All of those molecules, which contain only one Brønsted acid/base group, showed ORR activity. The 2-pyridyl derivative was the most active and most stable during prolonged electrolysis. While the addition of a proton relay greatly enhanced catalyst stability in comparison to chloroiron(5,10,15,20-tetraphenylporphyrin) (ClFeTPP) the overpotential (h) values were high. The aforementioned Co-porphyrins can reduce O2 with lower h, but the product is undesirable H2O2. Motivated by these observations, we hypothesized that incorporation of a 2-pryidly group into CoTPP could enhance O2 reduction kinetics and potentially change selectivity from a 2electron (H2O2) process to a 4-electron (H2O) process. In addition, because Co-porphyrins can


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activate O2 at more positive potentials than the corresponding iron complexes, an improvement in kinetics or selectivity with a simple ligand modification may provide a way to simultaneously improve the kinetics and the energetics of the O2 reduction reaction. Experimental All reagents were purchased from Sigma-Aldrich unless otherwise noted and used without further purification. Solvents were from J.T. Baker. Gases were obtained from Praxair Canada. Mass spectra were collected by using a Bruker microFlex MALDI-TOF or Bruker Maxis Impact electrospray ionization mass spectrometer. UV-Visible (UV-vis) spectra were recorded in CH2Cl2 solvent using a Cary100Bio UV-Visible spectrophotometer. The ligand, 5-(2-pyridyl)-10,15,20triphenylporphyrin (H2TPPy), was prepared using a literature procedure.22,23 5,10,15,20Tetraphenyl porphyrin (H2TPP) is a by-product of that reaction. Metalation of porphyrins with Co(II) was done by refluxing H2TPPy with excess CoCl2•4H2O in dimethylformamide for 3 hours. The reaction was then cooled to room temperature. A purple precipitate formed on addition of water. The solids were collected using vacuum filtration and washed thoroughly with water. The resulting complexes, CoTPPy or CoTPP, were characterized by UV-vis spectroscopy and MALDITOF (see Supporting Information).The iron(III) complex of H2TPPy (ClFeTPPy) was prepared as previously described.22 Electrochemical measurements were carried out using a Pine Instruments WaveDriver 20 bipotentiostat. Rotating disk electrochemistry (RDE) and rotating ring-disk electrochemistry (RRDE) experiments used the Pine Modulated Speed Rotator. Cyclic voltammetry (CV) and controlled potential electrolysis (CPE) experiments used conventional three-electrode cell with an edge plane graphite working electrode (5 mm by 3 mm surface area), Pt wire counter electrode, and a AgCl/Ag, saturated KCl reference electrode (CH Instruments). Control experiments also were carried out with a glassy carbon counter electrode.24 CV and CPE experiments yielded


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identical results, irrespective of the counter electrode used. Graphite working electrode surfaces were prepared by lightly abrading on 4000 SiC paper, washing thoroughly with deionized water, followed by sonication (120 s) in isopropyl alcohol, and briefly drying with a heat gun. RDE electrodes (Pine Instruments) were polished according to manufacturer specifications. Cobalt porphyrins were dissolved at 1 mM concentrations in CH2Cl2, deposited on electrode surfaces, and the solvent was removed under a gentle stream of air. Potassium ferricyanide was used as an external standard for all electrochemical experiments and all potentials are reported with respect to the normal hydrogen electrode (NHE). CVs were collected in 1 M aqueous H2SO4 unless otherwise noted. For experiments under O2, the headspace was flushed with O2 and the solution allowed to equilibrate. The concentration of O2 was measured as 0.56 mM using a Vernier optical dissolved O2 sensor. Results and Discussion Cobalt(II) 5-(2-pyridyl)-10,15,20-triphenylporphyrin (CoTPPy, Figure 1) was synthesized according to literature22 and characterized by using UV-vis spectroscopy and mass spectrometry. The UV-vis spectrum for the CoTPPy recorded in CH2Cl2 showed similar lmax and molar absorptivity (ɛ) to that of for cobalt(II) 5-10-15-20-tetraphenylporphyrin (CoTPP).25 This suggests that the electronic properties of the complexes are very similar. All characterization data are provided in the Supporting Information.

Figure 1. Structure of cobalt(II) 5-(2-pyridyl)-10,15,20-triphenylporphyrin (CoTPPy)


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Solutions of CoTPPy (1 mM in CH2Cl2) were drop cast on edge plane graphite (EPG) electrodes and investigated using cyclic voltammetry. In Ar-sparged aqueous 1 M H2SO4, cyclic voltammograms (CVs) showed a quasi-reversible wave centered at 0.52 V (Figure 2a) with 260 mV separation between the cathodic and anodic peaks, which is assigned to the CoIII/II couple. Reported values for the CoIII/II couple for CoTPP adsorbed on carbon electrodes are between 0.5 and 0.6 V.19,26 Again, this comparison indicates that addition of the 2-pyridyl group of CoTPPy does not exert a strong electronic effect. The reversible CoIII/II couple for CoTPPy on graphite was reproducible over the course of four CV sweeps at 100 mV s–1 in Ar-sparged solution. The scan rate dependence of CVs showed a linear relationship between the peak currents and the scan rate (see Supporting Information), which is an indication of efficient immobilization of the catalysts on the EPG surface.27 The electroactive CoTPPy concentration on the EPG surface was calculated from the total charge (QCV = 3.65 µC) passed at the reductive CoIII/II wave in Ar-sparged solution, Gcat = QCV/nFA = 2.52 ´ 10–10 mol cm–2, where n is the number of electrons (1 for CoIII/II couple), F is the Faraday constant, and A is the electrode surface area (= 0.15 cm2). The amount of electroactive CoTPPy is consistent with the literature for other adsorbed porphyrins,18 but still much lower than the amount deposited. Such behavior is known, but the origin of the discrepancy between electroactive and deposited catalysts is not known.18,19


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Figure 2. (a) CV for CoTPPy deposited on EPG in Ar-sparged 1 M H2SO4 solution. (b) Comparison of CVs collected in the Ar-(blue) and O2- (red) sparged solution. Four repeating CV sweeps are shown in both panels. CVs were recorded at a scan rate of 100 mV s–1.

CVs collected under 1 atm O2 were irreversible and showed a 15-fold current increase for CoTPPy on EPG, with limiting current observed at 0.38 V. The catalytic current density was steady, without any apparent degradation, during repeated four CV sweeps (Figure 2b). The onset of the catalytic current is between 0.6 and 0.7 V. This onset value is similar to another Coporphyrin ORR catalyst with a modest overpotential,21 and about 50 mV higher (i.e., lower overpotential) than a Co-hangman porphyrin ORR catalyst that has a prepositioned carboxylic acid proton relay.13 Control experiments carried out using a bare EPG electrode under O2 atmosphere show currents only at potentials more negative than –0.5 V (see Supporting Information). The stability of CoTPPy also was checked using controlled potential electrolysis (with 0.38 V applied potential, see Supporting Information). No decay in current was observed during 1.5 hours of electrolysis, similar to our observations for ClFeTPPy.22 The stability of CoTPPy and ClFeTPPy are noteworthy given other reports of the poor stability of electrode-adsorbed porphyrins in acidic media.16,28 The selectivity of O2 reduction by adsorbed CoTPPy was explored using rotating ring-disk


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electrode (RRDE) and rotating disk electrode (RDE) voltammetry experiments. First, RRDE experiments were carried out under 1 atm O2 in 1 M H2SO4 solutions using adsorbed CoTPPy or CoTPP on an EPG disk. The Pt ring electrode potential was held at 1.2 V to ensure complete oxidation of any partially reduced oxygen species (e.g., H2O2) generated upon reduction of O2 at the disk.15,29 The current detected during oxidation of reactive oxygen species provides valuable way to evaluate the selectivity for the full 4H+/4e– reduction of O2 to H2O. Results from typical experiments are shown in Figure 3.

Figure 3. RRDE results for (a) CoTPP and CoTPPy; (b) ClFeTPP and ClFeTPPy immobilized on the rotating EPG disk under 1 atm O2 in 1 M aqueous H2SO4 at 500 rpm rotation rate. The scan rate was 20 mV s–1. The potential at the Pt-ring was held at 1.2 V. The ring current for the CoTPPy was magnified by a factor of 2 for clarity. The number of electrons (n) involved in the O2 reduction process for the CoTPP and CoTPPy was calculated from the RRDE data by using equation (1): 𝑛 = 4 𝐼& ⁄(𝐼& + 𝐼) ⁄𝑁)

(1)

where Id and Ir are the limiting currents obtained for the EPG disk and Pt-ring electrodes, respectively, and N is the collection efficiency of the electrode (0.24, see Supporting Information). The value of n calculated from the RRDE experiment for the CoTPPy adsorbed on rotating EPG disk was 3.51 ± 0.02 at 0.4 V applied potential (h = 0.83 V). This is a striking result because related


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Co-tetraarylporphyrins commonly produce H2O2 during ORR experiments.18–20 We also note that the position of the pyridyl group appears to be important because related Co-porphyrins with functional groups in the 4-position produce H2O2 under similar experimental conditions.19,26,30 RRDE experiments for CoTPP (Figure 3) give n = 2.6 ± 0.1 under identical conditions. Our value of n is slightly higher than reported for CoTPP deposited on graphite under slightly different conditions (1 M HClO4 instead of the 1 M H2SO4 used here).19 Additional RRDE experiments using ClFeTPPy22 and ClFeTPP showed that iron-derivatives are selective for the 4e– reduction of O2. Analysis of the RRDE provides n = 3.6 for ClFeTPPy at 0.1 V applied potential (h = 1.1 V) and n = 3.6 for ClFeTPP at 0.1 V applied potential (h = 1.1 V). These values of n are consistent with reported values for related iron porphyrins.16 For Fe complexes, the 2-pyridyl group does not change selectivity, but greatly improves stability, limiting the presence of intermediates that can produce inert µ-oxo dimers.31,32 The Faradaic efficiencies (FE) for reduction of O2 to H2O can be calculated from RRDE data by using equation (2): -×/0 /2

100 – %H2O2 = 100 – /

3 4/0 /2

× 100

(2)

From the data shown in Figure 3, CoTPPy adsorbed on the EPG electrode has a FE for H2O of 70 ± 3%, with the remaining 30% attributed to H2O2 formation. For comparison, the FE for O2 reduction to H2O by CoTPP is 27 ± 3% (Figure 4) at an applied potential of 0.4 V (h = 0.83 V). The corresponding analysis for FeTPPy (see Supporting Information) gives a slightly higher yield of H2O (80%), but at higher h (h = 1.1 V). The yield of H2O is similar to the yield for ClFeTPP,16 but as noted above, the catalytic stability of ClFeTPPy is much better.22


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Figure 4. Percentage of H2O obtained from the RRDE experiments for the CoTPP, CoTPPy, or CoTPOH immobilized on the rotating EPG disk under 1 atm O2 in 1 M aqueous H2SO4 at 500 rpm rotation rate. The scan rate was 20 mV s–1 in all experiments. The change in O2 reduction selectivity for CoTPPy versus CoTPP was surprising. This change in selectivity could be specifically attributed to the 2-pyridyl group, or more generally due to the presence of an H-bond donating group. To test these ideas, the electrochemical O2 reduction activity of related Co(5-(2-hydroxyphenyl)-10,15,20-triphenylporphyrin (CoTPOH) was investigated.33 The calculated percent yield for H2O from RRDE experiments are shown in Figure 4 and other experimental details are presented in the Supporting Information. The presence of the 2-hydroxyphenyl group somewhat improves the selectivity for the 4e– reduction of O2 to H2O, but not nearly to the same extent as the pyridyl group. At an applied 0.50 V applied potential (h = 0.73 V), the calculated yield of H2O for CoTPOH is about half of that for CoTPPy. The above comparison is consistent with demonstrations that the presence of a hydrogen (H) bond, or H+ donor, can strongly affect O2 reduction selectively.11–15,29 The 2-pyridyl group is a special case. It is unlikely that it serves as a direct proton relay due to its large distance from the iron site or activated intermediates.15 Another importance factor is the difference in pKa of the


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pyridine and phenol. The pyridine group is likely a more suitable acid than phenol for promoting protonation of the distal O atom in activated intermediates with intact O-O bonds. Differences in pKa of other nitrogen containing proton relays in related iron porphyrins are proposed to play a central role in catalysis.29 To evaluate the effect of pH on the influence O2 reduction by CoTPPy, RRDE experiments were carried out at pH 0, 1.52, 2.37, and 6.25. Above pH 0, the current onset moved toward more negative potentials and much less H2O was formed (see Supporting Information). For experiments at pH 6.25 using CoTPPy effectively 100% H2O2 was formed at potentials above –0.1 V. At the same applied overpotential (1 V, corrected for pH), CoTPP makes no H2O while CoTPPy makes about 80%. We attribute this behavior to the protonation state of the pyridine. While the pKa of [CoTPPy-H]+ is not known, it is unlikely to be much larger than the pKa of pyridinium (5.17).34 As noted above, CoTPP and CoTPPy are electronically similar, so the difference in selectivity is most likely due to the protonation state of the 2-pyridyl group. The selectivity and kinetics of O2 reduction by CoTPPy were further explored using RDE experiments. Current-potential curves obtained from the RDE experiments for CoTPPy on an EPG disk are shown in Figure 5a. Koutecky-Levich (K-L) plots, jlim–1 vs. ω–1/2; where jlim is the limiting current density obtained at a given angular rotation rate (ω), also as are set in Figure 5, along with theoretical K-L plots corresponding to 2e– (to H2O2) and 4e– (to H2O) processes also are shown in Figure 5b. The linear fit for experimental K-L plot for CoTPPy is parallel to that obtained for the theoretical K-L plot for n = 4, corresponding to the 4-electron reduction of O2 to H2O. The value of n calculated from the linear fit of the experimental data was 4.0. The calculated values vary with applied potential, with a 2-electron pathway dominating at lower overpotential (see Supporting Information). The results from these RDE experiments mirror those described above using RRDE.


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Figure 5. (a) Linear sweep voltammograms obtained from RDE experiments under 1 atm O2 in 1 M aqueous H2SO4 for CoTPPy immobilized onto rotating EPG disk electrode at different rotations (100 rpm – 2000 rpm) (b) Koutecky-Levich (K-L) plot for CoTPPy using current density values at 0.4 V (n), and the theoretical K-L plots for 2e– () and 4e– (l) processes are shown for comparison. The kinetics of aqueous O2 reduction by CoTPPy and ClFeTPPy deposited on graphite were evaluated using the intercepts from K-L plots constructed from plateau currents (see Supporting Information). The second order catalytic rate constants, kcat, for CoTPPy and ClFeTPPy adsorbed onto EPG are calculated as 2.9 ´ 107 M–1s–1 and 1.4 ´ 107 M–1 s–1, respectively. These values are very similar to related iron tetraarylporphyrins with one functionalized phenyl group.29 It is noteworthy that CoTPPy achieved a factor of 2 higher kcat at 230 mV lower values of η than for ClFeTPPy under identical conditions. In comparison to related studies of immobilized ORR catalysts in water, these values are slightly larger than reported for more elaborate iron porphyrins containing one nitrogen-based Brønsted acid/base group (investigated at pH 7)29 and much faster than for simple cobalt porphyrins.18 K-L plots also were constructed at different potentials between the Ecat/235 and the plateau current as a means to probe the turnover kinetics at different applied potentials (see Supporting Information for further details). TOF values were determined using the calculated kcat and the


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concentration of O2 (0.56 mM). The resulting turnover-overpotential relationship for CoTPPy and ClFeTPPy are shown in Figure 6. The slope before the plateau region for CoTPPy is about 170 mV per decade and for ClFeTPPy is about 110 mV per decade. At the potentials investigated, the current is potential dependent. As previously pointed out for iron porphyrins, the reduction of FeIII to FeII is rate limiting under these conditions.36 Likewise, the slope of the plot for CoTPPy is consistent with rate limiting electron transfer, presumably associated with the large intrinsic barriers associated with the CoIII/II redox couple.37 We note that below overpotential values of 0.7 V, the mechanism is changing to favor 2-electron reduction of O2 (Figure 5)

Figure 6. Turnover-overpotential relationships for CoTPPy and ClFeTPPy. TOF values were determined from K-L plots constructed using RDE data collected in in 1 M aqueous H2SO4 under 1 atm O2.

Conclusions Heterogeneous electrochemical O2 reduction was investigated using graphite electrodes coated with CoTPPy and related metalloporphyrins. Analysis using RRDE and RDE experiments using CoTPPy showed that O2 is reduced to H2O, except at very low overpotentials where the yield of


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H2O2 increases. A 2-electron process was confirmed under our conditions for closely related CoTPP. The change in selectivity by replacing one phenyl group of the TPP ligand with a 2-pyridyl group is remarkable. It is unlikely that the 2-pyridyl group can serve as a direct proton relay,15 but RRDE experiments at different pH values show that the protonation state of the pyridyl group has an important impact on selectivity for H2O versus H2O2. In addition to the change in selectivity enhancement for CoTPPy, >200 mV improvement in overpotential (at TOFmax) with respect to ClFeTPPy. Cobalt porphyrins typically have higher metalIII/II reduction potentials than do Feporphyrins. As such, Co porphyrins give access to O2 reduction chemistry at lower potentials.20,21,26,30 In this work, the shift in overpotential is accompanied by a factor of 2.5 increase in TOFmax. The incorporation of a single, strategically placed nitrogen in CoTPPy gives rise to selective 4electron reduction of O2. Analogies can be draw with highly active O2-reducing materials that incorporate a first row metal and nitrogen on a carbon support.38–40 While CoTPPy and ClFeTPPy are not as active as those materials, such immobilized molecular catalysts serve as a point of connection between homogeneous catalysts and heterogeneous materials. Importantly, we think that incorporation of features from molecular catalysts (or catalysts themselves) into functional materials provide new avenues for proton-coupled redox catalysis. The modifiable features of coordination complexes may provide route to more active materials for electrocatalysis.

ASSOCIATED CONTENT Supporting Information is available free of charge via the Internet at http://pubs.acs.org. The following files are available free of charge: PDF with additional characterization data, cyclic voltammetry experiments, rotating ring-disk electrochemistry experiments, rotating disk electrochemistry experiments, and related analyses.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Jeffrey J. Warren: 0000-0002-1747-3029 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Simon Fraser University, the Natural Sciences and Engineering Research Council (NSERC, RGPIN05559 to J.J.W.), and the Canadian Institute for Advanced Research Bio-Inspired Solar Energy and Azrieli Global Scholars Programs supported this work. Indian Institute for Science and Education Research (IISER-Kolkata) provided travel support for M.G. Notes There are no conflicts to declare. ACKNOWLEDGMENT We thank E. K. Berdichevsky for assistance with MALDI data acquisition. REFERENCES (1) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729–15735.


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TOC Graphic


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Changing the selectivity of O2 reduction catalysis with one ligand

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