A Bifunctional Electrocatalyst for OER and ORR based on a Cobalt(II) Triazole Pyridine Bis‐[Cobalt(III) Corrole] Complex

Abstract As alternative energy sources are essential to reach a climate‐neutral economy, hydrogen peroxide (H2O2) as futuristic energy carrier gains enormous awareness. However, seeking for stable and electrochemically selective H2O2 ORR electrocatalyst is yet a challenge, making the design of—ideally—bifunctional catalysts extremely important and outmost of interest. In this study, we explore the application of a trimetallic cobalt(II) triazole pyridine bis‐[cobalt(III) corrole] complex CoIITP[CoIIIC]2 3 in OER and ORR catalysis due to its remarkable physicochemical properties, fast charge transfer kinetics, electrochemical reversibility, and durability. With nearly 100 % selective catalytic activity towards the two‐electron transfer generated H2O2, an ORR onset potential of 0.8 V vs RHE and a cycling stability of 50 000 cycles are detected. Similarly, promising results are obtained when applied in OER catalysis. A relatively low overpotential at 10 mA cm−2 of 412 mV, Faraday efficiency 98 % for oxygen, an outstanding Tafel slope of 64 mV dec−1 combined with superior stability.


Materials and methods
Chemicals were purchased from Fluka, Alfa Aesar, Sigma-Aldrich, and Merck.Pyrrole was freshly distilled before use.5-(4-tert-Butylphenyl)-dipyrromethane [1] , 3-triisopropylsilylpropynal [2] , and 2-(azidomethyl)pyridine [3] were synthesized according to reported literature procedures.DCM was obtained using a M. Braun Inert Gas System GmBH where it is stored over molecular sieve MB-SPS-7 under argon atmosphere.NMR solvents were purchased from Sigma-Aldrich.TLC was performed on Macherey-Nagel silica gel 60 (0.20 mm) with fluorescent indicator UV254 on aluminium plates and on Merck aluminium oxide 60 (0.20 mm) with fluorescent indicator UV254 on aluminium plates.For chromatography, silica-gel columns were prepared with silica-gel 60 (0.070-0.20 mesh) from Grace and aluminium oxide 60, basic, activity level II from Acros. 1 H and 13 C NMR spectra were recorded on a Bruker DRX 500 MHz spectrometer and a Bruker Advance 300 MHz NMR spectrometer.Chemical shifts are given in parts per million (ppm) on the delta scale (δ) and are referenced to the used deuterated solvent for 1 H-NMR.High resolution mass spectra were obtained using an Agilent 6520 Q-TOF mass spectrometer with an ESI source and an Agilent G1607A coaxial sprayer and a Thermo Fisher Scientific LTQ Orbitrap XL with an Ion Max API Source.MALDI-TOF was measured on a .UV-Vis absorption spectra were collected on a Varian CARY 300 Bio spectrophotometer from 200 to 900 nm.

Synthesis of 5-(4-t-Butylphenyl)-dipyrromethane:
4-t-butylbenzaldehyde (0.9234 g, 5.7 mmol), and pyrrole (39.5 ml, 570 mmol) were mixed under inert atmosphere.InCl3 (0.154 g, 5.7mmol) was added and the mixture was stirred for 2h at room temperature.After that, powdered NaOH (0.7 g) was added and the mixture was stirred for an additional 1h, the mixture was diluted with heptane and evaporated to dryness to obtain the product as off white powder (1.39 g, 87%), 1
The mixture is diluted with DCM and washed with H2O thrice.Organic fraction was collected, dried with Na2SO4 and solvents evaporated under reduced pressure.Purification with column chromatography (silica, DCM/Heptane, [1:2]) to obtain the product as red solid (150 mg, 87%) Rf=0.55, 1

For
the EIS measurements of OER, first two platinum electrodes were measured in a one cell compartment with the corresponding electrolyte as a control experiment to determine the electrolyte resistance.Afterwards, the setup was transferred to a H-cell configuration with Nafion membrane inbetween.By this way the resistance of the membrane is found and immediately substracted from the electrolyte resistance.Next, one platinum electrode was replaced by a carbon paper electrode as a working electrode.Finally, the carbon paper coated with the Co II TP [Co III C]2 3 was set as working electrode, to evalute the complete electrochemical cell by EIS.All fitted and calculated impedance data are summarized in the table S5.The bode plot for the two-electrode system is shown in the manuscript.All resistance values for each cell components, i.e., electrolyte solution, membrane, carrier electrode are shown in tables S4 and S5 for ORR and OER cell systems, respectively.Based on EIS, the applied electrochemical cells were characterized in detail indicating negligible losses of the systems.

Figure S21 .Figure S22 .Figure S23 .
Figure S21.Chronoamperometric measurements of Co II TP[Co III C]2 3 under ambient atmosphere at 1600 rpm to prove the long-term stability of the catalyst.

Table S1 .
Elemental identification and quantification from XPS analysis of Co II TP [Co III C]2 3.

Table S5 .
Cell parameter extracted via electrochemical impedance measurements for OER.