A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Abstract Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H+/4 e− process, while oxygen can be fully reduced to water by a 4 e−/4 H+ process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2 −. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

an argon atmosphere and stored over molecular sieves (4 Å) upon use. 1-Phenyloctane used for the liquid-STM measurements was vacuum distilled prior to use. TLC was performed by using Fluka silica gel (0.2 mm) on aluminium plates. Silica-gel columns for chromatography were prepared with silica gel 60 (0.060-0.20 mesh ASTM) from Acros.
Edge plane graphite (EPG) electrodes were purchased from Pine Instruments, USA. Highly oriented pyrolytic graphite was purchased from NT-MDT (ZYB grade, 1 cm 2 ).

Instrumentation
Proton ( 1 H NMR) and carbon ( 13 C NMR) spectra were recorded on a Bruker Ascend 700 spectrometer equipped with a cryogenically cooled probe (TXI), or on a Bruker Avance 500 MHz spectrometer. 19 F NMR spectra were recorded on a Bruker Avance 300 MHz at 282.4 MHz. The chemical shifts are given in parts per million (ppm) on the delta scale (δ) and are referenced to the residual non-deuterated solvent for 1 H and TFA for 19 F. MALDI-TOF measurements were collected with a Bruker Autoflex III Smartbeam spectrometer and on an Agilent atmospheric pressure photoionization (APPI) source on an Agilent 6520 quadrupole time-of-flight (QTOF) in the positive mode. UV-Vis absorption spectra were measured on a Varian CARY 100 Bio spectrophotometer. All electrochemical experiments were performed using a CH instruments (CHI720D) electrochemical analyzer. Bipotentiostat, reference electrodes, and counter electrodes were purchased from CH instruments. RRDE data was collected using the RRDE setup from Pine Research Instrumentation (E6 series with Changedisk tips with AFE6M rotor)

Construction of modified electrodes
Physisorption of the catalyst on EPG electrode: 100 µL of 1 mM catalyst in chloroform was deposited on a freshly cleaned EPG electrode mounted on a RRDE setup. Once solvent was fully evaporated, the surface was thoroughly dried with N 2 gas and rinsed with chloroform and ethanol. Finally, the surface was rinsed with triply distilled water before using it in electrochemical experiments.

Cyclic Voltammetry experiments
All homogeneous CV experiments were performed in acetonitrile solution containing 1 mM catalyst and 100 mM TBAP (supporting electrolyte). All heterogeneous CV experiments were performed in buffer solution as mentioned. Buffer solution contained 100 mM Na 2 HPO 4 and 100 mM KPF 6 . The mentioned pH was adjusted by adding NaOH and H 3 PO 4 . In all cases platinum and Ag/AgCl were used as counter electrodes and reference electrodes, respectively.

Coverage Calculation
The coverage for a particular species was determined by integrating the oxidation and reduction currents of the respective species. [1,2] Partially Reduced Oxygen species A Platinum ring was polished first by alumina powder of grit size of 1, 0.3, 0.05 µm and then electrochemically cleaned. An EPG electrode was also cleaned by polishing and both were inserted into the RRDE tip. Then the RRDE tip was mounted on the rotor and immersed into a cylindrical glass container equipped with platinum and Ag/AgCl electrodes. The collection efficiency (CE) of the platinum in the RRDE setup was estimated in a 2 mM K 3 Fe(CN) 6 and 100 mM KNO 3 solution at a rotation speed of 300 rpm and 10 mVps scan rate. Generally, a 15 ± 2% CE was observed during these experiments. For the detection of H 2 O 2 by these experiments platinum was held at a constant potential was taken from literature. [1][2][3] For normal measurement of PROS, the ratios of ring and disk currents were taken at the potential where the Pt ring electrode exhibits the maximum current during RRDE experiments.

STM of manganese corroles on Ag(111) and HOPG
For the single-molecule investigations presented here, we chose a single-crystal surface of the silver as substrat. The Ag(111) substrate was prepared by cycles of Ar + ion sputtering (600 eV) and thermal annealing at 720 K. Mn(TpFPC) was sublimated at ultra-high vacuum conditions (base pressure <10 −9 mbar) from a quartz crucible at 500 K onto the substrate kept at room temperature and subsequently annealed at 360 K for 10 minutes. After precooling to 100 K, the samples were in-situ transferred into the STM chamber. STM experiments were performed at 5 K and a base pressure of <10 −10 mbar employing electrochemically etched tungsten tips that were thermally deoxidized by flash-annealing to above 1070 K. STM images were analyzed using the program WSxM. [4] The dI/dV signal was obtained with lockin technique adding a sinusoidal modulation (V=20 mV and ν=757 Hz) to the dc bias. Total-energy density functional calculations (DFT) have been performed using the Vienna Ab Initio Simulation Package [5] including the projector-augmented wave method [6] for describing electron-ion interactions and the generalized gradient approximation (PW91 functional) [7] for modeling electron-electron exchange-correlation interactions. STM simulations were obtained via the Tersoff-Hamann model. [8] The STM measurements at the solid-liquid interface were performed at the HOPG-1-phenyloctane interface using a homebuilt liquid-STM. [9] A 10 µL droplet of manganese corrole solution ([2] =10 -5 M) was brought onto a freshly cleaved HOPG surface and the STM tip (Pt/Ir 80/20) was immersed in it.