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.

Abstract: Oxygen reduction and water oxidation are two key processes in fuel cell applications.T he oxidation of water to dioxygen is a4 H + /4 e À process,w hile oxygen can be fully reduced to water by a4e À /4 H + process or partially reduced by fewer electrons to reactive oxygen species such as H 2 O 2 and O 2 À .W edemonstrate that anovel manganese corrole complex behaves as ab ifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore,o ur combined kinetic,s pectroscopic,a nd electrochemical study of manganese corroles adsorbed on different electrode materials (down to as ubmolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.
Innature,the oxidation of water is catalyzed by the Mn 4 Ca inorganic unit embedded in the D1 protein subunit of photosystem II (PSII). [1] Thew ater oxidation complex of PSII has inspired aw ide range of model molecular water oxidation catalysts (WOCs). As in the WOCo fP SII, high oxidation states of the metals are generally stabilized by introducing electron-donating ligands with oxygen and nitrogen donor atoms. [2] Thed evelopment of synthetic catalysts which are able to mediate water oxidation and dioxygen reduction under mild conditions with am inimized energy demand has become an appealing challenge. [3] Recently, many different examples of these molecular catalysts were described in detail by kermark and co-workers. [4] Homogeneous and heterogeneous molecular catalysts offer attractive features,s uch as controllable redox properties,e ase of mechanistic investigation, and available strategies for the characterization of reactive intermediates. [5] Av ariety of homogeneous and heterogeneous water-oxidation catalysts based on transition metals have been developed, including complexes of Mn, Ru, Ir, and Co. [4,6] All of these catalyst systems are proposed to proceed through ah igh-valent intermediate. [7] Corroles are trianionic ligands known to stabilize metal ions in their high-valent oxidation states. [8] Nocera and co-workers reported ah angman Co corrole for the efficient oxidation of water. [9] Co corrole complexes are also reported for selective 4e À /4 H + oxygen reduction reactions. [10] In contrast, Mn corroles are vastly unexplored as potential O 2 evolution catalysts,a lthough Mn is the metal responsible for natural water-oxidation processes.The corrole macrocycle can stabilize the oxidation state of Co only as high as VI, but that of Mn up to VII. Moreover,t he corrole macrocycle,incontrast to the closely related porphyrin-based systems,t ends to be involved as an on-innocent ligand, forming p-radical species. [11] Thes tabilization of higher oxidation states determines the extraordinary properties of manganese(V)-oxocorroles as oxygen atom transfer reagents for the epoxidation of alkenes [12] or the O À Obond-formation step in the artificial photosynthetic oxidation of water. [13] Water oxidation is considered to be the bottleneck of the water splitting reaction. [14] Hence,there is astrong motivation to develop catalysts that can oxidize water efficiently under ambient conditions.The development of efficient bifunctional catalysts has been afast evolving area. [15] Although there are some reports of bifunctional catalysts for the reversible conversion of protons into H 2 ,r eports on nonprecious metal based electrocatalysts which can efficiently oxidize water to oxygen and also reduce oxygen are rare.Such catalysts are in high demand. [16] In the present study we have investigated the adsorption performance of manganese 5,10,15-tris(penta-fluorophenyl)corrole (MnTpFPC) analogues [17] on different electrode materials in athin-film phase as well as down to the scale of individual molecules.W eh ave developed aw atersoluble bifunctional electrocatalyst 1 with asingle Mn site for the homogeneous and heterogeneous oxygen evolution reaction (OER) and homogeneous and heterogeneous oxygen reduction reaction (ORR) in aqueous solution ( Figure 1).
Cyclic voltammetry (CV) studies of complex 1 in CH 3 CN solution shows two one-electron oxidations corresponding to the Mn III /Mn IV (0.53 Vv ersus Ag/AgCl) and Mn IV /Mn V (0.78 Vv ersus Ag/AgCl) processes.T here is an irreversible catalytic wave at ap otential E p,a = 1.38 Vw hich appears following the oxidation of Mn III to Mn V ,t hus suggesting the presence of an irreversible anodic process.T he waveform for the Mn IV /Mn V couple at E 1/2 = 0.73 Vi si ndependent of the scan rate (n)u nder these conditions.I ts peak current (i p ) varies linearly with n 1/2 (inset of Figure 2) and is consistent with diffusion-limited electron transfer at the electrode. [18] Thed iffusion coefficient, D cat = 2.32 10 À7 cm 2 s À1 ,w as obtained from the scan-rate dependence of i p (inset of Figure 2) using the equation i p = 0.4463 n p FAC cat. (n p FnD/ RT) 1/2 ,w here i p , F, A, C cat. , n, D, R,a nd T are the maximum noncatalytic current, Faraday constant, area of the electrode, bulk concentration of the catalyst, scan rate,d iffusion coefficient of the catalyst in solution, ideal gas constant, and temperature,r espectively. [19] At slow scan rates,n early ideal plateau wave shapes were observed, reaching ac urrent maximum at 1.3 V. Reproducible CV measurements further indicate that the catalyst is stable following multiple catalytic turnovers.
Thecatalytic evolution of oxygen by 1 was investigated in acetonitrile solution containing 100 mm Bu 4 NClO 4 (TBAP) by adding different concentrations of abase (NaOH in water; Figure 3A). As the concentration of the base was gradually increased from 1mm to 25 mm,asharply increasing irreversible current was obtained. Thep lot of (i c /i p ) 2 versus the concentration of the base ( Figure 3B)shows alinear relationship,thus indicating that the process is pseudo-first order with respect to the base.E volution of O 2 was confirmed by cathodic scans,where the O 2 evolved in the anodic sweep was detected during the cathodic sweep (where it was reduced by 1). Using the equation [20] the pseudofirst order rate constant (k cat. )i sc alculated to be 11.4 s À1 at    ) showing homogeneously increasing catalytic oxygen evolution with increasing base concentration from 1mm NaOH to 25 mm NaOH at ascan rate of 50 mVs À1 .Aglassy carbon electrode was used as the working electrode. B) Plot of (i c /i p ) 2 versus conc. of NaOH (in mm)in ah omogeneous OER in acetonitrile at ascan rate of 100 mVs À1 ;linear fit (y = 22.227 x, R 2 = 0.966) Angewandte Chemie aconcentration of 25 mm NaOH and ascan rate of 10 mV s À1 . In this equation, I c is defined as the maximum catalytic current and the other terms are as mentioned above.
With the aim of testing manganese corroles as heterogeneous catalysts,i nt he next step we investigated 1) the adsorption behavior and 2) the electronic properties of Mn corroles on different solid surfaces by cyclic voltammetry, scanning tunneling microscopy (STM), and spectroscopy.W e selected substrate materials such as graphite and silver,which are highly relevant as electrode materials for heterogeneous catalysis.M oreover,w eh ave studied 2 and 3 at the solidliquid, [21,22] and the solid-vacuum interface-thus covering aw ide range of catalytic interfaces for application. A prerequisite for STM studies under ultrahigh vacuum conditions is that the molecules can be carefully evaporated onto the substrate without any destruction of the molecules.W e chose compound 3 for these experiments.O ctadecylc hains need to be attached to the corrole macrocycle (compound 2) to obtain proper orientation and adsorption for the STM experiments at the solid-liquid interface. [22] Figure 4A shows at ypical STM image obtained at the solid-liquid interface of as elf-assembled monolayer of 2 on the basal plane of highly ordered pyrolytic graphite (HOPG). Them onolayer was prepared by exposing the HOPG substrate to a1 0 mm solution of 2 in 1-phenyloctane at 295 K. In the image,l amellar arrays are visible in rotational domains of approximately 1208 8.Inthese lamellae,which have ap eriodicity of 4.4 AE 0.2 nm, single molecules of 2 can be distinguished (marked by ar ed circle in Figure 4A). The molecular layer clearly exhibits regions with diameters > 20 nm and short-range order,b ut it is rather disordered at longer distances.T he individual molecules are imaged as elongated disklike shape features,t hus suggesting their aromatic planes are oriented approximately parallel with respect to the surface.O ccasionally,b righter features are observed with an apparent height of approximately twice that of the majority of the other molecules in the layer, which may indicate the presence of face-to-face stacked dimers. Repeated scanning of the same area did not result in detectable changes in the appearance of the molecular layer, thus indicating that the molecules remain stable at the solid-liquid interface.
In contrast, Mn corroles were found to form highly ordered monolayer films at as olid-vacuum interface.T he STM image of Figure 4B shows am onolayer of manganese corroles 3 immobilized on as ingle-crystal Ag(111) surface under ultrahigh vacuum at 5K.Ahigh degree of order is evident. Figure 4C-E depict STM images recorded with increased magnification at different sample bias voltages. Compared to the results obtained at the solid-liquid interface, intramolecular features of the individual Mn corrole molecules within the layer are clearly resolved. As ag uide to the eye,wehave overlaid the structure model on the STM images to facilitate the assignment of the STM topography to different structural elements.T he different structural elements of the manganese corrole molecule are discernible (metal center,p yrrole rings of the macrocycle,p entafluorophenyl substituents;s ee also Figure S5 and XPS data in Figure S6). Similar to our results on HOPG,w eh ave found that manganese corrole molecules adsorb nondissociatively on Ag(111), with their macrocycles oriented approximately parallel to the substrate surface.T he high spatial resolution and exceptional drift stability obtained at the solid-vacuum interface at low temperatures ( Figures 4B-E) facilitates local spectroscopy of the frontier orbital electronic structure of different functional groups on single Mn corrole molecules within the layer. Figure 4F (bottom) shows typical tunneling  Figure S5). [26] Angewandte Chemie Communications 2352 www.angewandte.org conductance spectra obtained with the STM tip fixed at ac onstant height over the macrocycle (black curve) and the Mn center (red curve) of single manganese corrole molecules 3 adsorbed on Ag(111).
Forcomparison, the top part of Figure 4F shows the cyclic voltammogram of 1 in solution. Notice that the energy axes of both methods are related and, thus,c an be interconverted. Foraproper comparison, the energy values from the two methods have to be referred to ac ommon zero level, for example,a sr epresented by the common vacuum level. A well-established referencing procedure has been described by Mazur et al. [23] Thei nterconversion between different reference cathodes is explained in Ref. [24];t he work function of Ag(111) is 4.74 eV. [25] Ad etailed analysis reveals that the half-wave potential energies of the first oxidation (observed by cyclovoltammetry, upper curve) lie close to the onset of the broad dI/dV peak, which originates from tunneling out of occupied metalcentered molecular orbitals lying close below the Fermi level of Ag(111) (lower red curve and simulated STM image in Figure 4F). This finding indicates that the active center of the surface-immobilized catalyst exists as Mn III and, most probably,exhibits similar catalytic properties as the dissolved catalyst in solution. This is ap romising result in view of expanding the potential applications of manganese corroles towards heterogeneous catalysis at the solid-gas interface.
Indeed, catalytic oxygen evolution was confirmed by varying the pH value of the buffer ( Figure 5) after immobilizing the catalyst 1 on an edge plane pyrolytic graphite (EPG) electrode.T he sample was purged with argon before each scan to remove O 2 .O nly instantly generated O 2 during oxidative scans was detected on reverse scans at À0.3 V versus NHE (peak potential (E p )f or the O 2 /O 2 À couple). At pH 7.0, very slow OER kinetics were detected at potentials > 1.3 V. However,a st he pH value of the buffer was increased, enhanced electrocatalytic OER currents were observed, and at pH 11.0 high, mass transfer limited, catalytic current was observed. 1 is capable of reducing the O 2 which was produced during the OER (see Figure 5, inset). The generated electrocatalytic current increases at ap otential of about À0.35 V. Evidence of oxygen evolution also resulted from rotating ring-disk electrochemistry (RRDE, Figure 6A) studies on physisorbed catalyst 1 on an edge plane graphite (EPG) electrode under anaerobic conditions,w ith the platinum at ac onstant potential of 0.3 V. Theg enerated oxygen was reduced by platinum in situ. TheR RDE experiments suggest an ormal substrate diffusion limited current. The hydroxide oxidation current increases in accordance with the Koutecky-Levich Equation discussed below.Alinear plot was observed when i À1 was plotted against the inverse square root of the angular rotation rate (w À1/2 ;i nset of Figure 6B). From the intercept of this plot, the second order rate constant was determined to be 1.55 10 4 m À1 s À1 .During the process of oxygen evolution, RRDE experiments were performed to detect whether any partially oxidized species were generated. To do so,aPt ring was kept at ac onstant potential of 0.2 V, where Pt can oxidize the generated partially oxidized species, Figure 5. Anaerobic cyclic voltammogramof1 immobilized on an edge plane pyrolytic graphite (EPG) electrode on varying the pH value from pH 7.0 to pH 11.0, which indicates the bifunctional nature of the catalyst. The inset shows the zoomed portion of the ORR where produced oxygen during the OER gets reduced. Figure 6. A) RRDE data of 1 physisorbed on an EPG electrode in pH 11.0 buffer at aconstant rotation of 300 rpm and scan rate of 10 mVs À1 ,w ith platinum held at aconstant potentialo f0.3 Vwhere it reduced the oxygen generated during the OER ( Figure S8). B) Linear sweep voltammograms of immobilized catalyst on an EPG electrode at ascan rate of 50 mVs À1 on varying the rotation rate from 250 to 500 rpm. The inset shows the Koutecky-Levich plot (I À1 versus n À1/2 ) from which k cat. was calculated;linear fit (y = À0.0045 x, R 2 = 0.993).
such as H 2 O 2 and O 2 À .F rom the RRDE data we could determine the generation of partially oxidized species at ayield as low as 2.1 %( Figure S7). To calculate the turnover number (TON) and the turnover frequency (TOF),w e physisorbed the catalyst on an edge plane graphite electrode and performed controlled potential electrolysis at apotential of 1.4 Vv ersus Ag/AgCl for 11.1 h.
AT ON of 1.90 10 4 over 11.1 hw as obtained, thus illustrating that the catalyst is highly stable at the electrode surface.T he TOFw as calculated to be 0.47 s À1 ( Figure S9). Finally,w em easured the generated oxygen by means of an inverted burette technique.AFaradaic efficiencyof82% was calculated ( Figure S10).
When the catalyst was physisorbed on an edge plane graphite electrode in the absence of oxygen at pH 7.0, ar eversible one-electron reduction was conducted at E8 8 1/2 % À0.25 V( versus Ag/AgCl;F igure 7A). In air-saturated buffer, al arge irreversible oxygen reduction current superseded the reversible process.The onset potential of this large reduction current is responsible for the catalytic reduction of oxygen. As econd electrocatalytic ORR was observed at À0.6 V, which also catalyzes the ORR process.Anormal substrate diffusion limited current was observed at potentials below À0.6 Vw hen rotating disk electrochemistry (RDE; Figure 7B)w as performed. When the rotation rate was increased, the O 2 reduction current increased according to the Koutecky-Levich Equation, i À1 = i K (E) À1 1 + i L À1 ,w here i K (E) is the potential-dependent kinetic current and i L is the Levich current. i K (E) and i L are defined as nFA[O 2 ]k cat. G catalyst and 0.62 nFA[O 2 ](D O 2 ) 2/3 w 1/2 n À1/6 ,respectively,where n is the number of electrons transferred to the substrate, A is the macroscopic area of the disk (0.096 cm 2 ), [O 2 ]i st he concentration of O 2 in an air-saturated buffer (0.26 mm)at258 8C, k cat. is the second order rate constant of the electrocatalytic reduction of O 2 , G catalyst is the catalyst concentration in moles cm À3 , D O 2 is the diffusion coefficient of O 2 (1.9 10 À5 cm 2 s À1 )a t2 58 8C, w is the angular velocity of the disc, and n is the kinematic viscosity of the solution (0.009 cm 2 s À1 ) at 25 8 8C. [19] Al inear plot was observed when i À1 values at multiple rotation rates were plotted against the inverse square root of the angular rotation rate (w À1/2 ;i nset of Figure 7B). The number of electrons (n)i nvolved in the O 2 reduction by ac atalytic species may be calculated from the slope,a nd the second order rate of catalysis (k cat. )i so btained from the intercept of this linear plot. Thes lopes obtained from the experimental data at different potentials in the substrate diffusion controlled region are consistent with the theoretical slope presumed for a2.3 e À process.This is in accordance with the RRDE data, which shows approximately 90 %p artially reduced oxygen species (PROS) and is consistent with previous reports on manganese corroles. [13,27] Thei ntercept of the Koutecky-Levich plot can be used to evaluate the k cat. value (i K = nFA[O 2 ]k cat. G catalyst ). At À0.64 V( versus Ag/ AgCl), the rate of the 2e À reduction of O 2 is 3.81 10 3 m À1 s À1 .
To conclude,t he manganese corrole complex 1 exhibits bifunctional character in aqueous solution by oxidizing hydroxide ions through af our-electron process in weak to moderate alkaline conditions to molecular oxygen and reducing O 2 in at wo-electron process to hydrogen peroxide. Thec omplex can produce as teady OER current with af aradaic efficiency of > 80 %o ver 11 h. Thes electivity, fast kinetics,a nd stability of this complex suggest that manganese corroles should definitely be considered as potential candidates for bifunctional catalysts for the reversible conversion of oxygen into water. Our combined STM results-obtained at the solid-liquid and solid-vacuum interface-have revealed 1) the nondissociative and regular adsorption of manganese corrole molecules on electrode surfaces and 2) intact electronic properties of manganese corroles.B oth results are crucial for heterogeneous catalysis at the solid-liquid as well as solid-gas interfaces. The dotted and dashed lines are used to denote the theoretical plots for 2e À and 4e À ,r espectively.