CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402533

Screen-Printed Calcium–Birnessite Electrodes for Water Oxidation at Neutral pH and an “Electrochemical Harriman Series” Seung Y. Lee,[a] Diego Gonzlez-Flores,[b] Jonas Ohms,[a] Tim Trost,[a] Holger Dau,[b] Ivelina Zaharieva,*[b] and Philipp Kurz*[a] A mild screen-printing method was developed to coat conductive oxide surfaces (here: fluorine-doped tin oxide) with micrometer-thick layers of presynthesized calcium manganese oxide (Ca–birnessite) particles. After optimization steps concerning the printing process and layer thickness, electrodes were obtained that could be used as corrosion-stable water-oxidizing anodes at pH 7 to yield current densities of 1 mA cm2 at an overpotential of less than 500 mV. Analyses of the electrode coatings of optimal thickness (  10 mm) indicated that composition, oxide phase, and morphology of the synthetic Ca–birnessite particles were hardly affected by the screen-printing procedure. However, a more detailed analysis by X-ray absorp-

tion spectroscopy revealed small modifications of both the Mn redox state and the structure at the atomic level, which could affect functional properties such as proton conductivity. Furthermore, the versatile new screen-printing method was used for a comparative study of various transition-metal oxides concerning electrochemical water oxidation under “artificial leaf conditions” (neutral pH, fairly low overpotential and current density), for which a general activity ranking of RuO2 > Co3O4  (Ca)MnOx  NiO was observed. Within the group of screened manganese oxides, Ca–birnessite performed better than “Mnonly materials” such as Mn2O3 and MnO2.

Introduction In biology, the processes of photosynthesis harvest and convert solar energy and store it chemically in the form of different reduced carbon species.[1, 2] Within this complicated chain of events, the oxidation of water to molecular oxygen is one of the key reactions [Eq. (1)], as it yields the reduction equivalents needed for CO2 fixation from the following reaction: 2 H 2 O ! O 2 þ 4 H þ þ 4 e

ð1Þ

In vivo, only one single catalyst for this reaction has been developed through biological evolution, that is, the CaMn4Ox cluster, which forms the oxygen-evolving complex (OEC) of photosystem II.[2–5] Many scientific details concerning the structure and mechanism of the OEC have emerged over the last decades.[3, 6] This scientific basis led to the development of bioinspired manganese-based catalysts for water splitting, for which the materials could be a part of a device to produce O2 [a] S. Y. Lee, J. Ohms, T. Trost, Prof. Dr. P. Kurz Institut fr Anorganische und Analytische Chemie Albert-Ludwigs-Universitt Freiburg Albertstrasse 21, 79104 Freiburg (Germany) E-mail: [email protected] Homepage: www.bioanorganik.uni-freiburg.de [b] D. Gonzlez-Flores, Prof. Dr. H. Dau, Dr. I. Zaharieva Fachbereich Physik, Freie Universitt Berlin Arnimallee 14, 14195 Berlin (Germany) E-mail: [email protected] Homepage: www.physik.fu-berlin.de/einrichtungen/ag/ag-dau Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402533.

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and H2 from the virtually unlimited resources of solar (or wind) energy and water. The H2 product is envisioned to act as a renewable, storable, and convertible energy unit for society. If generated by using the energy of photons from the sun, such H2 would be a prime example of a so-called “solar fuel”.[3, 4, 7, 8] For most approaches following this line of thinking, the catalysis of the water-oxidation reaction has proved to be a major hurdle.[3, 7, 9–11] Concerning heterogeneous water-oxidation catalysts for (photo)electrochemical devices, it has been known for a long time that oxides of ruthenium and iridium are very good anode materials to catalyze the reaction outlined in Equation (1).[11–13] However, their high costs and limited availabilities most likely prevent their large-scale use. Oxides of cobalt and/or nickel have been identified as the second-best options already in early reports,[10–12] and because they are much cheaper than RuOx and IrOx, these materials have attracted much attention in recent years for possible applications in renewable energy-conversion devices. In consequence, significant advances in both the performance and the understanding of CoOx and NiOx anodes for water oxidation have been made.[10, 14] For manganese oxides, early experiments mostly reported unimpressive electrocatalytic properties.[12, 13] On the other hand, the research team of Harriman performed a screening of a number of metal oxides in 1988 concerning their water-oxidation activities in a photochemical reaction system, in which Mn2O3 performed very well.[11, 15] In their setup, [Ru(bpy)3]3 + (bpy = 2,2’’-bipyridyl) acted as a photochemically generated oxidation agent to drive the reaction in Equation (1). In the reChemSusChem 0000, 00, 1 – 11

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sulting ranking of promising oxide catalysts (the “Harriman age to, one, be able to optimize the properties of the oxides in series”), manganese(III) oxide ranks next to the oxides of cobalt separate synthesis steps and, two, coat the electrodes with and nickel as the second-best choice behind IrO2 and a thoroughly characterized material and thereby to define RuO2.[11, 15] A key difference between most electrochemical a clear starting point for future mechanistic studies. screenings and the photochemical experiments of Harriman is the pH regime: whereas electrochemical studies are mostly performed in either very alkaline or very acidic electrolytes, photochemical experiments require near-neutral conditions (pH 4–8) to avoid side reactions of the [Ru(bpy)3]3 + photosensiResults and Discussion tizer.[16] Electrode preparation We performed a number of investigations on the synthesis, characterization, and catalytic performance of manganese Fortunately, a large number of methods yielding oxide coatoxides in recent years to develop affordable and efficient cataings already exist. For these, it is known that parameters such lyst materials for water oxidation inspired by the elemental as thickness, binder, and sintering temperature have to be composition of the biological OEC.[17–19] In model reactions varied for each application. To find a suitable method for the with chemical oxidants such as Ce4 + and [Ru(bpy)3]3 + , layered preparation of Ca–birnessite-coated electrodes, we started manganese oxides from the mineral family of birnessites from two published procedures for screen printing of metal emerged as the most promising synthetic manganese-based oxides,[27, 28] one of which had already been used for MnOx [20] catalyst materials. In analogy to the OEC, we also observed anodes.[27] 2+ that the best results were obtained if Ca ions were incorpoIn a first round of experiments, we prepared four different rated into such birnessite oxides.[19] Furthermore, variations in types of electrodes by either spray painting or screen printing the birnessite synthesis protocol showed that a Ca/Mn ratio of synthetic Ca–birnessite onto fluorine-doped tin oxide (FTO) approximately 1:5 and a sintering temperature of about 400 8C substrates. For this, two different suspensions were used, in were very important to obtain high catalytic rates in water oxieach case containing 100 mg of particles of the same synthetic dation.[17] The result of these optimization rounds was a syntheCa–birnessite batch per 1.1 mL of suspension (oxide formula sis protocol for Ca–birnessites that show rates in chemical approx. K0.20Ca0.21MnO2.21·1.4 H2O; concerning the preparation, water-oxidation runs that are approximately 50 times faster characterization, and properties of the oxide materials, see than those of Mn2O3 (which already performed well in the HarRefs. [17, 19, 20]). In addition to the H2O and/or EtOH solvents, riman study). Of course, one would now like to use these adthese “inks” contained either polyethylene oxide (PEO; for elecvanced calcium manganese oxide catalysts in technical devices trode types 1 and 2) or ethyl cellulose (EC; for 3 and 4) as orsuch as electrolyzers and “artificial leaves”,[3, 21, 22] but to do so, ganic binders. The fabrication sequence for the electrodes is a mild fabrication method for durable Ca–birnessite-coated shown schematically in Figure 1 (for details, see the Experimenelectrodes has to be developed first. In most recent reports, tal Section and the Supporting Information, especially manganese oxide anodes for water oxidation have been preFigure S1). In the first experiments, we discovered that the anpared by electrosynthesis, that is, by the direct formation of nealing step, in which the organic components burn off and oxide films on an electrode kept at oxidizing potentials and the contact between the oxide and the electrode is estabplaced into aqueous solutions containing the appropriate lished, should be performed for at least 60 min at 450 8C to metal ions. This has been a very successful approach for cobalt obtain mechanically stable electrodes. As a result, we kept oxides,[23, 24] and the first promising results (good current densithese annealing conditions identical for the preparation of all ties, reasonable overpotential, etc.) have also been obtained the electrodes used in this study. for electrochemically produced MnOx electrodes.[25, 26] However, this route significantly limits the possible synthetic parameters for oxide preparation, especially concerning the controlled incorporation of secondary ions (such as Ca2 + ). In the study presented herein, we thus developed and compared protocols to deposit presynthesized calcium–birnessite particles (prepared according to the mentioned optimized protocol[17]) on electrode surfaces. In comparison to electrodeposited Figure 1. Fabrication sequence for Ca–birnessite-coated FTO electrodes by either screen printing or spray painting films, we consider it an advant- of oxide suspensions.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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CHEMSUSCHEM FULL PAPERS Electrocatalytic properties of Ca–birnessite anodes We then tested the four electrode types in commonly used electrochemical setups for water-oxidation catalysis (Figure S2).[24, 26, 27] For electrolysis, we focused on a neutral pH regime, because extremely acidic or alkaline electrolytes will most likely be incompatible with the delicate, multilayer semiconductor architectures envisioned for “artificial leaf” devices.[21, 22] First, cyclic voltammograms at oxidizing potentials were recorded with the electrodes immersed in a neutral phosphate buffer electrolyte. As shown in Figure 2 (left), in all four

www.chemsuschem.org EC anode 3 (Figure 2, c) also demonstrates nicely how little the current densities detected during CV measurements (total length of experiment  180 s) might translate into long-term chronoamperometric data that can be used for Tafel plots (300 s electrolysis time for each data point): whereas up to 10 mA cm2 is observed in the CV, not even a tenth of this value is reached in the longer term measurements of the “Tafel steps” (Figure S4). Even for the better binder PEO, we found that sprayed electrodes had much lower catalytic activity than printed ones. This might be explained by the thickness of the resulting Ca– birnessite layer on the substrate. A visual inspection showed that the catalyst film formed was thicker for printed electrodes than for sprayed ones (Figure S1). Recent papers suggest catalytic “bulk activity” of the entire porous birnessite material,[26, 30] and thus, our results strongly support this model, as we find that thicker birnessite layers result in higher catalytic currents.

Figure 2. Cyclic voltammograms (left) and Tafel plots (right) of Ca–birnessite electrodes fabricated along the route shown in Figure 1. Color code (see also Figure 1): c: printed PEO suspension (electrode type 1); c: sprayed PEO suspension (2); c: printed EC suspension (3); c: sprayed EC suspension (4), and c: FTO electrode treated with an oxide-free ink. CV scan rate: 20 mV s1, electrolyte: 1/15 m phosphate buffer, pH 7. See the Experimental Section and the Supporting Information for details.

cases catalytic water oxidation well above the current detectable for an FTO substrate coated by an oxide free ink can be observed for potentials above approximately + 1.3 V. Thus, all four electrode types are “active” but yield markedly different current densities [i, all potentials in this paper are given vs. normal hydrogen electrode (NHE)]. Tafel plots for electrocatalytic water oxidation were calculated from chronoamperometric experiments, for which the applied potential was increased stepwise from + 1.2 to + 1.6 V in 100 mV steps and held for 300 s at each step (Figure S4). The obtained current density traces are shown in Figure 2 (right). Clearly, the best results were found for printed PEO anodes, for which currents of 1 mA cm2 could be reached at overpotentials (h) for water oxidation of h = 500 mV. For electrolysis at pH 7, these numbers rank among the best that have so far been reported for any MnOx electrode.[26, 27, 29] In comparison, the currents measured for sprayed PEO electrodes were approximately a factor of five smaller. Worse still, both printed and sprayed electrodes originating from EC-containing suspensions quickly lost their electrocatalytic activity for potentials higher than + 1.2 V so that real Tafel data could not be determined (Figures 2 and S4). Thus, the recently reported coating method based on EC that was successfully used for alkaline conditions[27] seems unsuitable for electrolysis at neutral pH values (for which Ref. [28] lacks long-term data). A comparison of the cyclic voltammetry (CV) curve and Tafel plot for printed  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Influence of the oxide layer thickness on catalytic activity

To investigate the relationship between film thickness and catalytic activity in more detail, we prepared five PEO suspensions as before, but these suspensions now contained different amounts of Ca–birnessite powder (12.5–200 mg oxide per 1.1 mL of ink). Using these inks, electrodes of type 1 were printed, and the method that clearly emerged as the best preparation procedure from the first round of optimizations was followed. Investigating the anode coatings by scanning electron microscopy (SEM), one finds a generally smooth oxide layer on the millimeter scale (Figure 3 A) but then a highly irregular arrangement of the individual 1–10 mm oxide particles at higher magnification (Figures 3 B, C). Additionally, cross-section images of the coatings indicate that there are rather large spaces between the birnessite particles, and thus, the layers are clearly porous materials. If different amounts of oxides were printed, these general properties were very similar and only the thickness of the birnessite layer was altered (Figures 3 E, F). The approximate coating thicknesses obtained from SEM cross-section images show a strict linear correlation to the birnessite concentration of the applied ink (Figure S5). Finally, we found that the described irregular microstructure of the catalyst coatings was not significantly affected by the water-electrolysis process, as shown by a comparison of the SEM images taken of an electrode with a 10 mm coating before (Figure 3 C) and after 100 min of electrolysis at + 1.3 V (Figure 3 D). To evaluate their electrocatalytic properties in water oxidation, Figure 4 (left) shows Tafel plots for a series of type 1 elecChemSusChem 0000, 00, 1 – 11

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www.chemsuschem.org Thus, apparently an unusually large optimal thickness for the catalyst coatings of approximately 10 mm exists for our preparation method. This result might be explained as the result of the favorable combination of a large thickness of the porous oxide layer (Figure 3) acting as a volume catalyst on the one hand, whereas diffusion paths for the electrons that are too long are avoided on the other hand. As clearly visible in the cross-section micrographs shown in Figure 3, electrons have to pass multiple birnessite particle boundaries on their way from the catalytic sites to the FTO back contact. For layers that are too thick (> 10 mm), this negative effect apparently dominates, which results in the observed decrease in the current despite larger catalyst volumes deposited on the anodes. Long-term stability and Faradaic efficiency

For technical applications, the long-term stability of the anodes is of great importance. This point proved to be a limiting factor in earlier studies of MnOx electrodes at neutral pH.[26, 27] A type 1 10 mm electrode, the best product that emerged from the described optimization steps, was thus used for 2 h as an anode in a water-electrolysis cell, and it was kept Figure 3. SEM micrographs of type 1 Ca–birnessite-coated electrodes at at a constant potential of + 1.3 V (h  480 mV). The resulting three different magnifications (A–C). A section of uncoated FTO substrate is visible in panel A at the bottom right of the picture. D) Electrode surface current density trace is shown in Figure 4 (right) and features after 100 min of electrolysis at a constant potential of + 1.3 V in pH 7 elec(after currents that are  25 % higher during the first 15 min) trolyte. Bottom row) Cross-sections of type 1 electrodes showing the effect a stable water oxidation current of i  1.05 mA cm2 over the of varying the birnessite concentration in the ink used for screen printing: entire course of the experiment. Additionally, long-term elecE) “100 mg electrode”, F) “75 mg electrode”. trolysis also resulted in no significant change in the layer appearance by SEM, as already shown above (Figure 3 D). Both observations are in marked contrast to results for electrochemically prepared birnessite electrodes and those printed by using ethyl cellulose as the binder, for which the stability at pH 7 was poor (see Ref. [25] and Figure S4). To confirm that the currents Figure 4. Left) Comparison of Tafel plots for type 1 electrodes of differing oxide layer thickness. Birnessite coating recorded during electrolysis thickness: c:  0.5 mm; c: 1.2 mm; c: 3.6 mm; c: 9.6 mm, and c: 15 mm. Measurement conditions as indeed corresponded to water in Figure 2. Right) Chronoamperometry for a type 1 electrode with a  10 mm coating used as water-oxidation anode at + 1.3 V (h  480 mV) for 2 h in 1/15 m phosphate buffer electrolyte (pH 7). electrolysis and that they were not caused by other reactions (e.g., redox processes of the rather thick catalyst layer), we determined the Faradaic efficientrodes with different birnessite coating thicknesses. We obcy for O2 evolution. To do so, a gas-tight two-compartment cell served pronounced differences with an interesting trend concerning the relation between layer thickness and catalytic acwas used, and the oxygen formed on a type 1 electrode in the tivity: at first, currents gradually increased from a 0.5 mm layer anode compartment was detected by gas chromatography to more concentrated suspensions, and a maximum current analysis of the headspace above the solution (for experimental was reached for electrodes with a coating thickness of approxidetails, see the Supporting Information and Figure S3). Analysis mately 10 mm. At an applied potential of + 1.3 V, the current of samples taken at 10 min intervals during 1 h electrolysis exfor such an anode was approximately four times higher than periments showed Faradaic efficiencies > 90 % for the electrolthat for the 0.5 mm electrode (Figure 4, left). However, a further ysis (Figure 5). Taking into account that small gas leaks and O2 increase in the coating layer thickness to 15 mm did not result oversaturation of the solution can never be fully avoided in in any further gain but rather a decrease in the current to apsuch experiments, the results clearly show that electrochemical proximately half the maximum value. processes alternative to the oxidation of H2O to O2 do not  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 5. Correlation of O2 evolution measured by headspace gas chromatography (*) with the charge passed through the electrolysis system (c). a: 90 % Faradaic efficiency. Reaction conditions: Chronopotentiometrically monitored electrolysis with a fixed current of 1 mA passed through a type 1 10 mm anode immersed in 1/15 m phosphate buffer electrolyte (pH 7). See the Supporting Information for details.

seem to be of great importance for the anode in this electrolysis system. From the results presented so far, we can therefore conclude that Ca–birnessite-coated FTO electrodes prepared by screen printing of an ink of medium birnessite concentration and annealed at 450 8C can be used as efficient and stable anodes for water electrolysis in a neutral electrolyte. At pH 7, stable currents of 1 mA cm2 can be maintained at overpotentials of h  500 mV for hours (Figure S7), which thus meets the parameters currently often considered as requirements for water-oxidation catalysts in “artificial leaves”.[21, 22]

Figure 6. Top) XRD patterns of the presynthesized Ca–birnessite powder used for ink preparation (c) and a type 1 10 mm Ca–birnessite-coated FTO electrode (c); *: XRD reflections for SnO2 :F. Bottom) Raman spectra of the Ca–birnessite powder (c), a type 1 10 mm electrode (c), and the same anode after 100 min of electrolysis at + 1.3 V and pH 7 (c).

Characterization of Ca–birnessite electrode coatings It is known that Mn3 + /4 + oxides change their structures and compositions at elevated temperatures and, furthermore, might react as strong oxidation agents with organic matter (such as EtOH and/or the binders present in the inks).[31] Therefore, we investigated whether the fabrication procedure or extended electrocatalysis resulted in major changes in the previously established properties of the synthetic Ca–birnessite used herein.[17, 19, 26] As first indications, we studied the best-performing type 1 electrodes by using X-ray powder diffraction (XRD), vibrational spectroscopy (IR and Raman), and X-ray absorption spectroscopy (XAS). In contrast to common crystalline manganese oxides such as hausmannite (Mn3O4), b-Mn2O3, and a-MnO2, Ca–birnessites are highly amorphous materials.[32] For low Ca contents, an ordered arrangement of the layers sometimes makes it possible to observe at least the interlayer birnessite spacings of approximately 7  by XRD (reflections at 2q  128), but for synthetic birnessites containing larger amounts of Ca2 + (such as the material used in this study), even these signals cannot be detected.[17] We thus found no XRD pattern at all for the synthetic Ca–birnessite used herein for the preparation of the ink (Ca/ Mn ratio  1:5; Figure 6, top). As the red diffraction pattern in Figure 6 shows, this situation does not change for the fabricated electrodes. The characteristic XRD reflections of the underlying FTO substrate are clearly visible, whereas MnOx signals of significant intensity are absent; this indicates that the synthetic  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Ca–birnessite was not transformed into one of the common crystalline manganese oxides under the coating conditions. This conclusion is also supported by vibrational spectroscopy. Figure 6 (bottom) shows the Raman spectra of the synthetic Ca–birnessite, a freshly prepared type 1 10 mm electrode, and an electrode after 100 min of use as a water-oxidation anode. Despite the commonly low Raman activities of manganese oxides, both the MnO vibrational band of the [Mn(m-O)2]n chains and the symmetric stretching vibration of the [MnO6] octahedra can be clearly observed for all three samples at n˜  570 and 630 cm1, respectively.[33] Such MnO vibrations are also found in the corresponding attenuated total reflectance (ATR)-IR spectra, in which the broad signal from interlayer water is visible as well (Figure S6 a). Importantly, both Raman and IR spectra again do not show any significant changes after either the annealing or the electrolysis step, and thus, the XRD and vibrational spectroscopy data indicate that the structure and composition of the synthetic Ca–birnessite is most likely retained upon deposition onto the FTO electrode by using the “type 1 method”. To study the structural changes in the birnessite anode material under different conditions in more detail, we performed X-ray absorption measurements at the Mn K-edge of the presynthesized Ca–birnessite powder and also of screen-printed type 1 electrodes before and after electrochemical treatment. ChemSusChem 0000, 00, 1 – 11

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Figure 7. Top) Mn K-edge XANES. Bottom) Fourier-transformed EXAFS spectra of Ca–Mn-oxides.c: synthetic Ca–birnessite powder used for ink preparation; c: type 1 10 mm electrode; c: type 1 10 mm electrode used as water-oxidation anode for 5 h at + 1.35 V at pH 7.

www.chemsuschem.org ing results in a decrease in the amplitude of the second band (di-m-oxido MnMn distance; Figure 7, bottom, left arrow) and this change was not reversed after applying an oxidizing potential of + 1.35 V. In addition, a third band at higher reduced distance values appears that has also been observed previously for Mn anodes under water-oxidation conditions.[26, 34] The described changes can be quantitatively analyzed by simulations resulting in the data shown in Table 1. To obtain these values, we performed a joint fit of the three spectra keeping the number of shells and interatomic distances constant for all samples (see also Figure S7). The first band (MnO distance) was simulated by two shells (1.90 and 2.28 ) to account for the Jahn–Teller elongation typical for MnIII ions. The sum of the coordination numbers resulting from these two shells was fixed to 6 by assuming an octahedral coordination for all Mn centers. The small increase in the fraction of longer MnO distances after screen printing from 0.5 to 0.7 corresponds to the decrease in the average oxidation state (and thus an increase in the fraction of MnIII ions) already observed in the XANES spectra. The second signal corresponds to di-m-oxido bridges between the Mn atoms, and similar to previous analyses,[19, 34] it was also simulated with two shells (2.86 and 2.98 ). In an idealized birnessite structure consisting of infinite layers of di-moxido bridged [MnO6] octahedra,[32] the number of these short MnMn distances per Mn ion should be 6, as found in the presynthesized material (Table 1). For the electrode coatings, the number of these vectors is markedly decreased to a total of approximately 4.5, which indicates a disruption in the longrange order of the material. Unlike the oxidation state, this decrease in long-range order was not reversed under water-oxidizing conditions. The longer metal–metal distances were simulated by assuming backscattering by Ca2 + ions, but the possibility that Mn ions also contribute to these positions cannot be ruled out (if Mn is considered as a backscatterer, the resulting distances are  0.05  shorter). We used three MnCa shells for the simulations. The short (3.1 ) and long (3.8 ) distances were already previously identified in catalytically active MnCa oxide materials and were assigned to two different binding positions for Ca2 + relative to Mn: one, a cubane-type CaMn3O4 motif (short distance); two, the capping of vacancies of the birnessite layer

The XANES (X-ray absorption near-edge structure) spectra shown at the top of Figure 7 indicate that the screen-printing procedure caused a reduction in the average manganese oxidation state in the material from + 3.6 for the synthetic oxide powder to + 3.3 in the fabricated anode. However, electrochemical operation of the electrode at + 1.35 V in phosphate buffer (pH 7) for 5 h increased the oxidation state back to + 3.6 (average Mn oxidation states were determined by comparison of the Mn K-edge energy positions with those of MnO2, Mn2O3, MnCO3, and MnO reference compounds as reported previously[20]). The birnessite structure of the synthetic material can be confirmed by the characteristic shape of the Fourier-transformed EXAFS (extended X-ray absorption fine structure) spectrum. As illustrated in Figure 7 (bottom), the spectrum is dominated by two main bands that correspond Table 1. Interatomic distances R and coordination numbers obtained by joint-fit simulation of the k3-weighted to the MnO distance in the EXAFS spectra (Figure S7). The errors represent the 68 % confidence interval of the respective fit parameter. MnO6 octahedra and the short Coordination number Bond type R[a] MnMn distance for the di-m[] synthetic Ca–birnessite type 1 electrode as prepared electrode after 5 h at + 1.35 V oxido-bridged Mn ions. 1.903(2) 5.5(1) 5.3(1) 5.4(1) MnOshort After the screen-printing pro2.28(2) 0.5[b] 0.7[b] 0.6[b] MnOlong cedure and even after 5 h of MnMndi-m-O 2.861(8) 4.3(4) 2.5(5) 2.7(3) water electrolysis, the EXAFS 2.0(4) 2.0(4) 1.5(4) MnMndi-m-O 2.98(2) 3.19(1) 1.8(4) 3.2(4) 2.3(4) MnCashort data provides evidence that the 3.45(1) 0.0(3) 1.6(4) 0.9(4) MnCa middle materials, in general, keep their 3.86(2) 0.9(4) 1.0(4) 0.2(4) MnCalong amorphous birnessite-type struc[a] The Debye–Waller parameter s was set to 0.063  for all bond lengths to avoid overparametrization. [b] The tures, but there are also some sum of the coordination numbers of the first MnO shells was fixed to 6. modifications. First, screen-print 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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CHEMSUSCHEM FULL PAPERS created by missing Mn ions (long distance).[20] As can be seen in Table 1, there is a tendency to increase the number of these metal–metal vectors for the electrode coatings in comparison to the synthesized oxide. However, the simulation results do not allow an unambiguous statement about this change to be made. More significant is the appearance of the third, intermediate metal–metal distance (3.4 ), which is absent in the starting material and also in most of the previously investigated Mn–Ca oxides.[19, 20] Interestingly, such a third signal was observed in the EXAFS spectra of Mn–Ca oxides prepared by an alternative synthesis route[35] and also in non-Ca2 + -containing electrochemically deposited Mn oxide films.[26, 34] Therein, the intermediate metal–metal distance was assigned to mono-m-oxidobridged Mn ions typical for oxides with tunnel structures[32] and were found to affect the electrocatalytic properties (Tafel slopes) and probably also the proton conductivity of the Mn oxide catalysts.[34] In conclusion, the combined presented characterization data clearly shows that the manganese oxides used to prepare type 1 electrodes are birnessite-type, highly disordered, layered calcium manganese oxides with average Mn oxidation states of approximately + 3.6. These general characteristics are neither affected by the screen-printing process nor extended operation as water-oxidizing anodes at pH 7. However, from a detailed look at the XAS spectra it can be concluded that the fabrication method causes Mn reduction to approximately + 3.3 and a clear order decrease (less di-m-oxo-connected Mn ions), which are accompanied by the appearance of an additional structural motif (3.45  metal–metal distance). Upon extended operation as an anode for water oxidation, the original Mn oxidation state of + 3.6 was restored, but the structure remained less ordered in comparison to that of the starting material. Comparison of a series of screen-printed metal oxides As demonstrated, type 1 Ca–birnessite electrodes show promising activity and stability in water-oxidation electrocatalysis at pH 7. Furthermore, the coating method proved to be mild enough to retain the chemical characteristics of the applied calcium birnessites. We thus saw the possibility to use the process for a comparison of the catalytic water-oxidation activities of different d-block metal oxides to perform an electrochemical “Harriman style” catalyst screening at pH 7. Following the type 1 protocol, we coated FTO electrodes with seven metal oxides from the original Harriman series (RuO2, Co3O4, NiO, Mn2O3, MnO2, Fe2O3, and WO3),[15] in each case by using oxide batches from commercial sources. The IR spectroscopy and XRD data provided a first indication that the coating procedure again did not significantly change the oxide structures (Figures S6 and S8). More detailed analyses would be needed in each case, but the electrocatalytic performances were our first concern. As above, cyclic voltammograms as well as Tafel measurements were recorded for these electrodes in neutral phosphate buffer electrolyte (Figure S9). A look at the resulting Tafel plots (Figure 8) shows that typical Tafel behavior is observed for  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 8. Comparison of Tafel plots for type 1 electrodes of different d-block metal oxides at pH 7. From top to bottom: &: RuO2, *: Ca–birnessite, ~: Co3O4, ~: b-MnO2, 3 : NiO, ": Fe2O3, *: Mn2O3, &: WO3. See also Figure S9.

most electrodes. However, the accessible current densities differ for this series by as much as a factor of 100 depending on the printed oxide. Looking at the plots in Figure 8, the oxide-coated anodes can be roughly sorted into three groups: 1) RuO2, which clearly performs in a league of its own with current densities that are at least four times higher than those of the other tested oxides; 2) oxides of cobalt, manganese, and nickel all show similar current densities of 0.3–3 mA cm2 for overpotentials of 400–700 mV; 3) Fe2O3 and WO3 might be classified as “inactive” as a result of their low current densities. Key values obtained from the analysis of the plots in Figure 8 at the positions of the gray lines are listed in the second and third columns of Table 2. Comparison of the over-

Table 2. Comparison of overpotentials and water-oxidation current densities of different metal oxide anodes prepared following the “type 1” screen-printing protocol. Oxide for electrode preparation

h[a] [mV]

i[b] [mA cm2] (vs. RuO2 [%])

Ratio kcat/kRuO2 (Harriman study)[c] [%]

RuO2 Ca–birnessite Co3O4 Mn2O3 ß-MnO2 NiO Fe2O3 WO3 RuO2[f] Ca–birnessite[f]

325 480 500 500 560 670 n.d.[e] n.d.[e] 310 470

3.6(100) 1.2(33) 1.0(27) 1.0(27) 0.6(16) 0.2(6) 0.02 (0.6) 0.002 (0.06) 5.6(100) 1.2(21)

100 –[d] 140 80 n.d.[e] 7 2 14 –[d] –[d]

[a] For 1 mA cm2. [b] h = 500 mV. [c] Calculated from the data of Ref. [14]. [d] –: not measured. [e] n.d.: not detectable. [f] For electrolysis at pH 13 (0.1 m KOH electrolyte, see Figure S10).

potentials (needed to reach i = 1 mA cm2) and current densities (for h = 500 mV) put into numbers the general trend in performance already described above. The result is a clear “ranking” of catalytic activities at pH 7: RuO2 > Co3O4, (Ca)MnOx, NiO > Fe2O3, WO3. Surprisingly, the differences between the oxides of the second group are only small. Thus, our study supports today’s research approach in which oxides of these three metals are investigated as promising and affordable materials for water-oxidation catalysis, with no clear “winner” fixed to date. ChemSusChem 0000, 00, 1 – 11

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CHEMSUSCHEM FULL PAPERS Probably as expected (but far from clear), this ranking of the electrochemical performances at pH 7 is also very similar to the photochemical Harriman series reported at pH 5. In the last two columns of Table 2, the values of i from our study and the photochemical rate constants (kcat) of Ref. [15] are listed, in each case normalized to the performance of ruthenium dioxide. The absolute values differ, but for most oxides the order within the rankings is identical. However, there are two interesting exceptions: First, for the photochemical catalysis, Co3O4 showed better results than RuO2. The reason for this might be the ill-defined batches of ruthenium dioxide used for the photochemical experiments for which the results for different types of RuO2 differ greatly.[15] Second, NiO showed good electrocatalytic activity in this study, whereas it performed poorly in the Harriman study. There are other recent reports in which nickel oxides were very successfully used as water-oxidation electrocatalysts,[36] and thus, its bad performance in the [Ru(bpy)3] experiments is difficult to understand. Anyway, our results support that NiOx should be counted into the group of “promising oxides”. Despite these small discrepancies, we consider it a remarkable confirmation of the value of the original Harriman study that it was possible to transfer the general activity trends from these 25-year old water-oxidation experiments by using oxide suspensions to electrolysis with oxide-coated anodes.

www.chemsuschem.org the initially observed current density of 8 mA cm2 to approximately 4 mA cm2 occurred during the first minutes. After 1 h of electrolysis, only approximately 2 mA cm2 was maintained (Figure 9, right). Thus, very high water-oxidation currents could be reached for short periods of time by using screen-printed Ca–birnessite electrodes at pH 13, but this regime (typical for alkaline electrolyzers) is not the one in which they might be preferentially used. A look at the Pourbaix diagram for manganese offers a possible explanation: the point representing the anode conditions of an alkaline electrolyzer (pH  13, E  1 V) lies well beyond the stability zone for manganese oxides.[37] The fast decay in electrocatalytic activity at pH 13 (Figure 9, right) might, thus, likely be caused by corrosion and/or unfavorable structural changes in the oxide, but this point needs further examination.

Conclusions

Three main conclusions can be drawn from the presented results concerning the use of screen-printed Ca–birnessites as electrocatalysts for “artificial leaves”. First, for electrolysis at pH 7, Ca–birnessites can clearly compete with the other “affordable oxides” (CoOx and NiOx) with regard to overpotentials and current densities (Table 2) but also with regard to stability (Figure 4). As both their cost and toxicities are lower, manganese oxides might in consequence become the materials of Performance under strongly alkaline conditions choice for the large-scale production of anodes for photoelectrochemical water-splitting devices. As mentioned above, a number of studies already describe the Second, our results confirm that the choice of manganese use of MnOx electrodes for water electrolysis, but they do so oxide for anode preparation is important. From MnO2 via mainly for strongly alkaline conditions (pH > 12).[12, 13, 25, 27] In Mn2O3 to Ca–birnessite, the overpotentials needed to reach i = some of these reports, much higher current densities of 1 mA cm2 dropped by 80 mV, whereas the current densities 10 mA cm2 and higher were found. To put our results into perfor h = 500 mV doubled (Table 2). In the context of electrocatalspective of electrolysis at high pH values, we performed cyclic ysis, these are large differences and stress the importance of voltammetry and potentiostatic electrolysis measurements for using the “right” oxide composition. type 1 Ca–birnessite electrodes by using 0.1 m KOH (pH 13) as Furthermore, the screen-printing method developed during the electrolyte. this study presents itself as a promising alternative to the curIndeed, very high current densities of 20 mA cm2 and rently much-studied electrodeposition route. By using the higher were reached in a cyclic voltammetry experiment at printing process, a presynthesized (and properly characterized) pH 13 (Figure 9). However, the currents observed in the CV for oxide powder can be printed homogeneously onto a conducalkaline electrolysis could be translated even less into longtive substrate by using nonaggressive chemicals and a modest term electrolysis currents than before at pH 7: for a printed birtemperature of 450 8C. If calcium birnessites are applied to FTO nessite electrode kept at + 0.96 V (h = 500 mV), a fast decay of electrodes in this way, durable anodes for electrochemical water oxidation at pH 7 can be obtained. Interestingly, this method can also be used very successfully to prepare printed cobalt oxide electrodes. Our “type 1 Co3O4 anode” prepared by using commercially available Co3O4 shows Tafel behavior that is virtually identical to that of an electrode prepared according to the currently much-used Nocera protocol[24] (Figure S11), and thus, the printing of synthetic cobalt oxides might also be a promising route to improve CoOx anodes. Figure 9. Cyclic voltammogram (left) and chronoamperometric electrolysis plot (right) for The fabrication concept of “synthesize first, then type Ca–birnessite electrodes under alkaline conditions. The inset on the right shows the print” has the advantage that the full array of solidfast decay of the current density during the first 3 min of electrolysis. CV scan rate: state preparation techniques can be explored to 20 mV s1, electrolysis potential: E = + 0.96 V (h = 500 mV), electrolyte: 0.1 m KOH, pH 13.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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CHEMSUSCHEM FULL PAPERS obtain oxides of different composition, crystallinity, and morphology before these are used as anode coatings. Characterization studies of the catalytic material are also facilitated: clear reference compounds exist in the form of the used oxide powder, which, for example, proved very useful in this study to interpret the Raman and IR spectroscopy, XRD, and XAS data. The small presented screening of different commercially available oxides already shows the potential of this approach and gives as an additional interesting result in the general confirmation of the “Harriman oxide series”, this time for electrochemical water oxidation at neutral pH. Despite numerous possibilities for further improvements in the electrocatalytic coatings, our future studies will also focus more on mechanistic aspects such as detailed studies of the influences of pH value and electrolyte, the electroactive surface area, and the importance of the amount of water incorporated into the oxide structure.

Experimental Section Preparation of “type 1 electrodes” The Ca–birnessite powder was synthesized as described (Ca0.21),[17] whereas other metal oxides were purchased from commercial sources. The inks were prepared by mixing an amount of metal oxide powder (12.5 to 200 mg) with PEO (30 mg), distilled water (1 mL), acetyl acetone (0.1 mL), and Triton X-100 (0.03 mL). The suspension was stirred for 12 h at room temperature, and then a layer of ink was applied to fluorine-doped tin oxide (FTO) coated glass slides (Sigma–Aldrich), covering an area of 1 cm2 in each case. To guarantee a reproducible thickness and size of the printing, adhesive foil (thickness  70 mm) was used to cover the rest of the electrode surface. The wet electrodes were put in a cabinet dryer for 60 min at 60 8C and then sintered at 450 8C for 60 min. Blank electrodes were prepared in the same way by omitting the step of adding the metal oxide powder.

Electrochemistry For voltammetry measurements, a typical three-electrode setup (Figure S2) was used consisting of a Ag/AgCl reference electrode (ALS, sat. KCl, + 197 mV vs. NHE at 25 8C), a Pt rod as counter electrode (Metrohm, diameter  2 mm), and the 1 cm2 FTO electrode coated with the metal oxide as the working electrode. A 1/15 m phosphate buffer (pH 7, prepared from KH2PO4 and Na2HPO4·2 H2O) served as the electrolyte solution. All electrochemical measurements were performed at room temperature by using a computercontrolled potentiostat/galvanostat (Princeton Applied Research, VersaSTAT 4). For comparisons of electrode types and oxide coatings, electrochemical measurements (cyclic voltammograms and Tafel plots) were recorded at least five times, in each case starting with a freshly prepared electrode. To determine Faradaic efficiencies for water oxidation, a gas-tight electrolysis cell was used, and gas samples taken from the headspace of the anode compartment were analyzed for their O2 content by using a PerkinElmer Clarus 480 gas chromatograph.

X-ray absorption spectroscopy (XAS) XAS was performed at the KMC1 beamline of the BESSY synchrotron operated by the Helmholtz-Zentrum Berlin (HZB).  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org Please consult the ESI for experimental details of both the sample preparations as well as the various analytical techniques.

Acknowledgements We would like to thank Dipl.-Ing. Daniel Hertkorn and M.Sc. Uwe Gleißner (IMTEK, ALU Freiburg) for XRD measurements in reflection configuration and Dipl.-Chem. Amalia Wagner (IAAC, ALU Freiburg) for taking the scanning electron micrographs. Raman spectra were kindly recorded by Dr. Anne Westphal at the Fraunhofer IFAM in Oldenburg (Germany). Furthermore, Dr. F. Schfers, M. Mertin, and M. Gorgoi provided technical support at the KMC-1 beamline of the BESSY synchrotron operated by the Helmholtz-Zentrum Berlin (HZB). This work was generously supported by a research grant of the Deutsche Forschungsgemeinschaft (projects KU 2885/2-1 and DA402/7-1, both part of the DFG priority program SPP 1613). Keywords: bioinspired catalysis · electrolysis · manganese · oxidation · water chemistry [1] L. Taiz, E. Zeiger, Plant Physiology, Sinauer Ass., Sunderland, 2010. [2] J. Barber, Chem. Soc. Rev. 2009, 38, 185 – 196. [3] H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, P. Strasser, ChemCatChem 2010, 2, 724 – 761. [4] W. Lubitz, E. J. Reijerse, J. Messinger, Energy Environ. Sci. 2008, 1, 15 – 31. [5] a) J. P. McEvoy, G. W. Brudvig, Chem. Rev. 2006, 106, 4455 – 4483; b) Y. Umena, K. Kawakami, J.-R. Shen, N. Kamiya, Nature 2011, 473, 55 – 60; c) P. E. Siegbahn, Acc. Chem. Res. 2009, 42, 1871 – 1880. [6] N. Cox, D. A. Pantazis, F. Neese, W. Lubitz, Acc. Chem. Res. 2013, 46, 1588 – 1596. [7] N. Armaroli, V. Balzani, Energy for a Sustainable World, Wiley-VCH, Weinheim, 2011. [8] D. G. Nocera, ChemSusChem 2009, 2, 387 – 390. [9] a) Chemical Energy Storage (Ed.: R. Schlçgl), de Gruyter, Berlin, 2013; b) A. Thapper, S. Styring, G. Saracco, A. W. Rutherford, B. Robert, A. Magnuson, W. Lubitz, A. Llobet, P. Kurz, A. Holzwarth, S. Fiechter, H. de Groot, S. Campagna, A. Braun, H. Bercegol, V. Artero, Green 2013, 3, 43 – 57; c) Energy Production and Storage (Ed.: R. H. Crabtree), Wiley, Chichester, West Sussex, 2010. [10] C. C. L. McCrory, S. Jung, J. C. Peters, T. F. Jaramillo, J. Am. Chem. Soc. 2013, 135, 16977 – 16987. [11] A. Harriman, Eur. J. Inorg. Chem. 2014, 573 – 580. [12] P. Rasiyah, A. C. Tseung, J. Electrochem. Soc. 1984, 131, 803 – 808. [13] Y. Matsumoto, E. Sato, Mater. Chem. Phys. 1986, 14, 397 – 426. [14] a) D. K. Bediako, B. Lassalle-Kaiser, Y. Surendranath, J. Yano, V. K. Yachandra, D. G. Nocera, J. Am. Chem. Soc. 2012, 134, 6801 – 6809; b) Y. Surendranath, M. Dinca, D. G. Nocera, J. Am. Chem. Soc. 2009, 131, 2615 – 2620; c) J. B. Gerken, J. Y. C. Chen, R. C. Mass, A. B. Powell, S. S. Stahl, Angew. Chem. Int. Ed. 2012, 51, 6676 – 6680; Angew. Chem. 2012, 124, 6780 – 6784; d) K. Klingan, F. Ringleb, I. Zaharieva, J. Heidkamp, P. Chernev, D. Gonzalez-Flores, M. Risch, A. Fischer, H. Dau, ChemSusChem 2014, 7, 1301 – 1310. [15] A. Harriman, I. J. Pickering, J. M. Thomas, P. A. Christensen, J. Chem. Soc. Faraday Trans. 1 1988, 84, 2795 – 2806. [16] M. Hara, C. C. Waraksa, J. T. Lean, B. A. Lewis, T. E. Mallouk, J. Phys. Chem. A 2000, 104, 5275 – 5280. [17] C. E. Frey, M. Wiechen, P. Kurz, Dalton Trans. 2014, 43, 4370. [18] M. M. Najafpour, T. Ehrenberg, M. Wiechen, P. Kurz, Angew. Chem. Int. Ed. 2010, 49, 2233 – 2237; Angew. Chem. 2010, 122, 2281 – 2285. [19] M. Wiechen, I. Zaharieva, H. Dau, P. Kurz, Chem. Sci. 2012, 3, 2330 – 2339. [20] I. Zaharieva, M. M. Najafpour, M. Wiechen, M. Haumann, P. Kurz, H. Dau, Energy Environ. Sci. 2011, 4, 2400 – 2408. [21] D. G. Nocera, Acc. Chem. Res. 2012, 45, 767 – 776.

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Received: June 10, 2014 Revised: August 23, 2014 Published online on && &&, 0000

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FULL PAPERS S. Y. Lee, D. Gonzlez-Flores, J. Ohms, T. Trost, H. Dau, I. Zaharieva,* P. Kurz* && – &&

Ca-lling on oxides: Powders of calcium manganese oxide (Ca–birnessite) are screen printed onto conductive substrates that are used as anodes in water electrolysis. Screening of various transi-

tion-metal oxides for electrochemical water oxidation under “artificial leaf conditions” is possible and confirms the suitability of Ca–Mn-oxides for this task.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Screen-Printed Calcium–Birnessite Electrodes for Water Oxidation at Neutral pH and an “Electrochemical Harriman Series”

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