Electrolytic water splitting using a carbon-supported MnOx-composite

Abstract

The present invention relates to the electrolytic splitting of water using a carbon-supported manganese oxide (MnO.sub.x) composite. Specifically, the present electrolytic splitting of water is carried under neutral electrolyte conditions with a high electrolytic activity, while using an oxygen evolution reaction (OER)-electrode comprising the present carbon-supported MnO.sub.x composite. Next, the present invention relates to a process for producing such a carbon-supported MnO.sub.x composite as well as to a composite obtainable by the present process for producing the same and to an OER-electrode comprising the carbon-supported MnO.sub.x composite obtainable by the present process.

Claims

1. A process comprising: forming a composite comprising a carbon-based support and a manganese oxide (MnO.sub.x) by a symproportionation precipitation method, wherein the manganese oxide is nanoparticular and comprises a mixture of Mn (+III) and Mn (+IV) or a mixture of Mn (+II) and Mn (+III); and electrolytically splitting water having a pH in the range of 4.5 to 8.5 using the composite.

2. The process of claim 1, wherein the MnO.sub.x is homogenously dispersed and deposited on the carbon-based support.

3. The process of claim 1, wherein the carbon-based support is a carbon-nanotube (CNT).

4. The process of claim 1, wherein the water comprises waste water, seawater and/or freshwater, and wherein the composite is used as an oxygen evolution electrode.

5. The process of claim 1, wherein the symproportionation precipitation method comprises the step of: (a) providing the carbon-based support with a manganese oxide precursor, (b) adding a symproportionation precipitation agent, (c) drying, and (d) calcinating.

6. The process of claim 5, wherein the manganese oxide precursor is a Mn (+II) salt.

7. The process of claim 5, wherein step (b) is carried out at a temperature of below 100° C.

8. The process of claim 5, wherein the manganese oxide precursor is a Mn (+II) salt, and the symproportionation precipitation agent is a Mn (+VII) salt.

9. The process of claim 5, further comprising the step of: functionalizing the carbon-based support prior to step (a) to create linker groups for the manganese oxide precursor.

10. The process of claim 5, wherein the calcination temperature in step (d) is in a range from 150° C. to 450° C.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1: SEM micrographs of s-MnO.sub.x (A) and (B), s-MnO.sub.x/MWCNT.sub.ox (C) and (D), and i-MnO.sub.x/MWCNT.sub.ox (E) and (F).

(2) FIG. 2: XRD patterns of the symproportionation precursor s-MnO.sub.x after calcination at different temperatures, i.e. 110° C., 170° C., 250° C., 350° C., 550° C. and 700° C. from lower to upper pattern, and after heating to 700° C. during TGA. The final product can be identified as Mn.sub.2O.sub.3 (Bixbyite, black triangles).

(3) FIG. 3: TG-MS curves for the thermal decomposition of manganese oxide (s-MnO.sub.x) in air.

(4) FIG. 4: TPR profiles of s-MnO.sub.x (19.0 mg) (upper signal), s-MnO.sub.x/MWCNT.sub.ox (124.8 mg) (middle signal) and MWCNT.sub.ox (111.5 mg) (lower signal).

(5) FIG. 5: TEM images of the manganese oxide sample (s-MnO.sub.x) under the electron beam; at the beginning (A), after 2 minutes (B), after 13 minutes (C), after 26 minutes (D) and the corresponding Fourier transform (E).

(6) FIG. 6: Evolution of the Mn species with temperature as determined by fitting of the L-NEXAFS spectra of s-MnO.sub.x in 0.5 mbar O.sub.2 (a), s-MnO.sub.x/MWCNT.sub.ox at 0.25 mbar O.sub.2 (b), and i-MnO.sub.x/MWCNT.sub.ox at 0.25 mbar O.sub.2 (c). In (a), (b) and (c) is -.diamond-solid.-Mn.sup.2+; -.square-solid.- Mn.sup.3+; -.box-tangle-solidup.- Mn.sup.4+.

(7) FIG. 7: Mn L-NEXAFS spectra of s-MnO.sub.x in 0.5 mbar O.sub.2 (a), s-MnO.sub.x/MWCNT.sub.ox at 0.25 mbar O.sub.2 (b), and i-MnO.sub.x/MWCNT.sub.ox at 0.25 mbar O.sub.2 (c), each at 25° C. (upper spectrum) and 400° C. (lower spectrum). In (a), (b) and (c) is •••• sum; -•- Mn.sup.2+; - - Mn.sup.3+ and -••- Mn.sup.4+.

(8) FIG. 8: XRD pattern of the dried MWCNT supported manganese oxides s-MnO.sub.x/MWCNT.sub.ox (upper pattern) and i-MnO.sub.x/MWCNT.sub.ox (middle pattern) and oxidized MWCNTs (lower pattern).

(9) FIG. 9: TG-MS-DSC curves for the thermal decomposition of manganese oxide (s-MnO.sub.x/MWCNT.sub.ox, 6.5 wt % Mn/C) in air.

(10) FIG. 10: Pore size distributions of MWCNT -.square-solid.-, MWCNT.sub.ox -□-, s-MnO.sub.x/MWCNT.sub.ox -- and i-MnO.sub.x/MWCNT.sub.ox -- samples.

(11) FIG. 11: TEM images of s-MnO.sub.x/MWCNT.sub.ox (A) and i-MnO.sub.x/MWCNT.sub.ox (B, C).

(12) FIG. 12: Mn K-XANES of as-prepared s-MnO.sub.x/MWCNT.sub.ox. The spectrum is shown relative to the edge position of Mn metal (6539 eV).

(13) FIG. 13: Mn L-NEXAFS spectra of the MWCNT.sub.ox supported manganese oxide (i-MnO.sub.x/MWCNT.sub.ox) and its decomposition products in 0.5 mbar O.sub.2, at different temperatures, i.e. vacuum, 25° C., 150° C., 200° C., 250° C., 300° C., 350° C. and 400° C. from the lower spectrum to the upper spectrum.

(14) FIG. 14: Raman of s-MnO.sub.x/MWCNT.sub.ox (- -) and i-MnO.sub.x/MWCNT.sub.ox (- -) and difference spectrum in light gray.

(15) FIG. 15: Electrocatalytic activity for OER of s-MnO.sub.x/MWCNT.sub.ox, i-MnO.sub.x/MWCNT.sub.ox and MWCNT.sub.ox. Rotating disk electrode measurements has been conducted at 1600 rpm in 0.1 M potassium phosphate buffer at pH 7. Electrode potential has been sweeped between 1.0V and 2.0V vs. RHE with rate of 6 mV/s.

(16) FIG. 16: Electrocatalytic stability for OER of s-MnO.sub.x/MWCNT.sub.ox (.square-solid.) and i-MnO.sub.x/MWCNT.sub.ox (.box-tangle-solidup.) before and after stability test as compared to MWNT.sub.ox (◯) and Pt (*). Stability test consisted of 22 potential cycles between 1.0 V and 2.0 V vs. RHE with rate of 100 mV s.sup.−1. RDE measurements has been conducted at 1600 rpm in 0.1 M potassium phosphate buffer at pH 7.

(17) FIG. 17: Electrocatalytic activity for OER of s-MnO.sub.x/MWCNT.sub.ox (.square-solid.) and i-MnO.sub.x/MWCNT.sub.ox (.box-tangle-solidup.) as compared vis-à-vis different calcination and drying conditions during preparation, i.e. i-MnO.sub.x/MWCNT.sub.ox calcinated at 400° C. (◯), i-MnO.sub.x/MWCNT.sub.ox directly calcinated at 300° C. without predrying (□) and i-MnO.sub.x/MWCNT.sub.ox solely dried at 110° C. (.circle-solid. and ∇). Moreover, comparision of electrocatalytic activity vis-à-vis a 4 wt.-% () and a 10 wt.-% MnO.sub.x/SBA-15 () samples.

EXAMPLES

(18) In the following the invention and its beneficial effects is further described by means of Examples and Comparative Examples. Specifically, the characteristics of an i-MnO.sub.x/CNT composite and an s-MnO.sub.x/CNT composite are compared vis-à-vis an unsupported s-MnO.sub.x material and vis-à-vis i-[???] MnO.sub.x/SBA-15 being silica-supported MnO.sub.x samples.

(19) Sample Preparation

(20) Preparation of an Unsupported s-MnO.sub.x Reference Sample

(21) An unsupported manganese oxide reference sample was prepared by a symproportionation reaction in alkaline media. The pH of a 100 ml solution of 0.015 M manganese(II) nitrate tetrahydrate (Mn(NO.sub.3).sub.2.4H.sub.2O, Merck) and 0.09 M NH.sub.4Cl was adjusted to 8.0 with ammonia. 100 ml of a 0.01 M potassium permanganate (KMnO.sub.4, Carl Roth) solution were added dropwise. The pH of the solution was kept constant at 8.0 with NH.sub.3. After filling up the volume to 500 ml with distilled water the solution was stirred for 2 hours. Subsequently, the precipitate was filtered and washed with distilled water until the conductivity of the filtrate was <0.5 mS/cm. The product was dried for 16 h at 110° C. in air followed by calcination in air at various temperatures to form the manganese oxide (s-MnO.sub.x).

(22) Functionalization of Carbon Nano-Tubes (CNTs)

(23) Multiwalled carbon nanotubes (MWCNTs) were supplied by Bayer MaterialScience AG Germany (Baytubes, >95% C, outer diameter 15 nm). Their initial specific surface area, measured by N.sub.2 adsorption (BET) is 291 m.sup.2/g. The pristine Baytubes were treated with concentrated nitric acid (65% HNO.sub.3, 500 ml for 10 g CNTs) under vigorous magnetic stirring at 100° C. for 16 h, in order to remove the residual growth catalyst and amorphous carbon impurities and to functionalize the nanotube inner and outer surfaces by oxidation. After cooling down to room temperature, the suspension was filtered and extensively rinsed with distilled water until reaching a neutral pH of the filtrate. The product (MWCNT.sub.ox) was dried at 110° C. over night. The specific surface area increased from 291 to 316 m.sup.2/g after a 16 h treatment at 100° C., most probably because part of the inner walls, which blocked the inner channel, were oxidized.

(24) Preparation s-MnO.sub.x/MWCNT.sub.ox Sample

(25) For the symproportionation precipitation sample, the pH of a 100 ml solution of 0.015 M manganese(II) nitrate tetrahydrate (Mn(NO.sub.3).sub.2×4H.sub.2O, Merck) and 0.09 M NH.sub.4Cl was adjusted with ammonia to 8.0. For example, for a 10 wt % loading of Mn on MWCNT, 1.373 g of the oxidized CNTs (MWCNT.sub.ox) as prepared above were first dispersed in 200 ml distilled water and treated with ultrasound for a few seconds to improve dispersion. The CNT suspension was then slowly added to the Mn(NO.sub.3).sub.2 solution under vigorous stirring. Finally, 100 ml of a 0.01 M potassium permanganate (KMnO.sub.4, ROTH) solution were added dropwise. The pH of the dispersion was kept constant at 8.0 with NH.sub.3 throughout the synthesis. After filling up the volume to 500 ml with distilled water, the solution was stirred for 2 hours at room temperature in order to allow MnO.sub.4.sup.− to react with Mn.sup.2+. Subsequently, the solid was filtered and washed with distilled water until the conductivity of the filtrate was <0.5 mS/cm. The product was dried for 16 h at 110° C. in air followed by calcination in air at various temperatures to form the MWCNT-supported manganese oxide particles (s-MnO.sub.x/MWCNT.sub.ox).

(26) Preparation i-MnO.sub.x/MWCNT.sub.ox Samples

(27) For the incipient wetness impregnation sample, 0.5 g of the oxidized CNTs (MWCNT.sub.ox) as prepared above were first impregnated with 3 ml of distilled water in order to fill the inner channel of the CNTs and have the Mn deposition occur preferentially on the outer surface, similarly to the sample prepared by symproportionation. Two samples with a nominal loading of 5 and 10 wt.-% have been prepared by incipient wetness. For the 5 wt.-% MnO.sub.x/CNT sample prepared, 114 mg of manganese(II) nitrate tetrahydrate (Mn(NO.sub.3).sub.2.4H.sub.2O, Merck) were dissolved in 1 ml of distilled water. The solution was then slowly added to the wet MWCNTs while continuously stirring the paste. The mixture was dried at 60° C. for 6 h and 110° C. for 24 h in air. Then, it was calcinated in air at different temperatures in order to decompose the manganese precursor and form MWCNT-supported manganese oxide particles (i-MnO.sub.x/MWCNT.sub.ox). The same procedure was applied for the 10 wt.-% MnO.sub.x/CNT sample.

(28) Measurement Methods and Measurement Conditions

(29) The X-ray powder diffraction (XRD) measurements were performed with a STOE STADI-P transmission diffractometer equipped with a focusing primary Ge(111) monochromator and a 3° linear position sensitive detector (PSD) using Cu Kα radiation. The powder samples were fixed with small amounts of X-ray amorphous grease between two thin films of polyacetate foil.

(30) The concentration of manganese and the presence of metal impurities from the remaining catalyst were examined by wavelength dispersive X-ray fluorescence (WDXRF) using a Bruker AXS S4 PIONEER spectrometer. For this purpose, 0.5 g of sample was placed in a polystyrene holder (Ø 34 mm) and covered with a 4 μm thick polypropylene film.

(31) Specific surface areas of the calcined material and the precursors were carried out by N.sub.2 physisorption (Quantachrome Autosorb-1) and evaluated using the BET method. The samples were outgassed for 4 h at 100° C. Pore size distributions were determined from the desorption branches of the isotherms using the BJH method.

(32) The thermogravimetric analysis (TGA) and evolved gas analysis (EGA) of the decomposition reaction were achieved with a NETZSCH STA449 thermobalance under controlled gas flow (21% O.sub.2 in Ar, 100 ml/min) connected to a quadrupole mass spectrometer (QMS200 OMNISTAR, Balzers). The measurements were performed with approximately 15 mg sample in a temperature range of 30-700° C. (2 Kpm).

(33) Temperature-programmed reduction (TPR) of the samples was performed in a fixed bed reactor (TPDRO-1100, CE Instruments) in 5% H.sub.2/Ar (80 ml/min), with a heating rate of 6 Kpm, in a quartz tube. The H.sub.2 consumption was monitored with a thermal conductivity detector (TCD). The TCD detector was calibrated by reducing a known amount of CuO.

(34) For morphological studies of the materials, SEM images were acquired with a Hitachi S-4800 scanning electron microscope equipped with a field emission gun. The samples were loosely dispersed on conductive carbon tape (Plano). The SEM was operated at low accelerating voltage (1.5 kV) for a better resolution of the surface features of the samples. Elemental analysis by X-ray energy dispersive spectroscopy (EDX) was carried out at 15 kV using an EDAX detector connected to the SEM.

(35) The microstructure of the samples was examined using a Philips CM200 transmission electron microscope (TEM) equipped with a field emission gun (FEG). The samples were dispersed in chloroform and deposited on a holey carbon film supported on a copper grid. High-resolution images were taken with a CCD camera, and selected areas were processed to obtain the power spectra (square of the Fourier transform of the image), which were used for measuring inter planar distances and angles for phase identification.

(36) To characterized the CNT support materials after the different deposition processes, Raman spectroscopy was applied at room temperature on a LabRam spectrometer using 633 nm laser excitation from a HeNe laser, with a power of 20 mW at the laser output. All data were obtained with a BX40 Olympus microscope (objective 100). Acquisition times were typically 3.Math.180 s. Prior to the experiments, the Raman spectrometer was calibrated using a Si wafer.

(37) The near-edge X-ray absorption fine structure spectra (NEXAFS) were measured at the ISISS beamline at the BESSY II synchrotron facility of the Helmholtz-Zentrum Berlin (Germany) in the end station of the FHI-MPG. The beamline was operated with 60 μm exit slit. The Total Electron Yield (TEY) spectra were obtained in continuous driving mode of the monochromator (CMM) (0.178 eV/s). For the detection of the Mn L.sub.3,2-edges (transition from Mn2p core level to unoccupied Mn3d state) an energy region of 660-633 eV were measured. The measurements were performed in vacuum and in O.sub.2 with a partial pressure of 0.5 mbar (s-MnO.sub.x) and 0.25 mbar (s-MnO.sub.x/MWCNT.sub.ox, i-MnO.sub.x/MWCNT.sub.ox), while heated with a constant rate (6 Kpm) from 25 up to 550 K. The NEXAFS data were collected at room temperature, after each incremental increase in temperature of 50-100° C. and at the final temperature. The spectra were intensity normalized to unity at 648 eV and analyzed by fitting them with a linear combination of spectra representing Mn.sup.2+, Mn.sup.3+ and Mn.sup.4+ single valence sites. The WinXAS software package was used for data evaluation. All experimental spectra could be described reasonably well as a superposition of these three components, which have been extracted from the data set under the assumption, that pure Mn.sup.2+ is present in the sample impregnated with Mn(NO.sub.3).sub.2, Mn.sup.3+ is formed upon oxidation of this sample and that no Mn.sup.2+ is present after heating the symproportionated sample to 400° C. in oxygen. The resulting reference spectra are compared to those of reference Mn-oxide materials MnO and MnO.sub.2 and a reasonable agreement was found. Using these reference materials themselves for fitting the NEXAFS spectra, however, resulted in significantly poorer agreement, which can be explained by the contribution of the local geometry to the spectrum, which varies in different polymorphs of Mn-oxides and may not be representative for the prepared materials. This has also been observed in studies of other mixed valence systems. X-ray photoemission spectra were recorded at the same beamline.

(38) For the sample s-MnO.sub.x/MWCNT.sub.ox, the Mn K-edge X-ray absorption near edge structure was measured at HASYLAB (Hamburg, Germany) at the Xl beamline in the transmission mode. The energy scale was calibrated using a Mn metal foil with an edge position at 6539 eV. The edge region was evaluated using WinXAS by comparing the positions of the edge features on the energy scale to literature data. The edge position was determined by fitting a modified atan step-function to the near-edge region. In this work, the acronym NEXAFS is used for spectra recorded with surface-sensitive soft X-ray measurements at the Mn L-edge, while the bulk-sensitive hard X-ray measurements at the Mn K-edge are termed XANES.

(39) Carbon-supported s-MnO.sub.x and i-MnO.sub.x with an effective loading of 6.4 and 6.1 wt.-%, respectively (dried at 110° C.) have been tested for electrocatalytic activity and stability.

(40) Electrocatalytic measurements were conducted using a 3-electrode/electrolyte compartment. Pt gauze and a commercial reversible hydrogen electrode (RHE) was used as counter and reference electrode, respectively. 0.1 M potassium phosphate buffer (pH 7) was employed as an electrolyte. Catalyst inks were prepared by mixing 5 mg catalyst with 1.99 ml distilled water, 500 μl isopropanol and 10 μl of Nafion solution (5 wt %, Aldrich), followed by homogenization with a hornsonicator (5 W, 15 min) in a water bath. The catalyst ink (10 μl) was dispersed onto glassy carbon electrodes (Ø 5 mm, 0.196 m.sup.2) and dried at 60° C. in air. The glassy carbon electrodes and Pt cylinder (Ø 5 mm, 0.196 m.sup.2) were polished and cleaned stepwise in ultrasonic bath using water and acetone before usage. Preceding the electrochemical measurements, the electrolyte was de-aerated with N.sub.2 for at least 15 min before switching to a N.sub.2 blanketing flow during measurement. OER activities were measured using anodic linear sweep voltammetry between 1.0 and 2.0 V vs. RHE with a sweep rate of 6 mV/s. All potentials were IR-corrected after measurements using electrochemical impedance spectroscopy. Electrochemical stability was tested by cycling the electrode potential 20 times in the mentioned potential range with a sweep rate of 100 mV s.sup.−1. All potentials are referred to the reversible hydrogen electrode.

(41) Experimental Results

(42) s-MnO.sub.x Sample

(43) SEM images of s-MnO.sub.x after drying at 110° C. are shown in FIG. 1-A, B. It can be seen that very small particles of dimensions in the 10 nm-range form larger aggregates exhibiting a high porosity. SEM-EDX analysis shows that the Mn:O ratio is between 0.6 and 0.73, thus too high for the target MnO.sub.2 compound. The BET-specific surface area was 232 m.sup.2/g, which is relatively high for a transition metal oxide.

(44) The dried precipitate was calcined at different temperatures, the XRD patterns of the resulting samples are presented as FIG. 2 together with the XRD pattern of the sample after heating to 700° C. in the thermobalance. Oxide phases do not crystallize until the calcination temperature has reached 550° C. The appearing reflections can be assigned to Mn.sub.2O.sub.3 (Bixbyite, JCPDS 89-4836), in agreement with the temperature range of the transformation of MnO.sub.2 to Mn.sub.2O.sub.3. Calcination below this temperature yields X-ray amorphous samples.

(45) Simulation of the calcination by thermogravimetric measurements in synthetic air coupled with mass spectrometry is shown in FIG. 3. The thermal decomposition can be divided into four major steps. Initially, at temperatures below the drying temperature of 110° C. an MS signal at m/e=18 indicated emission of weakly bonded re-absorbed water molecules. The next dehydration peak at 173° C. is assigned to the decomposition of hydroxyl groups. The small dehydration peak centered at 303° C. is assigned to emission of water, located in the (2×2) channels of α-MnO.sub.2, which was reported to occur near 300° C. According to the TGA data, all hydroxyl groups can be assumed as decomposed at 303° C. The mass loss observed at 502° C. is assigned to the oxygen loss due to the conversion of MnO.sub.2 to Mn.sub.2O.sub.3, in agreement with the XRD results. The total weight loss up to 700° C. is 18 wt %. Assuming that at temperatures above 502° C. pure Mn.sub.2O.sub.3 is present, which evolved entirely from MnO.sub.2, the calculated mass difference to the state after dehydration at 303° C. should be twice as high, than the experimentally determined difference of just 5.5%. This indicates that part of the manganese has been in a lower oxidation state than IV from the beginning.

(46) The TPR profile of the sample calcinated for 2 h at 170° C., in which the majority of the OH-groups should be decomposed, is shown in FIG. 4. The TPR profile shows two structured peaks occurring below and above approximately 380° C. The low temperature peak has an early onset near 150° C., a shoulder to the low temperature side and a narrow peak at 267° C. The presence of nanoparticles of manganese oxide in the sample may cause the low onset and the shoulder as dispersed manganese oxide is more easily reduced than bigger particles. The first peak can be assigned to the reduction of Mn.sup.4+ to Mn.sup.3+ (MnO.sub.2 to Mn.sub.2O.sub.3). The reduction peak in the temperature range 300-400° C. is a superposition of two reduction steps. The first is assigned to the reduction of Mn.sub.2O.sub.3 to Mn.sub.3O.sub.4 and the second to that of Mn.sub.3O.sub.4 to MnO. The presence of pure MnO after reduction at 700° C. is confirmed by XRD (not shown). Integration of the TPR curve and comparison with the reduction profile of a pure CuO standard reveals that the average oxidation state of manganese in the starting material is Mn.sup.3.42+ (MnO.sub.1.71), in agreement with the Mn:O ratio calculated from the EDX spectra.

(47) Further characterization of the dried sample was attempted by transmission electron microscopy investigations. The s-MnO.sub.x sample consists of an amorphous material. The TEM micrographs in FIG. 5 show, that under the reducing conditions of prolonged electron beam irradiation the material slowly crystallizes. After 13 minutes (FIG. 5-C) first lattice planes can be observed, which are well pronounced after 26 minutes (FIG. 5-D). The Fourier transform (FFT) of the TEM image after 26 minutes, is shown in FIG. 5-E. The interplanar distances correspond to the (011) (112) (013) (121) and (220) lattice planes of tetragonal α-Mn.sub.3O.sub.4 (Hausmannite), with lattice constants of a=5.76 Å and c=9.42 Å. However, this result gives hardly any information about the initial oxidation state of the amorphous starting material as not only crystallization but also reduction may have been triggered by the electron beam in the microscope.

(48) In-situ NEXAFS measurements in the total electron yield mode, which are sensitive to near-surface oxidation states, have been performed to characterize the change of the Mn valence in oxidizing conditions, i.e. with heat treatment in oxygen atmosphere. The Mn L-edges (L3-edge around 640-645 eV, L2 at 650-655 eV) show a rich fine structure due to a strong multiplet splitting that is very sensitive to the local chemical environment.

(49) The evolution of the Mn valence with temperature is shown in FIG. 6(a). FIG. 7(a) presents the experimental and fitted NEXAFS spectra at the Mn L absorption edges of the dried manganese oxide at room temperature and 400° C. in oxygen atmosphere (0.5 mbar). In agreement with the EDX, TG and TPR data, the major component in the as-prepared sample is Mn.sup.4+, but lower oxidation states are also abundant. From the NEXAFS fit an average Mn oxidation state of a hypothetically homogeneous material of +3.40 can be calculated, which is in good agreement with the TPR data. The Mn.sup.4+ component decreases with increasing temperature, while the Mn.sup.3+ component increases. The Mn.sup.2+ component is relatively stable, which may indicate that Mn.sup.2+ is not associated to the Mn.sup.3+ component in a mixed valence compound like Mn.sub.3O.sub.4, but that it may rather be attributed to a surface termination in a reduced state. Between 300 and 400° C. the average reduction of the material stops after an average valence of +2.90 has been reached at 300° C. Heating in oxygen at 400° C. results again in oxidation with an increase in the Mn.sup.4+ site fraction reaching an average oxidation state of +3.92. At further heating above 500° C. a reduction to lower oxidation states takes place. At a temperature above 550° C., a mixture of Mn.sup.2+ and Mn.sup.3+ is existent with an average oxidation state of +2.80. The formation of crystalline Mn.sub.3O.sub.4 (Hausmannite, average valence +2.67) at 550° C. is confirmed by XRD (not shown).

(50) In summary, for the sample s-MnO.sub.x, it can be concluded that the symproportionation of Mn(II) nitrate and permanganate (VII) solutions under the given conditions yielded an amorphous hydrated manganese oxide with an average oxidation state slightly below IV (MnO.sub.1.7.n H.sub.2O). Despite the absence of a stabilizing support, the particle size of the precipitate is very small while the porosity and specific surface area are high. Upon heating in oxygen, the sample loses water and is first reduced to an average oxidation state near +3, before being oxidized to +4 around 400° C. At higher temperature, thermal reduction takes place and leads to formation of stoichiometric manganese oxides. The reactivity of the material depends on the oxygen partial pressure. In addition, TEM has shown that the material is sensitive towards electron beam irradiation, which makes the material somewhat hard to comprehensively characterize. However, the results obtained in this section will be used as reference for the supported material prepared under analogous conditions in presence of MWCNTs.

(51) s-MnO.sub.x/MWCNT.sub.ox Sample

(52) A s-MnO.sub.x/MWCNT.sub.ox sample was prepared with a nominal content of 10.0 wt.-% Mn/C (considering that Mn from both Mn(NO.sub.3).sub.2 and KMnO.sub.4 will be deposited on the CNTs). However, XRF analysis has shown, that the effective concentration of Mn in the sample amounts to only 6.5±0.5 wt.-% Mn/C. Considering that KMnO.sub.4 is a strong oxidizing reagent, we surmise that a fraction did not react with the pre-adsorbed Mn.sup.2+ ions, but was spent to oxidize defects of carbon, which have not been completely oxidized previously by the HNO.sub.3 treatment.

(53) The X-ray diffraction pattern of the dried manganese oxide (110° C.), supported on MWCNTs (s-MnO.sub.x/MWCNT.sub.ox) with 6.5 wt.-% Mn/C does not show any sharp peaks (FIG. 8). The intensity bumps at 2θ=25.6 and 43.4° correspond to the graphite of the MWCNT.sub.ox walls. As for the unsupported reference sample, reflections of crystalline manganese oxide cannot be observed.

(54) Calcination of the dried, MWCNT.sub.ox supported manganese oxide was simulated by thermogravimetric measurements in synthetic air coupled with mass spectrometry. The resulting curves are shown in FIG. 9. The thermal decomposition shows just one major step. Up to 300° C., no significant weight loss is observed. At 403° C. a strong mass loss is seen, caused by the total oxidation of MWCNT.sub.ox, identified by the release of large amounts of CO.sub.2 and by a sharp exothermic DTA peak. This combustion temperature, i.e. the temperature of the half weight loss in the TG curve, is significantly below the combustion temperature of pure oxidized Baytubes (502° C.), which is attributed to the dispersed Mn oxide phase acting as an oxidation catalyst. The total weight loss up to 480° C. is 86 wt %. From the remaining weight of the ashes, an upper limit of the Mn loading of 8.8 wt % Mn/C can be calculated assuming MnO.sub.2 as the decomposition product. This is in reasonable agreement with the concentration determined by XRF analysis.

(55) While the XRD and TGA results are strongly dominated by the carbon support phase, temperature programmed reduction (TPR) allows for characterization of the Mn oxide phase (FIG. 4). For comparison, a TPR profile of pure MWCNT.sub.ox was also recorded. It shows one broad peak at 680° C., due to decomposition of oxygen-containing surface groups. The TPR profile of the s-MnO.sub.x/MWCNT.sub.ox shows three pronounced peaks. Similar to the pure s-MnO.sub.x sample, the first peak at 283° C. is assigned to the reduction of Mn.sup.4+ to Mn.sup.3+ (MnO.sub.2 to Mn.sub.2O.sub.3) and possesses a shoulder to the lower temperature side. In contrast to the pure manganese oxide (s-MnO.sub.x), the reduction signal assigned to Mn.sup.3+.fwdarw.Mn.sup.2+ at 397° C. does not split and the intermediate formation of Mn.sub.2O.sub.3 cannot be resolved. The third broad peak at 555° C. is attributed to the reduction of surface groups of the oxidized MWCNTs. Again, the increased reactivity compared to the transition metal-free system is tentatively attributed to a catalytic function of the highly dispersed Mn oxide during thermal treatment. The relative increase of this high temperature TPR signal may be explained with the creation of additional defects on the CNTs due to the presence of the strong oxidant KMnO.sub.4 during synthesis (see above). Due to the apparent similarity of the reduction temperatures and profiles of the first two TPR signals to pure manganese oxide, the Mn oxide phase in the supported sample is assumed to be identical or very similar to the s-MnO.sub.x material in the unsupported case. Assuming that reduction to MnO is completed after the second peak, a similar average oxidation state of Mn in the starting material of Mn.sup.3.56+ (MnO.sub.1.78) can be calculated by Integration of the first two TRP signals. The presence of pure MnO after reduction at 700° C. is confirmed by XRD (not shown).

(56) SEM images of the s-MnO.sub.x/MWCNT.sub.ox after calcination at 150° C. are shown in FIG. 1-C,D. Areas with large manganese agglomerates cannot be observed. In accordance with the TG-MS data, there is no damage of the MWCNTs due to starting combustion detectable even at high magnifications. SEM-EDX studies confirm that the oxidized tubes are relatively homogeneously coated with manganese oxide particles with local concentrations between 4.5 and 7.0 wt.-%, in agreement with the integral XRF analysis.

(57) The BET surface area decreased from 316 m.sup.2/g for the pure MWCNT.sub.ox to 240 m.sup.2/g after deposition of Mn oxide. The s-MnO.sub.x/MWCNT.sub.ox sample presents a typical type IV isotherm, with a clear hysteresis at high P/P.sub.0. The pore size distribution calculated from the desorption branch of the isotherm using the BJH method shows a bimodal distribution (FIG. 10). Pores below a diameter of 3 nm are assigned to the inner channel of the nanotubes, whereas larger pores >10 nm are formed between the entangled nanotubes. FIG. 10 reveals that the volume of the small pores decreased after deposition of the Mn oxide. The most likely explanation is that MnO.sub.x particles also formed at the tubes tips as well as inside the CNTs, thus blocking the inner channel of some of the CNTs.

(58) The TEM image of the s-MnO.sub.x/MWCNTs in FIG. 11 shows a single particle of Mn.sub.3O.sub.4 (Hausmannite) with a size of approximately 5 nm. However, following the experience of pure manganese oxide it cannot be safely concluded whether the Mn.sub.3O.sub.4 has formed during the preparation or under the reducing conditions of the electron beam in the microscope. Nevertheless it can be assumed that the particle size would not change significantly during such beam-induced reduction, and the successful nanostructuring of the Mn oxide phase by dispersion on MWCNTs is evidenced by this result.

(59) Further insight into the initial oxidation state of the sample s-MnO.sub.x/MWCNT.sub.ox was provided by Mn K-edge XANES (FIG. 12). In contrast to the L-edge NEXAFS, K-edge XANES is bulk sensitive. The edge position was determined to be shifted by 12.2 eV compared to that of metallic Mn reference at 6539 eV. With respect to Mn oxide references such a shift is characteristic for an average valence of ˜3.4. Thus, the valence information extracted from the Mn K-edge XANES is well consistent with the results obtained from TPR analysis. The estimated depth of information in the NEXAFS experiment is here at 5-8 nm, which is in the range of particle sizes observed in TEM. Although NEXAFS is a surface sensitive method, it can be assumed, that the spectra are representative for the bulk of the present nanostructured Mn-phase. FIGS. 6b and 7b present the Mn L-edge during the calcination of the MWCNT.sub.ox supported manganese oxide in 0.25 mbar O.sub.2. In the as-prepared state, the NEXAFS spectrum can be simulated with Mn.sup.2+ and Mn.sup.4+ species. The former may be an effect of reduction in the near-surface region of the nanoparticles. In agreement with the previous results, the average oxidation state of the sample is estimated to +3.55. Alike the bulk reference sample s-MnO.sub.x, the Mn.sup.4+ component firstly decreases with increasing temperature and later comes back at 400° C. at the expanse of Mn.sup.2+ and Mn.sup.3+. This intermediate oxidation is considerably weaker compared to s-MnO.sub.x and an average oxidation state of only +3.04 is reached, while it was +2.83 at 350° C. before the re-oxidation took place. At 450° C. already thermal reduction sets in and the fraction of Mn.sup.4+ lowered again. The lower reaction temperature compared to s-MnO.sub.x can the attributed to the smaller particle size.

(60) In summary, the results obtained on the s-MnO.sub.x/MWCNT.sub.ox sample show that by symproportionation it was possible to produce a nanostructured, MWCNT.sub.ox supported manganese oxide, with similar redox properties as the unsupported manganese oxide sample s-MnO.sub.x. The TPR, K-edge XANES and L-edge NEXAFS analyses show that a large part of the sample is present in the oxidation state +IV. In-situ NEXAFS measurements confirm the variability of the oxidation state of Mn in the electrode material with variation of the pre-treatment temperature and the oxygen partial pressure.

(61) i-MnO.sub.x/MWCNT.sub.ox Sample

(62) An i-MnO.sub.x/MWCNT.sub.ox sample was prepared with a nominal content of 5.0 wt.-% Mn/C. The sample was prepared by incipient wetness impregnation. Accordingly, the oxidized MWCNTs were first impregnated with water to fill their inner channel, followed by the addition of an aqueous solution of manganese(II)nitrate. The sample was dried following a 2-step process: first at 60° C. to allow the Mn nitrate to further spread over the surface of the sample, then at 110° C. to favor its slow dehydration and reaction with the OH groups present on the surface of the oxidized CNTs. Finally, the material was calcined at 300° C. to fully decompose the Mn precursor.

(63) XRD revealed that the formed MnO.sub.x was X-ray amorphous like the other samples (FIG. 8).

(64) After calcination at 300° C., no areas with agglomerates of manganese could be found in the SEM images (FIG. 1-E,F). The SEM-EDX analysis detected a local manganese concentration of 6.4±0.6 wt %. Thus the manganese distribution is nearly homogeneous and close to the nominal 5 wt % Mn/C.

(65) The specific surface, measured by N.sub.2 physisorption, is 332 m.sup.2/g for the calcined sample (300° C.). Thus, the MWCNT.sub.ox surface modified with manganese oxide particles is slightly larger than the surface of pure oxidized MWCNTs (316 m.sup.2/g), which may be explained with the starting decomposition of the carbon material. The pore size analysis reveals a slightly increasing contribution corresponding to small pores in comparison to MWCNT.sub.ox, which is in line with the small damages observed in the SEM images and which arise from the catalyzed oxidation of the CNT around the MnO.sub.x particles. Both observations point towards a different redox behavior of i-MnO.sub.x/MWCNT.sub.ox compared to s-MnO.sub.x/MWCNT.sub.ox.

(66) The TEM micrographs in FIG. 11-B,C of i-MnO.sub.x/MWCNT.sub.ox shows single particles of Mn.sub.3O.sub.4 (Hausmannite) of approximately 5-10 nm. However, following the experience of pure manganese oxide it again cannot be decided whether the Mn.sub.3O.sub.4 has formed during the preparation or under the conditions in the microscope.

(67) FIG. 13 presents the NEXAFS study of i-MnO.sub.x/MWCNT.sub.ox and shows the Mn L-edge evolution during the simulated calcination in 0.25 mbar O.sub.2. The as-prepared material consists exclusively of Mn.sup.2+ as is expected by impregnation with manganese(II)nitrate solution. In contrast to the s-MnO.sub.x samples, this sample is first oxidized upon heating and the Mn.sup.3+ component increases with increasing temperature. Heating to 400° C. results in a mixture of Mn.sup.2+ and Mn.sup.3+. The average oxidation state gradually increases from +2.0 to +2.63 at 400° C.

(68) The Mn oxide particles obtained by incipient impregnation are of a size comparable to those prepared by symproportionation. However, from the NEXAFS results, it is evident that the oxidation state of manganese and its evolution with heat treatment is different. While in s-MnO.sub.x/MWCNT.sub.ox a large fraction of Mn.sup.4+ was present, a divalent manganese species were exclusively found in i-MnO.sub.x/MWCNT.sub.ox. While the Mn centers in s-MnO.sub.x/MWCNT.sub.ox were reduced upon heating, those in i-MnO.sub.x/MWCNT.sub.ox were found to be oxidized but without the formation of Mn.sup.4+.

(69) To investigate possible differences of the MWCNT support in the two supported electrode materials arising from the different preparation procedure, Raman spectroscopy and C is XPS was applied. The thermal analyses suggested that a higher defect concentration may have been generated by the use of strongly oxidizing MnO.sub.4.sup.− solution during preparation of s-MnO.sub.x/MWCNT.sub.ox. Such additional defects may affect the conductive properties of the composite electrode and its electrocatalytic properties and should be reflected in the Raman and carbon core level spectra. No significant difference could be observed in the Raman or C 1s spectra, which are presented as supporting information (FIG. 14). Thus, the amount of amorphous and graphitic carbon was unchanged in the CNT support and differences in the electrocatalytic performance are likely not related to the CNT support.

(70) Electrocatalytic Properties

(71) Both, the s-MnO.sub.x/MWCNT.sub.ox and the i-MnO.sub.x/MWCNT.sub.ox sample were tested for their electrocatalytic activity and stability with respect to the water splitting (oxygen evolution reaction) under neutral pH conditions using the electrocatalytic measurement conditions as described above.

(72) FIG. 15 compares the electrocatalytic OER activity of s- and i-MnO.sub.x/MWCNT.sub.ox during linear electrode potential sweeps at neutral pH with that of the carbon support and a Pt reference electrocatalyst. Both MnO.sub.x/MWCNT.sub.ox composites showed high catalytic activity during the anodic voltammetric scans. In particular, the activity of the Mn oxide materials significantly exceeded the catalytic activity of pure Pt. The overpotential of both MnO.sub.x/MWCNT.sub.ox at 2 mA cm.sup.−2 was 190 mV lower than for Pt. The MWCNT.sub.ox support showed only little catalytic activity and was comparable to pure Pt.

(73) Our results demonstrate that the electrocatalytic efficiency (overpotential) of Mn-catalyzed hydrogen/oxygen production in neutral pH conditions such as pH 7 (1.8 V/RHE at 8 mA/cm2) is comparable to that of the most active electrodeposited MnO.sub.x films in highly alkaline media (1.7 V/RHE).

(74) FIG. 15 also clearly highlights distinct differences in the electrochemical behavior between the two MnO.sub.x/MWCNT.sub.ox. At a current density of 1 mA cm.sup.−2 a 20 mV lower overpotential was obtained for the s-MnO.sub.x/MWCNT.sub.ox than i-MnO.sub.x/MWCNT.sub.ox. In contrast, at a technologically more relevant current density of 8 mA cm.sup.−2 i-MnO.sub.x/MWCNT.sub.ox showed a 120 mV lower overpotential than s-MnO.sub.x/MWCNT.sub.ox.

(75) Thus, s-MnO.sub.x/MWCNT.sub.ox reveals a lower, and hence more favorable onset potential for the OER, whereas i-MnO.sub.x/MWCNT.sub.ox is catalytically more active at higher electrode potentials and current densities.

(76) The following table summarizes the overpotentials achieved by the present composites, i.e. s-MnO.sub.x/CNT and i-MnO.sub.x/CNT, and composites obtained as indicated in the table.

(77) TABLE-US-00001 Overpotential* Number Sample description [mV] 9812 Pure CNT support — 11742 s-MnOx/CNT 0.543 11759 i-MnOx/CNT 0.544 13544 10% Mn, 2-step drying 0.576/0.613 (repro) 13669 10% Mn, 2-step drying, calc 300° C. 0.530 13543 10% Mn, 1-step drying, calc 300° C. 0.464 13542 10% Mn, 2-step drying, calc 400° C. 0.579 *OER overpotentials at 1 mA/cm.sup.2

(78) Our correction for the ohmic potential drops for each catalytic measurement excludes differences in specific conductivity of the samples as origin for the observed reactivity differences. We also note that the hypothetical faradaic current density corresponding to the conversion of all Mn species into the oxidation state +IV at the scan rate of FIG. 15 would be insufficient to explain the current densities observed. Atomic-scale geometric and electronic structural factors accounting for the difference in catalytic water splitting activity may include the difference in surface area between the two supported materials (332 m.sup.2/g for i-MnO.sub.x, while 240 m.sup.2/g for the s-MnOx material); also, the presence of a lower average initial oxidation state of Mn between +II and +III in the i-MnOx material compared to the s-MnOx catalyst, which could be associated with structural motifs involving more hydroxyl OH ligands rather than Mn-bridging 0 ligand may contribute to the observed catalytic activity trends. Based on this, we would predict that at prolonged OER relevant electrode potentials, where Mn is preferably present in the +IV state and would transform from the initial oxidation states, the two MnO.sub.x materials could gradually narrow their activity difference.

(79) The stability of both MnO.sub.x/MWCNT.sub.ox samples was investigated using cyclic voltammetry between 1.0 V and 2.0 V as described above and is shown in FIG. 16. The anodic linear sweep scan after 20 potential cycles led to a loss in activity for both MnO.sub.x/MWCNT.sub.ox composites, with s-MnO.sub.x/MWCNT.sub.ox showing a significantly higher overpotential increase (activity loss) than i-MnO.sub.x/MWCNT.sub.ox. At a current density of 2 mA cm.sup.−2 the overpotential for s-MnO.sub.x/MWCNT.sub.ox and i-MnO.sub.x/MWCNT.sub.ox increased by 140 mV and 40 mV, respectively. In contrast to the MnO.sub.x/MWCNT.sub.ox, the potential cycling of MWCNT.sub.ox led to an unexpected decrease in overpotential but without exceeding the catalytic activity of the aged s-MnO.sub.x/MWCNT.sub.ox.

(80) Comparative Examples

(81) Comparative 4 wt.-% and 10 wt.-% i-MnO.sub.x/SBA-15 samples have been prepared by the same procedure as outlined above for the i-MnO.sub.x/MWCNT.sub.ox except for using an SBA-15 mesoporous silica support.

(82) As is illustrated in FIG. 17, both SBA-15-supported MnO.sub.x samples do not show an electrocatalytic OER activity under neutral pH conditions using the electrocatalytic measurement conditions as described above. It is observed that the lacking OER activity of these SBA-15 supported samples is independent from the loading of MnO.sub.x, i.e. 4 wt.-% or 10 wt.-% MnO.sub.x, hence it is concluded that the silica support impairs the electrocatalytic OER activity, which might be due to an insufficient conductivity of the SBA-15 support.

CONCLUSIONS

(83) From the above it can be concluded that both the incipient wetness composite (i-MnO.sub.x/MWCNT.sub.ox) and the symproportionation precipitation composite (s-MnO.sub.x/MWCNT.sub.ox) are a viable water splitting electrocatalysts at neutral pH. However, the incipient wetness preparation of supported MnO.sub.x led to more active and more stable electrocatalyst than the symproportionation preparation. The variation in catalytic activity between both catalysts, i.e. i-MnO.sub.x/MWCNT.sub.ox and s-MnO.sub.x/MWCNT.sub.ox, could be caused by a variety of structural and electronic differences, such as oxidation states, Mn—O bonding and bond distances, crystallinity and long range order, or variations in surface area and dispersion on the support or in support interactions between the two MnO.sub.x materials. Catalyst degradation as observed in FIG. 15 can be rationalized by loss of the lower Mn oxidation states or Mn ion dissolution during oxidation-reduction cycles or the formation of inactive forms of the MnO.sub.2 species, e.g. due to changes in the crystallinity. Without wishing to be bound to a theory, at present, we relate the differences in performance of the two composites to the significant differences of initial oxidation states of Mn and their evolution (FIGS. 6b and 6c). Further, we conclude that an initially low oxidation state around +II already after preparation as in i-MnO.sub.x/MWCNT.sub.ox might be preferable compared to higher oxidation states near +IV as in s-MnO.sub.x/MWCNT.sub.ox. Moreover, the particularly beneficial effect of the i-MnO.sub.x/MWCNT.sub.ox might be caused by special effects of in-situ electro-oxidation of Mn(II) under working conditions on the material's crystallinity and defect structure compared to the effects of chemical oxidation during synthesis.

(84) Finally, the i-MnO.sub.x/MWCNT.sub.ox and the s-MnO.sub.x/MWCNT.sub.ox composite of the invention are both superior in the electrocatalytic splitting of water under neutral pH conditions as compared to SBA-15 silica-supported MnO.sub.x composites.