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Nanostructured MnxOy for oxygen reduction reaction (ORR) catalysts
Accepted Manuscript Title: Nanostructured Mnx Oy for Oxygen Reduction Reaction (ORR) catalysts Author: Luisa Delmondo Gian Paolo Salvador Jos´e Alejandro Mu˜noz-Tabares Adriano Sacco Nadia Garino Micaela Castellino Matteo Gerosa Giulia Massaglia Angelica Chiodoni Marzia Quaglio PII: DOI: Reference:
S0169-4332(16)30717-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.03.224 APSUSC 32990
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APSUSC
Received date: Revised date: Accepted date:
15-1-2016 7-3-2016 30-3-2016
Please cite this article as: Luisa Delmondo, Gian Paolo Salvador, Jos´e Alejandro Mu˜noz-Tabares, Adriano Sacco, Nadia Garino, Micaela Castellino, Matteo Gerosa, Giulia Massaglia, Angelica Chiodoni, Marzia Quaglio, Nanostructured MnxOy for Oxygen Reduction Reaction (ORR) catalysts, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.03.224 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanostructured MnxOy for Oxygen Reduction Reaction (ORR) catalysts Luisa Delmondo*a, Gian Paolo Salvadorb, José Alejandro Muñoz-Tabaresb, Adriano Saccob, Nadia Garinob, Micaela Castellinob, Matteo Gerosaa,b, Giulia Massagliaa,b, Angelica Chiodonib and Marzia Quagliob a
Department of Applied Science and Technology - DISAT, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino (Italy)
b
Center for Space Human Robotics @PoliTo, Istituto Italiano di Tecnologia, C.so Trento 21, 10129 Torino (Italy)
* Corresponding author (Luisa Delmondo). Tel.: + 39 011 5091931; Fax: +39 011 5091901; e-mail: [email protected]
Highlights
• Good performance catalysts for oxygen reduction reaction • Nanostructured low-cost catalysts respect to platinum ones • Synthesis using environmental benign chemical reagents
Abstract In the field of fuel cells, oxygen plays a key role as the final electron acceptor. To facilitate its reduction (Oxygen Reduction Reaction - ORR), a proper catalyst is needed and platinum is considered the best one due to its low overpotential for this reaction. By considering the high price of platinum, alternative catalysts are needed and manganese oxides (MnxOy) can be considered promising substitutes. They are inexpensive, environmental friendly and can be obtained into several forms; most of them show significant electro-catalytic performance, even if strategies are needed to increase their efficiency. In particular, by developing light and high-surface area materials and by optimizing the presence of catalytic sites, we can obtain a cathode with improved electro-catalytic performance. In this case, nanofibers and xerogels are two of the most promising nanostructures that can be used in the field of catalysis. In this work, a study of the morphological and catalytic behavior of MnxOy nanofibers and xerogels is proposed. Nanofibers were obtained by electrospinning, while xerogels were prepared by sol-gel and freeze drying techniques. Despite of the different preparation approaches, the obtained nanostructured manganese oxides exhibited similar catalytic performance for the ORR, comparable to those obtained from Pt catalysts.
Keywords
ORR catalysts; manganese oxides; nanofibers; xerogels. 1 Introduction Driven by growing concerns about global warming and the depletion of petroleum resources, the development of new and clean strategies for energy conversion and storage represents one of the major scientific challenges of the twenty-first century [1]. Fuel cells [2] and metal-air batteries [3] are, indisputably, among the most important energy-related technologies trying to give an answer to this demand [4,5]. The electrochemical Oxygen Reduction Reaction (ORR) occurring at the cathode of these devices is one of the key limits for their further development [6]. Indeed, the ORR is particularly slow and energetically not favored, requiring electrocatalysts to increase its efficiency. In devices operating at ambient conditions, platinum and related materials are the preferred choice to catalyze the ORR [7, 8], even thought the high price of this scarce precious metal has a decisive impact on the cost of the final devices. For this reason, during the last decade non-precious, highly durable and low-cost catalysts have been explored in order to reduce or eliminate platinum [8], while preserving comparable ORR activity. Currently two main classes of materials are under investigation as potential substitutes for Pt: metal-free ORR catalysts and non-precious metal compounds (NPMCs) and oxides [7]. Among metal-free ORR catalysts, N-doped carbon-based materials have a prominent role, showing a noticeable durability and cost-effective features. Pure carbon materials as nanostructured carbons, carbon nanotubes, and graphene sheets, generally show poor electrocatalytic activities. Doping them by different non-metals heteroatoms (e.g., N, B, S, O, P) and/or decorating their surfaces with metal oxides can significantly enhance their ORR activities [9]. Among the NPMCs investigated so far, metal oxides have a great potential as alternative catalysts for the ORR. A key limit exists, which is related to their low electrical conductivity. For this reason, their coupling to carbon materials is effective in enhancing the catalytic performances by the improved electron transfer [7, 10]. A common strategy for the enhancement of the catalytic performances of all these materials is to shape them in new structures able to significantly increase their surface area, resulting in an increased number of catalytic sites actually exposed to oxygen [7]. Given to their low cost, relatively high abundance, lower environmental impact and considerable electrocatalytic activity, manganese oxides are among the most interesting non-precious metal catalysts [6, 10, 11]. With the aim to design well performing ORR catalysts, in this work, manganese oxides were synthesized according to two key nanostructures: nanofibers and xerogels, by using biocompatible and biodegradable precursors such as PolyEthylene Oxide (PEO) and agar, respectively, with manganese acetate as the Mn source. PEO was chosen both because it is biocompatible and biodegradable and generally, because it is more environmental friendly than other common templating agents used to produce nanofibers. Indeed, it needs only water as solvent, differently from what is used for a widespread list of polymers, such as polyvinyl pirrolidone (PVP) [12,13] and polyacrylonitrile (PAN) [14,15]. Taking into account the same environmental approach, for xerogel synthesis, Agar was chosen as the gelling agent for the inorganic precursor, because it is a polysaccharide, extracted from a group of red-purple marine algae, that can become a gel at room temperature without the addition of catalysts, which are, usually, inorganic salts (inorganic chlorides, nitrates, etc.). Avoiding the use of these salts is particularly important since their excess removal requires post-treatments that are not environmentally benign in terms of energy (because of the thermal treatments) and/or solvents consumption. For example, other polysaccharides, such as k-carrageenan or sodium alginate, need catalysts for the gelling process and therefore lead to less green synthesis routes. Electrospun nanofibers and xerogels, are gaining increasing interest in the field of catalysis since they show very high surface area combined to well-controlled and reproducible synthesis routes [16-19]. Moreover they
offer two intrinsically different technological approaches to be exploited for their integration in final devices. Indeed nanofibers arrange themselves during the electrospinning process in thin mats or sheets collected on substrates, while xerogels can be shaped in 3-dimentional, self-standing structures by the proper use of molds. The concurrent development of these two nanostructures offers incredible advantages for new electrodes engineering and processing. As an example, the use of carbon-based materials as substrates for the synthesis of catalysts mats can be useful to demonstrate the possibility of direct fabrication of electrodes for electrochemical devices by the electrospinning technique [20]. In this paper we discuss the preparation of both nanofibers and xerogels-based manganese oxides, highlighting their crystalline structure and morphology. The catalytic behavior of the resulting materials with respect to the ORR is then evaluated through Rotating Ring Disk Electrode (RRDE) electrochemical measurements, and compared to the behavior of platinum. An impressive result of 3.6 electrons involved in the ORR is reported for both nanofibers and xerogels -based manganese oxides. 2 Materials and methods 2.1 Materials Polyethylene oxide (Mw = 600 kDa), bacteriological agar, manganese (II) acetate tetrahydrate (Mn(CH3COO)2·4H2O), ethanol, 2-propanol, Nafion solution (5 wt% in water and aliphatic alcohols), potassium hydroxide and Pt/C paste were purchased from Sigma-Aldrich and processed without any further purification. Silicon wafers (p type <100>, 1-30 Ω·cm) were supplied by MEMC. De-ionized water (Diwater) was used as solvent. 2.2 Methods 2.2.1 Manganese oxide nanofibers synthesis Firstly, 9 ml of aqueous PEO solution (5 wt%) were prepared by mixing PEO powder and Di-water. Afterwards, 3ml of 20 wt% aqueous manganese acetate solution were added to the polymeric solution and stirred for 24 h, at room temperature. The PEO/manganese acetate solution, thus prepared, was loaded in a 6 ml syringe, connected to a stainless steel needle (27 Gauge x 15 mm) and mounted into a NANON 01A electrospinning apparatus (MECC CO., LTD.) equipped with a high voltage power supply (HVU-30P100) and a syringe pump setup operating with a flow rate ranging from 0.1 ml/h up to 99.9 ml/h. During electrospinning, a voltage of about 14.5 kV was applied between the needle and a planar collector plate, covered with a substrate. A carbon-based material and monocrystalline silicon wafer were selected as substrates. In particular, silicon was selected in order to easily perform structural and morphological analysis on the mats, while the carbon-based substrate was selected to test the ability to directly assembly the electrode with the catalyst. The working distance, i.e. the distance from the tip of the needle to the collector, was 0,1 m in all cases. The electrospun PEO/manganese acetate nanofibers were then dried in vacuum at 70 °C for 24 h, in order to eliminate all the possible residues of solvent. Further on, the samples were calcinated by means of a vertical furnace (Carbolite, VST 12/300/3216) at 480 °C for 3 h in air, with a heating rate of 2.5 °C/min to ensure the complete decomposition of the PEO and the manganese acetate oxidation. 2.2.2 Manganese oxide xerogel preparation Manganese oxide xerogels were prepared by sol–gel method. The synthesis process included the dissolution of manganese acetate into the proper quantity of deionized water to prepare solutions with two different Mn concentrations (0.05 M and 0.1 M). Concurrently, a solution of agar (4%wt) was prepared by dissolving solid agar into hot water (about 85°C), under stirring. Once the agar was solubilized, the manganese acetate solution was added and let mixing for one hour. Afterwards, the manganese acetate/agar solution was poured into plastic moulds and let become a strong structured gel for a few hours.
The obtained gel was, then, dried by means of a freeze dryer (Lio-5PDGT – Cinquepascal srl) for two days. Next, the dried gels were calcinated in air (at 600°C for 6 hours, with an heating rate of 2.5 °C/min) in order to burn agar and transform manganese acetate into manganese oxide. Both kind of samples, nanofibers and xerogels, were stored at room temperature in a low-humidity container for their further characterization. 2.3 Characterizations The morphologies of both kinds of samples were examined by means of Field Emission Scanning Electron Microscopy (FESEM, ZEISS, Merlin and Supra 40), equipped with an Oxford Energy Dispersive X-ray detector. Samples for Transmission Electron Microscopy (TEM) were prepared by suspending a small quantity of material in ethanol, subsequently immersed in ultrasonic bath. A suspension droplet was then drawn and applied to a standard holey carbon TEM Cu grid, analyzing the specimen after the complete evaporation of the solvent. TEM observations were performed with a FEI Tecnai F20ST, equipped with a field emission gun (FEG) and operating at 200 kV. X-ray diffraction (XRD) with Bragg–Brentano symmetric geometry was performed with a PANalytical X'Pert Pro instrument using Cu-Kα (40 kV and 40 mA) radiation. Continuous scan mode was used to collect XRD data in a range of 10° to 74° and 0.02° as step size. Rietveld analysis was performed in order to analyze each spectrum by using Topas Academic software (version 4.1). Convolution-based method was used where the source emission profiles with full axial instrument contributions was modeled [21], while the background was fitted with a Chebyshev polynomial function with eight parameters. Average crystallite size was assumed to be isotropic in all cases and modeled by applying the integral breadth based method, whereas lattice strain was assumed to be zero because of the small crystal size found during the early refinement steps. Information about the surface area of the xerogel samples was obtained by measuring nitrogen adsorptiondesorption isotherms at -196°C using liquid nitrogen on a Quadrasorb Si from Quantachrome. Prior to the adsorption measurement, the samples were evacuated at 150°C under vacuum overnight. The surface area was determined in the relative pressure range from 0.1 to 0.3 of the Brunauer-Emmett-Teller (BET) plot. Decomposition behaviors of PEO and agar were analyzed by means of thermogravimetric analyses (TGA) under an air flux at a scan rate of 10°Cmin-1, using a TG209 F1 Libra from Netzsch. X-ray photoelectron spectroscopy (XPS) studies were carried out by a PHI 5000 Versaprobe scanning X-ray photoelectron spectrometer (monochromatic Al K-alpha X-ray source with 1486.6 eV energy, 15 kV voltage and 1 mA anode current), in order to investigate surface chemical composition. All the electrochemical measurements were carried out by means of a CHInstrument 760D electrochemical workstation and an ALS RRDE-3A rotating ring disk electrode apparatus. For all the measurements, the manganese oxide catalysts were deposited on the working electrode (a BioLogic glassy carbon disk/Pt ring) according to the following method. Before each catalyst deposition, the working electrode was properly polished with ethanol. The catalyst (2 mg of active mass) was dispersed in a solution containing 25 µl of water, 175 µl of Nafion© solution and 100 µl of 2-propanol. The mixture was ultrasonicated for 2 minutes to form a uniform black dispersion. 10 µl of this formulation were cast-coated onto the disk surface to form a uniform film. The resulting deposition was dried at room temperature for one day. For comparison purposes, commercial Pt/C paste was used as reference catalyst and deposited on the electrode by the same procedure described above. The final Pt loading was 0.5 mg/cm2. Pt was used as counter electrode and Ag/AgCl as reference electrode. The electrolyte used was 0.1 M KOH O2-saturated aqueous solution and the experiments were conducted at room temperature. All the potentials are reported with respect to the Ag/AgCl electrode. For the Rotating Ring Disk Electrode (RRDE) measurements, the disk electrode was scanned cathodically from 0.2 V to -0.8 V, with a rate of 5 mV/s and a fixed rotating speed of 2500 RPM, while the ring electrode was maintained at a fixed potential of 0.2 V. Electrochemical Impedance Spectroscopy (EIS) measurements were carried out at 0, -0.3 and 0.6 V potentials and 2500 RPM rotating speed, with a small signal of 10 mV and frequency range 10-2 – 104 Hz. 3 Results and discussion
3.1 Nanofibers Thin random mats of nanofibers were synthesized starting from manganese (II) acetate as Mn precursor and PEO as templating agent. Morphological characterizations were performed on the electrospun nanofibers before the drying step, in order to verify the quality of the obtained mat. FESEM images of the as-electrospun nanofibers at two different magnifications are reported in Fig. 1(A-B). They show fibers with a smooth surface and with diameters ranging from 120 to 200 nm. Branching effect, caused by a charge accumulation inside the jet, can seldom be observed [22].
Fig. 1. FESEM images of the nanofibers after drying on carbon-based material at two different magnifications.
The most critical step to obtain the final manganese oxide nanofibers was the calcination of the aselectrospun nanofibers. This criticality is related to the need to retain the shape of the nanofibers, to totally remove PEO and to completely convert the manganese acetate nanofibers into manganese oxide ones, at the same time. Even if the manganese acetate oxidation can be obtained at lower temperatures [13], the calcination step was performed at 480 °C in air, since at this temperature PEO could almost be completely removed, as demonstrated by TG analysis reported in Fig. 2.
Fig. 2. TGA plot of a water solution of 5wt% PEO 600kDa.
Fig. 3(A-B-C) show FESEM images at different magnifications of the calcinated electrospun manganese oxide nanofibers supported on carbon-based substrate, while Fig. 3(D) shows a FESEM image of the fibers supported on Si substrate. The first substrate was selected as conducting support for the fibers in a possible electrode, the second one for easiness of characterizations. In these images it is possible to observe how the manganese oxide well retains the 1D fibrous morphology on both the tested substrates, exhibiting a nanograin structure (Fig. 3(C-D)). Because of the thermal treatment, a slightly collapse of the nanofiber mats is generally observed, as evidenced in Fig. 3(A), if compared to Fig. 1(A), caused by PEO thermal removal. Despite the different arrangement of the fibers after calcination, we can assume that the effective surface area is preserved because of the presence of nanograins (Fig. 3(C-D)).
Fig. 3. FESEM images at different magnifications of nanofibers after calcination on carbon based (A-C) and on silicon (D) substrates.
The grain structure was further analyzed by the High Resolution (HR)TEM. Fig. 4(A) shows a Bright Field (BF) image of a fiber portion deposited on the Cu-grid: this presents the characteristic longed shape of the fibers, in addition to the presence of very small grains. The Selected Area Electron Diffraction (SAED) pattern obtained from such a fibers was composed by complete low intense rings, which were indexed as Mn3O4 (inset in Fig. 4(B)). The Dark Field (DF) image on another fiber portion, reported in Fig. 4(B), shows that the grains composing the fiber are actually round crystals with an average size of 10 nm, randomly distributed through the fiber. This DF image was obtained by selecting the electrons from one of the most intense ring on the SAED pattern, that corresponds to the {211} family planes for Mn3O4. Finally, Fig. 4(C) and 4(D) show an HRTEM image of one of these crystals (white frame denoted (i)) and its corresponding Fast Fourier Transform (FFT), respectively. The FFT was indexed as Mn3O4 along the [100] axis zone. The above result points out the polycrystalline structure of the electrospun nanofibers.
Fig. 4. TEM images of the calcinated nanofibers in (A) bright field, (B) dark field, SAED pattern in the inset (C) (HR)TEM micrograph and its corresponding (D) FFT.
Fig. 5 shows the XRD spectrum obtained from Manganese Oxide nanofibers supported on Si. In this spectrum, the diffraction peaks at 28.9°, 32.5° and 36.1° (2θ) correspond to (112) (103) and (211) planes for Mn3O4 with lattice parameters equal to a = b 5.764 ± 0.002 Å and c = 9.445 ± 0.004 Å. In addition to the main manganese oxide phase, some sharp peaks were found above 45° (2θ) which correspond to the silicon substrate on which the fibers were supported. These results are a clear indication of the monophasic nature of the sample, which is composed by Mn3O4 only, in agreement with TEM observations. The Bragg positions for M3O4, which fit well in position and intensity with the experimental data and the model obtained from Rietveld refinements, are also reported in Fig. 5. The fitting between the experimental data and the model is remarkably good, with χ2 equal to 1.45 and a Rbragg below 1%. The crystal size was estimated in 11.3 ± 0.3 nm that, also, fits well with what found by (HR)TEM, indicating a narrow crystal size distribution [23].
Fig. 5. XRD pattern of the manganese oxide nanofibers supported on Si after calcination. In the graph are reported the experimental measure (black curve), the fitting results (red curve) and their difference (gray curve).
3.2 Xerogels A massive nanostructured material was synthesized starting from manganese (II) acetate as Mn precursor. The process involved the preparation of a gel, using agar as gelling agent. The agar-manganese acetate (AMA) gel was dried by means of the freeze dryer technique, in which the samples were frozen at -50°C and then the solvent (water) was removed by sublimation, using a vacuum pump. This drying method lets the removal of the solvent within the gel network, avoiding, as much as possible, the collapse of the structure, giving rise to 3D light nanostructures. A calcination step in air was necessary in order to have a manganese oxide material. The calcination experimental conditions were chosen taking into account the parameters tested in the nanofiber preparation, even if some variations were necessary. This because of the differences in volume and geometry of the obtained materials and the higher complexity of the agar molecule with respect to PEO. For example, PEO can be removed by thermal treatment at temperatures around 400°C as determined by TG analysis (Fig. 2), while a relevant amount of agar (~20%) at temperatures above 700°C is still present (Fig. 6).
Fig. 6. Thermal decomposition behavior of agar gel measured by means of TGA.
Once the samples were prepared, a morphological characterization was due for evaluating which kind of crystalline structures and types of manganese oxide were synthesized. First of all, the structure determination was carried out taking into account that the volume shrinkage due to the drying and calcination step was not so excessive (about 15%) and from this point of view the samples could be considered as aerogels. On the other hand, surface area calculation by means of nitrogen adsorption technique and the analysis of the adsorption isotherms by BET equation (not shown here) led to values (about 40m2/g) that are lower than those usual for aerogels, even if the inorganic aerogels have, typically, lower surface area values than the organic ones [24,25]. From FESEM images it is also possible to appreciate that the agar morphology is predominant before the oxidation step (Fig. 7(A)), while after the thermal treatment two different types of grains are clearly visible (Fig. 7(B-C)). The first one presents a dendritic morphology while the second one is granulated, suggesting nanosized grains. In both cases, anyway, it was not possible to appreciate the presence of pores, confirming the low surface area estimated by BET analysis. For this reason, it is more appropriate to name the samples as xerogels than aerogels.
Fig. 7. FESEM images of (A) xerogels before thermal treatment and of samples (B) Mn1, (C) Mn2, after thermal treatment.
The presence of grains with different sizes and morphology (crystallographic habit) could suggest the existence of different types of manganese oxides. Different crystallization (growth) kinetics could be
observed during the calcination process, rather than one, as obtained for the nanofibers, where Mn3O4 was the only phase found. In order to verify this hypothesis, XRD measurements were carried out. Fig. 8(A-C) show the XRD spectra for the samples prepared starting from 0.5M (Mn1) and 1M (Mn2) manganese acetate water solutions, respectively. According to the obtained data, in the Mn1 sample a mix of two oxides (Mn3O4 and Mn2O3) was confirmed by the presence of the main diffraction peaks for Mn3O4 at 28.9°, 32.5° and 36.1° (2θ) that correspond to the (112) (103) and (211) planes, and the intense peaks at 32.9°, 38.3° and 55.2° indexed as Mn2O3, (222), (104) and (044) planes. Fig. 8 also shows the model obtained from the Rietveld refinement of Mn1 sample (χ2 equal to 3.82 and Rwp of 3.93), from which it was possible to estimate a proportion between the two oxides (Mn3O4:Mn2O3) of ≈ 1:4. Additionally to the difference in composition, a major difference was also found in the crystal size, where the average size for Mn2O3 was 65.8±0.8 nm against 21± 1 nm for Mn3O4. The above results, together with the FESEM images, allow us to postulate that, effectively, the big dendritic grains are associated to Mn2O3 phase, while the nanosized grains correspond to the Mn3O4, similar to what found in electrospun nanofibers. Regarding the Mn2 sample, XRD spectrum (Fig. 8(B)) also shows a mix of Mn3O4 and Mn2O3 oxides. In this case, however, it was found a proportion Mn3O4:Mn2O3 of 1:1. This change could be really ascribable to the increase of manganese amount in the AMA, that leads to an increase in the Mn3O4 formation. Rietveld refinement (χ2 equal to 2.29 and Rwp of 2.95) of the Mn2 samples allows us also estimate the crystal size for the oxides mix and a similar trend, as Mn1 samples, was found. Here, the estimated average sizes were 71±3 nm for Mn2O3 and 21± 1 nm for Mn3O4. Finally, it is important to mention that, additionally to the peaks related to the two manganese oxides, some low intense peaks were found. These peaks could be attributed to impurities due to manganese oxalate formation during the calcination step.
Fig. 8. XRD fitting of samples Mn1 (A) and Mn2 (B) after calcination. In the graph are reported the experimental measures (black curve), the fitting results (red curve) and their difference (gray curve).
To further support the XRD and FESEM analyses, TEM characterization of the xerogels has been performed. The results related to the Mn2 sample are shown in Fig. 9. The two different morphologies observed by FESEM (Fig. 7), i.e. the large dendritic structures and nanosized grains, were also observed by TEM as presented in the BF image of Fig. 9(A) (frame (i) for the nanosized grains and frame (ii)) for the dendritic structure). The SAED pattern obtained from the area observed in Fig. 9(A) is presented in Fig. 9(B). This SAED pattern is composed by two different patterns, that are clearly distinguished. The first corresponds to complete and low-intensity rings, which was indexed as Mn3O4, and that could, a priori, be assigned to the nanosized grains (frame (i) in Fig. 9(A)). The second pattern, which corresponds to the bright spots, was assigned to the large dendritic structure (frame (ii)) and indexed as Mn2O3 along the [201] axis zone. The correlation between these two kinds of pattern and the two different morphologies was confirmed by the DF image presented in Fig. 9(C-D). The DF image in Fig. 9(C) was obtained by selecting the electrons from one of the most intense ring on the SAEDP pattern, that corresponds to the {211} family planes for Mn3O4. In this figure, it can be observed that the nanograins are actually rounded Mn3O4 crystals with average size of 25 nm (see the HRTEM image in the inset of Fig. 9(C)). On the other hand, the dendritic structure in DF imaging, which was obtained by selecting the electrons from one of the closest spots to the SAED pattern center, is presented in Fig.9(D). In this DF image, the dendritic structure presents a bright contrast, even though the central part is darkened because of the particle thickness; however, a closer look at this DF image makes evident the presence of rectangular domains of less of 100 nm (highlighted by the white arrow on Fig. 9(D)), which fit well with the crystal size found by means of XRD. The above results suggest that the entire dendritic structure in Fig. 9(A) is composed by Mn2O3 alternate domains oriented along the <201> axis zones (see also the HRTEM image in the inset of Fig. 9(D)).
Fig. 9. TEM images of xerogels in (A) bright field, (B) SAED pattern, (C) and (D) Dark Field and (HR)TEM micrographs of the nanosized grains and the dendritic structure, respectively.
Taking into account the calcination temperature (600°C) employed for both Mn1 and Mn2 samples and the TG analysis of agar, it is very probable the presence of a residual amount of carbon in the sample. This feature is not detrimental for the catalytic performance of these samples, as shown in the following paragraph. Manganese oxides, on the other hand, do not have a good electrical conductivity. In order to prepare well-performing electrodes for electrochemical devices, it is, therefore, necessary to increase the oxide electrical conductivity. It is well known that carbon-based materials are used as electrodes [29], for this reason the presence of carbon impurities, in a graphitic shape, in our samples (as confirmed by Raman spectrum not shown here) is advantageous in the perspective of fabricating ORR electrodes. In order to confirm this statement, an Energy Dispersive X-ray Spectroscopy (EDS) analysis of both precursor concentrations was carried out. It was possible to point out that, actually a residual amount of carbon is still present (Fig. 10(A)). Other elements like Na, Mg, Ca and S, evidenced by the EDS measurements, can be attributed to impurities in agar. Residual carbon can be due to agar degradation residues, because of the presence of C-C, C=O, C-O bonds, as pointed out by XPS measurements (Fig. 10(B)).
Fig. 10. A) EDS plot of samples Mn1 and Mn2 after calcination. B) XPS HR C1s spectra of samples Mn1 and Mn2. The C1s peaks were deconvoluted (curves fit not reported) with three components due to C-C, C-O and –C=O bonds.
3.3 Catalytic performance The catalytic performance of the presented manganese oxide-based materials was evaluated by means of different electrochemical techniques. The RRDE technique allows validating the catalytic pathways of the proposed materials by using 4electrodes measurements. While scanning the disk potential at fixed rotation rate, the current at disk and ring electrode is measured. The disk current is related to the four-electrons ORR current of the analyzed catalyst, while the current of the ring electrode (whose potential is fixed at a large value) is associated to the twoelectrons ORR (intermediate) peroxide species [26]. In Fig. 11 the results for the samples under investigation are reported. It can be observed that for both nanofibers and xerogels, the ring currents (Fig. 11(A)) appear very low if compared to the corresponding disk currents (Fig. 11(B)). This means that the preferred reduction pathway relies on the four electrons reaction, and, as consequence, the current related to the two electrons reduction is low. In addition, by looking at the disk currents, a larger conductivity can be inferred for Mn2 and nanofibers samples, leading to increased cathodic currents with respect to that of Mn1 xerogel. For completeness, also the currents related to the reference Pt/C catalyst are reported in Fig. 11(A) and in the inset of Fig. 11(B). Starting from these curves, and by applying equations (1) and (2) below, (1)
(2)
(where IR and ID are the ring and disk currents, respectively, and N is the current collection efficiency of the Pt ring) [27], it is possible to estimate both the percentage of peroxide species formation (HO2- %) and the electron transfer number n, and to compare the ORR catalytic performance of all the samples under investigation. The dependence of peroxide species percentage and electron transfer number on the applied potential is reported in Fig. 11(C). For the proposed catalysts, n values comprised between 3.5 and 3.7 were obtained for potentials lower than -0.4 V, with the corresponding peroxide species percentages lower than 25%, in line with other low-cost catalyst proposed in the literature [28]. In addition, it has to be highlighted that the obtained performance for the manganese oxide-based nanofibers and Mn2 xerogel samples are just slightly lower than the reference Pt sample ones, as reported in Fig. 11(C).
Both the materials showed a good catalytic behavior for the oxygen reaction reduction, being good candidates as low-cost and green catalysts for applications in electrochemical devices. Slightly lower performance was obtained for the Mn1 sample. In order to further investigate this aspect, EIS analysis was carried out and the results are discussed below.
Fig. 11. Ring current (A) and Disk current (B) obtained from the RRDE measurements of manganese oxide-based and reference Pt/C samples at a rotating speed of 2500 rpm and a potential of the ring electrode of 0.2 V. (C) Comparison of electron transfer number (left axis) and peroxide percentage (right axis) evaluated from RRDE measurements of manganese oxide-based samples and of the reference Pt/C catalyst at 2500 rpm rotating speed and different potentials.
In Fig. 12 the Nyquist plots of the impedances related to all the manganese oxide-based and reference Pt samples are reported. For all the materials, a typical spectrum composed by a high-frequency main arc (related to the charge transfer at the oxide/electrolyte interface) and a low-frequency side arc (related to the diffusion of species) [30] can be observed. In accordance with the RRDE measurement reported in Fig. 11, a larger impedance is exhibited by Mn1 xerogels, while the other samples are characterized by quite similar behaviors. In order to quantify this discrepancy, impedance spectra were fitted with the equivalent circuit shown in the inset of Fig. 12, which represents the well-known Randles’ circuit [31]: it is composed by the series resistance Rs (accounting for the electrolyte conductivity), the charge transfer resistance Rct (related to the electron transfer at the solid/liquid interface), the double layer capacitance Qdl (accounting for the charge accumulation at the same interface, here modeled through a constant phase element [32]), and the Warburg element Ws (modeling the charge diffusion). The fitted curves are reported in Fig. 12, superimposed to the experimental data: as can be clearly observed, a good matching between measured and calculated curves was obtained, thus witnessing an optimal choice of the equivalent circuit used for the fitting procedure. The main parameter obtained from this procedure is the charge transfer resistance, which can be directly connected to the catalytic properties: as expected, a larger value (9850 Ω) was obtained for Mn1 sample with respect to those of the other two samples (6082 Ω and 6492 Ω, for Mn2 sample and nanofibers, respectively), while quite similar values of the other parameters were obtained for all the catalyst materials (Rs of about 55 Ω, Qdl in the range 11-13 μF). This justifies the slightly lower disk current, and a correspondingly lower electron transfer number, was obtained for the Mn1 xerogel sample. Moreover, in accordance with the electrochemical measurements reported above, the reference Pt/C catalyst exhibits the lowest resistance value, namely 2484 Ω.
Fig. 12. Nyquist plot of the impedance of the different samples measured at 2500 rpm rotating speed and -0.3 V. The points are experimental data while the continuous lines are the curves obtained from a fitting procedure using the equivalent circuit shown in the inset.
4 Conclusions
Nanostructured manganese oxides were prepared by using biocompatible and biodegradable precursors, by means of the electrospinning and sol-gel plus freeze drying techniques, respectively. These materials were obtained in the form of nanofibers and xerogels. In particular, thanks to the optimization of the thermal treatment, reproducible samples of nanofibers composed by Mn3O4, and xerogels composed by a mix of Mn3O4 and Mn2O3, with relative proportion depending on the initial Mn precursor concentration, were obtained. All the nanostructured MnxOy-based catalysts showed extremely good catalytic performance for the oxygen reduction reaction, with n values between 3.5 and 3.7, at potentials lower than -0.4 V, which is in line with other cost-effective catalysts proposed in the literature. Both the nanofibers and xerogels resulted to be good candidates as low-cost and green catalysts for applications in electrochemical devices.
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S0169-4332(16)30717-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.03.224 APSUSC 32990
To appear in:
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Received date: Revised date: Accepted date:
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Please cite this article as: Luisa Delmondo, Gian Paolo Salvador, Jos´e Alejandro Mu˜noz-Tabares, Adriano Sacco, Nadia Garino, Micaela Castellino, Matteo Gerosa, Giulia Massaglia, Angelica Chiodoni, Marzia Quaglio, Nanostructured MnxOy for Oxygen Reduction Reaction (ORR) catalysts, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.03.224 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanostructured MnxOy for Oxygen Reduction Reaction (ORR) catalysts Luisa Delmondo*a, Gian Paolo Salvadorb, José Alejandro Muñoz-Tabaresb, Adriano Saccob, Nadia Garinob, Micaela Castellinob, Matteo Gerosaa,b, Giulia Massagliaa,b, Angelica Chiodonib and Marzia Quagliob a
Department of Applied Science and Technology - DISAT, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino (Italy)
b
Center for Space Human Robotics @PoliTo, Istituto Italiano di Tecnologia, C.so Trento 21, 10129 Torino (Italy)
* Corresponding author (Luisa Delmondo). Tel.: + 39 011 5091931; Fax: +39 011 5091901; e-mail: [email protected]
Highlights
• Good performance catalysts for oxygen reduction reaction • Nanostructured low-cost catalysts respect to platinum ones • Synthesis using environmental benign chemical reagents
Abstract In the field of fuel cells, oxygen plays a key role as the final electron acceptor. To facilitate its reduction (Oxygen Reduction Reaction - ORR), a proper catalyst is needed and platinum is considered the best one due to its low overpotential for this reaction. By considering the high price of platinum, alternative catalysts are needed and manganese oxides (MnxOy) can be considered promising substitutes. They are inexpensive, environmental friendly and can be obtained into several forms; most of them show significant electro-catalytic performance, even if strategies are needed to increase their efficiency. In particular, by developing light and high-surface area materials and by optimizing the presence of catalytic sites, we can obtain a cathode with improved electro-catalytic performance. In this case, nanofibers and xerogels are two of the most promising nanostructures that can be used in the field of catalysis. In this work, a study of the morphological and catalytic behavior of MnxOy nanofibers and xerogels is proposed. Nanofibers were obtained by electrospinning, while xerogels were prepared by sol-gel and freeze drying techniques. Despite of the different preparation approaches, the obtained nanostructured manganese oxides exhibited similar catalytic performance for the ORR, comparable to those obtained from Pt catalysts.
Keywords
ORR catalysts; manganese oxides; nanofibers; xerogels. 1 Introduction Driven by growing concerns about global warming and the depletion of petroleum resources, the development of new and clean strategies for energy conversion and storage represents one of the major scientific challenges of the twenty-first century [1]. Fuel cells [2] and metal-air batteries [3] are, indisputably, among the most important energy-related technologies trying to give an answer to this demand [4,5]. The electrochemical Oxygen Reduction Reaction (ORR) occurring at the cathode of these devices is one of the key limits for their further development [6]. Indeed, the ORR is particularly slow and energetically not favored, requiring electrocatalysts to increase its efficiency. In devices operating at ambient conditions, platinum and related materials are the preferred choice to catalyze the ORR [7, 8], even thought the high price of this scarce precious metal has a decisive impact on the cost of the final devices. For this reason, during the last decade non-precious, highly durable and low-cost catalysts have been explored in order to reduce or eliminate platinum [8], while preserving comparable ORR activity. Currently two main classes of materials are under investigation as potential substitutes for Pt: metal-free ORR catalysts and non-precious metal compounds (NPMCs) and oxides [7]. Among metal-free ORR catalysts, N-doped carbon-based materials have a prominent role, showing a noticeable durability and cost-effective features. Pure carbon materials as nanostructured carbons, carbon nanotubes, and graphene sheets, generally show poor electrocatalytic activities. Doping them by different non-metals heteroatoms (e.g., N, B, S, O, P) and/or decorating their surfaces with metal oxides can significantly enhance their ORR activities [9]. Among the NPMCs investigated so far, metal oxides have a great potential as alternative catalysts for the ORR. A key limit exists, which is related to their low electrical conductivity. For this reason, their coupling to carbon materials is effective in enhancing the catalytic performances by the improved electron transfer [7, 10]. A common strategy for the enhancement of the catalytic performances of all these materials is to shape them in new structures able to significantly increase their surface area, resulting in an increased number of catalytic sites actually exposed to oxygen [7]. Given to their low cost, relatively high abundance, lower environmental impact and considerable electrocatalytic activity, manganese oxides are among the most interesting non-precious metal catalysts [6, 10, 11]. With the aim to design well performing ORR catalysts, in this work, manganese oxides were synthesized according to two key nanostructures: nanofibers and xerogels, by using biocompatible and biodegradable precursors such as PolyEthylene Oxide (PEO) and agar, respectively, with manganese acetate as the Mn source. PEO was chosen both because it is biocompatible and biodegradable and generally, because it is more environmental friendly than other common templating agents used to produce nanofibers. Indeed, it needs only water as solvent, differently from what is used for a widespread list of polymers, such as polyvinyl pirrolidone (PVP) [12,13] and polyacrylonitrile (PAN) [14,15]. Taking into account the same environmental approach, for xerogel synthesis, Agar was chosen as the gelling agent for the inorganic precursor, because it is a polysaccharide, extracted from a group of red-purple marine algae, that can become a gel at room temperature without the addition of catalysts, which are, usually, inorganic salts (inorganic chlorides, nitrates, etc.). Avoiding the use of these salts is particularly important since their excess removal requires post-treatments that are not environmentally benign in terms of energy (because of the thermal treatments) and/or solvents consumption. For example, other polysaccharides, such as k-carrageenan or sodium alginate, need catalysts for the gelling process and therefore lead to less green synthesis routes. Electrospun nanofibers and xerogels, are gaining increasing interest in the field of catalysis since they show very high surface area combined to well-controlled and reproducible synthesis routes [16-19]. Moreover they
offer two intrinsically different technological approaches to be exploited for their integration in final devices. Indeed nanofibers arrange themselves during the electrospinning process in thin mats or sheets collected on substrates, while xerogels can be shaped in 3-dimentional, self-standing structures by the proper use of molds. The concurrent development of these two nanostructures offers incredible advantages for new electrodes engineering and processing. As an example, the use of carbon-based materials as substrates for the synthesis of catalysts mats can be useful to demonstrate the possibility of direct fabrication of electrodes for electrochemical devices by the electrospinning technique [20]. In this paper we discuss the preparation of both nanofibers and xerogels-based manganese oxides, highlighting their crystalline structure and morphology. The catalytic behavior of the resulting materials with respect to the ORR is then evaluated through Rotating Ring Disk Electrode (RRDE) electrochemical measurements, and compared to the behavior of platinum. An impressive result of 3.6 electrons involved in the ORR is reported for both nanofibers and xerogels -based manganese oxides. 2 Materials and methods 2.1 Materials Polyethylene oxide (Mw = 600 kDa), bacteriological agar, manganese (II) acetate tetrahydrate (Mn(CH3COO)2·4H2O), ethanol, 2-propanol, Nafion solution (5 wt% in water and aliphatic alcohols), potassium hydroxide and Pt/C paste were purchased from Sigma-Aldrich and processed without any further purification. Silicon wafers (p type <100>, 1-30 Ω·cm) were supplied by MEMC. De-ionized water (Diwater) was used as solvent. 2.2 Methods 2.2.1 Manganese oxide nanofibers synthesis Firstly, 9 ml of aqueous PEO solution (5 wt%) were prepared by mixing PEO powder and Di-water. Afterwards, 3ml of 20 wt% aqueous manganese acetate solution were added to the polymeric solution and stirred for 24 h, at room temperature. The PEO/manganese acetate solution, thus prepared, was loaded in a 6 ml syringe, connected to a stainless steel needle (27 Gauge x 15 mm) and mounted into a NANON 01A electrospinning apparatus (MECC CO., LTD.) equipped with a high voltage power supply (HVU-30P100) and a syringe pump setup operating with a flow rate ranging from 0.1 ml/h up to 99.9 ml/h. During electrospinning, a voltage of about 14.5 kV was applied between the needle and a planar collector plate, covered with a substrate. A carbon-based material and monocrystalline silicon wafer were selected as substrates. In particular, silicon was selected in order to easily perform structural and morphological analysis on the mats, while the carbon-based substrate was selected to test the ability to directly assembly the electrode with the catalyst. The working distance, i.e. the distance from the tip of the needle to the collector, was 0,1 m in all cases. The electrospun PEO/manganese acetate nanofibers were then dried in vacuum at 70 °C for 24 h, in order to eliminate all the possible residues of solvent. Further on, the samples were calcinated by means of a vertical furnace (Carbolite, VST 12/300/3216) at 480 °C for 3 h in air, with a heating rate of 2.5 °C/min to ensure the complete decomposition of the PEO and the manganese acetate oxidation. 2.2.2 Manganese oxide xerogel preparation Manganese oxide xerogels were prepared by sol–gel method. The synthesis process included the dissolution of manganese acetate into the proper quantity of deionized water to prepare solutions with two different Mn concentrations (0.05 M and 0.1 M). Concurrently, a solution of agar (4%wt) was prepared by dissolving solid agar into hot water (about 85°C), under stirring. Once the agar was solubilized, the manganese acetate solution was added and let mixing for one hour. Afterwards, the manganese acetate/agar solution was poured into plastic moulds and let become a strong structured gel for a few hours.
The obtained gel was, then, dried by means of a freeze dryer (Lio-5PDGT – Cinquepascal srl) for two days. Next, the dried gels were calcinated in air (at 600°C for 6 hours, with an heating rate of 2.5 °C/min) in order to burn agar and transform manganese acetate into manganese oxide. Both kind of samples, nanofibers and xerogels, were stored at room temperature in a low-humidity container for their further characterization. 2.3 Characterizations The morphologies of both kinds of samples were examined by means of Field Emission Scanning Electron Microscopy (FESEM, ZEISS, Merlin and Supra 40), equipped with an Oxford Energy Dispersive X-ray detector. Samples for Transmission Electron Microscopy (TEM) were prepared by suspending a small quantity of material in ethanol, subsequently immersed in ultrasonic bath. A suspension droplet was then drawn and applied to a standard holey carbon TEM Cu grid, analyzing the specimen after the complete evaporation of the solvent. TEM observations were performed with a FEI Tecnai F20ST, equipped with a field emission gun (FEG) and operating at 200 kV. X-ray diffraction (XRD) with Bragg–Brentano symmetric geometry was performed with a PANalytical X'Pert Pro instrument using Cu-Kα (40 kV and 40 mA) radiation. Continuous scan mode was used to collect XRD data in a range of 10° to 74° and 0.02° as step size. Rietveld analysis was performed in order to analyze each spectrum by using Topas Academic software (version 4.1). Convolution-based method was used where the source emission profiles with full axial instrument contributions was modeled [21], while the background was fitted with a Chebyshev polynomial function with eight parameters. Average crystallite size was assumed to be isotropic in all cases and modeled by applying the integral breadth based method, whereas lattice strain was assumed to be zero because of the small crystal size found during the early refinement steps. Information about the surface area of the xerogel samples was obtained by measuring nitrogen adsorptiondesorption isotherms at -196°C using liquid nitrogen on a Quadrasorb Si from Quantachrome. Prior to the adsorption measurement, the samples were evacuated at 150°C under vacuum overnight. The surface area was determined in the relative pressure range from 0.1 to 0.3 of the Brunauer-Emmett-Teller (BET) plot. Decomposition behaviors of PEO and agar were analyzed by means of thermogravimetric analyses (TGA) under an air flux at a scan rate of 10°Cmin-1, using a TG209 F1 Libra from Netzsch. X-ray photoelectron spectroscopy (XPS) studies were carried out by a PHI 5000 Versaprobe scanning X-ray photoelectron spectrometer (monochromatic Al K-alpha X-ray source with 1486.6 eV energy, 15 kV voltage and 1 mA anode current), in order to investigate surface chemical composition. All the electrochemical measurements were carried out by means of a CHInstrument 760D electrochemical workstation and an ALS RRDE-3A rotating ring disk electrode apparatus. For all the measurements, the manganese oxide catalysts were deposited on the working electrode (a BioLogic glassy carbon disk/Pt ring) according to the following method. Before each catalyst deposition, the working electrode was properly polished with ethanol. The catalyst (2 mg of active mass) was dispersed in a solution containing 25 µl of water, 175 µl of Nafion© solution and 100 µl of 2-propanol. The mixture was ultrasonicated for 2 minutes to form a uniform black dispersion. 10 µl of this formulation were cast-coated onto the disk surface to form a uniform film. The resulting deposition was dried at room temperature for one day. For comparison purposes, commercial Pt/C paste was used as reference catalyst and deposited on the electrode by the same procedure described above. The final Pt loading was 0.5 mg/cm2. Pt was used as counter electrode and Ag/AgCl as reference electrode. The electrolyte used was 0.1 M KOH O2-saturated aqueous solution and the experiments were conducted at room temperature. All the potentials are reported with respect to the Ag/AgCl electrode. For the Rotating Ring Disk Electrode (RRDE) measurements, the disk electrode was scanned cathodically from 0.2 V to -0.8 V, with a rate of 5 mV/s and a fixed rotating speed of 2500 RPM, while the ring electrode was maintained at a fixed potential of 0.2 V. Electrochemical Impedance Spectroscopy (EIS) measurements were carried out at 0, -0.3 and 0.6 V potentials and 2500 RPM rotating speed, with a small signal of 10 mV and frequency range 10-2 – 104 Hz. 3 Results and discussion
3.1 Nanofibers Thin random mats of nanofibers were synthesized starting from manganese (II) acetate as Mn precursor and PEO as templating agent. Morphological characterizations were performed on the electrospun nanofibers before the drying step, in order to verify the quality of the obtained mat. FESEM images of the as-electrospun nanofibers at two different magnifications are reported in Fig. 1(A-B). They show fibers with a smooth surface and with diameters ranging from 120 to 200 nm. Branching effect, caused by a charge accumulation inside the jet, can seldom be observed [22].
Fig. 1. FESEM images of the nanofibers after drying on carbon-based material at two different magnifications.
The most critical step to obtain the final manganese oxide nanofibers was the calcination of the aselectrospun nanofibers. This criticality is related to the need to retain the shape of the nanofibers, to totally remove PEO and to completely convert the manganese acetate nanofibers into manganese oxide ones, at the same time. Even if the manganese acetate oxidation can be obtained at lower temperatures [13], the calcination step was performed at 480 °C in air, since at this temperature PEO could almost be completely removed, as demonstrated by TG analysis reported in Fig. 2.
Fig. 2. TGA plot of a water solution of 5wt% PEO 600kDa.
Fig. 3(A-B-C) show FESEM images at different magnifications of the calcinated electrospun manganese oxide nanofibers supported on carbon-based substrate, while Fig. 3(D) shows a FESEM image of the fibers supported on Si substrate. The first substrate was selected as conducting support for the fibers in a possible electrode, the second one for easiness of characterizations. In these images it is possible to observe how the manganese oxide well retains the 1D fibrous morphology on both the tested substrates, exhibiting a nanograin structure (Fig. 3(C-D)). Because of the thermal treatment, a slightly collapse of the nanofiber mats is generally observed, as evidenced in Fig. 3(A), if compared to Fig. 1(A), caused by PEO thermal removal. Despite the different arrangement of the fibers after calcination, we can assume that the effective surface area is preserved because of the presence of nanograins (Fig. 3(C-D)).
Fig. 3. FESEM images at different magnifications of nanofibers after calcination on carbon based (A-C) and on silicon (D) substrates.
The grain structure was further analyzed by the High Resolution (HR)TEM. Fig. 4(A) shows a Bright Field (BF) image of a fiber portion deposited on the Cu-grid: this presents the characteristic longed shape of the fibers, in addition to the presence of very small grains. The Selected Area Electron Diffraction (SAED) pattern obtained from such a fibers was composed by complete low intense rings, which were indexed as Mn3O4 (inset in Fig. 4(B)). The Dark Field (DF) image on another fiber portion, reported in Fig. 4(B), shows that the grains composing the fiber are actually round crystals with an average size of 10 nm, randomly distributed through the fiber. This DF image was obtained by selecting the electrons from one of the most intense ring on the SAED pattern, that corresponds to the {211} family planes for Mn3O4. Finally, Fig. 4(C) and 4(D) show an HRTEM image of one of these crystals (white frame denoted (i)) and its corresponding Fast Fourier Transform (FFT), respectively. The FFT was indexed as Mn3O4 along the [100] axis zone. The above result points out the polycrystalline structure of the electrospun nanofibers.
Fig. 4. TEM images of the calcinated nanofibers in (A) bright field, (B) dark field, SAED pattern in the inset (C) (HR)TEM micrograph and its corresponding (D) FFT.
Fig. 5 shows the XRD spectrum obtained from Manganese Oxide nanofibers supported on Si. In this spectrum, the diffraction peaks at 28.9°, 32.5° and 36.1° (2θ) correspond to (112) (103) and (211) planes for Mn3O4 with lattice parameters equal to a = b 5.764 ± 0.002 Å and c = 9.445 ± 0.004 Å. In addition to the main manganese oxide phase, some sharp peaks were found above 45° (2θ) which correspond to the silicon substrate on which the fibers were supported. These results are a clear indication of the monophasic nature of the sample, which is composed by Mn3O4 only, in agreement with TEM observations. The Bragg positions for M3O4, which fit well in position and intensity with the experimental data and the model obtained from Rietveld refinements, are also reported in Fig. 5. The fitting between the experimental data and the model is remarkably good, with χ2 equal to 1.45 and a Rbragg below 1%. The crystal size was estimated in 11.3 ± 0.3 nm that, also, fits well with what found by (HR)TEM, indicating a narrow crystal size distribution [23].
Fig. 5. XRD pattern of the manganese oxide nanofibers supported on Si after calcination. In the graph are reported the experimental measure (black curve), the fitting results (red curve) and their difference (gray curve).
3.2 Xerogels A massive nanostructured material was synthesized starting from manganese (II) acetate as Mn precursor. The process involved the preparation of a gel, using agar as gelling agent. The agar-manganese acetate (AMA) gel was dried by means of the freeze dryer technique, in which the samples were frozen at -50°C and then the solvent (water) was removed by sublimation, using a vacuum pump. This drying method lets the removal of the solvent within the gel network, avoiding, as much as possible, the collapse of the structure, giving rise to 3D light nanostructures. A calcination step in air was necessary in order to have a manganese oxide material. The calcination experimental conditions were chosen taking into account the parameters tested in the nanofiber preparation, even if some variations were necessary. This because of the differences in volume and geometry of the obtained materials and the higher complexity of the agar molecule with respect to PEO. For example, PEO can be removed by thermal treatment at temperatures around 400°C as determined by TG analysis (Fig. 2), while a relevant amount of agar (~20%) at temperatures above 700°C is still present (Fig. 6).
Fig. 6. Thermal decomposition behavior of agar gel measured by means of TGA.
Once the samples were prepared, a morphological characterization was due for evaluating which kind of crystalline structures and types of manganese oxide were synthesized. First of all, the structure determination was carried out taking into account that the volume shrinkage due to the drying and calcination step was not so excessive (about 15%) and from this point of view the samples could be considered as aerogels. On the other hand, surface area calculation by means of nitrogen adsorption technique and the analysis of the adsorption isotherms by BET equation (not shown here) led to values (about 40m2/g) that are lower than those usual for aerogels, even if the inorganic aerogels have, typically, lower surface area values than the organic ones [24,25]. From FESEM images it is also possible to appreciate that the agar morphology is predominant before the oxidation step (Fig. 7(A)), while after the thermal treatment two different types of grains are clearly visible (Fig. 7(B-C)). The first one presents a dendritic morphology while the second one is granulated, suggesting nanosized grains. In both cases, anyway, it was not possible to appreciate the presence of pores, confirming the low surface area estimated by BET analysis. For this reason, it is more appropriate to name the samples as xerogels than aerogels.
Fig. 7. FESEM images of (A) xerogels before thermal treatment and of samples (B) Mn1, (C) Mn2, after thermal treatment.
The presence of grains with different sizes and morphology (crystallographic habit) could suggest the existence of different types of manganese oxides. Different crystallization (growth) kinetics could be
observed during the calcination process, rather than one, as obtained for the nanofibers, where Mn3O4 was the only phase found. In order to verify this hypothesis, XRD measurements were carried out. Fig. 8(A-C) show the XRD spectra for the samples prepared starting from 0.5M (Mn1) and 1M (Mn2) manganese acetate water solutions, respectively. According to the obtained data, in the Mn1 sample a mix of two oxides (Mn3O4 and Mn2O3) was confirmed by the presence of the main diffraction peaks for Mn3O4 at 28.9°, 32.5° and 36.1° (2θ) that correspond to the (112) (103) and (211) planes, and the intense peaks at 32.9°, 38.3° and 55.2° indexed as Mn2O3, (222), (104) and (044) planes. Fig. 8 also shows the model obtained from the Rietveld refinement of Mn1 sample (χ2 equal to 3.82 and Rwp of 3.93), from which it was possible to estimate a proportion between the two oxides (Mn3O4:Mn2O3) of ≈ 1:4. Additionally to the difference in composition, a major difference was also found in the crystal size, where the average size for Mn2O3 was 65.8±0.8 nm against 21± 1 nm for Mn3O4. The above results, together with the FESEM images, allow us to postulate that, effectively, the big dendritic grains are associated to Mn2O3 phase, while the nanosized grains correspond to the Mn3O4, similar to what found in electrospun nanofibers. Regarding the Mn2 sample, XRD spectrum (Fig. 8(B)) also shows a mix of Mn3O4 and Mn2O3 oxides. In this case, however, it was found a proportion Mn3O4:Mn2O3 of 1:1. This change could be really ascribable to the increase of manganese amount in the AMA, that leads to an increase in the Mn3O4 formation. Rietveld refinement (χ2 equal to 2.29 and Rwp of 2.95) of the Mn2 samples allows us also estimate the crystal size for the oxides mix and a similar trend, as Mn1 samples, was found. Here, the estimated average sizes were 71±3 nm for Mn2O3 and 21± 1 nm for Mn3O4. Finally, it is important to mention that, additionally to the peaks related to the two manganese oxides, some low intense peaks were found. These peaks could be attributed to impurities due to manganese oxalate formation during the calcination step.
Fig. 8. XRD fitting of samples Mn1 (A) and Mn2 (B) after calcination. In the graph are reported the experimental measures (black curve), the fitting results (red curve) and their difference (gray curve).
To further support the XRD and FESEM analyses, TEM characterization of the xerogels has been performed. The results related to the Mn2 sample are shown in Fig. 9. The two different morphologies observed by FESEM (Fig. 7), i.e. the large dendritic structures and nanosized grains, were also observed by TEM as presented in the BF image of Fig. 9(A) (frame (i) for the nanosized grains and frame (ii)) for the dendritic structure). The SAED pattern obtained from the area observed in Fig. 9(A) is presented in Fig. 9(B). This SAED pattern is composed by two different patterns, that are clearly distinguished. The first corresponds to complete and low-intensity rings, which was indexed as Mn3O4, and that could, a priori, be assigned to the nanosized grains (frame (i) in Fig. 9(A)). The second pattern, which corresponds to the bright spots, was assigned to the large dendritic structure (frame (ii)) and indexed as Mn2O3 along the [201] axis zone. The correlation between these two kinds of pattern and the two different morphologies was confirmed by the DF image presented in Fig. 9(C-D). The DF image in Fig. 9(C) was obtained by selecting the electrons from one of the most intense ring on the SAEDP pattern, that corresponds to the {211} family planes for Mn3O4. In this figure, it can be observed that the nanograins are actually rounded Mn3O4 crystals with average size of 25 nm (see the HRTEM image in the inset of Fig. 9(C)). On the other hand, the dendritic structure in DF imaging, which was obtained by selecting the electrons from one of the closest spots to the SAED pattern center, is presented in Fig.9(D). In this DF image, the dendritic structure presents a bright contrast, even though the central part is darkened because of the particle thickness; however, a closer look at this DF image makes evident the presence of rectangular domains of less of 100 nm (highlighted by the white arrow on Fig. 9(D)), which fit well with the crystal size found by means of XRD. The above results suggest that the entire dendritic structure in Fig. 9(A) is composed by Mn2O3 alternate domains oriented along the <201> axis zones (see also the HRTEM image in the inset of Fig. 9(D)).
Fig. 9. TEM images of xerogels in (A) bright field, (B) SAED pattern, (C) and (D) Dark Field and (HR)TEM micrographs of the nanosized grains and the dendritic structure, respectively.
Taking into account the calcination temperature (600°C) employed for both Mn1 and Mn2 samples and the TG analysis of agar, it is very probable the presence of a residual amount of carbon in the sample. This feature is not detrimental for the catalytic performance of these samples, as shown in the following paragraph. Manganese oxides, on the other hand, do not have a good electrical conductivity. In order to prepare well-performing electrodes for electrochemical devices, it is, therefore, necessary to increase the oxide electrical conductivity. It is well known that carbon-based materials are used as electrodes [29], for this reason the presence of carbon impurities, in a graphitic shape, in our samples (as confirmed by Raman spectrum not shown here) is advantageous in the perspective of fabricating ORR electrodes. In order to confirm this statement, an Energy Dispersive X-ray Spectroscopy (EDS) analysis of both precursor concentrations was carried out. It was possible to point out that, actually a residual amount of carbon is still present (Fig. 10(A)). Other elements like Na, Mg, Ca and S, evidenced by the EDS measurements, can be attributed to impurities in agar. Residual carbon can be due to agar degradation residues, because of the presence of C-C, C=O, C-O bonds, as pointed out by XPS measurements (Fig. 10(B)).
Fig. 10. A) EDS plot of samples Mn1 and Mn2 after calcination. B) XPS HR C1s spectra of samples Mn1 and Mn2. The C1s peaks were deconvoluted (curves fit not reported) with three components due to C-C, C-O and –C=O bonds.
3.3 Catalytic performance The catalytic performance of the presented manganese oxide-based materials was evaluated by means of different electrochemical techniques. The RRDE technique allows validating the catalytic pathways of the proposed materials by using 4electrodes measurements. While scanning the disk potential at fixed rotation rate, the current at disk and ring electrode is measured. The disk current is related to the four-electrons ORR current of the analyzed catalyst, while the current of the ring electrode (whose potential is fixed at a large value) is associated to the twoelectrons ORR (intermediate) peroxide species [26]. In Fig. 11 the results for the samples under investigation are reported. It can be observed that for both nanofibers and xerogels, the ring currents (Fig. 11(A)) appear very low if compared to the corresponding disk currents (Fig. 11(B)). This means that the preferred reduction pathway relies on the four electrons reaction, and, as consequence, the current related to the two electrons reduction is low. In addition, by looking at the disk currents, a larger conductivity can be inferred for Mn2 and nanofibers samples, leading to increased cathodic currents with respect to that of Mn1 xerogel. For completeness, also the currents related to the reference Pt/C catalyst are reported in Fig. 11(A) and in the inset of Fig. 11(B). Starting from these curves, and by applying equations (1) and (2) below, (1)
(2)
(where IR and ID are the ring and disk currents, respectively, and N is the current collection efficiency of the Pt ring) [27], it is possible to estimate both the percentage of peroxide species formation (HO2- %) and the electron transfer number n, and to compare the ORR catalytic performance of all the samples under investigation. The dependence of peroxide species percentage and electron transfer number on the applied potential is reported in Fig. 11(C). For the proposed catalysts, n values comprised between 3.5 and 3.7 were obtained for potentials lower than -0.4 V, with the corresponding peroxide species percentages lower than 25%, in line with other low-cost catalyst proposed in the literature [28]. In addition, it has to be highlighted that the obtained performance for the manganese oxide-based nanofibers and Mn2 xerogel samples are just slightly lower than the reference Pt sample ones, as reported in Fig. 11(C).
Both the materials showed a good catalytic behavior for the oxygen reaction reduction, being good candidates as low-cost and green catalysts for applications in electrochemical devices. Slightly lower performance was obtained for the Mn1 sample. In order to further investigate this aspect, EIS analysis was carried out and the results are discussed below.
Fig. 11. Ring current (A) and Disk current (B) obtained from the RRDE measurements of manganese oxide-based and reference Pt/C samples at a rotating speed of 2500 rpm and a potential of the ring electrode of 0.2 V. (C) Comparison of electron transfer number (left axis) and peroxide percentage (right axis) evaluated from RRDE measurements of manganese oxide-based samples and of the reference Pt/C catalyst at 2500 rpm rotating speed and different potentials.
In Fig. 12 the Nyquist plots of the impedances related to all the manganese oxide-based and reference Pt samples are reported. For all the materials, a typical spectrum composed by a high-frequency main arc (related to the charge transfer at the oxide/electrolyte interface) and a low-frequency side arc (related to the diffusion of species) [30] can be observed. In accordance with the RRDE measurement reported in Fig. 11, a larger impedance is exhibited by Mn1 xerogels, while the other samples are characterized by quite similar behaviors. In order to quantify this discrepancy, impedance spectra were fitted with the equivalent circuit shown in the inset of Fig. 12, which represents the well-known Randles’ circuit [31]: it is composed by the series resistance Rs (accounting for the electrolyte conductivity), the charge transfer resistance Rct (related to the electron transfer at the solid/liquid interface), the double layer capacitance Qdl (accounting for the charge accumulation at the same interface, here modeled through a constant phase element [32]), and the Warburg element Ws (modeling the charge diffusion). The fitted curves are reported in Fig. 12, superimposed to the experimental data: as can be clearly observed, a good matching between measured and calculated curves was obtained, thus witnessing an optimal choice of the equivalent circuit used for the fitting procedure. The main parameter obtained from this procedure is the charge transfer resistance, which can be directly connected to the catalytic properties: as expected, a larger value (9850 Ω) was obtained for Mn1 sample with respect to those of the other two samples (6082 Ω and 6492 Ω, for Mn2 sample and nanofibers, respectively), while quite similar values of the other parameters were obtained for all the catalyst materials (Rs of about 55 Ω, Qdl in the range 11-13 μF). This justifies the slightly lower disk current, and a correspondingly lower electron transfer number, was obtained for the Mn1 xerogel sample. Moreover, in accordance with the electrochemical measurements reported above, the reference Pt/C catalyst exhibits the lowest resistance value, namely 2484 Ω.
Fig. 12. Nyquist plot of the impedance of the different samples measured at 2500 rpm rotating speed and -0.3 V. The points are experimental data while the continuous lines are the curves obtained from a fitting procedure using the equivalent circuit shown in the inset.
4 Conclusions
Nanostructured manganese oxides were prepared by using biocompatible and biodegradable precursors, by means of the electrospinning and sol-gel plus freeze drying techniques, respectively. These materials were obtained in the form of nanofibers and xerogels. In particular, thanks to the optimization of the thermal treatment, reproducible samples of nanofibers composed by Mn3O4, and xerogels composed by a mix of Mn3O4 and Mn2O3, with relative proportion depending on the initial Mn precursor concentration, were obtained. All the nanostructured MnxOy-based catalysts showed extremely good catalytic performance for the oxygen reduction reaction, with n values between 3.5 and 3.7, at potentials lower than -0.4 V, which is in line with other cost-effective catalysts proposed in the literature. Both the nanofibers and xerogels resulted to be good candidates as low-cost and green catalysts for applications in electrochemical devices.
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