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Ethanol tolerant precious metal free cathode catalyst for alkaline direct ethanol fuel cells
Accepted Manuscript Title: Ethanol tolerant precious metal free cathode catalyst for alkaline direct ethanol fuel cells Authors: Ilena Grimmer, Paul Zorn, Stephan Weinberger, Christoph Grimmer, Birgit Pichler, Bernd Cermenek, Florian Gebetsroither, Alexander Schenk, Franz-Andreas Mautner, Brigitte Bitschnau, Viktor Hacker PII: DOI: Reference:
S0013-4686(17)30087-7 http://dx.doi.org/doi:10.1016/j.electacta.2017.01.087 EA 28754
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
14-12-2016 12-1-2017 14-1-2017
Please cite this article as: Ilena Grimmer, Paul Zorn, Stephan Weinberger, Christoph Grimmer, Birgit Pichler, Bernd Cermenek, Florian Gebetsroither, Alexander Schenk, Franz-Andreas Mautner, Brigitte Bitschnau, Viktor Hacker, Ethanol tolerant precious metal free cathode catalyst for alkaline direct ethanol fuel cells, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.01.087 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.
Ethanol tolerant precious metal free cathode catalyst for alkaline direct ethanol fuel cells
Ilena Grimmer*1, Paul Zorn1, Stephan Weinberger1, Christoph Grimmer1, Birgit Pichler1, Bernd Cermenek1, Florian Gebetsroither1, Alexander Schenk1, Franz-Andreas Mautner2, Brigitte Bitschnau2, Viktor Hacker1
1
Graz University of Technology, Institute of Chemical Engineering and Environmental
Technology, Fuel Cell Systems Group, NAWI Graz, Inffeldgasse 25C, 8010 Graz, Austria 2
Graz University of Technology, Institute of Physical and Theoretical Chemistry, NAWI Graz,
Stremayrgasse 9/I, 8010 Graz, Austria
Email-addresses of the authors: Ilena Grimmer: [email protected] Paul Zorn: [email protected] Stephan Weinberger: [email protected] Christoph Grimmer: [email protected] Birgit Pichler: [email protected] Bernd Cermenek: [email protected] Florian Gebetsroither: [email protected] Alexander Schenk: [email protected] Franz-Andreas Mautner: [email protected] Brigitte Bitschnau: [email protected] Viktor Hacker: [email protected]
*Corresponding author: Ilena Grimmer, [email protected], phone: +43 316 873 8798, fax: +43 316 873 8782 1
Graphical abstract
Highlights
Selective ORR catalysts are presented for alkaline direct ethanol fuel cells Perovskite based cathode catalysts show high tolerance toward ethanol A membrane-free alkaline direct ethanol fuel cell is presented
Abstract La0.7Sr0.3(Fe0.2Co0.8)O3 and La0.7Sr0.3MnO3 -based cathode catalysts are synthesized by the sol-gel method. These perovskite cathode catalysts are tested in half cell configuration and compared to MnO2 as reference material in alkaline direct ethanol fuel cells (ADEFCs). The best performing cathode is tested in single cell setup using a standard carbon supported Pt0.4Ru0.2 based anode. A backside Luggin capillary is used in order to register the anode 2
potential during all measurements. Characteristic processes of the electrodes are investigated using electrochemical impedance spectroscopy. Physical characterizations of the perovskite based cathode catalysts are performed with a scanning electron microscope (SEM) and by
X-ray diffraction showing phase pure materials. In half cell setup,
La0.7Sr0.3MnO3 shows the highest tolerance toward ethanol with a performance of 614 mA cm-2 at 0.65 V vs. RHE in 6 M KOH and 1 M EtOH at RT. This catalyst outperformed the state-of-the-art precious metal-free MnO2 catalyst in presence of ethanol. In fuel cell setup, the peak power density was 27.6 mW cm-2 at a cell voltage of 0.345 V and a cathode potential of 0.873 V vs. RHE.
Keywords: alkaline direct ethanol fuel cell, membrane free, ethanol tolerant cathode catalysts, precious-metal free, oxygen reduction reaction
1. Introduction The simpler transportation and storage of liquids compared to gases like hydrogen, as well as the safer and more favorable handling, gives liquid fuel cells an advantage over H2/O2 fuel cells. Due to the high availability of ethanol and the already existing large infrastructure concerning transport and storage, direct ethanol fuel cells (DEFC) are one of the most promising candidates of direct liquid fuel cells for power generation. At the anode of the DEFC, ethanol is oxidized, whereas at the cathode, oxygen is reduced comparable to conventional H2/O2 fuel cells. The electrode reactions of a DEFC in alkaline media are shown in eq. 1-3.
Anode: CH3CH2OH + 4 OH- → CH3COOH + 3 H2O + 4 e-
(1)
Cathode: O2 + 2 H2O + 4 e- → 4 OH-
(2)
Overall: CH3CH2OH + O2 → CH3COOH + H2O
(3)
3
In DEFC research, most literature focuses on investigating novel anode catalysts, since ethanol oxidation is more complex than the oxygen reduction reaction (ORR). Typically, the complete oxidation of ethanol to CO2 is not accomplished due to insufficient catalysts with respect to the C-C bond cleavage. Although much effort has been invested in this area, there is still necessity for further optimization of the anode catalysts in alkaline media toward complete ethanol oxidation [1–5]. Despite the lack of highly active anode catalysts, selective and tolerant cathode catalysts are of major importance as well, since ethanol crossover is still an issue in alkaline direct ethanol fuel cells (ADEFCs) [6]. Currently, the state-of-the-art cathode catalysts for ADEFCs are Pt or MnO2 [7,8]. Since Pt is not catalytically selective and ethanol crossover cannot be completely avoided even if a membrane is used, Pt cannot be the ORR catalyst of choice. In this work precious metal free cathode catalysts with high tolerance toward ethanol as well as high activity toward ORR are investigated. La0.7Sr0.3MnO3 and La0.7Sr0.3(Fe0.2Co0.8)O3 catalysts were synthesized and tested as cathode catalysts in ADEFC application. In order to compare the activity of the synthesized perovskites, MnO 2 was used as reference material, since it is already widely used for ORR in alkaline media [7]. The MnO2 was synthesized according to a previous published procedure [9,10]. The cathode catalysts were tested ex situ using a rotating disk electrode (RDE). Afterwards, electrodes were manufactured using a cross-rolling procedure [10,11] and then subjected to intensive in situ measurements in half cell configuration. Furthermore, single-cell tests were conducted with the best performing cathode in presence of ethanol, in order to characterize the behavior of the catalyst in a fuel cell setup with a carbon supported Pt0.4Ru0.2 based anode. Due to the high selectivity toward ORR of the La0.7Sr0.3MnO3 perovskite based catalyst, no membrane was used. Measurements were performed in the so-called mixed electrolyte approach according to Grimmer et al. [12]. Electrochemical impedance spectroscopy (EIS) was used to characterize processes taking place at the electrodes or rather the cell. 2. Experimental 4
2.1 Cathode catalyst syntheses and electrode manufacturing The catalysts La0.7Sr0.3(Fe0.2Co0.8)O3 and La0.7Sr0.3MnO3 were prepared via sol-gel method with citric acid (CA) as complexing agent [13–16]. In short, the required stoichiometric amounts of the respective metal nitrates were dissolved in ultrapure water. CA was added in the double molar concentration of the metal salts (CA/metal, 2/1). After intensive stirring for 1 h at 80 °C, ethylene glycol, acting as surfactant, was added dropwise to the mixtures (CA/EG, 1/1). The precursor solutions were stirred at 80 °C until the gelation process was completed. The viscous mixtures were dried and subsequently calcined at 850 °C. Finally, the catalysts were grinded and screened to a certain catalyst particle size fraction. MnO2 catalyst was prepared by dispersing Vulcan XC72R (Cabot Corporation) or BP2000 (Cabot Corporation) in H2O/ethanol. After adding Mn(NO3)2* 4 H2O, the suspension was stirred overnight at 60 °C. Once the solvent was removed completely, it was heated up to 400 °C under inert atmosphere. The resulting MnO 2/C (30wt.% MnO2 on Vulcan XC72R or BP2000) was grinded and screened to a catalyst particle size fraction <45 µm [9,10].
The cathodes were prepared via cross-rolling method. For the active layers (AL), the respective catalyst was mixed with carbon powder (Vulcan XC72R and BP2000) and PTFE as well as Nafion solution (5 wt.%, from Quintech) as binders in n-propanol and rolled until a leathery texture was achieved. For the gas diffusion layer (GDL), acetylene black and PTFE was suspended in isopropanol/water (1/1). AL and GDL were rolled separately and subsequently rolled together. After dry-pressing, the electrodes were hot-pressed at 300 °C onto a nickel-mesh, acting as current collector [17]. The catalyst loadings and thicknesses of the cathodes are listed in Table 1.
2.2 Anode manufacturing The anode was fabricated via drop coating of the catalyst ink onto a carbon cloth substrate material [18]. For this, PtRu/C (40wt.% Pt, 20wt.% Ru on carbon black from Cabot 5
Corporation) and Nafion-solution (5 wt.%, from Quintech) were dispersed in n-propanol. The carbon cloth was coated with an appropriate amount of the ink resulting in a catalyst loading of 1.5 mgPt cm-2.
2.3 Ex situ characterization of cathode catalysts Ex situ measurements of the catalysts were conducted with a Reference 600 potentiostat (Gamry, supplied by C3 Analysentechnik), in a three electrode setup with a reversible hydrogen electrode (RHE) as reference electrode, a platinum counter electrode and a glassy carbon working electrode coated with a thin catalyst layer. The catalyst ink was prepared, with the respective catalyst and a Nafion solution (5 wt.%, from Quintech) as binder, dispersed in ethanol and ultrapure water (volume ratio Nafion:EtOH:H2O = 2:49:49) [13,19]. Thereby, no additional carbon was used. The glassy carbon electrode with an active area of 0.196 cm2 was coated with 10 µl of the catalyst ink resulting in a catalyst loading of 210 µg cm-2geo. The electrode was rotated during evaporation of the solvent, in order to achieve a homogeneous and uniform catalyst film. The measurements were conducted in 1 M KOH electrolyte at 30 °C that was intertized with a continuous flow of ultrapure nitrogen. The internal resistance (5 Ω) was determined by the standard procedure provided by Gamry Instruments (single frequency impedance measurement at OCV usually at 10 kHz). According to the low internal resistance, no IR correction was performed. Potential cycling between 0.1 V and 1.0 V vs. RHE at a scan rate of 100 mV s-1 was performed until stable cyclic voltammograms were obtained. The ORR was investigated after the electrolyte was saturated with oxygen. For this, linear sweep voltammograms at electrode rotation speeds of 400, 600, 900, 1200 and 1600 rpm in the potential range of 0.1 V to 1 V vs. RHE and at a scan rate of 10 mV s-1 were performed. To determine the number of exchanged electrons (n) and the heterogeneous rate constant (k h), the Levich (L) (eq. 4) and Koutecky-Levich (KL) (eq. 5) equations were applied. ⁄3
𝑖𝑙 = 0.62 𝑛 𝐹 𝐴 𝐷𝑟2
𝜔 1⁄2 𝜈 −1⁄6 𝐶𝑟
(4) 6
1 𝑖𝑘𝑙
=
1 2⁄3
0.62 𝑛 𝐹 𝐴 𝐷𝑟
𝜔 1⁄2 𝜈 −1⁄6 𝐶𝑟
+
1 𝑛 𝐹 𝐴 𝑘ℎ 𝐶𝑟
(5)
The required parameters, regarding a 1 M KOH electrolyte, are listed in table 2 [20]. In case of the Levich analysis, the number of electrons (n) was derived for each rotation speed in the diffusion limiting region and given as the mean value. During KL analysis the heterogeneous rate constant (kh) was calculated and the number of electrons was determined additionally. This was done graphically by plotting the reciprocal current density versus the reciprocal of the square root of the rotational rate (j -1 vs. ω−1/2). The intercept and slope of this plot allows the determination of kh and n [21,22]. The linearity of Levich and Koutecky-Levich plots confirms the compatibility of the selected potential region.
2.4 In situ characterization 2.4.1 Half-cell measurements Half-cell measurements were performed using a multichannel BaSyTec CTS (Cell Test System), in a three electrode setup with a RHE as reference electrode and a stainless steel counter electrode. The RHE was connected to the electrode via Luggin capillary with a distance of approx. 2 mm. Measurements were performed with pure oxygen at a gas flow rate of 20 ml min-1 in 6 M KOH + 1 M EtOH at room temperature. The polarization curves of the electrodes were recorded by increasing the current until a potential of 0.9 V vs. RHE was reached, afterwards the measurements were conducted potentiostatically to a potential of 0.65 V vs. RHE, in 50 mV steps holding each step for 3 minutes. Each value represents the mean value of the last 30 seconds at the corresponding operation point. The internal resistances were determined via electrochemical impedance measurements.
2.4.2 Fuel-cell measurements The polarization curve of the single-cell was recorded galvanostatically using a Reference 600 potentiostat (Gamry, supplied by C3 Analysentechnik) holding each current step for 2 7
minutes. Each value represents the mean value of the last 30 seconds at the corresponding operation point. The electrolyte/fuel consisting of 6 M KOH + 1 M EtOH was temperature-controlled to 30 °C and supplied to the anode at a flow rate of 10 ml min-1. The cathode, La0.7Sr0.3MnO3, was fed with O2 with a gas flow rate of 20 ml min -1. In order to record the anode potential during measurements, a Luggin capillary was connected to the backside of the anode. In-between the electrodes a PP separator (Freudenberg 700/77) was placed to ensure the avoidance of short circuits. The internal (anode vs. cathode) as well as the uncompensated (anode vs. RHE) resistances were investigated via the standard Gamry procedure, as discussed above.
2.4.3 Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) characterizations were performed in galvanostatic mode at different current densities using a Gamry Reference 600 potentiostat. Besides standard EIS recordings of the single cell (anode vs. cathode) additional measurements of anode vs. RHE and cathode vs. RHE were carried out in the same single cell configuration. EIS was used in order to investigate characteristic processes of the electrodes at 4 operating points and current densities of 50 mA cm-2, 25 mA cm-2, 10 mA cm-2 and 5 mA cm-2, respectively. The amplitudes were set to 2% of the applied d.c. current densities (rms) to obtain a high signal to noise ratios maintaining linearity requirements. The investigated frequency range was 100 kHz and 0.4 Hz.
3. Results and discussion 3.1 Physical characterization Figure 1 shows the SEM pictures as well as the XRD patterns of the synthesized cathode catalysts; XRD patterns are compared to data from literature [23,24]. The XRD data of the 8
MnO2 catalyst are discussed elsewhere [10], showing a composition of monoclinic 𝛾–MnO2 with a small part of Mn3O4 (not shown). XRD patterns show phase pure, homogeneous materials with negligible impurities of both catalysts. Despite both catalysts were synthesized similarly, the morphology of the manganese based perovskite differs completely from FeCo-based catalyst.
While the
La0.7Sr0.3(Fe0.2Co0.8)O3 catalyst exhibits a very smooth distinct structure with well-defined grains, La0.7Sr0.3MnO3 catalyst shows a very fringed surface morphology without precise delimitation of the particles. The La 0.7Sr0.3MnO3 provides a greater surface area compared to the FeCo-based catalyst.
3.2 Ex situ measurements The general ORR mechanism in aqueous alkaline electrolyte can be divided into two pathways. The direct four-electron reduction, which leads to the production of water (eq. 6) and the less efficient, indirect two-electron reaction which results in the formation of alkaline stabilized hydrogen peroxide intermediates (eq. 7) [25–27]. This peroxide intermediate can further be converted into a harmful radical species with detrimental effects on catalysts. This species is vulnerable for reduction [26], or can undergo a disproportionation reaction (eq. 9). 𝑂2 + 2 𝐻2 𝑂 + 4 𝑒 − → 4 𝑂𝐻−
(6)
𝑂2 + 𝐻2 𝑂 + 2 𝑒 − → 𝐻𝑂2− + 𝑂𝐻 −
(7)
𝐻𝑂2− + 𝐻2 𝑂 + 2 𝑒 − → 3𝑂𝐻 −
(8)
𝐻2 𝑂2 → 𝑂2 + 𝐻2 𝑂
(9)
The cyclic voltammograms (CVs) below (figure 2-3) show the faradaic currents of the cathodic sweep as a function of applied potential at various rotation rates. The curves are corrected with CVs in N2 atmosphere. Thereby, the catalysts exhibit onset potentials of approx. 0,83 - 0.85 V vs. RHE.
9
A single step reduction process should appear as a straightforward sigmoidal current response [27]. On the contrary, the recorded curve indicates the combination of two sigmoidal curves, which is explained by the parallel occurrence of two and four electron reduction processes. The two electron reduction process is supplemented by a catalyzed disproportion of HO2- (eq. 8), resulting in a pseudo four-electron reduction process [25,27]. As expected, the current increases at higher rotation rates due to thinner diffusion layers. This diffusion limiting correlation is expressed in linear Levich plots over a wide potential range of 0.1 to 0.5 V vs. RHE (data not shown). In potential regions with occurring kinetic limitations, the Koutecky-Levich expression was used for analysis. Besides, the number of transferred electrons was determined by the Levich and Koutecky-Levich analysis. Thereby, the La0.7Sr0.3MnO3 perovskite results indicate exchanged electrons from around 2 and above. This confirms most likely a combined two and four electron transfer as expected from Tulloch and Donne [27]. On the contrary, the La0.7Sr0.3(Fe0.2Co0.8)O3 catalyst exhibits a lower ORR activity which is also reflected in the low number of exchanged electrons (approximately 1.3) in the Levich analysis. If the kinetic regions are considered, both catalysts show similar results. Overall, the RDE results can be confirmed in the literature [27–29]. Table 3 shows selected results from the analysis.
In general, the La0.7Sr0.3MnO3 catalyst demonstrated a higher performance. This is indicated at higher current densities and the higher number of transferred electrons and the higher heterogeneous rate constant. 3.3 In situ Measurements 3.3.1 Half-cell measurements of the cathodes Polarization curves of the cathodes tested in half-cell configurations are shown in figure 4-6. Measurements were performed in potentiostatic mode from OCP to 0.65 V vs. RHE. 10
The internal resistances, peak current densities at 0.65 V vs. RHE of all cathodes in 6 M KOH and in 6 M KOH + 1 M EtOH as well as the associated iR corrected cell potentials are listed in table 4.
As expected, the open circuit potentials of all three catalyst materials were in the range of 1 V vs. RHE, independent of the presence of ethanol. The state-of-the-art MnO2 catalyst had the highest performance in 6 M KOH, although the catalyst loading was less than half compared to the perovskite-based catalyzed cathodes. However, due to the limited tolerance of MnO2 toward ethanol, the current density dropped by 68% in EtOH containing electrolyte making it rather less suitable for ADEFC application, as can be seen in figure 6 and table 4. In contrast to that the synthesized perovskite catalyst showed superior ethanol tolerance. Among all tested catalysts, the manganese based perovskite La 0.7Sr0.3MnO3 exhibited comparably high current densities in 6 M KOH as MnO2, but additionally also the highest tolerance toward ethanol crossover. In presence of ethanol even an increasing performance of up to 614 mA cm-2 at 0.65 V vs. RHE was observed. The increasing performance is attributed to enhanced moistening of the electrode in ethanol environment. An appropriate combination of hydrophobic binder materials, carbon and catalyst ensured an optimal formation of the three-phase-boundary in presence of hydrophilic EtOH. Therefore, a larger catalyst surface became available for the ORR. The perovskite-based La0.7Sr0.3(Fe0.2Co0.8)O3 cathode had the lowest performance in pure KOH electrolyte, but still it outperformed the MnO2-based electrode in presence of ethanol. The higher catalytic activity of the manganese based perovskite is assigned to the different morphology as can be seen in the SEM images in 3.1. The greater surface area leads to higher ORR activity.
3.3.2 Fuel cell measurements
11
Due to the good performance of the cathode catalyzed with La0.7Sr0.3MnO3, single cell measurements were conducted with the same cathode and a PtRu catalyzed anode. The polarization curve (without iR correction) of the single-cell, the recorded anode potentials and the thereof calculated cathode potentials as well as the corresponding power densities are shown in figure 7 and associated results are given in table 5.
The internal resistance of the fuel cell was 0.13 Ω. The OCV of the fuel cell was unexpected high with 0.935 V, with a calculated cathode potential of > 1 V vs. RHE. With a peak power density of 27.6 mW cm-2 the cell shows quite good performance. As can be seen from the electrode potentials, the limitation of the ADEFC is the anode performance rather than the cathode. The anode performance can be optimized by higher operating temperatures, increased ethanol concentrations in the fuel as well as by identifying suitable anode electrode structures; which reduce the mass transport limitations. As far as the obtained results show, the cathode performs excellent with low-cost materials. It showed a calculated potential of 0.873 V vs. RHE at the maximum power density, at 80 mA cm-2.
3.4 Electrochemical Impedance spectroscopy All EIS results exhibited two distinguishable limiting processes taking place at each electrode, shown as Nyquist diagram with two semicircles for the iR corrected cathode in figure 8 exemplary. Anode EIS measurements show similar behavior, being limited by two characteristic processes. Nevertheless, the fuel cell EIS series exhibited only two processes since the characteristic frequencies were too close to each other to be observable as separate semicircles. Therefore, the high frequency semicircles and low frequency semicircles partially mask each other. Additionally, the cathode EIS at 50 mA cm -2 clearly indicated a low frequency relaxation behavior and a low frequency inductive loop, respectively [30,31]. 12
Complex nonlinear least square fitting was applied utilizing the following equivalent circuit: -(Lcable)-(Rel)-(RHF||CPEHF)-((RLF||CPELF)- to obtain the 8 fitting parameters [30,31]. Abbreviations: Lcable
Wire inductance
Rel
Electrolyte resistance
RLF
Resistance low frequency
RHF
Resistance high frequency
CPEHF
Constant phase elements high frequency
CPELF
Constant phase elements low frequency
The values in table 6 show the resistance parameters of the semicircles in low and high frequency ranges of the cathode, the anode and the fuel cell, at respective current densities. The first semicircle is attributed to the porosity of the electrode since R HF was found to be constant with respect to the applied current density. The second semicircle is ascribed to activation overpotential due to the exponentially growing RLF with respect to decreasing current density. As can be seen in table 6, the anode causes a large part of the performance losses, in proportion 3 to 1 (anode to cathode).
4. Conclusion Three cathode catalysts were characterized in-situ in half cell configurations. The most promising candidate as ORR catalyst in ADEFC application is La0.7Sr0.3MnO3 as can be seen from ex-situ and in-situ results. In in situ half-cell measurements it also showed the highest tolerance toward ethanol with a current density of 614 mA cm -2 at 0.65 V vs. RHE at room temperature in 6 M KOH + 1 M EtOH. MnO2 catalyst which is widely used as precious metal free ORR catalyst was outperformed in presence of ethanol by perovskite based catalysts. 13
Tests in single cell setup showed very good performance exhibiting a peak power density of 27.6 mW cm-2, without iR correction. EIS spectra confirmed the findings of the VI measurements exhibiting a cathode to anode performance loss ratio of approx. 3. Furthermore, it was depicted that two limiting processes for both electrodes occur. The first semicircle is attributed to the porosity of the anode and cathode, respectively. The second and more dominant low frequency semicircles was found to be activation limited processes and may be attributed to charge transfer at the anode and cathode. In alkaline media, precious metal free catalysts show extraordinary performance and therefore great potential for usage in ADEFCs. Besides ethanol, which can easily be obtained e.g. from biomass, only oxygen from air is needed. With ORR selective cathodes, cost-intensive membranes are redundant, which makes an ADEFC one of the most convenient technologies for low power applications. With this findings preliminary work has been done for low-cost ADEFC stacks.
Acknowledgements We want to thank Apollo Energy Systems for providing cathode materials. Financial support by the Austrian Federal Ministry of Science, Research and Economics (BMWFW), the Austrian Research Promotion Agency and the IEA research cooperation are gratefully acknowledged.
14
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Communications Errors in the use of the Koutecky – Levich plots, Electrochem. Commun. 15 (2012) 42–45. doi:10.1016/j.elecom.2011.11.017. [22]
K.J.J. Mayrhofer, D. Strmcnik, B.B. Blizanac, V. Stamenkovic, M. Arenz, N.M. Markovic, Measurement of oxygen reduction activities via the rotating disc electrode method : From Pt model surfaces to carbon-supported high surface area catalysts, 53 (2008) 3181–3188. doi:10.1016/j.electacta.2007.11.057.
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A.V. Sathe, V.G.; Paranjpe, S.K.; Siruguri, V.; Pimpale, Novel magnetic phases in La0.7Sr0.3Co1−yFeyO3: a neutron diffraction study, J. Phys. Condens. Matter. 10 (1998) 4045–4055.
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A.N. Petrov, V.I. Voronin, T. Norby, P. Kofstad, Crystal Structure of the Mixed Oxides La0.7Sr0.3Co1−zMnzO3±y(0≤z≤1), J. Solid State Chem. 57 (1999) 52–57.
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T. Nagai, S. ichi Yamazaki, M. Asahi, Z. Siroma, N. Fujiwara, T. Ioroi, Metalloporphyrin-modified perovskite-type oxide for the electroreduction of oxygen, J. Power Sources. 293 (2015) 760–766. doi:10.1016/j.jpowsour.2015.06.004.
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J. Tulloch, S.W. Donne, Activity of perovskite La1-xSrxMnO3 catalysts towards oxygen reduction in alkaline electrolytes, J. Power Sources. 188 (2009) 359–366. doi:10.1016/j.jpowsour.2008.12.024.
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T. Poux, F.S. Napolskiy, T. Dintzer, G. Kéranguéven, S.Y. Istomin, G.A. Tsirlina, E. V. Antipov, E.R. Savinova, Dual role of carbon in the catalytic layers of perovskite/carbon composites for the electrocatalytic oxygen reduction reaction, Catal. Today. 189 (2012) 83–92. doi:10.1016/j.cattod.2012.04.046.
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K. Elumeeva, J. Masa, J. Sierau, F. Tietz, M. Muhler, W. Schuhmann, Electrochimica Acta Perovskite-based bifunctional electrocatalysts for oxygen evolution and oxygen reduction
in
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electrolytes,
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Bernard T. Mark E. Orazem, Electrochemical Impedance Spectroscopy, Wiley, 2008.
[31]
J.R.M. Evgenij Barsoukov, Impedance Spectroscopy, Wiley, 2005.
18
Figure 1. SEM images and XRD patterns of a, b, e) the La0.7Sr0.3MnO3 c, d, f) the La0.7Sr0.3(Fe0.2Co0.8)O3 perovksite catalysts
19
Figure 2. RDE measurements of the La0.7Sr0.3MnO3 perovskite. The insert represents the corresponding Koutecky-Levich plot
20
Figure 3. RDE measurements of the La0.7Sr0.3(Fe0.2 Co0.8)O3 perovskite. The insert represents the corresponding Koutecky-Levich plot
21
RT, 20 mlO2/min
1
Potential / V
0.9
0.8 0.7 VI, 6 M KOH VIiR comp, 6 M KOH
0.6
0.5
VI, 6 M KOH + 1 M EtOH VIiR comp, 6 M KOH + 1 M EtOH
0
200
400 600 Current density / mA cm-2
Figure 4. Polarization curve of La0.7Sr0.3MnO3 in 6 M KOH and 6 M KOH + 1 M EtOH, with and without iR correction
22
RT, 20 mlO2/min
1
VI, 6 M KOH + 1 M EtOH VIiR comp, 6 M KOH + 1 M EtOH
0.9
Potential / V
VI, 6 M KOH VIiR comp, 6 M KOH
0.8 0.7 0.6
0.5
0
200
400 600 Current density / mA cm-2
800
Figure 5. Polarization curve of La0.7Sr0.3(Fe0.2Co0.8)O3 in 6 M KOH and 6 M KOH + 1 M EtOH, with and without iR correction
23
RT, 20 mlO2/min
1
VI, 6 M KOH + 1 M EtOH VIiR comp, 6 M KOH + 1 M EtOH
0.9 Potential / V
VI, 6 M KOH VIiR comp, 6 M KOH
0.8
0.7
0.6 0
200
400 600 Current density / mA cm-2
800
Figure 6. Polarization curve of MnO2 in 6 M KOH and 6 M KOH + 1 M EtOH, with and without iR correction
24
VI curve Anode potential Cathode potential calculated
Cell voltage / V
1
Power density
35 30
25
0.8
20 0.6 15
0.4
10
0.2
Power density / mW cm-2
1.2
5 6 M KOH 1 M EtOH, 30 °C
0
0
50 100 Current density / mA cm-2
0 150
Figure 7. Polarization curve and corresponding power densities without iR correction, recorded anode potential and calculated cathode potential
25
0.5 50 mA cm-² 25 mA cm-² 10 mA cm-² 5 mA cm-²
1.58 Hz
Im{Z} / Ohm
0
1 Hz 0.25 Hz
−0.5 3.98 kHz −1
0.13 Hz
−1.5 −2
0
1
2 Re{Z} / Ohm
3
4
Figure 8. Nyquist diagram of the iR corrected cathode
26
Table 1. Parameters of the tested cathodes
MnO2/C
La0.7Sr0.3MnO3
La0.7Sr0.3(Fe0.2Co0.8)O3
Thickness / mm
0.9
0.9
0.9
Catalyst loading / mg cm-2
20
45
45
Vulcan XC72R content / wt.%
*
10
10
BP2000 content / wt.%
*
10
10
PTFE content / wt.%
10
10
10
Nafion content / wt.%
10
10
10
*
MnO2 deposited on carbon support
The cathodes had an active area of 4 cm².
27
Table 2. Parameters for the Levich and Koutecky-Levich evaluation [20]
F
Faraday constant
96485
C mol-1
A
Area of electrode
0.196
cm²
Dr
Diffusion coefficient
1.8∙10-5
cm² s-1
ω
Rotation rate
42-168
rad s-1
ν
Kinematic viscosity
0.01
cm² s-1
Cr
Bulk concentration
7.8∙10-7
mol cm-3
28
Table 3. Results from the Levich and Koutecky-Levich analysis of the two cathode catalysts
Catalyst
Levich /n
Potential /V
KouteckyLevich / n
Potential Rate /V constant kh
La0.7Sr0.3MnO3
2.2
0.45
2.8
0.63
0.011
La0.7Sr0.3(Fe0.2Co0.8)O3 1.1
0.45
2.1
0.63
0.0055
29
Table 4. Internal resistances and current densities at 0.65 V vs. RHE (without iR correction) of the cathodes in 6 M KOH and 6 M KOH + 1 M EtOH
iR / Ω
La0.7Sr0.3MnO3
La0.7Sr0.3(Fe0.2Co0.8)O3
MnO2
6 M KOH
6 M KOH
6 M KOH
6 M KOH 1 M EtOH
0.083
Current density at 0.65 V vs. RHE 530 / mA cm-2 iR corrected 0.826 potential / V vs. RHE
6 M KOH 1 M EtOH
0.087
6 M KOH 1 M EtOH
0.095
614
399
256
611
196
0.854
0.789
0.739
0.852
0.713
30
Table 5. Substantial values at open circuit voltage and maximum (power density)
Current density / mA cm-2
Cell voltage /V
Anode potential /V
Cathode potential, Power density calculated / mW cm-2 /V
0.00
0.934 (OCV)
0.066 (OCP)
1.000 (OCP)
0.00
80.0
0.345
0.527
0.873
27.6 (max.)
31
Table 6. Resistance parameters of the semicircles in low (RLF) and high (RHF) frequency ranges
Current density
RHF / Ω cm²
[mA cm-2] 50 25 10 5
Fuel cell 0.551 0.476 0.557 0.503
RLF / Ω cm² Anode 0.578 0.552 0.727 0.725
Cathode 0.201 0.222 0.228 0.221
Fuel cell 3.70 5.25 10.9 21.4
Anode 4.26 5.18 8.60 15.8
Cathode 0.93 1.12 2.42 4.73
32
S0013-4686(17)30087-7 http://dx.doi.org/doi:10.1016/j.electacta.2017.01.087 EA 28754
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
14-12-2016 12-1-2017 14-1-2017
Please cite this article as: Ilena Grimmer, Paul Zorn, Stephan Weinberger, Christoph Grimmer, Birgit Pichler, Bernd Cermenek, Florian Gebetsroither, Alexander Schenk, Franz-Andreas Mautner, Brigitte Bitschnau, Viktor Hacker, Ethanol tolerant precious metal free cathode catalyst for alkaline direct ethanol fuel cells, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.01.087 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.
Ethanol tolerant precious metal free cathode catalyst for alkaline direct ethanol fuel cells
Ilena Grimmer*1, Paul Zorn1, Stephan Weinberger1, Christoph Grimmer1, Birgit Pichler1, Bernd Cermenek1, Florian Gebetsroither1, Alexander Schenk1, Franz-Andreas Mautner2, Brigitte Bitschnau2, Viktor Hacker1
1
Graz University of Technology, Institute of Chemical Engineering and Environmental
Technology, Fuel Cell Systems Group, NAWI Graz, Inffeldgasse 25C, 8010 Graz, Austria 2
Graz University of Technology, Institute of Physical and Theoretical Chemistry, NAWI Graz,
Stremayrgasse 9/I, 8010 Graz, Austria
Email-addresses of the authors: Ilena Grimmer: [email protected] Paul Zorn: [email protected] Stephan Weinberger: [email protected] Christoph Grimmer: [email protected] Birgit Pichler: [email protected] Bernd Cermenek: [email protected] Florian Gebetsroither: [email protected] Alexander Schenk: [email protected] Franz-Andreas Mautner: [email protected] Brigitte Bitschnau: [email protected] Viktor Hacker: [email protected]
*Corresponding author: Ilena Grimmer, [email protected], phone: +43 316 873 8798, fax: +43 316 873 8782 1
Graphical abstract
Highlights
Selective ORR catalysts are presented for alkaline direct ethanol fuel cells Perovskite based cathode catalysts show high tolerance toward ethanol A membrane-free alkaline direct ethanol fuel cell is presented
Abstract La0.7Sr0.3(Fe0.2Co0.8)O3 and La0.7Sr0.3MnO3 -based cathode catalysts are synthesized by the sol-gel method. These perovskite cathode catalysts are tested in half cell configuration and compared to MnO2 as reference material in alkaline direct ethanol fuel cells (ADEFCs). The best performing cathode is tested in single cell setup using a standard carbon supported Pt0.4Ru0.2 based anode. A backside Luggin capillary is used in order to register the anode 2
potential during all measurements. Characteristic processes of the electrodes are investigated using electrochemical impedance spectroscopy. Physical characterizations of the perovskite based cathode catalysts are performed with a scanning electron microscope (SEM) and by
X-ray diffraction showing phase pure materials. In half cell setup,
La0.7Sr0.3MnO3 shows the highest tolerance toward ethanol with a performance of 614 mA cm-2 at 0.65 V vs. RHE in 6 M KOH and 1 M EtOH at RT. This catalyst outperformed the state-of-the-art precious metal-free MnO2 catalyst in presence of ethanol. In fuel cell setup, the peak power density was 27.6 mW cm-2 at a cell voltage of 0.345 V and a cathode potential of 0.873 V vs. RHE.
Keywords: alkaline direct ethanol fuel cell, membrane free, ethanol tolerant cathode catalysts, precious-metal free, oxygen reduction reaction
1. Introduction The simpler transportation and storage of liquids compared to gases like hydrogen, as well as the safer and more favorable handling, gives liquid fuel cells an advantage over H2/O2 fuel cells. Due to the high availability of ethanol and the already existing large infrastructure concerning transport and storage, direct ethanol fuel cells (DEFC) are one of the most promising candidates of direct liquid fuel cells for power generation. At the anode of the DEFC, ethanol is oxidized, whereas at the cathode, oxygen is reduced comparable to conventional H2/O2 fuel cells. The electrode reactions of a DEFC in alkaline media are shown in eq. 1-3.
Anode: CH3CH2OH + 4 OH- → CH3COOH + 3 H2O + 4 e-
(1)
Cathode: O2 + 2 H2O + 4 e- → 4 OH-
(2)
Overall: CH3CH2OH + O2 → CH3COOH + H2O
(3)
3
In DEFC research, most literature focuses on investigating novel anode catalysts, since ethanol oxidation is more complex than the oxygen reduction reaction (ORR). Typically, the complete oxidation of ethanol to CO2 is not accomplished due to insufficient catalysts with respect to the C-C bond cleavage. Although much effort has been invested in this area, there is still necessity for further optimization of the anode catalysts in alkaline media toward complete ethanol oxidation [1–5]. Despite the lack of highly active anode catalysts, selective and tolerant cathode catalysts are of major importance as well, since ethanol crossover is still an issue in alkaline direct ethanol fuel cells (ADEFCs) [6]. Currently, the state-of-the-art cathode catalysts for ADEFCs are Pt or MnO2 [7,8]. Since Pt is not catalytically selective and ethanol crossover cannot be completely avoided even if a membrane is used, Pt cannot be the ORR catalyst of choice. In this work precious metal free cathode catalysts with high tolerance toward ethanol as well as high activity toward ORR are investigated. La0.7Sr0.3MnO3 and La0.7Sr0.3(Fe0.2Co0.8)O3 catalysts were synthesized and tested as cathode catalysts in ADEFC application. In order to compare the activity of the synthesized perovskites, MnO 2 was used as reference material, since it is already widely used for ORR in alkaline media [7]. The MnO2 was synthesized according to a previous published procedure [9,10]. The cathode catalysts were tested ex situ using a rotating disk electrode (RDE). Afterwards, electrodes were manufactured using a cross-rolling procedure [10,11] and then subjected to intensive in situ measurements in half cell configuration. Furthermore, single-cell tests were conducted with the best performing cathode in presence of ethanol, in order to characterize the behavior of the catalyst in a fuel cell setup with a carbon supported Pt0.4Ru0.2 based anode. Due to the high selectivity toward ORR of the La0.7Sr0.3MnO3 perovskite based catalyst, no membrane was used. Measurements were performed in the so-called mixed electrolyte approach according to Grimmer et al. [12]. Electrochemical impedance spectroscopy (EIS) was used to characterize processes taking place at the electrodes or rather the cell. 2. Experimental 4
2.1 Cathode catalyst syntheses and electrode manufacturing The catalysts La0.7Sr0.3(Fe0.2Co0.8)O3 and La0.7Sr0.3MnO3 were prepared via sol-gel method with citric acid (CA) as complexing agent [13–16]. In short, the required stoichiometric amounts of the respective metal nitrates were dissolved in ultrapure water. CA was added in the double molar concentration of the metal salts (CA/metal, 2/1). After intensive stirring for 1 h at 80 °C, ethylene glycol, acting as surfactant, was added dropwise to the mixtures (CA/EG, 1/1). The precursor solutions were stirred at 80 °C until the gelation process was completed. The viscous mixtures were dried and subsequently calcined at 850 °C. Finally, the catalysts were grinded and screened to a certain catalyst particle size fraction. MnO2 catalyst was prepared by dispersing Vulcan XC72R (Cabot Corporation) or BP2000 (Cabot Corporation) in H2O/ethanol. After adding Mn(NO3)2* 4 H2O, the suspension was stirred overnight at 60 °C. Once the solvent was removed completely, it was heated up to 400 °C under inert atmosphere. The resulting MnO 2/C (30wt.% MnO2 on Vulcan XC72R or BP2000) was grinded and screened to a catalyst particle size fraction <45 µm [9,10].
The cathodes were prepared via cross-rolling method. For the active layers (AL), the respective catalyst was mixed with carbon powder (Vulcan XC72R and BP2000) and PTFE as well as Nafion solution (5 wt.%, from Quintech) as binders in n-propanol and rolled until a leathery texture was achieved. For the gas diffusion layer (GDL), acetylene black and PTFE was suspended in isopropanol/water (1/1). AL and GDL were rolled separately and subsequently rolled together. After dry-pressing, the electrodes were hot-pressed at 300 °C onto a nickel-mesh, acting as current collector [17]. The catalyst loadings and thicknesses of the cathodes are listed in Table 1.
2.2 Anode manufacturing The anode was fabricated via drop coating of the catalyst ink onto a carbon cloth substrate material [18]. For this, PtRu/C (40wt.% Pt, 20wt.% Ru on carbon black from Cabot 5
Corporation) and Nafion-solution (5 wt.%, from Quintech) were dispersed in n-propanol. The carbon cloth was coated with an appropriate amount of the ink resulting in a catalyst loading of 1.5 mgPt cm-2.
2.3 Ex situ characterization of cathode catalysts Ex situ measurements of the catalysts were conducted with a Reference 600 potentiostat (Gamry, supplied by C3 Analysentechnik), in a three electrode setup with a reversible hydrogen electrode (RHE) as reference electrode, a platinum counter electrode and a glassy carbon working electrode coated with a thin catalyst layer. The catalyst ink was prepared, with the respective catalyst and a Nafion solution (5 wt.%, from Quintech) as binder, dispersed in ethanol and ultrapure water (volume ratio Nafion:EtOH:H2O = 2:49:49) [13,19]. Thereby, no additional carbon was used. The glassy carbon electrode with an active area of 0.196 cm2 was coated with 10 µl of the catalyst ink resulting in a catalyst loading of 210 µg cm-2geo. The electrode was rotated during evaporation of the solvent, in order to achieve a homogeneous and uniform catalyst film. The measurements were conducted in 1 M KOH electrolyte at 30 °C that was intertized with a continuous flow of ultrapure nitrogen. The internal resistance (5 Ω) was determined by the standard procedure provided by Gamry Instruments (single frequency impedance measurement at OCV usually at 10 kHz). According to the low internal resistance, no IR correction was performed. Potential cycling between 0.1 V and 1.0 V vs. RHE at a scan rate of 100 mV s-1 was performed until stable cyclic voltammograms were obtained. The ORR was investigated after the electrolyte was saturated with oxygen. For this, linear sweep voltammograms at electrode rotation speeds of 400, 600, 900, 1200 and 1600 rpm in the potential range of 0.1 V to 1 V vs. RHE and at a scan rate of 10 mV s-1 were performed. To determine the number of exchanged electrons (n) and the heterogeneous rate constant (k h), the Levich (L) (eq. 4) and Koutecky-Levich (KL) (eq. 5) equations were applied. ⁄3
𝑖𝑙 = 0.62 𝑛 𝐹 𝐴 𝐷𝑟2
𝜔 1⁄2 𝜈 −1⁄6 𝐶𝑟
(4) 6
1 𝑖𝑘𝑙
=
1 2⁄3
0.62 𝑛 𝐹 𝐴 𝐷𝑟
𝜔 1⁄2 𝜈 −1⁄6 𝐶𝑟
+
1 𝑛 𝐹 𝐴 𝑘ℎ 𝐶𝑟
(5)
The required parameters, regarding a 1 M KOH electrolyte, are listed in table 2 [20]. In case of the Levich analysis, the number of electrons (n) was derived for each rotation speed in the diffusion limiting region and given as the mean value. During KL analysis the heterogeneous rate constant (kh) was calculated and the number of electrons was determined additionally. This was done graphically by plotting the reciprocal current density versus the reciprocal of the square root of the rotational rate (j -1 vs. ω−1/2). The intercept and slope of this plot allows the determination of kh and n [21,22]. The linearity of Levich and Koutecky-Levich plots confirms the compatibility of the selected potential region.
2.4 In situ characterization 2.4.1 Half-cell measurements Half-cell measurements were performed using a multichannel BaSyTec CTS (Cell Test System), in a three electrode setup with a RHE as reference electrode and a stainless steel counter electrode. The RHE was connected to the electrode via Luggin capillary with a distance of approx. 2 mm. Measurements were performed with pure oxygen at a gas flow rate of 20 ml min-1 in 6 M KOH + 1 M EtOH at room temperature. The polarization curves of the electrodes were recorded by increasing the current until a potential of 0.9 V vs. RHE was reached, afterwards the measurements were conducted potentiostatically to a potential of 0.65 V vs. RHE, in 50 mV steps holding each step for 3 minutes. Each value represents the mean value of the last 30 seconds at the corresponding operation point. The internal resistances were determined via electrochemical impedance measurements.
2.4.2 Fuel-cell measurements The polarization curve of the single-cell was recorded galvanostatically using a Reference 600 potentiostat (Gamry, supplied by C3 Analysentechnik) holding each current step for 2 7
minutes. Each value represents the mean value of the last 30 seconds at the corresponding operation point. The electrolyte/fuel consisting of 6 M KOH + 1 M EtOH was temperature-controlled to 30 °C and supplied to the anode at a flow rate of 10 ml min-1. The cathode, La0.7Sr0.3MnO3, was fed with O2 with a gas flow rate of 20 ml min -1. In order to record the anode potential during measurements, a Luggin capillary was connected to the backside of the anode. In-between the electrodes a PP separator (Freudenberg 700/77) was placed to ensure the avoidance of short circuits. The internal (anode vs. cathode) as well as the uncompensated (anode vs. RHE) resistances were investigated via the standard Gamry procedure, as discussed above.
2.4.3 Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) characterizations were performed in galvanostatic mode at different current densities using a Gamry Reference 600 potentiostat. Besides standard EIS recordings of the single cell (anode vs. cathode) additional measurements of anode vs. RHE and cathode vs. RHE were carried out in the same single cell configuration. EIS was used in order to investigate characteristic processes of the electrodes at 4 operating points and current densities of 50 mA cm-2, 25 mA cm-2, 10 mA cm-2 and 5 mA cm-2, respectively. The amplitudes were set to 2% of the applied d.c. current densities (rms) to obtain a high signal to noise ratios maintaining linearity requirements. The investigated frequency range was 100 kHz and 0.4 Hz.
3. Results and discussion 3.1 Physical characterization Figure 1 shows the SEM pictures as well as the XRD patterns of the synthesized cathode catalysts; XRD patterns are compared to data from literature [23,24]. The XRD data of the 8
MnO2 catalyst are discussed elsewhere [10], showing a composition of monoclinic 𝛾–MnO2 with a small part of Mn3O4 (not shown). XRD patterns show phase pure, homogeneous materials with negligible impurities of both catalysts. Despite both catalysts were synthesized similarly, the morphology of the manganese based perovskite differs completely from FeCo-based catalyst.
While the
La0.7Sr0.3(Fe0.2Co0.8)O3 catalyst exhibits a very smooth distinct structure with well-defined grains, La0.7Sr0.3MnO3 catalyst shows a very fringed surface morphology without precise delimitation of the particles. The La 0.7Sr0.3MnO3 provides a greater surface area compared to the FeCo-based catalyst.
3.2 Ex situ measurements The general ORR mechanism in aqueous alkaline electrolyte can be divided into two pathways. The direct four-electron reduction, which leads to the production of water (eq. 6) and the less efficient, indirect two-electron reaction which results in the formation of alkaline stabilized hydrogen peroxide intermediates (eq. 7) [25–27]. This peroxide intermediate can further be converted into a harmful radical species with detrimental effects on catalysts. This species is vulnerable for reduction [26], or can undergo a disproportionation reaction (eq. 9). 𝑂2 + 2 𝐻2 𝑂 + 4 𝑒 − → 4 𝑂𝐻−
(6)
𝑂2 + 𝐻2 𝑂 + 2 𝑒 − → 𝐻𝑂2− + 𝑂𝐻 −
(7)
𝐻𝑂2− + 𝐻2 𝑂 + 2 𝑒 − → 3𝑂𝐻 −
(8)
𝐻2 𝑂2 → 𝑂2 + 𝐻2 𝑂
(9)
The cyclic voltammograms (CVs) below (figure 2-3) show the faradaic currents of the cathodic sweep as a function of applied potential at various rotation rates. The curves are corrected with CVs in N2 atmosphere. Thereby, the catalysts exhibit onset potentials of approx. 0,83 - 0.85 V vs. RHE.
9
A single step reduction process should appear as a straightforward sigmoidal current response [27]. On the contrary, the recorded curve indicates the combination of two sigmoidal curves, which is explained by the parallel occurrence of two and four electron reduction processes. The two electron reduction process is supplemented by a catalyzed disproportion of HO2- (eq. 8), resulting in a pseudo four-electron reduction process [25,27]. As expected, the current increases at higher rotation rates due to thinner diffusion layers. This diffusion limiting correlation is expressed in linear Levich plots over a wide potential range of 0.1 to 0.5 V vs. RHE (data not shown). In potential regions with occurring kinetic limitations, the Koutecky-Levich expression was used for analysis. Besides, the number of transferred electrons was determined by the Levich and Koutecky-Levich analysis. Thereby, the La0.7Sr0.3MnO3 perovskite results indicate exchanged electrons from around 2 and above. This confirms most likely a combined two and four electron transfer as expected from Tulloch and Donne [27]. On the contrary, the La0.7Sr0.3(Fe0.2Co0.8)O3 catalyst exhibits a lower ORR activity which is also reflected in the low number of exchanged electrons (approximately 1.3) in the Levich analysis. If the kinetic regions are considered, both catalysts show similar results. Overall, the RDE results can be confirmed in the literature [27–29]. Table 3 shows selected results from the analysis.
In general, the La0.7Sr0.3MnO3 catalyst demonstrated a higher performance. This is indicated at higher current densities and the higher number of transferred electrons and the higher heterogeneous rate constant. 3.3 In situ Measurements 3.3.1 Half-cell measurements of the cathodes Polarization curves of the cathodes tested in half-cell configurations are shown in figure 4-6. Measurements were performed in potentiostatic mode from OCP to 0.65 V vs. RHE. 10
The internal resistances, peak current densities at 0.65 V vs. RHE of all cathodes in 6 M KOH and in 6 M KOH + 1 M EtOH as well as the associated iR corrected cell potentials are listed in table 4.
As expected, the open circuit potentials of all three catalyst materials were in the range of 1 V vs. RHE, independent of the presence of ethanol. The state-of-the-art MnO2 catalyst had the highest performance in 6 M KOH, although the catalyst loading was less than half compared to the perovskite-based catalyzed cathodes. However, due to the limited tolerance of MnO2 toward ethanol, the current density dropped by 68% in EtOH containing electrolyte making it rather less suitable for ADEFC application, as can be seen in figure 6 and table 4. In contrast to that the synthesized perovskite catalyst showed superior ethanol tolerance. Among all tested catalysts, the manganese based perovskite La 0.7Sr0.3MnO3 exhibited comparably high current densities in 6 M KOH as MnO2, but additionally also the highest tolerance toward ethanol crossover. In presence of ethanol even an increasing performance of up to 614 mA cm-2 at 0.65 V vs. RHE was observed. The increasing performance is attributed to enhanced moistening of the electrode in ethanol environment. An appropriate combination of hydrophobic binder materials, carbon and catalyst ensured an optimal formation of the three-phase-boundary in presence of hydrophilic EtOH. Therefore, a larger catalyst surface became available for the ORR. The perovskite-based La0.7Sr0.3(Fe0.2Co0.8)O3 cathode had the lowest performance in pure KOH electrolyte, but still it outperformed the MnO2-based electrode in presence of ethanol. The higher catalytic activity of the manganese based perovskite is assigned to the different morphology as can be seen in the SEM images in 3.1. The greater surface area leads to higher ORR activity.
3.3.2 Fuel cell measurements
11
Due to the good performance of the cathode catalyzed with La0.7Sr0.3MnO3, single cell measurements were conducted with the same cathode and a PtRu catalyzed anode. The polarization curve (without iR correction) of the single-cell, the recorded anode potentials and the thereof calculated cathode potentials as well as the corresponding power densities are shown in figure 7 and associated results are given in table 5.
The internal resistance of the fuel cell was 0.13 Ω. The OCV of the fuel cell was unexpected high with 0.935 V, with a calculated cathode potential of > 1 V vs. RHE. With a peak power density of 27.6 mW cm-2 the cell shows quite good performance. As can be seen from the electrode potentials, the limitation of the ADEFC is the anode performance rather than the cathode. The anode performance can be optimized by higher operating temperatures, increased ethanol concentrations in the fuel as well as by identifying suitable anode electrode structures; which reduce the mass transport limitations. As far as the obtained results show, the cathode performs excellent with low-cost materials. It showed a calculated potential of 0.873 V vs. RHE at the maximum power density, at 80 mA cm-2.
3.4 Electrochemical Impedance spectroscopy All EIS results exhibited two distinguishable limiting processes taking place at each electrode, shown as Nyquist diagram with two semicircles for the iR corrected cathode in figure 8 exemplary. Anode EIS measurements show similar behavior, being limited by two characteristic processes. Nevertheless, the fuel cell EIS series exhibited only two processes since the characteristic frequencies were too close to each other to be observable as separate semicircles. Therefore, the high frequency semicircles and low frequency semicircles partially mask each other. Additionally, the cathode EIS at 50 mA cm -2 clearly indicated a low frequency relaxation behavior and a low frequency inductive loop, respectively [30,31]. 12
Complex nonlinear least square fitting was applied utilizing the following equivalent circuit: -(Lcable)-(Rel)-(RHF||CPEHF)-((RLF||CPELF)- to obtain the 8 fitting parameters [30,31]. Abbreviations: Lcable
Wire inductance
Rel
Electrolyte resistance
RLF
Resistance low frequency
RHF
Resistance high frequency
CPEHF
Constant phase elements high frequency
CPELF
Constant phase elements low frequency
The values in table 6 show the resistance parameters of the semicircles in low and high frequency ranges of the cathode, the anode and the fuel cell, at respective current densities. The first semicircle is attributed to the porosity of the electrode since R HF was found to be constant with respect to the applied current density. The second semicircle is ascribed to activation overpotential due to the exponentially growing RLF with respect to decreasing current density. As can be seen in table 6, the anode causes a large part of the performance losses, in proportion 3 to 1 (anode to cathode).
4. Conclusion Three cathode catalysts were characterized in-situ in half cell configurations. The most promising candidate as ORR catalyst in ADEFC application is La0.7Sr0.3MnO3 as can be seen from ex-situ and in-situ results. In in situ half-cell measurements it also showed the highest tolerance toward ethanol with a current density of 614 mA cm -2 at 0.65 V vs. RHE at room temperature in 6 M KOH + 1 M EtOH. MnO2 catalyst which is widely used as precious metal free ORR catalyst was outperformed in presence of ethanol by perovskite based catalysts. 13
Tests in single cell setup showed very good performance exhibiting a peak power density of 27.6 mW cm-2, without iR correction. EIS spectra confirmed the findings of the VI measurements exhibiting a cathode to anode performance loss ratio of approx. 3. Furthermore, it was depicted that two limiting processes for both electrodes occur. The first semicircle is attributed to the porosity of the anode and cathode, respectively. The second and more dominant low frequency semicircles was found to be activation limited processes and may be attributed to charge transfer at the anode and cathode. In alkaline media, precious metal free catalysts show extraordinary performance and therefore great potential for usage in ADEFCs. Besides ethanol, which can easily be obtained e.g. from biomass, only oxygen from air is needed. With ORR selective cathodes, cost-intensive membranes are redundant, which makes an ADEFC one of the most convenient technologies for low power applications. With this findings preliminary work has been done for low-cost ADEFC stacks.
Acknowledgements We want to thank Apollo Energy Systems for providing cathode materials. Financial support by the Austrian Federal Ministry of Science, Research and Economics (BMWFW), the Austrian Research Promotion Agency and the IEA research cooperation are gratefully acknowledged.
14
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18
Figure 1. SEM images and XRD patterns of a, b, e) the La0.7Sr0.3MnO3 c, d, f) the La0.7Sr0.3(Fe0.2Co0.8)O3 perovksite catalysts
19
Figure 2. RDE measurements of the La0.7Sr0.3MnO3 perovskite. The insert represents the corresponding Koutecky-Levich plot
20
Figure 3. RDE measurements of the La0.7Sr0.3(Fe0.2 Co0.8)O3 perovskite. The insert represents the corresponding Koutecky-Levich plot
21
RT, 20 mlO2/min
1
Potential / V
0.9
0.8 0.7 VI, 6 M KOH VIiR comp, 6 M KOH
0.6
0.5
VI, 6 M KOH + 1 M EtOH VIiR comp, 6 M KOH + 1 M EtOH
0
200
400 600 Current density / mA cm-2
Figure 4. Polarization curve of La0.7Sr0.3MnO3 in 6 M KOH and 6 M KOH + 1 M EtOH, with and without iR correction
22
RT, 20 mlO2/min
1
VI, 6 M KOH + 1 M EtOH VIiR comp, 6 M KOH + 1 M EtOH
0.9
Potential / V
VI, 6 M KOH VIiR comp, 6 M KOH
0.8 0.7 0.6
0.5
0
200
400 600 Current density / mA cm-2
800
Figure 5. Polarization curve of La0.7Sr0.3(Fe0.2Co0.8)O3 in 6 M KOH and 6 M KOH + 1 M EtOH, with and without iR correction
23
RT, 20 mlO2/min
1
VI, 6 M KOH + 1 M EtOH VIiR comp, 6 M KOH + 1 M EtOH
0.9 Potential / V
VI, 6 M KOH VIiR comp, 6 M KOH
0.8
0.7
0.6 0
200
400 600 Current density / mA cm-2
800
Figure 6. Polarization curve of MnO2 in 6 M KOH and 6 M KOH + 1 M EtOH, with and without iR correction
24
VI curve Anode potential Cathode potential calculated
Cell voltage / V
1
Power density
35 30
25
0.8
20 0.6 15
0.4
10
0.2
Power density / mW cm-2
1.2
5 6 M KOH 1 M EtOH, 30 °C
0
0
50 100 Current density / mA cm-2
0 150
Figure 7. Polarization curve and corresponding power densities without iR correction, recorded anode potential and calculated cathode potential
25
0.5 50 mA cm-² 25 mA cm-² 10 mA cm-² 5 mA cm-²
1.58 Hz
Im{Z} / Ohm
0
1 Hz 0.25 Hz
−0.5 3.98 kHz −1
0.13 Hz
−1.5 −2
0
1
2 Re{Z} / Ohm
3
4
Figure 8. Nyquist diagram of the iR corrected cathode
26
Table 1. Parameters of the tested cathodes
MnO2/C
La0.7Sr0.3MnO3
La0.7Sr0.3(Fe0.2Co0.8)O3
Thickness / mm
0.9
0.9
0.9
Catalyst loading / mg cm-2
20
45
45
Vulcan XC72R content / wt.%
*
10
10
BP2000 content / wt.%
*
10
10
PTFE content / wt.%
10
10
10
Nafion content / wt.%
10
10
10
*
MnO2 deposited on carbon support
The cathodes had an active area of 4 cm².
27
Table 2. Parameters for the Levich and Koutecky-Levich evaluation [20]
F
Faraday constant
96485
C mol-1
A
Area of electrode
0.196
cm²
Dr
Diffusion coefficient
1.8∙10-5
cm² s-1
ω
Rotation rate
42-168
rad s-1
ν
Kinematic viscosity
0.01
cm² s-1
Cr
Bulk concentration
7.8∙10-7
mol cm-3
28
Table 3. Results from the Levich and Koutecky-Levich analysis of the two cathode catalysts
Catalyst
Levich /n
Potential /V
KouteckyLevich / n
Potential Rate /V constant kh
La0.7Sr0.3MnO3
2.2
0.45
2.8
0.63
0.011
La0.7Sr0.3(Fe0.2Co0.8)O3 1.1
0.45
2.1
0.63
0.0055
29
Table 4. Internal resistances and current densities at 0.65 V vs. RHE (without iR correction) of the cathodes in 6 M KOH and 6 M KOH + 1 M EtOH
iR / Ω
La0.7Sr0.3MnO3
La0.7Sr0.3(Fe0.2Co0.8)O3
MnO2
6 M KOH
6 M KOH
6 M KOH
6 M KOH 1 M EtOH
0.083
Current density at 0.65 V vs. RHE 530 / mA cm-2 iR corrected 0.826 potential / V vs. RHE
6 M KOH 1 M EtOH
0.087
6 M KOH 1 M EtOH
0.095
614
399
256
611
196
0.854
0.789
0.739
0.852
0.713
30
Table 5. Substantial values at open circuit voltage and maximum (power density)
Current density / mA cm-2
Cell voltage /V
Anode potential /V
Cathode potential, Power density calculated / mW cm-2 /V
0.00
0.934 (OCV)
0.066 (OCP)
1.000 (OCP)
0.00
80.0
0.345
0.527
0.873
27.6 (max.)
31
Table 6. Resistance parameters of the semicircles in low (RLF) and high (RHF) frequency ranges
Current density
RHF / Ω cm²
[mA cm-2] 50 25 10 5
Fuel cell 0.551 0.476 0.557 0.503
RLF / Ω cm² Anode 0.578 0.552 0.727 0.725
Cathode 0.201 0.222 0.228 0.221
Fuel cell 3.70 5.25 10.9 21.4
Anode 4.26 5.18 8.60 15.8
Cathode 0.93 1.12 2.42 4.73
32