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  • 1
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    Transition Metal Dissolution in State-of-the-Art and Next Generation Li-Ion Batteries Studied By Spatially Resolved Operando X-Ray Absorption Spectroscopy
    The Electrochemical Society ; 2018
    In:  ECS Meeting Abstracts Vol. MA2018-02, No. 4 ( 2018-07-23), p. 179-179
    Details
    In: ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2018-02, No. 4 ( 2018-07-23), p. 179-179
    Abstract: High energy Li-Ion batteries rely on the pairing of a transition metal oxide cathode with a graphite anode. Pushing the upper cut-off voltage and state of charge (SOC) to higher values, the cycle-life of the battery is drastically reduced. Besides electrochemical electrolyte oxidation leading to poor coulombic efficiencies and a large growth of cell impedance, leaching of transition metal ions from the cathode active materials is one of the major obstacles to increase the power density of the cell without change of active materials. While the capacity loss which can be ascribed to the overall loss of active material is minor, cell aging is largely aggravated by liberated transition metal ions present in the electrolyte, as they diffuse through the separator and deposit on the graphite anode due to its low potential. A severe impedance growth of the anode and loss of active lithium by chemical delithiation of the graphite has been observed by many studies. Especially for manganese, the amount of active lithium loss has been found to be a multifold higher than the total amount of manganese ions present in the electrolyte and deposited onto the graphite anode, indicating a catalytic role of transition metal ions on active lithium loss (1). Comparisons of the transition metal leaching behavior for different cathode active materials is missing so far, as studies vary widely in test procedures (potential range, SOC) and environment (electrolyte and temperature). We herein present operando analysis of the intrinsic stability of several cathode active materials upon charge to high SOC and potential with our unique spectroscopic cell enabling spatially resolved operando X-ray absorption spectroscopy, monitoring the concentration and oxidation state of transition metals in the electrolyte and in the graphite anode (2, 3). Aside layered mixed transition metal oxide materials (Li 1+w [Ni x Co y Mn z ] 1-x O 2 ) also the spinel structure is tested to better understand the effect of crystal destabilization at high SOC. The formerly standard layered oxide NCM111 (x=y=z=0.33) was already studied earlier, focusing on manganese dissolution (3). In this talk we present time and potential resolved analysis of all transition metals released from NCM111 (see Fig. 1) in comparison to a nickel-rich NCM811 (x=0.8, y=z=0.1), a lithium- and manganese-rich layered oxide material (HE-NCM; w=0.17, x=0.22, y=0.12, z=0.66 (4)) and the high-voltage spinel LiNi 0.5 Mn 1.5 O 4 , (LNMO). This data allows conclusions on the extent of the two major causes for transition metal leaching from oxide based cathode materials: chemical decomposition induced by protic electrochemical electrolyte oxidation products (5) and crystal lattice destabilization of layered transition metal oxides caused by high degrees of delithiation (high SOC). References J. A. Gilbert, I. A. Shkrob and D. P. Abraham, Journal of The Electrochemical Society , 164 , A389 (2017). Y. Gorlin, A. Siebel, M. Piana, T. Huthwelker, H. Jha, G. Monsch, F. Kraus, H. A. Gasteiger and M. Tromp, Journal of the Electrochemical Society , 162 , A1146 (2015). J. Wandt, A. Freiberg, R. Thomas, Y. Gorlin, A. Siebel, R. Jung, H. A. Gasteiger and M. Tromp, J. Mater. Chem. A , 4 , 18300 (2016). B. Strehle, K. Kleiner, R. Jung, F. Chesneau, M. Mendez, H. A. Gasteiger and M. Piana, Journal of The Electrochemical Society , 164 , A400 (2017). M. Metzger, B. Strehle, S. Solchenbach and H. A. Gasteiger, Journal of The Electrochemical Society , 163 , A798 (2016). Acknowledgements Financial support for S. S. and B. S. by the BASF SE through its Research Network on Electrochemistry and Batteries is gratefully acknowledged. XAS data were gathered at the European Synchrotron Radiation Facility (ESRF) and SOLEIL Synchrotron (France).We are grateful to Dr. Debora Motta Meira at the ESRF, Grenoble (France) for providing assistance in using beamline BM 23 as well as Dr. Emiliano Fonda for assistance on the SAMBA beamline at SOLEIL Synchrotron. Fig. 1: Transition metal dissolution of an NCM111 (5.5 mAh/cm 2 )//C 6 cell upon cycling in the conventional potential range (0-6.5 h: 60% SOC until ≈4.2 V Li ) and upon high degrees of delithiation (7-11 h: ≈100% SOC until 5.1 V Li ) measured in our operando XAS cell using glass fiber separators and standard LP57 electrolyte (1 M LiPF 6 in EC:EMC [3:7]) at 25 °C. The concentration of transition metal ions within the graphite electrode (filled symbols) and in the electrolyte (hollow stars, shown as electrolyte share in the graphite pores [concentration measured in the separator multiplied by the porosity of the graphite anode] ) is monitored operando. Figure 1
    Type of Medium: Online Resource
    ISSN: 2151-2043
    URL: Article
    DOI: 10.1149/MA2018-02/4/179
    Language: Unknown
    Publisher: The Electrochemical Society
    Publication Date: 2018
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  • 2
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    (Vittorio de Nora Award Address) Analysis of the Catalyst Requirements with Regards to Catalyst Structure and Catalyst Durability Studies for PEM Water Electrolysis
    The Electrochemical Society ; 2020
    In:  ECS Meeting Abstracts Vol. MA2020-02, No. 38 ( 2020-11-23), p. 2454-2454
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    In: ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2020-02, No. 38 ( 2020-11-23), p. 2454-2454
    Abstract: Proton exchange membrane (PEM) based water electrolyzers are a promising technology for the large-scale production of hydrogen from renewable energy. The most efficient and durable catalysts for PEM water electrolysis are platinum to catalyze the hydrogen evolution reaction (HER) and iridium oxide to catalyze the oxygen evolution reaction (OER). While the currently rather high noble metal loadings (≈0.3-0.5 mg Pt /cm 2 and ≈1-3 mg Ir /cm 2 ) have only a minor impact on the overall water electrolyzer system cost [1], the large-scale global implementation of PEM water electrolysis would require a substantial reduction of the noble metal loadings due to the limited availability of Pt and Ir. This is easy to achieve for the Pt cathode catalyst, owing to its fast HER kinetics and the fact that a high dispersion of Pt is readily achieved with carbon-supported Pt (Pt/C). However, lowering the Ir loading below ≈0.5 mg Ir /cm 2 is not possible with currently available Ir catalysts (Ir black, nano-IrO 2 , or IrO 2 supported on a TiO 2 substrate), as it results in too thin, non-contiguous electrode layers with poor in-plane conductivity, owing to their low iridium packing density of ≈2.3 g Ir /cm 3 electrode (this is in contrast to ≈0.05-0.10 g Pt /cm 3 electrode for a typical Pt/C catalyst) [2]. Thus, to reach the long-term target of an Ir-specific power density of ≤0.01 g Ir /kW [2], novel catalysts with a lower iridium packing density are required, which could be achieved, e.g., by supporting iridium or iridium oxide nanoparticles on conductive supports or by other approaches [3] . A detailed analysis leading to this conclusion will be discussed in this presentation. The development and implementation of such novel catalysts also requires the evaluation of their long-term stability. Conventionally, this is either done by measurements in liquid electrolyte, typically using the rotating disk electrode (RDE) technique, or testing in an actual PEM electrolyzer. Unfortunately, recent studies revealed that the RDE based approach does not yield reliable catalyst durability data, since for yet unknown reasons, the catalyst life-times obtained by the RDE method are three to four orders of magnitude lower than what is observed in PEM electrolyzers [3, 4]. On the other hand, catalyst durability testing in a PEM electrolyzer requires very long measurement times, so that accelerated durability test protocols are necessary [5] . These aspects will also be discussed. References: [1] K. E. Ayers, N. Danilovic, R. Ouimet, M. Carmo, B. Pivovar, M. Bornstein; Annual Review of Chemical and Biomolecular Engineering 10 (2019) 219. [2] M. Bernt, A. Siebel, H. A. Gasteiger; J. Electrochem. Soc. 165 (2018) F305. [3] M. Bernt, A. Weiß, M. F. Tovini, H. El-Sayed, C. Schramm, J. Schröter, C. Gebauer, H. A. Gasteiger; Chem. Ing. Tech 92 (2020); https://doi.org/10.1002/cite.201900101. [4] H. A. El-Sayed, A. Weiß, L. F. Olbrich, G. P. Putro, H. A. Gasteiger; J. Electrochem. Soc. 166 (2019) F458. [5] A. Weiß, A. Siebel, M. Bernt, T. H. Shen, V. Tileli, H. A. Gasteiger; J. Electrochem. Soc. 166 (2019) F487.
    Type of Medium: Online Resource
    ISSN: 2151-2043
    URL: Article
    DOI: 10.1149/MA2020-02382454mtgabs
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    Publisher: The Electrochemical Society
    Publication Date: 2020
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  • 3
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    (Invited) Hydrogen Oxidation and Evolution Reaction (HOR/HER) on Pt Electrodes in Acid vs. Alkaline Electrolytes: Mechanism, Activity and Particle Size Effects
    The Electrochemical Society ; 2014
    In:  ECS Transactions Vol. 64, No. 3 ( 2014-08-18), p. 1069-1080
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    In: ECS Transactions, The Electrochemical Society, Vol. 64, No. 3 ( 2014-08-18), p. 1069-1080
    Abstract: In this study, we provide an overview regarding our recent finding on the mechanism, activity and particle size effect for the hydrogen oxidation and evolution reaction (HOR/HER) on Pt-group metal electrodes, under both acid and alkaline conditions. We show that there is an activity decrease of about two orders of magnitude when going from acid to base conditions on electrodes which are able to form a H-UPD layer like Pt, Ir, Pd and Rh. Similarities in the HOR/HER process between acid and base conditions have been found: the rate determining step, which has been identified by means of electrochemical impedance spectroscopy measurements to be the Volmer reaction, remains the same and there is no particle size dependency on the reactivity of Pt electrodes. In the light of these finding, Volcano plots of the HOR/HER activities in acid and base have been proposed.
    Type of Medium: Online Resource
    ISSN: 1938-5862 , 1938-6737
    URL: Article
    DOI: 10.1149/06403.1069ecst
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    Publisher: The Electrochemical Society
    Publication Date: 2014
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  • 4
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    Accelerated Degradation Test in PEM Water Electrolysis
    The Electrochemical Society ; 2017
    In:  ECS Meeting Abstracts Vol. MA2017-02, No. 37 ( 2017-09-01), p. 1648-1648
    Details
    In: ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2017-02, No. 37 ( 2017-09-01), p. 1648-1648
    Abstract: Proton exchange membrane water electrolysis (PEM-WE) is a suitable technology for producing hydrogen via electricity generated from renewable but fluctuating energy sources, such as wind or solar energy. Due to their modest activity but sufficiently high stability in the acidic environment of a PEM system, iridium oxides (IrO x ) are the most commonly used catalysts for the oxygen evolution reaction (OER). 1 However, recent studies revealed a strong correlation between the OER activity and the stability of IrO x depending on its surface morphology and hydration state (i.e., between highly crystalline thermal IrO 2 and amorphous, hydrous oxides). Bulk IrO 2 provides the best compromise regarding long lifetime requirements, since its OER activity decreases with inreaseing IrO x crystallinity, whereas its stability improves 2,3 . Recent studies from our lab revealed that IrO x can easily be reduced to metallic iridium (Ir) when IrO x based membrane electrode assemblies (MEAs) are held at open circuit voltage (OCV) in a PEM-WE, where crossover H 2 from the cathode side reduces the surface of the IrO x catalyst at the anode. This reduction step is indicated by the formation of H-UPD features in the recorded cyclic voltammograms (CVs). Interestingly, the polarization curve recorded directly after this reduction shifts towards lower cell voltages, corresponding to an improved OER activity ( Figure 1 ). Moreover, in a subsequent CV (recorded after the latter polarization curve), the H-UPD features disappeared while the characteristic oxide formation and reduction peaks evolved, pointing towards a change of the catalyst surface properties to a state closer to less crystalline, hydrous IrO x . Considering the fact, that hydrous IrO x exhibits less stability 4 and that this partial IrO x reduction and re-oxidation can occur during cycles of extended OCV periods, these findings must be considered as potential degradation mechanism in PEM-WE operation. During the lifetime of an electrolyzer, especially if coupled with a fluctuating power supply, operation interruptions can be expected to occur frequently, thereby altering the form of the IrO x . Therefore, we apply an accelerated test protocol cycling between operation and OCV periods to investigate this degradation mechanism further as well as the effect of potential mitigation strategies. Acknowledgements : This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) and by the German Ministry of Education and Research (funding number 03SFK2V0, Kopernikus-project P2X). References   (1) C. Rozain, E. Mayousse, N. Guillet and P. Millet, Appl. Catal. B , 182 , 123 (2016) (2) T. Reier, D. Teschner, T. Lunkenbein, A. Bergmann, S. Selve, R. Kraehnert, R. Schlögl, and P. Strasser, J. Electrochem. Soc. , 161 , F876 (2014). (3) S. Cherevko, T. Reier, A. R. Zeradjanin, Z. Pawolek, P. Strasser, and K. J. J. Mayrhofer, Electrochem. Commun. , 48 , 81 (2014). (4) S. Geiger, O. Kasian, B. R. Shrestha, A. M. Mingers, K. J. J. Mayrhofer and S. Cherevko, J. Electrochem. Soc. , 163 , F3132–F3138 (2016) Figure 1 Polarization curves (A) at initial state (black) and after the reduction step (blue); IV plots were recorded galvanostatically at 80 °C and ambient pressure on a MEA (Nafion 212) with 5 cm² active area, fed with 5 mLmin -1 liquid H 2 O to the anode. HFR values estimated from impedance spectra recorded during the IV plots are displayed in (B). Figure 1
    Type of Medium: Online Resource
    ISSN: 2151-2043
    URL: Article
    DOI: 10.1149/MA2017-02/37/1648
    Language: Unknown
    Publisher: The Electrochemical Society
    Publication Date: 2017
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  • 5
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    Operando Characterization of Lithium-Sulfur Battery Intermediates Using X-Ray Absorption Spectroscopy
    The Electrochemical Society ; 2015
    In:  ECS Meeting Abstracts Vol. MA2015-01, No. 2 ( 2015-04-29), p. 243-243
    Details
    In: ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2015-01, No. 2 ( 2015-04-29), p. 243-243
    Abstract: The improvement of the energy density of current Li-Ion batteries and the development of novel, post-Li-Ion energy storage devices is largely hindered by a poor understanding of the cell chemistry and degradation mechanisms which ultimately lead to a low cycle life and/or poor rate capability. For example, some controversy still exists regarding the formation of Li 2 S and is related to the fact that intermediates can’t be easily characterized due to their low stability. In-situ speciation using common characterization techniques including X-ray diffraction (XRD) [1, 2] and Ultraviolet-visible spectroscopy (UV-VIS) [3, 4] was previously attempted but yet unable to yield a clear understanding of intermediates involved in the electrochemical conversion of sulfur to Li 2 S during discharge of the battery. This is mainly due to the fact that they detect either solid or liquid species but not both at the same time. X-ray absorption spectroscopy (XAS) eliminates these restrictions and thus provides a powerful tool for monitoring compositional changes during operation of a battery. Different sulfur species exhibit characteristic features, dependent on their chemical composition, in the x-ray absorption near-edge structure (XANES) at the S K-edge, exploited in previous studies [5-8]. However, a drawback in these previous measurements was the data collection geometry in which the x-ray beam was penetrating the cell through the backside of the sulfur electrode. In this work, we introduce a novel spectro-electro­chemical cell which allows direct observation of intermediates in the separator and use it to explore the influence of the electrolyte solvent’s dielectric constant on the conversion of polysulfides to Li 2 S by comparing solvents with a low-dielectric constant (DOL:DME, 1:1 v/v) and a high-dielectric constant (DMAC), the first discharge curves of which are shown in Figure 1. Figure 1: Galvanostatic discharge curves measured in operando mode at a C-rate of 0.1 h -1 using either DOL-DME (1:1 v/v) or DMAC, each containing 1M LiClO 4 and 0.5M LiNO 3 . Our operando XAS measurements show that the formation of Li 2 S is faster in DOL:DME compared to DMAC, and also suggest that Li 2 S 2 species is present at the end of discharge in DOL:DME. Furthermore, contrary to other reports, we do not detect any elemental sulfur at the end of discharge [5]. References: [1] N. A. Cañas, S. Wolf, N. Wagner and K. A. Friedrich, J. Power Sources , 226 , 313 (2013). [2] S. Walus, C. Barchasz, J.-F. Colin, J.-F. Martin, E. Elkaim, J.-C. Lepretre and F. Alloin, Chem. Commun. , 49 , 7899 (2013). [3] R. Bonnaterre and G. Cauquis, J. Chem. Soc., Chem. Commun. , 293 (1972). [4] N. A. Cañas, D. N. Fronczek, N. Wagner, A. Latz and K. A. Friedrich, J. Phys. Chem. C , 118 , 12106 (2014). [5] M. Cuisinier, P.-E. Cabelguen, S. Evers, G. He, M. Kolbeck, A. Garsuch, T. Bolin, M. Balasubramanian and L. F. Nazar, J. Phys. Chem. Lett. , 4 , 3227 (2013). [6] M. E. Fleet and X. Liu, Spectrochim. Acta B , 65 , 75 (2010). [7] T. A. Pascal, K. H. Wujcik, J. Velasco-Velez, C. Wu, A. A. Teran, M. Kapilashrami, J. Cabana, J. Guo, M. Salmeron, N. Balsara and D. Prendergast, J. Phys. Chem. Lett. , 5 , 1547 (2014). [8] M. U. M. Patel, I. Arčon, G. Aquilanti, L. Stievano, G. Mali and R. Dominko, ChemPhysChem , 15 , 894 (2014). Figure 1
    Type of Medium: Online Resource
    ISSN: 2151-2043
    URL: Article
    DOI: 10.1149/MA2015-01/2/243
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    Publisher: The Electrochemical Society
    Publication Date: 2015
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  • 6
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    Nanometric Fe-substituted ZrO 2 on Carbon Black: a Novel PGM-Free ORR Catalyst for PEMFCs
    The Electrochemical Society ; 2018
    In:  ECS Meeting Abstracts Vol. MA2018-01, No. 30 ( 2018-04-13), p. 1721-1721
    Details
    In: ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2018-01, No. 30 ( 2018-04-13), p. 1721-1721
    Abstract: The independence from Platinum-Group-Metal (PGM) is one of the major goals towards the commercialization of Proton Exchange Membrane Fuel Cells (PEMFCs), due to the limited availability of PGM and their considerable loading on the Oxygen-Reduction-Reaction (ORR), increasing their cost contribution to the overall system. On the other hand, the challenges for PGM-free ORR catalysts are an activity approaching that of Pt and a long-term operational stability in the strong acidic environment of a PEMFC. Low-cost FeNC catalysts fulfill the high activity requirement, but they still face fast degradation in acidic environment [1, 2]. On the contrary, pure valve-metal oxides as non-PGM ORR catalysts need a significant improvement in ORR activity and H 2 O yield but they are very promising in terms of stability during PEMFC operation [3, 4]. The aim of this contribution is to combine the intrinsic high ORR activity of Fe with the stability of valve-metal oxides, via partial substitution of Zr 4+ with Fe 3+ in the ZrO 2 structure [5]. The formation of a solid solution of oxides of the two aliovalent-metals should also provide oxygen vacancies or uncoordinated metal sites at the oxide surface, hypothesized to be correlated to an increased ORR activity of valve-metal oxides [6, 7] . In this study, to obtain Fe-substituted ZrO 2 we used a homogeneous mixture of the two metals at the molecular level, by employing two soluble organometallic precursors, i. e., zirconium (IV) tetra-tert-butyl dichloro phthalocyanine (ZrCl 2 Pc(tBu) 4 ) and iron(II) tetra-tert-butyl phthalocyanine (FePc(tBu) 4 ). Graphitized Ketjen-Black carbon is first impregnated with the precursors and then heat-treated as described in [8, 9]. XRD and TEM characterization show that our catalysts consist of ZrO 2 particles of about 3 nm (Fe-substituted) and 7 nm (pure ZrO 2 ). Mössbauer spectroscopy reveals that the Fe moieties are isolated and in Fe 3+ electronic configuration at high spin, typical of an oxidic environment. This is confirmed by soft XAS data at the Fe L-edge and XPS data, pointing to the desired Fe x Zr 1-x O 2-δ phase. We already published an evaluation of the ORR mass activity of Fe x Zr 1-x O 2-δ in comparison to Fe-only and ZrO 2 -only catalysts, using a thin-film rotating (ring) disk electrode (RRDE) setup [9]. Using the preferred catalyst Fe 0.07 Zr 0.93 O 2-δ , the variation of its mass activity and selectivity as a function of the loading is further discussed, with a H 2 O 2 yield even lower in comparison to an FeNC catalyst [10]. The catalyst mass activity and its Arrhenius analysis in a single PEMFC at ≈0.4 mg cat /cm² is compared to the RDE results (Figure 1) [11]. The Arrhenius analysis resulted in an ORR activation energy of 16 kJ/mol (RDE) and 18 kJ/mol (PEMFC) at 0.4 V RHE , significantly lower than our best pure-ZrO 2 catalyst reported (21 kJ/mol from RDE and 29 kJ/mol from PEMFC data) [4]. Stability tests and loading optimization of nanometric Fe x Zr 1-x O 2-δ will be the future outlook. Acknowledgements: This work was supported by the Bayerische Forschungsstiftung (Project ForOxiE², AZ 1143-14). References: [1] M. Lefèvre, J. P. Dodelet, ECS Transactions 2012 , 45(2) , 35-44. [2] B. Piela, T. S. Olson, P. Atanassov, P. Zelenay, Electrochimica Acta 2010 , 55 , 7615-7621. [3] A. Ishihara, S. Yin, K. Suito, N. Uehara, Y. Okada, Y. Kohno, K. Matsuzawa, S. Mitsushima, M. Chisaka, Y. Ohgi, M. Matsumoto, H. Imai, K. Ota, ECS Transactions 2013 , 58 , 1495-1500. [4] T. Mittermeier, P. Madkikar, X. Wang, H. A. Gasteiger and M. Piana, J. Electrochem. Soc. 2016 , 163 , F1543-F1552. [5] D. Sangalli, A. Lamperti, E. Cianci, R. Ciprian, M. Perego, A. Debernardi, Phys. Rev. B 2013 , 87 , 085206. [6] A. Ishihara, M. Tamura, Y. Ohgi, M. Matsumoto, K. Matsuzawa, S. Mitsushima, H. Imai, K.-i. Ota, J. Phys. Chem. C , 2013 , 117 , 18837–18844. [7] Y. Ohgi, A. Ishihara, K. Matsuzawa, S. Mitsushima, K.-i. Ota, M. Matsumoto, H. Imai, J. Electrochem. Soc. 2013 , 160 , F162-F167. [8] P. Madkikar, X. Wang, T. Mittermeier, A. H. A. Monteverde Videla, C. Denk, S. Specchia, H. A. Gasteiger, M. Piana, J. Nanostruct. Chem. 2017 , 7 , 133-147. [9] P. Madkikar, T. Mittermeier, H. A. Gasteiger, M. Piana, J. Electrochem. Soc. 2017 , 164(7) , F831-F833. [10] A. Bonakdarpour, M. Lefevre, R. Yang, F. Jaouen, T. Dahn, J.-P. Dodelet, J. R. Dahn, Electrochem. Solid-State Lett. 2008 , 11(6) , B105-B108. [11] P. Madkikar, D. Menga, G. Harzer, T. Mittermeier, F. E. Wagner, M. Merz, S. Schuppler, P. Nagel, A. Siebel, H. A. Gasteiger, M. Piana, manuscript in preparation . Figure 1
    Type of Medium: Online Resource
    ISSN: 2151-2043
    URL: Article
    DOI: 10.1149/MA2018-01/30/1721
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    Publisher: The Electrochemical Society
    Publication Date: 2018
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  • 7
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    Impact of Intermittent Operation on the Lifetime and Performance of a PEM Water Electrolyzer
    The Electrochemical Society ; 2018
    In:  ECS Meeting Abstracts Vol. MA2018-02, No. 46 ( 2018-07-23), p. 1606-1606
    Details
    In: ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2018-02, No. 46 ( 2018-07-23), p. 1606-1606
    Abstract: In times of an increasing demand for energy production through renewable but fluctuating energy sources, such as wind or solar energy, hydrogen as an energy carrier becomes more and more important. Proton-exchange membrane water electrolysis (PEM-WE) is a suitable and already quite advanced technique for sustainable production of hydrogen. 1 However, coupling a PEM-WE with intermittent renewable energy sources will induce frequent current interrupts of the PEM-WE system. These events can potentially lead to rapid degradation of the membrane electrode assemblies (MEAs) and hence, a thorough understanding of the underlying mechanisms is crucial to assess the stability and lifetime of a PEM-WE and to choose appropriate operating conditions. In this work, we present a test protocol involving operation at high (3 Acm -2 geo ) and low (0.1 Acm -2 geo ) current density, alternating with current interrupts during which the system remains at the open circuit voltage (OCV). Previous studies in our lab revealed that the permeation of hydrogen through the membrane into the anode compartment during extended OCV periods can cause the reduction of IrO x 2 , the most commonly used anode catalyst for the oxygen evolution reaction (OER) owing to its decent activity and high stability. During a subsequent start-up of the PEM-WE, metallic Ir is oxidized to a hydrous Ir-oxide. The transformation of the catalyst surface was probed by cyclic voltammetry (CV) during the degradation test. While the initial CV (Fig. 1, black curve) typical for crystalline IrO x is essentially featureless, CVs recorded after ten current-interrupt cycles revealed the formation of hydrogen under-potential-deposition (H-UPD) features (region 1, blue curve), which are characteristic for metallic Ir electrodes. 3 The redox-features evolving at ≈0.8 V are characteristic of an amorphous, hydrous Ir-oxide (region 2). 4 The appearance of these hydrous Ir-oxide features indicates a change in hydration state as well as in surface chemistry, which is known to affect both the OER activity and the stability of IrO x . 5 Amorphous hydrous Ir-oxide exhibits higher OER activity but lower stability compared to crystalline thermally grown IrO x. Interestingly, the polarization curve recorded directly after IrO x reduction during an OCV period shows a lower cell voltage (i.e., improved OER activity), thus supporting the formation of a hydrous Ir-oxide. However, since this hydrous oxide is less stable, a rapid decay of cell performance over an extended number of OCV cycles due to Ir dissolution/re-precipitation occurs. In summary, this study will provide a better understanding of the MEA degradation mechanism occurring over an extended number of OCV cycles, which could result in a PEM-WE system when operated with intermittent renewable energy sources. This implies that hybridization strategies are required to maximize PEM-WE durability. In addition, PEM-WE load cycles to OCV may serve as an accelerated aging test. Acknowledgements : This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) and by the German Ministry of Education and Research (funding number 03SFK2V0, Kopernikus-project P2X). References Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A Comprehensive Review on Pem Water Electrolysis. International Journal of Hydrogen Energy 2013 , 38 , 4901-4934. Weiß, A.; Bernt, M.; Siebel, A.; Rheinländer, P. J.; Gasteiger, H. A., ECS Meet. 232 2017 , Abstr. # I01F-1648. Woods, R., Hydrogen Adsorption on Platinum, Iridium and Rhodium Electrodes at Reduced Temperatures and the Determination of Real Surface Area. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1974 , 49 , 217-226. Pickup, P. G.; Birss, V. I., A Model for Anodic Hydrous Oxide Growth at Iridium. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1987 , 220 , 83-100. Reier, T.; Teschner, D.; Lunkenbein, T.; Bergmann, A.; Selve, S.; Kraehnert, R.; Schlögl, R.; Strasser, P., Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species. Journal of The Electrochemical Society 2014 , 161 , F876-F882. Figure 1 Cyclic Voltammograms (CVs) recorded at 50mV/s during the accelerated degradation before cycling (black) and after 10 cycles (blue) at 80 °C, ambient pressure and 5 mL min -1 H2O (anode)/ 50 mL min -1 H 2 (cathode) for an MEA with ~1.6 mg Ir cm - ² MEA anode and ~0.3 mg Pt cm - ² MEA cathode loading using a Nafion ® 212 (~50 µm) membrane Figure 1
    Type of Medium: Online Resource
    ISSN: 2151-2043
    URL: Article
    DOI: 10.1149/MA2018-02/46/1606
    Language: Unknown
    Publisher: The Electrochemical Society
    Publication Date: 2018
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    X-Ray Absorption Spectroscopy Study of the Pd-H System during HOR in an Operating PEMFC
    The Electrochemical Society ; 2016
    In:  ECS Meeting Abstracts Vol. MA2016-01, No. 35 ( 2016-04-01), p. 1710-1710
    Details
    In: ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2016-01, No. 35 ( 2016-04-01), p. 1710-1710
    Abstract: Volcano plots are frequently used in electrocatalysis to categorize and compare different catalysts and ultimately allow to predict the activity of novel materials on the basis of their binding energy for adsorbed intermediates. 1 An accurate computation of the binding energy requires a good knowledge of the catalyst bulk and surface structure under reaction conditions. Applied to the hydrogen oxidation/evolution reaction (HOR/HER), calculations suggested that Pd should have an activity comparable to that of Pt due to their similar M-H bond strength. 2 This suggestion, however, was recently disproven by unravelling the true HOR activity of the platinum-group metals (PGM): in fact, Pd is ≈100 times less active than Pt. 3 This might be related to the fact that Pd is known to form a bulk hydride phase when exposed to H 2 which is accompanied by an expansion of its crystal lattice. The stoichiometry x of this Pd-H x phase depends on the gas phase activity of H 2 : an increasing pressure leads to a larger x . 4 Moreover, the H 2 partial pressure is related to the electrode potential via a simple Nernst relationship, and thus, it is possible that the hydride phase disappears after a certain overpotential is reached and the Pd lattice reverts to its original state leading to a change of the active surface. 5 While there are examples of HOR polarization curves measured using the rotating-disk electrode (RDE) method, their results remain inconclusive, which might be a consequence of the limited mass-transport in liquid electrolytes compared to the gas phase. 3,6 Thus, it is necessary to study the Pd-H interaction in a fuel cell where diffusion of H 2 is much faster, in a so-called H 2 -pump configuration. 7 In inert atmosphere, x can be evaluated from the anodic charge in a CV after electrolytic charging. However, in H 2 atmosphere this is not possible due to large HOR currents masking a potential hydride desorption feature. Consequently, alternative characterization techniques are necessary to determine whether palladium catalysts under HOR/HER conditions are present as metallic hydrogen or as Pd-H x phase. The absorption of H 2 leads to a change of the palladium lattice parameter which can be obtained from the Extended X-ray Absorption Fine Structure (EXAFS), measured at the K-edge of Pd. In this work, we show the correlation between gas-phase activity of H 2 and electrode potential by recording absorption isotherms in N 2 atmosphere at temperatures between 20-100 °C (cf. fig. 1). Subsequently, we apply this methodology to study the Pd-H phase composition during the HOR in a H 2 -pump experiment at 80 °C. In our operando measurements in H 2 atmosphere, we clearly show that at overpotentials up to 70 mV RHE (RHE = reversible hydrogen electrode), a bulk hydride phase with a composition of ≈Pd-H 0.6 is present with an associated lattice parameter which is ≈3.2 % larger than in metallic Pd. Thus, the HOR on palladium catalysts takes places on the hydride phase of palladium rather than on a palladium metal phase. References: 1           J. Greeley, T. F. Jaramillo, J. Bonde, I. B. Chorkendorff and J. K. Nørskov, Nat. Mater. , 2006, 5 , 909–913. 2           S. Trasatti, J. Electroanal. Chem. Interfacial Electrochem. , 1972, 39 , 163–184. 3           J. Durst, C. Simon, F. Hasché and H. A. Gasteiger, J. Electrochem. Soc. , 2014, 162 , F190–F203. 4           T. B. Flanagan and W. A. Oates, Annu. Rev. Mater. Sci. , 1991, 21 , 269–304. 5           L. Birry and A. Lasia, Electrochim. Acta , 2006, 51 , 3356–3364. 6           S. Henning, J. Herranz and H. A. Gasteiger, J. Electrochem. Soc. , 2014, 162 , F178–F189. 7           K. C. Neyerlin, W. Gu, J. Jorne and H. A. Gasteiger, J. Electrochem. Soc. , 2007, 154 , B631. Figure 1
    Type of Medium: Online Resource
    ISSN: 2151-2043
    URL: Article
    DOI: 10.1149/MA2016-01/35/1710
    Language: Unknown
    Publisher: The Electrochemical Society
    Publication Date: 2016
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    A Platinum Micro-Reference Electrode for Impedance Measurements in a PEM Water Electrolysis Cell
    The Electrochemical Society ; 2021
    In:  Journal of The Electrochemical Society Vol. 168, No. 11 ( 2021-11-01), p. 114511-
    Details
    In: Journal of The Electrochemical Society, The Electrochemical Society, Vol. 168, No. 11 ( 2021-11-01), p. 114511-
    Abstract: We present a platinum wire micro-reference electrode (Pt-WRE) suitable for recording individual electrochemical impedance spectra of both the anode and the cathode in a proton exchange membrane water electrolyzer (PEM-WE). For this purpose, a thin, insulated Pt-wire reference electrode (Pt-WRE) was laminated centrally between two 50 μ m Nafion® membranes, whereby the potential of the Pt-WRE is determined by the ratio of the local H 2 and O 2 permeation fluxes at the tip of the Pt-WRE. Impedance analysis with the Pt-WRE allows determination of the proton sheet resistance of the anode, the anode catalyst layer capacitance, and the high-frequency resistance (HFR) of both electrodes individually, using a simple transmission-line model. This new diagnostic tool was used to analyze performance degradation during an accelerated stress test (AST), where low and high current densities were alternated with idle periods without current (i.e., at open circuit voltage (OCV)), mimicking the fluctuating operation of a PEM-WE with renewable energy. Our analysis revealed that the increasing HFR that was observed over the course of the OCV-AST, which is the main cause for the observed performance decrease, can unequivocally be assigned to an increasing contact resistance between the anode electrode and the porous transport layer.
    Type of Medium: Online Resource
    ISSN: 0013-4651 , 1945-7111
    URL: Article
    DOI: 10.1149/1945-7111/ac3717
    RVK:
    VA 4000
    Language: Unknown
    Publisher: The Electrochemical Society
    Publication Date: 2021
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  • 10
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    (Invited) Design, Performance Characterization, and Durability of an Iridium-Based OER Catalyst for PEM Water Electrolysis
    The Electrochemical Society ; 2022
    In:  ECS Meeting Abstracts Vol. MA2022-01, No. 33 ( 2022-07-07), p. 1339-1339
    Details
    In: ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2022-01, No. 33 ( 2022-07-07), p. 1339-1339
    Abstract: One of the building blocks to transition to a fully renewable energy supply is the utilization of hydrogen as a replacement of fossil fuels and as a chemical energy storage/carrier medium. This requires the economical and sustainable generation of hydrogen by water electrolysis, whereby proton exchange membrane (PEM) water electrolyzers would enable much higher power densities compared to conventional electrolyzers based on liquid alkaline electrolytes [1]. However, one of the short-comings of PEM water electrolyzers (PEMWEs) is the need for expensive and resource-limited iridium based catalysts for the oxygen evolution reaction (OER), so that the large-scale global deployment of PEMWEs would require a substantial reduction of the iridium loading from currently ~1-2 mg Ir /cm 2 elelctrode to below ~0.05 mg Ir /cm 2 elelctrode [2]. In this contribution, we will discuss the technical challenge to reduce the iridium loading using currently employed iridium catalysts, which is related to the high iridium packing density in the electrode (in units of g Ir /cm 3 electrode ), so that for iridium loadings below ~0.4 mg Ir /cm 2 the electrode becomes too thin to allow for a homogenous electrode with sufficient in-plane electrical conductivity [3]. We will then present a catalyst concept that results in much lower iridium packing densities and that thus enables lower iridium loadings [4] . While such a catalyst exhibits a lower electrical conductivity than a currently employed benchmark catalyst, this drawback can be mitigated by utilizing porous transport layers at the anode that have a highly conductive coating [4]. The long-term stability of this novel type of iridium based OER catalyst will be examined in a 30 cm 2 active area short-stack over ~3700 h; comparing the evolution of the OER mass activity and of the high frequency resistance corrected cell voltage with that of a benchmark catalyst that is evaluated in the same short-stack, which allows for mechanistic insights into the observed degradation rates [5]. References: [1] A. Buttler, H. Spliethoff; "Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review"; Renewable and Sustainable Energy Reviews 82 (2018) 2440. [2] M. Bernt, A. Weiß, M. Fathi Tovini, H. El-Sayed, C. Schramm, J. Schröter, C. Gebauer, H. A. Gasteiger; "Current Challenges in Catalyst Development for PEM Water Electrolyzers"; Chem. Ing. Tech. 92 (2020) 31. [3] M. Bernt, A. Siebel, H. A. Gasteiger; "Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings"; J. Electrochem. Soc. 165 (2018) F305. [4] M. Bernt, C. Schramm, J. Schröter, C. Gebauer, J. Byrknes, C. Eickes, H. A. Gasteiger; "Effect of the IrO x Conductivity on the Anode Electrode/Porous Transport Layer Interfacial Resistance in PEM Water Electrolyzers"; J. Electrochem. Soc. 168 (2021) 084513. [5] M. Möckl, M. Ernst, M. Kornherr, F. Allebrod, M. Bernt, J. Byrknes, C. Eickes, C. Gebauer, A. Moskovtseva, H. A. Gasteiger; "Durability investigation and benchmarking of a novel iridium catalyst in a PEM water electrolyzer at low iridium loading"; manuscript to be submitted. Acknowledgements: This work was conducted within the framework of the Kopernikus P2X project funded by the German Federal Ministry of Education and Research (BMBF).
    Type of Medium: Online Resource
    ISSN: 2151-2043
    URL: Article
    DOI: 10.1149/MA2022-01331339mtgabs
    Language: Unknown
    Publisher: The Electrochemical Society
    Publication Date: 2022
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