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Research

Low temperature fuel cells

Involved researchers: Daniel Sandbeck, Konrad Ehelebe, Florian Speck.

Membrane Electrode Assembly (MEA)Membrane Electrode Assembly (MEA) and a magnified view of the Pt/C anode or cathode catalyst layer
Copyright: C. Baldizzone, Thesis, Ruhr-University Bochum, 2015

In our low temperature fuel cells activities, we concentrate on both acidic proton exchange membrane (PEM) and alkaline anion exchange membrane (AEM) fuel cell (FC) research.
In PEMFC, stability and activity of Pt and Pt-based oxygen reduction reaction (ORR) catalysts constitutes one of the group’s main core research directions. In this we rely on a set of methods and techniques available in the group including high-throughput catalyst screening for fast discovering of the more advanced catalysts, on-line ICP-MS for time- and potential-resolved analysis of dissolution kinetics, identical location TEM to track degradation on the nanoscale, etc.


Following collaborating projects (see list of collaborators and projects) are established in the group in order to understand mechanisms governing activity and stability of both unsupported and supported catalysts:

  • Mechanism of platinum oxidation and dissolution – research at platinum single crystals;
  • Effect of Pt particle size and Pt density in the catalyst layer on the stability – research at 2D and 3D model catalyst systems;
  • Ionic liquids modified Pt and Pt-alloys – research at carbon supported catalysts;
  • Advanced supports – research at non-carbon conductive oxide supports;
  • Shape controlled Pt nanoparticles for ORR – research at supported and unsupported Pt particles of different shape and size.

Also non-PGM (platinum group metals) catalysts are within the research interests of our group. The most promising non-PGM ORR catalyst is Fe-C-N. In collaboration with specialists in the Fe-C-N synthesis and characterization we investigate degradation of Fe-C-N in both PEM and AEM environments.
In AEMFC, we address both ORR and HOR (hydrogen oxidation reaction) electrocatalysis. Stability of non-PGM catalysts is a central part of our activity in the H2020 CREATE project. Essential interests are in the area of non-PGM Ni-based and low-PGM ceria oxide supported HOR electrocatalysts.

Further reading:

  1. S. Cherevko “Stability and dissolution of electrocatalysts: Building the bridge between model and “real world” systems”, Current Opinion in Electrochemistry 8 (2018) 118-125. (https://doi.org/10.1016/j.coelec.2018.03.034)
  2. E. Pizzutilo, S. Geiger, J.-P. Grote, A. Mingers, K.J.J. Mayrhofer, M. Arenz, S. Cherevko “On the need of improved accelerated degradation protocols (ADPs): examination of platinum dissolution and carbon corrosion in half-cell tests”, Journal of The Electrochemical Society, 163(14) (2016) F1510-F1514. (http://dx.doi.org/10.1149/2.0731614jes)
  3. S. Cherevko, N. Kulyk, K.J.J. Mayrhofer “Durability of platinum-based fuel cell electrocatalysts: Dissolution of bulk and nanoscale platinum”, Nano Energy, 26 (2016) 275-298. Special issue on Electrocatalysis. (https://doi.org/10.1016/j.nanoen.2016.03.005)



Electrochemical hydrogen production via Electrolysis


Involved researchers: Dr. Daniel Escalera, Julius Knöppel

Studying OER on a IrRu gradient librarySimultanious detection and quantification of Ir and Ru catalyst dissolution during oxygen evolution reaction
Copyright: Cherevko

In our electrochemical hydrogen production (water electrolysis) activities, we concentrate on both acidic proton exchange membrane (PEM) and alkaline anion exchange membrane (AEM) water electrolysis (EL) research.


In PEMEL, we aim at understanding mechanism of oxygen evolution reaction (OER) on Ir and Ir-based electrocatalysts and its influence on catalyst degradation. Our recent results show that, in general, OER on Ir is destructive. Depending on the structure of Ir catalyst (amorphous hydrous vs. crystalline rutile iridium oxides) at least two OER routes are possible. Due to leaching of less noble elements, advanced Ir-based materials like Ir single and double perovskites and mixtures like Ir-Ni and Ir-Sn tend to form hydrous Ir oxide and thus are unstable during acidic OER. With help of on-line ICP-MS and OLEMS techniques and with involvement of isotope labelling experiments we clarified that for all Ir-based catalysts besides rutile IrO2 there is participation of lattice oxygen in the OER mechanism. The introduced by our group stability metric (stability number) shows that hydrous oxide is unstable, while IrO2 is very stable. With this metric it is possible to put stability of novel OER materials on the stability scale of hydrous and rutile oxides.


Currently, in particular within our group collaboration within DFG SPP 2080 and BMBF Kopernikus research projects, we concentrate on understanding of OER at Ir-Ru and OER at Ir on advanced supports.

In AEMEL, we explore OER on novel highly active materials. Besides activities within group’s high-throughput synthesize and characterization projects, we actively cooperate with leading groups in synthesis and characterization of advanced OER materials, which include single and double Ir perovskites; epitaxial Ni and Co based perovskites for OER; epitaxial and powdered Mn-based perovskites for OER.

Further reading:

  1. S. Geiger, O. Kasian, M. Ledendecker, E. Pizzutilo, A. Mingers, W.T. Fu, O. Diaz-Morales, Z. Li, T. Oellers, L. Fruchter, A. Ludwig, K.J.J Mayrhofer, M. Koper, and S. Cherevko “The Stability-number as a metric for electrocatalyst stability benchmarking”, Nature Catalysis 1 (2018) 508-515. (http://dx.doi.org/10.1038/s41929-018-0085-6)
  2. T. Li, O. Kasian, S. Cherevko, S. Zhang, S. Geiger, C. Scheu, P. Felfer, D. Raabe, B. Gault, K.J.J. Mayrhofer “Atomic-scale insights into surface species of electrocatalysts in three dimensions”, Nature Catalysis 1(4) (2018) 300-305. (http://dx.doi.org/10.1038/s41929-018-0043-3)
  3. O. Kasian, J.-P. Grote, S. Geiger, S. Cherevko, K.J.J. Mayrhofer∗ “The common intermediates of oxygen evolution and dissolution reactions during water electrolysis on iridium”, Angewandte Chemie 57(9) (2018) 2488–2491. (http://dx.doi.org/10.1002/anie.201709652)



High-throughput electrochemistry


Involved researchers: Florian Speck, Vishnu Prataap

Effect of catalyst layer thickness on Pt dissolutionEffect of catalyst layer thickness on Pt dissolution
Copyright: Cherevko

The vast majority of the electrocatalysts currently used in technologically important processes are still far away from been optimal. As most of them are composed of expensive and rare noble metals, there is a high economical reasoning for replacing current catalysts with more abundant materials. Moreover, many reactions still lack a catalyst which would provide desired performance in term of activity, selectivity, and stability. Thus, design and synthesis of novel electrocatalysts and optimization of state-of-the-art electrocatalysts occupies a significant part of the modern electrocatalysis research.
Since theoretical tools for prediction of optimal catalysts are still to be developed, we still rely on the trial-and-error approach. Although, knowledge accumulated in fundamental studies is used as a guidance. In this situation, combinatorial approaches of catalysts synthesis and characterization become an important tool in discovery of novel electrocatalysts.

Currently, our group pursue three directions in synthesis of high-throughput catalyst libraries:

  • Physical vapour deposition (PVD) of gradient libraries: model flat surfaces are prepared using two and more sources (sputtering targets, e-beam evaporated materials). Representative examples are Ir-Ru libraries for OER in PEMEL;
  • Electrodeposition of material libraries: model flat to applied porous surfaces can be obtained by controlling electrodeposition parameters. Combinatorial mode is realized by using scanning flow cell (SFC) with two and more electrolyte sources and electrolyte premixing. Representative example is NiFe:Au and NiFe:Ru libraries for OER in AEMEL;
  • Ink-jet printing of metal precursors with additional thermal treatment: applied particulate surfaces of high-surface-area are prepared by parallel and/or consecutive two and more metal salt precursors printing. Representative examples are Ni-based alloys for HOR in AEMEL.


The high-throughput catalyst libraries synthesis methods are combined with automated physicochemical characterization (XPS, EDS, etc) and fast electrochemical screening with SFC based techniques.

Further reading:

  1. O. Kasian, S. Geiger, M. Schalenbach, A. Mingers, A. Savan, A. Ludwig, S. Cherevko, K.J.J. Mayrhofer “Using instability of a non-stoichiometric mixed oxide oxygen evolution catalyst as a tool to improve its electrocatalytic performance”, Electrocatalysis 9 (2018) 139-145. (https://doi.org/10.1007/s12678-017-0394-6)
  2. O.Kasian, S. Geiger, P. Stock, G. Polymeros, B. Breitbach, A. Savan, A. Ludwig, S. Cherevko, and K.J.J. Mayrhofer “On the origin of the improved ruthenium stability in RuO2 − IrO2 mixed oxides”, Journal of The Electrochemical Society, 163(11) (2016) F3099-F3104. (http://dx.doi.org/10.1149/2.0131611jes)
  3. G.P. Keeley, S. Cherevko, K.J.J. Mayrhofer∗ “The stability challenge on the pathway to low and ultra-low platinum loading for oxygen reduction in fuel cells”, ChemElectroChem, 3(1) (2016) 51-54. (http://dx.doi.org/10.1002/celc.201500425)


Photoelectrochemistry


Involved researchers: Julius Knöppel

Stability of photoabsorbers and photo(electro)catalystsInvestigation of WO3 corrosion during photoelectrochemical water splitting
Copyright: Cherevko

Photoelectrochemical water splitting is considered as an alternative technology to classical harvesting electricity by photovoltaics and hydrogen production water splitting in electrolyzers. Even though technological, environmental or economical advantage of this technology is still to be proven, stability of photoabsorbers and electrocatalysts during photoelectrochemical water splitting is of great scientific interest. Our group recently demonstrated that even materials typically considered as stable, suffer from dissolution. Thus, due to high anodic potential of the photogenerated holes and presence of complexing sulphate ions in electrolyte, WO3 photoanodes dissolve severely during light illumination. Moreover, dissolution was found to scale with main anodic current. Currently, we concentrate at understanding WO3 photocorrossion and its dependence on different electrolytes, presence of hole skavengers in electrolyte, presence of electrocatalyst such as Ir.
Another promising photoanode is BiVO4. In collaboration with MPIE (Germany) we investigate degradation of BiVO4 in different electrolytes.

Further reading:

  1. J. Knöppel, S. Zhang, F.D. Speck, K.J.J. Mayrhofer, C. Scheu, S. Cherevko “Time-resolved analysis of dissolution phenomena in photoelectrochemistry - A case study of WO3 photocorrosion”, Electrochemistry Communications 96 (2018) 53-56. (http://dx.doi.org/10.1016/j.elecom.2018.09.008)


Noble metals recycling


Involved researchers: Dr. Michael Paul

Noble metals recycling using novel transient dissolution processNoble metals recycling using novel transient dissolution process
Copyright: Hodnik

Disruptive in most electrochemical processes, dissolution can also be useful. The so-called transient dissolution which takes place during noble metals oxidation and reduction can be employed in recycling of noble metals. In this process, metal oxidation and reduction is achieved by repetitive enrichment of an electrolyte with oxidative, e.g. ozone, and reductive, e.g. hydrogen, gases or liquids. Unlike traditional hydrometallurgical processes, recycling via transient dissolution can be performed at relatively mild conditions, i.e. low base or acid concentration, room temperate, etc.


Our group concentrates on the fundamental understanding of processes taking place at electrolyte/electrode interface during dissolution. In this case our goal is to maximize the dissolution yield and selectivity. We aim at developing optimal green solutions in recycling of the platinum group metals (PGM) and noble metals like Au and Ag. A DAAD funded program is devoted to this research.


Further reading:

  1. N. Hodnik, C. Baldizzone, G. Polymeros, S. Geiger, J-P. Grote, S. Cherevko, A. Mingers, A. Zeradjanin, K.J.J. Mayrhofer “Platinum recycling going green via induced surface potential alteration enabling fast and efficient dissolution”, Nature Communications, 7 (2106) 13164. (http://dx.doi.org/10.1038/ncomms13164)



Fundamental research on electrochemical stability of materials


Responsible persons: Florian Speck

Fundamental research on metals dissolutionThermodynamic Pourbaix diagrams showing regions of passivation (red) and dissolution (blue). With help of on-line ICP-MS, we study kinetics of dissolution.
Copyright: Cherevko

Exposed to an electrochemical environment and in response to this environment, most of technologically important materials tend to degrade. Classical example is the corrosion of unprotected steel in air. Corrosion takes also place in the electrochemical energy conversion technologies. Electrocatalyst dissolution, viz. Pt in proton exchange membrane (PEM) fuel cell (FC) and Ir in water electrolysis (EL), is a well-known degradation process. In order to understand mechanisms governing catalysts dissolution we focus on the fundamental research on model systems (single- and poly-crystalline electrodes).


Unlike research topics described more in detail in other topical sections, here we concentrate on understanding oxidation and dissolution in general. Our new results on dissolution kinetics complement the existing thermodynamic data. In our research, we aim at discovering stability descriptors and their further application in designing more stable materials, independent on technology. Our preliminary on-line ICP-MS results reveal that materials tendency towards oxidation/passivation and dissolution can be predicted based on the strength of metal-metal and/or metal-oxygen bonds.

Further reading:

  1. S. Cherevko “Electrochemical dissolution of noble metals.” In: The Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry. Elsevier, 5 (2018) 68-75. (http://dx.doi.org/10.1016/B978-0-12-409547-2.13569-3)
  2. M. Schalenbach, O. Kasian, M. Ledendecker, F.D. Speck, A. Mingers, K.J.J. Mayrhofer, S. Cherevko “The electrochemical dissolution of noble metals in alkaline media”, Electrocatalysis 9 (2018) 153-161. (http://dx.doi.org/10.1007/s12678-017-0438-y)

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