Electrochemistry & Catalysis
Prof. Robert Francke
The efficient use of energy and materials and the introduction of renewable resources into material cycles are core objectives of a circular economy. Electrochemistry is becoming a key technology for achieving these goals. On the one hand, the share of green electrical energy from wind power and photovoltaics is growing. On the other hand, electrosynthesis allows for substitution of chemical oxidizing or reducing agents by green electrical energy.
One of the major challenges in electrosynthesis is the kinetic inhibition of the electron transfer, which oftentimes leads to selectivity problems and increased energy consumption (“overpotential”). To address this issue, our work is focused on the development of homogeneous and heterogeneous electrocatalytic systems for the conversion of organic compounds and small molecules such as CO2, H2O, and N2. The use of sub-stoichiometric charge quantities for the catalysis of redox-neutral reactions (electrochemical catalysis) offers a further promising and thus far little investigated approach, which we currently explore in the context of molecular rearrangements. To enable a knowledge-based optimization of our reactions, the mechanistic understanding of the catalytic process is of special interest for us. For this purpose, we employ electroanalytical techniques and, if necessary, combine them with spectroscopic methods and control experiments.
The necessity for using large quantities of supporting electrolytes represents a further challenge in electrosynthesis. Once the reaction is complete, the salt must be separated from the reaction mixture. Since the salt additive is both a potential source of waste and a significant cost factor, its recycling is highly desirable. Similarly, molecular catalysts pose a separation and waste problem. In this context, we are placing further emphasis on the topic of multifunctional electrolytes.
Over the past years, we have established a research program that is focused on the utilization of organometallic catalysts for electrosynthesis [1-5]. In one case we have studied a system based on cyclopentadienone iron complexes, [1,2] which allows for CO2-to-CO conversion in nearly quantitative Faradaic yield and high current density. Using a combination of cyclic voltammetry, controlled potential electrolysis, FTIR spectroelectrochemistry (in collaboration with the group Vibrational Spectroscopy in Catalysis) and quantum chemical calculations (in collaboration with the Roemelt Group, HU Berlin), we were able to obtain mechanistic insights and to propose a sequence that proceeds via the key-intermediate shown in the figure below (top right). In another project we have investigated a diimine manganese framework under electro- and photochemical conditions (in collaboration with the group Catalysis for Energy) .
 E. Oberem, A. F. Roesel, A. Rosas-Hérnandez, T. Kull, S. Fischer, A. Spannenberg, H. Junge, M. Beller, R. Ludwig, M. Roemelt, R. Francke, Organometallics 2019, 38, 1236–1247.
 A. Rosas-Hernández, H. Junge, M. Beller, M. Roemelt, R. Francke, Catal. Sci. Technol. 2017, 7, 459−465.
 C. Steinlechner, A. F. Roesel, E. Oberem, A. Paepcke, N. Rockstroh, F. Gloaguen, S. Lochbrunner, R. Ludwig, A. Spannenberg, H. Junge, R. Francke, M. Beller, ACS Catal. 2019, 9, 2091−2100.
 R. Francke, M. Roemelt, B. Schille, Chem. Rev. 2018, 118, 4631–4701.
 H. Liang, T. Beweries, R. Francke, M. Beller, Angew. Chem. Int. Ed. 2022, e202200723.
Compared to electrocatalysis, in which a chemical interaction between a substrate and a catalyst facilitates the electrode process, electrochemical catalysis represents exactly the opposite case. Here, the injection (or removal) of an electron into (or from) a substrate triggers a redox-neutral reaction that may otherwise require harsh conditions and/or the use of reagents . Such reactions involve the electrogeneration of an ionic or radical ionic species, which after a coupled chemical step either undergoes a backward electron exchange with the electrode (ECEb mechanism) or triggers a chain process in the bulk solution. Under these circumstances, sub-stoichiometric amounts of charge are sufficient to achieve a full conversion and conceptionally, the electrons and holes can be understood as being catalysts. In this context, we have developed a research program focusing on electrochemically catalyzed redox-neutral reactions. A major accomplishment was the development of the electrochemical Newman-Kwart rearrangement (ENKR) of O-aryl thiocarbamates to the corresponding S-aryl compounds (see figure below). Generally, the NKR represents the key-reaction in the three-step synthesis of thiophenols from their phenol derivatives and proceeds between 200 and 300 °C. Electrochemical catalysis, however, allows for operation at room temperature and provides a complementary reactivity with respect to the arene substitution [2,3]. Carrying out the reaction in a microflow reactor allows for omission of salt additives.
 R. Francke, R. D. Little, ChemElectroChem. 2019, 6, 4373.
 T. Broese, A. F. Roesel, A. Prudlik, R. Francke, Org. Lett. 2018, 20, 7483.
 A. F. Roesel, M. Ugandi, N. T. T. Huyen, M. Májek, T. Broese, M. Roemelt, R. Francke, J. Org. Chem. 2020, 85, 8029-8044.
During the past five years, we have investigated several approaches toward simplifying the workup of indirect electrosyntheses. One of these approaches is based on merging redox mediator and supporting electrolyte into a single molecularly defined species (see Figure below). Due to the wide range of applications of hypervalent iodine reagents in organic synthesis, we decided to use the iodine(I)/iodine(III) couple as test case. A series of investigations on ionically tagged iodoarenes showed that under certain conditions, electrolysis in the batch cell is possible without the use of salt additives and that the separation and reuse of the mediator is significantly facilitated.[2-4] Using 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as solvent, we found that the active dialkoxy-λ3-iodanes are formed in nearly quantitative Faradaic efficiencies. The electrolyzed solutions served as reactive media for several oxidative C-O, C-C, and C-N couplings. The electrochemical approach was also successfully applied to hypervalent bromine compounds .
Another approach is based on attaching catalyst molecules and supporting electrolyte units to soluble polymers (resulting in “polymediators and polyelectrolytes”), which allows for separation of both components in a single step using size-exclusive membranes. Using the example of TEMPO-catalyzed alcohol oxidation as a test case (see Figure below), we have demonstrated for the first time that indirect electrosynthesis can be efficiently coupled to dialysis or ultrafiltration using polyelectrolyte HP-1 and polymediator HP-2 [6,7] The polymer solutions are sufficiently conductive, exhibit a high electrocatalytic activity toward oxidation of various alcohols, and can be reused for multiple times.
 R. Francke, Curr. Opin. Electrochem. 2021, 28, 100719.
 A. F. Roesel, T. Broese, M. Májek, R. Francke, ChemElectroChem. 2019, 6, 4229–4237.
 O. Koleda, T. Broese, J. Noetzel, M. Roemelt, E. Suna, R. Francke, J. Org. Chem. 2017, 82, 11669–11681.
 T. Broese, R. Francke, Org. Lett. 2016, 18, 5896–5899.
 I. Sokolovs, N. Mohebbati, R. Francke, E. Suna, Angew. Chem. Int. Ed. 2021, 60, 15832-15837.
 B. Schille, N. O. Giltzau, R. Francke, Angew. Chem. Int. Ed. 2018, 57, 422.
 N. Mohebbati, A. Prudlik, A. Scherkus, A. Gudkova, R. Francke, ChemElectroChem 2021, 8, 3837-3843.
In view of the use of regenerative energy sources and the storage of surplus electricity, electrochemical technologies are increasingly becoming the focus of research, industry, and politics. In this context, we are developing active heterogeneous electrocatalysts, particularly composites with reduced content of precious metals such as iridium or platinum (or, if possible, noble metal-free catalysts that contain only readily available components). Our work was originally funded by the AiF and the Ministry of Economics of the state of Mecklenburg Vorpommern with the participation of regional SMEs, subsequently also in cooperation with industry (Evonik Industries, Siemens Energy AG) financed by the BMBF (e.g. Kopernikus, H2Giga) or with international participation (CADIAC). Our work includes the development of electrocatalytically active materials for oxygen reduction (ORR) in fuel cells and for hydrogen generation (HER) and oxygen generation (OER) in alkaline and acidic water electrolysis. For example, the SusHy project succeeded in developing a highly active oxidation catalyst for alkaline water electrolysis [1,2]. In addition, we are developing and testing "solid oxide" composites as cathode materials for "Solid State Ammonia Synthesis" (SSAS), in which nitrogen is electrochemically reduced to ammonia between 400 and 600°C (WIR!-Campfire). As an energy-relevant topic, we are also interested in electrochemical CO2 reduction (eCO2RR) using heterogeneous electrocatalysts. The subject of our current work in the field is the development of materials that preferentially reduce CO2 to C2+ products despite the presence of water.
The developed materials, such as the chitosan-derived gels shown in the figure below (from left to right loaded with Mn, Fe, Cu, Ni, Co), are comprehensively characterized in cooperation with the LIKAT-department Analytical Services, for instance by SEM and TEM (see figure, bottom left and right).
 M. Polyakov, A.-E. Surkus, A. Maljusch, S. Hoch, A. Martin, ChemElectroChem 2017, 4, 2109-2116; Impact of the Co : Cu Ratio in CoCu-Containing Oxidic Solids on their Activity for the Water-Splitting Reaction.
 A. Maljusch, A. Martin, M. Polyakov, A.-E. Surkus, S. Hoch, M. Blug, W. Deis, Verfahren zur Herstellung eines in der Wasseroxidation einsetzbaren Elektrokatalysators, 2015E00266, EP2017/074926, WO2018/065339 A1.
The area "Electrochemistry & Catalysis" is divided into two topics: