Overcoming cathode poisoning from electrolyte
impurities in alkaline electrolysis by means of
self-healing electrocatalyst films
Stefan Barwea, Bastian Meia,b, Justus Masaa, Wolfgang Schuhmanna,* and Edgar Ventosaa,c,*
a) Analytical Chemistry – Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum,
Universitätsstr. 150, D-44780 Bochum, Germany. E-mail: wolfgang.schuhmann@rub.de
b) Photocatalytic Synthesis Group, MESA+ Institute for Nanotechnology,
University of Twente; Meander 229, P.O. Box 217, 7500 AE Enschede, The Netherlands c) IMDEA Energy, Avda. Ramón de la Sagra 3, E-28935 Móstoles, Madrid, Spain.
Abstract
The performance of electrolyzers for hydrogen production is strongly influenced by electrolyte impurities having either a positive or negative impact on the activity of electrocatalysts. We show that cathode deactivation by zinc impurities present in the electrolyte can be overcome by employing catalyst immobilization based on self-assembled and self-healing films. During electrolysis zinc impurities deposit as dendritic films on the cathode electrode increasing the overpotential for the hydrogen evolution reaction (HER), however, continuous self-assembling and self-healing of HER catalyst films subsequently mask the zinc dendrites restoring the advantageous HER overpotential. Zn electrolyte impurities are turned from having a negative to a positive impact leading to an enhanced performance of the cathode due to the increase in surface area caused by the growth of the Zn dendrites.
Graphical Abstract
1. Introduction
Increasing the amount of electrical energy produced from renewable sources requires the integration of large-scale energy storage systems. Among various possible strategies, transforming excess electrical energy into chemical energy by production and storage of hydrogen is considered a very promising solution. [1,2] Stored hydrogen can be used as an energy carrier, transforming it back into electrical energy on demand by its oxidation in fuel cells [3] or by using it as a chemical feed stock for other industrial purposes. [4] Electrochemical hydrogen production from water consists of water reduction and oxidation at the cathode and anode, respectively, using an electrolyzer. [5] However, the slow kinetics of the hydrogen evolution reaction (HER) and even more importantly of the oxygen evolution reaction (OER) lowers the the energy efficiency of the overall process. Despite the fact that a large number of electrocatalysts have been shown to enhance the kinetics of the HER and the OER, Ni still remains the state of the art electrode material in large scale alkaline water electrolyzers. [6] The lack of suitable approaches to stably fix catalytically active powder materials on current collectors especially at the harsh conditions of industrial electrolysis hampers faster implementation of newly developed catalysts into practice. Another serious issue of alkaline water electrolysis is the continuous decay of the HER and OER activity. In the case of Ni-based cathodes, [7,8] four major reasons for the loss in HER activity are generally discussed: i) The high concentration of molecular hydrogen can lead to an absorption of hydrogen at the catalyst, in the nickel structure in this specific case, changing the intrinsic HER activity. ii) The likely adsorption of organic molecules originating from membrane degradation on the surface of the catalyst or current collector can decrease the HER activity over time. iii) A loss of catalytically active material can lead to a reduced HER activity. iv) The deposition of metal cations,
predominantly electrolyte impurities, can form catalytically less active surface coatings, hence decreasing the HER activity.
Recently, we proposed an innovative approach for attaining very stable catalytic films with self-healing capability based on in-situ self-assembly of catalyst particles during active electrolysis. [9] The catalyst particles are directly added to the electrolyte forming a suspension that is pumped through a running electrolyzer cell. The self-assembly process is driven by electrostatic forces as particles with negatively charged surfaces stick to the anode, while those with positively charged surfaces stick to the cathode. In this previous work, we demonstrated that activity decay due to the loss of active materials immobilized on the cathode and anode is overcome by this strategy (i.e. issue (iii) above). In the present study, we show that the self-assembling and self-healing catalyst films also prevent HER active cathodes from deactivation by formation of passivating layers, i.e. issue (iv) above.
2. Results and discussion
The feasibility of this approach is further explored for the case of detrimental deposition of trace metal impurities. These impurities can originate from a variety of different sources, e.g. impurities in the commercial electrolyte (KOH) or corrosion products of cell parts under the highly corrosive environment, i.e. highly alkaline electrolyte, high temperature and presence of molecular oxygen. The deposition of such impurities can either occur upon reduction of the ions to metallic species at the applied cathodic potentials that are needed to produce hydrogen, or by simple chemical precipitation of their salts or hydroxides. [10] The former is determined by the redox potential of the dissolved ions and their corresponding metallic forms, while the latter is related to the solubility products of the metal ions and their salts.
The formation of self-assembling and self-healing catalytically active films [9] was used to overcome cathode deactivation triggered by impurities. Scheme 1a illustrates the formation mechanism of self-assembling and self-healing catalyst films that is governed by electrostatic interaction between the surface of the polarized electrode and the suspended particles upon intimate contact evoked by forced convection. Particles with positively charged surfaces collide with the negatively (vs. Epzc) polarized cathode and electrostatic attraction leads to particle adhesion. Upon contact between electrode and particle, the particle adapts the electrode potential and, thus, its polarization (Scheme 1a 1). Since the immobilized particle changes its polarization, further immobilization of suspended particles continues to take place (Scheme 1a 2&3). Immobilization of several particles leads to the formation of insoluble particle agglomerates bestowing higher stability to the particle film (Scheme 1a 4). The high ionic strength of the electrolyte (3 M KOH) resulting in a short Debye-length favors the formation of particle agglo-merates due to reduction of the inter-particle repulsion. [11] However, aggregates in the solution phase are continuously broken down by the induced forced convection (stirring and pumping through the tubes). Other suspended particles replace the detached ones, which leads to self-hea-ling of the catalyst film.
Trace metal impurities in solution, regardless of their source, slowly deposit on the electrode surface due to either precipitation or electrochemical reduction, leading to passivation of the catalytically active sites concomitantly increasing the cell voltage at the applied current density (Scheme 1b I&II). Our hypothesis is that addition of catalytically active material in the electrolyte reservoir leads to a newly self-assembled catalyst film which would cover the deactivated electrode leading to restoration of its catalytic activity, indicated by a decrease of the cell voltage, even below the initial value (Scheme 1b III). Additionally, the self-healing
capabilities of the self-assembled film would prevent further deactivation by continuous renewal of the catalytically active layer.
Zinc (Zn) was chosen as model impurity to demonstrate the proof of concept for the inhibition of cathode deactivation by means of self-assembling films. Zn impurities can originate from different sources. Degradation of plastics, e.g. tubing or gaskets, can release Zn ions since Zn is heavily used in plasticizers of almost every industrially relevant plastic. [12] Zn can also be present as an impurity in the electrolyte (KOH). [10] Moreover, Zn is one of the main components of brass, which may be present in some elements of the system due to its low cost. Thus, Zn is considered to be a highly interesting model element.
An electrolyzer with Ni as both anode and cathode material (referred as Ni//Ni) was assembled and a current density of 50 mA cm-2 was applied. The cell voltage at this current density reached a steady value of ~ 2.6 V (not corrected for membrane or solution resistances) (Figure 1 green triangles). Controlled amounts of Zn(NO3)2 x 6 H2O were directly added to the electrolyte to accelerate cathode deactivation. Addition of Zn(NO3)2 x 6 H2O (0.067 mg mL-1) to the catholyte (3 M KOH) resulted in a sharp increase of the cell voltage until a steady cell voltage of around 3.1 V was reached, which remained constant for at least 6 h (Figure 1 grey rectangles).
Post-mortem analysis of the deactivated Ni cathode by means of SEM and XPS revealed clear evidence of the formation of a Zn layer. The initially smooth surface of the Ni cathode was covered by dendrite-shaped structures (Figure 2a&b). XPS analysis of the dendrite modified Ni cathode showed that the covering layer consisted of Zn (Figure 2d). The Ni 2p peaks which were clearly visible for the bare Ni foil at binding energies of 852.3 and 896.5 eV, suggesting metallic Ni species [13], were not detectable by XPS for the cathode used in the electrolyzer in the presence of Zn, indicating the formation of a relatively thick and dense layer of Zn on the Ni
electrode. Zn signals at binding energies of 1021.2 and 1044.4 eV dominated the XP spectrum, revealing the deposition of metallic Zn [14]. Linear sweep voltammetry shows that the HER potential at a current density of -50 mA cm-2 on Ni electrodes is -1.55 V (vs. Ag/AgCl/3 M KCl) (Figure S1). Considering that the standard potential for the reduction of zincates in solution to metallic Zn is -1.43 V (vs. Ag/AgCl/3 M KCl) [15], the electrodeposition of Zn takes place at the applied HER potential of -1.55 V (vs. Ag/AgCl/3 M KCl) (Figure S1).
After deactivation of the cathode by the electrodeposited Zn, NixB powder as catalyst for HER was added to the catholyte. The addition of NixB resulted in an instantaneous voltage drop of ca. 0.7 V, reaching a stable cell voltage below 2.4 V at an applied current density of 50 mA cm-2 (Figure 1 red triangles). The preparation and characterization of the NixB as HER catalyst were recently described. [16] The obtained cell voltage was not only lower than that of the Ni//Ni electrolyzer in the absence of impurities, but also lower than that of a Ni//NixB @ Ni electrolyzer without Zn layer (Figure 1 yellow rectangles). This fact is attributed to an increase in surface area since the deposited Zn dendrites roughen the electrode surface on which the NixB catalyst can be immobilized. According to the suggested mechanism of particle immobilization from suspension [9], the electrode has to be polarized negatively with respect to its potential of zero charge to enable the immobilization of positively charged particles like NixB (zeta potential = +104.3 ± 3.1 mV). This prerequisite was already shown to be fulfilled for the Ni cathode / NixB catalyst [9]. The potential of zero charge (Epzc) of the cathode after Zn deposition should be dominated by the Zn layer and thus change from 0.08 V to around -1.09 V vs Ag/AgCl/3 M KCl. [17] Consequently, the cathode is still negatively polarized with respect to Epzc at an applied current density of 50 mA cm-2 (E = -1.55 V vs. Ag/AgCl/3 M KCl, see Figure S1). The voltage difference of 200 mV between Ni//Ni and Ni//NixB @ Ni originates from the 200 mV lower
overpotential for HER. Hence, the necessary potential for the HER should be above that of zincate reduction preventing cathode deactivation, while the healing properties of the self-assembled film should ensure that the potential for HER remains above that for Zn deposition [9]. SEM analysis of the cathode after subsequent additions of Zn impurities and NixB revealed a change in the surface morphology. The dendrite shaped surface observed after the addition of Zn changed to a particulate morphology after the subsequent addition of NixB (Figure 2c, Figure S4).
XPS analysis showed that a film of NixB was formed on the Zn covered Ni electrode. The Zn signals at binding energies of 1021.2 and 1044.4 eV that dominated the spectrum before the addition of NixB were decreased to a level below the XPS detection limit. Instead, the XP spectrum displayed strong signals from Ni in an oxidized state at binding energies of 855.2 and 872.4 eV and a weak B signal at 191.4 eV (Figure 2d). [16,18] The oxidized state of the elements presumably arose from surface oxidation of the very oxygen sensitive metal boride species under storage at atmospheric conditions prior to XPS analysis.17 Thus, XPS confirmed that the particulate film formed on the Zn covered electrode consisted of NixB particles.
As case study of the capability of self-assembling films to prevent deactivation of the cathode by electrolyte impurities, we evaluated the performance of an electrolyzer which contained Viton® gaskets instead of highly stable PTFE gaskets [19] as used in the previous section. Viton® is a highly chemically resistant fluoropolymer elastomer that is widely used as gasket material. [20] A Ni//Ni electrolyzer using Viton® gaskets was operated in 3 M KOH at a current density of 50 mA cm-2. The cell voltage reached a steady value of 3.2 V, which was close to the cell voltage of the intentionally poisoned Ni//Zn @ Ni electrolyzer (Figure 3a). After disassembly of the electrolyzer cell, optical inspection of the electrode revealed a slight change in color of the Ni
cathode (Figure S5). Post-mortem XPS analysis revealed the presence of a covering layer of Zn, which is indicated by the dominant Zn signals at binding energies of 1022.4 and 1046 eV (Figure 3b). Furthermore, optical inspection of the used gaskets especially of the anode compartment showed a change in the physical form of the material upon exposure to the harsh conditions. The former soft and flexible gaskets became harder and brittle, suggesting a loss of plasticizers. Although the electrolyte circuits were separated, the dissolved Zn species presumably crossed the anion exchange membrane as zincates ([(Zn(OH)4)]2-) to the cathode side, where Zn was deposited on the cathode. [21] In addition, release of Zn from the Viton® gaskets of the cathodic compartment cannot be ruled out. Thus, the presence of impurities released from the gaskets caused a 0.6 V increase in the cell voltage as compared to the model Ni//Ni PTFE sealed electrolyzer at the same current density (Figure 3a).
The addition of NixB powder to the catholyte led to the formation of a particle suspension, which was pumped through the cathode compartment. The cell voltage significantly dropped after the addition of NixB until it reached a steady voltage of around 2.45 V, which was a similar value to that of a Ni//NixB @ Ni electrolyzer in the absence of Zn impurities (Figure a). Optical inspection directly after disassembly of the electrolyzer confirmed the formation of a film as already reported elsewhere (Figure S5). [9] The difference in the final cell voltage between the NixB-modified cathodes contaminated with intentionally added Zn(NO3)2 and spontaneously released Zn species from Viton® gaskets is presumably due to the differences in the Zn films originating from the different Zn sources and concentrations (Figure S6). The Zn dendrites formed by deposition from the intentionally added Zn salt result in a rougher surface than the inadvertently deposited Zn layer originating from the gasket.
3. Conclusion
In conclusion, the intrinsic characteristics of the self-assembling film formation that was previously proposed for attaining very stable catalytic films in electrolyzers were exploited to tackle cathode deactivation issues arising from electrolyte impurities. Zn impurities in the electrolyte were investigated as a case study since traces of this element can be easily found in the electrolyte and deactivate Ni based cathodes. We showed that when a Zn layer is formed on the cathode, its passivating effect on the HER activity is even overcompensated by the subsequent self-assembly of the catalyst particle film on top of the Zn layer, restoring the activity of the electrode. Noteworthy, the formation of the catalyst film on the poisoning dendritic Zn layers leads to improved performance of the electrolyzer manifested by a cell voltage smaller than the value before the passivation which is attributed to roughening of the electrode surface upon Zn deposition allowing more catalyst particles to be immobilized. Hence, the continuous self-renewal of the self-assembled catalyst film prevents electrode passivation due to the presence of Zn impurities in the electrolyte. Our work aims at opening the door towards the development of innovative strategies to overcome electrode deactivation caused by electrolyte impurities.
4. Experimental Methods 4.1 Synthesis of NixB
NixB was synthesized according to the procedure described elsewhere. [16,22] Briefly, aqueous NiCl2•6 H2O (20 mL, 0.5 M, Sigma-Aldrich) placed in a round-bottomed Schlenk flask was deaerated by means of Schlenk vacuum technique, flushed with Ar and maintained at 0 °C using
an ice-bath. NaBH4 (1.0 M, Sigma-Aldrich) in NaOH (0.1 M, Carl Roth), separately deaerated and flushed with Ar, was slowly added to the NiCl2 solution by means of a syringe.
As a precaution, vigorous frothing occurs if NaBH4 solution is rapidly added to NiCl2 containing solution. Instantaneous formation of a dark precipitate was observed. The precipitate was collected by filtration and washed with large amounts of pure Milli-Q water (SG Water), followed by washing with ethanol (J. T. Baker). The used material was characterized as shown elsewhere. [16]
4.2 Electrochemical measurements
All electrochemical measurements were performed in alkaline media (3 M KOH, Carl Roth) at room temperature. For the electrolyzer (a home-made, non-zero gap electrolyzer, see Supple-mentary information, Fig. S2 and Fig. S3), a VMP3 multi-channel potentiostat/galvanostat (Biologic) was used. Ni foil (0.1 mm thick, 99.2 % Ni and a surface area exposed to electrolyte of 4.1 cm2, Metall Jobst) acted as anode and cathode. Anodic and cathodic compartments were separated by an anion exchange membrane (Fumasep® FAA-3-PK-130, Fumatech). A two-channel Minipuls3 peristaltic pump (Gilson) was used to separately pump anolyte and catholyte through the system at a flow rate of 30 mL min-1. Catalyst powders were directly added to the catholyte solution. The concentration of catalyst in the electrolyte was 1.3 mg mL-1. The catalyst suspensions were stirred to prevent settling of the particles. The electrolysis cell was either sealed with gaskets made from PTFE (Teadit 24BB, TEadit) or Viton® (DuPont). For accelerated contamination, Zn(NO3)2 x 6 H2O (0.067 mg mL-1, Sigma Aldrich) was added to the cathode compartment.
XPS spectra were recorded using a UHV set-up equipped with a Gammadata-Scienta SES 2002 analyzer. The base pressure in the measurement chamber was 5 x 10−10 mbar. Monochromatic Al Kα (1486.6 eV; 14.5 kV; 30 mA) was used as incident radiation, and a pass energy of 200 eV was chosen resulting in an effective instrument resolution higher than 0.6 eV. Charging effects were compensated using a flood gun, and binding energies were calibrated based on positioning the main C 1s peak originating from carbon contaminations at 284.5 eV.
SEM images were taken using a Quanta 3D FEG scanning electron microscope (FEI) operated at 20.0 kV.
ACKNOWLEDGMENT
Financial support by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Cluster of Excellence RESOLV (EXC1069), the BMBF in the framework of the project NeMeZu (FKZ 03SF0497B) and Comunidad de Madrid in the framework of the talent attraction program (2017-T1/AMB-5190).
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Scheme 1. a) Schematic representation of the catalyst film formation. Starting with single particle collisions induced by forced convection under non-ideal laminar flow (1-3) and leading to the formation of insoluble particle agglomerates which bestows higher stability to the film (4). b) Schematic representation of the cathode deactivation due to deposition of trace metal impurities and the corresponding change in the overall cell voltage of the electrolysis cell. I) Bare nickel cathode electrolyte impurities. Slow deposition of impurities on the cathode leads to an increase in cell voltage. II) Inactive material covers the electrode surface area increasing the cell voltage. III) Addition of catalyst particles to the electrolyte. A self-assembling and self-healing catalyst film forms and masks the inactive layer with a catalytic layer which results in a decrease in cell voltage to a value below that prior to the deactivation.
Figure 1. Evolution of cell voltage with time of a Ni//Ni electrolyzer with and without Zn conta-mination and NixB.
Figure 1. SEM micrographs of electrolyzer cathodes. a) Bare Ni foil cathode. b) Ni foil cathode covered with Zn. c) NixB film formed on Zn covered Ni foil cathode. d) XPS survey spectra of the electrodes of a), b) and c).
Figure 3. a) Evolution of the cell voltage with time for a Ni//Ni electrolyzer in the absence and in the presence of Zn impurities and NixB. b) XPS survey spectra of bare Ni foil, Zn deposited on Ni foil and NixB deposited on Zn on Ni foil.
Stefan Barwe studied chemistry at the Ruhr-Universität Bochum. He
obtained his PhD with Wolfgang Schuhmann at Analytical Chemistry – Center for Electrochemical Sciences (CES) in 2017. His research is focusing on stable fixation of powder based, non-noble metal electrocatalysts on electrodes. Furthermore, he is interested in finding alternative anode reactions to replace the energy demanding oxygen evolution reaction, i.e. the anodic oxidation of biorefinery products to sustainable platform chemicals.
Bastian Mei studied chemistry with major in industrial chemistry at the
Ruhr-University Bochum (Germany), where he obtained his PhD with Martin Muhler in 2013. After two years as a Postdoctoral researcher at the Technical University of Denmark (group of Ib Chorkendorff) he joined the group of Guido Mul at the University of Twente, where he is currently an Assistant Professor. His research interests include (photo)electrochemistry and heterogeneous photocatalysis with an particular emphasis on sustainable hydrogen production, selective conversion of organic molecules and the development of structure-activity relations.
Justus Masa earned the Dr. rer. nat. with Wolfgang Schuhmann from
Ruhr-University Bochum in 2012. He received a B.Sc. degree in 2003 and a M.Sc. degree in Chemistry in 2008 both from Makerere University, Uganda. He is currently a group leader for electrocatalysis and energy conversion at the Center for Electrochemical Sciences (CES) at Ruhr-University Bochum. He was a visiting scholar at the University of Oxford in the group of Richard Compton in 2013. His research interests include electrocatalysis, especially the rational design of low-cost catalysts for fuel cells, electrolyzers and nanomaterials design for electrochemical energy systems.
Edgar Ventosa is a senior assistant researcher at IMDEA Energy (Spain).
He completed his Ph.D. in Chemistry at the University of Burgos (Spain) in 2009. Before joining IMDEA Energy in 2017, he was a postdoctoaral researcher in several research institutes, i.e. Ruhr-University-Bochum (Germany), CTME (Spain), University of Warwick (UK) and IREC (Spain). His research is focused on electrochemical energy storage and conversion, with emphasis on batteries (Li-ion, Zn-Ni, Zn-Air, Zn-MnO2, all-vanadium flow and semi-solid flow and organic flow batteries).
Wolfgang Schuhmann studied chemistry at the University of Karlsruhe,
and completed his PhD with F. Korte in 1986 at the Technical University of Munich. After finishing his habilitation at Technical University of Munich in 1993, he was appointed professor for Analytical Chemistry at the Ruhr-University Bochum in 1996. His research interests cover a broad spectrum of electrochemistry, including micro- and nanoelectrochemistry, scanning elec-trochemical microscopy, biosensors, biofuel cells, electrocatalyst
develop-ment, batteries, photoelectrochemistry, electrochemical deposition of catalyst nanoparticles and noble-metal free electrocatalysts among others.
