Redox behaviors of POMs or POM-based hybrids
Anne-Lucie Teillout (Associate Professor), Pedro de Oliveira (Professor), Israël Mbomekalle (Researcher), Anais Abrousse (PhD)
Polyhedral representations of the α1 (A) and α2 (B) isomers of the Dawson structure α-[X2M18O62]n−.
As molecular electrochemists, we are interested in the redox behaviors of PolyOxoMetallates (POMs) or POM-based hybrids that we synthesize within the team or that are provided to us by our collaborators. A recent example involves the study of the influence of pH on the electrochemical response of vanadium monosubstituted tungsten POMs from the Dawson family (Figure to the side), which revealed the existence of coupled proton-electron transfer (CPET) processes. This allowed us to rationalize the behavior of the α1 and α2 isomers and to predict their relative effectiveness as electro-catalysts for the oxidation of biologically relevant organic molecules [Manga2018].
The accumulated electrochemical data enable us to select the most promising species for concrete (photo-) electro-catalytic applications, such as, among others, water oxidation (OER: oxygen evolution reaction) and reductions of oxygen (ORR: oxygen reduction reaction) and protons (HER: hydrogen evolution reaction), aligned with the societal challenge of energy transition. A long-standing collaboration with the University of Porto has allowed us to observe certain trends regarding the electro-catalytic efficiency for ORR of hybrids composed of POMs and carbon-based nanomaterials (NMC) (graphene (GF) or carbon nanotubes (MWCNT)), doped with nitrogen or sulfur. Among the hybrids obtained with MWCNT previously doped with nitrogen at a temperature of 600°C and tungsten “sandwich” POMs from the Dawson family, having either 4 iron atoms or 4 nickel atoms or 2 iron and 2 nickel atoms in the equatorial plane, the most effective for ORR, according to the measured electrochemical parameters, is the mixed POM (Fe2Ni2@MWCNT_N6) [Marques2022]. Its effectiveness surpasses that of most reported POM@NMC composites to date, highlighting the importance of potential synergies between metal centers of different chemical natures.
In a similar vein, and in collaboration with CEA Saclay, we studied electro-active hybrid nanomaterials assembled through non-covalent layer-by-layer deposition of tungsten POMs from the Dawson family (both classic and “sandwich” types containing nickel centers) onto carbon nanotubes. Chemical functionalization allowed us to achieve densely and homogeneously covered nanotubes with POMs. The electrochemical characterization of the nanohybrids highlighted the beneficial contribution of the nanotube platform to the overall stability of the immobilized POMs and the enhancement of their catalytic performances when these materials are applied to HER (Figure below) [Jawale2022].
Left: Schematic of the three-dimensional assembly of hybrids consisting of CNTs, nitrilotriacetic acid diacetylene (DANTA), PDADMAC (polydiallyldimethylammonium chloride), and POMs. Right: Comparison of cyclic voltammograms obtained with POM/CNT hybrid materials deposited on a glassy carbon electrode. The cathodic process corresponds to the HER. The current density, J, has been corrected according to the surface concentration. Electrolyte: 0.5 M H2SO4, pH = 0.3. Counter electrode: Pt; reference electrode: SCE. Scan rate: 2 mV/s.
Theoretical approaches (e.g., DFT calculations) conducted by our collaborators from the University of Tarragona can not only confirm our experimental observations and elucidate the underlying reaction mechanisms (such as that of OER [Azmani2021]) but also guide the selection of POMs to be used. Additionally, there is a concern to replace catalysts containing certain rare and expensive chemical elements with others made from much more abundant and inexpensive elements, ideally exhibiting equivalent or even enhanced efficiencies.
Collaborations
Éric Rivière (ICMMO, U Paris Saclay); Edmond Gravel (Université Paris-Saclay, CEA, INRAE, Department of Medicines and Technologies for Health (DMTS), SCBM, Gif-sur-Yvette); Josep M. Poblet (Laboratory of Physical and Inorganic Chemistry, University Rovira i Virgili, Tarragona, Spain); Inès S. Marques and Cristina Freire (Department of Chemistry and Biochemistry, University of Porto, Portugal); Dhajani V. Jawale (Department of Chemistry, University of Delhi, India); Joseph Dika Manga (Department of Chemistry, Universities of Dschang, Douala, and Yaoundé 1, Cameroon).