Polyquinoid Dye-Cobalt(II,III) Oxide Hybrids: Novel Rare Metal-Free Dyadic Nanomaterials For Photoelectrochemical Water Oxidation
Ruggero Bonetto a b, Nuria Romero c, Marcos Gil-Sepulcre d, Federica Sabuzi e, Giulia Alice Volpato b, Mattia Forchetta e, Laia Francàs a, Raffaella Signorini b, Mirco Natali f, Jordi García-Antón a, Olaf Rüdiger d, Karine Philippot c, Danilo Pedron b, Serena DeBeer d, Pierluca Galloni e, Andrea Sartorel b, Xavier Sala a
a Department of Chemistry, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, Barcelona 08193, Spain
b Department of Chemical Sciences, University of Padova, Via F. Marzolo 1, 35131, Padova, Italy
c CNRS, LCC (Laboratoire de Chimie de Coordination), UPR8241, INPT, Université de Toulouse, UPS, Cedex 4, F-31077 Toulouse, France.
d Max Planck Institute for Chemical Energy Conversion (MPI-CEC), 45470 Mülheim an der Ruhr, Germany
e University of Rome Tor Vergata, Department of Chemical Science and Technologies, Italy, Via della Ricerca Scientifica, 1, Roma, Italy
f Department of Chemical and Pharmaceutical Sciences, University of Ferrara, Via Fossato di Mortara 17, 4421, Ferrara (IT)
Proceedings of The Future of Hydrogen: Science, Applications and Energy Transition (H2Future)
Ibiza, Spain, 2024 April 17th - 19th
Organizers: Carolina Gimbert Suriñach, Sixto Gimenez Julia and Emilio Palomares
Contributed talk, Ruggero Bonetto, presentation 020
Publication date: 27th February 2024

Artificial Photosynthesis is a promising strategy for conversion of solar energy into chemical feedstocks. In this context, Dye-Sensitized Photoelectrochemical Cells (DS-PECs) have achieved a key role as devices able to drive solar light-promoted water splitting. Indeed, due to its abundance and low toxicity, water is identified as the most appealing substance to be exploited in large-scale devices.

The core target of these modular devices is the photo-driven reduction of naturally abundant water to H2, the most appealing energy-dense vector. The Hydrogen Evolution Reaction (HER, E0H+/H2 = 0 V vs the Reversible Hydrogen Electrode, RHE) is typically coupled to the oxidation of water to dioxygen, providing the reducing equivalents to feed H2 fuel synthesis. A key design principle to maximize photon-to-fuel output therefore relies on effective water oxidation catalysts (WOCs), able to overcome the thermodynamically demanding (E0O2/H2O = 1.23 V vs RHE) and kinetically limiting 4e/4H+ Oxygen Evolution Reaction (OER) from water, requiring competent management of proton-coupled electron transfer (PCET) steps to selectively funnel photogenerated holes into multi-electron WOC activation at low overpotentials.[1,2]

The essential structure of DSPEC photoanodes is composed of a semiconductor, often a nanostructured n-type semiconducting metal oxide (SCO) porous film, interfaced with a transparent conducting oxide (TCO) and integrating a WOC. The SCO is a high-band gap semiconductor (typically SnO2 or TiO2), incorporating a molecular light harvesting unit (i.e., a dye) that extends its light absorption by direct or indirect excited state sensitization.[2,3]

We studied photoactive hybrid dyads composed by organic dyes and Co3O4 nanoparticles as WOC, with the goal of applying them on meso-SnO2 photoanodes. The dyes belong to the unique class of KuQuinones (KuQ), polyquinoid chromophores characterized by a wide absorption in the visible and a highly oxidizing excited state (E1*KuQ/KuQ•– > 2 V vs RHE), able to manage PCET events.[4–6] KuQ was bound through a phosphonate anchor to Co3O4heptOH, 3 nm spherical particles stabilized by a 1-heptanol shell.[7] Co3O4heptOH particles, synthesized via a finely controllable organometallic approach, have been reported as a rare metal-free WOC in electro- and photocatalytic systems.[8,9] The ability of the dyads to evolve O2 under visible light irradiation was associated with fast singlet excited state (1*KuQ) emission quenching via electron transfer to Co3O4, owing to the highly oxidizing character of 1*KuQ and to the direct chemical bond between the dye and the WOC. The hybrid dyads were then studied under visible light irradiation upon deposition on meso-SnO2 films. The photocurrent response was associated to selective O2 evolution through generator-collector chronoamperometric experiments. Indeed, the hybrid nanomaterials have been proven competent in photoelectrochemical OER, with Faradaic efficiency (FEO2) close to 90%. As a comparison, “unbound” photoanodes based on KuQ-sensitized SnO2 and Co3O4heptOH, lacking a direct dye-WOC interaction, displayed similar photocurrent densities, albeit with a poorly reproducible and lower (30–50%) FEO2. Furthermore, the photoaction spectrum, reaching 0.44% incident photon-to-current conversion efficiency (IPCE) at 510 nm, confirmed the role of KuQ as light-harvesting molecular component.

Finally, ex-situ resonance Raman and X-ray Absorption Spectroscopy (XAS) experiments allowed to map the evolution of the hybrid dyads under photoelectrochemical stress. While an expected overall oxidation state increase of the Co3O4 catalytic units was observed, indicating CoIIIOOH as steady-state intermediate in the OER, overall chemical stability of the dyads was also assessed. Remarkably, KuQ was proven unaffected with respect to phosphonate bond hydrolysis and oxidative damage to the chromophore core.

These findings support the applicability of the novel dyads to DS-PECs and reinforce the superior approach of managing the directionality of the electron transfer chain by means of chemical bonds as a key design principle for Artificial Photosynthesis.

R. B. kindly acknowledges Prof. Xavier Sala for hosting his visiting research period in the Autonomous University of Barcelona and the Department of Chemical Sciences of the University of Padova, the Doctoral School in Molecular Sciences, and Prof. Andrea Sartorel for support.

XAS experiments were performed at the CLAESS beamline at the ALBA Synchrotron (Cerdanyola del Vallès, Barcelona) under proposals 2021095409 and 2022086973, with the collaboration of ALBA staff. We especially acknowledge Giulio Gorni and Vlad Martin-Diaconescu (CLAESS at ALBA) for their assistance during XAS experiments.

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