A 2021 Solar RRL paper by Galliano and co-workers reported the first quasi-solid, truly aqueous dye-sensitized solar cell (DSSC) combining a cobalt polypyridyl redox mediator with a xanthan-gum hydrogel electrolyte (power conversion efficiency up to 4.47%, stable over five days), and explicitly deferred the mechanistic understanding of this system: the hypothesized interaction between xanthan gum and the additive N-methylbenzimidazole that could alter TiO2 surface passivation and recombination, the role of xanthan carboxyl groups in governing Co2+/Co3+ diffusion in the gel matrix and inside TiO2 mesopores, and an extended multivariate design-of-experiments study. These questions remain open: the group's own impedance-spectroscopy follow-up addressed the iodide xanthan system, subsequent hydrogel work moved to iodide or copper mediators and other biopolymers, and no study has quantified Co2+/Co3+ diffusion in xanthan gels or tested the xanthan-NMBI hypothesis, although a 2025 monolithic organic-solvent study identified photoinduced Co(III)-tert-butylpyridine complexation and water-induced redox reactions as relevant cobalt-electrolyte degradation routes.

Background

A dye-sensitized solar cell (DSSC) is a photoelectrochemical device in which a mesoporous, dye-coated titanium dioxide (TiO2) film on a transparent conducting electrode absorbs light; photoinjected electrons cross the TiO2 while a redox couple dissolved in an electrolyte regenerates the oxidized dye and shuttles charge to a counter electrode. Key figures of merit are the open-circuit voltage (VOCV_{\mathrm{OC}}), short-circuit current density (JSCJ_{\mathrm{SC}}), fill factor, and power conversion efficiency (PCE). Conventional DSSCs use volatile organic solvent electrolytes with the iodide/triiodide (I−/I3−\mathrm{I^-}/\mathrm{I_3^-}) couple, which is corrosive, opaque to part of the spectrum, and limits voltage. Two sustainability and performance threads motivated the source work. First, replacing the organic solvent with water gives truly aqueous DSSCs, which are safer and cheaper but harder to make efficient because most dyes and mediators are poorly soluble in water and water accelerates interfacial degradation (Bella, Gerbaldi, Barolo and Grätzel, Chemical Society Reviews 2015). Second, replacing I−/I3−\mathrm{I^-}/\mathrm{I_3^-} with cobalt polypyridyl complexes such as [Co(bpy)3]2+/3+[\mathrm{Co(bpy)}_3]^{2+/3+} (bpy = 2,2'-bipyridine) raises the achievable voltage and avoids corrosive iodine, at the cost of bulkier, slower-diffusing mediator species; the first aqueous electrolytes based on the [Co(bpy)3]2+/3+[\mathrm{Co(bpy)}_3]^{2+/3+} couple were demonstrated by Xiang, Huang, Cheng, Bach and Spiccia (Energy & Environmental Science 2013). To prevent leakage and solvent evaporation, liquid electrolytes can be jellified into quasi-solid gels. Xanthan gum (XG) is an anionic polysaccharide bearing carboxyl (carboxylate) side groups that jellifies water at the 1-3 wt% level into a thixotropic hydrogel, and had previously been shown by the same Torino group to stabilize iodide-based aqueous DSSCs (Galliano et al., Nanomaterials 2020). In DSSC electrolytes, N-methylbenzimidazole (NMBI) is a standard additive: it adsorbs on the TiO2 surface, passivating recombination sites and raising the voltage. Chenodeoxycholic acid (CDCA) is a co-additive that suppresses dye aggregation and improves wetting. The source paper (Galliano, Bella, Bonomo, Giordano, Grätzel, Viscardi, Hagfeldt, Gerbaldi and Barolo, Solar RRL 2021) combined these ingredients for the first time: a truly aqueous electrolyte with a [Co(bpy-pz)2]2+/3+[\mathrm{Co(bpy\text{-}pz)}_2]^{2+/3+} mediator (bpy-pz = a bipyridine-pyrazole ligand; two tridentate bpy-pz ligands coordinate the cobalt centre), NMBI, CDCA-saturated water, jellified with XG, and optimized by a design-of-experiments (DoE) approach, i.e. a multivariate factorial design over three factors (Co2+ concentration, Co2+/Co3+ ratio, XG amount) fitted by partial least-squares regression. The best cell reached PCE 4.47% (VOCV_{\mathrm{OC}} = 791 mV, JSCJ_{\mathrm{SC}} = 7.52 mA cm−2^{-2}, fill factor 75.1%), at the time the highest for any cobalt-based quasi-solid aqueous DSSC, and gelled cells retained >95% of initial PCE over five days while liquid cells lost more than 20%. The authors, however, observed a puzzling JSCJ_{\mathrm{SC}} drop for gelled versus liquid cells at high Co2+ concentration, which they tentatively ascribed to XG partially removing NMBI from the TiO2 surface (weakening passivation and increasing recombination) and to hindered mediator diffusion, and they explicitly deferred these mechanisms, the systematic study of the electrode/electrolyte interface, and an extended DoE to future work.

Problem Statement

For the quasi-solid truly aqueous DSSC built from a [Co(bpy-pz)2]2+/3+[\mathrm{Co(bpy\text{-}pz)}_2]^{2+/3+} cobalt mediator, NMBI, CDCA-saturated water and a xanthan-gum (XG) hydrogel matrix — the system that achieved ~4.5% efficiency with five-day stability in 2021 — the following interlinked questions remain open, as posed by the source work: (1) Interfacial mechanism: does XG, through its carboxylate-bearing chains and a possible XG-NMBI association, remove NMBI from the TiO2-dye/electrolyte interface and thereby weaken surface passivation and enhance recombination, as hypothesized to explain the reduced JSCJ_{\mathrm{SC}} of gelled versus liquid cells at high Co2+ concentration? (2) Transport mechanism: what are the diffusion kinetics of the Co2+\mathrm{Co^{2+}} and Co3+\mathrm{Co^{3+}} mediator species within the XG gel matrix and inside the dye-sensitized mesoporous TiO2 network, and how are these kinetics governed by the polymer's functional groups (e.g. carboxylic moieties)? (3) Composition and stability: how can electrolyte composition — considering factors such as cobalt concentration and ratio, gum content, additive identity, dye, and counter-electrode composition — be chosen to improve stability beyond the five-day window while maintaining or improving efficiency? Later organic-solvent findings point to photoinduced mediator-additive complexation and water-induced redox reactions as candidate degradation channels in cobalt electrolytes, but the source work itself deferred the extended design of experiments and noted that moderate fitting errors in its stability models limited the analysis of interactions between factors. The operative interfacial and mass-transport mechanisms remain to be identified — in a falsifiable form tied to the measurements that would confirm or refute them — and the way electrolyte composition governs efficiency and stability of the gelled aqueous cobalt electrolyte remains to be established.

References

  1. S. Galliano, F. Bella, M. Bonomo, F. Giordano, M. Grätzel, G. Viscardi, A. Hagfeldt, C. Gerbaldi, C. Barolo, 'Xanthan-Based Hydrogel for Stable and Efficient Quasi-Solid Truly Aqueous Dye-Sensitized Solar Cell with Cobalt Mediator', Solar RRL 5(7), 2000823 (2021). DOI: 10.1002/solr.202000823, https://doi.org/10.1002/solr.202000823 (source paper)
  2. W. Xiang, F. Huang, Y.-B. Cheng, U. Bach, L. Spiccia, 'Aqueous dye-sensitized solar cell electrolytes based on the cobalt(II)/(III) tris(bipyridine) redox couple', Energy & Environmental Science 6(1), 121-127 (2013). DOI: 10.1039/C2EE23317G, https://doi.org/10.1039/C2EE23317G
  3. F. Bella, C. Gerbaldi, C. Barolo, M. Grätzel, 'Aqueous dye-sensitized solar cells', Chemical Society Reviews 44(11), 3431-3473 (2015). DOI: 10.1039/C4CS00456F, https://doi.org/10.1039/C4CS00456F
  4. S. Galliano, F. Bella, M. Bonomo, G. Viscardi, C. Gerbaldi, G. Boschloo, C. Barolo, 'Hydrogel Electrolytes Based on Xanthan Gum: Green Route towards Stable Dye-Sensitized Solar Cells', Nanomaterials 10(8), 1585 (2020). DOI: 10.3390/nano10081585, https://doi.org/10.3390/nano10081585
  5. M. Bonomo, A. Y. Segura Zarate, L. Fagiolari, A. Damin, S. Galliano, C. Gerbaldi, F. Bella, C. Barolo, 'Unreported resistance in charge transport limits the photoconversion efficiency of aqueous dye-sensitised solar cells: an electrochemical impedance spectroscopy study', Materials Today Sustainability 21, 100271 (2023). DOI: 10.1016/j.mtsust.2022.100271, https://doi.org/10.1016/j.mtsust.2022.100271
  6. B. Selvaraj, G. Shanmugam, S. Kamaraj, E. Thirugnanasambandam, S. Peters, A. Gunasekeran, A. Sambandam, R. S. Pillai, 'Effect of Copper and Cobalt Metal Complex Redox Mediator Based Xanthan Gum Gel Electrolyte Materials on Performance of Dye Sensitized Solar Cells', ChemistrySelect 7(43), e202203197 (2022). DOI: 10.1002/slct.202203197, https://doi.org/10.1002/slct.202203197
  7. A. Y. Segura Zarate, L. Gontrani, S. Galliano, E. M. Bauer, D. T. Donia, C. Barolo, M. Bonomo, M. Carbone, 'Green zinc/galactomannan-based hydrogels push up the photovoltage of quasi solid aqueous dye sensitized solar cells', Solar Energy 272, 112460 (2024). DOI: 10.1016/j.solener.2024.112460, https://doi.org/10.1016/j.solener.2024.112460
  8. L. Casoli, A. Y. Segura Zarate, E. M. Bauer, M. Bonomo, S. Galliano, C. Barolo, A. Lembo, L. Gontrani, M. Carbone, 'Strategies for the Implementation of Chenodeoxycholic Acid in Natural Polymer-Based Hydrogels for Stable and Low-Light Effective Quasi-Solid Electrolytes for Aqueous Dye-Sensitized Solar Cells', ACS Applied Energy Materials 8(23), 17451-17466 (2025). DOI: 10.1021/acsaem.5c02940, https://doi.org/10.1021/acsaem.5c02940
  9. S. Domenici, R. Speranza, F. Bella, A. Lamberti, T. Gatti, 'A Sustainable Hydrogel-Based Dye-Sensitized Solar Cell Coupled to an Integrated Supercapacitor for Direct Indoor Light-Energy Storage', Solar RRL 9(6), 2400838 (2025). DOI: 10.1002/solr.202400838, https://doi.org/10.1002/solr.202400838
  10. J. Martins, A. M. V. M. Pereira, S. Emami, C. M. Silva, D. Ivanou, A. Mendes, 'Highly Stable Dye-Sensitized Solar Cell using Cobalt Tris(Bipyridyl) Complexes Via Monolithic Architecture', Solar RRL 9(14), 2500310 (2025). DOI: 10.1002/solr.202500310, https://doi.org/10.1002/solr.202500310
  11. G. Salerno, N. Manfredi, A. Abbotto, O. Bettucci, 'Sustainable Electrolyte Media in Dye-Sensitized Solar Cells: From Water-Based to Deep Eutectic Solvents and Biopolymeric Approaches', Molecules 31(12), 2037 (2026). DOI: 10.3390/molecules31122037, https://doi.org/10.3390/molecules31122037