The development of alternative energy sources, especially green energy sources for traditional fossile fuels is a major challenge that humankind is currently facing. The conversion of solar energy into chemical fuels is a promising method. Inspired by photosynthesis taking place in the green plants, artificial photosynthesis was proposed. The targeted reaction is as follows:

2 H₂O (l) → 2 H₂ (g) + O₂ (g)

Theoretically, the half redox equation related to water oxidation, also called oxygen evolution reaction (OER), which is a key step for water splitting, needs a high potential bias, as a consequence of the very slow kinetics of this reaction. This latter could be improved by employing a catalyst in order to make it energetically less expensive. Catalysts based on metal oxides have shown bright prospects for this application [1]. As a consequence of the determination of the fine structure of photosystem II in charge of this redox process in plants, it was revealed that the oxygen evolving complex (OEC) was a Mn₄CaO₅ entity resembling strongly the one that can be found in manganese oxides. These latter attracted consequently a considerable attention of scientists over the last years as possible OER photoelectrocatalysts. After comparison with traditional catalysts based on precious metals, manganese oxides, and mainly MnO₂, have the advantages of being abundant on Earth, and therefore inexpensive, and also environmentally friendly. In particular, many investigations reported in literature showed in a rather convincing manner that MnO₂ is a promising candidate as OER photoelectrocatalysis. In a work published in 2011, T. F. Jaramillo et al. showed that birnessite-type MnO₂ could be a good solar water splitting catalyst [2], whereas S.S. Stahl et al. found that the OER activity of manganese oxides depends on their crystalline structure [3]. D.G. Nocera and co-workers investigated the nucleation and growth mechanism of MnOx thin films electrodeposited in potentiostatic conditions using a potential value selected inside a narrow potential range [4]. Few studies were thus focused on large variations of the potential used during the electrodeposition of MnO₂thin films in potentiostatic conditions, and on the corresponding performances of these films towards photoelectrocatalysis of water oxidation.

In this work, manganese dioxide (MnO₂) thin films with different morphologies were prepared by one-step potentiostatic deposition using an aqueous electrolyte containing MnSO₄ and NaClO₄ in a traditional three-electrode cell. Fluorine doped tin oxide (FTO) substrates were used as working electrodes, and a platinum grid and a K₂SO₄ saturated Hg/Hg₂SO₄ electrode (SSE) were used as counter- and reference electrodes, respectively. The same setup was used during the OER tests except that a 0.1 M KOH Hg/HgO reference electrode (MOE) and a 0.1 M NaOH electrolyte were used instead of SSE and NaClO₄respectively.

Our results show that the value of the electrodeposition potential has a strong influence on the morphology and crystallinity of resulting MnO₂ thin films, as evidenced by SEM-FEG and XRD characterisations. The birnessite type crystalline structure was obtained only for films electrodeposited at potential values close to the peak potential related to MnO₂ electrodeposition, whereas MnO₂thin layers electrodeposited at potential values situated far beyond this peak potential value and before the foot of the anodic wall related to water oxidation were found to be amorphous, which clearly promoted their activity towards OER, by comparison with those of the birnessite variety. In the course of our investigations, chronoamperometry experiments were carried out for all the samples. They allowed us to find out that the stability of the electrocatalytic behaviour, as well as the adherence on the underlying substrate, were both better for amorphous films.  Electrochemical impedance spectroscopy (EIS) experiments were also carried out to develop a better understanding of their electrochemical properties in the course of water oxidation experiments. Moreover, their semi-conductor behaviour was evidenced using water oxidation experiments in the presence of a solar simulator, and deeply investigated with the help of Incident Photon to electron Conversion Efficiency (IPCE), UV-visible spectrophotometry and Current Sensing Atomic Force Microscopy (CS-AFM).

 

References:

[1] R. L.Doyle, I.J. Godwin, M.P. Brandon, M.E.G. Lyons, Phys. Chem.Chem. Phys., 2013, 15, 13737-13783

[2] B. A. Pinaud, Z. Chen, D. N. Abram, T. F. Jaramillo, J. Phys. Chem. C, 2011, 115 (23), 11830–11838

[3] R. Pokhrel, M. K. Goetz, S. E. Shaner, X. Wu, S. S. Stahl, J. Am. Chem. Soc., 2015, 137 (26), 8384–8387

[4] M. Huynh, D. K. Bediako, Y. Liu, D. G. Nocera, J. Phys. Chem. C, 2014, 118 (30), 17142–17152