Water oxidation is pivotal for the sustainable production of non-fossil fuels but also energetically highly demanding. In natural photosynthesis, light-driven water oxidation is catalyzed with high efficiency by a Mn₄CaO₅ cluster, embedded in a specific protein environment in an enzyme called Photosystem II (PSII) [1]. An attractive strategy to emulate this system is to combine an electrocatalytically active material with a solar cell. 

We tested different electrochemical protocols to coat ITO electrode with Mn oxide film starting from aqueous solution of Mn²⁺ ions. In all cases the resulting materials were layered-like amorphous Mn oxides which contained predominantly di-μ-oxido bridged Mn⁴⁺ ions and thus resembled the atomic structure of the Mn₄CaO₅ complex in PSII [2]. The catalytic activity however varied significantly depending on the protocol used for electrodeposition. 

Here we apply quasi in-situ X-ray absorption spectroscopy (XAS) to study the structure of the highest activity catalyst (MnCat) during the catalytic process. The potential-induced structural changes in the MnCat are compared to the potential-induced changes in the catalytically inactive Mn oxide as well as to the laser-flash induced structural changes in the Mn₄CaO₅ cluster in intact PSII complex studied previously using the same XAS technique [3]. We demonstrate that the Mn oxidation state and structural changes in MnCat astonishingly resemble those occurring in the natural photosynthetic cycle implying that the catalyst mimics the natural paragon not only structurally but also functionally. 

Surprisingly, similar potential-induced structural changes to those found in MnCat and in PSII are also found in the apparently inactive Mn oxide. We show that the prerequisite to have functional mimic of the natural paragon is also the presence of sufficient amount of Mn³⁺ ions stable in the oxide structure at overpotentials which are high enough to have a sizable oxygen evolution activity. Thus when the goal is to electrodeposit catalytically active Mn-based materials, either the deposition protocol must be designed such that Mn³⁺ ions are stabilized in the oxide structure during the deposition, or Mn³⁺ ions should be introduced in the structure by adequate post-treatment (heat, chemical reagents etc.). 

References: 

[1] H. Dau, I. Zaharieva, M. Haumann, Curr. Op. Chem. Biol. 16, 3 (2011).

[2] I. Zaharieva, P. Chernev, M. Risch, K. Klingan, M. Kohlhoff, A. Fischer, H. Dau, Energy Environ. Sci. 5, 7081 (2012).

[3] H. Dau, M. Haumann, Coord. Chem. Rev. 252, 272 (2008).