Activity and Stability of the Tetramanganese Polyanion [Mn4(H2O)2(PW9O34)2]10— during Electrocatalytic Water Oxidation

Sara Goberna-Ferrón,José Galán-Mascarós,Joaquín Soriano-López

doi:10.3390/inorganics3030332

Abstract

In natural photosynthesis, the oxygen evolving center is a tetranuclear manganese cluster stabilized by amino acids, water molecules and counter ions. However, manganese complexes are rarely exhibiting catalytic activity in water oxidation conditions. This is also true for the family of water oxidation catalysts (WOCs) obtained from POM chemistry. We have studied the activity of the tetranuclear manganese POM [Mn4(H2O)2(PW9O34)2]10—(Mn4), the manganese analog of the well-studied [Co4(H2O)2(PW9O34)2]10— (Co4), one of the fastest and most interesting WOC candidates discovered up to date. Our electrocatalytic experiments indicate that Mn4 is indeed an active water oxidation catalysts, although unstable. It rapidly decomposes in water oxidation conditions. Bulk water electrocatalysis shows initial activities comparable to those of the cobalt counterpart, but in this case current density decreases very rapidly to become negligible just after 30 min, with the appearance of an inactive manganese oxide layer on the electrode.

Highlights

Water oxidation catalysis is currently one of the hot topics in chemistry research [1,2,3], since it is regarded as the bottleneck in the development of an artificial photosynthesis scheme [4,5,6]
This is true for certain conditions, and it can put into question the quantification of its catalytic activity, it is important to note that the genuine water oxidation catalysts (WOCs) activity of Co4 has been confirmed beyond any reasonable doubt [21,22,23], and corroborated even by the most critical research teams [24]
In the presence of Mn4 (1 mM) a significant catalytic water oxidation wave is observed above 1.0 V, which is accompanied by significant gas bubbling at the anode

Summary

Introduction

Water oxidation catalysis is currently one of the hot topics in chemistry research [1,2,3], since it is regarded as the bottleneck in the development of an artificial photosynthesis scheme [4,5,6]. Some studies suggested that this POM may not be stable in water oxidation conditions [18,19,20] This is true for certain conditions, and it can put into question the quantification of its catalytic activity, it is important to note that the genuine WOC activity of Co4 has been confirmed beyond any reasonable doubt [21,22,23], and corroborated even by the most (initially) critical research teams [24]. The water electrochemistry of this polyanion [Mn4 (H2 O) (PW9 O34 )2 ]10− (Mn4 ) confirms its activity as a water oxidation catalysts, albeit its intrinsic instability in water oxidation conditions

Results and Discussion

Materials and Instrumentation

Synthesis and Characterization

Electrochemistry

Conclusions