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Spin-Coated vs. Electrodeposited Mn Oxide Films as Water Oxidation Catalysts.

ABSTRACT: Manganese oxides (MnO_x), being active, inexpensive and low-toxicity materials, are considered promising water oxidation catalysts (WOCs). This work reports the preparation and the physico-chemical and electrochemical characterization of spin-coated (SC) films of commercial Mn₂O₃, Mn₃O₄ and MnO₂ powders. Spin coating consists of few preparation steps and employs green chemicals (i.e., ethanol, acetic acid, polyethylene oxide and water). To the best of our knowledge, this is the first time SC has been used for the preparation of stable powder-based WOCs electrodes. For comparison, MnO_x films were also prepared by means of electrodeposition (ED) and tested under the same conditions, at neutral pH. Particular interest was given to α-Mn₂O₃-based films, since Mn (III) species play a crucial role in the electrocatalytic oxidation of water. To this end, MnO₂-based SC and ED films were calcined at 500 °C, in order to obtain the desired α-Mn₂O₃ crystalline phase. Electrochemical impedance spectroscopy (EIS) measurements were performed to study both electrode charge transport properties and electrode-electrolyte charge transfer kinetics. Long-term stability tests and oxygen/hydrogen evolution measurements were also made on the highest-performing samples and their faradaic efficiencies were quantified, with results higher than 95% for the Mn₂O₃ SC film, finally showing that the SC technique proposed here is a simple and reliable method to study the electrocatalytic behavior of pre-synthesized WOCs powders.

SUBMITTER: Hernandez S

PROVIDER: S-EPMC5502989 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Cobalt polyoxometalates (Co-POMs) have emerged as promising water oxidation catalysts (WOCs), with the added advantage of their molecular nature despite being metal oxide fragments. In comparison with metal oxides, that do not offer well-defined active surfaces, POMs have a controlled, discrete structure that allows for precise correlations between experiment and computational analyses. Thus, beyond highly active WOCs, POMs are also model systems to gain deeper mechanistic understanding on the oxygen evolution reaction (OER). The tetracobalt Weakley sandwich [CoII 4(H2O)2(B-α-PW9O34)2]10- (Co4-WS) has been one of the most extensively studied. We have compared its activity with that of the iron analog [FeIII 4(H2O)2(B-α-PW9O34)2]6- (Fe4-WS) looking for the electronic effects determining their activity. Furthermore, the effect of POM nuclearity was also investigated by comparison with the iron- and cobalt-monosubstituted Keggin clusters. Electrocatalytic experiments employing solid state electrodes containing the POMs and the corresponding computational calculations demonstrate that CoII-POMs display better WOC activity than the FeIII derivatives. Moreover, the activity of POMs is less influenced by their nuclearity, thus Weakley sandwich moieties show slightly improved WOC characteristics than Keggin clusters. In good agreement with the experimental data, computational methods, including pK a values, confirm that the resting state for Fe-POMs in neutral media corresponds to the S1 (FeIII-OH) species. Overall, the proposed reaction mechanism for Fe4-WS is analogous to that found for Co4-WS, despite their electronic differences. The potential limiting step is a proton-coupled electron transfer event yielding the active S2 (FeIV[double bond, length as m-dash]O) species, which receives a water nucleophilic attack to form the O-O bond. The latter has activation energies slightly higher than those computed for the Co-POMs, in good agreement with experimental observations. These results provide new insights for the accurate understanding of the structure-reactivity relationships of polyoxometalates in particular, and or metal oxides in general, which are of utmost importance for the development of new bottom-up synthetic approaches to design efficient, robust and non-expensive earth-abundant water oxidation catalysts.

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Spin-Coated vs. Electrodeposited Mn Oxide Films as Water Oxidation Catalysts.

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