Electrochemical water splitting by layered and 3D cross-linked manganese oxides: correlating structural motifs and catalytic activity

Abstract

a Manganese based precious metal-free electrocatalysts for the oxygen evolution reaction (OER) are promising materials for energy storage systems based on dark or photo-coupled water electrolysis, because they are active, inexpensive and of low toxicity. In this work, atomic scale structure-activity relationships of two different nano-structured manganese oxides, MnO x , are established using a combination of X-ray absorption, diffraction and electrochemistry. Prepared by chemical symproportionation (s-MnO x ) and impregnation (i-MnO x ), the s-MnO x catalyst consisted of a layered structure similar to d-MnO 2 while the i-MnO x catalyst displayed a mixture of tunnelled, 3D cross-linked b-and defective g-MnO 2 structures. During electrocatalytic oxygen evolution the structural motifs of both MnO x remain largely unchanged, but the oxidation state of Mn increases from 3.5 to 3.9-4.Kinetic parameters of the electrocatalytic oxygen evolution reaction were extracted using Tafel slope analysis and pH titration experiment, and the role of the protons abstracted was analyzed. The study reveals fundamental differences of general importance in the catalytic activity between layered and cross-linked structures. The exclusive presence of di-m-oxo-bridged Mn ions in the layered structure is coupled to a pronounced redox and charge capacity behaviour. This ensured efficient use of surface and bulk active sites, and resulted in a relatively large Tafel slope. Consequently, the intrinsic OER activity is especially high in s-MnO x . In contrast, 3D cross-linked structures with both mono-and di-m-oxo-bridged Mn ions resulted in lower intrinsic activity but smaller Tafel slope, and thus favourable activity at technological water-splitting rates. The insights from this comparative study will provide guidance in the structural design and optimization of other non precious metal oxide OER catalysts. Broader contextThe intermittency of the electrical output power of renewable solar and wind energy devices makes efficient, cost-effective, grid-scale energy storage mandatory. Electrical power storage using molecular bonds (synthesis of fuel molecules) offers a number of advantages over storage in solid state material lattices. As electrolytic synthesis of chemical fuels is a reductive chemical process, a corresponding electrooxidation process at the grid-scale level is essential. Electrochemical water oxidation to molecular oxygen is perhaps the only feasible oxidation process that can provide electrons and protons at the required grid-scale. However, inexpensive, efficient and stable water oxidation catalysts are rare, if non-existent. This is why the development of non-precious water oxidation catalysts has become a scientic priority. Manganese oxides attract special attention as water splitters, as they form the basis for Nature's oxygen evolving complex in the Photosystem II complex. Detailed insight into structure-activity relationships of inorganic Mn containing water splitting materials is still scarce.

Nanostructured Manganese Oxides as Highly Active Water Oxidation Catalysts: A Boost from Manganese Precursor Chemistry

“…[16,[55][56][57][58] Lately, electrochemical water splitting using layered and three dimensional crosslinked manganese oxides were studied by the same group; they correlated the catalytic activity with the responsible structural motifs. [59] In a similar manner, Nakamura et al have shown that electrochemical water oxidation could be efficiently carried out under neutral pH by stabilizing the surface-associated intermediate Mn III species. [60] Recently, Spiccia et al modeled water-oxidizing manganese(III/IV) oxide and prepared a screen-printed material containing nanostructured manganese by means of redox precipitation that was highly active towards electrochemical water oxidation whereas manganese oxide films electrodeposited from ionic liquids were also investigated for the same purpose.…”

Water Oxidation at Electrodes Modified with Earth‐Abundant Transition‐Metal Catalysts

“…[ 174 , 175 ] In general, MnOx catalysts exhibit lower activity than CoOx or NiOx, which has been attributed to the stable formation of inactive MnO2 domains. [ 176 ] Dau et al…”

Hierarchical structures constructed by BiOX (X = Cl, I) nanosheets on CNTs/carbon composite fibers for improved photocatalytic degradation of methyl orange

“…(Guan et al 2013;Mi et al 2014;Xie et al 2014;Li et al 2013) Highly reactive facets can effectively enhance surface properties and improve the separation of photo-generated carriers, so enormous effort has been devoted to the synthesis of (001) facet-dominated bismuth oxyhalides for the applications on photocatalysis through hydrothermal reaction, successive ionic layer adsorption and reaction, etc. (Guan et al 2013;Mi et al 2014;Jiang et al 2014;Bergmann et al 2013;Mayer et al 2012;Lee et al 2012;Chen et al 2011) However, nanosheets (NSs) or granular supported NSs are difficult to recycle from aqueous solution after photocatalytic reactions. (Jiang et al 2014) To solve this problem, one of effective ways is to grow NSs on carbon fibers (CFs) to form semiconductor-CFs hierarchical structures.…”