Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst.

Hyunah Kim,Sung Eun Jerng,Kisuk Kang,Inchul Park,Kyoungsuk Jin,Ki Tae Nam,Sun Hee Kim,Jimin Park

doi:10.1038/ncomms9253

Abstract

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems. Although extensive studies have focused on the metal-oxide catalysts, the effect of metal coordination on the catalytic ability remains still elusive. Here we select four cobalt-based phosphate catalysts with various cobalt- and phosphate-group coordination as a platform to better understand the catalytic activity of cobalt-based materials. Although they exhibit various catalytic activities and stabilities during water oxidation, Na2CoP2O7 with distorted cobalt tetrahedral geometry shows high activity comparable to that of amorphous cobalt phosphate under neutral conditions, along with high structural stability. First-principles calculations suggest that the surface reorganization by the pyrophosphate ligand induces a highly distorted tetrahedral geometry, where water molecules can favourably bind, resulting in a low overpotential (∼0.42 eV). Our findings emphasize the importance of local cobalt coordination in the catalysis and suggest the possible effect of polyanions on the water oxidation chemistry.

Highlights

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems
Na2CoP2O7, Li2CoP2O7, NaCoPO4 and LiCoPO4 compounds were synthesized using conventional solid-state methods, as described previously[41,42,43,44,45], and the X-ray diffraction (XRD) patterns of the synthesized compounds well matched those of previous reports without impurites[41,42,43,44,45] (Supplementary Fig. 1)
The Td polyhedrons are isolated by pyrophosphate groups, and with respect to the long-range ordering of the polyhedron the Td polyhedra in Na2CoP2O7 form a two-dimensional layer via pyrophosphate groups, which is alternately stacked with sodium layers

Summary

Introduction

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems. Splitting water into hydrogen and oxygen molecules via solar energy has been considered as one of the most environment-friendly ways to effectively use renewable energy resources from harvest to redistribution[1,2,3,4,5,6] It has been studied for more than a half century, the inefficiency of the oxygen evolution reaction (OER) has not yet been resolved and is still regarded as a bottleneck in the integration of an overall water splitting system[1,2,3,4,5,6,7,8]. Noble-metal catalysts, such as Pt, IrOx and RuOx, exhibit outstanding OER catalytic activity; the high costs of these catalysts prohibit their practical use[10,11,12,13,14] In this regard, the development of costeffective, abundant-element-based and efficient OER catalysts is highly demanded. The coordination and corresponding electronic structure of the transition metal in the catalysts is believed to greatly affect the OER activity[19,20,21,22,23,31,32,33,34], there is a lack of systematic information relating the local cobalt coordination to the OER catalysis

Methods

Results

Conclusion