Engineering Disorder into Heterogenite‐Like Cobalt Oxides by Phosphate Doping: Implications for the Design of Water‐Oxidation Catalysts

Abstract

Amongst the most promising materials designed to catalyse water oxidation from earth‐abundant materials are the metal oxides. Despite the success of these materials, understanding the relationship of structure to function has been very challenging. It has been noted that many metal oxide water‐oxidation catalysts function best in a proton‐accepting electrolyte, such as a borate or phosphate buffer. However, these same electrolytes are known to significantly affect the metal oxide structures by imparting a level of “disorder” or “molecular nature” to the materials. The most well‐known case is that of Nocera's Co–Pi catalyst (Pi: inorganic phosphorus). In this study, we have synthesised a series of “heterogenite‐like” cobalt oxides with different levels of phosphate doping (0–9 %P). Our synthetic method enables us to make “bulk materials”, the structural properties of which (as observed by X‐ray absorption spectroscopy and transmission electron microscopy) mimic those observed directly on electrode surfaces. The changes made to the bulk phases were directly correlated with the reactivity for water‐oxidation catalysis and the ability of the CoOx materials to act as sacrificial oxidants. The most disordered materials were most reactive for sacrificial oxidation but were less effective as water‐oxidation catalysts. These results help us understand how disorder changes the thermodynamic stability of metal oxides and how this impacts on efficiency for water oxidation.

The Oxidation of Peroxide by Disordered Metal Oxides: A Measurement of Thermodynamic Stability “By Proxy”

“…The trend for cobalt is particularly notable; the reduction observed in BET surface area is a function of phosphate doping, which is a strong indication that the dopant is playing a role blocking accessibility to interstitial pores, hence the negative impact on total surface area. This would be consistent with the HPO 4 2− ion being associated with an interlayer …”

“…This is discussed in full in Ref. . The key analytical characterization of this series of materials is summarized in Figure .…”

“…The proportion of phosphate incorporated into the CoO x material is described as a weight percent that is, CoO x (%P), where x is the wt % P. The level of phosphate doping (%P) was controlled by the ratio of cobalt to phosphate in the first synthetic step of the synthesis. Our previous research has shown that phosphate doping systematically increased the atomic and nanoscale structural disorder in the CoO x materials . It is known from the XANES of these materials that the whole series of Co III , therefore the Na + /H + content must increases slightly along with PO 4 3− to charge balance.…”

“…Four CoO x samples with different levels of phosphate doping were prepared for this study: 0 %P (“control”), 1 %P, 3 %P and 9 %P. The cobalt oxide materials were synthesised according to methods previously described by King et al . The synthesis involved mixing stock solutions of cobalt(II) (0.2 m , CoSO 4 ⋅7 H 2 O) and phosphate buffer (0.1 m or 1 m , pH 7, NaH 2 PO 4 .2 H 2 O) in a defined ratio and oxidising the resulting cobalt phosphate based floc with sodium hypochlorite (4 w/v%, 50 % by volume of the total reaction solution, NaOCl).…”

Design of Multi‐Metallic‐Based Electrocatalysts for Enhanced Water Oxidation

“…Compared with highly‐crystalline materials, the disordered structure caused by multiple doping possesses unstable Gibbs free energy. In other words, it is more likely to transform into an active intermediate with exact lattice orientation toward OER (Figure ) . Therefore, the energy needed to convert the pristine material to the active intermediate phase decreases in the chaotic structure, leading to the overall energy‐saving for OER .…”