• Energy Research

Ball milling is commonly used to reduce catalyst particle size. However, little attention is paid to further changes that ball milling can cause to the rest of the catalysts’ physicochemical properties, which can impact their intrinsic catalytic activity. The effect of ball milling on the physicochemical properties of NiCoO2, NiO, CoO, and NiO:CoO mixtures is reported and correlated with their electrochemical oxygen evolution reaction (OER) activity. It is also shown that particle fragmentation is an inherent consequence of ball milling, but some oxides can also experience a phase transformation. In the case of rocksalt‐structured CoO, it is partially or entirely transformed to spinel‐structured Co3O4. Additionally, NiCo2O4 with a spinel structure can be formed by ball milling NiO and CoO simultaneously (both rocksalt structures), but only in the absence of water. The changes impact the electrochemical activity of the initial oxides. Ball milled NiCoO2 exhibits the highest activity with a mean potential of 1.563 V at 10 mA cm−2, demonstrating the advantage of having Ni and Co in the same structure. Although NiCo2O4 is also a binary oxide, the results indicate that its metal coordination environment makes it intrinsically less active than NiCoO2 for the OER in alkaline media.

The interface between ceramic particles and a polymer matrix in a hybrid electrolyte is studied with high spatial resolution by means of Electrochemical Strain Microscopy (ESM), an Atomic Force Microscope (AFM)-based technique. The electrolyte consists of polyethylene oxide with lithium bis(trifluoromethanesulfonyl)imide (PEO6–LiTFSI) and Li6.5La3Zr1.5Ta0.5O12 (LLZO:Ta). The individual components are differentiated by their respective contact resonance, ESM amplitude and friction signals. The ESM signal shows increased amplitudes and higher contact resonance frequencies on the ceramic particles, while lower amplitudes and lower contact resonance frequencies are present on the bulk polymer phase. The amplitude distribution of the hybrid electrolyte shows a broader distribution in comparison to pure PEO6–LiTFSI. In the direct vicinity of the particles, an interfacial area with enhanced amplitude signals is found. These results are an important contribution to elucidate the influence of the ceramic–polymer interaction on the conductivity of hybrid electrolytes.