Cluster Validity Index for Uncertain Data Based on a Probabilistic Distance Measure in Feature Space.

Cluster validity indices (CVIs) for evaluating the result of the optimal number of clusters are critical measures in clustering problems. Most CVIs are designed for typical data-type objects called certain data objects. Certain data objects only have a singular value and include no uncertainty, so they are assumed to be information-abundant in the real world. In this study, new CVIs for uncertain data, based on kernel probabilistic distance measures to calculate the distance between two distributions in feature space, are proposed for uncertain clusters with arbitrary shapes, sub-clusters, and noise in objects. By transforming original uncertain data into kernel spaces, the proposed CVI accurately measures the compactness and separability of a cluster for arbitrary cluster shapes and is robust to noise and outliers in a cluster. The proposed CVI was evaluated for diverse types of simulated and real-life uncertain objects, confirming that the proposed validity indexes in feature space outperform the pre-existing ones in the original space.

Capturing Manganese Oxide Intermediates in Electrochemical Water Oxidation at Neutral pH by In Situ Raman Spectroscopy.

Electrochemical water splitting is a promising means to produce eco-friendly hydrogen fuels. Inspired by the Mn4 CaO5 cluster in nature, substantial works have been performed to develop efficient manganese (Mn)-based heterogeneous catalysts. Despite improvements in catalytic activity, the underlying mechanism of the oxygen evolution reaction (OER) is not completely elucidated owing to the lack of direct spectroscopic evidence for the active Mn-oxo moieties. We identify water oxidation intermediates on the surface of Mn3 O4 nanoparticles (NPs) in the OER at neutral pH by in situ Raman spectroscopy. A potential-dependent Raman peak was detected at 760 cm-1 and assigned to the active MnIV =O species generated during water oxidation. Isotope-labeling experiments combined with scavenger experiments confirmed the generation of surface terminal MnIV =O intermediates in the Mn-oxide NPs. This study provides an insight into the design of systems for the observation of reaction intermediates.

Complex Impedance Analysis on Charge Accumulation Step of Mn3O4 Nanoparticles during Water Oxidation.

The development of efficient water-oxidizing electrocatalysts is a key issue for achieving high performance in the overall water electrolysis technique. However, the complexity of multiple electron transfer processes and large activation energies have been regarded as major bottlenecks for efficient water electrolysis. Thus, complete electrochemical processes, including electron transport, charge accumulation, and chemical bond formation/dissociation, need to be analyzed for establishing a design rule for film-type electrocatalysts. In light of this, complex capacitance analysis is an effective tool for investigating the charge accumulation and dissipation processes of film-type electrocatalysts. Here, we conduct complex capacitance analysis for the Mn3O4 nanocatalyst, which exhibits superb catalytic activity for water oxidation under neutral conditions. Charge was accumulated on the catalyst surface by the change in Mn valence between Mn(II) and Mn(IV) prior to the rate-determining O-O bond forming step. Furthermore, we newly propose the dissipation ratio (D) for understanding the energy balance between charge accumulation and charge consumption for chemical O-O bond formation. From this analysis, we reveal the potential- and thickness-dependent contribution of the charge accumulation process on the overall catalytic efficiency. We think that an understanding of complex capacitance analysis could be an effective methodology for investigating the charge accumulation process on the surface of general film-type electrocatalysts.