Interfacial Phenomena at the Junction of Conductive Carbon with Pseudocapacitive Metal Oxides and Polymers: Implications for Electrochemical Capacitors

Abstract

The technological push to dramatically shorten the charge/discharge times of electrochemical energy-storage (EES) devices while still providing significant energy content requires rethinking how we design and assemble EES, from the choice of particular types of charge-storing materials to how those materials are spatially translated into a high-performance, yet still practical, electrode architecture. Materials that exhibit “pseudocapacitance,” with fast faradaic charge-storage mechanisms that mimic the current–voltage characteristics of a conventional capacitor, promise to satisfy both energy and power needs when incorporated into electrochemical capacitor (EC) devices. The poor electronic conductivity of most pseudocapacative materials (e.g., manganese oxides, MnOx) dictates that they must be integrated with a better electronic conductor, typically porous, nanostructured carbons, in order to minimize ohmic losses and to reasonably utilize innate faradaic capacity. The past decade has seen great advancements in the development of new electrode architectures that incorporate oxides or polymers at carbon substrates, accompanied by promising electrochemical results. Yet our experience with such multifunctional materials shows that we may still be “leaving on the table” too much energy and power performance, due to suboptimal electronic/chemical/physical interactions at the junction of carbon (with its semimetal character) and nanoscale overlayers of pseudocapacitive materials (often semiconductors or hopping conductors). In order to explore these fundamental questions, we step back from the complexity of 3D electrode architectures to planar electrode configurations that otherwise mimic the material characteristics of practical electrode structures. Carbon films prepared by chemical vapor deposition (CVD) that are based on the pyrolysis of benzene and related monomers serve as model substrates on which we deposit pseudocapacitive metal oxides and electroactive polymers. The resulting 2D interfaces are characterized by classical electroanalytical methods to determine fundamental properties such as electron-transfer kinetics. In conjunction with electrochemical characterization, we apply surface-sensitive techniques such as photoelectron spectroscopy and scanning probe microscopy to query electronic, chemical, and physical structure as those properties are tuned by variations in synthesis/deposition of each phase: carbon and pseudocapacitive coating. Lessons learned from these model 2D interfaces are readily applied to the redesign of practical 3D electrode structures in next-generation electrochemical capacitors and batteries.