MSE Seminar Series: Jeffrey Long, U.S. NRL
Friday, November 17, 2017
2108, Chemical and Nuclear Engineering Building
Jeffrey W. Long, U.S. Naval Research Laboratory, Washington D.C.
Abstract:Understanding Pseudocapacitive Charge Storage from Fundamental Interfacial Processes to 3D Electrode Design
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 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 achieve reasonable utilization of faradaic capacity. At the Naval Research Laboratory, we have focused on new electrode designs in which pseudocapacitive phases, particularly transition metal oxides (e.g., MnOx, FeOx), are integrated as ultrathin coatings at the surface of porous 3D carbon scaffolds, and demonstrated their advantages when deployed in mild-aqueous-electrolyte electrochemical capacitors. We are now extending the function of these electrode materials to “hybrid” devices and mixed-electrolyte systems (based on Zn2+, for example) that allow us to take advantage of both battery-like and pseudocapacitive charge storage advantages in a single electrode.
As we continue to develop and transition these next-generation electrode architectures, we have also found that more subtle design factors, such as nanoscale order/disorder in the metal oxide coating and the physicochemical nature of the carbon–metal oxide interface, can have a significant impact on electrochemical performance. For fundamental investigations of such phenomena, we use 2D pyrolytic-carbon substrates to mimic the surface properties of 3D carbon, but in form factors that are more readily characterized by conventional surface spectroscopy and scanning-probe microscopy. We apply nanoscale pseudocapacitive oxides and redox polymers to these planar carbon films and explore how the physical, chemical, and electronic structure of the resulting interface of carbon and pseudocapacitive material impacts electrochemical properties. Lessons learned from these model interfaces are readily translated to improved performance in practical 3D electrode architectures.