We present an exploratory study of the tribological properties between an AFM probe and a Au(111) surface in an aqueous environment while subjected to applied surface potentials. Using a three-electrode setup, the electrical potential and interfacial electric field on a Au(111) working electrode are controlled. Lateral force microscopy is used to measure the friction forces between the AFM probe and the Au surface. As the AFM probe approaches the surface, normal forces are also measured to gain insight into the interfacial forces. When a positive potential is applied to the Au surface, the friction is found to rise sharply at a critical potential and level off at a relatively high value. However, when a negative potential is applied, the friction forces are low, even lower compared to the open circuit potential case. These changes in friction, by a factor of approximately 35, as a function of the applied potential are found to be reversible over multiple cycles. We attribute the origin of the high friction at positive potentials to the formation of a highly confined, ordered icelike water layer at the Au/electrolyte interface that results in effective hydrogen bonding with the AFM probe. At negative potentials, the icelike water layer is disrupted, resulting in the water molecules acting as boundary lubricants and providing low friction. Such friction experiments can provide valuable insight into the structure and properties of water at charged surfaces under various conditions and can potentially impact a variety of technologies relying on molecular-level friction such as MEMs.
We explore the use of vertically aligned carbon nanotube (VACNT) arrays as an electrode in a triboelectric nanogenerator (TENG) that harvests mechanical energy and converts it to electrical energy. When VACNT arrays, 1 cm2 in area, are mechanically contacted with PET and PTFE counter electrodes in vertical contact-separation mode, currents up to 0.16 µA and 0.21 µA, and voltages up to 1.42 V and 3.20 V are obtained, respectively. The VACNT TENG output remains stable even after over 20,000 continuous contact cycles. A 0.47 µF capacitor is successfully charged to 4.5 V in 60 s using a VACNT-PTFE triboelectric nanogenerator (TENG) prototype.
Supercapacitors are notable for their ability to deliver energy at higher power (compared to batteries) and store energy at higher density (compared to capacitors) as well as exhibit a long cycle life. In our efforts to further the development of supercapacitors, our focus is on using vertically aligned carbon nanotubes (VACNT) as supports for pseudocapacitive and faradaic capacitive electrode materials. The objective is to develop electrodes that can function in an inexpensive aqueous environment with relatively small potential windows, and still store energy at a higher density than carbon materials alone. We describe the different approaches explored to overcome the challenges of non-uniform deposition, poor wetting and array collapse. Materials that are electrochemically anchored to VACNT supports include NixCoyOz, VOx, Fe2O3 and Co-Mn mixed oxides. In each case, the specific capacitance obtained using the VACNT templates is more than double that obtained by direct deposition onto current collectors or by using VACNT alone as electrodes. The ease of VACNT growth and the degree of coating control that can be achieved using electrodeposition means there is much potential in exploring them as supports for capacitive electrode materials.
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