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44225-G5
Fundamentals of Laser Direct Write Deposition for Electrochemically-Active Materials
Laser direct-write (LDW) printing has emerged as an important and successful method to produce mesoscale patterns of complex materials such as energy storage and conversion materials, biological molecules, and even living cells without destroying their desirable physical, chemical or biological properties. The development of this technique has been primarily applications driven, and thus little is known about the fundamental deposition process, its affect on the material, and the ability to control the interactions to produce novel phases and structures in the deposited materials. Armed with a better understanding of the laser-interaction and the materials changes that are induced, we can optimize the structures and chemistries for catalytic and/or charge storage performance. In this report, we focus on the effects of the incident laser energy on the properties of LDW printed ultracapacitor electrodes. The most notable result is that such effects can directly lead to performance improvements at high charge and discharge rates by independently modifying the protonic and electronic conduction in the material.
Hydrous ruthenium oxide (HRO) is a well-studied ultracapacitor electrode material with extremely high specific capacitance (~750 F/g). This performance metric is due to the dual-insertion properties of the material allowing both protons and electrons to move through the bulk electrode and contribute to the energy storage mechanisms. However, this behavior is strongly dependent on the amount of structural water in the system, with greater amounts of water leading to increased proton conductivity and decreased electron conductivity. Through traditional oven baking processes, the amount of structural water can be controlled, thereby affecting both proton and electron conductivity simultaneously and giving the specific capacitance its sensitivity to annealing temperature. Optimal capacitance is obtained when both electron and proton conductivity are optimized at approximately RuO2•0.5 H2O.
Using LDW, we find it is possible to independently control the electron and proton conductivity through modifications to the material during the laser transfer process. Under this technique, a UV laser generates a localized ablation event at the interface between an ink, composed of the active material in a liquid matrix, and a sacrificial (source) substrate. This local vaporization propels a droplet of ink forward, toward a receiving substrate where computer controlled positioning enables the formation of arbitrary patterns in two dimensions. In our experiments, an ink of HRO, composed of commercially available nanoscale powder and 5M sulfuric acid is deposited by LDW under varying incident laser energy.
Experimental results show that at low incident laser energies, the material maintains its structural, morphological, and electrochemical properties in comparison to control samples printed without using a laser. However, as the laser energy is increased, we observe a number of changes to the electrochemical performance. The most notable of these is an increase in the relative amount of charge storage at high discharge rates as demonstrated through cyclic voltammetry measurements. The effects on proton and electron conduction can be independently probed through electrochemical impedance spectroscopy where we find that the proton transport is largely unaffected by the laser energy while the electron resistivity shows a sharp reduction by almost a factor of 4 under optimal laser conditions (~0.5 J/cm2). As the laser energy is further increased beyond the optimal, there is a gradual increase in the material resistivity
The mechanism behind this distinctive processing capability is unique to the laser transfer method. Under furnace annealing, the changes in the transport properties are due to reduction in water content, whereas under the rapid heating and cooling associated with laser processing, there is insufficient time for significant reduction in water content. Direct imaging of the transfer process yields a flight time of only 50-100 μs. Therefore, we would not expect any significant changes to the proton transport in these materials. This assertion is further supported by TGA measurements on the transferred materials. However over this same time frame, there is opportunity for highly localized material diffusion and sintering of the HRO particles. Such sintering behavior results in a decrease of electron resistivity due to an increase in interparticle contact. Independent high resolution TEM measurements further support this assertion.
In fact, the amount of heating and subsequent sintering that occurs depends on the laser energy in a highly non-linear way. As the laser energy increases, there is an increase in the amount of heating to the material, however, there is also a change in the amount of cooling that occurs during the transfer process. At low laser energies, the transferred material stays primarily confined to a compact droplet whereas at high laser energies, there is more of an explosive behavior leading to increased cooling during transfer. Therefore we find that at higher laser energies, the extent of sintering is reduced and the resistivity begins to increase.
