AmericanChemicalSociety.com

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 induced materials changes, 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 (>800 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 separately 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.  In addition, the electrochemical measurements require a thin layer of Nafion for mechanical encapsulation and to allow for proton transport.  To accomplish this, we developed a pneumatic dispensing method that deposits a precise amount of liquid solution on top of the electrodes which results in a uniform film upon baking.

Experimental results show that at low incident laser energies, the material maintains its structural and morphological 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.  This effect is seen in all samples regardless of the pre-baking condition. 

For a given scan rate, the specific capacitance of the material either increases, decreases, or remains the same depending the prebake conditions.  For samples in which an excess of structural water is present, the increase in laser energy produces a monotonic increase in the capacitance.  However in the opposite case, there is a monotonic decrease in the specific capacitance.  For samples pre-baked to the “ideal” conditions, the material shows an increase in capacitance followed by a decrease with a maximum occurring at a laser energy of 25 uJ (~0.5 J/cm²). 

We examine these results by probing the effects on proton and electron conduction through EIS and DC conductance measurements.  Here we find that the kinetics of the proton reaction are marginally improved by the laser energy.  Conversely, the electrical resistance studies show a sharp reduction by almost a factor of 4 under optimal laser conditions.  As the laser energy is further increased beyond the optimal, there is a gradual increase in the material resistivity.

Based on this analysis, we are able to provide an explanation for the observed capacitance behavior.  For materials with an excess of water, there is a relative lack of electron conductivity leading to a lower overall capacitance.  With laser energy increases, there is a decrease in resistance and a corresponding improvement in the capacity.  In the converse case, the material is proton limited and so the change in electronic conductivity has a minimal impact on the overall electrochemical properties.  For an ideal material, the increase in laser energy causes an initial increase until the material becomes proton limited associated with a decrease in capacity.

The mechanism behind this distinctive processing capability is unique to LDW.  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 LDW, there is insufficient time for significant reduction in water content (~50-100ms).  Therefore, we would not expect any significant changes to the bulk properties.  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 associated with increases in interparticle contact. 

The amount of heating and 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, but 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 a more explosive response leading to increased cooling.  Therefore we find that at higher laser energies, the extent of sintering is reduced and the resistivity begins to increase.