Laser Direct Writing of Hydrous Ruthenium Dioxide Micro-Pseudocapacitors
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Laser Direct Writing of Hydrous Ruthenium Dioxide Micro-Pseudocapacitors Craig B. Arnold1, Ryan C. Wartena2, Bhanu Pratap1, Karen E. Swider-Lyons2, and Alberto Piqué1 1 Materials Science and Technology Division, Code 6372 2 Chemistry Division, Code 6171 Naval Research Laboratory, Washington, DC 20375, USA. ABSTRACT We are using a laser engineering approach to develop and optimize hydrous ruthenium dioxide (RuOxHy or RuO2·0.5 H2O) pseudocapacitors. We employ a novel laser forward transfer process, Matrix Assisted Pulsed Laser Evaporation Direct Write (MAPLE-DW), in combination with UV laser machining, to fabricate mesoscale pseudocapacitors and microbatteries under ambient temperature and atmospheric conditions. Thin films with the desired high surface area morphology are obtained without compromising their electrochemical performance. The highest capacitance structures are achieved by depositing mixtures of sulfuric acid with the RuO2·0.5 H2O electrode material. Our pseudocapacitors exhibit linear discharge behavior and their properties scale proportionately when assembled in parallel and series configurations.
INTRODUCTION The manufacture of thin film hydrous ruthenium dioxide pseudocapacitors is generally considered difficult due to the complex materials requirements for an effective, high capacitance power source [1]. The most advantageous morphology for a high capacity pseudocapacitor is a permeable, porous structure leading to a large effective surface area [2,3]. However, the RuO2·xH2O system has the additional concern that the capacitance of the material depends on the processing temperature [4,5,6]. These aspects of morphological constraints and processing temperature limitations, in addition to the presence of water in the system have made RuO2·0.5 H2O incompatible with standard vacuum techniques for thin film growth. Instead, recent studies of thin film pseudocapacitors have focused on anhydrous and metal oxide systems [7,8]. We take a unique approach under the Laser Engineered Advanced Power Sources (LEAPS) program at the Naval Research Laboratory, to overcome these difficulties by using a laser direct writing technique, MAPLE-DW [9,10]. This technique produces thin films of the hydrous material with the desired morphological properties and without the need for high temperature or lithographic processing following deposition. Instead, pseudocapacitors are arranged through the use of UV laser machining of the deposited films. A distinct advantage of our technique is the ability to operate under ambient conditions to deposit a mixture of RuO2·0.5 H2O and a liquid electrolyte. This technique of distributing electrolyte within the electrode is commonly used in the production of larger scale electrochemical power sources, but it is not possible with other thin film techniques such as PVD, CVD, or sol-gel methods. Additionally, the use of liquid sulfuric acid electrolyte as opposed to solid-state materials enables higher proton conductivity in the electrolyte and enhances pseudocapacitor properties [11]. In th
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