To assemble the battery, stencil printing was chosen based on its low cost and compatibility with various substrates including flexible plastics. Stencil printing is also better suited than other printing methods such as inkjet or gravure in order to print thick active layers (10s-100s µm) necessary to achieve high areal capacities. Processing temperatures of each battery component were below 150°C to remain compatible with low cost, flexible substrates. The anode ink was composed of zinc, zinc oxide, bismuth (III) oxide, and polyethylene glycol in a polyethylene oxide binder. A photopolymerizable polyacrylic acid (PAA) sol-gel was previously designed and implemented as the electrolyte and separator. The cathode ink was fabricated by combining Vulcan XC 72 carbon powder with a platinum catalyst in a polyvinylidene fluoride (PVDF) binder. PVDF was chosen based on its hydrophobicity to prevent the electrolyte solution from flooding the cathode and limiting the reduction of oxygen. The carbon-binder ratio was varied to tune the ink viscosity for stencil printing and to optimize the sheet resistance of the printed cathodes. Oxygen half-cells were designed to determine the effect of platinum concentration on the oxygen reduction reaction (ORR) overpotential and discharge performance. In addition, the ORR overpotential was studied as a function of potassium hydroxide concentration and volume used in the printed sol-gel electrolyte.
The fully printed zinc-air batteries demonstrated a steady discharge voltage of 1.25 to 1.30 V. High areal capacities of 2-5 mAh cm-2 were achieved with current densities between 1-5 mA cm-2, exceeding the performance of previously reported printed zinc-air batteries. Self-discharge lifetimes of one month were obtained with the use of a PDMS encapsulation layer. The internal resistance of these cells was typically less than 200 Ω while using low cost carbon based current collectors and less than 100 Ω with silver-carbon bilayer collectors. This work represents the first demonstration of a packaged, fully printed zinc-air battery with a printed cathode and catalyst layer. The batteries exhibit low internal resistances (< 100 Ω) and high areal capacities (> 2 mAh cm-2) at peak current requirements for printed electronics applications (> 1 mA cm-2), a step towards realizing integrated energy storage for printed electronics systems.