Acceleration Techniques for Recombination of Gases in Electrolysis Microactuators with Nafion®-Coated Electrocatalyst

Abstract

Recombination of electrolysis gases (oxidation of hydrogen and reduction of oxygen) is an important factor in operation efficiency of devices employing electrolysis such as actuators and also unitized regenerative fuel cells. Several methods of improving recombination speed and repeatability were developed for application to electrolysis microactuators with Nafion®-coated catalytic electrodes. Decreasing the electrolysis chamber volume increased the speed, consistency, and repeatability of the gas recombination rate. To further improve recombination performance, methods to increase the catalyst surface area, hydrophobicity, and availability were developed and evaluated. Of these, including in the electrolyte pyrolyzed-Nafion®-coated Pt segments contained in the actuator chamber accelerated recombination by increasing the catalyst surface area and decreasing the gas transport diffusion path. This approach also reduced variability in recombination encountered under varying actuator orientation (resulting in differing catalyst/gas bubble proximity) and increased the rate of recombination by 2.3 times across all actuator orientations. Repeatability of complete recombination for different generated gas volumes was studied through cycling.

Keywords: Electrolysis microactuators; gas recombination.

Figure 1

Operation concept for electrolysis microactuator developed as part of a micropump system for controlled site-specific drug delivery.

Figure 2

The angle of an actuator clamped in a test fixture was varied for each volume (0°, actuator facing up on the benchtop, 90°, actuator perpendicular to benchtop, and 180°, actuator facing down on the benchtop).

Figure 3

Hypothetical example: if the wait time (denoted with dashed vertical lines) between the first and second set of generated gas boluses (10 and 50 μL, respectively) is not sufficient for complete recombination, the bellows structure will exceed maximum inflation when the fourth 50 μL bolus is generated).

Figure 4

Recombination time course for a 5.33 μL bolus delivered with actuators fabricated utilizing different methods to accelerate recombination.

Figure 5

% Recombination vs. angular orientation for different delivered generated gas volumes for unmodified actuators with Nafion®-coated electrode catalyst and actuators with additional pyrolyzed Nafion® coating on suspended Pt wire segments.

Figure 6

(a) Repeated cycling (actuation and recombination), (b) averaged trends for actuation and recombination of for 10 and 5 μL boluses.

Figure 7

(a) Actuation and recombination results obtained for different generated gas volumes of 0.277, 1.11, 5.33, 10, 25.33, and 50 μL for an actuator facing up on the benchtop. (b) Data for smaller bolus volumes.