Performance evaluation of HCOOH micro-fluidic fuel cell using Ni wire electrode

Abstract

A membrane-free low cost microfluidic fuel cell (μFFC) consisting a trident shaped channel is molded into a poly(dimethyl siloxane) block and Ni wires affixed therein that doubled up as the catalyst and electrical connectors. Streams of aqueous acidic solutions of formic acid as the anolyte (fuel), KMnO4 as the catholyte (oxidant) and an acid electrolyte, flown through the respective channels at a constant rate ensured laminar flow across the length of the channels, while being in contact with the Ni wires, thereby tapping its’ catalytic activity for good electrochemical performance. The effect of varying fuel or oxidant concentration on the μFFC performance is studied. In the chronopotentiometric mode, the high catalytic activity of Ni allows high currents of the order of 1.25 mA to be sustained by the cell, particularly when the surface is fresh, and this current drops when the deposition of Mn, K, and S occurs. A flow rate of 150 μL/min. is found to be optimal, as the highest open-circuit voltage (OCV) of 1.33 V is attained at this flow rate. While the cell performance is largely unaffected by formic acid concentration, but it is controlled by KMnO4 concentration. Higher oxidant concentrations yielded higher OCVs, due to more amount of the five-electron reaction, occurring at the cathode enhancing the charge separation and hence the OCV. DRT studies of the EIS data resolved two different time constants for the anodic and cathodic processes. The μFFC delivers a maximum power density of 2.1 mW/cm2 and a stable current of 3.5 mA/cm2 for more than 10 min. at 0.6 V, thus validating its deployment in a variety of applications like diagnostic devices and as an independent power supply for MEMS devices.