A passive microfluidic hydrogen–air fuel cell with exceptional stability and high performance

Abstract

We describe an advanced microfluidic hydrogen-air fuel cell (FC) that exhibits exceptional durability and high performance, most notably yielding stable output power (>100 days) without the use of an anode-cathode separator membrane. This FC embraces an entirely passive device architecture and, unlike conventional microfluidic designs that exploit laminar hydrodynamics, no external pumps are used to sustain or localize the reagent flow fields. The devices incorporate high surface area/porous metal and metal alloy electrodes that are embedded and fully immersed in liquid electrolyte confined in the channels of a poly(dimethylsiloxane) (PDMS)-based microfluidic network. The polymeric network also serves as a self-supporting membrane through which oxygen and hydrogen are supplied to the cathode and alloy anode, respectively, by permeation. The operational stability of the device and its performance is strongly dependent on the nature of the electrolyte used (5 M H2SO4 or 2.5 M NaOH) and composition of the anode material. The latter choice is optimized to decrease the sensitivity of the system to oxygen cross-over while still maintaining high activity towards the hydrogen oxidation reaction (HOR). Three types of high surface area anodes were tested in this work. These include: high-surface area electrodeposited Pt (Pt); high-surface area electrodeposited Pd (Pd); and thin palladium adlayers supported on a "porous" Pt electrode (Pd/Pt). The FCs display their best performance in 5 M H2SO4 using the Pd/Pt anode. This exceptional stability and performance was ascribed to several factors, namely: the high permeabilities of O2, H2, and CO2 in PDMS; the inhibition of the formation of insoluble carbonate species due to the presence of a highly acidic electrolyte; and the selectivity of the Pd/Pt anode toward the HOR. The stability of the device for long-term operation was modeled using a stack of three FCs as a power supply for a portable display that otherwise uses a 3 V battery.

Optimization of a permeation‐based microfluidic direct formic acid fuel cell (DFAFC)

“…We report current and power densities for the DFAFCs using the geometric surface area of the electrode that is in contact with the liquid electrolyte, a scaling that better represents the planar trans-membrane-dependent fluxes of oxygen and fuel, as described in detail in previous reports [28,30]. Following the deposition, the electrodes were thoroughly rinsed with milli-Q water and electrochemically cleaned by potential cycling from À0.225 to 1.25 V (Ag/AgCl) at 5 mV/s in 0.5 M H 2 SO 4 solution.…”

“…The details of the fabrication procedures used to construct the device have been reported elsewhere [28]. In brief, we fabricated the microfluidic channel layer of the device (Fig.…”

“…Electrode fabrication is detailed in our previous work [28]. Briefly, the thin-film electrodes were obtained by electronbeam deposition of a 15 nm Ti adhesion layer, followed by 100 nm Pt layer onto a glass slide while maintaining a vacuum of $2 Â 10 À7 mbar.…”

“…We recently reported a passive design for a microfluidic hydrogen-air fuel cell that does not require the use of pumps or a water management system in order to provide a stable power output for over 100 days [28]. The device is very simple in design and incorporates a planar arrangement of electrodes that are fully immersed in a liquid electrolyte contained in PDMS-based microfluidic channels.…”

Microfluidic Electrochemical Cell Array in Series: Effect of Shunt Current

“…1,2 The first CLFC was introduced by Ferrigno et al in 2002, using vanadium redox species in a Y-shaped channel. 3 Since then, several reviews have been published which summarize the fundamental physics of these devices, 4 and also highlight some of the milestones achieved in device functionality, reactants and architectures, 2,5 for both fuel cells [6][7][8][9][10] and batteries. [11][12][13] The first architectures reported used planar electrodes on the sides or bottom of a micro-channel.…”