Membraneless Vanadium Redox Fuel Cell Using Laminar Flow

Abstract

This communication describes a small redox fuel cell fabricated using a design that omits the membrane normally used to separate anodic and cathodic compartments. This design exploits the laminar flow 1 that occurs in liquids flowing at low Reynolds number (Re) to eliminate convective mixing of fuels. Two separate streamsone oxidizing and one reducing -flow parallel to one another through the channel, and no membrane is needed to separate these streams (only diffusive exchange occurs across the interface between them). We demonstrate this concept by operating a millimeter-scale redox fuel cell that uses the redox couples V(V)/V(IV) (cathodic compartment) and V(III)/V(II) (anodic compartment) [2][3][4][5] and that presents no added mechanical nor electrical resistance between the two aqueous solutions.Previous work on fuel cells that do not require a membrane has used selective catalysts or enzymes to restrict reactions of oxidant and reductant present in a mixture to the appropriate electrode. [6][7][8][9][10][11] An example from Dyer 6 used a mixture of O 2 and H 2 ; another, by Willner et al., 9,10 was a biofuel cell with enzymes as catalysts. Figure 1 shows a schematic representation of a single cell. We used soft lithography to fabricate a channel with two inlets and one outlet either in poly(dimethylsiloxane) (PDMS) using soft lithography (for thick channels: h ≈ 200 µm) or in SU-8 photoresist using conventional lithography (for thin channels: h ≈ 50 µm); 12,13 details of fabrication are given in the Supporting Information. We used graphite rather than metals as the electrodes to reduce electrolysis of water during the operation of the cell. 14 The vanadium system has two redox couples with a large difference in formal potentials, ∼1.0 V/NHE for V(V)/V(IV) (as VO 2 + /VO 2+ ) and -0.25 V/NHE for V(III)/V(II). 15 We prepared these two redox species by electrolyzing an ∼1 M solution of VOSO 4 in 25% H 2 SO 4 in two half-cells separated by a Nafion membrane. (The concentrations used in the fluidic fuel cell were ∼1 M for both V(V) and V(II), and 10 -3 M for V(III) and V(IV)). When the V(V) and V(II) solutions were flowing at 25 µL s -1 in the channel, the cell generated a maximum open-circuit voltage (i.e., the potential when no net current was flowing) of 1.52 V in a 200-µm thick membraneless structure; it generated 1.59 V in a 50-µm thick system at a flow rate of 0.07 µL s -1 . 16 These voltages are approximately 90% of the experimental value (1.67 V) obtained using two platinum wires separated by a membrane in a twoelectrode configuration. The permeation of O 2 through the PDMS slab is sufficiently slow, as compared to the residence time of the solution in the channel, that it does not affect the open-circuit potential.As the two half-cells of the fuel cell are not physically separated by a membrane, they are defined only by the laminar flow of the two streams of fuel. For two fluids with the same viscosity (the case for our two solutions) and flow rate, the interface between the two miscible aqueous ...

Energy Harvesting and Remote Powering for Implantable Biosensors

“…Recently, a new kind of membrane-less fuel cell has been introduced [32], [33]. It exploits the laminar characteristics of microchannel flows to keep the two reactants separated, avoiding in this way the use of a membrane.…”

Interfacial mass transport in steady three-dimensional flows in microchannels

“…Zero velocity normal to the plane results in an interface across which solute only diffuses; there is no convective transport across this centre plane. The interface acts like the membrane in a coflowing membrane separator (Brody and Yager 1997), and such interfaces act like a 'virtual membrane' in the context of fluid-based fuel cells (Cohen et al 2005, Ferrigno et al 2002.…”

“…Heat and mass transfer to solid-liquid interfaces and across liquid-liquid interfaces are fundamental to heat exchanger design (Acharya et al 1992, 2001, Mokrani et al 1997, Peerhossaini et al 1993, electrochemical systems for analysis and energy production (Cohen et al 2005, Ferrigno et al 2002, Shrivastava et al 2008, separations with membranes (Shrivastava et al 2008) and without membranes (Aref andJones 1989, Brody andYager 1997), fabrication at fluid-fluid interfaces (Kenis et al 1999, and sensors involving interfacial reactions (Foley et al 2008, Golden et al 2007, Kamholz et al 1999, Squires et al 2008, Teles and Fonseca 2008, Vijayendran et al 2003. Many of these systems rely on the suppression of turbulence as a means of controlling fluid flow, whereas others are relegated to laminar flow by other constraints.…”

“…Although many have noted significant increases in rates of transfer with the introduction of chaotic trajectories (Bryden and Brenner 1999, Ganesan et al 1997, Lefevre et al 2003, others (ourselves included) have cautioned against equating increases in mixing efficiency due to chaotic flows with increased rates of transfer (Ganesan et al 1997, Ghosh et al 1998, Kirtland et al 2006. Experimentally, many investigators have harnessed the controllability of laminar flow to their advantage in systems depending on interfacial heat and mass transfer: liquid-liquid extraction (Brody and Yager 1997); homogeneous reaction of initially separate reactant streams , Kamholz et al 1999; microfabrication at fluid-fluid interfaces (Kenis et al 1999 and laminar flowbased fuel cells (Cohen et al 2005, Ferrigno et al 2002. All of these systems, however, suffer some loss of efficiency due to depleted regions near the interfaces at which transfer occurs.…”

Modeling and Behavior Analysis of a Membraneless Fuel Cell

“…Therefore, it is important to determine conditions to maintain stability on the flow for the proper functioning of the fuel cell 5 . Figure 1 shows the simplified model of a membraneless fuel cell assumed in present work, according to the basic geometry proposed by 2,6 . Reactants are fed from left input and flow in a laminar way through a channel with an anode on the fuel side and a cathode on the oxidant side.…”