Elsevier

Electrochimica Acta

Volume 54, Issue 2, 30 December 2008, Pages 698-705
Electrochimica Acta

An alkaline microfluidic fuel cell based on formate and hypochlorite bleach

https://doi.org/10.1016/j.electacta.2008.07.009Get rights and content

Abstract

An alkaline microfluidic fuel cell is demonstrated employing an alkaline version of a formic acid anode and a sodium hypochlorite cathode. Both sodium formate fuel and sodium hypochlorite oxidant are available and stable as highly concentrated solutions, thereby facilitating fuel cell systems with high overall energy density. Sodium hypochlorite is commonly available as hypochlorite bleach. The alkaline anodic half-cell produces carbonate rather than the less-desirable gaseous CO2, while sustaining the rapid kinetics associated with formic acid oxidation in acidic media. Both half-cells provide high current densities at relatively low overpotentials and are free of gaseous products that may otherwise limit microfluidic fuel cell performance. The microfluidic fuel cell takes advantage of a recently developed membraneless architecture with flow-through porous electrodes. Power densities up to 52 mW cm−2 and overall energy conversion efficiencies up to 30% per single pass are demonstrated at room temperature using 1.2 M formate fuel and 0.67 M hypochlorite oxidant. The alkaline formate/hypochlorite fuel and oxidant combination demonstrated here, or either one of its individual half-cells, may also be useful in conventional membrane-based fuel cell designs.

Introduction

A microfluidic fuel cell is defined as a device that incorporates all fundamental components of a fuel cell within a single microfluidic channel and its walls. Since its invention in 2002, microfluidic fuel cell technology has developed rapidly [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22] and is now considered a candidate for commercial small-scale portable power generation. Microfluidic fuel cells typically operate without a membrane, and the most common configurations rely on the laminar nature of flow in microstructures to maintain sufficient separation of fuel and oxidant streams, flowing side-by-side in a co-laminar format. Supporting electrolyte contained in both co-laminar streams facilitates ionic conduction between the electrodes. Mixing by diffusion is restricted to an interfacial width at the center of the channel, depending on mean velocity and channel geometry. The electrode spacing is typically an order of magnitude larger than the inter-diffusion width. Microfluidic fuel cells have several advantages as compared to traditional proton exchange membrane (PEM)-based fuel cells: fuel and oxidant streams may be combined in a single microchannel; fuel and/or oxidant crossover can be mitigated by adjusting the flow rate of the co-laminar streams; no ion exchange membrane is required; sealing, manifolding, and fluid delivery infrastructure requirements are reduced; and issues related to membrane hydration and water management are eliminated. In addition, microfluidic fuel cells may be manufactured using inexpensive microfabrication methods and low-cost materials.

Microfluidic fuel cells with room-temperature power densities on the order of 100 mW cm−2 have been demonstrated [15], [16]. Operation at high power densities has, however, resulted in low overall energy conversion efficiency due to low single-pass fuel utilization. To mitigate this problem, a new microfluidic fuel cell architecture with flow-through porous electrodes was recently developed by our group [15]. The flow-through design is based on cross-flow of reactant through the porous electrodes into a co-laminar center channel with orthogonally directed flow in which the waste solutions facilitate ionic charge transfer in a membraneless configuration. This cell architecture enables improved utilization of the three-dimensional active area inside the porous electrodes and provides enhanced rates of convective/diffusive transport to the active sites without increasing the parasitic loss required to drive the flow. When operated with 2 M vanadium solutions, the flow-through design demonstrated a high level of overall energy conversion efficiency, as a relatively high level of fuel utilization and cell voltage were achieved concurrently. With respect to overall energy density, the solubility and concentration of ions is a limitation of microfluidic fuel cell systems based on vanadium redox species; in addition, the associated redox reactions provide only one electron per ion.

In the context of fuel and oxidant combinations for microfluidic fuel cell systems with practical energy density, the following attributes are desirable: both reactants and products are available and stable at high concentration in the liquid phase; the reactants provide at least two electrons per molecule; spontaneous and/or electrochemically activated decomposition into gaseous products are prevented; the fuel and oxidant do not react upon mixing; and if the anodic and cathodic supporting electrolytes are different, both fuel and oxidant species are stable in both electrolytes.

Several different fuels have been employed in microfluidic fuel cells presented to date. These include hydrogen [7], [18], [19], methanol [5], [6], [11], formic acid [4], [8], [12], [13], vanadium redox species [9], [14], [15], [16], and hydrogen peroxide [10]. Most liquid hydrocarbon fuels produce carbon dioxide (CO2) as the end product after complete electrochemical conversion. Typically, CO2 evolves as a gaseous product in acidic or neutral media [13] unless the current densities are low enough to fully dissolve it in the fuel or electrolyte stream [4]. Formic acid is an established fuel with high energy density that has previously been used in both PEM-based direct formic acid fuel cells [23], [24] and microfluidic fuel cells [4], [8], [12], [13], and its electrochemical kinetics on palladium catalyst are fast, thereby allowing much higher power densities than other hydrocarbon fuels such as methanol [25]. In contrast to acidic and neutral media, alkaline media have the capability of absorbing large amounts of CO2 as carbonates with the additional benefit of automated carbon sequestration. An alkaline formate fuel from concentrated formic acid therefore accomplishes all the criteria for an all-liquid microfluidic fuel cell system with high energy density.

The oxidants employed in microfluidic fuel cells to date have been oxygen [4], [5], [6], [7], [8], [18], [19], air [11], [12], vanadium redox species [9], [14], [15], [16], and hydrogen peroxide [10], [13]. Among these, only hydrogen peroxide is available as a highly concentrated liquid. Direct hydrogen peroxide reduction on common catalysts such as platinum and palladium is however frequently accompanied by oxygen gas evolution from its decomposition [13]. Alternative liquid oxidants that have been employed in fuel cells or flow batteries include permanganate [4], [26] and hypochlorite [27], [28]. Permanganate is a strong oxidant that enables high current densities and highly positive reduction potentials in acidic media [29]. Reports have shown, however, that product manganese dioxide precipitate can clog the pores of a microporous electrode in the absence of stirring [26]. The use of permanganate oxidant in microfluidic fuel cells that incorporate both microporous electrodes and microscale channels should therefore be avoided. Hypochlorite, in contrast, does not produce any precipitate, and has shown benign electrochemical characteristics as an oxidant in magnesium, zinc and aluminum semi-fuel cells [27], [28] using palladium/iridium and platinum catalysts. Hypochlorite oxidant has many useful characteristics for microfluidic fuel cell implementation: relatively high standard reduction potential and rapid kinetics; high solubility in aqueous media; highly soluble reduction product (chloride); and finally, no gaseous decomposition reactions take place when the hypochlorite is used in an alkaline electrolyte [27]. Furthermore, hypochlorite solution is produced in large quantities as domestic bleach. Sodium hypochlorite bleach is considered relatively safe and is available at very low cost. Hypochlorite therefore meets all the criteria of an all-liquid microfluidic fuel cell with high energy density. Additionally, the product sodium chloride is common and non-toxic.

In this work, a microfluidic fuel cell based on alkaline formate and hypochlorite bleach is presented. Both palladium and gold porous electrodes are formed via electrodeposition on a porous carbon substrate. The electrochemical responses of the porous electrodes are characterized in a stationary three-electrode cell prior to in situ tests and fuel cell implementation.

Section snippets

Preparation of solutions

Alkaline formate fuel solution was obtained by adding 5 wt.% concentrated formic acid (HCOOH; Fisher Sci., Fair Lawn, NJ) to a 10 wt.% (2.8 M) sodium hydroxide (NaOH) electrolyte prepared by dissolving NaOH pellets (EMD Chemicals, San Diego, CA) in Millipore Milli-Q water (Millipore, Billerica, MA). The resulting solution contained 1.2 M formate and 1.6 M OH. Alkaline hypochlorite (ClO) oxidant solution (0.67 M) was prepared by dissolving 10 wt.% (2.8 M) NaOH pellets directly in a 5% sodium

Reaction scheme

At the pH used here, the fuel exists predominantly as formate (HCOO), the oxidant as hypochlorite (ClO) and the CO2 as carbonate. The reactions, written in terms of these species, lead to the following standard electrode potentials (vs. SCE) at 298 K ((1) calculated from Gibbs’ energies of formation in [31] and (2) taken from [32]):

  • Anode:HCOO + 3OH  CO32− + 2H2O + 2e, E0 = −1.17 V

  • Cathode:ClO + H2O + 2e  Cl + 2OH, E0 = 0.57 V

  • Overall cell reaction:HCOO+ClO+OHCO32+Cl+H2O,Ecell0=1.74V

Note that the actual

Conclusions

This work demonstrated the feasibility of an alkaline formate anode coupled with an alkaline hypochlorite cathode in a microfluidic fuel cell architecture with flow-through porous electrodes. Both half-cells were unique in terms of fuel cell implementation. In contrast to the vanadium redox system, formate and hypochlorite are both available in highly concentrated solutions, thereby enabling a microfluidic fuel cell system with high overall energy density. The proof-of-concept

Acknowledgments

The authors would like to acknowledge project funding by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Angstrom Power Inc., and infrastructure funding from Canada Foundation for Innovation (CFI) and British Columbia Knowledge Development Fund (BCKDF).

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