Isaac B. Sprague

Modeling of Diffuse Charge Effects in a Microfluidic Based Laminar Flow Fuel Cell

A mathematical model for laminar flow fuel cells including electrical double layer and ion transport effects is developed. The model consists of the Poisson-Nernst-Plank equations and the modified Navier-Stokes equations to account for the advection of species in the downstream direction. The generalized Frumkin-Butler-Volmer equation is used for the fuel cell kinetics. The finite-volume method is used to develop a system of algebraic equations from the governing partial differential equations, and a numerical algorithm is developed to obtain the results. The accuracy of the 2-D numerical simulation is validated against published results using a 1-D analytical solution. Numerical results show that the concentration distributions for both the neutral species and ions change in both the cross-stream and streamwise directions. An especially interesting result is the change in positive ion concentration within the electrical double layer along the streamwise direction. A study on the importance of the electric body force in the momentum conservation equations is also presented. It is found that the flow results are only affected by the electric body force term at the start of the electrodes and has a negligible impact on device performance results. This model allows us to study both kinetically active (electrodes) and inactive (insulated wall) regions for a microfluidic fuel cell. The mathematical model and numerical simulation will be particularly useful in analyzing the complex behavior that occurs in laminar flow electrochemical devices where a minimum of two spatial dimensions must be considered and the electrical double layer and ion transport cannot be neglected.

Characterization of a membraneless direct-methanol micro fuel cell

Abstract The performance of a membraneless laminar flow micro fuel cell was evaluated under different operating conditions. The fuel cell was microfabricated in polydimethylsiloxane using standard soft-lithography techniques. It used methanol solution as the fuel for the anode side, and oxygen saturated sulphuric acid for the cathode. The parameters studied were the methanol concentration, flowrate, device width, and the concentration of sulphuric acid in the anode stream. Performance was characterized by V—I plots, stability of open circuit potential (OCP), polarization resistances, and anode polarization curves. We observed behaviour different from that shown thus far by existing laminar flow fuel cells. Our results show that the power output of the device decreases with an increase in the methanol concentration. An increase in the flowrate also decreases the power output of the device. It is shown that these trends are likely caused by the cells internal resistance to proton transport. The addition of sulphuric acid to the fuel significantly decreases this resistance. It was found that the device OCP was not stable over extended operation, and could drop by more than 150 mV in 72 h.

Depth Averaged Analytic Solution for a Laminar Flow Fuel Cell with Electric Double Layer Effects

A comprehensive multidimensional analysis is presented for a laminar flow fuel cell with electric double layer (EDL) dependent kinetics in a planar microdevice. The EDL is described with the Stern model, and a generalized Frumkin--Butler--Volmer (gFBV) equation is used to describe the EDL dependent kinetics. The liquid electrolyte is modeled with the Poisson--Nernst--Planck (PNP) equations and the incompressible Navier--Stokes (NS) equations. For planar microchannel applications, the three-dimensional model is reduced to an in-plane depth averaged set of equations through an asymptotic analysis. The diffuse layers are resolved in the thin double layer limit through asymptotic matching by considering the Debye length to channel width ratio as a smallness parameter. This yields an outer problem for the bulk electrolyte and an inner problem for the anode and cathode diffuse regions. Fuel cell performance is then evaluated by introducing several specified local current density profiles. The resulting approxim...

Performance improvement of micro-fuel cell by manipulating the charged diffuse layer

A fuel cell device is presented based on a counter-flow microfluidic fuel cell (CFMFC) with nano-porous electrodes by developing an advection flux of ions within the electric double layer (EDL). Typically, in a microfluidic fuel cell, advection in the EDL is negligible because the near wall electrolyte velocity is zero. However, by using nano-pores, a non-negligible ion flux due to advection can be developed in the charged regions of the EDL which affects the structure of the EDL. In this article, we use a mathematical model to study how advection in the EDL affects the kinetic performance of fuel cells. Our model predicts that the peak power density can be increased by more than 2 fold in a CFMFC using this approach to kinetic enhancement.

Flow Through Nanoporous Electrodes in a Microfluidic Fuel Cell

In this paper we present how advection in the electric double layer (EDL) affects the kinetic performance of electrochemical cells. To accomplish this we use a laminar flow fuel cell model based on the Poisson-Nernst-Planck and Frumkin-Butler-Volmer equations. The model contains nonlinear physics with very disparate length scales due to the complex 3-dimensional nature of the nano-porous device. To account for these difficulties, the full mathematical model is solved numerically using a novel numerical algorithm developed based on domain decomposition method. Numerical results show that the presence of an advection flux through nano-pores on the order of the EDL width yields some novel physics that affect the structure of electrode-electrolyte interface. We also show that electrolyte advection within the EDL can be used to enhance the kinetic performance of electrodes in electrochemical cells. In the device presented the peak power density can be increased significantly with flow velocity.Copyright

The Electrode-Electrolyte Interface in Acidic and Alkaline Fuel Cells

This numerical study presents the role of diffuse region of the electric double layer in both acidic and alkaline fuel cells. The numerical model is based on the Poisson-Nernst-Planck (PNP) and generalized-Frumkin-Butler-Volmer (gFBV) equations. The Laminar Flow Fuel Cell (LFFC) is used as the model fuel cell architecture to allow for the appropriate and equivalent comparison of acidic and alkaline cells. In particular, we focus on how each device behaves to changing reactant supply at the electrodes, including the overall cell performance and individual electrode polarizations. It is found that the working ion concentration at the reaction plane contributes to differing performance behaviors in acidic and alkaline fuel cells, including activation losses and reactant transport overpotentials. This is due to the working ion, and the electrode where it’s consumed, being opposite for acidic and alkaline fuel cells.Copyright

A Numerical Model to Simulate Diffuse Effects in Microfluidic Fuel Cells

A 2D numerical model is developed for a laminar flow fuel cell considering ion transport and the electric double layer around the electrodes. The Frumkin-Butler-Volmer equation is used for the fuel cell kinetics. The finite volume method is used to form algebraic equations from governing partial differential equations. The numerical solution was obtained using Newton’s method and a block TDMA solver. The model accounts for the coupling of charged ion transport with the electric field and is able to fully resolve the diffuse regions of the electric double layer in both the stream-wise and cross-channel directions. Different operating phenomena, such as laminar flow separation and the development of the depletion boundary layers and electric double layers are obtained. These numerical results demonstrate the model’s ability to capture the complex behavior of a microfluidic fuel cell which has been ignored in previous 1D models.Copyright

Characterization of a Microfluidic Based Direct-Methanol Fuel Cell

The performance of a membraneless laminar flow micro fuel cell was evaluated under different operating conditions. The fuel cell was microfabricated in poly-dimethyl-siloxane using standard soft-lithography techniques. It used methanol solution as the fuel for the anode side, and oxygen saturated sulfuric acid for the cathode. The parameters studied were the methanol concentration and the concentration of sulfuric acid in the anode stream. The performance was characterized by V-I plots, stability of open circuit potential, and anode polarization curves. Our results show that the power output of the device decreases with increase in the methanol concentration. It is shown that these trends are caused by the cell’s internal resistance to proton transport. The addition of sulfuric acid to the fuel significantly decreases this resistance. The device open circuit potential was not stable over extended operation, and could drop by more than 150 mV in 72 hours.Copyright