A woven thread-based microfluidic fuel cell with graphite rod electrodes

A woven thread-based microfluidic fuel cell based on graphite rod electrodes is proposed. Both inter-fiber gaps and inter-weave spaces could provide flow channels for the liquid transport through the woven cotton thread. Therefore, no external pumps are required to maintain the co-laminar flow, benefiting for the integration and miniaturization. In the experiment, sodium formate and hydrogen peroxide are used as fuel and oxidant, respectively. To improve the electrochemical reaction kinetics, KOH and H₂SO₄ serve as supporting electrolyte at the anode and cathode, respectively. Na₂SO₄ solution is used as the electrolyte to separate the cathode and anode in the middle flow channel and alleviate the reactant crossover. The open circuit potential of the fuel cell achieves 1.44 V and the maximum current density and power density are 56.6 mA cm^?2 and 20.7 mW cm^?2, respectively. Moreover, the cell performance reduces with increasing the electrode distance due to a high ohmic resistance. With an increase in the fuel concentration from 1 M to 4 M, the performance increases and it reduces with further increasing to 6 M owing to a correspondingly low flow rate. The highest fuel utilization rate reaches 10.9% at 4 M fuel concentration.     

Computational modeling of microfluidic fuel cells with flow-through porous electrodes

In the current work, a computational model of a microfluidic fuel cell with flow-through porous electrodes is developed and validated with experimental data based on vanadium redox electrolyte as fuel and oxidant. The model is the first of its kind for this innovative fuel cell design. The coupled problem of fluid flow, mass transport and electrochemical kinetics is solved from first principles using a commercial multiphysics code. The performance characteristics of the fuel cell based on polarization curves, single pass efficiency, fuel utilization and power density are predicted and theoretical maxima are established. Fuel and oxidant flow rate and its effect on cell performance is considered and an optimal operating point with respect to both efficiency and power output is identified for a given flow rate. The results help elucidate the interplay of kinetics and mass transport effects in influencing porous electrode polarization characteristics. The performance and electrode polarization at the mass transfer limit are also detailed. The results form a basis for determining parameter variations and design modifications to improve performance and fuel utilization. The validated model is expected to become a useful design tool for development and optimization of fuel cells and electrochemical sensors incorporating microfluidic flow-through porous electrodes.     

Air-breathing membraneless laminar flow-based fuel cell with flow-through anode

This paper describes a detailed characterization of laminar flow-based fuel cell (LFFC) with air-breathing cathode for performance (fuel utilization and power density). The effect of flow-over and flow-through anode architectures, as well as operating conditions such as different fuel flow rates and concentrations on the performance of LFFCs was investigated. Formic acid with concentrations of 0.5 M and 1 M in a 0.5 M sulfuric acid solution as supporting electrolyte were exploited with varying flow rates of 20, 50, 100 and 200 μl/min. Because of the improved mass transport to catalytic active sites, the flow-through anode showed improved maximum power density and fuel utilization per single pass compared to flow-over planar anode. Running on 200 μl/min of 1 M formic acid, maximum power densities of 26.5 mW/cm² and 19.4 mW/cm² were obtained for the cells with flow-through and flow-over anodes, respectively. In addition, chronoamperometry experiment at flow rate of 100 μl/min with fuel concentrations of 0.5 M and 1 M revealed average current densities of 34.2 mA/cm² and 52.3 mA/cm² with average fuel utilization of 16.3% and 21.4% respectively for flow-through design. The flow-over design had the corresponding values of 25.1 mA/cm² and 35.5 mA/cm² with fuel utilization of 11.1% and 15.7% for the same fuel concentrations and flow rate.     

Sequential flow membraneless microfluidic fuel cell with porous electrodes

A novel convective flow membraneless microfluidic fuel cell with porous disk electrodes is described. In this fuel cell design, the fuel flows radially outward through a thin disk shaped anode and across a gap to a ring shaped cathode. An oxidant is introduced into the gap between anode and cathode and advects radially outward to the cathode. This fuel cell differs from previous membraneless designs in that the fuel and the oxidant flow in series, rather than in parallel, enabling independent control over the fuel and oxidant flow rate and the electrode areas. The cell uses formic acid as a fuel and potassium permanganate as the oxidant, both contained in a sulfuric acid electrolyte. The flow velocity field is examined using microscale particle image velocimetry and shown to be nearly axisymmetric and steady. The results show that increasing the electrolyte concentration reduces the cell Ohmic resistance, resulting in larger maximum currents and peak power densities. Increasing the flow rate delays the onset of mass transport and reduces Ohmic losses resulting in larger maximum currents and peak power densities. An average open circuit potential of 1.2 V is obtained with maximum current and power densities of 5.35 mA cm⁻² and 2.8 mW cm⁻², respectively (cell electrode area of 4.3 cm²). At a flow rate of 100 μL min⁻¹ a fuel utilization of 58% is obtained.     

Mass transfer and performance of membrane-less micro fuel cell: A review

Membrane-less micro fuel cells (MMFCs) are high potential alternative power sources compared to conventional batteries. They use the advantage of laminar flow without the presence of a membrane to separate the anode and the cathode. This article is a wide-ranging review of recent studies on mass transfer, performance, modelling advances and future opportunity in MMFCs research. The discussion focuses on the critical factors that limit the performance of MMFCs. Because MMFCs are diffusion-limited, most of this review focuses on design considerations to enhance the power density output. Moreover, the current status of computational modelling for MMFC systems to upgrade the cell performance will be presented. The review also identifies the challenges and opportunities available for increasing cell performance and making the MMFC a practical application device in the future.     

Counter flow membraneless microfluidic fuel cell

A counter flow membraneless microfluidic fuel cell is presented, where a non-reacting electrolyte separates the reacting streams. In this fuel cell design, vanadium reactants flow through porous carbon electrocatalysts. A sulfuric acid stream is introduced in the gap between the electrodes and diverts the reactants to opposite and independent outlets. This fuel cell differs from other membraneless designs in its ability to maintain a constant separation between the reactants without diffusive mixing.     

Glycerol oxidation in a microfluidic fuel cell using Pd/C and Pd/MWCNT anodes electrodes

Pd/C and Pd/MWCNT based electro-catalysts were prepared by impregnation and used as anodes for glycerol electro-oxidation in a microfluidic fuel cell. Average particle size and lattice parameters of the catalysts were determined by X-ray diffraction, resulting in 7.5 and 3.5 nm for Pd/C and Pd/MWCNT respectively. The electro-catalytic activity of Pd/C and Pd/MWCNT was investigated in 0.1 M glycerol. The results obtained by electrochemical studies in half cell configuration showed that the onset potential for glycerol oxidation on Pd/MWCNT was characterized by a negative shift ca. 40 mV compared to Pd/C. The maximum power density obtained was 0.51 and 0.7 mW cm⁻² for Pd/C and Pd/MWCNT respectively. These results are comparable with those obtained for a microfluidic fuel cell that uses glucose as fuel. The results of this work not only show that glycerol can be used as fuel in a microfluidic fuel cell but also its performance is similar to that obtained with others fuels.     

A membraneless microfluidic fuel cell stack

A membraneless microfluidic fuel cell stack architecture is presented that reuses reactants from one cell to a subsequent one, analogous to PEMFC stacks. On-chip reactant reuse improves fuel utilization and power densities relative to single cells. The reactants flow separately through porous electrodes and interface with a non-reacting and conductive electrolyte which maintains their separation. The reactants remain separated downstream of the interface and are used in subsequent downstream cells. This fuel cell uses porous carbon for electrocatalysts and vanadium redox species as reactants with a sulfuric acid supporting electrolyte. The overall power density of the fuel cell increases with reactant flow rate and decreasing the separating electrolyte flow rate. The peak power, maximum fuel utilization, and efficiency nearly double when electrically connecting the cells in parallel.     

Planar and three-dimensional microfluidic fuel cell architectures based on graphite rod electrodes

We propose new membraneless microfluidic fuel cell architectures employing graphite rod electrodes. Commonly employed as mechanical pencil refills, graphite rods are inexpensive and serve effectively as both electrode and current collector for combined all-vanadium fuel/oxidant systems. In contrast to film-deposited electrodes, the geometry and mechanical properties of graphite rods enable unique three-dimensional microfluidic fuel cell architectures. Planar microfluidic fuel cells employing graphite rod electrodes are presented here first. The planar geometry is typical of microfluidic fuel cells presented to date, and permits fuel cell performance comparisons and the evaluation of graphite rods as electrodes. The planar cells produce a peak power density of 35 mW cm⁻² at 0.8 V using 2 M vanadium solutions, and provide steady operation at flow rates spanning four orders of magnitude. Numerical simulations and empirical scaling laws are developed to provide insight into the measured performance and graphite rods as fuel cell electrodes.     

Analysis of Pt/C electrode performance in a flowing-electrolyte alkaline fuel cell

We characterize the performance of Pt/C-based electrodes under alkaline conditions using a microfluidic H₂/O₂ fuel cell as an analytical platform. Both anodes and cathodes were investigated as a function of electrode preparation procedures (i.e., hot pressing, acclimatization) and fuel cell operating parameters (i.e., electrolyte composition) via chronoamperometric and electrochemical impedance analyses. X-ray micro-computed tomography was employed to link electrode structure to performance. In addition, the flowing electrolyte stream is used to study the effects of carbonates on individual electrode and overall fuel cell performance. Our studies provide direct evidence that the performance of hydrogen-fueled room-temperature alkaline fuel cells (AFCs) is limited by transport processes to and from the anode primarily due to water formation. Furthermore, the presence of carbonate species in the electrolyte appears to impact only anode performance whereas cathode performance remains unchanged.     

Performance increase of microfluidic formic acid fuel cell using Pd/MWCNTs as catalyst

This paper shows that the combination of an O₂ saturated acidic fluid setup (O₂-setup) and a composite of Pd nanoparticles supported on multiwalled-carbon nanotubes (Pd/MWCNTs) as anode catalyst material, results in the improvement of microfluidic fuel cell performance. Microfluidic fuel cells were constructed and evaluated at low HCOOH concentrations (0.1 and 0.5 M) using Pd/V XC-72 and Pd/MWCNTs as anode and Pt/V XC-72 as cathode electrode materials, respectively. The results show a higher power density (2.9 mW cm⁻²) for this cell when compared to the value reported in the literature that considers a commercial Pd/V XC-72 and 3.3 mW cm⁻² using a Pd/MWCNTs with a 50% less Pd loading than that commercial Pd/V XC-72.     

Performance of Au and AuAg nanoparticles supported on Vulcan in a glucose laminar membraneless microfuel cell

Au and AuAg electrocatalysts were prepared by chemical reduction and supported on Vulcan XC-72 for their application in a laminar membraneless microfluidic fuel cell that operates with glucose as fuel in basic media. Average particle size and lattice parameters were determined by X-ray diffraction technique, resulting in 4 and 21 nm for Au and AuAg respectively. The composition ratio of Au and Ag in the mixture was estimated by X-ray fluorescence. X-ray photoelectron spectroscopy measurements were used to determinate oxidation states. The electrocatalytic activity of Au/C and AuAg/C materials was investigated in terms of glucose electrooxidation in 0.3 M KOH. The results obtained by electrochemical studies in a half cell configuration showed that the onset potential for glucose oxidation on AuAg/C presented a negative shift ca. 150 mV compared with Au/C. AuAg/C was evaluated in a microfluidic fuel cell operated with glucose as fuel showing good stability and higher performance when was compared with Au/C.     

Performance of a microfluidic microbial fuel cell based on graphite electrodes

A microfluidic microbial fuel cell utilizing the laminar flow to separate the anolyte and catholyte streams based on graphite electrode is proposed. The co-laminar flow of the two streams inside the microchannel is visualized under different flow rates. The effects of the acetate concentration and flow rate on the cell performance are investigated. The results show that the cell performance initially increases and then decreases with increasing influent COD concentration and the anolyte flow rate. The microfluidic microbial fuel cell produces a peak power density of 618 ± 4 mW/m² under the conditions of 1500 mg/L influent COD and an anolyte flow rate of 10 mL/h. The low internal resistance of fuel cell results from elimination of the proton exchange membrane and high surface-to-volume ratio of the microfluidic structure. Moreover, the thickness of biofilm decreases gradually along the flow direction of the microchannel due to the diffusive mixing of the catholyte.     

Energy and exergy analysis of microfluidic fuel cell

Microfluidic fuel cell (MFC) is a promising fuel cell type because its membraneless feature implies great potential for low-cost commercialization. In this study, an energy and exergy analysis of MFC is performed by numerical simulation coupling computational fluid dynamics (CFD) with electrochemical kinetics. MFC system designs with and without fuel recirculation are investigated. The effects of micropump efficiency, fuel flow rate and fuel concentration on the MFC system performance are evaluated. The results indicate that fuel recirculation is preferred for MFC to gain higher exergy efficiency only if the efficiency of the micropump is sufficiently high. Optimal cell operating voltage for achieving the highest exergy efficiency can be obtained. Parasitic effect will cause a significant reduction in the exergy efficiency. An increase in the fuel concentration will also lead to a reduction in the exergy efficiency. Increasing the fuel flow rate in a MFC with fuel recirculation will cause a fluctuating variation in the exergy efficiency. On the other hand, in a one-off MFC system, the exergy efficiency decreases with increasing fuel flow rate. The present work enables better understanding of the energy conversion in MFC and facilitates design optimization of MFC.     

Immobilization of Saccharomyces cerevisiae for application in paper-based microfluidic fuel cell

Microfluidic fuel cells that use microorganisms to oxidize different organic substances to generate electricity are gaining importance due to their versatility to use different fuels. Saccharomyces cerevisiae has used for various purposes due to its capacity to ferment broad spectrum of carbohydrates. In this research, the development of bioanodes based on the immobilization of this yeast was carried out to apply them in the evaluation of a paper lateral-flow microfluidic fuel cell. Immobilization was performed using two different supports, Vulcan carbon and graphene oxide, and four carbohydrates as fuel (saccharose, glucose, fructose, and maltose). The results indicated that the yeast is better distributed and reaches a higher capacity to oxidize carbohydrates when is immobilized on graphene oxide, this bioanode shows better performance in the microfluidic device, reaching a potential above 0.9V when saccharose are used as fuel, representing a promising approach to use microbial bioanodes in small energy conversion devices.     

PdCo supported on multiwalled carbon nanotubes as an anode catalyst in a microfluidic formic acid fuel cell

This work reports the synthesis of Pd-based alloys of Co and their evaluation as anode materials in a microfluidic formic acid fuel cell (μFAFC). The catalysts were prepared using the impregnation method followed by thermal treatment. The synthesized catalysts contain 22 wt.% Pd on multiwalled carbon nanotubes (Pd/MWCNT) and its alloys with two Co atomic percent in the sample with 4 at.% Co (PdCo1/MWCNT) and 10 at.% Co (PdCo2/MWCNT). The role of the alloying element was determined by XRD and XPS techniques. Both catalysts were evaluated as anode materials in a μFAFC operating with different concentrations of HCOOH (0.1 and 0.5 M), and the results were compared to those obtained with Pd/MWCNT. A better performance was obtained for the cell using PdCo1/MWCNT (1.75 mW cm⁻²) compared to Pd/MWCNT (0.85 mW cm⁻²) in the presence of 0.5 M HCOOH. By means of external electrode measurements, it was also possible to observe shifts in the formic acid oxidation potential due to a fuel concentration increment (ca. 0.05 V for both PdCo1/MWCNT and PdCo2/MWCNT catalysts and 0.23 V for Pd/MWCNT) that was attributed to deactivation of the catalyst material. The maximum current densities obtained were 8 mA cm⁻² and 5.2 mA cm⁻² for PdCo2/MWCNT and Pd/MWCNT, respectively. In this way, the addition of Co to the Pd catalyst was shown to improve the tolerance of intermediates produced during formic acid oxidation that tend to poison Pd, thus improving the catalytic activity and stability of the cell.     

Analyses of fuel utilization in microfluidic fuel cell

A microfluidic fuel cell is a miniature power source, which potentially could be used in micro electronic equipments, laptop computers, mobile phones and video cameras. In recent reports, the idea of a microfluidic fuel cell without using a polymer electrolyte membrane is proposed, whereby the laminar nature of the flow in the micro-channels is used to keep the anode and cathode streams separated such that adverse electrochemical reactions do not take place at the two electrode polarities. Since such cells are restricted by their size, improvement in fuel utilization would increase the cell efficiency by several degrees. In the present study, an improvement in fuel utilization is proposed by altering the design of the microfluidic fuel cell. In particular, a sulfuric acid stream is introduced between the fuel (HCOOH) and oxidizer (O₂ in H₂SO₄) streams to improve fuel utilization. Further improvement in fuel utilization is possible by changing the aspect ratio of the cell from 0.1 to 1. The fuel utilization of a cell with an aspect ratio of 0.1 is 14.1%, which increases to 16% when a sulfuric acid stream is introduced to prevent mixing of the fuel and oxidizer streams. The fuel utilization increases to 19% with the change in aspect ratio from 0.1 to 10, which further increases to 32% with the introduction of a sulfuric acid stream.