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.     

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.     

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.     

In situ visualization of biofilm formation in a microchannel for a microfluidic microbial fuel cell anode

A novel in situ approach is proposed to visualize biofilm formation in the microchannel for the microfluidic microbial fuel cell (MMFC) anode, which could reflect a more precise biofilm formation during start-up process in real-time. A microchannel reactor was designed and fabricated based on a transparent indium-tin-oxide (ITO) conductive membrane. In situ visualization of biofilm formation under various anolyte flow rates was captured by a phase contrast microscope combined with a custom long working distance objective. The results show that no steady biofilm is formed on the surface of anode under low flow rate of 50 μL min^?1 because of the insufficient nutrient supply. With increasing the anolyte flow rate, more attached bacteria on the anode surface and denser biofilm are observed in the microchannel. Less bacteria are attached on the surface of anode along flow direction due to the entrance effect. However, denser biofilm leads to larger mass transfer resistance of the anolyte and product in biofilm. Therefore, a superior bioelectrochemical performance is yielded for the biofilm formed under a moderate flow rate during start-up process.     

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.     

Chip-embedded thin film current collector for microfluidic fuel cells

Microfluidic fuel cells are an attractive candidate for low-power applications and provide a unique advantage over traditional fuel cells by elimination of the membrane. More importantly, microfluidic fuel cells enable a simple single-layer structure similar to common lab-on-chip devices, which makes conventional microfabrication or micromachining techniques readily applicable. Microfabrication is a preferable fabrication tool for microscale devices due to the benefits of high precision and repeatability at relatively low cost. However, the performance of most microfluidic fuel cells reported to date was negatively influenced by intrinsic contact resistances arising due to the highly porous nature of the electrodes. In the present work, a chip-embedded thin film current collector for vanadium fueled microfluidic fuel cells is proposed, fabricated, and evaluated as a potential mitigation strategy. The micromachining based thin film process is compatible with the overall cell fabrication, comprising photolithography and soft lithography, and does not require a substantial modification of the original cell design. Cells with and without current collectors are directly compared experimentally: the cell with current collectors demonstrates a 79% increase in peak power density, indicating that the contact resistance is significantly reduced by this approach. A volume specific peak power density of 6.2 W cm⁻³ is achieved, which is significantly higher than for previously reported microfluidic fuel cells. Electrochemical impedance spectroscopy (EIS) analysis is carried out to measure the combined ohmic cell resistance and confirmed a 32% reduction using the current collectors, which shows a good agreement with slope decrements in the polarization curves.     

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.     

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.     

A review on membraneless laminar flow-based fuel cells

The review article provides a methodical approach for understanding membraneless laminar flow-based fuel cells (LFFCs), also known as microfluidic fuel cells. Membraneless LFFCs benefit from the lamination of multiple streams in a microchannel. The lack of convective mixing leads to a well-defined liquid-liquid interface. Usually, anode and cathode are positioned at both sides of the interface. The liquid-liquid interface is considered as a virtual membrane and ions can travel across the channel to reach the other side and complete the ionic conduction. The advantage of membraneless LFFC is the lack of a physical membrane and the related issues of membrane conditioning can be eliminated or becomes less important. Based on the electrode architectures, membraneless LFFCs in the literature can be categorized into three main types: flow-over design with planar electrodes, flow-through design with three-dimensional porous electrodes, and membraneless LFFCs with air-breathing cathode. Since this paper focuses on reviewing the design considerations of membraneless LFFCs, a concept map is provided for understanding the cross-related problems. The impacts of flow and electrode architecture on cell performance and fuel utilization are discussed. In addition, the main challenges and key issues for further development of membraneless LFFCs are discussed.     

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.     

Microfluidic fuel cells: A review

A microfluidic fuel cell is defined as a fuel cell with fluid delivery and removal, reaction sites and electrode structures all confined to a microfluidic channel. Microfluidic fuel cells typically operate in a co-laminar flow configuration without a physical barrier, such as a membrane, to separate the anode and the cathode. This review article summarizes the development of microfluidic fuel cell technology, from the invention in 2002 until present, with emphasis on theory, fabrication, unit cell development, performance achievements, design considerations, and scale-up options. The main challenges associated with the current status of the technology are provided along with suggested directions for further research and development. Moreover, microfluidic fuel cell architectures show great potential for integration with biofuel cell technology. This review therefore includes microfluidic biofuel cell developments to date and presents opportunities for future work in this multi-disciplinary field.     

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.     

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.     

Electricity generation with a sediment microbial fuel cell equipped with an air-cathode system using photobacterium

The main purpose of this study is the characteristic and nature of current generation with a pure culture of single cell in a sediment microbial fuel cell. A sediment microbial fuel cell with an air-cathode system was studied for a prolonged period of time. The current maintained a steady increase throughout the entire time period and reached to its peak of 1.82 μA with power density of 29,024.65 μW/cm² at day 35. Water parameters such as salinity and pH were observed throughout the entire time period for better understanding. Operation of water parameter had been done after stabilization of current output for every measurement. The electron transfer pathway was assessed by cyclic voltammetry study. A low current density was observed due to profound internal resistance (141 Ω), and the reason for which was ohmic losses. A linear relationship was observed between current density and power density. Phylogenetic analysis was performed with 16S rRNA to identify the studied organism.     

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.     

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.     

Improved performance of a tubular microbial fuel cell with a composite anode of graphite fiber brush and graphite granules

In this study, a composite electrode combined of a graphite fiber brush and carbon granules (MFC-GFB/GG) was adopted as the anode of a tubular microbial fuel cell (MFC). Compared with an MFC with graphite granules (MFC-GG) and an MFC with a graphite fiber brush (MFC-GFB), MFC-GFB/GG showed a longer lag time during the start-up process, while it reached the highest operating voltage at 50 Ω. Furthermore, during the stable operation, the MFC-GFB/GG achieved the highest power density of 66.9 ± 1.6 W m⁻³, which was about 5.3 and 1.2 times as that of MFC-GG and MFC-GFB, respectively. The highest performance of the MFC-GFB/GG can be attributed to the highest active biomass content on the electrode and the smallest internal resistance of the MFC. The optimum COD concentration was found to be 500 mg COD L⁻¹ for MFC-GFB/GG.     

Orthogonal flow membraneless fuel cell

This communication reports the development and performance of a membraneless fuel cell that utilizes a convective electrolyte flow orthogonal to the plane of the cell electrodes. The orthogonal flow acts to convect reactants to the electrodes and as a pseudo-membrane between anode and cathode, preventing mixed potentials caused by back diffusion of oxidant onto the anode. The membraneless design allows the cell parameters to be defined by the fuel rather than the membrane. The convective electrolyte flow eliminates the mass transport limited current regime and allows the cell to run under semi-mixed conditions. Power densities as high as 46 mW cm⁻² were achieved with both hydrogen and methanol as fuels. Methanol had a fuel utilization of 42%.     

Analysis of microbial diversity of inocula used in a five-face parallelepiped and standard microbial fuel cells

This work had a double purpose: (i) to study the effect of sulphate-reducing (SR-In) and enriched (E-In) inocula on the characteristics of one-chamber standard microbial fuel cell (MFC-S) and parallelepiped cell and (ii) to analyze the bacterial communities in cells operated with either SR-In or E-In. The MFC-S of 150 mL consisted of one-chamber plexiglass cell with electrodes separated 7.8 cm. The MFC-P consisted of a parallelepiped built in plexiglass with a liquid volume of 270 mL. Five faces of this cell were fitted with ‘sandwich’ cathode–membrane–anode assemblages (CMA). The values of internal resistance (R_int) were 4602 and 687 Ω, for the MFC-S loaded with SR-In and E-In, respectively. The values of R_int were 400 and 84, and 292 and 80 Ω for the faces connected in series and parallel and the MFC-P loaded with SR-In and E-In, respectively. Parallel connection of cell faces also significantly improved the electrochemical characteristics of the P cell (higher powers). In general, use of E-In in both types of MFC lead to improved power densities compared to SR-In. Molecular biology analysis of microbial communities showed that the E-In was less diverse than SR-In (in terms of phyla). An electrochemically active bacterium Geovibrio ferrireducens belonging to phylum Deferribacteres was found in the E-In. Predominance of Deferribacteres was observed in the E-In. Members of this phylum were not found in the SR-In.     

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.