A direct formate microfluidic fuel cell with cotton thread-based electrodes

A direct formate microfluidic fuel cell with cotton thread-based electrodes is proposed. The palladium catalyst is directly coated on cotton threads by repeated dipping method to prepare electrodes, which integrates the flow channel and electrode together and provides exposed active sites for enhancing the mass transfer on the anode and cathode. The aqueous anolyte and catholyte transport through cotton threads by capillary force with aid of gravity, eliminating the use of any external pump and facilitating the integration and miniaturization of the whole system. In the experiment, a three-flow channel structure is employed. The fuel is sodium formate and the oxidant is hydrogen peroxide. 1 M Na₂SO₄ solution is introduced into the middle channel formed by cotton threads with no catalyst to alleviate the reactant crossover. Performance is evaluated under various catalyst loadings, fuel concentrations and differences in height between the inlet and outlet. Results show that the fuel cell produces an open circuit voltage (OCV) of 1.41 V. The maximum current density of 74.56 mA cm⁻² and the peak power density of 24.75 mW cm⁻² are yielded when the palladium loading is 1 mg cm⁻¹ and the difference in height between the inlet and outlet is 7 cm, using 4 M HCOONa as fuel. Furthermore, the performance of the fuel cell increases first and then decreases with increasing the palladium loading. The same variation is observed with increasing the fuel concentration. However, the performance gradually increases with increasing the difference in height from 3 cm to 7 cm. The proposed microfluidic fuel cell with cotton thread-based electrodes shows enormous potential as a micro power source for portable devices.     

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.     

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.     

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.     

Chrome and cobalt-based novel electrolyte systems for redox flow batteries

In the present work, novel redox ion-pairs (as cobalt and chromium) have been used in aqueous medium for the first time in the literature as electrolyte component of redox flow battery system. The electrochemical performance of the Co(II) and Cr(III) redox species as anolyte and catholyte was investigated by cyclic charge-discharge tests, respectively. Electrochemical behaviors of Cr(III) solutions in sulfuric acid solution were determined by using differential pulse voltammetry, electrochemical impedance spectroscopy and cyclic voltammetry via a typical three-electrode system. Morphological analyses of surface of pencil graphite electrode, which was used as anode in differential pulse voltammetric analysis, were done by scanning electron microscopy. Discharge capacity of the battery system consisting of 1.0 M Cr(III) as anolyte (negative electrolyte) and 1.0 M of Co(II) as catholyte (positive electrolyte) in 4.0 M of sulfuric acid was determined as 682.5 mAh (1.4 Ah L⁻¹) with 4 mA cm⁻² charge current density and 0.4 mA cm⁻² discharge current density. Voltage efficiency, energy efficiency and coulombic efficiency of the battery were 70.1%, 53.8% and 57.2%, respectively. The discharge cell potential of the battery was also determined as 1.40 V.     

High performance aluminum-air battery for sustainable power generation

Metal-air battery is receiving vast attention due to its promising capabilities as an energy storage system for the post lithium-ion era. The electricity is generated through oxidation and reduction reaction within the anode and cathode. Among various types of metal-air battery, aluminum-air battery is the most attractive candidate due to its high energy density and environmentally friendly. In this study, a novel polypropylene-based dual electrolyte aluminum-air battery is developed. Polypropylene pads are used as a medium to absorb the electrolyte, isolate the anode and cathode, control the hydrogen generation in the parasitic reaction. Potassium hydroxide is used as anolyte and sulfuric acid is used as catholyte. Parametric study is conducted to investigate the effect of electrolyte concentration and polypropylene separator thickness on the performance of the battery. The results show that the dual-electrolyte system can boost the open circuit voltage to 2.2 V as compared to the single electrolyte system for 5 M of anolyte while maintaining specific discharge capacity of about 1390.92 mAh.g⁻¹. The maximum peak power density has improved dramatically from 100 mW.cm⁻² to 350 mW cm⁻² for the dual electrolyte system.     

A flexible on-fiber H2O2 microfluidic fuel cell with high power density

To promote the simplification and integration of membraneless microfluidic fuel cell (MMFC) system and combine with flexible portable devices, a flexible on-fiber MMFC exploiting H₂O₂ as sole reactant is presented, eliminating the separation requirement of fuel and oxidant. Nickel (Ni) nano-particles and Prussian blue with multiwalled carbon nanotube (PB-MWCNT) are coated on hydrophilic braided carbon fibers (BCFs) to serve as the anode and cathode, respectively. The three-dimensional (3D) flow-through anode and cathode with a wealth of exposed electroactive sites improve reactant mass transfer. The anode and cathode are respectively wound on both sides of the middle cotton thread-based flow channel for separation. Under the combination of capillary force and gravity, reactants flow continuously through the fiber-based microchannels without external pumps. Importantly, the H₂O₂ MMFC achieves the highest maximum power density (MPD) of 14.41 mW cm^?2 so far in one-chamber or single-stream H₂O₂ fuel cells. Besides, no serious deterioration in the power-generation performance is observed in complex practical operating conditions including bending with various angles, repeated folding and dropping. Three presented flexible MMFCs are connected to power a handheld calculator, indicating the tremendous potential of developing micro power supplies based on abundant flexible materials as well as green and sustainable energy.     

Bipolar polymer electrolyte interfaces for hydrogen–oxygen and direct borohydride fuel cells

Direct borohydride fuel cells (DBFCs) using liquid hydrogen peroxide as the oxidant are safe and attractive low temperature power sources for unmanned underwater vehicles (UUVs) as they have excellent energy and power density and do not feature compressed gases or a flammable fuel stream. One challenge to this system is the disparate pH environment between the anolyte fuel and catholyte oxidant streams. Herein, a bipolar interface membrane electrode assembly (BIMEA) is demonstrated for maintaining pH control of the anolyte and catholyte compartments of the fuel cell. The prepared DBFC with the BIMEA yielded a promising peak power density of 110 mW cm⁻². This study also investigated the same BIMEA for a hydrogen–oxygen fuel cell (H₂–O₂ FC). The type of gas diffusion layer used and the gas feed relative humidity were found to impact fuel cell performance. Finally, a BIMEA featuring a silver electrocatalyst at the cathode in a H₂–O₂ FC was successfully demonstrated.     

Fabrication and evaluation of passive alkaline membraneless microfluidic DMFC

The fabrication and evaluation of a passive, air-breathing, membraneless microfluidic direct methanol fuel cell (ML-μDMFC) using a methanol-tolerant Ag/Pt/CP cathode is presented here. We previously proposed that due to its high tolerance to methanol and the good activity towards the oxygen reduction reaction in alkaline medium, this catalyst could be useful to reduce the methanol crossover effect in direct methanol fuel cells. Therefore, in order to demonstrate it, we designed and fabricated a microfluidic device that allowed the evaluation of the cathode in a high fuel concentration environment, using up to 5 M MeOH in 0.5 M KOH in passive mode. The results confirmed the high tolerance to MeOH and the ORR selectivity of the Ag/Pt/CP cathode, in contrast with a Pt/CP cathode, where performance decreased severely due to the methanol crossover. Employing the methanol-tolerant cathode, it was possible to obtain a power density of 2.4 mW cm⁻². Additionally, the durability studies revealed more stability for the ML-μDMFC using the bimetallic catalyst, compared with Pt/CP.     

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.     

Chaotic flow-based fuel cell built on counter-flow microfluidic network: Predicting the over-limiting current behavior

The membraneless microfluidic fuel cell (MFC) is a promising micro-scale power source with potentially wide applications. MFC commonly relies on the co-laminar microfluidic platform in which redox streams flow in parallel in a microchannel. The nature of this cell architecture limits the mass transport inside the cell, often resulting in low power density. To overcome the issues, we propose an innovative concept of chaotic flow-based fuel cell (CFFC), which is built on a counter-flow microfluidic platform with the flow channel patterned with micro-ridges. A CFD/electrochemical model is used to predict the performance and investigate the underlay mechanism of the CFFC. Two theoretical upper bounds, i.e., the limiting current and limiting fuel conversion for conventional MFC, are derived. Through the results, it is found that the generation of chaotic flow inside the patterned activation zone enables the CFFC to exceed the theoretical limitations and work with over-limiting current for high-power output. Meanwhile, the interfacial mixing and crossover is minimized by the counter-flow microfluidics, allowing for over-limiting fuel conversion to useful electricity output. The achievement of unprecedented operating regime demonstrated in this study open up a new direction towards optimization, operating and design of the MFC.     

A membraneless microscale fuel cell using non-noble catalysts in alkaline solution

This paper presents the development of a novel liquid-based microscale fuel cell using non-noble catalysts in an alkaline solution. The developed fuel cell is based on a membraneless structure. The operational complications of a proton exchange membrane lead the development of a fuel cell with the membraneless structure. Non-noble metals with relatively mild catalytic activity, nickel hydroxide and silver oxide, were employed as anode and cathode catalysts to minimize the effect of cross-reactions with the membraneless structure. Along with nickel hydroxide and silver oxide, methanol and hydrogen peroxide were used as a fuel at anode and an oxidant at cathode. With a fuel mixture flow rate of 200 μl min⁻¹, a maximum output power density of 28.73 μW cm⁻² was achieved. The developed fuel cell features no proton exchange membrane, inexpensive catalysts, and simple planar structure, which enables high design flexibility and easy integration of the microscale fuel cell into actual microfluidic systems and portable applications.     

A spiral shaped regenerative microfluidic fuel cell with Ni‐C based porous electrodes

Microchannel geometry, electrode surface area, and better fuel utilization are important aspects of the performance of a microfluidic fuel cell (MFC). In this communication, a membraneless spiral‐shaped MFC fabricated with Ni as anode and C as a cathode supported over a porous filter paper substrate is presented. Vanadium oxychloride and dilute sulfuric acid solutions are used as fuel and electrolyte, respectively, in this fuel cell system. The device generates a maximum open‐circuit voltage of ~1.2 V, while the maximum energy density and current density generated from the fuel cell are ~10 mW cm^?2 and ~51 mA cm^?2, respectively. The cumulative energy density generated from the device after five cycles are measured as ~200 mW after regeneration of the fuel by applying external voltage. The spiral design of the fuel cell enables improved fuel utilization, rapid diffusive transport of ions, and in‐situ regeneration of the fuel. The present self‐standing spiral‐shaped MFC will eliminate the challenges associated with two inlet membrane‐less fuel cells and has the potential to scale up for commercial application in portable energy generation.     

Performance optimization of microfluidic paper fuel-cell with varying cellulose fiber papers as absorbent pad

Miniaturization of devices, combined with other features such as portability, proneness to automation, rapid performance, amenability to integration, multiplexing, and cost-effectiveness, is rapidly increasing for various sensing and energy harvesting applications. One such emerging area is the development of microfluidic fuel cell on cellulose papers, which has enormous scope to optimize its performance. This is primarily because such devices eliminate the need for membranes as well as external pumps since they have built-in colaminar flow embedded capillaries. Such peripherals are usually used in conventional microfluidic fuel cells, which are fabricated using methods like photolithography, PDMS lithography, and 3D printing. This paper presents investigations on microfluidic paper–based fuel cells (MPFCs) with different cellulose absorbent pads for their performance optimization. Herein, the MPFC utilizes formic acid as fuel, oxygen from quiescent air as oxidant, and sulfuric acid as electrolyte for conducting ionic exchange under colaminar flow. The electrodes are realized through simple pencil strokes depositing a thin layer of graphite. The porous graphite electrodes act as diffusion agents breathing oxygen directly from the atmospheric air. Such an MPFC configuration, costing less than US $1, was optimized to achieve maximum energy density by examining various combinations of absorbent pads with different grades of cellulose papers. It is seen that the maximum open circuit potential is 0.46 V, while the maximum current and power densities are 1505.66 μAcm⁻² and 173.97 μWcm⁻², respectively, with a grade 6 absorbent pad. Such performance can be further enhanced by investigating MPFCs with various graphite pencils with a diverse number of strokes at different concentration levels.     

Studies of a granular aluminum anode in an alkaline fuel cell

A granular aluminum anode was investigated for use in an alkaline aluminum/hydrogen peroxide fuel cell. The fuel cell utilizes granules of aluminum (8–12 mm in diameter) as an anode, potassium hydroxide (KOH) as an anolyte and hydrogen peroxide as a catholyte. Granular anodes have a significantly higher surface area than planar surfaces, thereby resulting in higher utilization of the anode material. Polarization experiments were performed as well as closed circuit power production experiments. KOH concentrations were varied in the experiments. Polarization experiments achieved a current density of 10.02 mA/cm² using 2 M KOH and granular aluminum with a surface area of 205.6 cm². Power production experiments sustained a current density of 0.05 mA/cm² using 1.5 M KOH and granular aluminum with a surface area of 59.8 cm². Results indicate that granular metal anodes have potential for use in high energy density fuel cells.     

Effects of transport scale on heat/mass transfer and performance optimization for solid oxide fuel cells

A three-dimensional thermo-fluid–electrochemical model is developed to study the heat/mass transport process and performance of a solid oxide fuel cell (SOFC). The main objectives are to examine the transport channel size effects and to assess the potential of a thin-film-SOFC. A parametric study was performed to evaluate the channel scale effects on the temperature, species concentration, local current density and power density. The results demonstrate that decreasing the height of flow channels can lower the average solid temperature and improve cell efficiency. However, this improvement is rather limited for the smallest channels. Compared with the conventionally sized SOFC, the miniaturized SOFC with a thin-film electrolyte has the advantages of a lower operating temperature and a better performance. Based on our simulation results, the power density of a miniaturized SOFC could reach up to 5.461 W cm⁻³. However, an extremely small structure will lead to severe thermal stress induced by a large temperature gradient, a cell with a thicker rib width would have a higher efficiency and a lower average temperature. Numerical simulation is expected to help optimize the design of a solid oxide fuel cell.     

A study of the direct dimethyl ether fuel cell using alkaline anolyte

The electrooxidation behavior of dimethyl ether (DME) dissolved in acidic, neutral or alkaline anolyte has been studied. The cyclic voltammetry measurements reveal that DME in alkaline anolyte demonstrates higher electrooxidation reactivity than that in acidic or neutral anolyte. With increasing the NaOH concentration in the anolyte, the electrooxidation reactivity of DME can be further improved. Direct dimethyl ether fuel cells (DDFCs) are assembled by using Nafion membrane as the electrolyte, Pt/C as the cathode catalyst, and Pt-Ru/C as the anode catalyst. It is found that the use of alkaline anolyte can significantly improve the performance of DDFCs. A maximum power density of 60 mW cm⁻² has been achieved when operating the DDFC at 80 °C under ambient pressure.     

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.     

Performance of alkaline electrolyte-membrane-based direct ethanol fuel cells

A single alkaline direct ethanol fuel cell (alkaline DEFC) with an anion-exchange membrane and non-platinum (non-Pt) catalysts is designed, fabricated, and tested. Particular attention is paid to investigating the effects of different operating parameters, including the cell operating temperature, concentrations of both ethanol and the added electrolyte (KOH) solution, as well as the mass flow rates of the reactants. The alkaline DEFC yields a maximum power density of 60 mW cm⁻², a limiting current density of about 550 mA cm⁻², and an open-circuit voltage of about 900 mV at 40 °C. The experimental results show that the cell performance is improved on increasing the operating temperature, but there exists an optimum ethanol concentration under which the fuel cell has the best performance. In addition, cell performance increases monotonically with increasing KOH concentration in the region of low current density, while in the region of high current density, there exists an optimum KOH concentration in terms of cell performance. The effect of flow rate of the fuel solution is negligible when the ethanol concentration is higher than 1.0 M, although the cell performance improves on increasing the oxygen flow rate.     

The importance of OH− transport through anion exchange membrane in microbial electrolysis cells

In two-chamber microbial electrolysis cells (MECs) with anion exchange membranes (AEMs), a phosphate buffer solution (PBS) is typically used to avoid increases in catholyte pH as Nernst equation calculations indicate that high pHs adversely impact electrochemical performance. However, ion transport between the chambers will also impact performance, which is a factor not included in those calculations. To separate the impacts of pH and ion transport on MEC performance, a high molecular weight polymer buffer (PoB), which was retained in the catholyte due to its low AEM transport and cationic charge, was compared to PBS in MECs and abiotic electrochemical half cells (EHCs). In MECs, catholyte pH control was less important than ion transport. MEC tests using the PoB catholyte, which had a higher buffer capacity and thus maintained a lower catholye pH (<8), resulted in a 50% lower hydrogen production rate (HPR) than that obtained using PBS (HPR = 0.7 m³-H₂ m^?3 d^?1) where the catholyte rapidly increased to pH = 12. The main reason for the decreased performance using PoB was a lack of hydroxide ion transfer into the anolyte to balance pH. The anolyte pH in MECs rapidly decreased to 5.8 due to a lack of hydroxide ion transport, which inhibited current generation by the anode, whereas the pH was maintained at 6.8 using PBS. In abiotic tests in ECHs, where the cathode potential was set at ?1.2 V, the HPR was 133% higher using PoB than PBS due to catholyte pH control, as the anolyte pH was not a factor in the performance. These results show that maintaining charge transfer to control anolyte pH is more important than obtaining a more neutral pH catholyte.