Microfluidic Fuel Cell Research Articles

Converting Energy with Glycerol and CO2 in a Microfluidic Fuel Cell Equipped with CuBiO4/CuO Photocathode: Bypassing Bubbles Challenge of Concurrent Water Splitting.

The imperative to address CO2 emissions has prompted the search for alternative approaches to capture this gas with minimal energy consumption. In this context, leveraging the CO2 reduction reaction (CO2RR) as an oxidant in fuel cells has emerged as a sophisticated strategy to convert this gas into usable energy. This study introduces a hybrid microfluidic photo fuel cell (μPFC) designed for the efficient conversion of CO2 and glycerol into electrical energy. The prototype integrates 3D-printed components with glass sealing, enabling precise control over the reactant flow and the use of light-sensitive catalysts. The anodic glycerol electrooxidation was investigated on Pt/C dispersed on carbon paper (CP), while the CO2RR was carried out on CuBiO4/CP and CuBiO4/CuO/CP in the presence of solar light. Half-cell measurements demonstrate the photoactivity of CuBiO4/CuO/CP and CuBiO4/CP electrodes for the CO2RR under light exposure at low onset potential in a neutral pH solution, generating a positive theoretical open-circuit voltage of 0.89-0.91 V when coupled to glycerol electrooxidation in an alkaline medium. The use of the mixed medium in the membraneless system equipped with the photosensitive catalysts allowed the building of this galvanic cell. However, the feasibility of using CuBiO4/CP is hindered by the disruption of the colaminar channel caused by hydrogen bubbles produced during concurrent water splitting. In contrast, the μPFC equipped with a CuBiO4/CuO/CP photocathode demonstrates a stable and reproducible performance, delivering a maximum power density of 0.9 mW cm-2. The formation of the CuBiO4/CuO heterojunction effectively suppresses photocatalytic water splitting, allowing for efficient CO2 conversion without disruption of the laminar flow channel. This innovative approach highlights the potential of μPFCs as sustainable energy converters for the utilization of CO2 in aqueous solutions, offering a pathway toward carbon-neutral energy production.

Operando Spectroelectrochemical Imaging of Concentration Fields and Tafel Kinetics in Microfluidic Electrochemical Devices.

With the development of microtechnologies for energy conversion and storage, mass transfer and micromolar concentration variations need to be measured at the microscale. These advances need to be accompanied by novel imaging techniques with the capability of achieving high spatial resolution while detecting very small signal variations (less than 0.1%). Thus, in this study, a new microscopy technique is proposed based on a combination of electrochemical impedance spectroscopy (EIS) and visible imaging spectroscopy to measure the concentration fields at the micromolar scale in operando microfluidic fuel cells (MFCs). This technique exploits EIS modulation and Fourier analysis to reduce the noise during concentration field imaging. A mass transfer model in the periodic regime is derived to validate the measurements and to estimate the Tafel kinetics and mass diffusivities during potassium permanganate reduction from only one potential measurement. The proposed imaging method and mathematical framework presented in this study can be used to study binary electrochemical reactions without gas production.

Microfluidic fuel cell with arc-shaped electrodes to adapt to its mixing zone, a simulation study

Fuel cells are well known for their uninterrupted power supply, high energy density, and environmental friendliness. Among them there is an emerging type for portable applications called microfluidic fuel cell (MFC), which has caught attention during the last twenty years. An MFC generally employs two electrolytes, namely the anolyte containing fuel and the catholyte containing oxidant, which flow in parallel inside a microchannel. In the middle, a narrow mixing zone is formed which has a typical cross section of hourglass shape. To better utilize this specific shape, an MFC with innovative arc-shaped electrodes is proposed in this work and validated via numerical simulation. The protruding electrode surface towards the channel middle can not only better utilize the limited channel space for more reaction sites, but also reshape the mixing layer to further prevent reactant crossover. Benefited from the enhanced convective transport as well as diffusive transport, the maximum power density with electrode radius of 2 mm is improved by 18.9% at the flow rate of 1000 μL/min and 20.7% at the flow rate of 100 μL/min, compared with conventional flat electrodes. Besides, the fuel utilization at 0.8 V is also improved by 30.4% at 1000 μL/min and 32.6% at 100 μL/min. This work provides a brand-new idea of optimizing the shape of MFC electrode, which can improve MFC performance indexes of both power density and fuel utilization at the same time.

Defying Gravity to Enhance Power Output and Conversion Efficiency in a Vertically Oriented Four-Electrode Microfluidic Microbial Fuel Cell.

High power output and high conversion efficiency are crucial parameters for microbial fuel cells (MFCs). In our previous work, we worked with microfluidic MFCs to study fundamentals related to the power density of the MFCs, but nutrient consumption was limited to one side of the microchannel (the electrode layer) due to diffusion limitations. In this work, long-term experiments were conducted on a new four-electrode microfluidic MFC design, which grew Geobacter sulfurreducens biofilms on upward- and downward-facing electrodes in the microchannel. To our knowledge, this is the first study comparing electroactive biofilm (EAB) growth experiencing the influence of opposing gravitational fields. It was discovered that inoculation and growth of the EAB did not proceed as fast at the downward-facing anode, which we hypothesize to be due to gravity effects that negatively impacted bacterial settling on that surface. Rotating the device during the growth phase resulted in uniform and strong outputs from both sides, yielding individual power densities of 4.03 and 4.13 W m-2, which increased to nearly double when the top- and bottom-side electrodes were operated in parallel as a single four-electrode MFC. Similarly, acetate consumption could be doubled with the four electrodes operated in parallel.

Experimental Proof of Principle of 3D-Printed Microfluidic Benthic Microbial Fuel Cells (MBMFCs) with Inbuilt Biocompatible Carbon-Fiber Electrodes.

Microbial fuel cells (MFCs) represent a promising avenue for sustainable energy production by harnessing the metabolic activity of microorganisms. In this study, a novel design of MFC-a Microfluidic Benthic Microbial Fuel Cell (MBMFC)-was developed, fabricated, and tested to evaluate its electrical energy generation. The design focused on balancing microfluidic architecture and wiring procedures with microbial community dynamics to maximize power output and allow for upscaling and thus practical implementation. The testing phase involved experimentation to evaluate the performance of the MBMFC. Microbial feedstock was varied to assess its impact on power generation. The designed MBMFC represents a promising advancement in the field of bioenergy generation. By integrating innovative design principles with advanced fabrication techniques, this study demonstrates a systematic approach to optimizing MFC performance for sustainable and clean energy production.

A Comprehensive Investigation of Methanol Electrooxidation on Copper Anodes: Spectroelectrochemical Insights and Energy Conversion in Microfluidic Fuel Cells.

Here, we comprehensively investigated methanol electrooxidation on Cu-based catalysts, allowing us to build the first microfluidic fuel cell (μFC) equipped with a Cu anode and a metal-free cathode that converts energy from methanol. We applied a simple, fast, small-scale, and surfactant-free strategy for synthesizing Cu-based nanoparticles at room temperature in steady state (ST), under mechanical stirring (MS), or under ultrasonication (US). The morphology evaluation of the Cu-based samples reveals that they have the same nanoparticle (NP) needle-like form. The elemental mapping composition spectra revealed that pure Cu or Cu oxides were obtained for all synthesized materials. In addition to having more Cu2O on the surface, sample US had more Cu(OH)2 than the others, according to X-ray diffractograms and X-ray photoelectron spectroscopy. The sample US is less carbon-contaminated because of the local heating of the sonic bath, which also enhances the cleanliness of the Cu surface. The activity of the Cu NPs was investigated for methanol electrooxidation in an alkaline medium through electrochemical and spectroelectrochemical measurements. The potentiodynamic and potentiostatic experiments showed higher current densities for the NPs synthesized in the US. In situ FTIR experiments revealed that the three synthesized NP materials eletcrooxidize methanol completely to carbonate through formate. Most importantly, all pathways were led without detectable CO, a poisoning molecule not found at high overpotentials. The reaction path using the US electrode experienced an additional round of formate formation and conversion into carbonate (or CO2 in the thin layer) after 1.0 V (vs. Ag/Ag/Cl), suggesting improved catalysis. The high activity of NPs synthesized in the US is attributed to effective dissociative adsorption of the fuel due to the site's availability and the presence of hydroxyl groups that may fasten the oxidation of adsorbates from the surface. After understanding the surface reaction, we built a mixed-media μFC fed by methanol in alkaline medium and sodium persulfate in acidic medium. The μFC was equipped with Cu NPs synthesized in ultrasonic-bath-modified carbon paper as the anode and metal-free carbon paper as the cathode. Since the onset potential for methanol electrooxidation was 0.45 V and the reduction reaction revealed 0.90 V, the theoretical OCV is 0.45 V, which provides a spontaneous coupled redox reaction to produce power. The μFC displayed 0.56 mA cm-2 of maximum current density and 26 μW cm-2 of peak power density at 100 μL min-1. This membraneless system optimizes each half-cell individually, making it possible to build fuel cells with noble metal-free anodes and metal-free cathodes.

Interrelationship of Electric Double Layer Theory and Microfluidic Microbial Fuel Cells: A Review of Theoretical Foundations and Implications for Performance

Microbial fuel cells and their related microfluidic systems have emerged as promising greener energy alternatives for the exploitation of avenues related to combined power and wastewater treatment operations. Moreover, the potential for their application in biosensing technology is large. However, while the fundamental principles of science that govern the design and operation of microbial fuel cells (MFCs) and microfluidic microbial fuel cells (MMFCs) are similar to those found in colloid science, the literature shows that current research lacks sufficient reference to the electrostatic and electrokinetic aspects, focusing mostly on aspects related to the architecture, design, anodes, microbial growth and metabolism, and electron transfer mechanisms. In this regard, research is yet to consider MFCs and MMFCs in the context of electrostatic and electrokinetic aspects. In this extensive review, we show, for the first time, the interrelationship of MFCs and MMFCs with electric double layer theory. Consequently, we show how the analytical solution to the mean field Poisson–Boltzmann theory relates to these systems. Moreover, we show the interrelationship between MFC and MMFCs’ performance and the electric double layer and the associated electrostatic and electrokinetic phenomena. This extensive review will likely motivate research in this direction.

Modeling and Sizing of a Y-Shaped Laminar Flow Micro-Fluidic Fuel Cell

The high demand for portable and desktop electronic devices requires mobile energy sources. Electronic devices, in general, are small when they are portable and require autonomy, sustainability, efficiency, and large enough periods of operability. In that sense, microtechnology, a type of technology related to the manufacture of prototypes or three-dimensional high-precision products in various materials ranging in size from tenths of microns to a few millimeters, has reached a boom in microfabrication of devices that are not exclusively electronic. Applications in several fields show that micro-energy sources are a focal point for commercializing prototypes in the mobility sector.The energy demand to supply micro-devices has been increasing in recent years. Considering the power output of the laminar flow microfluidic fuel cell (LFFC), it is a suitable solution to provide the required electrical energy in small devices. The absence of electrolytes and no requirement of platinum as catalytic material are two of the most important advantages of this type of fuel cell. The current study provides detailed information about the design and characteristics of an LFFC. A complete analysis of the different channel shapes is considered in this study, with the Y-shaped channel being selected as the best option. The impact of the inclination degree for the inlet channels has been considered to evaluate the average velocity the flow acquires in the channel. In addition, the voltage-current behavior considering the materials, fuel/oxidant, and design characteristics has been obtained through modeling approaches. Figure 1. Velocity profile and Reynolds number in the mid-plane of the Y-channel design Figure 1

Abiotic, Hybrid, and Biological Electrocatalytic Materials Applied in Microfluidic Fuel Cells: A Comprehensive Review.

This article provides an overview of the work reported in the past decade in the field of microfluidic fuel cells. To develop appropriate research, the most commonly used electrocatalytic materials were considered and a new classification was proposed based on their nature: abiotic, hybrid, or biological. This classification allowed the authors to discern the information collected. In this sense, the types of electrocatalysts used for the oxidation of the most common fuels in different environments, such as glucose, ethanol, methanol, glycerol, and lactate, were presented. There are several phenomena presented in this article. This information gives an overview of where research is heading in the field of materials for electrocatalysis, regardless of the fuel used in the microfluidic fuel cell: the synthesis of abiotic and biological materials to obtain hybrid materials that allow the use of the best properties of each material.

Modeling and Sizing of a Y-Shaped Laminar Flow Micro-Fluidic Fuel Cell

The high demand for portable and desktop electronic devices requires mobile energy sources. The energy demand to supply micro-devices has been increasing in recent years. Considering the power output of the membraneless microfluidic fuel cell (MMFC), it is a suitable solution to provide the required electrical energy in small devices. The absence of electrolytes and no requirement of platinum as catalytic material are two of the most important advantages of this type of fuel cell. The current study provides detailed information about the design and characteristics of an MMFC. A complete analysis of the different channel shapes is considered in this study, with the Y-shaped channel being selected as the best option. The impact of the inclination degree for the inlet channels has been considered to evaluate the average velocity the flow acquires in the channel flow. In addition, the voltage-current behavior considering the materials, fuel/oxidant, and design characteristics has been obtained through modeling approaches.

Increasing Output Power of a Microfluidic Fuel Cell Using Fuzzy Modeling and Jellyfish Search Optimization

Increasing Output Power of a Microfluidic Fuel Cell Using Fuzzy Modeling and Jellyfish Search Optimization

A zero energy-input nitrogen removal reactor based on a short-circuited microfluidic fuel cell.

Excessive discharge of ammonia nitrogen would deteriorate water quality. In this work, we designed an innovative microfluidic electrochemical nitrogen-removal reactor (MENR) based on a short-circuited ammonia-air microfluidic fuel cell (MFC). The MENR utilizes the laminar characteristics of two flows (an anolyte containing nitrogen-rich wastewater and a catholyte of acid electrolyte solution) in a microchannel to establish an efficient reactor system. At anode, ammonia was catalyzed by a NiCu/C modified electrode to N2, while O2 in the air was reduced at cathode. In essence, the MENR reactor is a short-circuited MFC. Maximum discharge currents were achieved accompanied with strong ammonia oxidation reaction. Factors indicating electrolyte flow rate, initial nitrogen concentration, electrolyte concentration, and electrode geometry have various effects on the nitrogen removal performance of the MENR. Results indicate that the MENR showed efficient nitrogen removal properties. This work proposes an energy-saving process by using the MENR to remove nitrogen from ammonia-rich wastewater.

Carbon Shell-Confined Pd-Fe Bimetallic Nanoparticles in NCNTs as Anode Catalysts for Microfluidic Direct Ethylene Glycol Fuel Cells

Carbon Shell-Confined Pd-Fe Bimetallic Nanoparticles in NCNTs as Anode Catalysts for Microfluidic Direct Ethylene Glycol Fuel Cells

A breakthrough in the performance and fuel utilization of methanol microfluidic fuel cell via viscous co-flow electrolyte

Microfluidic fuel cells (MFCs) employ laminar flows to eliminate the expensive polymer electrolyte membrane and overcome the various drawbacks caused by it. However, there is a trade-off between their electrochemical performance and fuel utilization, especially with methanol fuel due to its high diffusivity in water. Also, the power density and fuel utilization of existing methanol MFCs still need improvement. In this work, a MFC with an alkaline anolyte and acidic catholyte co-flow configuration was designed to achieve high open-circuit voltage and high power density. To better form a steady alkaline-acid co-flow interface and to suppress the methanol crossover, gelling polymers were added to increase the density and viscosity of the electrolytes. This reduced the flow rate required from hundreds to 25 μL min−1. In this manner, not only the trade-off was resolved, but also the peak power density and fuel utilization were prominently promoted. The highest peak power density was nearly 43.5 and 90 mW cm−2 at 25 and 100 μL min−1, respectively, and the fuel utilization per pass was 7.5 times higher compared with that in the literature. Moreover, the cell robustness was greatly improved because of the viscous co-flow configuration.

Imaging concentration fields in microfluidic fuel cells as a mass transfer characterization platform

Microfluidic fuel cells (MFCs) are microfluidic electrochemical conversion devices that are used to power electrical equipment. Their performance relies on improving reactant mass transfer at the electrode interface. In this work, a MFC is developed to implement a novel imaging technique that allows the measurement of reactant concentration fields, featuring formic acid as the fuel and potassium permanganate as the oxidant. The concentration fields were imaged based on transmitted visible spectroscopy, which links the light intensity passing through the MFC to its local reactant concentration. An analytical model was developed to estimate the mass diffusivity and kinetic reaction rate coefficient. For the first time, mass transport and transfer coefficient were simultaneously measured during operation. These parameters estimated using the proposed technique can be implemented in a numerical model to predict the MFC performance and concentration distribution. This work paves the way towards advanced imaging tools for operando mass transfer characterizations in microfluidics and Tafel kinetic characterization in many electrochemical devices.

Perspective of Use of Pd/rGO in a Direct Urea Microfluidic Fuel Cell

The urine/urea oxidation reaction through catalysts with a higher performance in direct urea microfluidic fuel cells (DUµFC) is a promising method for power generation due to the large amount of human and animal urine containing 2–2.5 wt% urea. This paper presents a study that used urea as fuel in a DUµFC in the presence of palladium supported by reduced graphene oxide (rGO) for power generation. Some parameters, such as urea, KOH and H2SO4 concentration and flux rate, among others, are optimized in order to carry out the evaluation of urine samples as fuel in an air-breathing microfluidic fuel cell. The results show that the Pd/rGo catalyst mixed with Nafion® in the anodic compartment is dispersed and attached to the paper fibers, generating electrical contact and giving rise to the reactions of interest. In addition, XRD analysis confirmed the successful deposition of Pd and rGo on the substrate. These electrochemical results are promising, since, despite the decrease in the general performance of the DUµFC under ideal conditions with respect to normal cells, the generation of energy from urine was demonstrated.

Interchangeable Biomass Fuels for Paper-Based Microfluidic Fuel Cells: Finding Their Power Density Limits.

Paper batteries are self-pumping emerging tools for powering portable analytical systems. These disposable energy converters must be low-cost and must achieve enough energy to power electronic devices. The challenge is reaching high energy while keeping the low cost. Here, for the first time, we report a paper-based microfluidic fuel cell (PμFC) equipped with Pt/C on a carbon paper (CP) anode and a metal-free CP cathode fed by biomass-derived fuels to deliver high power. The cells were engineered in a mixed-media configuration, where methanol, ethanol, ethylene glycol, or glycerol is electro-oxidized in an alkaline medium, while Na2S2O8 is reduced in an acidic medium. This strategy allows for optimizing each half-cell reaction independently. The colaminar channel of the cellulose paper was chemically investigated by mapping the composition, which reveals a majority of elements from the catholyte and anolyte on each respective side and a mixture of both at the interface, assuring the existing colaminar system. Moreover, the colaminar flow was studied by investigating the flow rate by considering recorded videos for the first time. All PμFCs show 150-200 s to build a stable colaminar flow, which matches the time to reach a stable open circuit voltage. The flow rate is similar for different concentrations of methanol and ethanol, but it decreases with the increase in ethylene glycol and glycerol concentrations, suggesting a longer residence time for the reactants. The cells perform differently for the different concentrations, and their limiting power densities are composed of a balance among anode poisoning, residence time, and viscosity of the liquids. The sustainable PμFCs can be interchangeably fed by the four biomass-derived fuels to deliver ∼2.2-3.9 mW cm-2. This allows choosing the proper fuel due to their availability. The unprecedented PμFC fed by ethylene glycol delivered 6.76 mW cm-2, which is the benchmark output power for a paper battery fed by alcohol.

Two-phase mass transfer in a vapor-fed microfluidic fuel cell

Microfluidic fuel cells (MFCs) fed with methanol vapor are highly competitive in the field of high-efficient energy sources. However, the lack of a two-phase mass transfer theory within the porous electrode restricts the performance and scalability of this potential power option. Herein, a two-dimensional vapor-fed MFC model for two-phase mass transfer based on the mixed wettability of each porous layer is proposed. The effects of two-phase mass transfer on the reactant transport, electrochemical reaction, ion conduction, and cell performance are deeply discussed. This model provides a plausible theoretical explanation for maintaining adequate ion conductivity in catalyst layer and the transport limitations for methanol and oxygen at each electrode. The low fuel concentrations under high current densities lead to concentration polarization, indicating that the inefficient mass transfer at the anode dominates the cell performance. Under a peak power density of 35 mW cm−2, the vapor-fed MFC’s fuel utilization can reach up to 56%. For scaling up vapor-fed MFCs, the horizontal electrolyte channel and uniformly distributed evaporation surface are ideal for optimizing anode utilization and output power. The proposed model covers a gap in the theory of two-phase mass transfer within vapor-fed MFCs and facilitates its practical implementation for improved electrochemical performance.

Polymeric Microfluidic Fuel Cells with Controlled Printed Patterns.

Since its first appearance almost a couple of decades ago, microfluidic fuel cells (MFFCs) have gained considerable research momentum due to their potential applications in portable devices. The main focus has been on the effective fabrication of microfluidic channels with different materials, where the manufacturing limitations proved to be the main stumbling blocks. Paper-based MFFCs have been reported with some success, where the porosity of the flow channel medium drives the reactants, greatly reducing the need for elaborate external devices and complex manufacturing obstacles, although the longevity of these cells remains questionable. The current article addresses this issue by replacing the paper-based flow channels with 3D-printed substrates of different structural forms to serve as pathways for controlled flow and mixing responses of the reactant liquids without the use of other devices, such as micro pumps and valves. The line-by-line material consolidation mechanics of fused filament fabrication and the porous mesostructural responses of a commercial polymer filament are combined to build the microfluidic fuel channels of varying configurations. Numerical and experimental characterizations proved the cells to perform better than the current paper-based counterparts, apart from better longevity and possible new opportunities for future improvements based on more complex micro-, meso-, and macrostructural advances.

In‐Depth Analysis of Electrochemical Reaction Rate Distribution in Microfluidic Fuel Cell with Flow‐Through Electrodes

Nonuniform reaction rate distributions are commonly observed in the microfluidic fuel cell (MFC) systems, which bring a significant limit to the cell output. Herein, systematic analyses are performed to explore the electrochemical reaction rate distributions in the flow‐through electrodes of MFCs, and in‐depth understanding of the distribution mechanisms under various operating conditions is presented via a MFC performance simulation model. The results demonstrate that the high‐reaction‐rate regions locate at the inner parts of the flow‐through electrodes in the high flow rate and high‐reactant concentration cases which are ohmic‐resistance dominated, while moving toward the outer parts of the electrodes with the decreasing of flow rate and/or reactant concentration due to the increased concentration‐related activation resistance. A series of performance enhancement strategies are also proposed and examined. It is found that smaller electrode aspect ratio, reduced electrode distance, excess supporting electrolyte, and larger specific surface area are desirable in the high flow rate and high‐reactant concentration cases, which can boost the cell performances significantly. This work can contribute to the optimal designs of microfluidic fuel cells under various application scenarios in the future.