Microfluidics in Membraneless Fuel Cells

In the 1990s, the idea of developing miniaturized devices that integrate functions other than what normally are carried out at the laboratory level was conceived, and the so-called “lab-on-a-chip” (LOC) devices emerged as one of the most important research areas. LOC devices exhibit advantages related to the use of microfluidic channels such as small sample and reagent consumption, portability, low-power consumption, laminar flow, and higher surface area/volume ratio that enhances both thermal dissipation and electrochemical kinetics. Fuel cells are electrochemical devices that convert chemical energy to electrical energy. These are considered as one of the greener ways to generate electricity because typical fuel cells produce water and heat as the main reaction byproducts. The technical challenges to develop systems at the microscale and the advantages of microfluidics exhibited an important impact on fuel cells for several reasons, mainly related to avoid inherent problems of gaseous-based fuel cells. As a result, the birth of a new type of fuel cells as microfluidic fuel cells (MFCs) took place. The first microfluidic fuel cell was reported in 2002. This MFC was operated with liquid fuel/oxidant and had the advantage of the low laminar flow generated using a “Y” microfluidic channel to separate the anodic and cathodic streams, resulting in an energy conversion device that did not require a physical barrier to separate both streams. This electrochemical system originated a specific type of MFCs categorized as membraneless also called colaminar microfluidic fuel cells. Since that year, numerous works focused on the nature of fuels, oxidants and anodic/cathodic electrocatalysts, and cell designs have been reported. The limiting parameters of this kind of devices toward their use in portable applications are related to their low cell performances, small mass activity, and partial selectivity/durability of electrocatalysts. On the other hand, it has been observed that the cell design has a high effect on the cell performance due to internal cell resistances and the crossover effect. Furthermore, current technology is growing faster than last centuries and new microfabrication technologies are always emerging, allowing the development of smaller and more powerful microfluidic energy devices. In this chapter, the application of microfluidics in membraneless fuel cells is addressed in terms of evolution of cell designs of miniaturized microfluidic fuel cells as a result of new discoveries in microfabrication technology and the use of several fuels and electrocatalysts for specific and selective applications

Membraneless flow battery leveraging flow-through heterogeneous porous media for improved power density and reduced crossover

We propose and demonstrate a novel flow battery architecture that replaces traditional ion-exchange membranes with less expensive heterogeneous flow-through porous media. Compared to previous membraneless systems, our prototype exhibits significantly improved power density (0.925 W cm[superscript -2]), maximum current density (3 A cm[suprescript -2]), and reactant crossover, shown by the proposed experimentally-validated crossover model

Membrane-less hydrogen bromine flow battery

In order for the widely discussed benefits of flow batteries for electrochemical energy storage to be applied at large scale, the cost of the electrochemical stack must come down substantially. One promising avenue for reducing stack cost is to increase the system power density while maintaining efficiency, enabling smaller stacks. Here we report on a membrane-less hydrogen bromine laminar flow battery as a potential high-power density solution. The membrane-less design enables power densities of 0.795 W cm[superscript −2] at room temperature and atmospheric pressure, with a round-trip voltage efficiency of 92% at 25% of peak power. Theoretical solutions are also presented to guide the design of future laminar flow batteries. The high-power density achieved by the hydrogen bromine laminar flow battery, along with the potential for rechargeable operation, will translate into smaller, inexpensive systems that could revolutionize the fields of large-scale energy storage and portable power systems.American Society for Engineering Education. National Defense Science and Engineering Graduate FellowshipMIT Energy Initiative (Seed Fund

Toward a mechanistic understanding of microfluidic droplet-based extraction and separation of lanthanides

Droplet-based microfluidic extraction is a promising way for effective lanthanides extraction due to its outstanding mass transfer performance. The separation process can be greatly enhanced with the droplet-based microfluidic extraction technique. However, the interactions between mass transfer, microfluidic dynamics and extraction kinetics are still unclear, which has hindered further manipulation on microfluidic extraction to boost extraction performance. In this study, the mechanisms of microfluidic droplet-based extraction and separation intensification of lanthanides are for the first time unveiled by using a numerical simulation model. The limiting factors for the performance of droplet-based microfluidic extraction are identified through a model-based parametric analysis. The numerical analyses provide a comprehensive understanding of droplet-based microfluidic extraction systems and offer operation and optimization guidelines for future research in this area

Self Pumping Paper Based Formic Acid Fuel Cell

Fuel cells have been a source of green energy and are rapidly gaining applications in various fields. Laminar flow fuel cells are special type of fuel cell in which the oxidant and the fuel flow in parallel, mixed with the electrolyte, which substantially reduces losses due to mixing. This work focuses on the fabrication of paper based laminar flow fuel cells with a slit in between the two electrode areas to prevent further crossover. The capillary action of What man filter paper has been used which makes the cell self-pumping hence reducing the power requirements. Formic acid is used as fuel and potassium per magnate is used as oxidant. Concentration of the fuel has been varied and corresponding change in polarization curves, potentiometry, Impedance curves has been studied using Auto lab. The proposed cell can be used as a lateral flow in small diagnostic devices, which requires low power for their operation

2D dimensionless numbers in isothermal fuel cells with smooth electrocatalysts

One of the problems in the optimization of a fuel cell performance is the operation prediction for short and long-time behaviours. The employ of exact analytical functions for picturing the distribution of potential and current densities in 2D polymer electrolyte membrane fuel cells, generalizes the study and reduces large computational times for each experimental situation. Therefore, we foresee analytical solutions for mass balance equations using the asymptotic velocity equations (normal and tangential coordinates) to obtain a 2D concentration, current and overpotential profiles for smooth platinum catalysts. Dimensionless numbers are deduced, i.e. Wagner, Damkoehler and Graetz to characterize the fuel cell performance, first with a 1D approach and also along 2D coordinates. Besides, the complete polarization curve is predicted comparing the theoretical results with the proper variations of electrochemical magnitudes in a single home-made hydrogen/oxygen 200 cm2 polymer electrolyte membrane fuel cell.Agencia Nacional de Investigación e Innovació

Modeling of the electrochemical conversion of CO2 in microfluidic reactors

Today’s world faces immense challenges associated with meeting its energy needs, due to its current dependence on fossil fuels. At the same time, the world faces the threat of global climate change linked to CO2 emissions. Indeed, global energy consumption is expected to double in the next 50 years. This accelerates the depletion of conventional fossil fuels and leads to a steady increase in CO2 emission. Globally, CO2 emission through the combustion of fossil fuels has increased by about 1.6 times between 1990 (the Kyoto Protocol reference year) and 2013, with approximately 9.9 GtC added to the atmosphere in 2013. Taken together, the dual challenges of finding alternative energy sources and curbing CO2 emissions are very daunting. When it is powered by carbon-neutral electricity sources, the electrochemical conversion of CO2 into value-added chemicals offers an economically viable route to recycle CO2 towards reducing CO2 emissions and reducing dependence on fossil fuels. The majority of prior studies on the electrochemical conversion of CO2 are experimental in nature, focused on unravelling the mechanisms of known catalysts. As an alternative approach to the laborious experiments, first-principles modeling of the electrochemical reactors can complement the current experimental work by elucidating the complex transport and electrochemistry, particularly in the porous electrodes, and help in the design and optimization of such reactors. Currently, there is a lack of detailed modeling for the aqueous electrochemical reduction of CO2 in a microfluidic reactor, which has been demonstrated experimentally to be an effective reactor and a versatile analytical tool. This thesis focuses on developing a mathematical modeling framework for the electrochemical conversion of CO2 to CO in microfluidic reactors. Conversion of CO2 into CO is attractive due to the versatility of CO (with H2) as a feedstock for the production of a variety of products including liquid hydrocarbon fuels. A full model that takes into account of all the significant physics and electrochemistry in the cell, including the transport of species and charges, momentum and mass conservations, and electrochemical reactions, is first formulated. The full model that comprises of a system of coupled partial differential equations is solved using finite element method. It is then calibrated and validated using experimental data obtained for various inlet flow rates and compositions. Parametric studies for various design and operating variables are subsequently performed using the validated model. To reduce the computational time, yet preserve the geometric resolution and leading order behavior of the cell, narrow-gap approximation and scaling arguments are invoked which allow for significant reduction in the mathematical complexity of the full model and eventually approximate analytical solutions. The unit cell models are then extended to stack models for simulation and analysis of the electrochemical reduction of CO2 in a microfluidic cell stack