Microfluidics for Electrochemical Energy Conversion.

Electrochemical energy conversion is an important supplement for storage and on-demand use of renewable energy. In this regard, microfluidics offers prospects to raise the efficiency and rate of electrochemical energy conversion through enhanced mass transport, flexible cell design, and ability to eliminate the physical ion-exchange membrane, an essential yet costly element in conventional electrochemical cells. Since the 2002 invention of the microfluidic fuel cell, the research field of microfluidics for electrochemical energy conversion has expanded into a great variety of cell designs, fabrication techniques, and device functions with a wide range of utility and applications. The present review aims to comprehensively synthesize the best practices in this field over the past 20 years. The underlying fundamentals and research methods are first summarized, followed by a complete assessment of all research contributions wherein microfluidics was proactively utilized to facilitate energy conversion in conjunction with electrochemical cells, such as fuel cells, flow batteries, electrolysis cells, hybrid cells, and photoelectrochemical cells. Moreover, emerging technologies and analytical tools enabled by microfluidics are also discussed. Lastly, opportunities for future research directions and technology advances are proposed.

Evolution of water sorption in catalyst coated membranes subjected to combined chemical and mechanical degradation.

Catalyst coated perfluorosulfonic acid ionomer membranes (CCMs) were subjected to a combined chemical/mechanical accelerated stress test (AST) designed for rapid benchmarking of in situ membrane stability in polymer electrolyte fuel cells. In order to understand the evolution of the ionomer water sorption characteristics during combined chemical/mechanical degradation, CCM samples were periodically extracted from the AST and analyzed for ionomer mass fraction and water sorption properties. In spite of severe fluoride release and membrane thinning, the water uptake per unit mass of the partially degraded CCMs was found to be essentially constant. The mass fraction of ionomer in the CCM samples determined from thermogravimetric analysis (TGA) showed significant material loss throughout the AST process due to ionomer degradation and fluoride release, up to roughly 50% at end-of-life. The effects proceeding at different stages of degradation were therefore more accurately revealed by ionomer mass-normalized data. The water uptake per unit gram of ionomer was shown to increase significantly with degradation, in contrast to the previous results normalized by CCM dry mass. Although increased water sorption may indicate enlarged solvated hydrophilic domains in the membrane, which would be beneficial for enhanced proton mobility, the proton conductivity was found to decrease. This finding suggests that the additional water sorbed in the membrane was not contributing to proton conduction and was therefore likely situated in non-ionic cavities formed through degradation rather than in the ionic clusters.

An Organic Redox Flow Cell-Inspired Paper-Based Primary Battery.

A portable paper-based organic redox flow primary battery using sustainable quinone chemistry is presented. The compact prototype relies on the capillary forces of the paper matrix to develop a quasi-steady flow of the reactants through a pair of porous carbon electrodes without the need of external pumps. Co-laminar capillary flow allows operation Under mixed-media conditions, in which an alkaline anolyte and an acidic catholyte are employed. This feature enables higher electrochemical cell voltages during discharge operation and the utilization of a wider range of available species and electrolytes and provides the advantage to form a neutral or near-neutral pH as the electrolytes neutralize at the absorbent pad, which allows a safe disposal after use. The effects of the device design parameters have been studied to enhance battery features such as power output, operational time, and fuel utilization. The device achieves a faradaic efficiency of up to 98 %, which is the highest reported in a capillary-based electrochemical power source, as well as a cell capacity of up to 11.4 Ah L-1 cm-2 , comparable to state-of-the-art large-scale redox flow cells.

Four-dimensional joint visualization of electrode degradation and liquid water distribution inside operating polymer electrolyte fuel cells.

Understanding of degradation mechanisms present in polymer electrolyte fuel cells (PEFCs) is important to continue the integration of this clean energy technology into everyday life. Further comprehension of the interaction between various components during fuel cell operation is also critical in this context. In this work, a four-dimensional operando X-ray computed tomography method is developed for combined visualization of all PEFC components as well as transient water distribution residing in the cell, which results as a by-product of the electrochemical reaction. Time resolved, identical-location visualization through degradation stages is uniquely enabled by the non-invasive and non-destructive qualities of this method. By applying an accelerated stress test that targets cathode catalyst layer (CCL) corrosion, novel observations resulting from morphological changes of the CCL such as reduction in the water volume in the adjacent gas diffusion layer, CCL crack formation and propagation, membrane swelling, as well as quantification of local carbon loss is achieved. Additionally, insight into features that contribute to reduced fuel cell performance is enabled by the use of this specialized imaging technique, such as increased membrane undulation causing delamination and separation of the CCL from the microporous layer, which greatly affects liquid water pathways and overall device performance.

A microfluidic fuel cell with flow-through porous electrodes.

A microfluidic fuel cell architecture incorporating flow-through porous electrodes is demonstrated. The design is based on cross-flow of aqueous vanadium redox species through the electrodes into an orthogonally arranged co-laminar exit channel, where the waste solutions provide ionic charge transfer in a membraneless configuration. This flow-through architecture enables improved utilization of the three-dimensional active area inside the porous electrodes and provides enhanced rates of convective/diffusive transport without increasing the parasitic loss required to drive the flow. Prototype fuel cells are fabricated by rapid prototyping with total material cost estimated at 2 USD/unit. Improved performance as compared to previous microfluidic fuel cells is demonstrated, including power densities at room temperature up to 131 mW cm-2. In addition, high overall energy conversion efficiency is obtained through a combination of relatively high levels of fuel utilization and cell voltage. When operated at 1 microL min-1 flow rate, the fuel cell produced 20 mW cm-2 at 0.8 V combined with an active fuel utilization of 94%. Finally, we demonstrate in situ fuel and oxidant regeneration by running the flow-through architecture fuel cell in reverse.

"Bleaching" glycerol in a microfluidic fuel cell to produce high power density at minimal cost.

Glycerol/bleach flow-through microfluidic fuel cells are presented. Carbon paper-modified Pt/C nanoparticles were used as the anode and cathode. Glycerol oxidation in alkaline medium was tested against hypochlorite reduction in alkaline and acidic media. The mixed media system displayed a power density of 315 mW cm-2 and an open circuit voltage of 1.9-2.0 V.

Mitigation of chemical membrane degradation in fuel cells: understanding the effect of cell voltage and iron ion redox cycle.

Chemical membrane degradation through the Fenton's reaction is one of the main lifetime-limiting factors for polymer-electrolyte fuel cells. In this work, a comprehensive, transient membrane degradation model is developed to capture and elucidate the complex in situ degradation mechanism. A redox cycle of iron ions is discovered within the membrane electrolyte assembly, which sustains the Fe(II) concentration and results in the most severe chemical degradation at open circuit voltage. The cycle strength is critically reduced at lower cell voltages, which leads to an exponential decrease in Fe(II) concentration and associated membrane degradation rate. When the cell voltage is held below 0.7 V, a tenfold reduction in cumulative fluoride release is achieved, which suggests that intermediate cell voltage operation would efficiently mitigate chemical membrane degradation and extend the fuel cell lifetime.

A nanofluidic direct formic acid fuel cell with a combined flow-through and air-breathing electrode for high performance.

The use of three-dimensional flow-through nanoporous electrodes and the merging of a flow-through and air-breathing cathode were explored and successfully applied in a formic acid air-breathing nanofluidic fuel cell. The effects of fuel concentration, reaction stoichiometry and catalyst mass loading were investigated, resulting in power densities ranging from 28 to 100 mW cm(-2).

Microfluidic redox battery.

A miniaturized microfluidic battery is proposed, which is the first membraneless redox battery demonstrated to date. This unique concept capitalizes on dual-pass flow-through porous electrodes combined with stratified, co-laminar flow to generate electrical power on-chip. The fluidic design is symmetric to allow for both charging and discharging operations in forward, reverse, and recirculation modes. The proof-of-concept device fabricated using low-cost materials integrated in a microfluidic chip is shown to produce competitive power levels when operated on a vanadium redox electrolyte. A complete charge/discharge cycle is performed to demonstrate its operation as a rechargeable battery, which is an important step towards providing sustainable power to lab-on-a-chip and microelectronic applications.

A perspective on microfluidic biofuel cells.

This review article presents how microfluidic technologies and biological materials are paired to assist in the development of low cost, green energy fuel cell systems. Miniaturized biological fuel cells, employing enzymes or microorganisms as biocatalysts in an environmentally benign configuration, can become an attractive candidate for small-scale power source applications such as biological sensors, implantable medical devices, and portable electronics. State-of-the-art biofuel cell technologies are reviewed with emphasis on microfabrication compatibility and microfluidic fuel cell designs. Integrated microfluidic biofuel cell prototypes are examined with comparisons of their performance achievements and fabrication methods. The technical challenges for further developments and the potential research opportunities for practical cell designs are discussed.