Since its first appearance in 2002, microfluidic fuel cell has received great attention in the past two decades, which is mainly targeted at its use in portable electronics. This micro fuel cell technology utilizes microfluidic flows as electrolyte instead of conventional polymer membranes. To date, various fuels have been utilized in it, such as vanadium species, hydrogen, hydrocarbons, hydrogen peroxide, borohydride and nitrogenous materials, each of which has its specific merits and demerits. To optimize its power output and fuel utilization, innovative cell structures and advanced catalysts have been continuously developed for different fuels, with remarkable improvements achieved. The power output can be elevated from several mW cm−2 to several W cm−2 at room temperature, while the fuel utilization per single pass can reach 100% by using 3D flow-through electrodes. Also, investigations in recent years have shown that microfluidic fuel cell stacking increases the working voltage. In addition to cells with plastic channel, novel cell designs based on cellulose paper and fabric materials have also been proposed; apart from being lightweight, they are also free from pumping. These innovative cell designs represent a promising route for achieving real applications in areas such as medical diagnostic, wearable healthcare and smart logistics. As for the conventional plastic cells, they are currently less competitive against batteries and other fuel cells because of the extra pumping requirement, which should be resolved in future by developing passive pumps instead. Alternatively, they can be applied in specific circumstances where the extra pumping loss is tolerable.

Microfluidic fuel cells with different types of fuels: A prospective review

Application of porous biomass carbon materials in vanadium redox flow battery.

Cellulose: A Contribution for the Zero e-Waste Challenge

Flow batteries (FBs) currently are one of the most promising large-scale energy storage technologies for energy grids with a large share of renewable electricity generation Among the main technological challenges for the economic operation of a large-scale battery technology is its calendar lifetime, which ideally has to cover a few decades without significant loss of performance This requirement can only be met if the key parameters representing the performance losses of the system are continuously monitored and optimized during the operation Nearly all performance parameters of a FB are related to the two electrolytes as the electrochemical storage media and we therefore focus on them in this review We first survey the literature on the available characterization methods for the key FB electrolyte parameters Based on these, we comprehensively review the currently available approaches for assessing the most important electrolyte state variables: the state-of-charge (SOC) and the state-of-health (SOH) We furthermore discuss how monitoring and operation strategies are commonly implemented as online tools to optimize the electrolyte performance and recover lost battery capacity as well as how their automation is realized via battery management systems (BMSs) Our key findings on the current state of this research field are finally highlighted and the potential for further progress is identified

Trust is good, control is better: a review on monitoring and characterization techniques for flow battery electrolytes.

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. 

https://zenodo.org/record/7950216/files/Manuscript.pdf

Microfluidics for Electrochemical Energy Conversion

N‐ and P‐co‐doped Graphite Felt Electrode for Improving Positive Electrode Chemistry of the Vanadium Redox Flow Battery

Vanadium redox flow battery (VRFB) is an energy storage system, wherein V2+/V3+ and VO2+/VO2+ are used as negative and positive electrolyte respectively. It is well known that, V2+/V3+ redox reaction is sluggish in comparison to that of VO2+/VO2+ reaction. As the redox potential of V2+/V3+ redox-couple is more negative to that of H+/H2 redox-couple, during the V2+ formation hydrogen evolution occurs concomitantly, which affects the capacity retention of VRFB inducing redox couples concentration imbalance between the positive and negative electrolytes. In this study, we have explored the beneficial effect of D-fructose as an additive to the negative electrolyte. D-fructose (i) enhances the interfacial area of the graphite felt negative electrode-electrolyte interface by wetting, thereby the current due to V2+/V3+ redox reaction at a given overpotential, (ii) suppresses the H2 evolution at negative electrode during charging of VRFB (iii) controls the VO2+ accumulation at the positive electrolyte and (iv) chemically reduces the VO2+ arriving at negative electrode side through crossover, thereby avoiding the direct reaction between V2+ and VO2+. A capacity retention of 86 % and 25 % is achieved at the end of the 25th cycle in the presence and absence of D-fructose in the negative electrolyte, respectively. This way of in-situ redox balancing alleviates the requirement for external redox balancing of the electrolyte, and help VRFB to deliver constant capacity with cycles.

On In–situ Redox Balancing of Vanadium Redox Flow Battery Using D-Fructose as Negative Electrolyte Additive

This work deals about the development of paper-based fuel cells with high open-circuit voltages for application in powering the micro-nanosystems such as bio-sensors. The developed fuel cell employs Whatman paper as the ion conductor placed between the anode and the cathode to replace expensive ion-conducting membranes such as Nafion. A maximum open-circuit voltage of ∼2.1 V per single cell and a power output of ∼4.5 mW cm−2 at ∼6 mA cm−2 are obtained when employing 1 M NaBH4 in 20 wt% NaOH solution and 1 M VO2
 + in 2.5 M H2SO4 solution. Chronoamperometric measurement performed at 1 V indicates that the output power density varies from 6 to 1 mW cm−2 in about 100 min. The power delivered at the end of even 100 min is comparable to that of the peak power delivered by many of the micro-fuel cell systems reported in the literature.

Flexible paper-based borohydride-vanadium fuel cell for powering micro-nanosystems

Paper-Based Disposable Zinc-Vanadium Fuel Cell for Micropower Applications

Redox flow batteries (RFBs) are considered a vital part of the energy storage system for storing the energy generated from the renewable energy sources such as wind and solar. Limited by the water electrolysis potential window, the aqueous RFBs exhibit low energy density (<25 Wh L−1). In this context a non-aqueous organic solvent based RFB system is very attractive as it offers avenue to expand the operating potential window beyond 1.23 V, which has a direct impact on the RFB's energy and power densities. In this study, new redox systems based on tris(4-bromophenyl)amine (4-Br-TPA) and oxygen saturated N,N-dimethylformamide (DMF) containing tetra-n-butylammonium hexafluorophosphate (TBAPF6) are utilized as catholyte and anolyte respectively. Here, TBAPF6 performs dual role, both as supporting electrolyte and a stabilizer of superoxide anion radical.

Vasudevarao Pasala

Papers

N‐ and P‐co‐doped Graphite Felt Electrode for Improving Positive Electrode Chemistry of the Vanadium Redox Flow Battery

On In–situ Redox Balancing of Vanadium Redox Flow Battery Using D-Fructose as Negative Electrolyte Additive

Flexible paper-based borohydride-vanadium fuel cell for powering micro-nanosystems

Paper-Based Disposable Zinc-Vanadium Fuel Cell for Micropower Applications

A High Voltage Organic Redox Flow Battery with Redox Couples O2/Tetrabutylammonium Complex and Tris(4-bromophenyl)amine as Redox Active Species