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.

In this work, a novel glucose quantification strategy is presented in a self‐powered biosensing device. The analyte in the sample is oxidized in an enzymatic fuel cell and the generated charge is transferred to a capacitor. The built‐up capacitor voltage at a specific time can be directly correlated with the concentration of analyte. An electro‐fluidic switch placed on a paper‐based microfluidic channel connects a minimalistic electronic circuit to the fuel cell. The circuit modules discriminate the built‐up capacitor voltage, which corresponds to a particular glucose range. The digital semi‐quantitative result is visualized vía electrochromic displays. As a practical application of this working principle, the self‐powered single‐use device has been designed to perform screening of gestational diabetes mellitus. The device discriminates between healthy (<7.8 mm), pre‐diabetes (>7.8 mm), and diabetes (>11.1 mm) condition providing a reliable result. This single use, printable, and disposable self‐powered glucose biosensing device is autonomous and fully powered by the glucose contained in a 3.5 µL sample. It offers an energy‐saving, environmentally friendly, and low‐cost solution for Point‐of‐Care testing. By replacing the selective enzyme in the fuel cell, this strategy can be used for measuring other health parameters such as creatinine, cholesterol, or uric acid, among others.

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.

The proliferation of portable electronic devices has resulted in an increase of e‐waste that is generated after their use. One of the most hazardous components in e‐waste are batteries, due to their content of heavy metals and toxic chemicals. Fuel cells and redox flow batteries have been recognized as more sustainable alternatives to Li‐based batteries for powering portable applications. Although they provide comparable energy and power densities, they still face some challenges because they rely on proton exchange membranes based on nonenvironmentally friendly, high‐priced perfluorosulfonic acid copolymers that require energy‐intense manufacturing and recycling procedures. In this work, eco‐friendly and sustainable biopolymer electrolyte membranes (BioPEMs) are synthesized from biopolymers like chitosan, cellulose, and starch. These BioPEMs bring forth advantages in performance, sustainability, and cost. Additionally, they present good chemical and mechanical stability, high ionic conductivity in the same order of magnitude as Nafion membranes. Two alternatives of cellulose–chitosan based BioPEMs are successfully applied into primary redox batteries using benign eco‐friendly redox chemistries, delivering open circuit voltages above 0.75 V and power output up to 2.5 mW cm−2. These results demonstrate BioPEMs capability to improve biodegradable batteries in sectors requiring a transient electrical energy, such as environmental monitoring, agriculture, or packaging.