Microfluidics for Electrochemical Energy Conversion

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.

Microfluidic Platforms for Real-Time In Situ Monitoring of Biomarkers for Cellular Processes

Cellular processes are mechanisms carried out at the cellular level that are aimed at guaranteeing the stability of the organism they comprise. The investigation of cellular processes is key to understanding cell fate, understanding pathogenic mechanisms, and developing new therapeutic technologies. Microfluidic platforms are thought to be the most powerful tools among all methodologies for investigating cellular processes because they can integrate almost all types of the existing intracellular and extracellular biomarker-sensing methods and observation approaches for cell behavior, combined with precisely controlled cell culture, manipulation, stimulation, and analysis. Most importantly, microfluidic platforms can realize real-time in situ detection of secreted proteins, exosomes, and other biomarkers produced during cell physiological processes, thereby providing the possibility to draw the whole picture for a cellular process. Owing to their advantages of high throughput, low sample consumption, and precise cell control, microfluidic platforms with real-time in situ monitoring characteristics are widely being used in cell analysis, disease diagnosis, pharmaceutical research, and biological production. This review focuses on the basic concepts, recent progress, and application prospects of microfluidic platforms for real-time in situ monitoring of biomarkers in cellular processes.

Construction of bimetallic phosphide nanostructures with in situ growth, reduction, and phosphidation of ultra-thin graphene layers as highly efficient catalysts towards the OER

We present a novel approach for the in situ growth of bimetallic silicate onto ultrathin graphene, followed by in situ reduction and phosphorization to obtain uniformly dispersed bimetallic phosphides (rGO@FeNiP/rGO@FeCoP) on graphene layers. Unlike the traditional simple composites of single-metallic phosphides and carbon materials, the bimetallic synergy of rGO@FeNiP/rGO@FeCoP obtained through in situ growth, reduction, phosphorization, and alkaline treatment exhibits a large surface area, more nanopores and defects, and more active sites, facilitates electrolyte diffusion and gas release, accelerates electron transfer and enhances electrocatalytic oxygen evolution reaction (OER) performance. Furthermore, the continuous carbon layer architecture surrounding FeNiP/FeCoP provides structural support, improving catalyst stability. We have investigated the effect of different proportions of bimetals on electrocatalytic performance, providing a rational design and synthesis strategy for carbon-based bimetallic phosphides as a promising electrocatalyst for the OER.

Development of Electrochemical Cells and Their Application for Spatially Resolved Analysis Using a Multitechnique Approach: From Conventional Experiments to X-Ray Nanoprobe Beamlines

Real (electro)catalysts are often heterogeneous, and their activity and selectivity depend on the properties of specific active sites. Therefore, unveiling the so-called structure-activity relationship is essential for a rational search for better materials and, consequently, for the development of the field of (electro-)catalysis. Thus, spatially resolved techniques are powerful tools as they allow us to characterize and/or measure the activity and selectivity of different regions of heterogeneous catalysts. To take full advantage of that, we have developed spectroelectrochemical cells to perform spatially resolved analysis using X-ray nanoprobe synchrotron beamlines and conventional pieces of equipment. Here, we describe the techniques available at the Carnaúba beamline at the Sirius-LNLS storage ring, and then we show how our cells enable obtaining X-ray (XRF, XRD, XAS, etc.) and vibrational spectroscopy (FTIR and Raman) contrast images. Through some proof-of-concept experiments, we demonstrate how using a multi-technique approach could render a complete and detailed analysis of an (electro)catalyst overall performance.

Black TiO2 Photoanodes for Direct Methanol Photo Fuel Cells

Photocatalytic fuel cells (PFCs) are considered the next generation of energy converter devices, since they can harvest solar energy through relatively low-cost semiconductor material to convert the chemical energy of renewable fuels and oxidants directly into electricity. Here, we report black TiO2 nanoparticle (NP) photoanodes for simple single-compartment PFCs and microfluidic photo fuel cells (μPFCs) fed by methanol. We show that Ti3+ and oxygen vacancy (OV) defects at the TiO2 NPs are easily controlled by annealing in a NaBH4-containing atmosphere. This optimized noble-metal-free black TiO2 photoanode shows superior PFC performance for methanol oxidation and O2 reduction with a maximum power density (Pmax) ∼2000% higher compared to the undoped TiO2. At flow conditions, the black TiO2 photoanode showed a Pmax ∼90 times higher than the μFC equipped with regular TiO2 in the dark. The PFC and μPFC operate spontaneously with little activation polarization, and black TiO2 photoanodes are stable under light irradiation. The improved photoactivity of the black TiO2 photoanode is a consequence of the self-doping with Ti3+/OV defects, which significantly red-shifted the bandgap energy, induced intragap electronic states, and widened both the valence band and conduction band, enhancing the overall absorption of visible light and decreasing the interfacial charge transfer resistance.

Co-electrolysis of seawater and carbon dioxide inside a microfluidic reactor to synthesize speciality organics

We report co-electrolysis of seawater and carbon dioxide (CO₂) gas in a solar cell-integrated membraneless microfluidic reactor for continuous synthesis of organic products. The microfluidic reactor was fabricated using polydimethylsiloxane substrate comprising of a central microchannel with a pair of inlets for injection of CO₂ gas and seawater and an outlet for removal of organic products. A pair of copper electrodes were inserted into microchannel to ensure its direct interaction with incoming CO₂ gas and seawater as they pass into the microchannel. The coupling of solar cell panels with electrodes generated a high-intensity electrical field across the electrodes at low voltage, which facilitated the co-electrolysis of CO₂ and seawater. The paired electrolysis of CO₂ gas and seawater produced a range of industrially important organics under influence of solar cell-mediated external electric field. The, as synthesized, organic compounds were collected downstream and identified using characterization techniques. Furthermore, the probable underlying electrochemical reaction mechanisms near the electrodes were proposed for synthesis of organic products. The inclusion of greenhouse CO₂ gas as reactant, seawater as electrolyte, and solar energy as an inexpensive electric source for co-electrolysis initiation makes the microreactor a low-cost and sustainable alternative for CO₂ sequestration and synthesis of organic compounds.

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.

Immobilization of Glucose Oxidase on Glutathione Capped CdTe Quantum Dots for Bioenergy Generation

An efficient immobilization of Glucose oxidase (GOx) on an appropriate substrate is one of the main challenges of developing fuel cells that allow energy to be obtained from renewable substrates such as carbohydrates in physiological environments. The research importance of biofuel cells relies on their experimental robustness and high compatibility with biological organisms such as tissues or the bloodstream with the aim of obtaining electrical energy even from living systems. In this work, we report the use of 5,10,15,20 tetrakis (1-methyl-4-pyridinium) porphyrin and glutathione capped CdTe Quantum dots (GSH-CdTeQD) as a support matrix for the immobilization of GOx on carbon surfaces. Fluorescent GSH-CdTeQD particles were synthesized and their characterization by UV-Vis spectrophotometry showed a particle size between 5–7 nm, which was confirmed by DLS and TEM measurements. Graphite and Toray paper electrodes were modified by a drop coating of porphyrin, GSH-CdTeQD and GOx, and their electrochemical activity toward glucose oxidation was evaluated by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. Additionally, GOx modified electrode activity was explored by scanning electrochemical microscopy, finding that near to 70% of the surface was covered with active enzyme. The modified electrodes showed a glucose sensitivity of 0.58 ± 0.01 μA/mM and an apparent Michaelis constant of 7.8 mM. The addition of BSA blocking protein maintained the current response of common interferent molecules such as ascorbic acid (AA) with less than a 5% of interference percentage. Finally, the complex electrodes were employed as anodes in a microfluidic biofuel cell (μBFC) in order to evaluate the performance in energy production. The enzymatic anodes used in the μBFC allowed us to obtain a current density of 7.53 mAcm−2 at the maximum power density of 2.30 mWcm−2; an open circuit potential of 0.57 V was observed in the biofuel cell. The results obtained suggest that the support matrix porphyrin and GSH-CdTeQD is appropriate to immobilize GOx while preserving the enzyme’s catalytic activity. The reported electrode arrangement is a viable option for bioenergy production and/or glucose quantification.

Digital manufacturing for accelerating organ-on-a-chip dissemination and electrochemical biosensing integration

Organ on-a-chip (OoC) is a promising technology that aims to recapitulate human body pathophysiology in a more precise way to advance in drug development and complex disease understanding. However, the presence of OoC in biological laboratories is still limited and mainly restricted to laboratories with access to cleanroom facilities. Besides, the current analytical methods employed to extract information from the organ models are endpoint and post facto assays which makes it difficult to ensure that during the biological experiment the cell microenvironment, cellular functionality and behaviour are controlled. Hence, the integration of real-time biosensors is highly needed and requested by the OoC end-user community to provide insight into organ function and responses to stimuli. In this context, electrochemical sensors stand out due to their advantageous features like miniaturization capabilities, ease of use, automatization and high sensitivity and selectivity. Electrochemical sensors have been already successfully miniaturized and employed in other fields such as wearables and point-of-care devices. We have identified that the explanation for this issue may be, to a large extent, the accessibility to microfabrication technologies. These fields employ preferably digital manufacturing (DM), which is a more accessible microfabrication approach regardless of funding and facilities. Therefore, we envision that a paradigm shift in microfabrication that adopts DM instead of the dominating soft lithography for the in-lab microfabrication of OoC devices will contribute to the dissemination of the field and integration of the promising real-time sensing.

Renewable plant-derived lignin for electrochemical energy systems

Lignin, as one of the most abundant natural polymers, has been proved to be a promising material for the construction of high-performance electrochemical energy systems, including electrodes, electrolytes, and separators, because of their low-cost and sustainable natures and unique structure with abundant functional group. In this review article, we outline some key contributions in this field such as fundamental principles and various electrochemical energy systems including rechargeable batteries, supercapacitors, solar cells, and fuel cells. At the same time, we also point out the significant scientific discussion and critical barriers for lignin-based materials for electrochemical energy systems and also provides feasible strategies for preparing new sustainable energy materials.

A plant-like battery: a biodegradable power source ecodesigned for precision agriculture

The natural environment has always been a source of inspiration for the research community. Nature has evolved over thousands of years to create the most complex living systems, with the ability to leverage inner and outside energetic interactions in the most efficient way. This work presents a flow battery profoundly inspired by nature, which mimics the fluid transport in plants to generate electric power. The battery was ecodesigned to meet a life cycle for precision agriculture (PA) applications; from raw material selection to disposability considerations, the battery is conceived to minimize its environmental impact while meeting PA power requirements. The paper-based fluidic system relies on evaporation as the main pumping force to pull the reactants through a pair of porous carbon electrodes where the electrochemical reaction takes place. This naturally occurring transpiration effect enables to significantly expand the operational lifespan of the battery, overcoming the time-limitation of current capillary-based power sources. Most relevant parameters affecting the battery performance, such as evaporation flow and redox species degradation, are thoroughly studied to carry out device optimization. Flow rates and power outputs comparable to those of capillary-based power sources are achieved. The prototype practicality has been demonstrated by powering a wireless plant-caring device. Standardized biodegradability and phytotoxicity assessments show that the battery is harmless to the environment at the end of its operational lifetime. Placing sustainability as the main driver leads to the generation of a disruptive battery concept that aims to address societal needs within the planetary environmental boundaries.

Electrochemistry in Magnetic Fields

Developing new strategies to advance the fundamental understanding of electrochemistry is crucial to mitigating multiple contemporary technological challenges. In this regard, magnetoelectrochemistry offers many strategic advantages in controlling and understanding electrochemical reactions that might be tricky to regulate in conventional electrochemical fields. However, the topic is highly interdisciplinary, combining concepts from electrochemistry, hydrodynamics, and magnetism with experimental outcomes that are sometimes unexpected. In this Review, we survey recent advances in using a magnetic field in different electrochemical applications organized by the effect of the generated forces on fundamental electrochemical principles and focus on how the magnetic field leads to the observed results. Finally, we discuss the challenges that remain to be addressed to establish robust applications capable of meeting present needs.

Glycerol Is Converted into Energy and Carbonyl Compounds in a 3D-Printed Microfluidic Fuel Cell: In Situ and In Operando Bi-Modified Pt Anodes

The combination of energy and chemical conversion can be achieved by designing glycerol fuel cells. However, the anode must promote the reaction at onset potentials low enough to allow a spontaneous reaction, when coupled to the cathodic reaction, and must be selective. Here, we build a three-dimensional (3D)-printed glycerol microfluidic fuel cell that produces power concomitantly to glycolate and formate at zero bias. The balance between energy and the two carbonyl compounds is tuned by decorating the Pt/C/CP anode in situ (before feeding the cell reactants) or in operando (while feeding the cell with reactants) with Bi. The Bi-modified anodes improve glycerol conversion and output power while decreasing the formation of the carbonyl compounds. The in operando method builds dendrites of rodlike Bi oxides that are inactive for the anodic reaction and cover active sites. The in situ strategy promotes homogeneous Bi decoration, decreasing activation losses, increasing the open-circuit voltage to 1.0 V, and augmenting maximum power density 6.5 times and the glycerol conversion to 72% at 25 °C while producing 0.2 mmoL L^-1 of glycolate and formate (each) at 100 μL min^-1. Such a performance is attributed to the low CO poisoning of the anode, which leads the glycerol electrooxidation toward a more complete reaction, harvesting more electrons at the device. Printing the microfluidic fuel cell takes 23 min and costs ∼US$1.85 and can be used for other coupled reactions since the methods of modification presented here are applied to any existing and assembled systems.

Development and Challenges of Biphasic Membrane-Less Redox Batteries

Ion exchange membranes (IEMs) play important roles in energy generation and storage field, such as fuel cell, flow battery, however, a major barrier in the way of large-scale application is the high cost of membranes (e.g., Nafion membranes price generally exceeds USD$ 200 m^-2 ). The membrane-less technology is one of the promising approaches to solve the problem and thus has attracted much attention and been explored in a variety of research paths. This review introduces one of the representative membrane-less battery types, Biphasic membrane-less redox batteries that eliminate the IEMs according to the principle of solvent immiscibility and realizes the phase splitting in a thermodynamically stable state. It is systematically classified and summarizes their performances as well as the problems they are suffering from, and then several effective solutions are proposed based on the modification of electrodes and electrolytes. Finally, special attention is given to the challenges and prospects of Biphasic membrane-less redox batteries, which could contribute to the development of membrane-less batteries.