A wide range of applications are possible with paper-based analytical devices, which are low priced, easy to fabricate and operate, and require no specialized equipment. Paper-based microfluidics offers the design of miniaturized POC devices to be applied in the health, environment, food, and energy sector employing the ASSURED (Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment free and Deliverable to end users) principle of WHO. Therefore, this field is growing very rapidly and ample research is being done. This review focuses on fabrication and detection techniques reported to date. Additionally, this review emphasises on the application of this technology in the area of medical diagnosis, energy generation, environmental monitoring, and food quality control. This review also presents the theoretical analysis of fluid flow in porous media for the efficient handling and control of fluids. The limitations of PAD have also been discussed with an emphasis to concern on the transformation of such devices from laboratory to the consumer. 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9743133

Paper based microfluidic devices: a review of fabrication techniques and applications

In recent decades, significant progress has been achieved in rational developments of electrocatalysts through constructing novel atomistic structures and modulating catalytic surface topography, realizing substantial enhancement in electrocatalytic activities. Numerous advanced catalysts were developed for electrochemical energy conversion, exhibiting low overpotential, high intrinsic activity, and selectivity. Yet, maintaining the high catalytic performance under working conditions with high polarization and vigorous microkinetics that induce intensive degradation of surface nanostructures presents a significant challenge for commercial applications. Recently, advanced operando and computational techniques have provided comprehensive mechanistic insights into the degradation of surficial functional structures. Additionally, various innovative strategies have been devised and proven effective in sustaining electrocatalytic activity under harsh operating conditions. This review aims to discuss the most recent understanding of the degradation microkinetics of catalysts across an entire range of anodic to cathodic polarizations, encompassing processes such as oxygen evolution and reduction, hydrogen reduction, and carbon dioxide reduction. Subsequently, innovative strategies adopted to stabilize the materials' structure and activity are highlighted with an in-depth discussion of the underlying rationale. Finally, we present conclusions and perspectives regarding future research and development. By identifying the research gaps, this review aims to inspire further exploration of surface degradation mechanisms and rational design of durable electrocatalysts, ultimately contributing to the large-scale utilization of electroconversion technologies.

Surface engineering for stable electrocatalysis.

As an oxygen reduction reaction (ORR) catalyst, nitrogen-doped carbon (NC) is widely used in zinc-air batteries (ZABs). However, NC catalysts exhibit low conductivity and insufficient exposure of active sites. Therefore, a Co-based deep eutectic solvent (DES) was selected to modify NC catalyst (Co-NC) to improve ORR performances. Density functional theory (DFT) calculation shows that the modification of Co-based DES can change the electronic structure of NC and increase metallic active sites, which is beneficial to the desorption of reaction intermediates on Co-NC, further improving ORR performance. Co-NC shows excellent ORR performances and stability. Impressively, ZABs assembled with Co-NC manifest a high maximum power density of 177.4 mW cm−2, a high specific capacity of 726.12 mA h g−1 and a charge-discharge cycle life of 500 h. This study can provide practical reference for surface modified carbon-based electrocatalyst with DES to improve ORR performances. 

Cobalt-Based Deep Eutectic Solvent Modified Nitrogen-Doped Carbon Catalyst for Boosting Oxygen Reduction Reaction in Zinc–Air Batteries

The oxygen reduction reaction (ORR) represents a cornerstone for many clean energy conversion technologies such as fuel cells and metal‐air batteries. Nevertheless, the commercialization of these technologies is largely impeded by the slow kinetics of ORR, for which active, durable and cost‐effective ORR catalysts are needed. In recent years, zeolitic imidazolate framework (ZIF) derived carbon materials emerge as a new class of non‐precious metal catalysts (NPMCs) toward ORR, largely benefiting from their high surface area, abundant porosity, tunable chemical/electronic structure, and superior ORR activity which is comparable or even surpasses those state‐of‐the‐art Pt‐based ORR catalysts. This review provides an in‐depth overview of the latest developments in ZIF‐derived carbons toward ORR. The synthetic strategies of the ZIF‐derived carbon materials and the key factors that determine the ORR performance are highlighted. Future research directions and perspectives on exploring ZIF derived carbons as efficient ORR catalysts are provided with an emphasis on the principles in rationally engineering the coordination structures of active sites.

Recent Progress in ZIF‐Derived Carbons for Enhanced Oxygen Reduction Reaction Electrocatalysis

ZIF-derived carbons (ZDCs) represent a novel category of non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR). Nevertheless, establishing the correlation between their structure and performance is challenging due to their diverse structural properties and ill-defined active site moieties. Here we investigate the effect of ZIF-67 derived carbon's particle size and corresponding distribution of assorted nitrogen functionalities on the ORR. Our findings reveal a substantial increase in ORR activity with decreasing ZDC particle size, while the exact influence of nitrogen functionalities remains uncertain. Most importantly, the present work unveils a correlation between the ORR activity and the site density (SD) of Co-Nx within the ZDC materials, which can be attributed to the concurrently increasing SD and accessibility of Co-Nx on the ZDC particles with small sizes. The present work provides novel insights into the optimization of ZDCs as high-performance NPMCs for ORR in alkaline electrolytes. 

Toward quantification of active site density and size-dependent ORR activity of ZIF-derived carbons in alkaline electrolyte

Non-precious metal catalysts show great promise to replace the state-of-the-art Pt-based catalysts for catalyzing the oxygen reduction reaction (ORR), while their catalytic activity still needs to be greatly improved before their broad-based application. Here, we report a facile approach to improving the performance of zeolitic imidazolate framework-derived carbon (ZDC) toward the ORR by incorporating a small amount of ionic liquid (IL). The IL would preferentially fill the micropores of ZDC and greatly enhance the utilization of the active sites within the micropores, which are initially not accessible due to insufficient surface wetting. It is also disclosed that the ORR activity in terms of kinetic current at 0.85 V depends on the loading amount of the IL, and the maximum activity is obtained at a mass ratio of IL to ZDC at 1.2. The optimum stems from the counterbalance between the enhanced utilization of the active sites within the micropores and the retarded diffusion of the reactants within the IL phase due to its high viscosity.

Increasing Accessible Active Site Density of Non-Precious Metal Oxygen Reduction Reaction Catalysts through Ionic Liquid Modification

Paper‐based microfluidics emerges as an innovative platform for constructing miniaturized electrochemical devices, which mainly benefit from the spontaneous capillary action of paper. Nevertheless, the capillary‐driven flow dynamics on paper are determined exclusively by the intrinsic properties of paper and fluidics, thus lacking the controllability that conventional pump‐based microfluidics can provide. Herein, an approach to regulating the capillary flow on paper is introduced by conjugating the outlets of microfluidic channels with a photothermal module for water evaporation. The capillary flow rate on paper can be handily regulated from 4 to 37 µL min−1 under controllable illumination conditions. As a proof‐of‐concept, prototypical paper‐based microfluidic fuel cells integrated with the photothermal module are constructed. Their peak power density can be boosted from 0.3 up to 2.1 mW cm−2 under the simulated sunlight irradiation. The influence of capillary flow rate on the fuel cell performance is further validated using multiphysics simulations. The present work not only provides a practically feasible method to boost the performance of paper‐based microfluidic fuel cells using solar energy, but also opens a new avenue for modulating the performance of paper‐based microfluidics, which has long been a challenge in this field.

Solar‐Boosted Paper‐Based Microfluidic Fuel Cells for Miniaturized Power Sources

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Increasing Accessible Active Site Density of Non-Precious Metal Oxygen Reduction Reaction Catalysts through Ionic Liquid Modification

Toward quantification of active site density and size-dependent ORR activity of ZIF-derived carbons in alkaline electrolyte

Solar‐Boosted Paper‐Based Microfluidic Fuel Cells for Miniaturized Power Sources