Toward a Comprehensive Understanding of Cation Effects in Proton Exchange Membrane Fuel Cells.

Metal alloy catalysts (e.g., Pt-Co) are widely used in fuel cells for improving the oxygen reduction reaction kinetics. Despite the promise, the leaching of the alloying element contaminates the ionomer/membrane, leading to poor durability. However, the underlying mechanisms by which cation contamination affects fuel cell performance remain poorly understood. Here, we provide a comprehensive understanding of cation contamination effects through the controlled doping of electrodes. We couple electrochemical testing results with membrane conductivity/water uptake measurements and impedance modeling to pinpoint where and how the losses in performance occur. We identify that (1) ∼44% of Co2+ exchange of the ionomer can be tolerated in the electrode, (2) loss in performance is predominantly induced by O2 and proton transport losses, and (3) Co2+ preferentially resides in the electrode under wet operating conditions. Our results provide a first-of-its-kind mechanistic explanation for cation effects and inform strategies for mitigating these undesired effects when using alloy catalysts.

CO2 doping of organic interlayers for perovskite solar cells.

In perovskite solar cells, doped organic semiconductors are often used as charge-extraction interlayers situated between the photoactive layer and the electrodes. The π-conjugated small molecule 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) is the most frequently used semiconductor in the hole-conducting layer1-6, and its electrical properties considerably affect the charge collection efficiencies of the solar cell7. To enhance the electrical conductivity of spiro-OMeTAD, lithium bis(trifluoromethane)sulfonimide (LiTFSI) is typically used in a doping process, which is conventionally initiated by exposing spiro-OMeTAD:LiTFSI blend films to air and light for several hours. This process, in which oxygen acts as the p-type dopant8-11, is time-intensive and largely depends on ambient conditions, and thus hinders the commercialization of perovskite solar cells. Here we report a fast and reproducible doping method that involves bubbling a spiro-OMeTAD:LiTFSI solution with CO2 under ultraviolet light. CO2 obtains electrons from photoexcited spiro-OMeTAD, rapidly promoting its p-type doping and resulting in the precipitation of carbonates. The CO2-treated interlayer exhibits approximately 100 times higher conductivity than a pristine film while realizing stable, high-efficiency perovskite solar cells without any post-treatments. We also show that this method can be used to dope π-conjugated polymers.

Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical Devices: Insights into Structure-Property Relationships.

Rapid improvements in polymer-electrolyte fuel-cell (PEFC) performance have been driven by the development of commercially available ion-conducting polymers (ionomers) that are employed as membranes and catalyst binders in membrane-electrode assemblies. Commercially available ionomers are based on a perfluorinated chemistry comprised of a polytetrafluoroethylene (PTFE) matrix that imparts low gas permeability and high mechanical strength but introduces significant mass-transport losses in the electrodes. These transport losses currently limit PEFC performance, especially for low Pt loadings. In this study, we present a novel ionomer incorporating a glassy amorphous matrix based on a perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) backbone. The novel backbone chemistry induces structural changes in the ionomer, restricting ionomer domain swelling under hydration while disrupting matrix crystallinity. These structural changes slightly reduce proton conductivity while significantly improving gas permeability. The performance implications of this trade-off are assessed, which reveal the potential for substantial performance improvement by incorporation of highly permeable ionomers as the functional catalyst binder. These results underscore the significance of tailoring material chemistry to specific device requirements, where ionomer chemistry should be rationally designed to match the local transport requirements of the device architecture.

Quantum confinement in few layer SnS nanosheets.

Orthorhombic tin monosulfide (SnS) consists of layers of covalently bound Sn and S atoms held together by weak van der Waals forces and is a stable two-dimensional material with potentially useful properties in emerging applications such as valleytronics. Large-scale sustainable synthesis of few-layer (e.g., 1-10 layers) SnS is a challenge, which also slows progress in understanding their properties as a function of number of layers. Herein we describe solvothermal synthesis of SnS in water or ethylene glycol. The latter yields a flower-like morphology where the petals are SnS nanoplates and sonication and separation of these flowers via differential centrifugation yields 1-10 layer SnS nanoplates. The direct optical absorption edges of these SnS nanoplates blue-shift due to quantum confinement from 1.33 to 1.88 eV as the thickness (number of layers) is decreased from ∼5 nm (10 layers) to ∼2 nm (4 layers).

Functionalized ultrasound-propelled magnetically guided nanomotors: toward practical biomedical applications.

Magnetically guided ultrasound-powered nanowire motors, functionalized with bioreceptors and a drug-loaded polymeric segment, are described for "capture and transport" and drug-delivery processes. These high-performance fuel-free motors display advanced capabilities and functionalities, including magnetic guidance, coordinated aligned movement, cargo towing, capture and isolation of biological targets, drug delivery, and operation in real-life biological and environmental media. The template-prepared three-segment Au-Ni-Au nanowire motors are propelled acoustically by mechanical waves produced by a piezoelectric transducer. An embedded nickel segment facilitates a magnetically guided motion as well as transport of large "cargo" along predetermined trajectories. Substantial improvement in the speed and power is realized by the controlled concavity formation at the end of the motor nanowire using a sphere lithography protocol. Functionalization of the Au segments with lectin and antiprotein A antibody bioreceptors allows capture and transport of E. coli and S. aureus bacteria, respectively. Potential therapeutic applications are illustrated in connection to the addition of a pH-sensitive drug-loaded polymeric (PPy-PSS) segment. The attractive capabilities of these fuel-free acoustically driven functionalized Au-Ni-Au nanowires, along with the simple preparation procedure and minimal adverse effects of ultrasonic waves, make them highly attractive for diverse in vivo biomedical applications.

Self-propelled carbohydrate-sensitive microtransporters with built-in boronic acid recognition for isolating sugars and cells.

A new nanomotor-based target isolation strategy, based on a "built-in" recognition capability, is presented. The concept relies on a poly(3-aminophenylboronic acid) (PAPBA)/Ni/Pt microtube engine coupling the selective monosaccharide recognition of the boronic acid-based outer polymeric layer with the catalytic function of the inner platinum layer. The PAPBA-based microrocket is prepared by membrane-templated electropolymerization of 3-aminophenylboronic acid monomer. The resulting boronic acid-based microengine itself provides the target recognition without the need for additional external functionalization. "On-the-fly" binding and transport of yeast cells (containing sugar residues on their wall) and glucose are illustrated. The use of the recognition polymeric layer does not hinder the efficient propulsion of the microengine in aqueous and physiological media. Release of the captured yeast cells is triggered via a competitive sugar binding involving addition of fructose. No such capture and transport are observed in control experiments involving other cells or microengines. Selective isolation of monosaccharides is illustrated using polystyrene particles loaded with different sugars. Such self-propelled nanomachines with a built-in recognition capability hold considerable promise for diverse applications.