This communication reports the design and characterization of an air-breathing laminar flow-based microfluidic fuel cell (LFFC). The performance of previous LFFC designs was cathode-limited due to the poor solubility and slow transport of oxygen in aqueous media. Introduction of an air-breathing gas diffusion electrode as the cathode addresses these mass transfer issues. With this design change, the cathode is exposed to a higher oxygen concentration, and more importantly, the rate of oxygen replenishment in the depletion boundary layer on the cathode is greatly enhanced as a result of the 4 orders of magnitude higher diffusion coefficient of oxygen in air as opposed to that in aqueous media. The power densities of the present air-breathing LFFCs are 5 times higher (26 mW/cm2) than those for LFFCs operated using formic acid solutions as the fuel stream and an oxygen-saturated aqueous stream at the cathode ( approximately 5 mW/cm2). With the performance-limiting issues at the cathode mitigated, these air-breathing LFFCs can now be further developed to fully exploit their advantages of direct control over fuel crossover and the ability to individually tailor the chemical composition of the cathode and anode media to enhance electrode performance and fuel utilization, thus increasing the potential of laminar flow-based fuel cells.
Among the different polymorphic modifications of nickel hydroxide, the -form is widely used as the positive electrode material in all nickel based secondary cells. 1 -Nickel hydroxide consists of a hexagonal packing of hydroxyl ions with Ni 2ϩ occupying alternate rows of octahedral sites. This results in an orderly stacking of chargeneutral slabs of composition [Ni(OH) 2 ], with a basal spacing of 4.6 Å. When different types of defects such as growth faults and stacking faults are introduced, modified  phases differing from one another in their crystallinity are obtained. 2 These phases are referred to as  bc (bc: badly crystalline). 3 Further loss of crystallinity can come about by the interstratified growth of ␣-nickel hydroxide within the -phase. 4 The ␣-phase is a hydrated form of nickel hydroxide characterized by a higher basal spacing (7.6 Å) and turbostratic disorder. 5 These different modified forms of -nickel hydroxide can be distinguished from each other by the characteristic broadening of lines observed in the powder X-ray diffraction (PXRD) patterns. 2,6,7 As a part of our continuing effort 8,9 to understand the origin of the superior electrochemical activity of high performance electrode materials, we examined pasted electrodes fabricated from  and  bcnickel hydroxide samples. The objective of the present study is to examine the effect of crystallinity of the active material on the electrode performance. Pasted electrodes comprising  bc -nickel hydroxide exhibit a reversible discharge capacity 2.2 times higher than that exhibited by electrodes comprising -nickel hydroxide. We report our investigations on the possible causes for the superior performance of the badly crystalline material. Experimental  bc -Nickel hydroxide was obtained by chemical precipitation, by the addition of NaOH (2 M; 110 mL) to a solution of Ni(NO 3 ) 2 (3.4 M; 16 mL) under constant stirring at 80ЊC. The resulting precipitate was aged in the mother liquor at 60ЊC for 18 h. The precipitate was then filtered, washed free of alkali, and dried to constant weight at 100ЊC. The sample was stored without dessication.-Nickel hydroxide was obtained from a commercial source (Aldrich Chemical Co., USA) and used as such.All samples were characterized by powder X-ray diffractometry (JEOL JDX8P powder X-ray diffractometer, Cu K␣ source, ϭ 1.541 Å), infrared spectroscopy (Nicolet model Impact 400D Fourier transform infrared spectrometer, KBr pellets, 4 cm Ϫ1 resolution), electron microscopy (JEOL JEM 200CX electron microscope 200 kV, holey carbon grids: 200 mesh size), and thermogravimetric analysis (lab built system, heating rate, 5ЊC min Ϫ1 ). Drying studies were carried out by holding the samples in air in the thermogravimetric (TG) balance at 100ЊC for 30 min. The samples were cooled in the balance and the weight gained, if any, was registered.The PXRD patterns were simulated using the DIFFaX code (Version 1.8.1). 10 Positions of heavy atoms inside the hydroxide slabs
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.