Nitrogen-doped porous carbon materials show excellent water adsorption ability by forming strong hydrogen bonding between water molecules and the doped atoms. When these porous carbon materials are used to construct a water management layer (WML) of a passive direct methanol fuel cell (DMFC), high water concentration and hydraulic pressure formed inside the cathode catalyst layer would facilitate the water recovery from cathode to anode. In this paper, a highly hydrophilic nitrogen-doped carbon aerogel was synthesized by the carbonization of hydrogel precursors composed of resorcinol, formaldehyde, and graphene oxide under ammonia, and it was used for the first time to construct the WML for liquid-feed and vapor-feed passive DMFCs. The results show that the WML significantly improves the output performance of the liquid-feed DMFC by enhancing the water recovery, which is characterized and proved by the smaller cathode polarization, the slightly increased anode polarization, and a released cathode water flooding situation. A new method was also proposed to study the in situ methanol crossover of DMFCs, which confirmed that the methanol crossover during the discharge was reduced by the WML. As for the vapor-feed DMFCs, the WML reduces both the cathode and anode polarizations significantly, which increases the output performance greatly. This study opens a new window for the design and optimization of the membrane assembly electrode of DMFCs.
Although the hybrid power system that combines a photovoltaic cell and a lithium-ion battery is increasingly mature and practical, long-lifetime auxiliary power will be still needed in severe weather conditions. A small-volume hydrogen–oxygen fuel cell system based on the hydrolysis of NaBH4 is designed. The fuel cell system contains a tiny hydrogen generator, a hydrogen cleaner, and a small fuel cell stack consisting of three units in series. The relationship between the amount of catalyst and output performance is discussed. The long-time discharging results indicate that the fuel cell system has high power capacity. The compact design allows the fuel cell system to integrate the structure with a photovoltaic cell and lithium-ion cell and forms a hybrid power system with a small package. The power management circuit for these power sources without logic devices is designed and tested. The control strategy selects the photovoltaic–battery subsystem as the primary power source, and the fuel cell subsystem works as the backup power source to handle the circumstance when the energy stored in the battery is exhausted. The test results show that the power management system could switch the power supply automatically and timely under various emergency conditions, and the output voltage remains stable all the time.
Summary
In this paper, we report a facile method to synthesize a nitrogen‐doped carbon catalyst for oxygen reduction reaction (ORR). The catalyst was prepared via subsequent carbonization of the composite aerogel under NH3 and N2, which was synthesized by the hydrothermal treatment of resorcinol formaldehyde (RF) and graphene oxide (GO). The research focused on the effect of the heat treatment temperature on the carbon defects, doped nitrogen and ORR activity of the catalyst. The results show that more carbon defects can be formed under a relatively low temperature inside the catalyst which also possesses a high ratio of graphitic nitrogen. Interestingly, the catalyst shows a moderate ORR activity in alkaline (E1/2 = 0.78 V) but a good ORR activity in acids (E1/2 = 0.72 V), which should be attributed to its high content of carbon defects or graphitic nitrogen. Moreover, a direct methanol fuel cell with the nitrogen‐doped carbon as cathode catalyst was fabricated and tested, which gives peak power densities of 18.27 and 16.14 mW cm−2 at 4 and 14 M methanol solution, respectively. All the results reveal that the carbonization of RF‐GO composite aerogel is a promising facile method to prepare ORR catalysts with high activity.
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