Qixing Wu

Charge carriers in alkaline direct oxidation fuel cells

Contrary to conventional wisdom, this study demonstrates that the main charge carrier of alkaline direct oxidation fuel cells (DOFCs) running on ethanol with a cation exchange membrane (typically Nafion) is OH− ions, rather than Na+ ions.

A microfluidic-structured flow field for passive direct methanol fuel cells operating with highly concentrated fuels

Conventional direct methanol fuel cells (DMFCs) have to operate with excessively diluted methanol solutions to limit methanol crossover and its detrimental consequences. Operation with such diluted methanol solutions not only results in a significant penalty in the specific energy of the power pack, limiting the runtime of this type of fuel cell, but also lowers the cell performance and operating stability. In this paper, a microfluidic-structured anode flow field for passive DMFCs with neither liquid pumps nor gas compressors/blowers is developed. This flow field consists of plural micro flow passages. Taking advantage of the liquid methanol and gas CO2 two-phase counter flow, the unique fluidic structure enables the formation of a liquid–gas meniscus in each flow passage. The evaporation from the small meniscus in each flow passage can lead to an extremely large interfacial mass-transfer resistance, creating a bottleneck of methanol delivery to the anode CL. The fuel cell tests show that the innovative flow field allows passive DMFCs to achieve good cell performance with a methanol concentration as high as 18.0 M, increasing the specific energy of the DMFC system by about five times compared with conventional designs.

A hierarchical micro/mesoporous carbon fiber/sulfur composite for high-performance lithium–sulfur batteries

A carbon matrix with an appropriate porous structure plays a vital role in developing high-performance sulfur/carbon cathodes of lithium–sulfur batteries. In this work, a hierarchical porous carbon fiber (HPCF) with a few mesopores and abundant micropores was prepared via electrospinning with a SiO2 template and subsequent KOH activation. The HPCF with an ultra-high surface area and a large pore volume can construct a loose network structure to promise high sulfur utilization and sufficient sulfur loading. Mesopores can provide pathways for the infiltration of electrolyte to ensure fast transport of lithium ions during electrochemical reactions, whereas micropores can effectively suppress the diffusion of polysulfides by their strong adsorption capability. Due to such advantages, the proposed cathode, with 66 wt% sulfur content, can yield a high reversible capacity of 1070.6 mA h g−1 at 0.5C, and a stable cycle performance with a capacity retention of 88.4% after 100 cycles.

PEDOT-PSS coated sulfur/carbon composite on porous carbon papers for high sulfur loading lithium–sulfur batteries

Increasing the sulfur loading in the cathode of a lithium–sulfur battery is an important way to improve its capacity for practical applications. To achieve this, the present work proposes using PEDOT-PSS to encapsulate sulfur/carbon black (S/BP) by a facile solution mixing method and the formed composite is then coated on a porous carbon paper. It is believed that the formation of a core/shell structure in the PEDOT@S/BP composite can promote the electron transport and effectively impede the diffusion of polysulfides, and the porous carbon paper is able to retain the electrolyte containing the dissolved polysulfides in the cathode and alleviate the adverse effect of sulfur volumetric expansion. Due to such advantages, the proposed cathode, with a high sulfur loading of 3 mg cm−2, can yield a high reversible capacity of 1041 mA h g−1 and an excellent cycle stability with a capacity retention of 868 mA h g−1 after 100 cycles and an average coulombic efficiency of 99.4%.

Experimental Study of Single Phase Flow in a Closed-Loop Cooling System with Integrated Mini-Channel Heat Sink

The flow and heat transfer characteristics of a closed-loop cooling system with a mini-channel heat sink for thermal management of electronics is studied experimentally. The heat sink is designed with corrugated fins to improve its heat dissipation capability. The experiments are performed using variable coolant volumetric flow rates and input heating powers. The experimental results show a high and reliable thermal performance using the heat sink with corrugated fins. The heat transfer capability is improved up to 30 W/cm2 when the base temperature is kept at a stable and acceptable level. Besides the heat transfer capability enhancement, the capability of the system to transfer heat for a long distance is also studied and a fast thermal response time to reach steady state is observed once the input heating power or the volume flow rate are varied. Under different input heat source powers and volumetric flow rates, our results suggest potential applications of the designed mini-channel heat sink in cooling microelectronics.

High-absorption recyclable photothermal membranes used in a bionic system for high-efficiency solar desalination via enhanced localized heating

Desalination utilizing solar energy is an effective way to mitigate the crisis of water shortage, but its large-scale application in production has been largely limited by the low evaporation efficiency. Here, we report a plate thermal reduction (PTR) method for rapid preparation of plasmonic-active filter paper (PP) as the photothermal material with broadband solar absorption over 92%. By imitating the spontaneous water circulation mechanism of plants, we designed a high-efficiency bionic solar evaporation and desalination system. Air with excellent low thermal conductivity was used as the thermal insulation material. Because of the enhanced localized heating effect, the system features a high solar evaporation efficiency of up to 89% under 10 kW m−2. Furthermore, under natural sunlight, the bionic system has a drinkable freshwater production rate of 0.97 kg m−2 h−1 under about 0.9 kW m−2 with the highest evaporation efficiency of 79% for one day. With simple material preparation and efficient and stable performance, the bionic system is ideal for large-scale production to tackle energy, water resource and environmental issues.

Preparation and properties of branched sulfonated poly(arylene ether ketone)/polytetrafluoroethylene composite materials for proton exchange membranes

Branched sulfonated poly(arylene ether ketone)s (BSPAEKs) exhibit excellent oxidative stability and solubility, making them suitable for proton exchange membranes (PEMs). However, the mechanical properties of branched membranes cannot fully satisfy the requirements of PEMs. In this work, BSPAEK/polytetrafluoroethylene (PTFE) composite membranes are prepared by casting a BSPAEK solution onto porous PTFE films that contain different concentrations of Triton surfactant to reinforce their mechanical properties. The properties of the composite membranes, including their mechanical properties, proton conductivities, oxidative stabilities, water uptake, thermal stabilities and swelling ratios, are investigated experimentally. The tensile strength of BSPAEK/PTFE-7 (7 wt% Triton as a surfactant) is 26.0 MPa, which is 2.1 times higher than that of a pristine membrane. In addition, the BSPAEK/PTFE composite membranes exhibit excellent dimensional and oxidative stabilities. The BSPAEK/PTFE-5 (5 wt% Triton as a surfactant) composite membrane is tested in a direct methanol fuel cell (DMFC), and it can yield a peak power density of 69.70 mW cm−2 at 60 °C, which is somewhat comparable to those using Nafion membranes.

Catalytic performance of a pyrolyzed graphene supported Fe–N–C composite and its application for acid direct methanol fuel cells

In this work, a graphene supported Fe–N–C composite catalyst, synthesized by pyrolysis of graphene oxide (GO), graphitic carbon nitride (g-C3N4), ferric chloride (FeCl3) and carbon black (Vulcan XC-72), was evaluated for oxygen reduction reaction (ORR) in acid media. The introduction of carbon black was to separate the graphene sheets to enhance the specific surface area and thus improve the catalytic activity of the catalyst. The experimental results showed that the composite catalyst could yield an average electron transfer number of 3.85 and its onset and half-wave potentials for acidic ORR were only 56 and 69 mV smaller than those of Pt/C (40 wt% Pt) catalyst, respectively. The as-prepared catalyst was applied in an acid direct methanol fuel cell as the cathode catalyst and a peak power density of 11.72 mW cm−2 at 30 °C was demonstrated when feeding the anode and cathode with a 1 M methanol solution and air, respectively, suggesting its promising application.

Bio-inspired multiscale-pore-network structured carbon felt with enhanced mass transfer and activity for vanadium redox flow batteries

Simultaneously achieving fast mass transfer and high electrochemical activity has been a central issue of the porous electrode design of vanadium redox flow batteries (VRFBs). In this work, we present a bio-inspired multiscale-pore-network structured carbon felt electrode via chemistry of metal extraction for VRFBs. For the uniquely developed electrode, the micron-scale pores formed by the intersected carbon fibers are favorable for electrolyte permeation while the vanadium ion diffusion and redox reactions are promoted by the submicron-scale and nanoscale pores on the carbon fiber surfaces. Besides, oxygen and nitrogen functional groups are also introduced onto electrode surfaces to synergistically improve the catalytic activity and wettability. The superb structural and surface characteristics of the present multiscale electrode result in an encouraging VRFB performance with an energy efficiency of 81.9% at a current density of as high as 320 mA cm−2, which is 15.2% higher than that of pristine (single-scale) carbon felt electrodes. Furthermore, the battery with the multiscale electrodes can stably operate for 500 cycles without an obvious efficiency decay, verifying the excellent stability of such a structure. In principle, the multiscale electrode design can also be extended to other types of flow-cell architectured electrochemical systems that involve multiscale processes.

Study on the Mixed Electrolyte of N,N-Dimethylacetamide/Sulfolane and Its Application in Aprotic Lithium–Air Batteries

Aprotic lithium–air batteries have recently drawn considerable attention due to their ultrahigh specific energy. However, the chemical and electrochemical instability of the electrolyte is one of the most critical issues that need to be overcome. To increase the stability and maintain a relatively high conductivity of the lithium ion, a mixed electrolyte of sulfolane (TMS) and N,N-dimethylacetamide (DMA) was evaluated and tested in an aprotic lithium–air battery. The physical and chemical characterizations showed that the mixed electrolyte exhibited a relatively low viscosity, high ionic conductivity and oxygen solubility, and good stability. In addition, it was found that lithium–air batteries with an optimized electrolyte composition (DMA/TMS = 20:80, % v/v) showed a better cycle life and lower charge overpotential as compared to those with electrolytes with a single solvent, either DMA or TMS.