Development of CNTs reinforced metal matrix composite as a hydrogen source for PEMFC based auxiliary power unit

Abstract

An Auxiliary Power Unit (APU), an electrical power source for a vehicle system, operates independently from the main engine responsible for the propulsion. It has been reported that military tank equipped with the APU drastically saves fuel consumption by more than 70 % and hence significally increases the lifetime of engine by reducing the load of engine during idling. Fuel cell system is a promising device as an APU for military tank because it is highly efficient in power generation and very silent during its operation. Especially, the polymer electrolyte membrane fuel cell (PEMFC) has been considered as a power source for the APU based military tank due to its short starting time and low working temperature. Hyrogen station or the hydrogen storage and dispense system is essential to the operation of the fuel cell based APU. However the construction and maintainance of the hydrogen station needs a large investment cost. The on-board hydrogen production system in which hydrogen is produced on the just-on-site is very attractive because it removes all the processes for hydrogen transportation, dispense, and storage. The on-board hydrogen production system from the hydrolysis of chemical hydrides such as NaBH4 has been extensively studied. However, the price of the chemical hydrides is too expensive to be used for large scale applications such as armed vehicles and tanks. For the large scale applications, metallic fuels such as Al and Mg are more attractive as a hydrogen generating source due to its low cost and abundance. On-board hydrogen generation at a large scale can be achieved from the hydrolysis of metallic fuel or active metals such as Al and Mg. However, the hydrogen generation rates from the hydrolysis of the bulk Al and Mg are too low to be applied to fuel cells due primarily to oxides formed on their surfaces in air and water. The hydrogen generation rate from the hydrolysis of metallic fuels such as Al and Mg is proportional to their corrosion rate in aqueous solution where hydrolysis reaction occurs

alkaline water for Al and sea water for Mg. Hence, small amount addition of an electrochemically noble element to the active metal (metallic fuel) would significantly increase the hydrogen generation rate by causing a galvanic corrosion between the active metal and the electrochemicaly noble element. CNT reinforced metal matrix composite have been used as an armour plate and platform material in military applications due to its high mechanical strength. The CNTs in CNTs/Al composite would cause a strong galvanic corrosion of Al because of electrochemical noble nature of CNTs coupled to Al, hence would increase the hydrogen generation rate from the hydrolysis of Al. In the present work, CNT reinforced metal matrix composites such as CNTs/Al and CNTs/Mg were synthesized by spark plasma sintering (SPS) process, and then their potential use as hydrogen generating materials for the on-board hydrogen generation applications were examined. In the first part, we fabricated a CNTs reinforced Al composite by SPS and analyzed the effects of the CNTs addition and porous Al matrix on the hydrogen generation rate. The maximum hydrogen generation rate of CNTs/Al composite in the 10 wt. % NaOH solution at room temperature was 122 ml/min.g that is much fater than that (2.6ml/min.g) of the bulk Al. The fast hydrogen generation rate of the 5 vol.% CNT reinforced Al composite appears to be associated with the galvanic corrosion of Al coupled to the electrochemically noble CNTs and also with porous Al matrix with large reaction area. The galvanic current density between the CNTs and Al matrix in a 10 wt.% NaOH solution was $8.2 mA/cm^2$ that was 4 times higher than that $(1.89 mA/cm^2)$ of Al. Corrosion potential of CNT is 1.4 V noble to that of Al in 10 wt.% NaOH solution at a room temperature. Evidently, the strong galvanic corrosion of Al coupled to CNTs cause a significant increase in the hydrogen generation rate of the CNTs/Al composite. Furthermore, the porous matrix formed during SPS contributed to the increase in hydrogen generation rate of the CNTs/Al composite that is dependent on the pressure applied in the SPS process. The BET specific surface area of the SPS processed Al at 20 MPa was $1.0663 m^2/g$. This value is 5 times larger than that $(0.2176 m^2/g)$ of the SPS processed Al at 50 MPa. A porous Al structure with high specific surface area can be achievable by applying a low stress during SPS process. The Al sintered at 20 MPa exhibited 63 ml/min.g of hydrogen generation rate that is 3 times faster than that (20 ml/min.g) of the Al sintered at 50 MPa. Beneficially, the hydrogen generated from the hydrolysis of CNTs/Al composites did not contain any carbon monoxide (CO) that is very harmful to operation of PEMFC. We also examined the feasibility of CNTs/Al composite as a hydrogen source for PEMFC by single cell test. The hydrolysis reactor directly connected to anode of PEMFC, and then the hydrogen generated from the hydrolysis of CNTs/Al composite directly was fed to anode side of the cell. The 3.5 g of CNTs/Al sustained 13 min at 10 A, and 0.73 V, which is equivalent to 0.47 kWh per kg-CNTs/Al composite. In the second part, we fabricated CNTs reinforced Mg matrix composites as a hydrogen source in a neutral solution by SPS and analyzed the effects of the CNTs addition on the hydrogen generation rate. Hydrogen generation from the hydrolysis of Mg may occur in a neutral aqueous chloride solution such as sea water whereas the hydrolysis of Al occurs only in a strong alkaline solution. However, the hydrogen generation rate from the hydrolysis of Mg is extremely slow and linearly proportional to the corrosion rate of Mg in chloride water. Mg shows a fast corrosion rate when in galvanic contact with graphite in sea water. CNTs/Mg composite would exhibite a fast hydrogen generation rate because of strong galvanic corrosion between dispersed CNTs and Mg matrix in sea water. In fact, the hydrogen generation rate of 5 vol.% CNTs/Mg composite in the 10 wt.% NaCl solution at room temperature exhibited 15.8 ml/min.g which is 3300 times faster than that (0.0048 ml/min.g) of Mg as we expected. The galvanic current density of between Mg and graphite was measured to be $2.5 x 10^{-2} A/cm^2$ that is 50 times faster than that $(4.39x 10^{-4) A/cm^2)$ of Mg in 10 wt.% NaCl solution at room temperature. Evidently, the CNTs dispersed uniformly in the composite act as numerous cathodes, and hence they cause the hydrogen generation rate to be increased dramatically by enhancing the hydrolysis of Mg by galvanic corrosion wherever Mg is in contact with the CNTs. Further, CNTs did not produce any undesirable carbon compounds during the SPS process or during hydrolysis. The 5 vol.% CNTs/Mg composite also can be used as a on-board hydrogen generating source for a portable fuel cell system due to its excellent hydrogen generation properties in aqueous chloride solution such as sea water.