Hydrogen storage alloys for remote area power supply

Abstract

To supply electricity to a remote area community from national power grids is expensive and technically difficult. One of the possible solutions is to build a self-sustaining power generation system by harvesting the free renewable energy. However, the key issue to address in utilizing renewable energy is its intermittent nature that cannot guarantee a non-interruptible power supply at all time. Hence, this work proposed to store the energy in the form of hydrogen because of its superior energy density. The excess energy from renewable energy source is converted to hydrogen energy via electrolysis. The hydrogen is stored and used by the fuel cells to generate electricity. Nevertheless, hydrogen has a very low density at ambient pressure and temperature, which increases the complexity to store it in safe and economical manner.

Metal hydrides can be used to address this issue because of its extremely high volumetric hydrogen storage capacity. The aim of this work was to develop a new type of alloy that can be used in Remote Area Power Supply (RAPS). The designed alloys should have a hydrogen storage capacity of more than 1.00 wt% and have a capability to store and reverse the hydrogen within the pressure range of 0.10 to 1.00 MPa at room temperature. In addition to the fast absorption kinetics (less than 100 s for 1 g of sample), the alloy should also have the capability to retain at least 50% of hydrogen storage efficiency with at least 1.00 wt% of hydrogen storage capacity after 500 charge-discharge cycles. It is expected that the newly designed alloys can save at least 10% of raw materials cost as compared to the AB5 type alloys.

In this work, La-Mg-Ni based AB3 type Hydrogen Storage Alloy (HSA) was selected as the candidate. It was found that the hydrogen storage capacity was 1.67 wt%. An AB5 HSA has also been chosen for comparison. The hydrogen storage capacity of the La-Mg-Ni based AB3 was approximately 40% higher than the conventional AB5 type alloys. The effects of partial substitutions of both Ce and Al on the hydrogenation properties of La(0.65-x)CexCa1.03Mg1.32Ni(9-y)Aly were investigated simultaneously using factorial design. Both Ce and Al additions greatly improve the reversibility of hydrogen storage capacity. However, the maximum hydrogen storage capacity and absorption kinetics can be affected by the additions. As Ce and Al give opposite effects on the absorption and desorption plateaus, response surface methodology can be used to tune and optimize the properties of the HSA to the desired operating conditions for fuel cell applications.

The Johnson-Mehnl-Avrami-Kolmogorov model was used to understand the kinetics and hydrogen absorption mechanisms of La-Mg-Ni based HSA. Nonetheless, the experimental data cannot fit into the model with a single slope line, demonstrating that there was more than one mechanism operating. Hence the results were split into two regions according to their slopes. The results showed that the dominant rate-limiting step of samples with Al addition were interface-controlled absorption. On the other hand, a diffusion-controlled reaction is applicable to all other fast absorbing samples, as well as the second region of the absorption where hydrides formation are closed to saturation.

The effects of Ce and Al on the cycle stability of the La-Mg-Ni based HSA have also been investigated. The cycle stability was mainly enhanced by Al additions; unfortunately excessive addition of Al deteriorated the hydrogen storage capacity unanimously. Hence, even though La-Mg-Ni based HSA is more price-competitive than AB5 type HSA, its commercial readiness is limited by its efficiency. A hybrid system between AB5 type and La-Mg-Ni based HSA could be a solution. This work indicated that a composite with 50 wt% of each type of HSA had a superior cycle stability with a reasonable capacity retention and operating pressure plateaus, as well as, a 10% cost saving in raw materials. This work has successfully demonstrated the viability of HSA as the energy storage medium for RAPS application.