Structural and electrochemical properties of TiFe alloys synthesized by ball milling for hydrogen storage

The binary TiFe alloy was synthesized by mechanical alloying (MA) under argon atmosphere at room temperature. The effect of ball to powder weight ratio on the microstructures was characterized by X-ray diffraction (XRD). The effect of milling time on the electrochemical and activation properties was investigated by scanning electron microscope (SEM), galvanostatic charging and discharging, constant potential discharge, and potentiodynamic polarization techniques. Relationships between electrochemical properties, such as polarization, variation of electrochemical discharge capacity, ratio exchange current density, and Nernst potential and alloy compositions were evaluated. XRD results showed that with increasing ball to powder weight ratio, the amorphization process is accelerating and powders milled with a ratio of 1:8 have the highest conversion rate to TiFe. SEM observations reveal that particles show cleavage fracture morphology and size distribution is generally normalized. TiFe milled during 40 h was easily activated within 5 cycles and showed the best discharge capacity equal to 147 mAh g(-1). A good cycling was observed after 20 cycles at ambient temperature for the alloy milled for 30 h. A correlation between alloy composition, report, exchange current density, and Nernst potential on one hand and the variation of the electrochemical discharge capacity during cycling for different milling times on the other hand was observed.