Designing And Synthesis Of TiFe-based Hydrogen Storage Materials Via Substitution Of Transition Metal Additives

Energy is the basic requirement of the modern technological world which can sustain the development of a country,however,the existent energy resources are going to be limited with the increasing demand and utilization of energy with rapid growth of population.On another hand,the undesirable transformation of climate with high utilization of the fossil fuels is becoming the toughest challenge for the human beings,therefore,an alternative energy source is essential which could be reliable,sustainable,eco-friendly and cost effective.Hydrogen energy is considered as an alternate choice to replace the traditional fossil fuels owing to the clean and renewable source of energy which can only produce water as a waste product while using in combustion engines.However,prior to consider the hydrogen as a future energy choice,some major challenges must be needed to overcome such as low energy density and high volume of the container in order to provide an efficient and safe storage.The traditional methods such as liquidous or gaseous form of hydrogen storage do not qualify for the practical applications due to lower energy density and high volume,therefore,the solid-state materials for hydrogen storage open new promises of safe and efficient hydrogen storage for stationary and on-board applications.Among the solid-state materials,metal hydrides such as AB type Ti Fe intermetallic compound absorb high content of hydrogen reversibly 1.89 wt% at moderate temperature and pressure,however,the problems associated with Ti Fe alloy are the sluggish activation performance,oblique plateau pressure for the absorption/desorption and lower content of hydrogen.Therefore,it is essential to consider the above mentioned problems and identify the way to improve the performance of Ti Fe based hydrogen storage intermetallic compounds.In this study,the substitution method is employed in order to modify and synthesize novel compositions of Ti Fe intermetallic compounds via partial substitution of the transition metals such as Mn,Cu,Co and Y,which can improve the hydrogen storage and activation performance.The proposed sample Ti0.95Y0.05Fe0.86Mn0.05Cu0.05 is prepared via water-cooled copper crucible under the argon atmosphere to observer the effects of Cu and Y on the hydrogen storage and activation performances.The structural and phase analysis of the as-synthesized samples are characterized by x-ray diffraction(XRD),whereas the morphology and elemental composition are measured by scanning electron microscopy/energy dispersive spectroscopy(SEM/EDS)respectively,thereafter,the hydrogen storage performance,kinetic rates and initial activation reaction are measured by using pressure composition temperature(P–C–T)testing machine.However,some shortcomings(low capacity and oblique platform)are existed in the previous study,therefore,further systematic study on the hydrogen storage and activation performance with modification of microstructure via simultaneous substitution of Cu and Y in Ti Fe alloy and the designed samples Ti Fe0.86Mn0.1Y0.1-x Cux(where x = 0.01,0.03,0.05,0.07,0.09)are prepared.Nevertheless,the Cu addition degrades the hydrogen capacity with improving the activation mechanism,therefore,Co and Y are introduced in Ti Fe alloy with appropriate amount of Mn and the designed samples Ti Fe0.86Mn0.1Co0.1-x Yx(where x=0.02,0.04,0.06)are synthesized.The main conclusions are mentioned herein.Ⅰ.The Cu and Y additives are partially substituted in Ti Fe alloy with an appropriate amount of Mn in order to facilitate the activation mechanism and improve the hydrogen storage performance,dynamic rates and plateau pressure platform.The XRD results show that the modified Ti0.95Y0.05Fe0.86Mn0.05Cu0.05 sample exhibits Cs Cl order structure and the surface morphology studies reveal that the alloy shows biphasic nature,which composes of Ti Fe alloy matrix and Cu2 Y secondary phase.The addition of element Y could be enhanced significantly the hydrogen storage capacity due to the enlargement of lattice parameters and the maximum capacity 1.85 wt% at 20 oC is attained,whereas the higher affection of element Y towards the hydrogen atom than Ti leads to the formation of stable hydrides.Ⅱ.In addition,the activation and kinetic performances of the hydrogenated alloy could be improved significantly due to the catalytic performance of element Cu in the parent alloy and the intermetallic secondary phase Cu2 Y,which accelerate the propagation of hydrogen flux into the alloy via new fresh surfaces.This may presume from the above results that Ti0.95Y0.05Fe0.86Mn0.05Cu0.05 alloy exhibits high hydrogen storage and well activation performance.Ⅲ.The elements Cu and Y are simultaneously substituted with an appropriate amount of element Mn in Ti Fe alloy in order to further enhance the hydrogen storage performance,improve the activation mechanism and kinetics rates,plateau pressure platform and thermodynamic characteristics,therefore the designed alloys Ti Fe0.86Mn0.1Y0.1-x Cux(where x= 0.01,0.03,0.05,0.07,0.09)are synthesized.The results show that the hydrogen storage capacity increases first and then decreases slightly with increase of Y additives,the sample Ti Fe0.86Mn0.1Y0.05Cu0.05 achieves the highest hydrogen capacity 1.89 wt% at 10 ?C,whereas the absorption/desorption plateau pressure and slope are decreased.The element Y exhibits the tendency to elongate and stabilize the first hydrogen plateau platform,whereas due to higher affection of Y towards the hydrogen atoms the stability is increased with the increase of Y content.Ⅳ.The element Cu causes the deterioration in hydrogen capacity with the increase of Cu content,however it improves the activation performance significantly.Furthermore,the activation performance and kinetic rates of each alloy are improved with secondary phase particles(Cu Y and Cu4Y)which provide alternative channels for hydrogen flux to penetrate into the matrix and the Ti Fe0.86Mn0.1Y0.05Cu0.05 alloy shows the fastest kinetic rates at 10 ?C.The interfaces between the matrix and secondary phase particles are very active for hydrogen absorption and improve hydrogen absorption performance remarkably,which may infer that Cu and Y additives in Ti Fe alloys improve the hydrogen storage and activation performance which could be suitable for practical applications.Ⅴ.The Cu substitution results in the reduction of hydrogen storage performance,therefore,element Co is partially substituted simultaneously with elements Y and Mn in Ti Fe alloy in order to improve the activation performance without degradation of hydrogen storage capacity and the designed samples Ti Fe0.86Mn0.1Co0.1-x Yx(where x=0.02,0.04,0.06)are synthesized.X-ray diffraction results elucidate that samples at x=0.02 and 0.04 are mainly composed of Ti Fe single phase which exhibits Cs Cl order structure,whereas sample at x=0.06 shows minor peaks corresponding to Y-rich particles.The SEM/EDS results show that each sample comprise of Ti Fe matrix with Y-rich secondary phase particles(SPPs),whereas sample x=0.04 is further characterized by transmission electron microscope which confirms that the secondary phase is Y-rich particle.Ⅵ.The samples mentioned-above are activated easily,the hydrogen storage capacity increases first and then deteriorates with the increasing of x,whereas sample at x=0.04 exhibits the highest hydrogen storage capacity 1.96 wt% at 10 °C owing t o Co and Y incorporation.Furthermore,the plateau platform(α+β)region is flattened and the slope reduces significantly,whereas the dynamic rates are registered faster.The SPPs and interfaces between SPPs and matrix play an important role in promoting hydrogenation performances,which may imply that Co and Y additives are promising candidates for improving hydrogenation performance of Ti Fe alloy and could be used for practical applications.