Studies on Catalytic Hydrogen Generation from Sodium Borohydride

Abstract

Solid-state hydrogen storage has acknowledged substantial concern as a potential for hydrogen source for portable fuel cell applications. This method involves storage of hydrogen in complex chemical hydrides. These hydrides have high hydrogen content and hydrogen can be released through several chemical pathways. Sodium borohydride (NaBH4) stands out as preeminent among chemical hydrides owing to its high hydrogen storage capacity (10.8 wt%) and potentially safe operational uses. However, NaBH4 hydrolysis system also suffers from some major drawbacks: variance between theoretical and practical gravimetric hydrogen storage densities, solubility of residue (NaBO2 based by-products) and cost of NaBH4.

This study basically focuses on reducing the gap between theoretical and experimental hydrogen storage densities that increases the overall efficiency of NaBH4 based hydrogen generation (HG) system. Solubility of NaBO2 is the major hindrance that repels in globalizing the use of NaBH4 based HG system. Therefore, present study also highlights the analysis of residue by various characterization techniques. These techniques assist in determining various reactions that could occur in the system and that effect hydrogen generation rate (HGR). Considering the cost of NaBH4, this work is based on combining NaBH4 with appropriate catalyst promoter that additionally promotes HGR of the system. This combined dual-solid-fuel system is highly efficient in terms of hydrogen storage capacities compared with single hydride based system. It effectively decreases the use of NaBH4 and reduces its cost. Kinetics of NaBH4 hydrolysis reaction without addition of catalyst is poor and slow therefore an appropriate catalyst is required to improve the reaction kinetics. Consequently, after the selection of catalyst, parameters that affect the rate of reaction like concentrations of all the reactants and temperature are studied and kinetic parameters are determined. Therefore, in the present work detailed kinetic study is carried out on two hydrogen generation systems (with and without addition of catalyst promoter). Cobalt chloride hexahydrate (CoCl2.6H2O) is chosen as a most efficient and low cost catalyst for NaBH4/H2O system, compared with various platinum and ruthenium based catalysts that are not suitable for long term use due to the cost issues. Moreover, strong cationic charge on cobalt and high solubility of chlorine are the reasons behind high reactivity of CoCl2 for NaBH4 hydrolysis as compared to other Co based salts.

The reaction rate of NaBH4 hydrolysis is affected by the concentrations of reactant (NaBH4), stabilizer (NaOH), catalyst (CoCl2.6H2O) and reaction temperature. All these factors are observed and it is concluded that HGR increases with increase concentration of each factor. After observing these parameters, the kinetic studies are performed using power law kinetic model. The order and individual rate constants for each reactant are calculated. HGR is also investigated at different temperatures (293, 303, 313 and 323 K) using Arrhenius equation, at constant NaBH4 (1.25 moles/L), NaOH (1.6 moles/L) and CoCl2 (0.02 moles/L) concentrations. The activation energy is calculated for this system is 46 kJ/moles. Hydrogen generation densities at different NaBH4 concentrations are compared. For example, experimental hydrogen density of 7.9 wt% and theoretical hydrogen density of 10.8 wt% is observed for NaBH4 at room temperature and atmospheric pressure. The overall efficiency at this NaBH4 concentration is 73%. It is observed that experimental hydrogen density is quite lower as compared to the theoretical one. Thus, to make the system more competent and proficient improvement in terms of overall efficiency is required. The next section of the work deals with addition of catalyst promoter in the solution to increase the overall efficiency of the system. The concept to enhance hydrogen storage densities through coupling reactions or by using dual-solids for hydrogen generation is further studied in this work. Various catalyst promoting materials like γ-Al2O3 nanoparticles, γ-Al2O3 particles, CNT, MMT clay, SiO2, zeolite and zirconia sand are compared with respect to hydrogen generation rate (HGR). Maximum HGR is obtained on addition of Al2O3 nanoparticles (20 nm) in NaBH4/H2O system with CoCl2 as catalyst. Maximum HGR obtained is 19.47 moles/L.sec for NaBH4 (1.26 moles/L), Al2O3 nanoparticles (0.12 moles/L) and CoCl2.6H2O (0.02 moles/L) as catalyst at room temperature and atmospheric pressure. Due to the high surface to volume ratio of alumina nanoparticles hydrogen desorption rates are enhanced from the reactants. Additionally, owing to reactive nature of alumina nanoparticles and its hydrophilic and amphoteric nature, it is selected as catalyst promoter for NaBH4/ H2O based HG system. For NaBH4/γ-Al2O3 nanoparticles/H2O based HG system, various catalysts like CoCl2.6H2O, CoSO4.7H2O, (CH3COO)2Co.4H2O, Co(NO3)2.H2O, CdSO4 and CuSO4.5H2O are evaluated in terms of HG and HGR. Following order for maximum and overall HGR is observed: CoCl2.6H2O > CoSO4.7H2O > (CH3COO)2Co.4H2O > Co(NO3)2.H2O for NaBH4 (1.26 moles/L)/Al2O3 nanoparticles (0.12 moles/L)/H2O and catalyst (0.02 moles/L) at room temperature and atmospheric pressure. Nil amount of hydrogen evolved with CdSO4 and CuSO4.5H2O. Comparative studies are also performed between γ-Al2O3 (100-200 μm) and γ-Al2O3 (20 nm) to observe the difference in hydrogen generation (HG) with respect to particle size. Higher HG is observed with γ-Al2O3 having particle size of 20 nm than γ-Al2O3 having particle size of 100-200 μm, therefore, it is selected as promoter for the present NaBH4/H2O system with CoCl2 as catalyst. Hydrogen generation rate is monitored with change in concentration of NaBH4, γ-Al2O3 nanoparticles, CoCl2 and NaOH. It is concluded that hydrogen generation rate increases with increases in NaOH, Al2O3, NaBH4 and CoCl2 concentrations. After observing each parameter individually, the order for each component is calculated. Hydrogen generation rate is investigated at different temperatures (303, 313, 323 and 333 K) for constant NaBH4 (1.25 moles/L), NaOH (1.4 moles/L), CoCl2 (0.02 moles/L) and Al2O3 (0.09 moles/L) concentrations. Thus activation energy is calculated is 29 kJ/moles and A is 18.62×108 (sec)-1(L/moles)2.15. This value is less than activation energy calculated for NaBH4/H2O system without addition of γ-Al2O3 nanoparticles. Maximum hydrogen generation efficiencies are calculated at different mass ratios of Al2O3/NaBH4. Very high efficiency of 99.34% is achieved at mass ratio of 0.09 : 0.7 for Al2O3 : NaBH4 with theoretical hydrogen density of 10.76 wt% and experimental hydrogen density of 10.69 wt%. Therefore, it is observed that efficiency with addition of γ-Al2O3 nanoparticles is higher compared with the efficiency of previous HG (NaBH4/H2O) system. The residue obtained from the optimum system consisting of NaBH4 (1.26 moles/L), Al2O3 (0.12 moles/L), NaOH (0.93 moles/L), and CoCl2 aqueous solution (0.02 moles/L) is analyzed. The residue is characterized using EDS, XRD and FTIR to predict the possible reactions that could occur between different reactants. The work is extensive study of two different HG systems (NaBH4/γ-Al2O3 nanoparticles/H2O & NaBH4/H2O) including optimization of catalyst, operating parameters followed by kinetic studies and analysis of residue obtained of both the HG systems. On account of HGR, activation energy and efficiency, NaBH4/γ-Al2O3 nanoparticles/H2O based HG system with CoCl2 as catalyst is better than NaBH4/H2O based hydrogen generation and therefore, it can be considered as an efficient system to be used in practical application for hydrogen generation and storage.