Preparation Of Nanoporous Ni（Co） And Their Improvement Effect On Hydrogen Production From NaBH₄ Hydrolysis

NaBH₄,as a solid hydrogen storage material,has attracted much attention due to its high hydrogen storage capacity,environmental friendly products and convenient storage.However,pure NaBH₄is difficult to hydrolyze at room temperature.In order to solve this problem,we need to find a cheap,efficient and stable catalyst.Based on the study of NaBH₄hydrolysis catalyst,nanoporous Ni（NP-Ni）,three-dimensional interconnected porous Co（3DP-Co）and Ni-Co bimetal catalysts were prepared by dealloying and hydrogen reduction,respectively.The hydrolysis performance of NaBH₄/NP-Ni,NaBH₄/3DP-Co,NaBH₄/Ni-Co was studied by water displacement method,and the dehydrogenation mechanisms were analyzed by means of scanning electron microscope（SEM）,energy dispersive spectroscope（EDS）,X-ray diffraction（XRD）.NP-Ni obtained by acid etching with Mn₃₀Ni₄₀Al₃₀alloy as precursor has flocculent porous structure.After studying the hydrogen generation property of NaBH₄/NP-Ni,it was found that the catalytic effect of NP-Ni on NaBH₄was obviously higher than that of commercial nickel foam and 6-7 mm nickel powder,and the amount of hydrogen released after 55 minutes reached 92%of the theoretical value.The better catalytic performance of NP-Ni may be attributed to its higher specific surface area which brings more active sites and reduces the binding energy of B-H bond.And the activation energy of the NaBH₄/NP-Ni was determined to be 67.18 k J·mol^-1.In addition,the experimental results of NaBH₄/NP-Ni system show that the dehydrogenation rate remained 88%of the initial value after five cycles of catalysis,and NP-Ni exhibits high cycle stability.3DP-Co was successfully prepared by reduction of Co₃O₄nano-rods.Hydrolysis test results showed that 3DP-Co exhibited higher catalytic activity for NaBH₄hydrolysis than micron electrolytic cobalt powder.The high specific surface area provides more active sites for 3DP-Co,which can complete the hydrolysis of NaBH₄in 15 min.The influence of hydrolysis conditions on the performance of hydrogen release has also been discussed.After optimizing the reaction conditions,the hydrogen generation rate was 865ml·min^-1·g^-1,which was much higher than NaBH₄/NP-Ni.It was attributed to the lower activation energy of NaBH₄/3DP-Co:Ea=42.56 k J·mol^-1.Meanwhile,the results of cyclic catalytic tests of NaBH₄/3DP-Co system show that the cyclic stability of 3DP-Co is weaker than that of NP-Ni.After five times of catalysis,the dehydrogenation rate remains at 61%of the initial value.The main reason that affects the cycle stability is that the boron oxide layer formed on the surface of 3DP-Co will block the pores after repeated catalysis,resulting in the decrease of specific surface area and activity of 3DP-Co.In view of the high stability of NP-Ni and the high catalytic activity of 3DP-Co,in this paper,Ni-Co bimetal was prepared by introducing Ni element into the preparation process of 3DP-Co,and its catalytic performance for hydrolysis of NaBH₄was investigated.The results showed that with the increase of nickel content,the hydrogen generation rate decreased from 450 ml·min^-1·g^-1to 160 ml·min^-1·g^-1.According to Arrhenius equation,the activation energy increased from 44.9 k J·mol^-1to 61.6 k J·mol^-1.The results of cycle stability test show that the increase of Ni content can improve the catalytic performance of Co.When the nickel content is as high as 15 at%,the hydrogen production rate remains75%of the initial value after five times of catalysis.On the one hand,nickel is beneficial to inhibit the formation of oxides on the surface of the catalyst.On the other hand,it stabilizes the porous structure of the catalyst.As a result,the pores of the Ni-Co catalyst do not close before and after catalysis,thus exhibiting good cycle stability.