The present study reports original experimental work on generation of hydrogen, by hydrolysis of solid sodium borohydride with stoichiometric amount of distilled water (H2O/NaBH4: 2, 2.84 and 3 mol/mol), in the presence of a powder unsupported Ni-Ru based catalyst, reused about 320 times. The experiments, performed in two batch reactors with equal internal volume but with different bottom shapes (flat and conical), revealed - for the conical bottom shape with any excess of water - 8.1 H2 wt% and 92 kg H2/m3 (materials-only basis), and a H2 rate of 87.4 L(H2) min-1g-1 catalyst. The role of reactor bottom geometry on the solid NaBH4 hydrolysis - with any excess of water - is, as the authors are aware, for the first time here referred

Ferreira, M. J. F.

Pinto, A. M. F. R.

Rangel, C. M.

Repositório do LNEG

BATCH SOLID SODIUM BOROHYDRIDE HYDROLYSIS FOR HYDROGEN GENERATION: THE ROLE OF REACTOR BOTTOM SHAPE M.J.F. Ferreira1, C.M. Rangel2, A.M.F.R. Pinto1,# 1CEFT-DEQ, Faculdade de Engenharia, Universidade do Porto, Porto, Portugal 2LNEG, Fuel cell and Hydrogen Unit, Lisbon, Portugal #Corresponding author: apinto@fe.up.pt (A.M.F.R. Pinto); Tel.: +351 22 508 1675  SUMMARY The present study reports original experimental work on generation of hydrogen, by hydrolysis of solid sodium borohydride with stoichiometric amount of distilled water (H2O/NaBH4: 2, 2.84 and 3 mol/mol), in the presence of a powder unsupported Ni-Ru based catalyst, reused about 320 times. The experiments, performed in two batch reactors with equal internal volume but with different bottom shapes (flat and conical), revealed - for the conical bottom shape with any excess of water - 8.1 H2 wt% and 92 kg H2/m3 (materials-only basis), and a H2 rate of 87.4 L(H2) min-1g-1 catalyst. The role of reactor bottom geometry on the solid NaBH4 hydrolysis - with any excess of water - is, as the authors are aware, for the first time here referred. INTRODUCTION The quest for abundant energy is the problem of ensuring the sustainability of the world’s energy supply as population and the search for affluence continue to grow.  Many scientists and engineers agree that the solution to these global problems would be to replace the existing fossil fuel system by the hydrogen (H2) energy system. Hydrogen is a very efficient and clean energy vector, to be directly converted into electricity by proton exchange membrane fuel cell (PEMFC). Promising PEMFC applications in transportation, stationary and portable electronic devices (laptops, mobile telephones, iPods, iPads, etc) are more over next to reality. However, some drawbacks need to be solve for the plenty usage of H2 as the main energy carrier, and the H2 storage has been placed at the top of the schedule (low mass density, usage security, high cost, the need for a distribution network, etc) [1].  Boron and its compounds are considered as most capable candidates to be used as hydrogen storage and production material [2]. Sodium borohydride (NaBH4), which is a boron chemical, is being studied extensively, because H2 can be easily and controllably produced by NaBH4 catalytic hydrolysis, giving no harmful by-products [3]. NaBH4 reacts with water to generate molecular hydrogen according to the hydrolysis reaction (1):  NaBH4 + (2 + x) H2O ⎯⎯ →⎯catalyst  NaBO2.xH2O + 4H2 + heat                       (1)                                                                          where x is the hydration factor. Ideal hydrolysis is attained for x = 0 [4] where two moles of water are required to react with one mole of solid NaBH4 to liberate four moles of hydrogen, but in practice excess of water is necessary to account for the fact that the solid by-product (NaBO2.xH2O) can exist with varying degrees of hydration [5]. In fact, most schemes for hydride hydrolysis require a large excess of water (x >> 2) to pre-dissolve the hydride for storage or to keep the by-products in solution. Therefore, this water handling (x) issue, directly related to the H2 storage system capacity, is of utmost importance, to accepted NaBH4-based H2 generator for fuelling PEMFCs. In truth, the effective gravimetric hydrogen storage capacity (GHSC) of either NaBH4 or the system NaBH4-H2O is well below the ideal value (the theoretical value is 10.8 wt%). This led the US Department of Energy (DOE) recommending no-go for the stabilized solution of NaBH4 for on-board vehicle applications [6]. However, optimization of effective GHSC could be achieved, using solid NaBH4 with water vapour or with stoichiometric amount of pure liquid water [7-13] (in the absence of alkali inhibitors), allied to the catalyst efficiency. This latter issue has been extensively studied in the hydrolysis of NaBH4, in terms of catalyst performance, but unsatisfactorily with respect to the catalyst durability and operability (in severe conditions).  In a previous work [13], the authors study the activity of a nickel-ruthenium based catalyst (Ni-Ru catalyst/NaBH4: 0.2-0.4 g/g), re-used up to 150 times, in three different batch reactors, with solid NaBH4 and stoichiometric amounts of pure liquid water of H2O/NaBH4: 2–8 mol/mol. It was reported 6.3 wt% and 70 kg/m3, respectively, for GHSC and volumetric hydrogen storage capacity (VHSC) (materials-only basis), for the compressed H2 system of 1.26 MPa, in a batch reactor of 229 cm3 with conical bottom shape (being x=2 and H2 yield=98%). In the present work, the activity and stability of the same Ni-Ru based catalyst is further investigated, also in the topic of hydrolysis of solid NaBH4 with pure liquid water. The main objective is to exclude any excess of water (x = 0), producing H2 with even higher GHSC to supply on-demand a PEMFC.  EXPERIMENTAL TECHNIQUE/DISCUSSION A small amount of solid sodium borohydride (NaBH4) – 1.2 g, is mixed with 0.24 g of the reused powder catalyst (Ni–Ru based/NaBH4: 0.2 g/g), the latter with the impressive long-life cycling of reutilizations between 310 to 320 times. This mixture is placed inside two batch reactors, both with internal volume of 229 cm3, but with different bottom geometries: flat and conical, see Figure 1 (a) and (b). A stoichiometric amount of deionised liquid water, H2O/NaBH4: 2, 2.84 and 4 mol/mol (or x = 0, 0.84 and 1) is rapidly added to the solid mixture inside the reactor, by means of a syringe with a long needle, to ensure that the water is delivered very close to the powder moisture (NaBH4 plus Ni-Ru catalyst). Then, the catalyzed solid NaBH4 hydrolysis evolved, in the absence of an inhibitor, at room temperature (∼22 ºC). Specific details of the experimental rig and procedure are explained on earlier work [13]. The temperature of the reactor medium is read by a k-thermocouple (see T.Top in Fig.1) and recorded simultaneously with a data acquisition system using LabView software. To monitor the H2 generation rate (HGR), the gas pressure inside the reactor is followed with an appropriate pressure probe.  The tests with x = 0, which are equivalent to 45.2 wt% NaBH4, 45.7 wt% H2O and 9.1 wt% catalyst, performed in batch reactor SRc – with a conical bottom shape, shown the highest experienced HGR of 3.12 bar/s (see Fig.1c) or 87.4 L(H2) min-1g-1 catalyst or ∼2958 L(H2) min-1g-1active metal (the reused catalyst contained 1.42 At% of Ru). This value is the utmost HGR ever reported in the open literature [3].     T. Top Internal volume ≈ 229 cm3  (a)  T. Top Internal volume ≈ 229 cm3  (b) 0369120 20 40 60 80 100Time / secPressure / bar04080120160Temperature / ºC (c) Figure 1: Schematic view of the inside of the two batch reactors: (a) reactor SRf/0.229 L with flat bottom shape, (b) reactor SRc/0.229 L with conical bottom shape and (c) hydrogen evolution in SRc batch reactor for 1.2 g of NaBH4 and H2O/NaBH4: 2 mol/mol (x=0), and with Ni-Ru catalyst/NaBH4: 0.20 g/g.  Values of 8.1 wt% and 92 kg/m3 were obtained, respectively, for GHSC and VHSC, on a materials-only basis. The present results, performed in the batch reactor with conical bottom shape, are in conformity with our prior study [13], which shows the positive influence of reactor bottom shape on HGR, induction time and yield, in solid NaBH4 hydrolysis.  CONCLUSIONS The present study reveals the catalytic capability of the extreme used Ni-Ru based catalyst (∼320 time’s reused) to generate hydrogen with 8.1 wt% (materials-only basis), at 87.4 L(H2) min-1g-1 catalyst, in a batch reactor with a conical bottom shape, with exclusion of any excess of water (x = 0; being 80 % de H2 yield), at room temperature. Decreases of ∼40 % in these values were observed in the reactor with flat bottom geometry. At the best of authors knowledge, the role of reactor bottom geometry, for H2 production (and storage), by solid NaBH4 hydrolysis with any excess of water, is for the first time exposed on the present manuscript. It could be concluded that our compressed H2 system is a potential energy/hydrogen carrier candidate for portable applications. REFERENCES [1] P. Kruger, “Alternative energy resources: the quest for sustainable energy ”, John Wiley & Sons, New Jersey, 2006. [2] M. Balat, “Boron as an alternate engine fuel”, Energy Sources, 29 (2007), pp79-83. [3] U.B. Demirci, O. Akdim, J. Andrieux, J. Hannauer, R. Chamoun, P. Miele, “Sodium Borohydride Hydrolysis as Hydrogen Generator: Issues, State of the Art and Applicability Upstream from a Fuel Cell”, Fuel Cells, 10, 3 (2010), pp335-350. [4] H.I. Schlesinger, H.C. Brown, A.E. Finholt, J.R. Gilbreath, H.E. Hoeskstra, K. Hyde, “Sodium Borohydride, its Hydrolysis and Its Use as a Reducing Agent and in the Generation of Hydrogen”, J. Am. Chem. Soc., 75 (1953), pp215-219. [5] E.Y. Marrero-Alfonso, J.R. Gray, T.A. Davis, M.A. Matthews, “Minimizing water utilization in hydrolysis of sodium borohydride: The role of sodium metaborate hydrates”, Int. J. Hydrogen Energy, 32 (2007), pp4723-4730. [6] U.B. Demirci, O. Akdim, P. Miele, “Ten-year efforts and a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage”, Int. J. Hydrogen Energy, 34 (2009), pp2638-2645. [7] V.C.Y. Kong, F.R. Foulkes, D.W. Kirk, J.T. Hinatsu, “Development of hydrogen storage for fuel cell generators. I: Hydrogen generation using hydrolysis hydrides”, Int. J. Hydrogen Energy, 24 (1999), pp665-675. [8] R. Aiello, J.H. Sharp, M.A. Matthews, “Production of hydrogen from chemical hydrides via hydrolysis with steam”, Int. J. Hydrogen Energy, 24 (1999), pp1123-1130. [9] Y. Kojima, Y. Kawai, H. Nakanishi, S. Matsumoto, “Compressed hydrogen generation using chemical hydride”, J. Power Sources, 135 (2004), pp36-41. [10] E.Y. Marrero-Alfonso, J.R. Gray, T.A. Davis, M.A. Matthews, “Hydrolysis of sodium borohydride with steam”, Int. J. Hydrogen Energy, 32 (2007), pp4717-4722. [11] B.H. Liu, Z.P.Li, S. Suda, “Solid sodium borohydride as a hydrogen source for fuel cells”, J. Alloys Comp., 468 (2009), pp493-498. [12] P. Gislon, G. Monteleone, P.P. Prosini, “Hydrogen production from solid sodium borohydride”, Int. J. Hydrogen Energy, 34 (2009), pp929-937. [13] M.J.F. Ferreira, L. Gales, V.R. Fernandes, C.M. Rangel, A.M.F.R. Pinto, “Alkali free hydrolysis of sodium borohydride for hydrogen generation under pressure”, Int. J. Hydrogen Energy, 35 (2010), pp9869-9878. 

Batch solid sodium borohydride hydrolysis for hydrogen generation : the role of reactor bottom shape

Laboratório Nacional de Energia e Geologia Repository

http://repositorio.lneg.pt/bitstream/10400.9/1477/1/WHTC2011_0123_paper%5b1%5d.pdf

Batch solid sodium borohydride hydrolysis for hydrogen generation : the role of reactor bottom shape

Abstract

Similar works

Full text

Available Versions

Repositório do LNEG

Laboratório Nacional de Energia e Geologia Repository