Sodium Borohydride Production and Utilisation for Improved Hydrogen Storage Sean S. Muir BE (Chemical) Hons I, BSc A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in July 2013 Australian Institute for Bioengineering and Nanotechnology Abstract Sodium borohydride (NaBH4) has been the subject of extensive investigation as a potential hydrogen storage material. Its advantages include its ability to be stored as a stabilised aqueous solution and hydrolysed catalytically on demand, providing hydrogen safely, controllably, and at mild conditions. However, several drawbacks were identified by the US Department of Energy (DOE) in their “No-Go” recommendation in 2007; namely, the limitations on its gravimetric hydrogen storage capacity (GHSC) and its prohibitive manufacturing cost. This thesis aims to address some of the major issues associated with the use of NaBH4 for hydrogen storage. Efforts have been spread across three key research areas, which were identified in a review of the recent literature: Synthesising an efficient, durable, and inexpensive catalyst for the hydrolysis of aqueous NaBH4 solution The design of a hydrogen storage system based on NaBH4 which can overcome the GHSC constraints imposed by the solubility of its hydrolysis by-product, NaBO2 The development of a novel recycling process for regenerating NaBH4 from NaBO2, with the goal of reducing its production cost to within the DOE’s target range of $2-4 per gallon of gasoline equivalent (gge) To achieve the first goal, a new electroless plating method was developed for preparing cobalt-boron (Co-B) catalysts supported on shaped substrates. This method involves mixing the plating solutions at low temperature (<5 oC), in contrast to previously employed methods, which are generally carried out at room temperature or higher. The new method requires only one plating step to achieve the desired catalyst loading, and has higher loading efficiency than processes requiring multiple plating steps, due to reduced catalyst wastage. The catalysts produced by this method show significantly higher NaBH4 hydrolysis activity than those prepared by conventional methods, with a hydrogen o generation rate over 24000 mL/min/g recorded at a temperature of 30 C and a NaBH4 concentration of 15 wt%. The improved activity of these catalysts was thought to be due to their increased boron content and nanosheet-like morphology, both of which are products of the low temperature preparation method. i The stability of these catalysts was examined over several usage and deactivation cycles, which involved long-term exposure to alkaline solution. It was found that the deactivated catalysts were able to recover 80-90% of their initial activity even after 10 cycles, if operated at 40 oC or higher. Characterisation of the deactivated catalysts revealed that a layer of NaBO2 had formed on the surface, blocking the catalytically active Co sites. The improved cyclic stability is most likely due to the dissolution of this layer at higher temperatures. A new reactor design for a NaBH4-based hydrogen storage system was developed, with the goal of overcoming the GHSC limitations encountered by conventional system designs. The new design is based on a fed-batch configuration in which solid NaBH4 is added gradually to a container of alkaline solution and reacted as it is needed, eliminating the need for high NaBH4 concentrations. The reactor is separated into high and low temperature zones, each of which serves a different purpose; the hydrolysis of NaBH4 occurs in the former, while crystallisation of the by-product NaBO2 occurs in the latter. This separation helps to prevent the crystallised NaBO2 from blocking and damaging the catalyst, which is a major issue encountered in previous designs. A 2.0 L-scale prototype reactor was built and tested, with a maximum GHSC of 4.10 wt% achieved on a reactants- only basis. This was increased to 4.74 wt% with simulation of water recycling from an attached fuel cell. Finally, a novel process for the low-cost regeneration of NaBH4 was proposed. This process uses organometallic hydrides to produce NaBH4 from an alkoxylated derivative of NaBO2, a new reaction which is reported for the first time in this thesis. The production of NaBH4 was confirmed by XRD, XPS, and NMR analyses, and an unoptimised yield of 45% was obtained. The organometallic hydrides can themselves be regenerated within the process, by thermal decarboxylation of the corresponding organometallic formate. If all intermediates are recycled, the only key input into the process is formic acid, a relatively inexpensive organic reagent. It is anticipated that this process can provide cost and energy savings of an order of magnitude relative to current NaBH4 production methods. ii Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the General Award Rules of The University of Queensland, immediately made available for research and study in accordance with the Copyright Act 1968. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis. iii Publications during candidature Journal Publications Muir, SS & Yao, XD 2011, ‘Progress in sodium borohydride as a hydrogen storage material: Development of hydrolysis catalysts and reaction systems’, International Journal of Hydrogen Energy, vol. 36, no. 10, pp. 5983-97. Muir, SS, Chen, ZG, Wood, BJ, Wang, LZ, Yao, XD & Lu, GQ 2013, ‘New electroless plating method for preparation of highly active CoB catalysts for NaBH4 hydrolysis’, International Journal of Hydrogen Energy (submitted). Provisional Patents The University of Queensland 2012, Process, AU Patent 2012904699. The University of Queensland 2012, Substrate, AU Patent 2012904700. The University of Queensland 2012, Reactor, AU Patent 2012904953. Conference Presentations Muir, SS 2010, ‘New method for synthesising Co-B catalysts for NaBH4 hydrolysis with high cyclic stability’, presented to the 7th Annual Conference of the ARC Centre for Functional Nanomaterials, Gold Coast, 25-26 November. Muir, SS, Chen, ZG, Wang, LZ, Yao, XD & Lu, GQ 2012, ‘New electroless plating method for preparation of highly active CoB catalysts for NaBH4 hydrolysis’, paper presented to the 8th Annual Conference of the ARC Centre for Functional Nanomaterials, Sydney, 15-16 November. Publications included in this thesis No publications included. iv Contributions by others to the thesis Prof Max Lu, A/Prof Xiangdong Yao and A/Prof Lianzhou Wang all contributed to the thesis in an advisory capacity, particularly in the conception and design of the project and its goals. Further contributions in this area were made by Mr Angus Lilley and Mr Edo Dol of Control Technologies International. Mr Dol also contributed to the designing and testing of the NaBH4 hydrolysis reactor. Cyclic stability testing of the Co-B catalysts (Chapter 4) was carried out by Mr Xinyang Li as part of his Masters of Engineering program. The SEM, TEM, and EDX analyses in Chapter 3 were carried out by Dr Zhigang Chen. XPS experiments (Chapters 3, 4, and 6) were conducted by Dr Barry Wood, who also contributed to the interpretation of the data for compositional analysis. ICP-OES analysis in Chapter 3 was carried out by Mr David Appleton. NMR experiments in Chapter 6 were conducted by Dr Ekaterina Strounina (11B) and Dr Lynette Lambert (1H and 29Si), both of whom also contributed to the analysis and interpretation of the data. Statement of parts of the thesis submitted to qualify for the award of another degree None. v Acknowledgements I have been privileged to receive the support and assistance of so many people over the past four years. First and foremost, thanks must go to my advisory team of Prof Max Lu, A/Prof Xiangdong Yao and Prof Lianzhou Wang, for giving me the opportunity to undertake this PhD, and for their support and guidance along the way. Thanks must also go to my collaborators at Control Technologies International: Mr Angus Lilley, Mr Edo Dol, Mr Stephen Brown, and Mr Matthias Abram. It has been a pleasure to work with you all, and I hope you have enjoyed seeing this project come to fruition as much as I have. I wish to express my gratitude to those who provided technical support and assistance with my experimental work: Mr Xinyang Li, Dr Zhigang Chen, Dr Barry Wood, Mr David Appleton, Dr Ekaterina Strounina, and Dr Lynette Lambert. Thanks must also go to all those who built or provided support with lab equipment: Mr Robin Berlyn of UQ Glassblowing Services, Mr Robert Orr of Labglass, Mr Malcolm Marker and Mr Peter Bleakey of the EAIT Instrumentation Support Group, and Mr Ian Livingstone of AVT Services. During 2010 I had the privilege of working at the Institute of Metal Research at the Chinese Academy of Sciences in Shenyang, China P.R.