3D Metallic Foams as Catalysts for Hydrolysis and Electrooxidation of Sodium Borohydride

Environmental and social challenges that result from the burning of fossil fuels demand clean energy for the world future sustainability. Fuel cells are devices that convert a fuel source into electricity, providing a clean energy solution for both portable and stationary (e.g., electricity production and heating) applications more than any other currently available power source [1]. Amongst low temperature fuel cells, polymer electrolyte fuel cells (with a typical operation temperature of 80 ºC) are receiving great attention due to their characteristically high power density. However, its large-scale development has been hindered by cost competitiveness, engineering design and safety concerns for hydrogen-fueled devices. These drawbacks can be minimized by the use of high specific energy liquid fuels, such as methanol or sodium borohydride (NaBH 4 ) solutions. NaBH 4 is of high interest as it can be a direct or indirect fuel to power a direct borohydride fuel cell (DBFC) or a typical proton exchange membrane fuel cell (PEMFC), respectively [1]. In fact, International Energy Agency supports NaBH 4 as a sustainable route for fuel cell systems. NaBH 4 is a non-fossil derivative and nonflammable compound with high gravimetric hydrogen storage capacity (10.8 wt.%) [2]. As an indirect fuel, NaBH 4 produces clean hydrogen through hydrolysis in alkaline medium and ambient conditions, after which the hydrogen (H 2 ) fuel is channeled to a PEMFC where it is oxidized to generate electricity [3]. As a direct fuel, NaBH 4 is oxidized in the DBFC anode at ambient conditions, releasing eight electrons at quite negative electrode potentials [3]. In addition, no CO 2 is produced, unlike hydrocarbon fuels, which turns the overall NaBH 4 route a green approach suitable for portable applications [3]. A catalyst is always required, either to accelerate the hydrogen release or for the borohydride direct electrooxidation, so that the fuel cell can generate sufficient power [3,4]. Precious metals, such as Pt, Ru and their related alloys, can generate high hydrogen rates toward NaBH 4 hydrolysis. In the case of borohydride electrooxidation, most catalysts are built from precious metals, such as Au or Ag. Since these materials are expensive, there has been much focus on the use of transition metals [3]. Moreover, transition metals in the alloy form can be beneficial as they can enhance catalytic activity due to synergetic effects [4]. Recently, three-dimensional catalysts with very high porosity and surface areas based on Co have demonstrated high catalytic activity for borohydride hydrolysis [5], while those based on Ni have shown electroactivity for borohydride electrooxidation [6]. The main goal of this work is to develop low-cost catalysts, based on Ni, Co, Cu, and Fe, to be applied both on borohydride hydrolysis for H 2 generation and on borohydride electrooxidation, in order to find in which system, PEMFC or DBFC, they are more suitable. The catalysts were produced by electrodeposition technique, a simple one-step and fast process, using the dynamic hydrogen bubble template to create foam-like nanostructures. The ternary Ni-Cu-Fe foams presented the best performance towards borohydride oxidation in terms of the current density per unit gram of catalyst. On the other hand, Co-based foams are superior in fast H 2 demand applications by borohydride hydrolysis. Figure 1. SEM image of Ni 4 Cu 48 Fe 30 foam catalyst. References [1] D.M.F. Santos, C.A.C. Sequeira, Sodium borohydride as a fuel for the future, Renew. Sustain. Energy Rev. 15 (2011) 3980-4001. [2] 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 (2010) 335-350. [3] I. Merino-Jiménez, C.P. León, A.A. Shah, F.C. Walsh, Developments in direct borohydride fuel cells and remaining challenges, J. Power Sources 219 (2012) 339-357. [4] N. Patel, A. Miotello, Progress in Co-B related catalyst for hydrogen production by hydrolysis of boron-hydrides: A review and the perspectives to substitute noble metals, Int. J. Hydrogen Energy 40 (2015) 1429-1464. [5] S. Eugénio, U.B. Demirci, T.M. Silva, M.J. Carmezim, M.F. Montemor, Copper-cobalt foams as active and stable catalysts for hydrogen release by hydrolysis of sodium borohydride, Int. J. Hydrogen Energy 41 (2016) 8438-8448. [6] D.M.F. Santos, S. Eugénio, D.S.P. Cardoso, B. Šljukić, M.F. Montemor, Three-dimensional nanostructured Ni-Cu foams for borohydride oxidation, Russ. J. Phys. Chem. A 89 (2015) 178-183. Figure 1