Development of an electrochemical generator based on the oxidation of a solid fuel that can substitute to gaseous hydrogen: sodium borohydride, that can compete with lithium-ion batteries and proton exchange membrane fuel cell for portable/mobile applications of small power and high energy density.
Sodium borohydride is a fuel of high-energy density, easier to store and transport than gaseous hydrogen. Its oxidation in a direct borohydride fuel cell (DBFC) would lead to an electrochemical generator capable to compete (or even overwhelm) lithium batteries in terms of energy density and therefore of autonomy. However, high performance for these systems requires that the fuel is quantitatively oxidized at sufficiently low potential. To that goal, the electrode materials and structure must be optimized, which requires to understand the very complex reaction mechanisms at stake. Although such understanding has been reached in model conditions (diluted reactants, room temperature, simple catalyst surface), this is not the case, yet, in the experimental conditions encountered in a practical DBFC. The MobiDiC project will fill this gap and aims to unveil the reaction mechanisms in these conditions, which shall enable to develop optimized electrocatalysts and electrode structures for high-performance DBFCs.
Results originating from characterizations of the borohydride oxidation in model conditions do often not apply to practical direct borohydride fuel cell systems, because the reactions at stake strongly depend on the experimental conditions at stake. It is therefore mandatory to isolate the reaction mechanisms in the experimental conditions that are encountered in an operating DBFC. Our methodology to that goal, is to transpose experimental methods usually used in model fundamental conditions (e.g. in situ coupling of spectroscopies to electrochemistry), for complex nanostructured electrodes in operating conditions that are encountered in practical DBFC (concentrated reactants, temperature above room temperature, complex electrocatalyst surface). In that way, we shall unveil the elementary steps and reaction intermediates, which shall enable to design appropriate electrocatalysts and relevant nanostructured electrodes.
Thanks to this methodology, we already succeeded in isolating reaction intermediates and therefore steps of the borohydride oxidation reaction. This knowledge enabled to propose a relevant model for this complex reaction, model that describes both electrochemical and spectroscopic experimental data and is valid both in diluted and concentrated electrolytes. The model, initially set for Pt has been extended to Au and Pd surfaces. The next step will be to use this model to design performant electrocatalysts (if possible free of noble metals) and electrode structures, that can lead to optimized reaction kinetics, pathway and efficiency.
This project led to an international partnership with a leading laboratory in the USA, the creation of ties with a small company specialized in electromobility (e.g. fuel cell-powered electric bikes) and is supported by the pole of competitiveness “Tenerrdis” of the Auvergne Rhône Alpes region in France.
The methodology developed in this project could be applied to any complex electrochemical reactions; the fundamental outcome of the project is therefore obvious. More practically, the project shall lead to the design of direct borohydride fuel cell systems for mobile applications, if we indeed succeed in developing efficient electrocatalysts and electrode structures for the borohydride oxidation reaction. In that case, the potential market would be very large, and one could forecast that DBFCs start to compete with Li-ion batteries, at least for niche applications.
A scientific paper has already been published (J. Power Sources 375 (2018) 300-309), another one is accepted in Electrochimical Acta (https://doi.org/10.?1016/?j.?electacta.?2018.?04.?068) and 2 conference papers have been made, notably concerning the determination of the borohydride oxidation mechanisms by the coupling between spectroscopic/spectrometric techniques and electrochemistry as well as reaction modelling.
Other papers will follow on optimized electrocatalyst materials and electrode structures.
Direct borohydride fuel cells (DBFC) are a promising alternative to PEMFCs for mobile applications. They benefit from the advantages of the NaBH4 fuel (NaBH4 is easy to store and transport as a dry material, is dense in energy and can easily be fed as a stable fuel in alkaline anolyte solutions), but also from the fact it can use non-noble catalysts (cheaper and more abundant than platinum, the classical catalyst in low-temperature fuel cells). However, the anodic reaction in a DBFC (the borohydride oxidation reaction: BOR) is complex and still insufficiently mastered. In particular, the knowledge derived from lab-scale experiments (in model conditions, dilute anolyte solutions, low temperature) is insufficient to predict the behavior of DBFC systems. The MobiDiC project will provide further insights into the fundamentals of the electrochemical BOR and of the chemical BH4- catalytic decomposition in real DBFC conditions of small generators for portable applications (concentrated electrolytes, T = 10-40°C, three-dimensional porous electrodes); we will particularly focus on model surfaces of increasing complexity, mostly based on non-Pt electrocatalysts (e.g. Pd, Co, Ni), and increase our understanding of the processes involved in DBFC anodes by coupling classical electrochemical techniques and state-of-the-art in situ physico-chemical techniques. This knowledge will be strengthened by the modelling of the mechanisms of (electro)chemical reactions, the model and experiments being looped to optimize the (electro)catalysts developed in the project.
Then, we will elaborate and characterize model DBFC anodes of increasing complexity using segmented fuel cells. Our strategy to optimize the fuel consumption and maximize the energy output is highly innovative. It consists of building heterogeneous electrodes, e.g. multifunctional gradient anodes, for the optimized and combined heterogeneous hydrolysis and electrooxidation of the BH4-, including by promoting the desired catalytic decomposition of BH4- into BH3OH- (the latter compound being much easier to oxidize at low potential than the former) at the inlet on one catalyst, and the valorization at low potential of this compound (and of the unavoidable molecular H2) on relevant electrocatalysts throughout the outlet. As such, the anode will bear different regions across its thickness and/or along the gas channel to complete the fuel oxidation. The unique methodology that consists in using various surfaces of increasing complexity in experimental conditions that are characteristic of real DBFC operation will enable us to bridge the fundamental and engineering approaches that are complementary, but often opposed in the literature. From this, we will propose a model of the processes at stake in 3D (practical) electrodes for the complex BOR, model that will take into account the interplay of mass-transfer of reactants and intermediates, their adsorption/desorption, as well as chemical and electrochemical reactions.
The third objective of the project is to capitalize on these fundamental outcomes to build optimized electrodes for a portable DBFC demonstrator (using the relevant (electro)catalyst materials and electrode structures determined in the project), and to test their long-term operation to assess their durability, understand their degradation mechanisms and propose mitigation strategies.
Lastly, a mobile DBFC demonstrator will be built and field-tests will be carried out.
Monsieur Marian CHATENET (Institut Polytechnique de Grenoble)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IEM Université de Montpellier, IEM
PRAGMA INDUSTRIES PRAGMA INDUSTRIES
LEMTA Laboratoire d'Energétique et de Mécanique Théorique et Appliquée
Grenoble-INP, LEPMI Institut Polytechnique de Grenoble
ICPEES Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé
Help of the ANR 741,351 euros
Beginning and duration of the scientific project: October 2016 - 42 Months