MobiDiC

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.