Hydrogen: a major energy vector
In opposition to batteries, the hydrogen produced by the renewable production surplus, using water splitting electrolyzers, is not limited to be converted back to electricity in the power grid and could be instead used as a fuel in fuel cell powered portable and mobile devices and vehicles. A recent review has fully described all the different processes involved in the hydrogen energy vector, which can be summarized by Figure I.7, from the available sources, generation and storage options and the several hydrogen fueled final applications .
In the last decade, hydrogen was often described as (near) future fuel [69–71] because of its important energy density per unit mass (three times the one of common fossil fuels). As presented on Figure I.7, the production methods can be separated in two distinct categories: fossil fuel based, and sustainable water electrolysis. Unfortunately, around 96% of the hydrogen is produced from fossil resources today (49% from natural gas, 29% from liquid hydrocarbons, and ca. 18% from coal) , which contribute to significant GHG and other chemicals emission in the atmosphere . The main reason is obviously economic: because of the important energy demand of water electrolysis, the resulting hydrogen production cost is high (around 4 – 8 $/kg-1 versus 1 – 2 $.kg-1 for non-sustainable production, in 2019 [72,73]). The sustainability of hydrogen is thus still questionable nowadays, but current important investments and a real trend in the “green hydrogen production”, induced by the global warming targets discussed above, can already be observed from the amount of studies published recently in this domain chemical reactions in Europe and over 95% for ammonia synthesis in Japan. Overall the large majority of the world hydrogen is used for petroleum refining and ammonia and methanol synthesis , which will also contribute to the pollution of the atmosphere with GHG or other chemicals. Again, carbon-free transportation, other mobile devices and stationary electric generation are under consequent investigations and will be presented in more detail in the following section. However, to this day, the technologies are only emerging and this energy vector is a non-negligible part of the overall GHG emissions and several years or decades will be necessary for the hydrogen vector to be fully “green”.
Storage and transportation of hydrogen
In addition to the current issues just described, it is also necessary to store the produced hydrogen and then transport it from the production to utilization sites. The large energy density of hydrogen was mentioned previously as one of the main property to consider it as an energy carrier. However, when converted to volume, this energy density is only 5.6 MJ.L-1 for a given pressure of 700 bar at 25°C (which is now three times lower than hydrocarbon fuels). This value obtained only at high pressure asserts the difficulty to store hydrogen compared to natural gas or other liquid fuel. Several methods were developed to such end, where the state of hydrogen varies from gaseous to liquid or even solid.
• Gaseous hydrogen
The main storage system is the compression of gaseous hydrogen, since it is relatively easy and straightforward. It allows to reach acceptable densities for several applications: 23.3 kg.m-3 at 350 bar and 39.2 kg.m-3 at 700 bar, which corresponds to the common pressures of fuel cell buses and commercial fuel-cell cars respectively. It is also possible to perform cryo-compression at T = 100 K to reach a density of 39.5 kg.m-3 at relatively low pressure of 200 bar . Standardized tanks are already commercialized following strict norms to limit safety issues. This storage method presents the main drawback to require an energy-consuming compression system that decreases the overall energy efficiency of the hydrogen cycle. In addition, the high pressure most often considered is too high for standard fuel cell and electrolyzer and it must be adapted. Moreover, even after compression, the volumetric energy density of pressurized hydrogen is still low compare to gasoline, thereby increasing the transportation costs .
• Liquid hydrogen
Another method to obtain a higher volumetric energy density is to change the state of hydrogen from gaseous to liquid (10.1 MJ.L-1). However, this process requires even more energy than compression due to hydrogen very low density (0.089 kg.m-3) and boiling point (20.2 K). It was investigated for small scale storage and even considered for fueling vehicles. However, this technology is, to this day, not viable compared to pressurized hydrogen .
• Solid hydrogen
Because it is already complex to obtain liquid hydrogen, the methods to obtain a solid storage of hydrogen involve inserting it in other materials using physisorption or chemisorption (Figure I.7). It was proven to be an efficient method in addition to be much safer than commonly-used compressed hydrogen. Several materials can be used as insertion matrix to hydrogen such as metal hydrides , magnesium-based alloys , carbon based materials  or boron-based compounds , such as the sodium borohydride (NaBH4). This specific molecule will be thoroughly discussed in the rest of the manuscript but as a fuel and not as a hydrogen storage medium. This method presents the advantage of being the easiest way to transport hydrogen.
Indeed, as briefly evoked above, the transportation of pressurized hydrogen is a real challenge. It is possible to distribute it to the user using either trucks or pipelines. It is also possible to use trucks with adapted tanks to deliver liquefied hydrogen. These three options present both advantages and disadvantages related to the delivery cost and also the deployment time required. Pipelines are more suited for very large transport capacity, but will be very expensive and difficult to deploy. The cheapest method is obviously to use a network of trucks, but it is not suited to handle a large production . Because of the development of hydrogen related technologies and quantities produced in the upcoming years, it is mandatory to address this issue. In fact, it is nowadays one of the main barriers to the development of hydrogen vehicles in Europe: the hydrogen network is not sufficiently developed to motivate vehicle manufacturers to invest in the hydrogen technology and in return, governments do not develop the hydrogen distribution network because the demand is not important enough . However, significant funding is now unlocked and numerous hydrogen stations are now under development [81,82].
To conclude on hydrogen, it is a really promising energy vector that would unlock low GHG energy production and transportation due to its interesting energetic properties and its possible sustainable production. However, much efforts are still required to reach such a sustainable network since the main sources and utilization of hydrogen are currently directly contributing to the atmosphere pollution by either GHG or other chemicals. Nonetheless, the recent governmental engagements and technology developments are encouraging to see the hydrogen vector potential correctly used in a near future.
Development of fuel cells
Hydrogen can be converted back to electrical energy using systems called fuel cells (FC). There exist several families of FCs which are used for different purposes ranging from homogenization of the renewable energy production, electricity and heat generator for domestic purposes, transportation applications and powering portable or mobile electronic devices (Figure I.7), that can be more generally separated in stationary and portable systems. Overall, all FCs operate with the same operation principle: a fuel (a gas or liquid) is injected in a system composed of two electrodes separated by an electrolyte. This fuel is oxidized at the anode (negative electrode), while an oxidant (O2 in most cases) is being reduced at the cathode. In order to have an operating FC, the electrolyte, which also serves as the separator (except for liquid FC) must provide a high ionic conduction (to transport the charge carriers from one electrode to the other) but be an electronic insulator, otherwise the system will be short-circuited. The six families of FC: Proton Exchange Membrane Fuel Cell (PEMFC); Solid Oxide Fuel Cell (SOFC); Molten Carbonate Fuel Cell (MCFC); Direct FC like the Direct Methanol Fuel Cell (DMFC); Alkaline Fuel Cell (AFC); Phosphoric Acid Fuel Cell (PEFC), are operating with different characteristics. The primary ones of each FC family are regrouped in Table I.1.
Table of contents :
CHAPTER I: Why the Direct Borohydride Fuel Cell
I.1 Global warming and climate change
Consequences of the global warming
I.2 Hydrogen: a major energy vector
Storage and transportation of hydrogen
I.3 Development of fuel cells
Towards portable and mobile systems
I.4 The Direct Borohydride Fuel Cell
I.4.1.1 Production of borohydride
I.4.1.2 Properties of borohydride
I.4.2.1 Anode materials
I.4.2.2 Cathode materials
I.4.2.3 Different membrane configurations
I.5 Conclusion and context and objectives of the PhD
CHAPTER II: Materials and methods
II.1 Electrochemical measurements
Electrodes and catalyst inks preparation
II.1.2.1 Bulk polycrystalline electrodes
II.1.2.2 Nanoparticles ink preparation and deposition
II.2 Metallic nickel catalyst preparation
Electrochemical Surface Area Determination
Intentional oxidation of Ni surfaces
Surface treatments on 3D structured Ni supports
II.2.4.1 Hydrogen reduction treatment
II.2.4.2 Acid Etching procedures
II.2.4.3 Ni electrodeposition on etched Ni support
II.3 Characterization techniques
II.3.1.1 Transmission Electron Microscopy
II.3.1.2 Scanning Electron Microscopy
II.3.2.1 X-Ray Energy Dispersive Spectroscopy
II.3.2.2 X-Ray Diffraction Spectroscopy
II.3.2.3 Inductively coupled plasma atomic emission spectrometry
II.3.3.1 Differential Electrochemical Mass Spectrometry
II.3.3.2 In situ Fourier Transform Infra-Red spectroscopy
II.4 Fuel Cell tests
II.4.1.1 NiED/GDE anode elaboration procedure
II.4.1.2 NiED-based anodes elaboration using Ni support
II.4.1.3 Pt/GDL anode elaboration
II.4.1.4 Preparation of the SEBS55 polymer and deposition on Ni-based anodes. 63 Unit Direct Borohydride Fuel
II.4.2.1 BH4-/O2 DBFC setup
II.4.2.2 BH4-/H2O2 DBFC setup
II.5 Computational methods
Microkinetic model calculations
CHAPTER III: BOR mechanism on noble and model surfaces
III.1 Current understanding of the BOR on Pt electrodes
III.2 Influence of the NaBH4 concentration on the reactions pathway
Existence of additional reaction pathways
III.2.2.1 Hydrogen escape at different borohydride concentrations
III.2.2.2 Detection of BH3OH- species using RRDE
III.2.2.3 Adapting the proposed BOR model
III.3 Towards a kinetic model of the BOR on Pd surfaces
Tentative explanation of the BOR mechanism on Pd
The effect of Pd structure modification
III.4 Conclusion and moving to non-noble BOR catalysts
CHAPTER IV: Towards efficient Ni based BOR catalysts
IV.1 Importance of the state of surface of Ni
Activity of polycrystalline Ni for the BOR
IV.2 Bimetallic “NixM” BOR catalysts
Is there an effect of the co-element?
IV.3 Metallic NiED/C: the best BOR catalysts?
BOR activity compared to most used catalysts
IV.4 Tentative BOR mechanism on Ni
Coupled spectroscopic characterization on metallic Ni
IV.4.2.1 Hydrogen escape measurements using DEMS
IV.4.2.2 Fourier Transform Infra-Red spectroscopy
IV.4.3.1 Binding energy of intermediate species by DFT calculations
IV.4.3.2 Kinetic simulation of the BOR on metallic Ni
CHAPTER V: Integration of Ni electrodes and optimization of DBFC parameters
V.1 Elaboration of NiED/C anodes
V.1.1.1 Adapting the electrodeposition procedure
V.1.1.2 DBFC performance using NiED/GDE anodes
V.2 Optimization of the anode properties
V.2.1.1 Nickel structure characterization
V.2.1.2 Tuning of the state of surface of the NFM and NFT.
V.2.1.3 Nickel nanoparticle electrodeposition on etched Ni felt
V.3 Improving the DBFC system
Towards efficient anion exchange DBFCs
V.3.2.1 Investigating anion exchange membranes
V.3.2.2 DBFC tests using only non-noble catalysts
GENERAL CONCLUSION & PERSPECTIVES