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Global warming and pollution
Today’s world is the place of many challenges and questioning. Since the beginning of the industrial area, mid-19th century, the world has undergone so many changes and developments that it is impossible to compare this period to any other period. Globalization, life quality improvement, lifespan improvement, development of transport, communications, energy sources… No area is left behind.
Of course, such a development comes with a price, though it was only recently discovered [1]. In many places around the world, the climate is changing, shifting. This process is unequal, affecting some regions more (notably the poles and temperate areas) than others, but nonetheless present.
The increase of temperatures was measured in many countries in the recent years [2,3]. In fact, 2020 and 2016 were graded the warmest years ever measured, since record-tracking began in 1880 [4]. In 2019, the GIEC estimated the global warming since the pre-industrial area of 1.5°C [5] Figure I.1. The climate however, is a vast and complex system with many interactions, with often an escalating impact through a chain of consequences. Such a fast and global temperature increase then leads to more frequent and more intense natural disasters, resulting in local floods, droughts, forest fire, etc., but also to chronic and large-scale desertification or ice melting.
Figure I.1: Evolution in air and global (land-ocean) temperatures, relative to the mean temperature between 1850 and 1900 [6].
At the north pole, ice melting is accelerating. Specialists calculated its possible complete disappearance in summers as soon as 2035 [7]. And this comes with many consequences: ocean level rising, endangered species natural habitat disappearance, melting of Permatfrost (a type of land that stays frozen for more than 2 years) [8]. The latter could provoke the liberation of a large quantity of greenhouse gases, as well as ancient biogic agents, some of them related to lethal diseases (pest, spanish flu, etc.), which cannot leave us stoic in these times of pandemic.
Meteorologists and scientists of the whole World now agree on the reality of global warming. Though many climate changes happened before in Earth’s history, the current one was declared too fast and too consequent to be due only to natural causes [9,10], especially when its exact beginning can be traced back to the industrial revolution period, when coal industries were massively implanted.
The increase of greenhouse gases in particular, has been closely monitored since that period. The greenhouse effect is a natural phenomenon which permits the presence of life on earth. It is provoked by the accumulation of several gases in the atmosphere: mainly carbon dioxide (74%), methane (17%) and nitrous oxide (6%) [11]. All of them have a different impact, that can be normalized as CO2-equivalent. Their concentration is directly linked to the global surface temperature of Earth. An increase in the concentration of those gases, especially CO2, was measured over the recent years. Figure I.2 represents the amount of CO2 emissions by year by countries. Since 1950, our global CO2 emissions has been multiplied by 7.
Figure I.2: Annual CO2 emissions by world regions from fossil fuels and cement production (land use change is not included), based on the Global Carbon Project [12].
The major actors of this increase are the developed countries, with China and United States in the lead, and the groups of European and Asian countries close behind. Those emissions come from many different sources (Figure I.3). In first position, the energy sector makes 72% of these overall emissions, electricity and heat generation being the main causes. Then comes transportation and manufacturing domains, and agriculture.
As a consequence, a global consensus is that the global energetic mix needs to be changed. All around the world organisations and countries are moving ahead to try and decrease if not stop the phenomena.
In 1997, The Kyoto protocol, with today 192 signing countries [10], aimed to limit the total amount of CO2 emission per country. More recently, in 2015, the Paris Agreements were signed by 175 nations (now 182), which engaged to take all actions to limit the temperature increase below 2°C [14]. Following that , the European commission as well as Japan exposed their wish to respect these agreement and attain a objective of zero emission in 2050 [15,16]
For that, there are different possibilities: one of them is to reduce CO2 emissions.
Towards a new energetic mix
The energetic mix is the repartition of the sources for the energy production in the world. Figure I.4 presents the global energetic mix in 2018. This mix was dominated by coal, oil and natural gas, three fuels qualified as “fossil” based on their consumption of a limited material formed by geologic processes over time. These fuels all contain carbon, and their usage rejects a large amount of CO2. The portion of renewable sources (which are carbon-neutral) is of 14%, biofuels in first position, which is by far insufficient if one wants to reach the ambitious goals mentioned above.
The idea is then to reduce the part of polluting sources such as coal industries to replace them by greener energies. Renewable energies are obvious candidates. They harvest energies from natural, non-limited sources (solar light, wind, …) and can be used at the local scale. However, their production process must also be taken into account. Off shore wind turbine for example, contains around 400 kg of neodymium. As a rare earth material, this element is only located in some specific places in the world (mostly China, India and …) and its extraction process is very polluting, not speaking from the safety of the extraction/processing/use of the ore [18,19].
Renewable energies are still at their development, though, and still have a wide margin for improvement, notably for their choice of key materials, their efficiency and their usage (smart grid, …). One of their specificity, compared to coal and nuclear based industries, lies in their inherent intermittency for electricity production. Solar panels mostly produce electricity during the day and in summer, the production of wind turbines also scaling with the wind force, that also depends on daylight; so, in order to keep the electricity for later consumption (both on daily and seasonal scales), electricity storage is mandatory.
There are several possibilities to store electricity. One of them are batteries, that already exist in our everyday life: from mobile devices to transport, to stationary storage. However, it seems hardly feasible to deploy them at the large scale (because they are based on critical materials [20], and for long-term seasonal storage (because they self-discharge) [21]. Reversible dams are also popular, but most sites where it can be implemented are already saturated (at least in developed countries), not speaking from their non-negligible environmental impact. Another very actual solution consists of power-to-gas and notably in power-to-hydrogen. Hydrogen is the first element of the periodic table and, despite being very abundant on the planet, dihydrogen (H2, it will be referred in this manuscript as simply “hydrogen”) is only merely present naturally on Earth. H2 can however be produced from water and energy, stored in multiple ways, and then reused to produce electricity in a fuel cell. It is wise to say that these multiple means complement each other, and all have their own advantages and drawbacks, depending on when and for what they are implemented.
Hydrogen in industry and as an energy vector
A virtuous hydrogen economy works around one main technology, capable of converting hydrogen and oxygen into electricity, with only water as an outlet: the fuel cell. In the reverse reaction, hydrogen can be produced by providing electricity (from renewable energies for example) and water. This promising concept brought the attention as one of the solutions to attain carbon neutrality.
This leads to a global enthusiasm towards the possible use of hydrogen as an energy vector. France recently voted a budget of 7 billion euros or the next 10 years [22] to support research and development on the hydrogen vector. Globally, the European commission estimates a cumulative investment for hydrogen (production and usage) up 470 billion euros until 2050 [15,23], so as to develop the hydrogen grid, and encourage both research and applications.
Many of such application are concentrated in the transport sector, second sector in terms of CO2 emissions behind electricity production [24], but also in H2 for the industry (fertilizer, metallurgy, etc.).
The first hydrogen vehicle commercialized was the Toyota Mirai, in 2014. After 3000 vehicles sold each year, the new version, the Mirai 2, is very promising [25,26]. With its stored capacity of 5.6 kg of Hydrogen, the new model can drive for 650 km with a refill of only 3 minutes [27]. Honda and Hyundai now also have their own hydrogen model. In France, Symbio [28] adds an hydrogen tank to commercial electric vehicle to increase its autonomy (range extender). Buses [29], trucks [25] or even garbage trucks fleets (HECTOR European Project) [30] now have an hydrogen version, taking profit from their fixed journey to efficiently recharge them.
Road vehicles are not the only ones being converted to fuel cells. In 2018, Alstom lunched its ILint Coradia, the first hydrogen train [31,32]. In aeronautics, the European Group Airbus announced its ambition to commercialize the first non-emission aircraft (based on hydrogen technology) by 2035. Several boats also work with a fuel cell device, as the Energy Observer for example [33].
Fuel Cells
The first fuel cell was first experimented by Sir William Grove and Christian F. Schoenbein in 1838. They measured a voltage between two platinum plates, each associated to either a hydrogen or oxygen-containing tube and separated by acidified water [34]. It had to wait nearly a century, before Francis Thomas Bacon made it into a full stack prototype, in 1932 [35]
During the 60’s 70’s, an alkaline type of fuel cell was used for both Gemini and Apollo mission, preferred to batteries for their light weight and convenient water cycle.
Since that period, the fuel cell concept of oxidizing a fuel at the anode while reducing an oxidant (often oxygen) at the cathode, has been applied to several systems. The major ones are detailed in Table I.1 [35–38]. There are in fact many types of fuel cells, depending on the nature of the fuel, the electrolyte and the operating temperature. Instead of hydrogen, the direct ethanol fuel cells (DEFC) and direct methanol fuel cells (DMFC) use ethanol and methanol as a fuel, respectively, which eases the fuel handling and storing and makes them suited for portable applications, but their performances are severely hampered by the too complex, hence sluggish fuel oxidation reaction. High temperature fuel cells such as solid oxide fuel cells (SOFC), or molten carbonate fuel cells (MCFC) are more adapted to stationary power plant, and present the advantage of possible direct use of reformate gas. Their operational durability is however not granted.
PEMFC is the technology mostly used in transports today. Because a unit cell only operates at a voltage of ca. 1 V, PEMFC systems usually consists of stacks of several unit cells, each cell containing 4 main components:
The ionic membrane is made of a proton conducting polymer. It enables proton conductivity and electrode separation (both electrical and in terms of reactants).
The catalyst layers, on both sides of the membrane, are made of carbon, proton conducting ionomer, and electrocatalyst. Their role is to provide triple contact regions between gas, electrons and ions, a prerequisite for the reactions to take place.
The gas diffusion layers (GDL), on both sides of those catalyst layers, are porous carbon matrices, enabling both gas distribution, product draining and electronic/thermal conductivity.
These three components (and (sub)gaskets) constitute the membrane-electrodes assembly (MEA).
The bipolar plates maintain the MEA under compressive strain (hence gas tightness) and are in charge of electronic conductivity, gas distribution and single cell separation. They bring oxygen on one side, and hydrogen to the other side of each MEA.
Figure I.5 gives a schematic representation of the functioning of a PEMFC. The anode is furnished with hydrogen, that will be oxidized into protons in the catalyst layer. The electrons flow into the electronic circuit to reach the cathode side, thus creating an electrical current. The protons cross the polymer membrane, and participate in oxygen reduction. Pure water is produced at the cathode.
Table of contents :
General Introduction
Chapter I General context and state-of-the-art on the EHC technology
I.1. From climate change to fuel cells
I.1.1. Global warming and pollution
I.1.2. Towards a new energetic mix
I.1.3. Hydrogen in industry and as an energy vector
I.1.4. Fuel Cells
I.1.5. Hydrogen Production
I.2. Compression and Purification of Hydrogen
I.2.1. Main compression/purification technologies of today
I.2.2. Hydrogen electrochemical compressor (EHC)
I.2.3. Effect of impurities on common PEMFC anode catalysts
I.3. Countering strategies for impurities presence
I.3.1. Recovery techniques
I.3.2. Tolerant materials
I.3.3. HOR electrocatalysts materials and relevant kinetics study
I.4. Conclusion and Thesis objectives
Chapter II Experimental Procedures
II.1. Cleaning protocol
II.2. Synthesis of electrocatalysts
II.2.1. Polyol synthesis
II.3. Physical and chemical characterizations.
II.3.1. Electronic Microscopy
II.3.2. STEM
II.3.3. Environmental transmission electron microscopy (ETEM)
II.3.4. X-ray diffraction
II.3.5. X-ray photoelectron spectroscopy
II.3.6. Thermogravimetric analysis
II.3.7. AAS and ICPMS elemental analysis
II.4. Electrochemical characterizations
II.4.1. Electrochemical setup
II.4.2. Basic Electrochemical protocol
II.4.3. Differential electrochemical mass spectrometry
II.4.4. Gas diffusion electrode
II.5. Conclusion
Chapter III Un-alloyed PtRu and their CO tolerance/oxidation properties
III.1. PtRu alloy: A little state of the art
III.1.1. Advantages
III.1.2. Drawbacks
III.1.3. Interest to keep Pt and Ru in contact, while in distinct phases
III.2. Synthesis of the electrocatalysts
III.2.1. Adaptation of the polyol process to Ru nanoparticles
III.2.2. Electrocatalysts studied in this chapter
III.3. Physical and chemical characterizations
III.3.1. Metal content in electrocatalyst powder
III.3.2. Structural properties
III.3.3. Microscopy imaging
III.3.4. Summary of the physicochemical analyses
III.4. Electrochemical characterizations
III.4.1. Electrochemical signature
III.4.2. CO oxidation behaviour
III.4.3. HER/HOR activity
III.4.4. Discussion
III.4.5. Conclusion
Chapter IV Tungsten-oxide supported Pt electrocatalysts
IV.1. Why tungsten-oxide supported Pt electrocatalysts?
IV.2. Synthesis process for Pt/WO3 electrocatalysts
IV.2.1. Adaptation of the polyol process
IV.2.2. Electrocatalysts studied in this chapter
IV.3. Physical and chemical characterizations
IV.3.1. Metal content in electrocatalyst powder
IV.3.2. Structural and surface properties
IV.3.3. Microscopy analysis
IV.3.4. Discussion on physico-chemical characterizations
IV.4. Electrochemical characterizations
IV.4.1. Ink stability
IV.4.2. Electrochemical signature
IV.4.3. CO oxidation behaviour
IV.4.4. Activity for the hydrogen reactions
IV.4.5. Discussion
IV.5. Conclusion on tungsten-supported electrocatalysts
Chapter V Hydrogen oxidation in pure/impure environment
V.1. Basic experiments in GDE
V.1.1. Electrochemical signature of Pt/C
V.1.2. CO-stripping and influence of parameters for Pt/C
V.2. Electrocatalyst behaviour in pure H2
V.2.1. HOR in mass-transport-free setup
V.2.2. Exchange current density
V.2.3. Chronoamperometry in pure hydrogen
V.3. Electrocatalyst behaviour in polluted H2
V.3.1. H2 + 10 ppm CO
V.3.2. H2 + 50 ppm CO
V.4. Conclusion
General Conclusion
References
Annexes
