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Hydrolysis for a green future economy

Hydrolysis for a green future economy

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Supervisor: Dr. Dara Fitzpatrick

Student name: Si Lok Ko

Student No: 116342401

Date of completion: 12/01/2019

Table of Contents

1. Introduction

2. A brief History of hydrogen

3. How electrolysis work

4. Hydrogen Storage

5. Hydrogen Propulsion; fuel cells and Wankel rotary engine

6. Hydrogen vs. Battery

7. Hydrogen Infrastructure and Japan’s hydrogen society

8. Comments and conclusion

9. References

Introduction

The topic of this report is hydrolysis for a green future economy. It will discuss the following; its history, the main use of hydrogen, the evolution of hydrogen, problems encountered, its market competitors and a brief discussion.

Hydrogen is the simplest, lightest and most abundant substance in the universe. Approximately 99% of the universe is composed of this simple little diatomic gas, and all energy sources comes from hydrogen. The fusion energy of hydrogen atoms in the sun creates generates energy, and helium.

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Since it is a highly abundant element, in theory, it should be able to extract, or find “free” hydrogen. However, the air we breathe isn’t composed of “free” diatomic hydrogen gases. Most of the hydrogen are used in organic/biomolecules. So harvesting this element is not a simple task.

The 2 major industrial scaled method to abstract hydrogen is 1) Steam reformation of hydrocarbons 2) Electrolysis of water.

Steam reforming of hydrocarbon is a very common method in harvest hydrogen, since hydrocarbons are composed of carbon and hydrogen. However, it is not an environmentally, nor a long term viable method as hydrocarbons are finite, and its by-product, carbon monoxide, is as harmful as a fossil fuelled vehicle exhaust emissions.1

In relation to electrolysis, it seems more like a viable option, splitting water to hydrogen and oxygen. It’s environmentally friendly, water is highly abundant, and electricity can be generated in various environmentally friendly ways e.g. wind turbines. However, it is crucial to remember, hydrolysis can be considered as a form of green energy as long as electricity is generated using green energy.

This poses the question. Why not just use electricity as the main source of energy?

It is fair to say electric cars are already being manufactured, and are undergoing rigorous research and development, and one was even sent to space.2 But it is not the ideal form of energy storage in terms of weight. While it is applicable for smaller vehicles like cars or buses, it is not economical nor practical for other types of transit like railways, planes or boats. Battery life varies depending on usage, manufacturing and disposal of the battery itself is an equally strong argument against batteries.

Hydrogen carries more energy per mass, and has a quicker replenishing time. This is hydrogen’s strongest point as a means of fuel storage. As it carries more unit energy per mass, it implies the vehicle will be much lighter in comparison to battery powered vehicle, and this will reduce energy used to power the vehicle. Hydrogens can be stored in any sealed vessel, so making the container and disposal will be easier than disposing a battery.

While hydrogen technology appears to be more of a futuristic technology, its roots and interest has been around since the 19th century.

“I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable.”3 – Jules Verne.

It is important to gain an insight into the history of hydrogen briefly before we move on to the modern day technology and see how this fascination revolving this simple diatomic evolved throughout the centuries.

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Figure49 shows a Japanese manga strip of the new Toyota Mirai. Mirai means future in Japanese. And the characters in red translate to the (Toyota) ‘“Mirai” is here’.

A brief history of hydrogen

Hydrogen, as a gaseous substance, was often exploited for its lightness in the early days of science and science fiction. As mentioned previously, Jules Verne, a French novelist from the Victorian era, was already using hydrogen as a mode of transport for his novel “Five weeks in a balloon.”4 And another infamous use, was the zeppelin named Hindenburg.

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Figure5 of Hindenburg disaster captured

Since the incident, there has been a wain in interest for this light, highly flammable gas. But research still continued despite its associated reputation and hazards.

However, its uses were not limited to explosive flying balloons.

The humble history of a hydrolysis/fuel cell can be dated back to the mid-19th century. A middle class Welshman, Robert Grove, applied voltage to acidified water, and generated hydrogen and oxygen in a 2:1 ratio, and subsequently combined the two gases to get water and electricity.6

And most astonishingly, the hydrogen powered vehicle is not a new invention. The first hydrogen powered vehicle pre-dates Robert Grove’s gas powered battery and Verne’s balloon. The vehicle, built by a Swiss named Francois Isaac de Rivaz in 1807, ran on hydrogen using an internal combustion engine.7

It’s clear to see, hydrogen was used in several ways before the recent resurgence in interest as an alternative green energy.

How electrolysis work

Traditionally, a Hoffman voltammeter is used to demonstrate electrolysis of acidified water, splitting the molecule to 2-parts hydrogen and 1-part oxygen. Water is acidified to increase conductivity with the ionic species from the acid as water is a poor conductor. However, this traditional method is limited to demonstration purposes and holds no real economic value nor commercial interest. But its principle is still relevant.

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Figure50 demonstrates how hydrogen can be produced via electrolysis using a Hoffman voltammeter.

Currently, various research and studies are investigating different metal based electrodes and their effects to electrolysis. These effects include Oxygen Evolution Reaction (OER), surface layer of the electrode when electrolysis occurs, and most critically, cost of the electrode.

OER is a very important step in electrolysis in harvesting the hydrogen. If oxygen cannot be evolved, the reaction cannot proceed, and the hydroxyl species can revert and react with a proton to form water. Furthermore, the OER is a very energy demanding step, and is comparably slower electrochemical reaction than Hydrogen Evolution Reaction (HER).8

The choice of metal for the electrode is crucial for electrolysis. As the metal will be placed in harsher conditions and expose itself to anions, understanding the chemistry of that metal in such conditions will be key in designing a good electrode. A class of electrodes are called Dimensionally Stable Anodes (DSA) has been of great interest. “A dimensionally stable anode is one for which the degradation processes are much slower and limited to the electro-active surface layer. Since the shape and structural integrity of the electrode are preserved, the surface layer, usually a coating with appropriate catalytic properties, can be regenerated.”10 They are comprised of an underlying inert metal like titanium, which is then coated with an electrocatalytically active oxides of platinum group metals. 11

In addition, the structure of the electrode is important as it allows electrolyte movement. These can be classified into two main types. Flat and three dimensional. Flat electrodes allow for one surface for penetration, and are often based on copper or titanium foil. This limits the activity and results in a slower process. Three dimensional electrodes have multiple surfaces for penetration, resulting in better and faster process.11

Platinum group metals such as IrO2 and RuO2 has been studied extensively, and have shown the lowest overpotentials to complete the OER step. (Overpotential being the extra potential required to complete the reaction electrically.) Implying this type of electrode is efficient as it doesn’t require much energy to complete hydrolysis. Furthermore, due to the nature of platinum group metals, they are much less prone to oxidation reduction reactions, meaning they more durable. While these metals combat the two fundamental problems of OER and surface layer degradation, its cost of production is the greatest challenge posed as they are highly sought after.11

That said, studies have undergone with first row transition metal elements for their low cost and long term corrosion resistance in alkaline solution. An example of using these metals are bulk oxide / hydroxide electrodes.

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Metals such as Nickel, Cobalt, Manganese and Iron has been studied extensively, and used in the form of supercapacitors. This then, lays a better understanding for these metal electrodes in terms of electron transfer when a current is applied. One of the chemicals that revived the interest for bulk metal oxide material for catalysis is CoCF, thanks to its efficiency at low pH and low cost of assembly.11 However, like all electrodes, when current is applied, the reactions that occur in solution often results in the thickening of the film. This lowers the activity of the electrode as it there are less surface for the electrode to interact. In the CoCF case, it has a more nodular surface.

Surfaquo groups, are hydrated surface species that often arises from the redox reactions of the oxy groups, such as the rapid exchange of protons, electrons and hydroxyl groups.11 This often comprises of many complicated, variable reactions for different set parameters. Key anion species identified comprise of; oxides, hydroxides and hydroperoxide anions, in which all are bonded to the metal. In some cases, the film can be held together by oxy or hydroxyl bridges.

In relation to the previous, it is important how the film thickening can affect the overall process of electrolysis. The reason why there has been extensive research in the electrodes is, they affect the process directly, from its production method to the reactions that undergoes in water. For any electrodes that can carry out electrolysis efficiently, the following factors are highly desirable; high electrochemical surface area, fast electron transfer, facile and reversible ion diffusion.

As mentioned previously, the OER is the greatest downfall for “abstracting” hydrogen from water as it disrupts two main things. 1), it is the rate limiting step of the process. Hydrogen can only be evolved if the OER is fast enough, and 2), the extra energy required to overcome this process. This is essentially the raison d’etre for the dedication and involvement in developing better electrodes, to overcome the OER.

However, by preparing different transition metal electrodes is not a wise idea. The reason being, OER is still an unknown process mechanistically. It has been proven and argued, it being a very difficult process as it involves the movement of 4 electrons from a hydroxyl anion over the entire process to evolve one oxygen molecule. Hoare and Kinoshita have highlighted, by admitting to a number of assumptions, there are still possibly 10 to 11 possible mechanisms.11 And this possibility of numerous mechanisms is enhanced further when a different electrode of a different preparation or metal is introduced.

To elaborate further, there is a certain point to note. As these ions are solvated in solution, their activity of interest will most certainly revolve around the electrode. As a direct consequence, it is safe to assume these ions will have some sort of affinity towards the electrodes due to their charges, and may bind to the surface of the electrode. As these ions build up and form a film on the surface, the rate at which these species desorb may be the rate determining step to the entire step of electrolysis, or OER.11

To reiterate, there are many challenges in the entire process of electrolysis. Most of the difficulties arises from the electrode, but is not restricted to it. While one can increase the potential and theoretically eradicate all problems, it fundamentally defeats the purpose of “making” hydrogen as a cleaner alternative source of stored energy.

Even if hydrogen could be evolved with ease, there should be a method in storing hydrogen that is both safe and compact. Otherwise, the hydrogen gas evolved is just going to go up in flames before one can use it for transport.

Hydrogen Storage

Hydrogen storage is one of the three main hurdles to cross in order to achieve a potential hydrogen economy. It is arguably one that has been research the most, and one that has several applications in place. For starters, hydrogen was used as rocket fuel in the Saturn V, the modern Toyota Mirai and the experiment hydrogen powered Mazda MX-5. And interestingly, they had different forms of hydrogen storage.

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There are two main types of hydrogen storage. Physical, which pertains to compression or physisorption. The other from being, chemical. Chemical storage is storing hydrogen using chemical compounds such as metal hydrides.

Chemical storage was used in the experimental Mazda MX-5 in 1995. It used “powdered metal hydride to hold and absorb the hydrogen in a safe state.”12

In general, hydrogen incorporated chemical storage is either bonded or as complexes. The media of storage can be in a liquid or solid state. To release the hydrogen, it must be subjected to thermal or catalytic decomposition. It was reported a metal hydride storage tank using palladium, achieved 5.5wt% in 2013.15 However, there is a problem of the reversibility of the said reaction.13

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Figure53 of the prototype Mazda MX-5 hydrogen storage system

But overall, there are more questions to chemical storage of hydrogen than it answers. Not only is it less practical, due to the enhanced weight of the metal, price of the metal, associated atoms such as nitrogen if ammonia is used, extreme pressures and temperatures are required to make said hydrides, hampers this overall possibility of commercial viability.13 Thus, chemical storage is rarely, if even considered as a storage means.

If physical storage is applied, this is one of the strongest argument for considering hydrogen powered vehicles. When compared to the popular alternative means of energy storage, there are reasons as to why hydrogen technology should be continued.

Batteries (Lithium ion)

Hydrogen

278 Wh/kg

40″,000 Wh/kg

No pressurised containers

Extreme pressures for storage

3 tons + for a 400 mile journey

¬1.25 tons for the same range

Extra mass for distance

Lighter weight, less energy used

Quick charge / 3 hours full charge (depending on vehicle)

Quick refuels

Table 1 contrasts the main differences between the two energy storage mediums 14

In the case of the Toyota Mirai, the physical storage method is employed. Hydrogen is conventionally stored in its pure form as a compressed gas (CGH2) or as a cryogenic liquid. In the case of standard mobility vehicle fuel tanks, hydrogen is stored at 700bars, and have an energy density of approximately 4.8MJ/L.

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Figure61 shows the hydrogen tank used in Toyota Mirai

For liquid hydrogen, energy density is increased at 8.5MJ/L. However, the storage system must be subjected to lower temperatures than compressed gas, and it always runs the risk of evaporation from external heat sources; conduction, convection, radiation. While there are no existing permanent solutions, the method in resolving this issue is by delaying its evaporation by sealing the liquid hydrogen in a double hull vessel, where the outer layer is a vacuum.15

The downfall for hydrogen in comparison with petrol, is its poor energy density. For example, a fuel cell vehicle (FCV) requires 4-6 kg of hydrogen to travel 500km. This equates to a storage tank of 100-150 L. A typical mid-sized car has a tank size of 60L.15

As an alternative, physisorption, or molecular adsorption, is used to store hydrogen. This method of adsorption is reversible, and will not undergo decomposition of loss of gas. The slightly exothermic adsorption reaction would be suited towards low temperature refuelling for mobile storage technologies.13

Several materials used for physical storage are used, such as organic polymers, metals, zeolites (microporous aluminosilicates minerals), but in this report, carbon based material, specifically fullerene, will be discussed. Fullerene, also known as graphene has seen a surge in popularity for the choice of material, mostly used for its durability, lightweight and low cost.

Many carbon material has a high adsorption capacity for hydrogen. At room temperature, it is limited to approximately 0.2wt%, meaning 0.2g of hydrogen per 100g of carbon. At extreme pressures of 500 bars, 2.7% can be recovered. It was reported that, on repeated trials, 5% was recovered at -196°C at 20 bars for AX21, an activated carbon with 3000m2 per gram specific surface area.13

Similar to electrodes, by examining the structure in a nanoscale, decreasing the pore size can increase hydrogen adsorption immensely, preferably

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