Volume 4 fuel cells and hydrogen technology 4 01 – preface and context to hydrogen and fuel cells

Volume 4 fuel cells and hydrogen technology 4 01 – preface and context to hydrogen and fuel cells Volume 4 fuel cells and hydrogen technology 4 01 – preface and context to hydrogen and fuel cells Volume 4 fuel cells and hydrogen technology 4 01 – preface and context to hydrogen and fuel cells Volume 4 fuel cells and hydrogen technology 4 01 – preface and context to hydrogen and fuel cells Volume 4 fuel cells and hydrogen technology 4 01 – preface and context to hydrogen and fuel cells

4.01 Preface and Context to Hydrogen and Fuel Cells AJ Cruden, University of Strathclyde, Glasgow, UK © 2012 Elsevier Ltd 4.01.1 4.01.2 4.01.2.1 4.01.2.2 4.01.2.3 4.01.2.4 4.01.2.5 4.01.2.6 4.01.2.7 4.01.2.8 4.01.2.9 4.01.2.10 4.01.2.11 4.01.2.12 4.01.2.13 4.01.2.14 4.01.3 4.01.3.1 4.01.3.2 4.01.3.3 4.01.4 4.01.4.1 4.01.4.2 4.01.4.3 4.01.4.4 4.01.5 References Introduction An Overview of This Volume Chapter 4.01: Introduction Chapter 4.02: Current Perspective on Hydrogen and Fuel Cells Chapter 4.03: Hydrogen Economics and Policy Chapter 4.04: Hydrogen Safety Engineering: The State-of-the-Art and Future Progress Chapter 4.05: Hydrogen Storage: Compressed Gas Chapter 4.06: Hydrogen Storage: Liquid and Chemical Chapter 4.07: Alkaline Fuel Cells: Theory and Application Chapter 4.08: PEM Fuel Cells: Applications Chapter 4.09: Molten Carbonate Fuel Cells: Theory and Application Chapter 4.10: Solid Oxide Fuel Cells: Theory and Materials Chapter 4.11: Biological and Microbial Fuel Cells Chapter 4.12: Hydrogen and Fuel Cells in Transport Chapter 4.13: H2 and Fuel Cells as Controlled Renewables: FC Power Electronics Chapter 4.14: Future Perspective on Hydrogen and Fuel Cells Hydrogen and Fuel Cell Technology – Supplementary Material Flow Cells or Regenerative Fuel Cells Hydrogen Production – Electrolysis Hydrogen Demonstration Units – State of the Art Introduction to Basic Electrochemistry Redox Reactions Electrochemical Series Gibbs Energy – Useful Work Practical Fuel Cells Conclusions 7 7 7 7 8 8 8 10 11 17 18 21 22 25 26 26 4.01.1 Introduction Hydrogen is the most abundant material in the Universe, forming over 75% of known matter; however, it does not commonly exist on Earth in its natural form, due to its highly reactive nature, but within other compounds, most notably water and hydrocarbons The discovery of hydrogen gas is credited to the famous English philosopher Henry Cavendish (although he was actually born in Nice, France!) who, in 1766, wrote a seminal paper entitled ‘Experiments on Factitious Airs’ [1] (Figure 1) after experiments dissolving different metals (such as zinc) in acidic solutions These experiments produced a gas, the ‘factitious air’ that “takes fire, and goes off with an explosion” (see a further exert from the Cavendish paper of 1766 shown in Figure 2), which is now a common high-school test for hydrogen gas – set fire to it and it goes ‘pop’! Cavendish went on to determine that this gas was significantly lighter than air and, although credited with isolating this new inflammable gas, it was another Frenchman, Antoine Lavoisier, who named this gas as hydrogen in 1783 Indeed, the name ‘hydrogen’ itself is from the Greek words ‘hydros’ (meaning ‘water’) and ‘generos’ (meaning ‘to make’ or ‘to create’); hence, the name hydrogen means ‘to make water’ or ‘water former’ The story of hydrogen took a further step forward around this time when the Englishman, William Nicholson, correctly identified it following his early experiments on electrolysis Figure shows an extract of Nicholson’s famous paper of 1800 [2] where he correctly determines that water is composed of hydrogen and oxygen Of course, the discovery and naming of hydrogen at this time is all the more challenging due to its properties which, at standard temperature and pressure (STP), render it odorless, colorless, tasteless, nontoxic yet highly flammable (within its flammability limits of 4–74% in air) It is a highly reactive substance (hence, it does not naturally occur but is found bonded within many other compounds) and is the lightest element in the periodic table Hydrogen at STP is in the form of a molecular gas It was not until 1898 that the Scotsman, Sir James Dewar, liquefied hydrogen for the first time (see Figure showing a repeat of this first experiment in 1899), achieving temperatures of 20 K or –253 °C Even at such extreme low temperatures, hydrogen formed a colorless liquid Dewar continued his pursuit of ever colder temperatures and was the first to produce solid hydrogen, at temperatures below 14 K (–259 °C) in 1899 Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00401-7 Preface and Context to Hydrogen and Fuel Cells Figure Cavendish’s paper on ‘factitious air’ from 1766 From http://www.theworldsgreatbooks.com/images/Science/cavtext.jpg Figure Extract from Cavendish’s 1766 philosophical transactions paper http://books.google.co.uk/books?id=1kJFAAAAcAAJ&pg=PA141&lpg= PA141&dq=philosophical+transactions+of+the+royal+society+1766+cavendish&source=bl&ots=IGBnzpS7_e&sig=5cTcFhZXXJQZLHV7Nf6NVx3sJfo &hl=en&ei=alO6TsywKMi3hAfinZjBBw&sa=X&oi=book_result&ct=result&resnum=7&ved=0CEQQ6AEwBg#v=onepage&q=philosophical% 20transactions%20of%20the%20royal%20society%201766%20cavendish&f=false From around this time, hydrogen gas had been in use as a constituent of coal gas Coal gas, as opposed to natural gas (naturally occurring gas containing methane as the majority component), was a manufactured gas from the ‘cracking’ of coal, that is, the breakup of the long coal hydrocarbon molecules into different compounds This cracking was achieved by controlled combustion (with limited air) to produce a gas containing up to 50% hydrogen, methane, carbon monoxide, and other gaseous elements This gas was produced locally and used for lighting and heat/cooking [3] The manufacture of coal gas in the United Kingdom decreased rapidly following the discovery and extraction of natural gas, principally methane, from the North Sea, which could be both extracted and used in a much environmentally clean fashion, with less remediation required, than coal gas production [4] The use of coal gas was widespread and very visible in many towns and cities, principally due to the large gas holders (or gasometers as they became known as) that stored the coal gas These tanks (an example is shown in Figure 5) were a common site and are only now being replaced by high pressure storage in modern, underground, plastic high pressure natural gas pipelines So hydrogen gas has been in use, albeit in a mixed dilute form within a coal gas mix, for well over a century Hydrogen has also many industrial and speciality uses: as a product in semiconductor processing, petroleum refining, ammonia production for fertilizer production, heat treatment of metals, as a coolant in large electrical generators in power stations, and as a rocket fuel for space missions! However, this volume will concentrate on the technologies that aim to use pure hydrogen as a fuel Hydrogen is not a source of ‘primary energy’, as hydrogen requires to be produced/released or manufactured as a pure gas, and also requires further treatment to liberate energy when being converted to useful work It is also not a form of renewable energy, Preface and Context to Hydrogen and Fuel Cells Figure Extract from Nicholson’s paper of 1800, determining the composition of water http://books.google.co.uk/books? id=TggAAAAAMAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false Figure Painting of Sir James Dewar demonstrating liquefaction of hydrogen to the Royal Institution, 1899 rather it should be viewed as an ‘energy vector’, in a similar fashion to electricity which does not naturally occur, and requires to be produced (generated) from primary energy sources/fuels, and is then reconverted to useful work in our electric lights, heaters, machinery, computers, and so on However, hydrogen is a unique type of energy vector in that it can be stored, in large volumes, unlike electricity, and this unique feature will enable its use to support development and implementation of the other forms of renewable energy reported in this Comprehensive Renewable Energy series Preface and Context to Hydrogen and Fuel Cells Figure Coal gas storage tank (gasometer) http://www.igg.org.uk/gansg/12-linind/gasworks.htm This capacity for hydrogen to be used as both an energy store and a fuel is particularly relevant to the transport sector, a sector dominated by fossil fuels and hence carbon emission concerns, and a sector that forms of renewable energy like wind or solar (unless biofuels) is not frequently linked to The use of hydrogen as an intermediate energy store and fuel will allow wind energy and other forms of renewables to power water electrolysis plants, to produce hydrogen gas for use in vehicles, thereby creating a ‘double benefit’: increase the usage of renewable energy and replace vehicle fossil fuel consumption with a zero emission alternative A significant contributory factor in the pursuit of hydrogen use as a fuel has been the parallel development of fuel cells, a DC electrical source that can be thought of as a ‘continuously operating battery’ The fuel cell was first discovered by William Grove in 1839 [5], and was reported as a postscript in a paper focussing on developments on ‘voltaic series’, at the time an area of feverish scientific development in the new field of electricity, following Alessandro Volta’s creation of the zinc/copper pile battery Grove’s postscript to his paper is shown in Figure 6, and marks the time when Grove, in a subsequent publication in 1842 [6], presented further details of this initial fuel cell and included an image of the experimental arrangement, an image which has become redolent with the history of fuel cells A full copy of this paper is shown in Figure 7, first to illustrate the image of Grove’s fuel cell and second to highlight the spirit of research at this time (Note the comments on p 418 regarding the detection of electric potential by means of an electric shock felt across five persons joining hands!) A principal difference between a ‘battery’ and a ‘fuel cell’ is that a battery contains (or holds internally) the ‘fuel’ or chemical compounds with which to generate electricity within the casing of the battery: by contrast, the ‘fuel’ for a fuel cell is both held and supplied externally, and hence can be continuously supplied or replenished if desired 4.01.2 An Overview of This Volume This volume, entitled ‘Hydrogen and Fuel Cell Technology’, part of the overall Comprehensive Renewable Energy Major Reference Work, covers a range of technologies spanning the hydrogen and fuel cell sector This particular chapter presents the context to this Preface and Context to Hydrogen and Fuel Cells Figure William Grove’s postscript describing the first recorded operation of a hydrogen fuel cell sector and an overview of hydrogen and fuel cell technology The remaining chapters of this volume will now be briefly introduced, in the order they are presented within the volume: 4.01.2.1 Chapter 4.01: Introduction The history and background to the production and use of hydrogen as a fuel within fuel cells is presented in this chapter, in addition to providing an introduction to the fundamental thermodynamics of how fuel cells operate This is as a precursor to the further, more technically detailed, chapters that follow and includes an overview and context to the hydrogen and fuel cell sector It also Preface and Context to Hydrogen and Fuel Cells Figure Grove’s ‘Gaseous Voltaic Battery’ – the first reported hydrogen fuel cell presents information on examples of existing hydrogen production technology, namely, large-scale alkaline electrolysis, and of one current transportable proton exchange membrane (PEM)-based hydrogen production system for vehicle refuelling Finally, some detail of a regenerative fuel cell, a novel type of fuel cell that is gaining traction as a potential energy storage technology, is presented to illustrate that some fuel cell technologies not require hydrogen gas to operate Preface and Context to Hydrogen and Fuel Cells 4.01.2.2 Chapter 4.02: Current Perspective on Hydrogen and Fuel Cells This chapter presents a historical perspective and the current state of the utilization of hydrogen as a fuel, and of fuel cells as a power source Given the impetus to develop and use hydrogen and fuel cells within the space exploration sector, this chapter gives a particular perspective of these early developments and an overview of current uses 4.01.2.3 Chapter 4.03: Hydrogen Economics and Policy Developing hydrogen gas as a new form of energy vector requires significant technical development, as later chapters within this volume will discuss; however, it will also require significant financial investment Additionally, to break into the classic ‘chicken and egg’ cycle that tends to inhibit new technology uptake (e.g., users will not purchase hydrogen vehicles until there is an established network of hydrogen refuelling stations, while energy companies will not develop or invest in a network of hydrogen refuelling stations until there is a significant population of hydrogen vehicles to utilize this investment), the nature and use of government policy instruments to stimulate this development is presented and discussed 4.01.2.4 Chapter 4.04: Hydrogen Safety Engineering: The State-of-the-Art and Future Progress A key issue surrounding the utilization of hydrogen as a fuel centers on the safe use of this highly reactive, flammable gas The subject of hydrogen safety is simultaneously both technical and emotive, with considerable ongoing technical work studying hydrogen flammability and safety equipment, while also addressing public perceptions and concerns through publicity campaigns and demonstration programs This chapter presents technical information surrounding the physical properties of hydrogen and its characteristic behavior during leakage, combustion, and explosion It also details current international standards in this area surrounding the safe use of hydrogen as a fuel 4.01.2.5 Chapter 4.05: Hydrogen Storage: Compressed Gas A critical element in the development of a potential ‘hydrogen economy’ is the ability to store and transport hydrogen gas as a fuel The storage of hydrogen has, and remains most commonly, been in the form of a compressed gas Historically, this has typically been at ‘industrial’ gas pressures of around 200 bar, a typical pressure for a steel cylinder of hydrogen for laboratory or factory use; however, in recent times, the use of high pressure (up to 700 bar) storage of hydrogen within composite pressure vessels has been promoted particularly for vehicular use This chapter explores the issues and technologies involved in storing hydrogen gas at such pressures, and discusses the safe handling and use of hydrogen within such systems 4.01.2.6 Chapter 4.06: Hydrogen Storage: Liquid and Chemical Hydrogen storage in the form of compressed gas has a number of limitations and this chapter studies other possible forms of storing hydrogen as a fuel, namely, as a liquid at very low temperatures or within chemical media capable of absorption or reversible material reactions Some details of these alternative forms of storage are presented, and the issues surrounding reversibility, the dynamics and speed of storage/release, cost, storage density and cyclability are discussed within this context 4.01.2.7 Chapter 4.07: Alkaline Fuel Cells: Theory and Application There are several different types of fuel cell that can be broadly characterized by the type of electrolyte used within the cell In the late 1950s, the main electrolyte of interest was potassium hydroxide, an alkaline solution, and hence, it was this particular fuel cell technology that reinvigorated research and commercial interest following on from successful use in early space flights This chapter presents both further technical detail of alkaline fuel cell (AFC) technology and further historical information on past and current developments 4.01.2.8 Chapter 4.08: PEM Fuel Cells: Applications For transport applications of fuel cells, an obvious disadvantage to the use of AFCs was the requirement to contain and protect (in the event of collision/accident) a highly caustic liquid electrolyte A solution to this issue arose from the study of ion-conducting solid membranes, which obviated the need for a liquid electrolyte, thereby removing accompanying spill/leakage issues This work led to the development of positive ion-conducting membranes (e.g., proton conduction), as opposed to negative hydroxyl ion conduction within AFC, and spawned the PEM fuel cell sector This genre of fuel cell is now the most widely researched and most promising for utilization within the transport sector, and this chapter explores the material developments, capabilities, and demonstration of this technology 4.01.2.9 Chapter 4.09: Molten Carbonate Fuel Cells: Theory and Application As the chapter name suggests, this fuel cell technology is characterized by operation using a molten carbonate salt as the cell electrolyte This requires temperatures of several hundred degrees Celsius to produce the molten electrolyte and achieve appropriate Preface and Context to Hydrogen and Fuel Cells ion mobility, thereby enabling fuel cell operation, and this technology has developed to accommodate the resulting technical challenges This chapter presents details of the development of molten carbonate fuel cells, in particular the significant installed commercial capacity of these fuel cells, and discusses the future potential for this technology 4.01.2.10 Chapter 4.10: Solid Oxide Fuel Cells: Theory and Materials Of the range of electrolytes available for use within fuel cells, the highest temperature electrolyte currently used is a negative ion-conducting ceramic within the solid oxide fuel cell (SOFC) class Typically, operating temperatures of between 550 and 1000 °C are needed to produce the conditions necessary for appropriate ionic conduction in these materials; however, such high-temperature fuel cell technology is attractive in many process industries where high-grade waste heat may be employed to readily create the operating temperatures or direct use of fossil fuel combustion products that are available This chapter explains how an SOFC operates, discusses the material requirements and developments, and presents information on the current state of the art in this sector 4.01.2.11 Chapter 4.11: Biological and Microbial Fuel Cells Microbial and biological fuel cells are relatively new forms of fuel cell that typically use either hydrocarbons as a fuel, or utilize electrogenic bacteria to convert chemical energy to electrical energy rather than a more typical electrocatalyst These forms of fuel cell are gaining prominence in developing niche markets, such as wastewater treatment where electrical energy can be derived while simultaneously processing and cleaning the waste stream, and are potentially able to address the growing legislative requirements to clean hitherto neglected process streams This chapter explores the theory and materials (including enzymes and bacteria) employed to create these unique fuel cells and presents details of their resulting performance characteristics 4.01.2.12 Chapter 4.12: Hydrogen and Fuel Cells in Transport Of the many application areas for fuel cells, the sector that may provide the greatest advance in terms of commercial breakthrough and success is within the transport field Fuel cell utilization within electric vehicles, ships or as on-board electrical generators for load refrigeration or cab power, is increasing rapidly and offers efficiency and carbon emissions benefits compared to conventional fossil fuel options The transport sector offers both a huge market and a direct interface to the public (compared to the stationary power market for fuel cells which tends to ‘isolate’ the consumer from the technology, whereas, for example, transport fuel cells are accessible under the hood of a car); hence, fuel cells can gain rapid consumer acceptance and market traction within this key end-use sector This chapter presents details of fuel cell technology relevant to the transport sector and illustrates several examples of typical use 4.01.2.13 Chapter 4.13: H2 and Fuel Cells as Controlled Renewables: FC Power Electronics Neither hydrogen nor fuel cells are formally defined as ‘renewable sources’ of power within current legislation [7]; however, they are commonly viewed as an integral element of any future clean energy mix Uniquely, within the context of renewable energy sources where many of these sources are stochastic (i.e., variable) and uncontrolled in terms of their energy delivery, hydrogen and fuel cell technology offers the user direct control of the power delivered at any instant The ability to control the power and energy delivery from a fuel cell, and to control the storage and utilization of hydrogen as a fuel, conveys significant benefits to the utilization of hydrogen and fuel cell technologies This chapter explores the use of power electronics as a mechanism to control the output electrical power from a fuel cell, and examines the type and nature of the power electronic interface between the electrochemical cell and the electrical load 4.01.2.14 Chapter 4.14: Future Perspective on Hydrogen and Fuel Cells The final chapter of this volume presents a perspective of how hydrogen and fuel cell technology may develop in the future, and how hydrogen use as a fuel may pervade more aspects of our lives This concept, of a future hydrogen-based economy, is a view that many researchers and analysts believe it is inevitable as the world’s fossil fuel resources are depleted and environmental pressures compound the shift toward a cleaner energy supply 4.01.3 Hydrogen and Fuel Cell Technology – Supplementary Material It has not been possible within this volume to adequately cover ‘all’ aspects of hydrogen and fuel cell technology, and this brief section aims to introduce the reader to areas considered by the author as important with appropriate references to allow further reading as required 4.01.3.1 Flow Cells or Regenerative Fuel Cells The first technology of interest is the flow cell or flow battery, where the energy capacity of the system is stored external to the cell generally in the form of oppositely charged liquid electrolytes These electrolytes are reacted within a fuel cell, in the form of two half Preface and Context to Hydrogen and Fuel Cells cell reactions (see Section 4.01.4.1) separated by an ion-selective membrane (i.e., a membrane that allows only specific ions to pass through it but prevents the liquid electrolytes from mixing directly) that convert the chemical energy in the electrolytes fuels to electrical energy This technology is viewed as different from a ‘conventional’ hydrogen and oxygen (or air) fuel cell This technology can be encountered under a number of different names, for example, regenerative fuel cell or redox flow fuel cell This technology is exemplified by the technology such as the sodium polysulfide bromide-based system developed by a UK company called Regenesys, from the late 1990s to the early 2000s Regenesys aimed at commissioning a demonstration plant rated at 120 MWh, 12 MW at a site near Little Barford, Cambridgeshire, UK, to evaluate regenerative fuel cell technology as a large-scale form of electrical energy storage The test site was almost completed, as per Figure 8, but ultimately never fully commissioned, as explained in Reference [8] Figure shows the actual regenerative fuel cell modules within the Little Barford test site As with other forms of fuel cell, this technology produces DC electrical power, and hence, a power electronic inverter was required to convert this to AC electrical power suitable for delivery into the electricity grid For the Little Barford test site, the inverter was developed by ABB and was rated at 18.25 MVA [9] although the fuel cells were rated to deliver only up to 12 MW The Regenesys technology employed two electrolytes: sodium bromide (NaBr) and sodium polysulfide (NaS2) These two liquid electrolytes were circulated through the regenerative fuel cell as per the diagram shown in Figure 10, where the half-cell redox reactions occurred, generating electrical energy if pumped in one direction, and capable of reversal to store electrical energy if the electrolytes were pumped through the cell in the opposite sense (with electrical energy input to the cell in this instance) Figure Exterior view of Regenesys Technologies Ltd pilot utility scale energy storage plant at Little Barford, Cambridgeshire, UK, July 2003 From “Regenesys Utility Scale Energy Storage – Project Summary”, DTI, Contract Number: K/EL/00246/00/00, URN Number: 04/0148, 2004 Figure Interior view of the plant – stream of XL modules From “Regenesys Utility Scale Energy Storage – Project Summary”, DTI, Contract Number: K/ EL/00246/00/00, URN Number: 04/0148, 2004 10 Preface and Context to Hydrogen and Fuel Cells Electrolyte tank Electrode Ion-selective membrane Regenerative fuel cell Electrolyte Pump Power source/load Electrolyte tank Electrolyte Pump Figure 10 Flow diagram of Regenesys flow cell Price, A, et al A novel approach to utility scale energy storage IEE Power Engineering Journal, June 1999 This technology, along with other chemical types such as the zinc bromide battery [10], vanadium redox battery [11], and iron– chromium [12] system, can all act as types of redox flow cell, and the reader is directed to the various websites for further information 4.01.3.2 Hydrogen Production – Electrolysis Currently, the most common method of producing hydrogen gas on a large scale is through the process of steam methane reformation, where methane gas (CH4) is reacted with steam (H2O) to, ultimately, produce carbon dioxide (CO2) and hydrogen (H2) However, within the context of this volume, focussing on renewable energy, the preferred method of producing hydrogen is via electrolysis of water (see Section 4.01.1) where electrical energy, preferably from clean sources such as wind or solar power, is used to split water into its constituent gases, namely, hydrogen and oxygen This process, and accompanying electrolyser technology, has been extensively developed by companies such as Norsk Hydro [13] which, since 1920s, has been using cheap hydroelectric electricity in Norway to split water into hydrogen and oxygen gases, and then combine the hydrogen with nitrogen extracted from the air, to form ammonium, a key component of fertilizer The technology surrounding large-scale electrolysers, even today, is based on alkaline technology employing potassium hydro­ xide as an electrolyte, and such units are available in capacities up to MW rating, as shown in Figure 11 Figure 11 shows the electrical connections on the left-hand side of this image to supply DC electrical power to the fuel cells, with the white lye (potassium hydroxide) tanks shown toward the rear right-hand side of the image Figure 11 NEL Hydrogen, MW, alkaline electrolyser NEL Hydrogen Ltd., www.nel-hydrogen.com (accessed March 2012) [14] Preface and Context to Hydrogen and Fuel Cells 13 Figure 16 Hydro’s electrolyser plant for ammonia-production in Glomfjord, Norway (NEL Hydrogen Ltd.) Figure 17 Water treatment Courtesy of Dr A Cruden [15] HOST (Hydrogen On-Site Trial) unit, which demonstrates on-site hydrogen production via PEM electrolyser technology, compression and high pressure storage of hydrogen gas, and finally dispensing of this fuel gas to on-vehicle storage for use within a hydrogen internal combustion engine (HICE) on a van Some of the technology used within the HOST unit is illustrated in the following series of images For example, Figure 17 shows the water processing plant requiring to clean up a typical potable water supply for use within high purity PEM electrolysers Figure 18 shows a palladium dryer unit that is used to help purify the hydrogen gas produced from the PEM electrolysers prior to gas compression and storage This stage helps prevent corrosion issues within the compressor and storage vessels The purified hydrogen gas is subsequently stored in a tiered system of pressure vessels from ‘low’ pressure (up to 250 bar) to ‘medium’ pressure (up to 350 bar, shown in Figure 19) tanks, and finally up to 450 bar in the ‘high’ pressure tanks (the top four tanks shown in Figure 20) The compressor used is shown in Figure 21 The high pressure hydrogen stored is subsequently dispensed via a high pressure nozzle connector, as shown in Figure 22, which directly mates to a vehicle-mounted receptacle, shown in Figure 23 A vehicle refueller control system (i.e., a dispensing refueller system) is shown in Figure 24, and is used to control and monitor the quantity of hydrogen fuel gas dispensed to the vehicle Figure 24 effectively illustrates a future hydrogen refuelling pump, akin to a conventional ‘petrol pump’ that would be found on any petrol station forecourt Hence, Figure 24 shows the type of technology that is being developed for use within a future hydrogen economy for dispensing vehicle fuel 14 Preface and Context to Hydrogen and Fuel Cells Figure 18 Dryer/heater/separator Courtesy of Dr A Cruden Figure 19 Storage tanks Courtesy of Dr A Cruden Figure 25 illustrates a typical commercial van that employs a modified spark ignition combustion engine that can utilize hydrogen gas as a fuel Thus, the HOST trial system demonstrates and evidences all the technology required to produce, compress, store, dispense, and utilize hydrogen as a clean vehicle fuel Such systems are being trialled around the world [16] and highlight the progress that has been made in the underlying technology areas Preface and Context to Hydrogen and Fuel Cells 15 Figure 20 High pressure storage cylinders and bottom buffer store Courtesy of Dr A Cruden Figure 21 Compressor Courtesy of Dr A Cruden A further image of a test site is the NEL Hydrogen park at Porsgrunn (see Figure 26) in Norway, where demonstration of latest generation alkaline electrolyser technology, compression and storage, and vehicle refuelling equipment is under active trial 16 Preface and Context to Hydrogen and Fuel Cells Figure 22 WEH refueller nozzle Courtesy of Dr A Cruden Figure 23 Compressed hydrogen gas vehicle receptacle Courtesy of Dr A Cruden Preface and Context to Hydrogen and Fuel Cells 17 Figure 24 Hydrogen compressed gas vehicle refuelling station Courtesy of Dr A Cruden Figure 25 Hydrogen internal combustion engine (HICE) van Courtesy of Dr A Cruden 4.01.4 Introduction to Basic Electrochemistry As discussed earlier, a hydrogen fuel cell is effectively a ‘continuously operating battery’; that is, as long as ‘fuel gases’ (in this case, hydrogen and oxygen) are fed to the anode and the cathode, respectively, the fuel cell will continue to produce DC electrical power at its terminals In particular, a fuel cell performs a direct energy conversion from the chemical energy in the fuel gases to electrical energy This differs from conventional electricity generation within a thermal power station, say coal or gas fired, where the chemical energy in the fuel is first converted to heat via combustion, and the heat is used to produce superheated high pressure steam within a boiler, which in turn is then used to produce rotating kinetic energy via a steam turbine, which in turn then drives the main rotating shaft of a synchronous AC electrical generator As there are several energy conversion stages in the conventional thermal power station process, each with their own loss mechanism, electricity produced by this process is typically ∼40% efficient, that is, typically 18 Preface and Context to Hydrogen and Fuel Cells Figure 26 NEL Hydrogen Porsgrunn Hydrogen Station Courtesy of Dr A Cruden 60% of the input energy in the primary fuel (coal or gas) is ultimately not converted to electrical energy, but is lost, mostly in the form of waste heat Finally, this rather poor efficiency can only be achieved by building thermal power stations at a large scale, typically >1000 MW rating, as the thermal losses tend to increase at a smaller scale As a fuel cell undertakes a direct energy conversion, its efficiency is higher, typically in the range of ∼50% for conversion of chemical to electrical energy Further, this efficiency is broadly independent of the scale of the fuel cell, from a few hundred watts to several megawatts, leading to a highly efficient modular electricity generating technology The basic electrochemical theory of a fuel cell will now be introduced 4.01.4.1 Redox Reactions To begin with some electrochemical terms requires definition First, an ‘oxidation’ process is defined as the addition of oxygen or removal of hydrogen from a substance A ‘reduction’ process is the addition of hydrogen or removal of oxygen from a substance, that is, the opposite of the oxidation process Further, no oxidation process can take place without a corresponding reduction reaction For example, Oxidation ½1Š 2H2 + O2 ⇒ 2H2O Reduction In eqn [1], the hydrogen is oxidized, that is, oxygen is added to form water, while simultaneously the oxygen is reduced, that is, hydrogen is added to form water On this basis, the oxygen acts as the ‘oxidizing agent’, while the hydrogen acts as the ‘reducing agent’ However, there is a more general form of redox reaction which can be further explained by way of another example reaction (see eqn [2]): Oxidation H2S + CI2 ⇒ 2HCI + S ½2Š Reduction In eqn [2], it is clear that the hydrogen sulfide is the oxidizing agent, that is, it removes hydrogen to form sulfur; however, according to the premise stated above, there must be an accompanying reduction reaction Hence, the chlorine must act as the reducing agent An alternative definition of redox reactions may be formed from eqn [2] if the ionic reaction equations are considered: H2 S ỵ Cl2 2Hỵ Cl ỵ S H2 S 2Hỵ ỵ S ỵ 2e Cl2 ỵ 2e 2Cl oxidation reduction ½3Š Preface and Context to Hydrogen and Fuel Cells 19 A KNO3/H2O Cu Zn CuSO4/H2O ZnSO4/H2O Figure 27 Simple zinc/copper electrochemical cell http://en.wikipedia.org/wiki/Salt_bridge From the ionic reactions, it is clear that the oxidation reaction occurs with both the removal of hydrogen from the hydrogen sulfide and the ‘loss of electrons’ Similarly, the reduction reaction occurs with the ‘addition of electrons’ This therefore leads to a more general definition of redox reactions and following mnemonic to remember this: Oxidation is the removal of electrons from a substance Reduction is the addition of electrons to a substance ‘LEO says GER – Lose Electrons Oxidation, Gain Electrons Reduction’ All such ionic reactions can be realized under conditions where an electric current, that is, the flow of electrons, is present in an external electric circuit (as opposed to an internal current within a closed beaker say) Under these conditions, an ‘electrochemical cell’ is created A simple realization of this is a basic zinc/copper electrochemical cell, formed with a zinc rod placed in a beaker of solution of its own ions (say zinc sulfate, ZnSO4), and similarly a copper rod is placed in a beaker of its own ions (say copper sulfate, CuSO4) Connecting the two beakers is a ‘salt bridge’, a device used to allow ionic (as opposed to electronic) conduction between the two ‘half cells’ This arrangement is shown in Figure 27 From Figure 27, it is found that the ammeter will register current flow, the flow of electrons, from the zinc electrode to the copper electrode This current flow occurs while the zinc electrode slowly dissolves into the solution, and the copper electrode undergoes plating of fresh copper deposits on its surface The corresponding redox ionic reactions are as follows (also know as the half-cell reactions): Zn ỵ Cu2 ỵ Zn2 ỵ ỵ Cu Zn Zn2 ỵ ỵ 2e oxidation anode Cu2 ỵ ỵ 2e Cu reduction cathode ½4Š Equation [4] indicates that the zinc is being oxidized, while the copper is being reduced, and the electrons are ‘lost’ from the zinc flow through the external electrical circuit to the copper electrode where they are ‘added’ to the copper ion as part of the reduction process Within the fields of electrochemistry and physics, convention states that the electrode where oxidation occurs is termed the ‘anode’ and the electrode where reduction occurs is termed the ‘cathode’ Hence, for a galvanic cell (i.e., battery or fuel cell), the anode is the negatively charged electrode (from Figure 27 and eqn [4], the zinc electrode ‘dissolves’ to a solution of zinc ions (Zn2+) and becomes negatively charged due to accumulation of electrons), and the cathode is the positively charged electrode (as the copper ions (Cu2+) in solution accept electrons and deposit copper on the electrode surface, creating a deficit of electrons (a positive charge)) The amount of both zinc lost and copper plating added is proportional to the amount of electrical charge (i.e., electrons) that flows between the electrodes, where the amount of electrical charge, Q (coulombs), is given by Q ¼ Ià t ½5Š where I is the current (amps) and t is the time (seconds) If the electrical current is not allowed to flow (e.g., by breaking the electrical circuit via the ammeter), then there is no loss of zinc nor fresh plating of the copper electrode, proving that the electron transfer of the equations in [4] is intrinsic to the redox process If 20 Preface and Context to Hydrogen and Fuel Cells + 3V − S R Resistance wire A A A Ammeter − + E-cell Sliding contact Cu/Cu2+⏐⏐Zn2+/Zn Figure 28 Experimental setup to determine the open-circuit potential of an E-cell the ammeter in Figure 27 was replaced by a voltmeter, a potential difference (voltage) of 1.1 V would be measured between the two electrodes An alternative experimental arrangement that helps illustrate the operation of an electrochemical cell (E-cell), and determine the potential difference of the cell, is shown in Figure 28 The V battery shown in Figure 28 drives current through the resistance wire (RS) One terminal of the E-cell is connected to the resistance wire (R) and the other terminal of the E-cell is connected to the resistance wire via a sliding contact By varying the position of the sliding contact, a variable potential (voltage) is applied to the E-cell terminals At position ‘2’, it is assumed that the applied voltage via the battery and resistance wire is exactly equal and opposite to the potential difference generated by the E-cell, at which point the ammeter shows zero current At this position, measuring the voltage across the E-cell (voltage across R2) is a direct measure of the ‘open-circuit’ potential of the cell This is characterized as the potential where zero electronic current flows For the simple zinc/copper E-cell of Figure 27, this equals a voltage of 1.1 V Considering the equations in [4], electrochemically this means that the reactions are occurring at exactly the equal and opposite rate, for example, for each molecule of zinc that is oxidized to Zn2+ and liberates two electrons, a molecule of zinc ions (Zn2+) combines with two electrons (2e−) to produce zinc (Zn) The arrows in eqns [6] and [7] indicate these ‘reversible’ reactions, and at open circuit, these competing reactions are balanced, that is, they are at equilibrium Zn Zn2 ỵ ỵ 2e ẵ6 Cu2 ỵ ỵ 2e− ⇔Cu ½7Š If the sliding contact is now moved to position ‘1’, the ammeter will now show current flow from the E-cell, and the voltage measured across the E-cell terminals will be 1.1 V At this position, the reaction in eqn [6] will progress much faster in the reverse (left-hand) direction, and zinc ions from the solution will combine with free electrons from the supplied electric current to deposit fresh zinc on the electrode surface This is shown in eqn [10]: Zn2 þ þ 2e− ⇒Zn ½10Š Similarly, the copper electrode reaction in eqn [7] will also be driven in the reverse (left-hand) direction, and the copper electrode will gradually dissolve into its salt solution, as shown in eqn [11]: Cu ⇒Cu2 þ þ 2e− ½11Š Preface and Context to Hydrogen and Fuel Cells 4.01.4.2 21 Electrochemical Series The voltage measured between the electrodes of the zinc/copper cell, shown in Figure 27, on open circuit was measured as 1.1 V If both the electrode materials were changed, then it is highly likely that a different voltage would be measured, although it would not be easy to readily compare the performance of these two electrochemical cells as there is no common reference between them Indeed, the need to allow comparative measure of the electrochemical performance of different materials led to the development of the ‘standard hydrogen electrode’ (SHE) that is given an arbitrary potential of 0.00 V (Figure 29) The SHE is given the potential of V only at certain conditions: a hydrogen gas pressure of bar, bubbling over a platinum electrode foil, immersed in a solution of molar H+ ions (i.e., acid), at a temperature of 25 °C (298 K) This is embodied in eqn [12]: 2Hỵ aqị ỵ 2e H2 g ị ẵ12 0:00 V By definition of this arbitrary reference, all other materials can be electrochemically compared to the SHE, for example, magnesium, as shown in Figure 30 The voltmeter in Figure 30 will measure an open-circuit potential of 2.37 V, with the magnesium electrode Hydrogen at bar Temperature = 298 K Platinum wire Platinum foil covered in porous platinum Dilute sulfuric acid [H+] = mol dm−3 Figure 29 Example of a standard hydrogen electrode http://www.chemguide.co.uk/physical/redoxeqia/introduction.html V Temperature = 298 K High resistance voltmeter Hydrogen at bar Magnesium Platinum wire Salt bridge Platinum foil covered in porous platinum Dilute sulfuric acid [H+] = mol dm−3 Magnesium sulfate solution [Mg2+] = mol dm−3 Figure 30 Use of an SHE within an electrochemical cell to determine the potential of magnesium http://www.chemguide.co.uk/physical/redoxeqia/ introduction.html 22 Preface and Context to Hydrogen and Fuel Cells Table Series of electrochemical potentials E° (V) Reaction Li ⇔ Li+ + e− Mg ⇔ Mg2+ −3.04 − + 2e Al ⇔ Al3+ + 3e− 2H2 + 4OH− ⇔ 4H2O + 4e− −2.37 −1.66 −0.83 Zn ⇔ Zn2+ + 2e− −0.76 Fe ⇔ Fe2+ + 2e− −0.44 − −0.25 + 2e Ni ⇔ Ni2+ H2 ⇔ 2H+ + 2e− Cu2+ + 2e− ⇔ Cu Ag+ + e− ⇔ Ag Standard hydrogen electrode +0.40 +0.80 O2 + 4H+ + 4e− ⇔ 2H2O ⇔ Au 0.00 +0.34 O2 + 2H2O + 4e− ⇔ 4OH− Au3+ + 3e− Most likely to oxidize +1.23 Most likely to reduce +1.50 being determined (as per the experiments using the apparatus in Figure 27) as the electrode experiencing oxidation, that is, the magnesium, liberates electrons and is the anode, as per eqn [13]: Mg Mg2 ỵ ỵ 2e −2:37 V ½13Š If the electrochemical cell of Figure 30 is used as a battery, then the overall electrode reactions are as shown in eqn [14]: Mg Mg2 ỵ ỵ 2e 2:37 V 0:00 V 2Hỵ aqị ỵ 2e H2 g ị ẵ14 The testing of a range of different electrode materials has been undertaken and defined in Tables of Electrochemical Potentials, see Table Table is shown with all reactions at open circuit; however, the preferred direction of each reaction versus a SHE is as indicated, reading each equation from left to right Further, the reactions at the top of the table indicate materials best suited for oxidation, that is, the strongest oxidizing agents, while materials at the bottom of the table are best suited for reduction, that is, the strongest reducing agents This table can also be used to determine the open circuit potential, and anode and cathode, of any given electrochemical cell For example, taking zinc and copper again (as per Figure 27), Table indicates that zinc will oxidize (i.e., form the anode of the cell) with a standard potential of –0.76 V, while copper will reduce and form the cathode at a standard potential of +0.34 V Hence, overall, the zinc/copper electrochemical cell will produce an open-circuit potential of +0.34 – (–0.76) = 1.1 V (as noted previously) Similarly, taking an aluminum/zinc cell, this time the aluminum will oxidize and form the anode at a standard potential of –1.66 V, while this time the zinc will reduce and form the cathode at a standard potential of –0.76 V Hence, overall, the aluminum/ zinc electrochemical cell will produce an open-circuit potential of –0.76 – (–1.66) = 0.9 V 4.01.4.3 Gibbs Energy – Useful Work The equation for power, P (watts), produced by an electric circuit working with a potential difference of V (volts) and current I (amps) is given by eqn [15]: P ¼ V ÃI ½15Š The energy E (joules) consumed while the electric circuit is working at this power is given by eqn [16]: E ẳ P t ẵ16 where t is the time (seconds) that the circuit is working for Hence, by substituting [15] in [16], and using eqn [5] E ¼ P à t ¼ V à Ià t ¼ V à Q ½17Š Hence, the units of energy (joules, J) can be expressed in terms of volts Coulombs (VC), or joule of energy, is required to move coulomb of charge through a potential difference of volt Hence, as the open circuit voltage for the zinc/copper E-cell is 1.1 V, and from eqns [6] and [7], it is clear that there are two electrons involved in the accompanying redox reactions, then for mole of zinc and mole of copper reacting to completion in an E-cell as per Figure XX, would result in a useful energy production per mole given by Preface and Context to Hydrogen and Fuel Cells Wuseful ¼ nFE ¼2à 96485à 1:1 ¼ 212267 Joules ẳ 212:3 kJ mol 23 ẵ18 where n is the number of moles of electrons that flow during the reaction (= from eqns [6] and [7]), F is the Faraday constant that defines the total electric charge per mole of a substance (96 485 °C mol−1), and E is the open-circuit voltage of the E-cell (V) Equation [18] is of the same form as eqn [17], where nF is equivalent to Q, the total charge transferred From classic thermodynamics, the useful work, Wuseful, is more commonly denoted as ΔG, the ‘Gibbs free energy’ where, by convention, a decrease in free energy, –ΔG, occurs when useful work is done (i.e., the zinc/copper E-cell reaction causes an electric current to flow that can be used for performing ‘useful work’, e.g., powering a light bulb); hence, eqn [18] can more correctly be rewritten as −ΔG ¼ Wuseful ¼ 212:3 kJ mol − ΔG ¼ −212:3 kJ mol − ½19Š As ΔG is negative, by definition, useful work has been carried out, and the reaction will tend to occur in this sense By contrast, if a chemical reaction results in a positive ΔG, then this implies that the reaction is not feasible without input of energy, and it is not feasible to produce ‘useful work’ via such a reaction The first law of thermodynamics states that energy must be conserved, that is, that energy can change state (i.e., from chemical energy to heat) but cannot be created nor destroyed As the internal energy of a system cannot be determined in an absolute sense, then the first law states that the change in the internal energy, dU, of a system can be expressed as the change in heat, dQ, and the change in useful work, dW Mathematically, this can be expressed as dW ẳ dU dQ ẵ20 For a reversible process in a closed system, eqn [20] can be rewritten in terms of the change in the total energy of the system, more commonly known as the change in ‘enthalpy’ of the system, ΔH (kJ mol−1); the change in useful work also known as the Gibbs free energy, ΔG (kJ mol−1); and the change in heat of the system calculated as dQ = TΔS, where T is the temperature in units of kelvin (K) and ΔS is the change in ‘entropy’ of the system (J (K mol)−1) [17]: G ẳ H TS ẵ21 Each system, thermodynamically, is trying to achieve the minimum of free energy, that is, it is trying to achieve its lowest, most stable energy state For example, eqn [21] tells us that reactions that release free energy (i.e., ΔG is a ‘negative’ value) are generally spontaneous and result in more stable resulting state, whereas reactions that result in a ‘positive’ ΔG require energy to be input to the system to force the reaction to occur and generally result in system that is less stable Values for the enthalpies and entropies of reactants and products for a given reaction can be found in many data tables, and the calculation of ΔH and ΔS can be best illustrated by way of an example, considering the zinc/copper E-cell from Section 4.01.3.1 The reaction for the zinc/copper E-cell is Zn s ị ỵ CuSO4 aq ị ZnSO4 aq ị ỵ Cu s ị ẵ22 The values of enthalpy and entropy for these materials and compounds are [18] as follows: SZn s ị ẳ 41:6 J K mol ị SZnSO4 aq ị ẳ 120 J K mol Þ − SCu ðs Þ ¼ 33:2 J ðK mol ị SCuSO4 aq ị ẳ 109:2 J K mol ị HZnSO4 ẳ 982:8 kJ mol − ΔHCuSO4 ¼ −771:4 kJ mol − Hence, the values for ΔS, TΔS, and ΔH, at a temperature of 25 °C (298 K) are as follows: � � S ẳ Sproducts Sreactants ẳ SZnSO4 aq ị þ SCu ðs Þ – SZn ðs Þ þ SCuSO4 aq ị ẳ120 ỵ 33:2ị41:6 ỵ 109:2ị ẳ 153:2150:8 ẳ 2:6 J K mol ị TS ẳ 298 2:6ị ẳ 774:8 J mol H ẳ DHZnSO4 HCuSO4 ẳ 982:8771:4ị ẳ 211:4 kJ mol Replacing these values for TΔS and ΔH in eqn [21] gives ΔG ¼ ΔH− TΔS ¼ −211400 –774:8 ¼ −212174:8 ¼ −212:2 kJ mol − ½23Š This is the same as the value for ΔG (within the accuracy of the thermodynamic data from the data tables) from eqn [19], however, derived from fundamental properties In this case, the value of dQ = TΔS is positive, implying that a small amount of heat is actually Preface and Context to Hydrogen and Fuel Cells Electrolyte aqueous solution K+ OH− O2 H2O O2 compartment + Electrode Electrode − H2 H2 compartment H2O 24 2H2 + 4OH − ⇒ 4H 2O + 4e− O2 + 2H2O + 4e− ⇒ 4OH − Figure 31 Operation of an AFC absorbed from the surroundings during this reaction and that a significant amount of energy is available as useful work (in this case, electrical energy) However, consider the theoretical case of the alkaline hydrogen fuel cell, as first illustrated in Figures and The AFC works commonly employing a potassium hydroxide electrolyte, where the negative hydroxide ion (OH−) enables internal fuel cell operation by facilitating charge transfer between cathode and anode as illustrated in Figure 31 With reference to eqn [24] and Figure 31, it is evident that the hydrogen gas, delivered to the H2 compartment, permeates through the electrode membrane assembly and reacts with the hydroxyl ion, producing water (which exits the cell from the H2 compartment) and releases electrons Hence, the hydrogen electrode becomes the negative electrode, or anode, where oxidation occurs Further, with reference to eqn [25] and Figure 31, it is evident that the oxygen gas, delivered to the O2 compartment, permeates through the electrode membrane assembly and reacts with the water molecules present in the aqueous electrolyte and electrons delivered from the electrode, producing further hydroxyl ions Hence, the oxygen electrode becomes the positive electrode, or cathode, where reduction occurs Thus, an equilibrium process of hydroxyl ion consumption at the anode, and production on the cathode, is established Overall, there is no net loss or gain of hydroxyl ions; however, it is clear that the charge transfer within the fuel cell is facilitated by this negative ion With reference to the electrochemical potentials in Table 2, the half cell reactions for the alkaline hydrogen fuel cell are 2H2 ỵ 4OH 4H2 O ỵ 4e 0:83 V ẵ24 O2 ỵ 2H2 O þ 4e− ⇔4OH− þ0:4 V ½25Š which gives an overall reaction for an AFC of − − − − 2H2 þ O2 þ / 2H2 O þ 4O/H þ 4/e 4/H2 O ỵ 4e/ ỵ 4OH / 2H2 ỵ O2 2H2 O H2 ỵ O2 H2 O ẵ26 From eqns [24] and [25], the corresponding theoretical terminal voltage for an AFC is = 0.4 V – (–0.83 V) = 1.23 V, at STP As per eqn [19], this corresponds to a ‘free energy’ of −ΔG ¼ Wuseful ¼ nFE ¼ 2à 96485à 1:23 ¼ 237353 Joules ¼ 237:4 kJ mol − ΔG ¼ −237:4 kJ mol − ½27Š However, the actual theoretical available energy from the reaction given in eqn [26] can be calculated from the ‘enthalpy of combustion’ (which states the energy released from complete combustion of hydrogen in the presence of oxygen to form water (as a liquid, i.e., at temperatures below 100 °C), as per eqn [26]) which is ΔH = –286 kJ mol−8 The recognition that the value of ΔH for hydrogen is greater than that of the free energy, ΔG, indicates that not all the energy from the ‘reactants’ (i.e., the hydrogen and Preface and Context to Hydrogen and Fuel Cells 25 oxygen gases) produces electrical energy From the same thermodynamic data tables, for the reaction stated in eqn [26], the corresponding values of enthalpy and entropy are as follows: ¼ 130:7 J K mol ị ẳ 205:0 J K mol ị ẳ 70 J K molị − ¼ −285:8 kJ mol − � � � � � ΔS ¼ Sproducts – Sreactants ¼ SH2 O l ị SH2 g ị ỵ SO2 l ị ẳ70ị130:7 ỵ 205:0ị ¼ 70 –233:2 ¼ −163:2 J ðK mol Þ − SH2 ðg Þ SO2 ðg Þ SH2 O ðl Þ ΔHH2 O ðl Þ TΔS ¼ 298à ð−163:2Þ ¼ 48 633:6 J mol ẵ28 H ẳ HH2 O l ị ẳ 285:8 kJ mol Hence, the theoretical free energy, ΔG, is ΔG ¼ ΔH−TΔS ¼ 28580048633:6ị ẳ 237166:4 ẳ 237:2 kJ mol which is comparable with the value from eqn [27] Equation [28] now indicates that theoretically a fair proportion of the available energy from the hydrogen and oxygen fuel gases is released as heat: 48 633.6/285 800 = 17% It is worth noting that the product from the hydrogen fuel cell, that is, water, can be present in either liquid or gaseous form, dependent on the operating temperature of the fuel cell In the case of the AFC, the operating temperature is ∼70 °C which implies that the water will be present in liquid form For higher temperature fuel cells, such as SOFCs, which tend to operate at temperatures from 600 to 1000 °C, the water product would be present in a gaseous state, that is, steam, and an alternative value for the enthalpy and entropy of water would change to SH2 O g ị ẳ 188:8 J K mol ị HH2 O g ị ẳ 241:8 kJ mol Hence, for the case of an SOFC at 1000 °C, the corresponding heat loss can be calculated as � � � � � ΔS ¼ Sproducts – Sreactants ¼ SH2 O l ị SH2 g ị ỵ SO2 l ị ẳ188:8ị 130:7 ỵ 205:0ị ẳ 188:8 233:2 ẳ 44:4 J K mol Þ − TΔS ¼ 1273à ð−44:4Þ ¼ −56521 J mol ẵ29 G ẳ HTS ẳ 241 80056 521ị ẳ 185279 ẳ 185:3 kJ mol ẵ30 For this case, the resulting free energy is This gives rise to a theoretical open-circuit cell voltage of, by rearranging eqns [18] and [19] G ẳ nFE G 185300ị Eẳ ¼ ¼ 0:960 V nF 2à 96485 ½31Š This helps illustrate that as the working temperature of the fuel cell increases, the theoretical open-circuit voltage decreases (compared to the voltage of 1.23 V at a temperature of 25 °C, from eqn [27]) and the ratio of energy ‘lost’ as waste heat increases 4.01.4.4 Practical Fuel Cells The theoretical open-circuit voltage from a hydrogen/oxygen fuel cell was considered and defined as 1.23 V at 25 °C However, from a practical perspective, a user would not be able to measure this voltage but rather measure a voltage typically < 1.1 V on open circuit and, as an increasing electrical load is applied to the fuel cell, the terminal voltage decreases further Figure 32 illustrates the typical voltage versus current density (proportional to load current) characteristic for a fuel cell From Figure 32, it is clear that there are three distinct ‘regions’ to the fuel cell V/I characteristic [19]: Activation polarization This region experiences the initial energy loss required to activate the reactants and initiate the anode and cathode half-cell reactions This is evident at low currents and is nonlinear Ohmic polarization This region exhibits the expected linear V/I characteristic apparent from electrically loading a nonideal voltage source This linear drop is caused by the voltage drop caused from a finite internal resistance (created by the resistances of the electrolyte, electrode materials, electrical connections, and so forth) Concentration polarization This region is apparent only at high current densities (i.e., at or beyond full load) and is caused by the particular cell design reaching its limit in terms of its ability to supply sufficient reactants to the cell and remove the products In essence, the cell cannot support the high level of reaction rate and effectively ‘throttles’ itself 26 Preface and Context to Hydrogen and Fuel Cells Theoretical voltage (Nernst limit) 1.23 Cell voltage Activation polarization Ohmic polarization Concentration polarization Current density Figure 32 Typical voltage vs current density characteristic of a fuel cell 4.01.5 Conclusions Hydrogen and fuel cell technologies, which are not forms of renewable energy in their own right, offer significant benefits to assist in the utilization of primary renewable energy sources such as wind, wave, and solar, through zero emission production of hydrogen for use as an energy store and vector The storage of energy in the form of hydrogen gas (or other forms such as within a chemical hydride or as a supercooled liquid) enables subsequent reconversion of the hydrogen to electrical energy for, for example, stationary grid power or on-vehicle traction power via fuel cells or hydrogen combus­ tion engines [20] or turbines [21] Using hydrogen and fuel cells therefore allows currently stochastic or variable forms of renewable energy, such as wind and solar for example, to be regulated in some fashion through appropriate control of an energy storage buffer, comprising typically hydrogen production, gaseous storage, and reconversion to electrical energy In each case, the actual control implementation is facilitated via use of power electronics to regulate power to the electrolyser plant, and regulate the power from the fuel cell plant to a given load This introductory chapter presents the initial history of the discovery of hydrogen gas and its role in both electrolysis and fuel cell units It does not set out to fully detail the chronological development of this technology but merely introduce the basic electrochemistry, further detailed chapters of this volume, and illustrate aspects of hydrogen and fuel cell technology that invariably have had to be left out the detailed chapter discussions due to time and space constraints These latter technologies include the mature technology of alkaline electrolysis and discussion of regenerative fuel cells, a relatively new technology that will impact the larger electrical energy storage field in future This entire volume, on hydrogen and fuel cell technologies, presents significant technical and economic detail of this exciting technology field, supported by extensive referencing, and outlines the potential for a future hydrogen economy [22], where integrated use of this technology is commonplace and accepted References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] Cavendish H (1766) Experiments on factitious air Philosophical Transactions LVI: 141–184 Nicholson W (1800) Experiment with a new electrical or galvanic apparatus Journal of Natural Philosophy, Chemistry and the Arts 4: 179–187 Cleveland CJ (ed.) (2009) Concise Encyclopedia of the History of Energy Elsevier, ISBN 978-0-12-375117-1 Department of Environment (1995) Gas Works, Coke Works, and Other Coal Carbonisation Plants Crown copyright, ISBN 1-85112-232-X Grove WR (1839) On voltaic series and the combination of gases by Platinum Philosophical Magazine and Journal of Science XIV: 127–130 Grove WR (1842) Gaseous voltaic battery Philosophical Magazine and Journal of Science XXI: 417–420 The Energy Act (2011) http://www.legislation.gov.uk/ukpga/2011/16/pdfs/ukpga_20110016_en.pdf Regenesys Utility Scale Energy Storage – Project Summary, DTI, Contract Number: K/EL/00246/00/00, URN Number: 04/0148, 2004 http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/c4f6ee8cddd1bf08c1256e2700424e70/$file/prs%20little%20barford_reva.pdf ZBB EnerStore Battery, www.zbbenergy.com (accessed March 2012) VRB Battery, www.pdenergy.com (accessed March 2012) Deeya Energy, www.deeyaenergy.com (accessed March 2012) http://www.hydro.com/en/About-Hydro/Our-history/ (accessed March 2012) www.nel-hydrogen.com (accessed March 2012) www.itm-power.com (accessed March 2012) www.H2Stations.org (accessed March 2012) This website attempts to list all currently active or installed hydrogen refuelling stations worldwide http://en.wikipedia.org/wiki/Gibbs_free_energy Lide DR (2004–2005) Standard thermodynamic properties of chemical substances CRC Handbook of Chemistry and Physics, 85th edn, ch ISBN 0-8493-0485-7 Fuel Cell Handbook, http://www.netl.doe.gov/technologies/coalpower/fuelcells/seca/pubs/fchandbook7.pdf (accessed March 2012) Preface and Context to Hydrogen and Fuel Cells 27 [20] Sun Z, et al (2012) Research and development of hydrogen fuelled engines in China International Journal of Hydrogen Energy 37(1): 664–681 doi:10.1016/j ijhydene.2011.09.114 [21] Strohle J, et al (2007) An evaluation of detailed reaction mechanisms for hydrogen combustion under gas turbine conditions International Journal of Hydrogen Energy 32: 125–135 doi:10.1016/j.ijhydene.2006.04.005 [22] Jeremy R (ed.) (2002) The Hydrogen Economy Penguin, ISBN: 1-58542-193-6 ... energy storage technology, is presented to illustrate that some fuel cell technologies not require hydrogen gas to operate Preface and Context to Hydrogen and Fuel Cells 4. 01. 2.2 Chapter 4. 02:... ISBN 0- 849 3- 048 5-7 Fuel Cell Handbook, http://www.netl.doe.gov/technologies/coalpower/fuelcells/seca/pubs/fchandbook7.pdf (accessed March 2012 ) Preface and Context to Hydrogen and Fuel Cells 27... the context to this Preface and Context to Hydrogen and Fuel Cells Figure William Grove’s postscript describing the first recorded operation of a hydrogen fuel cell sector and an overview of hydrogen