CE60017 - Sustainable Energy Technologies Flashcards

Question

What is coulombic efficiency?

Answer 1

A measure of the capacity that you can get out of a battery, compared to the capacity that you put in during charge An important consideration when balancing electrode capacities, and for considering overall specific energy or capacity of the cell.

Answer 2

The ratio between the average discharge voltage to the average charge voltage (resistance of the cell, polarisation losses) = E (discharge) / E (charge) * 100

Answer 3

= Q*E (discharge) / Q*E (charge) *100 = voltaic efficiency * coulombic efficiency It is the ratio of the energy retrieved from the battery, to the energy provided to the battery, when coming back to the same SOC state = Columbic efficiency X Voltaic Efficiency

Answer 4

A reaction is spontaneous if it results in an increase in free energy. This is marked by a negative dG value in the Gibbs Free Energy Equation. Since ΔG=-nFVcell... A reaction is spontaneous if (cell potential) Vcell > 0

Answer 5

The tendency for a material to be reduced. The potential for Fe 3+ to Fe 2+ is more positive than Fe 2+ to Fe. This means that Fe 3+ has a greater tendency to gain electrons and be reduced to Fe 2+ compared to Fe 2+ to be reduced further to Fe. Under standard conditions, the reduction of Fe 3+ is favourable. Metal ions at the top of the series are good at picking up electrons. They are good oxidising agents. (for example Cu2+ is good at picking up electrons and becoming Cu) The oxidising ability of the metal ions increases as you go up the series. Metals at the bottom of the series are good at giving away electrons. They are good reducing agents. (for example Fe is good at giving away electrons and becoming Fe2+) The reducing ability of the metal increases as you go up the series.

Answer 6

Specific energy (or gravimetric energy density)= the stored energy per unit mass ( Wh/Kg) Energy density (or volumetric energy density) = the stored energy per unit volume ( Wh/l) Specific power = power per unit mass ( W/Kg) Specific volume =power per unit volume ( W/L) Gravimetric Battery Capacity = Ah/g or mAh/g

Answer 7

The gravimetric capacity can be maximized by finding lighter materials, that can store a charge with a smaller weight. The cell potential can be maximized by choosing cathode and anode materials with the biggest potential difference possible.

Answer 8

The potential difference between the electrodes when not drawing any current, this should be equal to the theoretical cell potential. In reality, the potential we measure is different, it has an overpotential.

Answer 9

- Li is lightweight - High power density - Good energy density compared to other batteries - Great cyclability - More environmentally friendly than lead acid All the elements on the first group have a single electron on the outer electron sphere, therefore: - Removing the electron is “easy” and reversible - They have an oxidation state of +1, meaning they can store one charge and being transported across a battery as +1 ion. This means Li, Na, K are all good candidates Mg and Ca even more so, as they can store 2 charges per atom Out of all of them, Li is the lightest! All other battery chemistry will intrinsically be heavier and therefore have a lower energy density.

Answer 10

1. Current collectors (conductive materials e.g. Cu or Al connected to the electrodes (anode and cathode) that allow the flow of electrons to the external circuit) 2. Electrodes (anode often made of graphite and cathode made of a metal oxide) 3. Electrolyte (medium allowing ion flow - can be solid, liquid, or a gel) 4. Separator (permeable membrane that prevents direct contact between the anode and cathode while allowing the flow of ions) 5. Container (holds all the components together and provides structural support and protection for the battery) 6. Terminal (points of connection where the battery is connected to an external circuit to deliver or receive electrical energy)

Answer 11

To separate the anode from the cathode to keep them electrically separated. Must be porous to allow Li ions to pass back and forth between the anode and the cathode Must be insulating to electrons. Can be made from: - Thin polymer films - Ceramic - Ceramic/polymer blends

Answer 12

Copper or carbon-coated Aluminium Choice based on costs and compatibility.

Answer 13

Binders are materials that hold together the components within the battery cell, maintaining the structural integrity of the electrodes and ensuring proper electrical contact between the active materials and current collectors. Binders are typically polymers that possess adhesive properties and chemical stability within the electrolyte environment of the battery. They help to immobilize the active materials (such as lithium cobalt oxide, graphite, etc.) onto the current collectors, preventing them from detaching or undergoing undesirable reactions during charge/discharge cycles. Common binder materials used in lithium-ion batteries include polyvinylidene fluoride (PVDF) and its copolymers, such as PVDF-HFP (hexafluoropropylene).

Answer 14

The main requirement of an electrolyte is that it needs to be stable at the potential of the cathode and anode.

Answer 15

Solid Electrolyte Interface The SEI is the layer of electrolyte degradation products that is formed at the surface of the electrode The SEI can be electrically conductive and/or ionically conductive. The Solid Electrolyte Interface (SEI) is a protective layer that forms on the surface of electrodes in lithium-ion batteries, consisting of electrolyte decomposition products. It prevents further electrolyte decomposition and enables stable lithium-ion transport while allowing efficient battery operation.

Answer 16

Electrons will be transported to the new interface, where the electrolyte will keep degrading indefinitely.

Answer 17

If a totally insulating layer is formed the degradation reaction stops, but so does the desired lithium reaction, so the battery is dead.

Answer 18

This is the ideal scenario. Since the electrons cannot be transport through the SEI, they can’t reach the electrolyte and further electrolyte degradation is not possible. At the same time, Li-ions can travel through the SEI to meet the electrons at the electrode surface and react with the electrode, so the battery is still working.

Answer 19

The solid electrolyte interface being ionically conductive but not electrically conductive is ideal. Since the electrons cannot be transport through the SEI, they can’t reach the electrolyte and further electrolyte degradation is not possible. At the same time, Li-ions can travel through the SEI to meet the electrons at the electrode surface and react with the electrode, so the battery is still working.

Answer 20

The Solid Electrolyte Interface (SEI) forms on the surface of lithium-ion battery electrodes through a series of electrochemical reactions between the electrolyte and the electrode materials during the initial charging and discharging cycles. These reactions result in the deposition and rearrangement of electrolyte decomposition products, creating a thin, passivating layer that protects the electrode surface from further degradation and promotes stable battery performance. The SEI is typically formed in the first cycle of the battery by decomposition of electrolyte at the electrode surface. The formation of the SEI prevents further degradation, but changes the energy levels. It consists of many organic and inorganic compounds with thickness of around 20 nm. Composition and structure depends on cathode, anode, electrolyte and cycling conditions used. Low Coulombic efficiency due to irreversible Li consumption. Efficiently prevents further chemical reactions between electrolyte and anode.

Answer 21

Lowest unoccupied molecular orbitals (LUMO) and highest occupied molecular orbitals (HOMO) of the electrolyte. If μA of anode > LUMO of electrolyte The electrolyte is reduced at the anode If μC of cathode < HOMO of electrolyte The electrolyte is oxidized at the cathode Electrolytes have a stable operating voltage ‘window’ in which they can be cycled without degrading. This ‘window’ is the energy gap between the lowest unoccupied molecular orbitals (LUMO) and highest occupied molecular orbitals (HOMO) of the electrolyte. (μ is the Fermi level - A and C refer to anode and cathode)

Answer 22

1. High ionic conductivity to reduce Li ion diffusion resistance. 2. Appropriate thickness to allow easy diffusion of Li ions into the electrode, but prevent further electrolyte decomposition. 3. Robust mechanical performance to accommodate non-uniform volume fluctuation, and withstand repeated Li deposition and plating processes. 4. Excellent stability with respect to structure, shape, morphology and chemistry during long-term cycling performance.

Answer 23

Graphite is the most common anode for Li-ion batteries Lithium in stored via intercalation between the graphene sheets It can store one lithium atom per 6 carbons: LiC6, with a theoretical capacity of Q=370 mAh g-1

Answer 24

Metallic Li anode in the rechargeable battery is plated/stripped repeatedly upon cycling. The initially present nucleation sites and positions contribute an imperative role in the following deposition nature/behaviour of Li. Protrusions with large curvature possess a significantly higher electric field at their tip sites that are prone to attract more Li ions for deposition, creating more local inhomogeneity. With repeated cycling, these finally grow into dendritic morphologies. On repeated recharge, dendrites grew across the electrolyte from the anode to the cathode, leading to dangerous short-circuits in the cell in the presence of the flammable organic liquid electrolyte.

Answer 25

The following approaches have been investigated to control the different reactions occurring on the lithium surface to suppress the formation of dendritic growths. Electrolyte additives: salts, reactive organic compounds Artificial SEI Solid Electrolytes Structuring the electrode architecture Polymer coatings Coatings with carbon materials

Answer 26

Providing a conductive 3D host structure which can stabilise Li metal deposition by: - Creating a larger, favourable surface area for Li nucleation and growth, - Providing porosity to accommodate Li metal growth, - Regulating localised current density in the electrode. It is done to protect the Li anode and prevent dendritic growths. Carbon is often chosen as it is cheap, lightweight, safe, and conductive.

Answer 27

Electrolyte additives can react with the metallic Li anode to form a more robust and passivating SEI film than that formed by Li salts in pure organic solvents, inhibiting further reaction between surface of Li and electrolyte. A more chemical and stable SEI can be formed. Salt anions in nonaqueous electrolytes strongly affect the surface chemistry of metallic Li anodes. The inner layer of SEI close to the surface of Li metal consists of inorganic salts. The composition is determined by the reduction of the anions present in solution. LiPF6, LiSO3CF3, LiBF4 and LiTFSI salts are typically the more reactive salts with Li surface and consequently adversely influence the cycling efficiency of Li than LiClO4. The thicker interphase layers formed may lead to higher resistance, and decreased cell performance. As the availability of salts/solvents is limited, additives that possess a higher reduction voltage compared to electrolyte salts and solvents can be employed, to contribute to stable SEI formation. These additives are normally added in small amounts (ppm level) to the electrolyte.

Answer 28

Liquid: - Poor chemical stability - Leakage - Flammable - Dendrites form easily - High ionic conductivity Solid: - Safe - Chemically stable - No leakage - Prevent dendrite formation - Poor ionic conductivity Higher ionic conductivities, but brittle, and poor interfacial contact with electrodes.

Answer 29

Ceramic Polymer Can combine the two. Can be true solid polymer, or gel polymer electrolytes. PEO based. Poly-ionic liquids. Biopolymers. Good interfacial adhesion with electrodes. Poor thermal stability.

Answer 30

Essentially the formation of an artificial SEI: By coating with a protective layer of e.g. carbon or CN. Introducing a thin layer of SE on the electrode. By inducing artificial SEI formation first before assembling the battery. This prevents the formation of dendrites.

Answer 31

A single S atom can store 2 Li atoms, and sulphur is quite lightweight. S + 2Li+ + 2e- ⇌ Li2S (E° ≈ 2.15 V vs Li/Li+) Overall conversion occurs via formation of polysulfides Li2Sx, 8

Answer 32

High theoretical capacity (1675 mAh g-1), High theoretical specific energy (2500 Wh kg-1), Current best is 500 Wh kg-1, compared to Li-ion 150-250 Wh kg-1, Relatively lightweight, Sulfur is cheap (Likely to enter the market where mass is the critical factor above all else, e.g. unmanned aerial vehicles, space and automotive (buses and trucks, rather than consumer vehicles where volume is a constraint).

Answer 33

Many reaction steps needed to form Li2S (S8 → Li2S8 → Li2S6 → Li2S4 → Li2S3 → Li2S2 → Li2S and many more) All reaction intermediates are soluble, which means they can enter the electrolyte. Polysulfide shuttle effect decreases capacity and rechargeability Large volume expansion from S to Li2S, Slow kinetics from Li2S2 to Li2S Uncontrolled Li2S precipitation Large amount of electrolyte needed Not compatible with conventional organic carbonate electrolytes Extremely low electrical conductivity of sulfur (5 × 10-30 S cm-1 at 25°C) Formation of lithium dendrites

Answer 34

1. Using separators A functional separator interlayer can: - Physically block or trap polysulfides to minimise the shuttle effect by tailoring nanostructure, - Chemically trap polysulfides by introducing functional groups or dopants, - Contain catalysts to accelerate polysulfide conversion to reduce shuttling. 2. Using carbon-sulphur composite cathodes Carbon provides: - Conductivity, - Structural scaffold, - Trapping sites for S and polysulfides, - Catalytic/functional surface. - As cathode conversion reactions involve dissolution and precipitation of active material species, electrolyte plays a much more active role than just providing ionic transport between electrodes.

Answer 35

E = Q.s*V*M.s / M.tot

Answer 36

Maximise sulfur loading: Higher sulfur loading increases the areal capacity of the cathode (mA h cm2), Increased fraction of active material offsets the ‘dead weight’ of inactive components, However, the coating and drying processes are problematic for thick electrodes, and thicker electrodes result in increased ionic and electronic resistance, and therefore lower rate capability. Reduce electrolyte loading: Electrolyte contributes the highest weight and volume fraction of the cell components and has a huge impact on specific energy at cell level. Reduce/eliminate weight of other inactive components (e.g. current collector, binder).

Answer 37

Sulfur use = m (s used) )/ m (s total) =Q (practical) / Q (theoretical) Sulfur utilisation can be affected by many factors including porosity, surface area, morphology, electrolyte loading. Achieving high sulfur loadings is not necessarily always desirable, if full conversion to Li2S does not occur: - Unutilised sulfur can cover and passivate the carbon surface, blocking charge transfer reactions, - Unused sulfur adds mass to the cell without contributing capacity, decreasing energy density.

Answer 38

- Highly reactive with Li - Water can corrode the electrodes and other components of the battery. Lithium reacts with water to form lithium hydroxide and hydrogen gas. - Pure water has poor conductivity for ions compared to other electrolytes commonly used in Li-ion batteries, such as lithium salts dissolved in organic solvents like ethylene carbonate (EC) or dimethyl carbonate (DMC).

Answer 39

Flame retardant SEI formation improvement and improved recyclability by suppressing anodic decomposition Cathode protection Improve thermal stability Improve wettability Improve corrosion resistance

Answer 40

Physical Confinement: 1. Hierarchical Porosity: Refers to the structure of the carbon material, which contains pores at different length scales (macro, meso, and micropores). This structure helps accommodate the volume changes that occur during the charge-discharge cycles of the battery. 2. Yolk-Shell Structure/Encapsulation: This involves enclosing the sulfur within a carbon shell (yolk-shell structure), which helps to prevent the dissolution of sulfur into the electrolyte and improves the stability of the electrode. 3. Capillary Absorption of Sulfur Species Through Porosity: The porous nature of the carbon material allows for the absorption and retention of sulfur species, which is beneficial for the battery's performance. 4. Polymeric Coating: Coating the carbon material with a polymer can further enhance the stability and performance of the electrode by providing additional protection and preventing the dissolution of sulfur. Chemical Confinement: 1. Heteroatom Doping (N, O): Introducing heteroatoms such as nitrogen (N) and oxygen (O) into the carbon structure can modify its properties, such as increasing its conductivity and enhancing its interaction with sulfur species. 2. Catalyst Decoration/Metal Additives: Adding catalysts or metal additives to the carbon material can improve the kinetics of the electrochemical reactions, leading to better battery performance. 3. Directed Nucleation: Controlling the nucleation of sulfur species on the carbon surface can help prevent the formation of undesirable products and improve the stability of the electrode.

Answer 41

Sustainability Energy equity Energy security The 2 main options to increase sustainability are battery recycling and different battery chemistry.

Answer 42

Na itself is cheaper and more abundant We can use Al instead of Cu as current collector (more abundant, cheaper, and lighter, though less conductive) [Potential to provide energy at 23/kWh/annum, less than half the cost of LIBs and a quarter that of lead-acid systems of the same size.] The electrode materials can be cheaper and more sustainable It is similar to Li-ion batteries (Same manufacturing and technology for development) Not as good as lithium in terms of performance, but second lightest atom in the 1st group and Sodium’s standard redox potential is only 340 mV higher than Li+/Li.

Answer 43

Sodium has: Higher standard redox potential Higher ionic radius Is heavier than lithium? Cell potential is lower to overall power will be lower. Larger ionic radius makes it harder for diffusion. Regarding insertion into electrode material, it will be harder as the ions are bigger and bulkier. [all compared to Li] Sodium’s higher standard redox potential (Na -2.71 V vs Li -3.04 V) and larger ionic radius (Na+ 1.02 Å vs Li+ 0.76 Å) generally mean that the capacity and voltage achieved will be lower, therefore projected energy density will be lower.

Answer 44

Advantages of Na-ion: Sodium is more abundant and cheaper than lithium Can use hard carbon anions, instead of graphite used in Li-ion. Do not require cobalt in the cathode Can replace Cu with Al as current collector Overall cheaper Disadvantages of Na-ion: Na-ion batteries have a shorter cycle life than Li-ion Lower gravimetric energy density Lower volumetric energy density Graphite is a widely-used anode material for lithium-ion batteries. However, Na-ions do not intercalate into graphite due to the larger ion size. NIB electrodes show slightly lower specific capacities than those for LIB, resulting in lower energy density. Therefore, the cost reduction is not significant in terms of the cost per energy.

Answer 45

Large-scale stationary energy storage and low-speed electric vehicles, despite lower energy density (than Li). This is due to the lower cost and greater abundance. To be competitive with Li-ion, improvements must be made in terms of cycle life and they must be easy to manufacture at larger scale. Anode materials with high specific capacities and appropriately low redox potentials to improve the energy density of NIBs could replace LIBs.

Answer 46

Metallic Sn is one of the most intensively investigated anode material for room-temperature NIBs, due to its ability to alloy up to 3.75 Na per Sn, corresponding to the high theoretical capacity of 847 mA h g−1. Sodiation (replacement of metal ions (typically lithium) with those of sodium) of Sn is a multi-step alloying process. However Sn prone to huge volumetric expansion (420%) which causes cracking and capacity fading.

Answer 47

Can coat Sn particles with various protective films, introduce additives into electrolyte e.g. FEC. Particle size control - smaller particles can better accommodate volume expansions Dispersion in carbon matrix -: for examples depositing tin on carbon nanofibers Particle encapsulation - for example in a bigger carbon particle, so that if the Sn expands it still has space Co-alloying to buffer volume changes - Other alloying materials: Sb, P, Ge, Bi, Si Structured electrodes to accommodate volume change

Answer 48

Pros: Abundant and low cost, Non-toxic, Stable, Low operation voltage, Low strain, Decent cyclability depending on the specific compound, Cons: Low specific capacity, Low electrical conductivity, Poor ion diffusivity.

Answer 49

Most cathode materials store Na+ ions by intercalation chemistry, which means that the number of storage sites is limited. This suggests that it will be difficult to greatly increase the specific capacity of the cathode materials. The use of cathode materials with high redox potentials is also limited because of electrolyte decomposition at high potentials. Most commercialized electrolytes even for Li‐ion batteries are unstable and decompose at over 4.8 V versus Li/Li+.

Answer 50

Oxides Polyanions Prussian Blue Analogue (PBA)

Answer 51

Oxides were initially pursued because of their dominance in Li-ion batteries. Na-based layered oxides show even richer crystal chemistry than Li-ion ones because Na ions can reside in more lattice sites. However, they are more prone to structural phase transitions, reducing lifetime and limiting power density. Meanwhile with polyanions, there is power switching from Li to Na for polyanion compounds due to open channels for ion diffusion. However, it relies on the use of toxic vanadium.

Answer 52

Cycling performance of NIBs still doesn’t compare to LIBs, implying continual consumption of electrolyte and an unstable SEI: - Less compact layer, - More resistive, - Mechanically less robust, - More soluble? Factors affecting the SEI: Type of solvent, Type of salt, Charging rate, Temperature of formation.

Answer 53

Similar redox potential to Li+ (-2.94 vs -3.04 V). Larger ion size, lower energy density. Fast ion diffusion – potentially faster charging batteries. K+ can intercalate into graphite (unlike Na+) ~279 mAh g-1. Combining graphite with soft carbon provides structural protection enhancing cycling behaviour. Challenges: Low capacity retention. Need rigid cathode structures that can accommodate stable and repeatable K+ ion insertion. High Lewis acidity of potassium results in fast electrolyte degradation. Need electrolytes that can operate in the higher voltage window.

Answer 54

Disassembly and discharging (onto direct recycling involving delamination and regeneration) Shredding Mechanical separation (onto indirect recycling involving hydrothermals, drying, and relithation) Hydrometallurgy (leaching, extraction, Li precipitation) Pyrometallurgy (heating and slag separation)

Answer 55

Discharge is the process of closing the battery circuit, allowing it to reach a stable open circuit voltage, to avoid short-circuiting later on. Pros: Takes electricity out of cells making them safe Mature and safe Electricity can be returned to the grid Ni need for Li neutralization Cons: Only valid for large modules and packs Li redeposition can occur if discharged too fast Prone to thermal run away

Answer 56

In the indirect route, the cells are shredded and mechanically separated to obtain a black mass, composed of cathode and anode material Pros: Easy and fast to do Cons: Purity of recovered material is intrinsically lower

Answer 57

Another pre-treatment process is freezing, which consists of putting the cell in liquid nitrogen to freeze the electrolyte. Pros: Frozen in liquid N2 where the electrolyte becomes unreactive Cons: Only suitable for batch processes Scale up difficult Expensive reagents needed Short processing time until the cells become active again

Answer 58

In the pyrometallurgy route, the black mass (which contains all the active cathode and anode material) is exposed to high temperature, to remove carbon, binders, solvents etc. What is left is a lithium-rich slag and an alloy containing Ni, Co etc. Pros: Allows charged cells to be processed Burns off the electrolyte taking away the biggest hazard Mature and scalable No post process neutralization Makes the hydrometallurgical process easier Cons: Inert atmosphere required-costly Large quantity of toxic gases and extensive scrubbing is needed to treat them vaporized electrolyte forms explosive mixture Low recovery efficiency and you can’t recover electrolyte and graphite, as they combust Large input of energy required CO2 emission via combustion processes Al current collectors will melt at these temperatures

Answer 59

Hydrometallurgy is a leaching process, done with a variety of acid or alkaline solutions. By adding other chemical precipitants or extractants, metals such as Co, Mn and NI can be separated via precipitation, extraction or adsorption. Pros: Higher recovery purity of active materials Lower energy consumptions and lower emission of toxic gases compared to pyrolymetallurgy Cons: Highly corrosive solvents needed for the extraction It requires longer pre-treatment compared to direct recycling

Answer 60

In the direct route, the battery cells are disassembled down to the individual electrodes. The separated electrodes can be then subject to delaminiation process (for example soaking in organic solvent) to separate the active material from the current collector. Pros: Higher Purity of the materials recovery Higher collection efficiency Lower energy consumption Fewer chemicals required Cons: Longer pre-treatment process It needs cells to be designed for easier disassembling

Answer 61

Materials: - Biodegradable binders - No current collectors - No critical materials - Single crystal cathodes Electrode/cell: - Aqueous formulations - Standardisation - Digital materials labels - Screw cap cells Pack/module: - No glues - Design for disassembly - Cell labels with components - Easy lock release

Answer 62

Grey: Hydrogen from reforming and gasification technology using natural gas and coal, with emission significant amount of CO2 in the atmosphere Blue: Similar to grey but with incorporation of CCS. Advanced reforming technology producing blue hydrogen from natural gas and biomass, with most CO2 emissions captured by carbon capture and storage (CCS). Green: Electrochemical or photochemical technology using electricity or solar power to produce green hydrogen, which is highly flexible and stores hydrogen by capturing excess generation.

Answer 63

Grey is from reforming and gasification of fossil fuels. Blue is similar but with CCS. Green is from electricity/solar power (e.g. electrolysis). Grey hydrogen is the cheapest option (€1.50/kg), but CO2 emissions may make grey hydrogen more costly Blue hydrogen can narrow the gap Green hydrogen it the cleanest but it’s price depends on renewable electricity and electrolysis system

Answer 64

- Use renewable energy (wind, solar, etc) - Fossil fuel free - No CO2 emission during the process - Low temperature, low pressure - Decentralised, on-demand on-site chemical synthesis - Fast start-up and shut-down

Answer 65

High cost in H2 production Challenges in Safe Storage Transportation and distribution Social awareness Competition with other energy Sources and storage systems

Answer 66

1. REVERSIBLE ON-BOARD APPROACHES Compressed gas Liquefied hydrogen Metal Hydrides High surface area porous materials 2. CHEMICAL HYDROGEN STORAGE: REGENERABLE OFF-BOARD Hydrolysis reactions Hydrogenation/dehydrogenation reactions Ammonia Boron hydrides

Answer 67

Volumetric capacity (0.039 kg/L at 700 bar) Limits of high pressure Costs Refueling or filling time Compression energy penalty (15-20% less than atmospheric pressure hydrogen) Heat management requirements

Answer 68

Advantages: Can store more H2 in a given volume than compressed tanks The volumetric capacity of liquid hydrogen is 0.07 kg/L compared with 0.039 kg/L at 700 bar Limitations: Liquid H2 boil-off Energy required for H2 liquefaction (-250C) Tank cost

Answer 69

Advantages High volumetric density Low pressure Safer Limitations: Very low gravimetric capacities 3-4 wt% (target for 2010 was 6 wt%) Hydrogen release kinetics are too slow Packing density of the powder is low Need to lower cost and increase cycle life

Answer 70

Fast hydrogen kinetics Low hydrogen binding energies Low thermal management issues during Hydrogen charging and discharging

Answer 71

Ammonia has a higher boiling point (-33.5C) than hydrogen, so it is easier to store as a liquid. Has a high capacity for H2 storage (17.6 wt%) The release of H2 from ammonia is endothermic Disadvantages: High fuel processing temperatures and large reactor mass and volume will be required Is toxic Traces of ammonia are incompatible with fuel cells Given all these challenges, most likely ammonia reactors will never meet the requirements for commercially viable hydrogen-powered fuel cell vehicles but this could happen off-board

Answer 72

E = E0 - RT/neF * lnQ = E0 - RT/neF * ln(a.red / a.ox) Where: E - cell potential E0 - standard cell potential R - gas constant T - temperature F - Faraday constant ne - number of moles of electrons transferred in the cell reaction Q - reaction quotient a - activity We want an electrolyser potential to be smaller as it means less energy input is needed.

Answer 73

Proton Exchange Membrane Water Electrolyzer Anion Exchange Membrane Water Electrolyzer

Answer 74

PEM: < 100 °C The separator is a proton-conducting membrane Polymeric Nafion membrane enables faster start up and shut down than traditional electrolysers Perfect for coupling with intermittent renewables Lower ohmic drop, higher current density More compact thanks to solid membrane Requires perfluorinated polymers and noble metal (Ir, Ru, Pt) catalysts AEM: < 100 °C The separator is an OH-conductive anion exchange membrane Least-mature technology PGM-free anodes available Catalyst and membrane stability limit the widespread Combines operational ease of PEM with low-cost catalysts

Answer 75

m = QM/Fn Where: m - mass of product produced during dt Q - charge (= current * time) F - Faraday constant M - product molecular weight n - number of e- transfer per unit product produced

Answer 76

It calculates overall current density, j j = j0*(exp⁡((a.a*ve*F)/RT*η) − exp(−(a.c*ve*F)/RT* η)) Where: j - overall current density (A/m2) j0 - exchange current density (A/m2) T - absolute temp (K) F - Faraday's constant (C/mol) R - gas constant (J/Kmol) ac - cathodic charge transfer coefficient aa - anodic charge transfer coefficient η - overpotential (V) ve- stoichiometric coefficient of the electrons

Answer 77

U is the real potential, and can be determined with I-V curves. U = E0.cell = E0.cathode - E0.anode This is only true if there is no current through the cell and the resistance through the electrolyte solution is neglected. Otherwise U ≠ E0.cell (the real potential is not equal to the thermodynamics potential) For electrocatalytic cells U > E0.cell

Answer 78

For electrocatalytic cells U > E0.cell In particular, U = E0 + η(H2) + η(O2) + iR Where: η(H2) - overpotential for hydrogen evolution η(O2) - overpotential for oxygen evolution is - ohmic drop, due to electrolyte resistance and calculated as current*resistance

Answer 79

If the reactant has a low concentration in the electrolyte, at high overpotential the rate of reaction becomes limited by the transport of reactant to the catalyst surface. As a result, at high overpotential, the current does not keep increasing with potential, but reaches a plateau which value is determined by the rate of reactant diffusion. For the case of electrolysers, this plateau is not observed because the reactant (water and proton) is readily available.

Answer 80

1. The current density: Normalized to the geometric surface area of the electrode 2. The specific activity: Current normalized to the surface area of the catalyst 3. The mass-specific activity: Current normalized to the mass of the catalyst (for noble metal-based catalyst it is usually normalized to the mass of the noble metal) 4. Onset Potential: Highest potential at which a faradaic current is measured 5. Overpotential η: Difference between the onset potential and the theoretical thermodynamic potential 6. Diffusion-limited current: Measured current in the diffusion region 7. Half-wave potential: Potential at which the current is half of the diffusion-limited current

Answer 81

PEM: Anode: 2H2O ⇌ O2 + 4H+ + 4ei Cathode: 4H+ + 4e- ⇌ 2H2 AEM: Anode: 4OH- ⇌ 2H2O + O2 + 4e- Cathode: 4H2O + 4e- ⇌ 2H2 + 4OH-

Answer 82

PEM: - requires precious metals for the catalysts - needs improvements in cost, stability, and conductivity of electrode materias Alkaline: - membrane stability and conductivity need improving - poor catalyst activity Photocatalysis: - More efficient and stable photoelectrode materials needed

Answer 83

Porous layers are used to remove the gaseous product from the surface of the catalyst and control bubble formation. On the HER (H2 evolution reaction) side, carbon can be used as a transport later On the OER (oxygen evolution reaction) side, the carbon would be quickly corroded so we need to use expensive and resistant materials, usually Ti Cost of bipolar plate alone is approximately 30% of total cost Since Ti passivates over time, currently coated with Pt and Au to prevent passivation

Answer 84

Per-fluoro sulfonic acid membranes (PFSA) are the state-of-the-art proton-exchange-membrane. The most commoly used, commercially available is Nafion It provides good proton conductivity (78 mS/dec) But has several disadvantages, such as: High production cost High sensitivity to temperature and humidity Long lasting in the environment

Answer 85

AEMs are solid polymer electrolyte membranes which contain positively charged cationic groups (e.g. quaternary ammonium functional groups) and mobile negatively charged anions (OH) as counter ions. Requirements: 1. High ionic conductivity and selectivity 2. High mechanical stability 3. Low swelling ratio 4. Good water permeability 5. Chemical long-term stability 6. Cost-efficient 7. Environmentally-friendly membrane production process.

Answer 86

In acid conditions, Pt and Pd are the most active, but expensive and unsustainable. In alkaline conditions, nickel alloys are active but the AEM electrolyzer has other limitations (i.e. the membrane). To improve kinetics and durability of the electrode, extensive studies for highly active and stable oxygen electrocatalysts have been performed. In acidic condition, noble metal compounds have been primarily utilized as electrocatalysts Transition metals are not stable in acidic media, but some have shown good stability and activity in alkaline.

Answer 87

Whilst the potential for RuO2 is lower than IrO2 at first, over time, its function deteriorates and more V is needed, whilst IrO2 remains stable. RuO2 can then dissolve into the electrolyte and deposit elsewhere.

Answer 88

Hydrogen evolution reaction is facile: - One intermediate: H* - Ideal catalyst simply requires ∆G(H)=0 - The theoretical minimum overpotential is 0V Oxygen evolution reaction is more challenging: - At least three intermediates: OH*, O*, OOH* - Suboptimal linear scaling relationships between ∆G(OH∗) and ∆G(OOH∗) impose at least 0.3 V - Overpotential on even the most active catalyst. - RuO2 is the most active OER catalyst in acid, but IrO2 is the most stable and hard to replace.

Answer 89

H2 is a clean energy resource, and a suitable carrier to storage intermittent renewable energy As a result, H2 production has been listed in the UK 10 points plan H2 can be classified as grey, blue and green hydrogen Water electrolysis constitute the cleanest option There are four types of water electrolysers, with different working principle, operation conditions and efficiency. The efficiency can be calculated by knowing the operation parameters. Developing active and stable HER and OER electrocatalysts is the most effective way to improve the electrolyser efficiency. Benchmark catalysts are already available, but new materials need to be developed to overcome the major challenges. Several factors can be used to evaluate the activity of catalysts: on-set potential, overpotential, faraday efficiency. Nifion is the only suitable membrane for PEM water electrolyser, while AEM is still semi-commercial. Besides catalysts and membranes, temperature and pressure can also affect efficiency.

Answer 90

Ion transport: PEM transfers protons (H+) whilst hydroxide transfers hydroxide (OH-) Temperature: PEM often operates at lower temps Catalyst: PEM typically uses precious metal-based (e.g., Pt) whilst AEM uses non-precious metal catalysts (e.g., Ni-Fe) Cost: PEM has higher cost due to precious metal catalysts and membrane Stabiltiy: PEM susceptible to degradation under certain conditions. AEM generally more stable due to alkaline environment.

Answer 91

The Fermi level helps describe how electrons behave in materials, especially in metals or semiconductors. It represents the highest energy level occupied by electrons at absolute zero temperature. The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by µ or EF for brevity. When the LUMO level of the organic solvent or electrolyte lithium salt is lower than the Fermi level of the negative electrode, electrons will be injected into the LUMO orbit by that driving force, causing the electrolyte component to be reduced. When the HOMO level is higher than Fermi energy level of the positive electrode, electrons are driven into the positive electrode, causing the solvent or lithium salt to be oxidized.

Answer 92

1. Transition metal hydrides - Low GSD (1-8%), High VSD (100-160 kg H2/m3) 2. Light element hydrides - Medium GSD (5-20%), High VSD (50-160 kg H2/m3) 3. Hydrocarbons - Medium GSD (15-25%), Medium VSD (80-120 kg H2/m3) 4. Liquid hydrogen - High GSD (~100%), Medium VSD (~80 kg H2/m3) 5. Sorption - Low GSD (1-5%), Low VSD (1-40 kg H2/m3) 6. H2 hydrate - Low GSD, Low VSD 7. Pressurised tanks - Low-Med GSD, Low VSD

Answer 93

Cell potential increases with gas feed pressure and decreases with temperature. Also, kinetics improve with temperature so that an increase in temperature usually leads to improved performance. pH makes no difference. For an electrolyser, we do not want a high cell potential.

Answer 94

Hydrogen: cathode (-ve potential) Oxygen: anode (+ve potential)

Answer 95

1. Specific energy (capacity) 2. Specific power 3. Safety 4. Performance 5. Life span 6. Cost

Answer 96

Irreversible capacity lost in first cycle due to SEI, imperfect charge balancing, mass of additional components in the cell not accounted for.

Answer 97

Electrolyte (solvent/salt) - solvents with higher chemical stability and lower reactivity with electrode materials tend to form more stable SEI layers C rate/formation cycle conditions - Higher C rates can accelerate SEI formation due to increased ionic flux Electrode porosity - Higher electrode porosity can enhance electrolyte infiltration, promoting SEI formation Type of Cathode - Different chemistries exhibit varying degrees of reactivity with electrolyte components, affecting SEI composition and stability Full vs Half Cell configuration - The presence of both electrodes can influence SEI composition and morphology

Answer 98

Pore structure Surface chemistry Graphitic structure Particle size Morphology Impurity content

Answer 99

The physical increase in volume experienced by the electrode material during the charge and discharge processes of a battery. This expansion is particularly common in materials used as anodes (negative electrodes). This can be due to: - Ion insertion - Structural changes - Phase transitions And can lead to: - Irreversible structural changes, degradation of the electrode material, and loss of active material, resulting in capacity fade and reduced battery performance over time. - Loss of electrical contact - Mechanical stress

Answer 100

Sn reacts with Na to form Na15Sn4, resulting in extremely high theoretical capacity (847 mAh/g). However, the alloying reaction accompanied by huge volume expansion. This results in electrode pulverisation, continual consumption of electrolyte, loss of active material resulting in capacity loss. To prevent this, we can coat Sn particles with various protective films, introduce additives into electrolyte e.g. FEC.

Answer 101

Alkaline This is due to: Lower overpotential Favourable reaction mechanisms Catalyst stability Availability of active sites

Answer 102

Band theory in semiconductors helps explain the behaviour of electrons within materials. In solids, electrons are not confined to individual atoms but instead form energy bands that extend throughout the material. These energy bands represent allowed energy levels that electrons can occupy. In semiconductors, the energy bands are separated by energy gaps, where no electron states are permitted. The two main energy bands in semiconductors are the valence band (lower energy) and the conduction band (higher energy). The energy gap, also known as the band gap, is the energy difference between the top of the valence band and the bottom of the conduction band. It represents the minimum amount of energy required to move an electron from the valence band to the conduction band.

Answer 103

N-type: donor impurity contributes free electrons P-type: acceptor impurity creates a hole These impact the Fermi level of the semiconductor

Answer 104

Photocatalysis - particle-based semiconductor dispersed in a reaction media working under light irradiation without applied potential. Photoelectrocatalysis - semiconductor films in a transparent electrode, immersed in an electrolyte and working under light irradiation and applied potential.

Answer 105

The semiconductor band gap must straddle the reduction and oxidation potentials for the proton reduction and water oxidation, respectively. I.e. the conduction band should be more negative than the H+/H2 potential (CB < 0) And the valence band should be more positive than the H20/O2 potential (VB > 1.23) The conduction band edge must be more negative than 0 V vs RHE and the valence band edge more positive than 1.23 V vs RHE.

Answer 106

Fast charge recombination and poor charge separation hinders the performance of photocatalysts.

Answer 107

Light absorption by a semiconductor (photocatalyst) Photoexcited states generation (electron and hole) Charge separation and migration Transfer of excited states to water molecules Charge recombination (unwanted)

Answer 108

Using co-catalysts. Electrons/holes are transferred from the CB/VB from the SC to the metal. Improves electron-hole separation and inhibits recombination.

Answer 109

Semiconductors in direct contact that allow the transfer of photoexcited states from one to another.

Answer 110

The ratio of electrons transferred toward a certain product relative to incident photos at a given wavelength. AQY = (Ne/Np) × 100% = (X M/Np) × 100% Ne: Number of reacting electrons Np: Number of incident photons M: Number of product molecules X: Number of reaction electrons

Answer 111

The ratio of converted chemical energy relative to the incident solar energy. AQE over the entire solar spectrum.

Answer 112

AM1.5 G refers to the standard spectrum at the earth’s surface, derived from an standard test from the American Society of Testing Materials to facilitate the comparison of solar conversion efficiency measurements.

Answer 113

1. Photoelectrode: The photoelectrode is the key component that absorbs light and initiates the photochemical reactions. It is typically made of a semiconductor material that can generate electron-hole pairs upon absorbing photons. Common semiconductor materials used for photoelectrodes include titanium dioxide (TiO2), silicon (Si), and metal oxides like hematite (Fe2O3) or bismuth vanadate (BiVO4). 2. Counter electrode (often Pt or IrO) 3. Electrolyte (often alkaline)

Answer 114

P-type: Electrons are the minority carriers (and holes are the majority) N-type: Holes / electron vacancies are the minority carriers (and electrons are the majority) Minority carrier (h+ in an n-type, e- in an p-type) transferred in the electrode surface. Minority carriers drive the catalysis on the surface of the semiconductor. The minority carrier is what brings about hydrogen evolution on the electrode surface. P type will a photocathode. N-type will be a photoanode.

Answer 115

N type: holes/h+ P type: electrons/e- Minority carriers drive the catalysis on the surface of the semiconductor. The minority carrier is what brings about hydrogen evolution on the electrode surface. P type will a photocathode. N-type will be a photoanode.

Answer 116

Working potential where the electrocatalytic current starts to take off from the background.

Answer 117

Ratio of produced photocurrent versus the incident photon flux at a given wavelength.

Answer 118

Ratio of charges transferred to a given product relative to the total charges passed through the circuit, indicates the selectivity of an electrocatalysts towards a certain product. Desirable >80% FE% = nFm/Q Where: n - # of reaction electrons m - moles product F - Faraday constant Q - total charge passed in Coulombs

Answer 119

The intentional introduction of impurities into an intrinsic (undoped) semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic semiconductor. There can be N-type (e- charge carriers) or P-type (hole charge carriers).

Answer 120

Narrower band gap = Better light harvesting, but also means that e- and h+ (charge) recombination is much faster (Note - insulators have a large gap, conductors have no gap, semiconductors are in the middle)

Answer 121

Pros: Cheap Simple Environmentally friendly Versatile Cons: Not very efficient Limited wavelength absorption Catalyst recovery and recycling

Answer 122

1. Absorption of Light: Light is absorbed by a semiconductor material. This semiconductor is typically titanium dioxide (TiO2) or another similar material with a wide bandgap. 2. Generation of Electron-Hole Pairs: When photons are absorbed by the semiconductor, they excite electrons from the valence band to the conduction band, leaving behind positively charged holes in the valence band. This process generates electron-hole pairs. 3. Migration of Charge Carriers: The excited electrons and holes are free to move within the semiconductor material due to its crystalline structure. Electrons move towards the surface of the material, while holes migrate towards the interior. 4. Reduction and Oxidation Reactions: Once the electrons and holes reach the surface of the semiconductor, they can participate in reduction and oxidation reactions, respectively. The electrons can reduce certain compounds by transferring their energy, while the holes can oxidize other compounds by accepting electrons. 5. Degradation of Contaminants: In photocatalysis applications such as water purification or air treatment, the excited electrons and holes react with organic pollutants or other contaminants adsorbed on the surface of the semiconductor. 6. Regeneration of Catalyst: After the photocatalytic reactions occur, the semiconductor material remains unchanged and can be reused for further cycles of photocatalysis.

Answer 123

2H+ + 2e- → H2 E = 0V vs RHE Reduction reaction

Answer 124

2H2O → O2 + 4H+ + 4e- E = 1.23 V vs RHE Oxidation reaction

Answer 125

CrOx-Rh SrTiO3:Al CoOx Improves electron-hole separation and inhibits recombination

Answer 126

The process in which the electronic band structure in a material curves up or down near a junction or interface. Imagine you have two different materials joined together, like a semiconductor and a metal. Each material has its own "energy levels" for its electrons, which are like steps on a staircase. Now, when you bring these materials close together to form a junction or interface, something interesting happens to the electron steps. They start to bend near the junction, like a ramp going up or down. This bending happens because the two materials have different properties. One might have more electrons available than the other, or they might have different abilities to conduct electricity. Because of this bending, the electrons in one material want to flow into the other material to balance things out, like water flowing downhill. This movement of electrons continues until a balance is reached, and the bending stops. When this balance is achieved, there's no more net movement of electrons, and we say that the potential difference, or the energy difference, between the materials has vanished. This state is called equilibrium. Schottky contacts have a potential barrier at the metal-semiconductor junction, allowing current flow predominantly in one direction, while Ohmic contacts have no barrier, enabling easy flow of current in both directions.

Answer 127

A region that forms at the interface between two materials, typically a semiconductor and another medium such as an electrolyte or another semiconductor with different doping characteristics. This region is characterized by an excess or deficit of charge carriers, which creates an electric field. N-type semiconductors have a Fermi level energy higher than the water chemical potential. Equilibration is achieved by electron flow away from the semiconductor-electrolyte interface. This loss of majority carriers in the SC leads to the formation of a space charge layer. A region at the surface of the photoanode that is depleted of electrons, which is balanced by an equal negative charge in the electrolyte at the electrode surface, termed the Helmholtz layer. The SCL bends the bands and gives directionality of excited charge-carrier movement. Electrons are repelled by the field and drawn to the bulk where holes are driven to the surface.

Answer 128

A material added to a semiconductor that helps oxidation. The scavenger is easier to oxidise, which helps collect holes faster than water formation, preventing the holes from recombining with the electrons. They're usually small organic molecules e.g. methanol and ethanol. Instead of oxidising water, the molecules are oxidised which are easier, faster, and require less oxidation potential than water.

Answer 129

Cocatalysts Scavengers Heterojunctions

Answer 130

I) Semiconductor with much wider band gap that straddles the CB and VB of the other (not very useful) II) Similar band widths (ok) Photooxidation in both Z-scheme) The CB of one semiconductor is more negative than the CB of the other. Following photooxidation in both SCs, the electron from one recombines with the hole of the other, so there remains an e- on one SC with a huge reducing power, and a hole on the other SC with a huge oxidation power, and the recombination change is hugely decreased as the e- and h+ are greatly separated.

Answer 131

Chalcogenides: Earth abundant, high absorption, stable, easy to make Cu2O: Easy to make, abundant, low stability.

Answer 132

Photocathode: H2 evolution reaction (HER) Photoanode: O2 evolution reaction (OER)

Answer 133

TiO2. Titanium dioxide (3.2 eV) Fe2O3. Hematite (2.1 eV). BiVO4. Bismuth Vanadate (2.4 eV)

Answer 134

Carrier lifetimes are typically in the range of pico to nanoseconds, whereas the timescale for water oxidation is in the range to milliseconds to seconds.

Answer 135

Additional thin layers of material deposited onto the surface of a semiconductor substrate. They can induce band bending at the junction, which, depending on the relative Fermi levels of the materials involved may aid or hinder the separation of photogenerated charges

Answer 136

Tandem photoelectrochemical cells (PECs) are a type of solar cell configuration designed to enhance the efficiency of light-to-electricity conversion by combining multiple photoelectrodes with complementary light absorption properties. In a tandem PEC, two or more photoelectrodes with different bandgap energies are stacked or arranged in series to capture a broader spectrum of sunlight and efficiently utilize photons with different energies. Each photoelectrode is typically made of a semiconductor material. Suitable photocathode material for water reduction (Cu2O) Suitable photoanode material for water oxidation (TiO2, Fe3O4, BiVO4) Evolution of H2 and O2 between applied vias Operating current limited by the photoelectrode producing the least photocurrent

Answer 137

CO2 has a high activation potential, requiring larger amounts of energy. (Electrochemical, Photochemical, or Thermochemical systems can be used to provide such energy.) Selectivity is also an issue - lower selectivity for larger products Hydrogen formation is easy and quick and competes with CO2 reaction processes. Overpotentials - Both OER and CO2R have very large overpotentials, meaning CO2R will run at a larger cell potential than this thermodynamic potential

Answer 138

Electrodes, catalyst, electricity needed. Reduction occurs at cathode - CO2 is bubbled into the system so, at the cathode, CO2 forms other products such as CH4, CH3OH etc. At the anode, water is oxidised to form oxygen.

Answer 139

Low overall selectivity Lower selectivity for larger products Hydrogen formation is competing reaction

Answer 140

Catholyte and anolyte separated by a membrane Usually neutral electrolyte (KHCO3 0.5 M) Low current densities (20 mA cm-2) owing to low CO2 solubility (34 mM at ambient conditions) (at high current densities, CO2 runs out and the process reverts to hydrogen evolution) Ag/AgCl reference electrode, water oxidation catalyst in the anode (IrO, Pt/C) Looks like a capital H shape

Answer 141

Linear sweep voltammetry can be used to calculate the peak current, calculate the peak current potential, and calculate the half-peak current potential. The current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly in time. In the context of linear sweep voltammetry, "swept" means that the potential between the working electrode and the reference electrode is changed continuously and linearly over time. Screening potential Recording current Catalyst comparison

Answer 142

The electric potential of the working electrode is stepped and the resulting current from faradaic processes occurring at the electrode (caused by the potential step) is monitored as a function of time. Constant potential Recording current Stability Product quantification

Answer 143

The current density used to produce one single product. Proportional to product formation (>200 mA cm-2 for industrially relevant application). Calculated multiplying the total geometric current density by the faradaic efficiency.

Answer 144

Defined as the rate of electrochemical conversion per electrocatalytic site at a certain potential. Highly challenging to calculate due to the structural ambiguity of the active sites and the difficulty of accurately quantifying them. TOF = J / (n*F*ASN) Where: J is the partial current density of the product n is the number of electrons F is the Faraday constant ASN is the active site number

Answer 145

Difference in electrical potential between anode and cathode, controlled by kinetic overpotentials of each half reaction and ohmic losses due to ion transport through the device. (<3 V)

Answer 146

It determines the extent to which CO2 is effectively converted to the product of interest. Defined as the molar ratio of the CO2 converted to the product of interest and the total CO2 inlet to the cathode. Hindered by the formation of carbonates (HCO3- and CO32-) by reaction of CO2 with OH-. CO2 UD = V(CO2 to product) / (V((CO2 to product) + V(CO2 lost)) *100 Where V are volumes

Answer 147

It possesses a porous electrode where the catalyst is coated and, instead of dissolving CO2 in the electrolyte, the CO2 goes through the porous layer, through the catalyst, into the electrolyte. GDL (gas diffusion layer) is hydrophobic. This means there is a triple phase boundary between catalyst, electrolyte, and product. No longer limited by solubility. Gaseous CO2 is reduced at a gas/electrode/electrolyte interface Deposition of a catalyst onto a GDL (typically a commercially available carbon-based substrate with hydrophobic treatment). GDL is more resistive compared to pure metals Conductive tape is applied around the electrode to minimize distance electrons need to travel Sufficient current collection is extremely important for high current density Flooding still an issue. Attributed to salt penetration into the GDL that compromises hydrophobicity

Answer 148

Gas diffusion electrodes (GDE) are electrodes with a conjunction of a solid, liquid and gaseous interface, and an electrical conducting catalyst supporting an electrochemical reaction between the liquid and the gaseous phase Enhanced mass transport. CO2 in contact with catalyst and electrolyte simultaneously. Large electrochemical surface area - Industrial current densities (200 mA cm-2) With or without electrolyte (flow cell vs zero gap electrolyser) Membrane and flowing aqueous electrolyte to separate the electrodes (hybrid electrolysers, flow electrolyser). Control cathode and anode pH, overcomes product crossover Deposition of a catalyst onto a GDL (typically a commercially available carbon-based substrate with hydrophobic treatment). Flooding still an issue. Attributed to salt penetration into the GDL that compromises hydrophobicity

Answer 149

Membrane electrode assembly (MEA) Humidified CO2, is fed to the cathode. Cathode and anode sandwich an ion-exchange membrane Polymer electrolyte membrane reduces ohmic losses by decreasing the interelectrode distance. Ionic species transport to and from the anode through a membrane

Answer 150

As current density, increases, CO2 conc decreases to 0 at ~ 20mA/cm2 in H cells, whilst is decreases more gradually in GDLs (above 200 mA/cm2). pH increases as more OH- ions are formed. GDL allows high CO2 concentration near the catalyst at high current densities, but the pH increases quicker as OH- is generated at a faster rate.

Answer 151

When CO2 reacts with OH- ions to form HCO3- and CO3 2- ions Carbonate crossover in CO2 electrochemical devices refers to the unintended transport of carbonate ions across the membrane or separator. During the electrochemical reduction of CO2, carbonate ions (CO3^2-) can be formed as intermediates. - Carbonate ions reaching the cathode can react with hydrogen ions or undergo reduction reactions, affecting the purity of the desired product. - Carbonate ions may interfere with the electrochemical reactions occurring at the cathode, reducing the efficiency of CO2 reduction or altering the selectivity of the process. - Prolonged exposure to carbonate ions can lead to degradation or fouling of the membrane, affecting the overall durability and performance of the CO2 electrochemical device. This is dealt with with bipolar membranes.

Answer 152

Bipolar membranes [Without bipolar membranes: The carbonate ions pass an AEM (anion exchange membrane) where the carbonates react with H+ ions to reform CO2. This CO2 formed is known as lost CO2.] WITH BIPOLAR MEMBRANES: An AEM (anion exchange membrane) and CEM (cation exchange membrane) are present, which separate the H+ and OH- ions from water reactions. Carbonate is formed in the cathode side. (Before, carbonate would travel to the anode and reform CO2). Now, CO2 is regenerated at the cathode, preventing CO2 loss, carbonate crossover, and maintaining alkaline conditions at the anode. However, there will be a lot of protons at the cathode, encouraging hydrogen formation. This reduces the Faradaic efficiency for CO2 formation.

Answer 153

Lower faradaic efficiency and higher cell voltage H+ flux benefits HER over CO2 Higher cell voltage arising from membrane resistance Higher CO2 utilization owing to lack of carbonate and product crossover

Answer 154

If C binds weakly to the catalyst, you have a catalyst that makes CO i.e. CO is released easily. If it binds strongly enough for another CO to come, the 2 CO molecules can be coupled and hydrogenated, forming hydrocarbons. If CO binding is too strong: Catalyst poisoning, leading to HER If CO* binding is not strong enough: CO* desorbs before C-C or C-H coupling. CO production Cu is the only catalyst that binds CO* strongly and H* mildly, therefore is the only material capable to make C2+/coupled products

Answer 155

H2 = Ni, Fe, Pt, Ti CO = Au, Ag, Zn, Pd, Ga Formate = Pd, Hg, Tl, In, Sn, Cd C1-C3 = Cu

Answer 156

Low pH (high H+ concentration) decreases the selectivity towards CO2R versus HER High pH (high OH- concentration) increases carbonate crossover Local pH changes during operation. Depletion of protons/production of hydroxide ions from HER and CO2RR. pH strongly affects CO2R selectivity CH4 production is favoured in a more acidic pH Alkaline conditions supress methane and hydrogen formation, promote C2+

Answer 157

Hard carbon Tin-based Sodium titanates

Answer 158

Inefficiencies (overpotentials η): 1. ion/electron transport (ηcond) 2. electrode kinetics (ηkin) 3. mass-transport induced thermodynamic effects (ηmt) 4. Cathode/membrane interface 5. Ohmic losses and resistance

Answer 159

* Adsorption of bent radial anion through oxygen (oxyphilic surfaces) and hydride transfer. * TRL 3-5, Sn NPs catalyst in GDE, AEM * Acidic centre compartment for direct production of FA and not the deprotonated form (HCOO-) * OER at anode, protons travel towards centre compartment * 2000 A m-2 (200 mA cm-2), 3.75 V, 73-91% FE * 10 wt% FA solution that is distilled-extracted to achieve 85 wt% * CO2UD 37-46%

Answer 160

TRL 5-6, GDEs in a membrane electrode assembly Humidified CO2 stream is fed at the cathode producing CO and (undesired) H2. Carbonate crossover in the cathode, carbonates cross membrane to anodic compartment. 2000 A m-2 (200 mA cm-2), 3 V, 98% FE CO2UD 49%

Answer 161

Copper is the only heterogeneous catalyst (to date) that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol from electrochemical CO2 reduction. Activity and selectivity is highly sensitive to catalyst surface structure, morphology, composition, electrolyte and pH. TRL 3-4, GDEs in a membrane electrode assembly (analogous to CO electrolyser) Humidified CO2 stream is fed at the cathode which is separated with an AEM from the anode. Alkaline aqueous stream is fed in the anode for the OER The cathode outlet contains CO2, H2O, C2H4, other gas and liquid products 1200 A m-2 (120 mA cm-2), 3.7 V, 64% FE CO2UD 20-33%

Answer 162

Fuel cells: Require a fuel Faster recharging time for fuel cells vehicles, compared to battery vehicles Higher specific energy than batteries Lower specific power than Li-ion batteries No self-discharge concerns

Answer 163

Pros: Fast start-up Low temperature Non-corrosive electrolyte High power density Operates at low pressure  safer Higher Performance Stable and active ORR catalyst available (Pt) Low Pt loading for HOR Durable and highly conductive membrane available (but perfluorosulfonate based) Cons: Relatively low tolerance to carbon dioxide (50ppm) and sulfur (few ppm) Need reactant gas humidification Need expensive and unsustainable platinum as ORR catalyst Need Nafion membrane, expensive, complex production Noble metals needed for OER (transition metals are stable but don’t perform well in acid)

Answer 164

Pros: Low temperature Fast start-up High efficiency Minimal corrosion Noble-metal free alternative for the ORR catalyst Noble metal-free ORR catalysts exists Cons: Intolerant to CO and CO2 (350 ppm) Liquid electrolyte Complex water management Short life-time Lower performace Slightly higher Pt loading needed for HOR Lower conductivity and durability of the membrane (but fluorine-free options available)

Answer 165

In PEMFCs, water is only generated (at the cathode as a product of the oxygen reduction reaction, ORR) and not electrochemically consumed. Water is also moved to the cathode from the anode by electro-osmotic drag as H2O produced by the hydrogen oxidation reaction (HOR) moves through the PEM. Thus, removing cathode water is the prime concern in the PEMFC to avoid flooding. To avoid cathode flooding and anode dry out, water need to back diffuse to the anode through the membrane. The role of water is also to hydrate the membrane and to facilitate H+ conduction.

Answer 166

Hydroxide Conductivity Alkaline Stability Water Management Despite recent advances, anion exchange membranes still show lower conductivity and stability compared to Nafion, hindering the development of AEMFCs. In addition, water management in AEMFCs is more complex than in PEMFCs

Answer 167

Water is both generated at the anode and consumed at the cathode. Water is moved from the cathode to the anode by electro-osmotic drag. Water management includes providing enough water to hydrate the membrane, while avoiding flooding and dry out at both electrodes. The balance between membrane hydration and flooding is thin and more complex than in PEMFCs

Answer 168

The state-of-the-art catalyst for HOR is platinum But with lower noble metal loading (0.05 𝑚𝑔_𝑃𝑡 𝑐𝑚^(−2)) compared to the oxygen reduction catalyst (0.13 𝑚𝑔_𝑃𝑡 𝑐𝑚^(−2)) The kinetics of hydrogen oxidation is slower in alkaline than acidic media, resulting in higher platinum loadings at the anode for AEMFCs compared to PEMFCs.

Answer 169

Several reactions possible, producing multiple intermediates. Peroxides may form which are reactive and may destroy the catalyst.

Answer 170

One approach to improving the sustainability of fuel cells is decreasing platinum loading, this can be achieved by: 1. Increasing the activity of platinum-based catalysts 2. Increasing platinum utilization

Answer 171

By alloying Pt with other metals one can modifying the binding energy of the intermediates and get closer to the top of the volcano. Pt3Ni(111) is the Pt alloy with the highest reported activity so far.

Answer 172

Improving the catalyst support Avoiding platinum aggregation

Answer 173

M-N-C catalysts consist of nitrogen and transition metals on a carbon support. They are generally synthesised by: 1. Carbon support, nitrogen-containing molecule or polymer and Transition metal precursor 2. Pyrolysis 3. Washing

Answer 174

To produce a fuel cell that can run for hundreds of hours, the stability of the oxygen reduction catalysts is of paramount importance. Unfortunately, all ORR catalysts lose activity with time to a certain degree. The loss in activity results from - The dissolution of active sites (the metal) - A rearranging of the structure of the active site to a less performing form - The degradation of the support In general, stability in fuel cells is less of an issue compared to electrolysers. Carbon can always be used as a support and porous layer, it does not corrode as for the case of oxygen evolution. In addition, the bipolar plates are less subject to corrosion, so it does not require Ti (as for the case of OER in electrolysers).

Answer 175

Opposite to electrolysers, the potential out of a fuel cell is always lower than what is thermodynamically predicted.

Answer 176

RDE: rotating disc electrode RRDE: rotating ring-disc electrode 3 electrode set-up: - Working electrode – where the catalyst is deposited and depending on the reaction we are looking at, we can observe oxygen reduction to water/peroxide - Counter electrode – to complete the cell circuit - Reference electrode – used to measure the potential at the working electrode Oxygen is bubbled through the electrolyte and is reduced at the catalyst, deposited on the working electrode. This set-up studies only the half reaction at the cathode. This electrodes have a conductive glassy carbon substrate where the catalyst is deposited . They can rotate, to provide defined mass transfer. The RRDE has in addition a platinum ring around the glassy carbon disc

Answer 177

Only looks at the half reaction at the cathode Has a different geometry from fuel cells, as oxygen diffuses through the electrolyte, instead of coming in gaseous form from the other side of the electrolyte Limited mass transport and current density

Answer 178

A reference electrode is an electrode that has a stable and well-known electrode potential. The overall chemical reaction taking place in a cell is made up of two independent half-reactions, which describe chemical changes at the two electrodes. To focus on the reaction at the working electrode, the reference electrode is standardized with constant (buffered or saturated) concentrations of each participant of the redox reaction.

Answer 179

This is the closest geometry to real fuel cells, it contains an anion/proton exchange membrane and a gas diffusion layer. Both the cathodic oxygen reduction and the anodic hydrogen oxidation are investigated and the gas transport is similar to that in fuel cells. However, they are very time consuming and complicated to assemble and test. MEA consists of three key elements: a proton-conducting membrane (typically made of perfluorosulfonic acid), an anode catalyst layer, and a cathode catalyst layer. The membrane allows ions to pass through while blocking electrons, facilitating the electrochemical reactions of hydrogen oxidation at the anode and oxygen reduction at the cathode. Catalyst layers on either side of the membrane enhance reaction rates by providing active sites for electrochemical reactions.

Answer 180

In between RDE and MEA. Has a more similar geometry to fuel cells, compared to RDE. Offers faster mass transport and higher current density, compared to RDE. Is easier and faster to assemble and used compared to MEA. Gas diffusion electrodes (GDE) are electrodes with a conjunction of a solid, liquid and gaseous interface, and an electrical conducting catalyst supporting an electrochemical reaction between the liquid and the gaseous phase.

Answer 181

GDEs have a continuous flow of gas, so do not experience mass transfer / diffusion limitations. Thus, as potential is decreased, current density continues to increase exponentially. RDEs experience a kinetic region as well as a mixed and diffusion region (regions explain the limiting steps).

Answer 182

In linear sweep voltammetry the potential is varied linearly. At the initial potential no reaction happens and the measured current is zero. As the potential is decreased, oxygen is reduced and a negative current is measured. The kinetic of reaction increases with overpotential and so does the current -> Kinetic Region. When the reaction kinetics is so high that all oxygen at the surface is immediately consumed, the reaction becomes controlled by the oxygen diffusion and the current is independent on the potential -> Diffusion Region

Answer 183

The current can be normalized with respect to different parameters, giving: The current density: Normalized to the geometric surface area of the electrode The specific activity: Current normalized to the surface area of the catalyst The mass-specific activity: Current normalized to the mass of the catalyst (for Pt-based catalyst it is usually normalized to the mass of platinum)

Answer 184

Onset Potential: Highest potential at which a faradaic current is measured Overpotential η: Difference between the onset potential and the theoretical thermodynamic potential Diffusion-limited current: Measured current in the diffusion region Half-wave potential: Potential at which the current is half of the diffusion-limited current

Answer 185

In a rotating ring-disc electrode, oxygen that has only partially reacted to hydrogen peroxide on the disc catalyst is further reduced to water on the platinum ring, with a collection efficiency ε. The current generated at the ring (𝐼_𝑅) and at the disk (𝐼_𝐷) are collected separately, allowing the determination of the hydrogen peroxide production and the average number of electrons (n). Equations: n = 4*I_D/(I_D + I_R/ε) Y_peroxide = 100*(2*I_R/ε)/(I_D + I_R/ε)

Answer 186

Its inherent energy comes from the sun It absorbs carbon dioxide as it grows Can be used directly to displace fossil fuels such as oil, coal, and natural gas for the production of energy or other products Can regrow in a relatively short time (unlike fossil fuels) The carbon that is released from the organic material was sequestered recently, compared to fossil fuels where the carbon was sequestered hundred of millions of years ago. Biomass residues are wastes that would be combusted or left to decompose.

Answer 187

Biomaterial (CxHzOz) oxidised at the anode to form H+, e-, and CO2, and the protons are then reduced at the cathode to form H2. Oxidising an organic material is much more favourable thermodynamically (more easily oxidised) than water. For biomass, thermodynamically < 1.23V, in practise < 1 V

Answer 188

V = ∆E.eq + Ƞa + Ƞc + IR Where: ∆E.eq is the difference between equilibrium electrode potentials Ƞa is the overpotential at the anode Ƞc is the overpotential at the cathode IR is the ohmic drop Oxidising biomass is much easier than water. ∆E.eq differs owing to the thermodynamics of an organic molecule oxidation. Biomass efficient transformation is governed by kinetics, which can be achieved with a suitable electrocatalyst to compensate for the involved overpotentials Thermodynamics of oxidation determines ∆E.eq.

Answer 189

Anode: CxHyOz + H2O → oxidation products + H+ + e- Cathode: H+ + e- → H2 Alkaline (NaOH, KOH) often used, though acid can also be used.

Answer 190

Complete biomass oxidation produces CO2 (under acidic conditions) or CO2-3 (under alkaline conditions). Partial oxidation generates value-added chemicals. OER involves multi-proton-coupled electron-transfer steps, suffering from the limitation of suboptimal scaling relation between OOH and OH adsorption energies, which is the major bottleneck and requires a cell potential above 1.23 V.

Answer 191

The volcano plot would have the same shape if plotted as a function of the binding energy of another intermediate, as the binding energies of key intermediates are linearly dependent.

Answer 192

No, the top of the volcano is not equal to zero overpotential. This is because the binding energy of O* and HO* cannot be tuned separately and even the optimal O* binding energy results in the presence of two ORR steps which are uphill in free energy. This results in the requirement of a finite overpotential for the reaction to occur.

Answer 193

As the temperature increases the electronic conduction decreases, while the ionic conduction in the electrolyte increases, resulting in an overall increase in the rate of reaction up to 80°C. Above this temperature, water starts to boil causing oxygen starvation at the catalyst and the conductivity in the Nafion membrane decreases significantly. As a result, the optimal operating temperature for PEMFC is 70-90°C. For the case of solid oxide fuel cell, the presence of a solid electrolyte removes the issues associated with water management and allows operating at higher temperature and reaction rates.

Answer 194

- The higher requirement of platinum for the HOR catalyst - The lower activity and stability of oxygen reduction catalysts compared to platinum in acidic conditions - The limited conductivity of anion exchange membranes.

Answer 195

Using linear sweep voltammetry, a polarisation curve can be made in oxygen saturated atmospheres. Impedance spectroscopy can be also used to characterize the main resistances in the system. Finally, a durability test should be performed to assess the stability of the catalyst.

Answer 196

Adding a co catalyst Enhancing the specific surface area of the semi conductor Creating a Z scheme heterojunction Adding a hole scavenger to the reaction media Doping the semiconductor

Answer 197

H2O ↔ HO* + H+ + e- HO* ↔ O* + H+ + e- O* + H2O ↔ HOO* + H+ + e- HOO* ↔ * + O2 + H+ + e-

Answer 198

At lower applied potentials, there's a process where a layer forms on the surface of the Ru catalyst. This layer, called a hydrous amorphous layer, makes it easier for the ruthenium ions to escape into the solution. This leads to catalyst degradation and loss of efficiency. The same happens for IrO2, but at a much higher applied potential. M²ⁿ⁺Oₙ²⁻ + nH2O <-> M²ⁿ⁺(aq) + 2n OH⁻(aq) O + 2n e-

Answer 199

Cell: H-type with anion exchange membrane, gas diffusion electrode Electrolyte: 0.1 M KHCO3 with CO2 inlet and outlet Working electrode: Au, Ag, or FeNC for CO and Cu for ethylene Counter electrode: Pt foil or IrO Reference electrode Ag/AgC The CO2 reduction performance would be assessed by performing chronoamperometry measurements at different negative working potentials (between -0.2 and -1 V approximately) for a significant time interval (15-60 min) and constant CO2 purge into the electrolyte. The head space would be analysed by gas chromatography to quantify the amount of product and the faradaic efficiency would be calculated by FE% = nFm/Q Where: * n is the number of reaction e- (2 for CO, 12 for C2H4) * m is the moles of product * Q is the total charge in coulombs (Q = It)

Answer 200

CO2 + e- → CO2*- The first electron transfer is employed to create a CO2- radical anion. When this happens the C-O bond gets weakened and the molecule reorganizes from the original linearity to a bent radical owing to the unpaired electron. Carbon dioxide is a highly thermodynamically stable molecule owing to the strong C=O bond, and therefore its reduction is kinetically sluggish. This radical intermediate can then undergo further reduction steps to produce various products depending on the reaction conditions and the specific catalyst involved in the CO2 reduction process.

Answer 201

Start with the Nerst equation: E = Eo − (RT/nF)*lnQ Simplifying and considering that ln x = 2.303 log x, E = Eo − (0.059/n)*log10(Q) And Q is the activity coefficients, a, of the products/reactants, = partial pressures: Q = p.products / p.reactants Considering Water at 1 bar and 1M solution: O2 + 4H+ + 4e- -> 2H20 (so 4e- used) E = 1.23 - (0.059/4)*log10([H2O]^2 / [O2][H+]^4) E = 1.23 - (0.059/4)*log10(1/[H+]^4) pH = -log10 (H+) E = 1.23 - 0.059pH

Answer 202

RDE consists of a glassy carbon disk where the catalyst is deposited. They allow study of a single reaction in a half-cell configuration and can rotate to provide defined mass transport. RRDE has a Pt ring where peroxide produced at the disc is transported. Oxygen travels to the first disc where it is reduced to peroxide (2e-) or water (4e-) If peroxide is produced, it goes to the disk where it is oxidised back to O2 Monitoring the current at the disc and ring, we can determine how much peroxide was produced at the disc

Answer 203

The average number of electron is a measure of how much of the reacted oxygen is converted to water (4 electron) or peroxide (2 electrons). The ideal ORR catalyst should have n=4, i.e. only produce water. This is because peroxide production accelerate the degradation of the catalyst.