Maximilian Stark (mail@dakror.de), WS2020
Stand: 25.02.2021
Energy Materials 1
Lecture 1
- Give a definition for the terms "primary" and "secondary" energy sources.
- primary: Fossil fuels, sun, water
- secondary: derived from primary, like biofuel, gasoline
- What is the global energy use by source?
- Total: 600 Exa Joule
- Oil: 190
- Coal: 160
- Gas: 140
- Renewables: 30
- Hydro: 40
- Nuclear: 25
- What is the energy density of some common fuels (give some examples).
- Volumetric vs Gravimetric
- Uranium: 1.5 PJ/L / 80 TJ/kg
- H2: 5 MJ/L / 140 MJ/kg
- Diesel: 40 MJ/L / 40 MJ/kg
- Wood: 3 MJ/L / 12 MJ/kg
- Analyze problems and alternatives of the current fossil fuel economy.
- Renewables fastest growing energy source: 11% annual growth (+3.2 EJ 2019)
- Fossil supply 80% of world energy through 2040
- Natural gas fastest growing fossil fuel: 1.7% annual
- What is the concept of "hydrogen economy"?
- Low QoL in cities due to emissions
- Hydrogen as main low-carbon energy source replacing natural gas or gasoline
- Zero emission H2 production through renewables
- What are current trends in electrical energy consumption?
- Coal stable #1
- Nuclear heavily declining
- Renewables gaining fast, overtook nuclear
- What are the renewable energy sources.
- Solar
- Wind
- H2
- Hydro / Wave
- What are possible ways to address the electricity generation versus consumption problem (analyze benefits and challenges)?
- Storage of excess generated power using functional energy materials -> Never lossless, expensive components
- Combining multiple energy sources to cover most time (Wind + Solar) -> Expensive
- Interconnector -> Exchange between power grids, already in use. 8 yr implementation time, weeks to months storage, 20 yr lifetime, 1.4 GW / cable
- Heat storage systems -> Thermal, already in use. 2 yr implementation time, <24h storage, 60 yr lifetime, 4GW
- Availability of fossil fuels.
- Political tension with limited resources
- Continuous growth in production, fluctuating prices
- Declining rate of oil discovery: 40 yr left, less than 20 top countries most of production
- Iran, Kuwait, Saudi, Russia, USA, Mexico
- Coal & Gas plentiful in Russia
- Uranium widely distributed with low quality
- What are the "solar fuels"?
- Methanol
- Ethanol
- Hydrogen
- Kohlenwasserstoff
- Ammoniak
- Energy storage in chemical form
- The role of material science in "energetics": what kind of materials do we need?
- Optimizing fossils
- Homo & Hetero catalysts: fuel conversion
- Ionic conductors: energy storage
- Hydrogen economy
- Electrocatalysts: electrolysis & fuel cells
- Ionic conductors: energy storage
- Hybrid systems
- Catalysts
- Ionic conductors
- Materials for energy storage
- Piezoeletrci materials
- Semi-conductors
Lecture 2
- Name the top 3 fossil fuels being used now.
- Oil: 33%
- Coal: 30%
- Gas: 24%
- What is the concept of rational use of fossil fuels?
- Continuous reduction of fossil fuel consumption
- Explain what hydrocracking, synthetic fuels, Fischer-Tropsch conversion, coal liquefaction (Bergius process), and the Sabatier process are.
- Hydrocracking
- Breaking up long HC-chains -> Production of jet fuel
- Catalysts + Hydrogen + Gas oil feed
- Reactor
- Separator -> Recycles hydrogen
- Fractionator -> Recycles gas oil & outputs products
- Highly dependant on catalysts
- Liquid fuel synthesis from coal & gas
- $CH_4 + H_2O \rightarrow CO + 3H_2$: syngas
- Coal liquefaction: $nC + (n + 1) H_2 \rightarrow C_nH_{2n+2}$
- Fischer Tropsch conversion: $(2n+1)H_2 + nCO \rightarrow C_nH_{2n+2}+nH_2O$
- 0.3 mil Barrels / day vs. consumption of 18.9 mil barrel / day US
- Poisioning sensitive catalysts
- Sabatier process: Methane from $CO_2$ and $H_2$
- $CO_2+4H_2 \rightarrow CH_4 + 2H_2O$
- Current application: ISS
- Application: cons. vs. gener. problem
- Poisoning sensitive catalysts with sulfur
- Name the most active catalysts for these processes.
- Copper
- Nickel
- Ruthenium
- Molybdenum
- What are heterogeneous catalysis and catalysts?
- Catalyst and reactants in different phases / aggregates
- Catalysts enhance rate of reaction, lowering energy barrier
- Catalyst surface resembles checkerboard
- What is the Sabatier principle?
- Interaction between catalyst and reaction intermediates should be "just right"
- Explain what is the concept of active sites.
- Hugh Scott Taylor
- Catalysis not even over catalyst surface but at few active sites / centers
- Why to use the volcano-plots and what is their meaning?
- Quantification of Sabatier principle
- Peak: Optimal catalyst configuration
- Plot of catalyst-induced reaction rate vs some parameter describing stability:
- Heat of adsorption of one reactions
- Heat of formation of bulk compound relative to surface compound
- Position of catalytic material (metal) along Periodic Table
- What are the scaling relations in heterogeneous catalysis and what is the physical origin of this phenomenon?
- Approximately linear scaling of binding energies of intermediates in multi-step processes
- Therefore any of the binding energies can be used as descriptor for volcano plot
- Origin: Adsorption of intermediates through same atoms (chain-building)
- What are general concepts allowing to design active, selective and stable heterogeneous catalysis?
- Fabrication systems using robots to produce thousands of candidate catalysts per hour
- Selectivity
- Preferring certain reaction pathways (by-product creation)
- More complex than activity-increase
- Identification or key intermediates largely responsible for selectivity
- Analysis of surface property causing process shift towards desired product
- Stability
- Ability of catalyst to maintain initial high activity and selectivity over time
- Only useful descriptor: Heat of formation of compounds $\Delta H_f$. The smaller the more stable
- Introduction and control of mesoporosity in zeolite support increases activity and stability
Lecture 3
- How is electrical current used to control redox reactions?
- Electrical current is used to increase or ease the transfer of electrons between spatially separated reactions instead of having to rely on complex environment setup
- Explain the basic principles of water electrolysis.
- Water is split into pure Hydrogen and oxygen gas
- Hydrogen reduction at cathode: $2 H_2O + 2e^- \rightarrow H_2 + 2OH^-$
- Oxygen oxidation at anode: $2OH^- \rightarrow \frac{1}{2}O_2 + H_2O + 2e^-$
- 1.23 V from environment to form reaction
- Give a definition: what is the overpotential?
- Potential difference between thermodynamically determined potential for redox event and the observed potential of the event
- Resistance against continuous flow of reaction by buildup of electrode-blocking molecules => electrolyte addition
- What are electrocatalysis and electrocatalysts?
- Watersplitting is in essence electrocatalysis as intermediates are forming at electrodes
- Electrocatalysts lower the overpotential of the reaction to occur
- Analyse the differences between the alkaline and acidic water electrolysis.
- Explain the working principles and name the main components of alkaline and PEM electrolysers. Name and describe key functional materials for them.
- Alkaline (KOH-electrolytes)
- 40-90°C
- Anode: $2OH^- \rightarrow \frac{1}{2} O_2 + H_2O + 2e^-$
- Cathode: $2H_2O+2e^- \rightarrow H_2 + 2OH^-$
- Transfer of $OH^-$ from Cathode to Anode
- Diaphragm separating electrodes
- 7d/yr maintenance
- Corrosion problems
- Scalable
- Low efficiency
- (acidic) PEM (polymer electrolyte membrane electrolytes)
- 20 - 100°C
- Anode: $H_2O \rightarrow 2H^+ + \frac{1}{2}O_2 + 2e^-$
- Cathode: $2H^+ + 2e^- \rightarrow H_2$
- Transfer: $H^+$ from Anode to Cathode
- Higher efficiency
- Expensive components (Iridium & Platinum)
- Corrosion
- 46d/yr maintenance
- Name the state-of-the-art electrocatalysts for alkaline and acidic water electrolysis.
- Alkaline: Ni (Anode), Ni (Cathode)
- Acidic: $IrO_2$ (Anode), Pt (Cathode)
- What are the reaction intermediates for the hydrogen evolution and oxygen evolution reactions?
- Hydrogen: H*
- Oxygen: OH, O, *OOH
- Explain the concept of "volcano plots" for electrocatalysis, taking H2 and O2 evolution reactions as examples.
- Binding energy of hydrogen species as descriptor
- Evaluating metal-hydrogen bond strength for electrode materials, only explanation, no prediction
- Prediction of activities using DFT calculations
- Analyse challenges and perspectives of electrochemical water splitting and CO2 electroreduction
- Expensive, net loss: overpotential, components
- $CO_2 + 2H^+ +2e^- \rightarrow CO + H_2O$
- $2CO + 7H_2O + 8e^- \rightarrow CH_3CH_2OH + 8HO^-$
Lecture 4
- What are fuel cells and what are basic principles of their operation?
- Electric generator using RedOx of fuel (Hydrogen, Methane) and oxidant
- Spatially separated sub-reactions at electrodes
- Anode: Fuel
- Cathode: Oxidant
- Ion transport in membrane
- What are main types of fuel cells?
- PEMFC: Polymer electrolyte membrane fuel cell
- low temperatures
- high power densities
- quick output variation
- SOFC: Solid oxide fuel cell
- very high temperature
- low-cost catalysts
- Name key functional materials for fuel cells.
- electrocatalysts
- electrolyte
- Name state of the art electrocatalysts for PEMFCs and SOFCs.
- PEMFC: Pt, Pt3Ni
- SOFC: Ni (Anode), Perovskite: Lanthanum-Strontium manganites and cobaltites (Cathode)
- What causes the main losses in PEMFCs and SOFCs?
- Slow kinetics of oxygen reaction
- Overpotential despite Pt-catalyst
- Oxygen vacancy concentration at cathode
- What is the influence of defects and particle size in heterogeneous and electrocatalysis (influence of on activity and stability of electrocatalysts)?
- Decreasing activity & stability
- Improved stability using mesoporous structures
- Analyse advantages and disadvantages of high temperature fuel cells.
- +: tolerance towards impurities
- +: low-grade catalysts
- What are trends in catalytic activity for SOFC anode materials and the what is the "activity descriptor" in this case?
- Rh, Ni, Ir highest
- $\Delta E_O$ as descriptor
- Analyse perspectives and challenges for fuel cell technologies.
- Same amount of platinum for FC as for catalytic converter in combustion engine
Lecture 5
- What are ionic conductors (definition)?
- Electrolyte
- Materials that transport conduct electricity via ion movement under an electrical field
- What is their role in energy conversion and storage devices?
- They are used to spatially separate the RedOx reaction
- Analyze differences "ionic versus electronic conductors".
- Ionic
- ions carry current
- conductivity increases exponentially with temperature increase (activated transport)
- Electronic
- electrons carry current
- Conductivity decreases linear with temperature increase (photon scattering)
- What are the main modes of ion transport in ionic conductors?
- Migration: ion movement to equalize potential gradients
- Convection: external move of material
- Diffusion: Chemical potential (concentration) induced movement of a species
- What are the main differences between solid and liquid electrolytes with respect to energy conversion and storage?
- Liquid possibility to add additional indifferent supporting electrolyte to minimize issues with non-uniform electric field distribution
- Solids have substantially higher electronic conductivity
- Solids may be amorphous, polychrystalline, single-crystal, immobile
- Liquid ions are surrounded by solvent molecules
- Liquid: easily controllable concentration and conductivity
- Ionic conductivity of liquids and solids: what are their typical dependencies on temperature?
- $\sigma = n e \mu$, $n$ number of charge ($e$) carriers, $\mu$ their mobility
- Solids: logarithmic $\sigma ~ \sigma_0\mathrm{exp}(-E_a / RT)$
- Fluids: non-Arrhenious behavior with increasing temperature: Many other parameters (dielectric constant) depend on temperature (unpredictable increase in conductivity)
- Typical origin of high ionic conductivity in solids.
- point, linear & planar defects in lattice structure
- Corporative behavior of some structural units in solids
- Superstructures consisting of conductive and nonconductive fragments
- Mixed ionic/electronic conductivity.
- What is the idea behind the homogeneous doping?
- Increase of conductivity if main origination through crystal structure defects
- Maintaining crystal stability
- Replacement of lattice atoms to create Oxygen vacancies and introduce ion conductivity
- What is the idea behind the heterogeneous doping?
- Creation of defects by insulator particles
- Name state of the art oxygen conducting electrolytes.
- Stabilized bismuth oxides (ESB, DWSB)
- Doped ceria (GDC, SNDC)
- Yttria-stabilized zirconia (YSZ)
- How to measure the ionic conductivity correctly (including explanations of the method principles)?
- Impedance spectroscopy
- Working, Counter, and Reference Electrode inside glass cell inside furnace
- Probing signals at different AC frequencies with some complex response
- Output: Modulus of impedance |Z| and phase shift between probing and response
- Black-Box interpretation, model fitting to output to explain system
- Basic interpretation: $WE \rightarrow Z_{WE} \rightarrow Z_{\text{electrolyte}} \rightarrow Z_{CE} \rightarrow CE$
Lecture 6
- What is the mechanism of proton conductivity in solid state proton conductors?
- H+ proton exchange of the structure with water channels via hydrogen bonds
- What is the origin of high ionic conductivity of Nafion?
- Presence of hydrophilic SO3H groups creates superstructure with hydrophobic fragments and water rich channels
- Explain what the Grotthuss mechanism of the proton movement in Nafion is
- Proton transport in aqueous electrolytes. H atom moves from Water molecule to water molecule via hydrogen bond
- What are main design principles of new ion conductors?
- Strong conductivity
- Mechanical strength
- Life span / durability
- What are general concepts for designing OH- conducting membranes?
- Platinum-free membranes, Ni, Co as electrocatalysts
- Air-cooled system
- Strategy 1: Forming of polymer superstructures (PVA + PDDA + GA)
- Strategy 2: Polymer superstructures + porous substrates for mechanical strength
- What are the "superionic" phase transitions?
- High conductivity at moderate temperature due to specific structural change
- Result of dynamically disordered hydrogen-bond network
- Proton transport through rapid reorientation of XO4 tetraheda
- Name state of the art proton conducting "solid acids".
- Analyze pros and cons for the OH- and H+ solid ion conductors.
- H+
- Pro
- Ready for commercial applications
- Reasonable chemical stability
- Higher current density
- Higher conductivities
- Wider temperature window
- Con
- No operation in intermediate temperatures (200 - 500)
- Weak thermal stability
- Weak long-term and mechanical stability
- OH-
- Pro
- More promising for energy conversion and storage
- Non-noble catalysts
- more flexible design
- reduced maintenance cost
- Con
- Low conductivities
- Low current densities
- Low long-term stability
- Give a definition: what are ionic liquids?
- Salt that is liquid at temperatures below 100°C
- Low melting point, composed of ions only
- Yielded by any salt melting without decomposing or vaporizing
- First: Ethylammonium Nitrate
- Different classes
- Aprotic: Suitable for lithium batteries & supercapacitors
- Protic: Suitable for FC
- Zweitterionic: Suitable for ionic-liquid-based membranes
- Analyze their possible prospective roles in energy science.
- Growing interest
- Advanced batteries (solid electrolyte)
- Dye-sensitized solar cells
- Double layer capacitors
- Actuators
- Fuel Cells
- Thermo-Cells
Lecture 7
- What are the working principles for supercapacitors?
- Energy storage in the double layer, at the interface between electrolyte and electrode
- $E=1/2CV^2$
- How to model the electric double layer between solid electron conductors and aqueous electrolytes?
- (inner + outer) Helmholtz layer, diffusion layer, electrolyte
- Creation of opposing polarized layers at interface with solvent barrier, acting as capacitor
- How to increase the double layer capacitance?
- $\displaystyle C = \frac{\varepsilon _r\varepsilon_0A}{d}$, $\varepsilon_r$ dielectric constant of electrolyte
- Increase surface area or decrease layer width
- What are the common requirements for the electrode materials used in supercapacitors?
- High conductivity
- High surface area range
- Good corrosion resistance
- High temperature stability
- Controlled pore structure
- Processability and compatibility in composite materials
- Cost
- What are the surface limited Faradaic processes? How to use them in order to increase the performance of supercapacitors?
- Double-layer capacitance leakage
- Specific adsorption of ions penetrating surface water layer and contacting electrode directly
- Capacitance increase due to additional current generated (shortlived due to limited surface contacts)
- Usage: Try to organize decay / leakage of shortlived effect to persistently increase capacitance
- Requirement of immediate redox response to potential change property of surface limited reactions
- Name state-of-the-art electrode materials for supercapacitors (double layer, pseudocapacitive materials). What are possible alternatives?
- Double-layer
- Carbon nanotubes to maximize surface area with light electron conductivity
- Pseudocapacitive, coating on the electrodes
- RuO2
- Alternatives: Co-, Ni-, Mn-Oxides
- What are typical composites and organic electrolytes for supercapacitors?
- H2SO4
- Aqueous electrolytes
- Ionic liquids (Activated Carbon)
- Analyse what are limitations and possible application areas of supercapacitors.
- Limitations
- Non-constant voltage
- Quick self-discharge
- High cost
- Low specific energy
- Possible applications
- Energy conversion
- Automotive
Lecture 8
- Give a definition of a primary cell or battery and a secondary cell or battery.
- Primary: non-rechargeable
- Secondary: rechargeable through reverse current direction
- Analyze what is the role and capacity of batteries vs other energy provision devices.
- medium specific energy over moderate time
- Explain working principles of the alkaline manganese dioxide batteries.
- 25 GEuro market annually
- Spatially separated spontaneous redox
- Ion-conductive separator between electrolyte chambers
- Stainless steel current collector -> MnO2 + Graphite (to increase conductivity) electrolyte -> Ion transfer -> Zinc + KOH electrolyte -> Brass current collector
- Cathode: $2MnO_2+H_2O + 2e^- \rightarrow Mn_2O_3 + 2OH^-$
- Anode: $Zn + 2OH^- - 2e^- \rightarrow ZnO + H_2O$
- Overall: $Zn + 2MnO_2 \rightarrow_{KOH} ZnO + Mn_2O_3$
- Electrons move only a bit beyond electrode surface into electrolyte
- What are state-of-the-art materials for these batteries?
- MnO2
- KOH electrolyte
- Gelling agent Carboxymethylcellulose
- Analyze advantages vs disadvantages of primary batteries.
- Higher specific energy than secondary
- Waste generation
- Low internal resistance
- Wide working temperature range
- Slow self discharge
- Low cost
- Explain the working principles of a simple lead acid battery.
- 16 GEuro market annually
- H2SO4 electrolyte
- Lead-Oxide Cathode coating (mixed e/ionic conductor)
- Lead Anode coating
- Discharge
- $\Delta E ~ 2.1V$
- Cathode: $PbO_2 + 4H^+ + SO_4^{2-} + 2e^- \rightarrow PbSO_4 + 2H_2O$
- Anode: $Pb + SO_4^{2-} - 2e^- \rightarrow PbSO_4$
- Building of $PbSO_4$ on electrode surfaces
- Charge
- $2.5V$
- Fast and almost fully reversible
- Cathode: $PbSO_4 + 2H_2O - 2e^- \rightarrow PbO_2 + 4H^+ + SO_4^{2-}$
- Anode: $PbSO_4 + 2e^- \rightarrow Pb + SO_4^{2-}$
- What are working principles and materials used in Zinc-Air batteries?
- $1.65V$
- Zinc Anode: $Zn + 4OH^- \rightarrow Zn(OH)_4^{2-} + 2e^-$
- Decay: $Zn(OH)_4^{2-} \rightarrow ZnO + H_2O + 2OH^-$
- Porous air cathode (O2 inflow): $O_2 + 4e^- + 2H_2O\rightarrow4OH^-$
- Overall reaction: $2Zn + O_2 \rightarrow 2ZnO$
- Parasitic reaction: $Zn + 2H_2O \rightarrow Zn(OH)_2 + H_2$
- Analyze advantages and disadvantages of Zinc-Air batteries (and issues with materials for them).
- Silver and Pt as catalysts for Oxygen reduction (ORR), not suitable for recharge (OER (evolution))
- Strategy: 3-electrode setup with different ORR & OER catalysts
- Precipitation of non-conduction ZnO
- Growth of Zn-dendrites with charge & discharge cycles
- Parasitic Hydrogen evolution
- How it is possible and why it is promising to use Zn-Air and Al-air batteries for automotive applications?
- Mechanically rechargeable batteries using solar power to recycle ZnO
- Al-Air more power, 1.2V potential difference with KOH electrolyte, but only mechanically rechargeable (almost as good as Li-Ion)
Lecture 9
- Give a definition to intercalation process.
- Reversible inclusion or insertion of molecule / ion into compounds with layered structures
- What are typical "intercalation compounds"?
- Name state of the art and varieties of materials for Li-ion batteries.
- What was the first battery using an intercalation compound?
- $xLi + TiS_2 \rightarrow Li_xTiS_2$
- Titanium disulfide cathode
- Lithium-aluminum anode
- $LiPF_6$ electrolyte dissolved in Propylene carbonate
- What are the working principles of Li-ion batteries?
- Transfer of Lithium ions from cathode into graphite anode
- What are typical LiMO2 compounds for battery applications?
- $LiCoO_2$
- $LiFePO_4$: longer life cycle and more environmentally friendly
- $LiMn_2O_4$: high thermal stability, low cost
- Analyze further possible directions for the development of Li-ion batteries.
- Increase of surface area for high capacity electrodes
- Overcoming mechanical issues: Changes in physical dimensions during intercalaction
- 3d nanostructures as key to ultrafast-charge batteries (3d-printing)
- Why is Si more promising material to replace graphite in Li-ion batteries?
- Over ten times larger mAh/g
- What are working principles of aqueous and nonaqueous Na-ion batteries?
- Na-Ion works the same as Li-Ion but also supports water as electrolyte, but is limited by electrochemical stability of water
- Name state of the art electrode materials for these devices.
- NASICONs
- $Na_3V_2(PO_4)_2F_3$
- $Na_2Ti_3O_7$
- Hard-Carbon anode
- Prussian blue based
- Analyze promises and challenges in the development of Li-air batteries.
- Energy density near gasoline
- Challenges
- Finding Electrocatalysts: Au/C vs Pt/C
- Finding Electrolytes
- Stability with lithium
- high oxidation potential
- low vapor pressure and high boiling point
- High lithium salt solubility and chemical stability
- Ionic liquids?
Lecture 10
- Explain what is the concept of "artificial leafs"?
- $2H_2O \rightarrow_{\text{sun}} O_2 + 2H_2$
- $2H_2 + 2CO_2 \rightarrow [CH_2O]$
- Synthetic photosynthesis with improvement in conversion to more usable fuels
- What are the main approaches in artificial photosynthesis?
- Mimicking natural process but replacing biological agents with solid state materials
- $2H_2O \rightarrow O_2 + 4H^+ +4e^-$
- Significant simplification of the system using semiconductors
- Attempts to immobilize natural objects (at surface of conducting solids)
- Simply (photo-)catalyst in solution
- Nanoparticles
- Self contained
- Difficult to extract H2
- How one can use semiconducting materials in water splitting without an external bias?
- Single crystal TiO2 photoanode and Pt cathode + bias: Honda-Fjuishima effect
- Give a definition to photocatalysts and corresponding co-catalysts.
- Catalyst accelerating photoreaction
- TiO2 + Pt
- What are the main factors affecting the activity of photocatalytic materials?
- Structure and electronic properties
- Requirements:
- band gap narrower than 3 eV
- band edge potientials suitable for water splitting
- stability under reaction conditions
- Relevant process steps
- Photocatalyst adsorption of photon energy larger than band gap energy, generation of photoexcited electronhole pairs
- Separation of photoexcited carriers and separate migration to surface
- Participation of H2O / H+ in redox process at surface for production of H2 and O2
- Explain the idea of the two-step photoexcitation systems.
- Spatially separated evolution of first O2 and then H2 using separate photo-redox with different catalysts
- Possibility to use visible light
- What is the idea behind the photo-driven water oxidation on protected semiconductors?
- TiO2 protective layer above second photo-electrode shielding from corrosion due to water contact and translucent enough to activate protected semiconductor like Ni
- Analyze promises and challenges with respect to the "artificial leaf" approach.
- Instability and low efficiency
- Required usage of co-catalysts when using semiconductor photocatalysts
- Use of molecular complexes
- Immobilizing of objects of nature
Lecture 11
- What is the "photovoltaic (PV) effect".
- Current induced into solid electrodes in contact with electrolyte under illumination
- Best results: UV light + AgCl coated electrodes
- What are intrinsic semiconductors (schematic energy diagrams)?
- Pure semiconductor with same number of excited electrons as holes
- Hole in valence band can carry current
- Electron in conduction band can carry current
- Band Gap $\Delta E_g = E_{\text{Conduction}} - E_{\text{Valence}}$
- $E_F$ Fermi level, expression of density and avg. energy of quantum states of electrons, 50% between $E_V and $E_C$
- Constant generation of electron-hole pairs through external / thermal energy
- What are doped semiconductors (schematic energy diagrams)?
- Si lattice
- P-type
- Boron doped, missing electron in lattice
- $E_F$ closer to $E_C$
- Positive electrode, negative electrolyte charge layer formation
- N-type
- Phosphorus doped, extra electron
- $E_F$ closer to $E_V$
- Negative electrode, positive electrolyte charge layer formation
- TiO2 usually n-doped due to stoechiometric oxygen deficiency
- Explain the mechanisms of the space charge layer formation at the semiconductor / electrolyte and semiconductor / semiconductor interfaces.
- Lack of free electrons at boundary between materials where holes from one side are completed by free electrons from other side
- Generation of electron-hole pairs
- High resistance since no possibility for current flow
- High drift: small depletion region, drift means atoms retain their electrons
- Diffusion: wandering of electrons
- Explain what the p-n junction is.
- Basically a diode
- Creation of driving force of electron flow upon photoexcitation
- Formation of electric field
- Contact plane between P and N-doped materials
- Applied voltage controls flow of current: With diffusion or with drift
- Explain the working principles of Si-based PV cells.
- Thin highly doped N layer on top
- Thick lightly doped P layer below
- Very thick depletion region
- What are the Shockley-Queisser Efficiency Limits?
- 33% at 1.34eV band gap
- Maximum theoretical efficiency of solar cell using p-n junction
- Assumptions
- 1 semiconductor material
- 1 pn junction
- unconcentrated light
- all energy is converted to heat from photons greater than band gap
- What are the main requirements for an ideal solar cell materials?
- direct band structure with band gap between 1.1 and 1.7 eV
- consisting or readily available and non-toxic materials
- good photovoltaic conversion efficiency
- long-term stability
- What are the common losses in solar cells and how to minimize these losses?
- recombination
- series resistance
- thermal (photon energy >> band gap): counteracted by increasing porosity
- metal / semiconductor contact
- reflection: Minimize through surface morphology with spikes
- Explain the principles of the photo-electrochemical water splitting.
- Need photon energy potential above water bond of 1.23V
- Photocatalyst + co-catalyst usage to split H2O into H2 and O2
- Hole evolves O2
- Electron evolves H2
- Issues: instability, suffering from corrosion
- Progress: using Cobalt materials to prolong durability
Lecture 12
- What is the concept of multiple junction (tandem) solar cells?
- Several cells to process different energy level photons
- High at the surface, lower below
- Maximizing energy yield at each layer -> Schockley-Queisser-Limit
- Approaches
- Independently connected layers
- Series connected
- Hybrid contacts (layers + external)
- What are the requirements to the materials for photovoltaic applications considering tandem solar cells?
- Atomic structures must be similar in atomic spacing (entire device)
- Requirement of multiple band gaps available
- High material quality for efficient charge carrier collection
- High quality intermediate materials as inactive layers for charge carrier transport
- Wide ranges of doping levels need to be attainable and controllable
- Tandem solar cells: what are the main difficulties in design and material choices?
- Design
- Series
- Transparent electrode
- High bandgap
- Recombination layer
- Low bandgap
- Back electrode
- Parallel
- Transparent electrode
- High bandgap
- Transparent middle electrode
- Low bandgap
- Back electrode
- Material
- Electron barrier
- Electron potential well
- High potential well for holes
- What are the "best-performing" state of the art multi-junction materials and structures?
- GaInP/GaAs//GaInAsP/GaInAs structures, 44% efficiency
- Four-junction solar cell
- What are thin film solar cells?
- What are the working principles of the polymer solar cells and what are the key properties determining their performance?
- Light entry through transparent anode
- Adsorbed in bulk heterojunction layer through generation of excitons
- Diffusion of excitons in bulk until recombination or reaching of donor-acceptor interface, separation into electrons & holes
- Movement of electrons & holes to respective anode (hole) & cathode (electron) through donor & acceptor material phase
- Polymer solar cells: what are advantages and challenges?
- Narrow repertoire of materials, especially acceptor
- lack of precise structural information on bulk heterojunction layer
- Fast degradation of key material properties
- Relatively cheap materials and manufacturing
- Flexibility in production and applications
- Efficiency: 5-12%
- Explain the working principles of dye-sensitized solar cells and perovskite solar cells.
- DSSC
- TiO2 coated with Ruthenium-material dye
- Electron in ruthenium die ejected from photoexcitation and transport out of conductive glass electrode
- Re-transport of electron from inert cathode via Ion transport (electrolyte)
- Perovskite
- Evolution over DSSC
- Perovskite material als powerful charge conductor material
- What are photovoltaic-thermal systems?
- Dual use of solar power for PV and heat energy collection
- PV + water heating tube
Lecture 13
- What is "the TW challenge"?
- Fossil fuels currently 85% of the 18 TW global power consumption
- Requirement for new energy sources to be scalable to TW level
- What is the rate of energy consumption worldwide?
- Currently 18 TW
- Estimated 30 TW by 2050
- What is the concept of important "technology elements"?
- Elements that are important for energy technologies but also very required for other field
- Analyze the availability of materials for photovoltaics.
- 4.5 yr current Si production for 200um thick cells for 1 TW
- 150 yr of current solar panel production for 1 TW
- Analyze availability of materials for fuel cells and electro-catalytic water splitting.
- SOFC
- YSZ: 21 moneth yttrium production
- 17000m3 ion conducting materials
- PEMFC
- 300t Pt: 20 months production
- Water splitting
- 20 month Pt
- Ni, Co, Pr abundant
- Iridium: 35 years
- Analyze electrochemical storage of energy in batteries as an example of a mass-specific problem: what are the state of the art materials and challenges?
- 24h of 1 TW storage: 6.8x10^16 J
- lead acid: 6.1x10^11 kg, 100 yr production
- Li-ion: 1.4x10^11 kg, 15 yr graphite + 160 Li production
- Na-ion: 8.6x10^8 kg, 9 yr Na, 20 yr graphite (100yr Ti for NASICON), 10 month Fe
- Supercapacitors: 4.3x10^11 kg
- Scalability to the 10+ TW level: analyze the current state-of-the-art.
- Wind energy: 2 months of steel production for 1 TW, but 50 yr Dy, 40 yr Nd (for magnets)
- Fischer-Tropsch synthesis on gasified biomass (cellulose)
- Fission: 10 yr U production