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
    1. Reactor
    2. Separator -> Recycles hydrogen
    3. 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.
- see below
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".
- CsHSO4
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"?
- Graphite derivatives
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
  1. Photocatalyst adsorption of photon energy larger than band gap energy, generation of photoexcited electronhole pairs
  2. Separation of photoexcited carriers and separate migration to surface
  3. 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
    1. Transparent electrode
    2. High bandgap
    3. Recombination layer
    4. Low bandgap
    5. Back electrode
  - Parallel
    1. Transparent electrode
    2. High bandgap
    3. Transparent middle electrode
    4. Low bandgap
    5. 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?
1. Light entry through transparent anode
2. Adsorbed in bulk heterojunction layer through generation of excitons
3. Diffusion of excitons in bulk until recombination or reaching of donor-acceptor interface, separation into electrons & holes
4. 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