Fuel Cells: Hydrogen Storage & Thermodynamics

• Fundamentals and Technology of Fuel Cells is summarized below

Hydrogen Storage

• Two methods are available for hydrogen storage: Gaseous and Liquid
• Gaseous hydrogen stored in containers (gas holders, tubes) for bigger storage and composite tanks (350/700 bar) for transportation
• Gaseous hydrogen storage can make use of rock salt caverns for bulk storage and depleted oil and gas fields
• Pipelines are not used for storing hydrogen due to its low volumetric energy density and low allowable pressure drop
• Liquid hydrogen uses compounds (methanol, ammonia), hydrates are niche, LOHC, and liquefication

Electrochemisty – Thermodynamics

• Renewable power for hydrogen is Capacity factor * Average power demand (~60 GW in Germany)
• During anode process hydrogen releases electrons and becomes negatively charged, resulting to lower potential
• During cathode process hydrogen uses electrons and becomes positively charged, resulting to higher potential
• Cell potential = Cathode potential - Anode potential

Half Cell Reactions and Electrolysis

• Proton Conduction Membrane (Cation conduction)
• Cations (H+) produced at the anode
• H2 → 2H+ + 2e- (anode)
• 2H+ + 2e- + ½O2 → H2O (cathode)
• 2H+ + 2e- + ½O2 + H2 → H2O + 2H+ + 2e-
• Oxygen ion Conduction Membrane (Anion conduction)
• Anions (O2-) consumed at the anode
• O2- + H2 → H2O + 2e- (anode)
• 2e- + ½O2 → O2- (cathode)
• 2e- + 2O2 + H2 → H2O + 2e- + O2-
• Oxidation takes place at the anode
• Electron donation requires energy input
• The electrical potential difference is the driving force for ionic transportation in electrolysis
• Electrons are transported to the cathode to force reduction

Half Cell Reactions Different Transport Ions

• Solid Oxide Fuel Cells (SOFC) reactions: H2 + O2 → H2O + 2e- ; ½O2 + 2e- → O2-
• Polymer Electrolyte Fuel Cell (PEFC) / Phosphoric Acid Fuel Cells (PAFC) reactions: H2 → 2H+ + 2e- ; ½O2 + 2H+ + 2e- → H2O
• Alkaline Fuel Cell (AFC) reactions: H2 + 2OH- → 2H2O + 2e- ; ½O2 + H2O + 2e- → 2OH-

Definition of Fuel Cells

• Fuel cells are electrochemical cells with continuous fuel and oxidant supply, ideally isothermal and used for electrical energy and heat production

Polarization Curve of a Fuel Cell

• Relevant equations for the graph are:
• E=-ΔH/zF
• EN = E° - RT/z . Σv . lna
• Polarization fuels have different Fuel Cell Type, Electrolyte, AdvantagesDisadvantages, and Operating temperature, given in more detail in following flashcards

Overview of Overpotentials in Fuel Cells

• Overpotential happens when the voltage is needed to drive a reaction in the fuel cell compared thermodynamically which reduces efficiency in cell's reactions
• Activation overpotential happens when limited velocity of transport at borders which reduces efficiency because of Reactants, Electrolyte, Electrodes, Temperature
• Technical Solutions: Higher temperature, Electrode material, Electrode structure
• Regime: Low current density; Nonlinear with current density
• Ohmic losses have influence on Material conductivity, caused by Ohmic resistances of Electrolyte, electrodes, which reduces efficiency
• Technical Solutions: Materials with higher conductivity; Higher temperature(ceramics, ionic conductor, molten salt)
• Regime: Average current density; Linear with current density
• Concentration overpotential because of reactant absence at electrodes occurs because of low concentrations
• Resulting low efficiencies can be solved by adding the technical solutions:
• Regime: High current density; Nonlinear w/ current density
• Higher porosity or thinner electrodes
• Lower current density to avoid reactant shortage
• Lower depletion of the fuel gas

Activation Overpotential and Activation Energy

• Overpotential: The lower the exchange current density is, the better the performance of electrode
• Lowering through optimization and improving catalyst

Nernst Potential and Open Circuit Voltage (OCV)

• Potential drops because of the consumption of reacting gases as it passes the gas channels, getting consumed electrochemically causing pressure and nernst potential drops
• Deviation happens because of internal short circuits can originate from electronic conductivity, Gas crossover and Significant leakage of a gas
• Concentration Polarization is caused when a reacting gas gets too low

Exchange Current Density

• Anodic and cathodic current densities, when the net current flow is equal to zero is known as Exchange current
• Exchange current is a characteristic meter of chemical reaction
• Exchange current cannot be measured directly, j0 is based on current dependence to overpotential
• Tefal Equation is used to calculate it jo = f
• Concentration of the species
• Temperature
• Electrode material type
• Anode Tefal Equation: j = j0 exp (+) and log (j)=log (j0)

Meaning of Transfer Coefficient

• The steepness of the current rise affected by Transfer coefficient alpha a < 0,5: Cathodic branch steeper than anodic branch a = 0,5: Anodic and cathodic branches are symmetrical alpha > 0,5: Anodic branch steeper than cathodic branch

System and Technical Basics

• Condensation begins in smaller pores earlier than in bigger pores than on a even surface
• Smaller dropers evaporate quickly

Nearly mainstream all fuels cells and fuels in energy technology needs hydrogen as a gas Fuel processing transforms fuel cells into hydrogren

• Includes CO so needs to be treated, gas clean up and CO conversion
• Reaction not complete

Gas Process Steps

• Reforming
• Desulfurization
• Reformation is always required to convert the fuel to an hydrogen rich gas that can be used as fuel for the fuel cell. It involves hydrocarbons reacted with steam to form CO, hydrogen and carbon dioxide

Partial Oxidation (POX)

• Process that converts hydrocarbon fuels like propane in a high-temperature catalized reactions
• Less CO results, and the CO can be converted to carbon dioxide

Physical Gas Treatment

• Water separation, dust separation, condensation
• Gas Scrubbers
• Adsorption Layers
• Membranes
• Gas treatment with chemical reactions for removal of sulfur and halogens

The Need for Two Stage Shift Reaction

• 2 step design: High temp smal then low temp - complete clean up

Le Chatelier's Principle

• Systems minimize energy therefore pressure shifts when reactions dont occur.

Desulferization

• Used to desulfurize the fuel gas

Pre Reforming for high Temp Fuel Cells

• Higher hydrocarbons from soote when disiintegrating
• In seperate steam reformer - reformer is front of stack and temp controls selectivity

Sulphur Components

• Odorants are added
• Minimal concentration of odorants
• Transmission without odours

More on Sulfur

• Sulfur components have their own characteristics. They are also adsorbed with Zinc

Direct/Liquid Fuel Cells

• the fuel does not need to be processed before electrocatalyzer which breaks dwon molecules. Lower temp, leads to sustancial drops

DMFC anode Catalysts

• Fuel Crossover can greatly reduce effiency

R&D on Mixed Potential Of DMFC

• Problem: fuel crossover
• Challeng: Reduce permiation of methanol

PAFCs - Materials and Cell Design

• Half of the marix is printed on eletrode , both sids lamiated togheter

PAFC and Corrosion

• The corrosion from electrolyte is because of the vaporized acid electrolyte
• The electrode shall never reach 0.8 V.

Operation and ADG

• Gas dryer chilled - organics removed
• stack maintined at temp
• cathode must be positive

Utiliziation Factors + More on DMFC Cathodes

• Too much reactants result to anode cathosrophe

Ammonia

• can harm cathodes

Polymer Electrolyte fuel Cells and Structures - DMFC anode Catalysts

Electryolyte Membrane

• micro
• macro
• homo

SOFC Materials

• Many types with pros and cons discussed

Overview of hydrogen storage methods including gaseous and liquid approaches. The lecture highlights container types, geological storage options, and the limitations of pipeline use. It also discusses electrochemistry, thermodynamics, anode and cathode processes, and cell potential.