Electrochemistry for Engineers 0581.5271 Electrochemistry for Engineers LECTURE 9 Lecturer: Dr. Brian Rosen Office: 128 Wolfson Office Hours: Sun 16:00

Fuel Cells

Important Definitions Coulombs – Unit of CHARGE Amperes – Unit of CURRENT [Coulombs per second] Volts – Unit of POTENTIAL [Joules per Coulomb] Watt – Unit of POWER [Joules per second] Joule – Unit of ENERGY [Watts x seconds] Watt-hour (Wh) is also a unit of energy [Watts x hours]

Important Definitions Pt 2 POWER DENSITY – Rate of Energy Transfer per unit volume or mass [kW/m3 or kW/kG]   ENERGY DENISTY – The amount of energy stored in a given system [kJ/m3 or kJ/kg]

Electrical Power Production Chemical Energy Nuclear Energy Combustion Fission Fusion Fuel Cells Heat Steam Engine IC Engine Mechanical Energy Thermoelectric Electrical Energy (current)  Solar and Wind Energy

Fuel Cell vs. IC Engine

When to use a Fuel Cell - Temp

Electrochemical Systems

Electrochemical Systems

Power Density of Systems

Power Density Cont’d

Fuel Cell-Based Technologies Stationary / Back-up Portable Transportation Direct Liquid Fuel Cells Liquid Fuel FC electronics Ancillaries Air

Common Types of Fuel Cells Alkaline (AFC) Polymer Electrolyte Membrane (PEMFC) Phosphoric Acid (PAFC) Polymer Electrolyte Membrane (PEMFC) Types of Fuel Cells Fuel cells are classified by the type of electrolyte they use (Table 1). The different electrolytes operate at different temperatures. Low-temperature fuel cells include the alkaline fuel cell (AFC), the proton exchange membrane fuel cell (PEMFC), and the phosphoric-acid fuel cell (PAFC). All of these fuel cells use hydrogen as a fuel. This hydrogen can be extracted from natural gas, biogas, methanol, or propane by the process of reformation. The hydrogen can also be created through the electrolysis of water. High-temperature fuel cells include the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC). These fuel cells offer the advantage that they can use either natural gas or untreated coal gas as a fuel directly without the use of a reformer through a process called "Direct Internal Reforming". The sixth type of fuel cell is the direct methanol fuel cell (DMFC). It is also a low temperature fuel cell, but it can use methanol as a fuel directly without the need of reforming. The primary limitation to the direct methanol fuel cell is that the rate of reaction on the anode is very slow. This results in a very low operating voltage output. For certain applications (e.g., portable power), the direct methanol fuel cell may be ideal. Molten Carbonate (MCFC) Direct Methanol (DMFC) Direct Methanol (DMFC) Solid Oxide (SOFC) Solid Oxide (SOFC)

Membrane Electrode Assembly PEM Fuel Cells Voltage = 0.6 V Cathode Reaction O2 + 4H+ + 4e-  2H2O Air Anode Cathode e- Membrane Electrode Assembly Gas Diffusion Layer H2 Bipolar plate Load H+ Anode Reaction 2H2  4H+ + 4e- NIST Platinum Catalyst Carbon black

Single Cell PEMFC

A small stack of about 10 cells PEM Fuel Cell Stacks NREL A small stack of about 10 cells NMSEA 3kW, 48V fuelcellstore.com

Advantages of Fuel Cells Challenges Facing Fuel Cells Pro’s and Cons of PEM FCs Advantages of Fuel Cells 1. Higher efficiency compared to IC engines 2. Zero emissions at the point-of-use (PEM) 3. No moving parts in the stack, so quieter Challenges Facing Fuel Cells 1. Cost (materials, labor, economy of scale) 2. Durability (membrane, catalyst) 3. Lack of H2 Infrastructure: H2 is difficult to produce, transport, and store

Pro’s and Con’s Pt 2 Advantages: High efficiency High energy density Fuel flexibility Environmental clean Limitations: Fuel Crossover Transport Issues Water Management (cathode flooding and anode dry-out) Litster et al., Journal of Power Sources,130, 2004, 61

Direct Methanol Fuel Cell (DMFC) (acidic conditions)

Progress Over the Years! This chart shows the dramatic increase in fuel cell investments for transportation from the governments in Canada, Europe and Japan. Primary focuses included research & development, demonstration & validation, providing market entry support, and investments in fueling infrastructure for fuel cell vehicles. This chart shows the relative recent domination of PEM fuel cell technology, a newer type of fuel cell technology, that holds the most promise for our first fuel cell vehicles. Some will tell you that fuel cells have been around for decades. While it’s true that fuel cell technology was invented in 1839, it was a much older technology. Fuel cell technology suitable for vehicles is a relatively recent achievement. Dramatic increase in public and private investment since 1983 as shown by the steady rise in Automotive Fuel Cell Patents

Reminder: Thermodynamic Potentials

Reminder: Products-Reactants

Fuel Cell Efficiency Where.. Where..

Overpotential Losses in Fuel Cell

Total Energy Lost as Heat Note that EH is not a real potential, but instead a theoretical potential assuming 100% thermodynamic Efficiency. Therefore, EH will be higher that the open circuit potential.

Power Density Curve

Activation Region

Exchange Density for H2 Oxidation + =

Exchange Density’s for PEM

Recall: Activation Barrier

Current at Sacrifice of Voltage Great catalyst Good catalyst Poor catalyst

Ohmic Region

Ohmic Overpotential Charge transport between the electrodes is not a frictionless process, therefore, there is a penalty of a loss of FC voltage j = charge flux σ = conductivity j = σ (dV/dx) = σ (V/x) V = i(L/Aσ ) = iR ηohm = i(Relec + Rionic) ≈ iRionic Rionic is on the order of 500-1000 mΩ V V=iR=jL/σ L x

Direction of Voltage Drop Anode (-) Cathode (+) Anode (-) Cathode (+) The voltage drop MUST be negative from the anode to the cathode in order to provide the driving force for migration towards the cathode.

Requirements for Electrolyte High ionic conductivity Low electronic conductivity High stability for oxidation and reduction Low fuel crossover Mechanical strength Easily manufactured

Sulfonic Acid (SO3- H+) Membranes Membranes contains phase-separated regions on the order of 5 nm in diameter These swell with water uptake.

Endurance (Durability) Test Results for Gore Primea 56 MEA at Three Current Densities (hydrogen fuel) Polarization Curves for 3M MEA (hydrogen fuel)

Laminar Flow Fuel Cells (LFFC) Depletion Depletion Diffusion

Mass Transport Region

Mass Transport at PEM Anode

Depletion Still Occurs: Harder to Model

A Familiar Solution

Constant Flow or Stoichiometry Constant Flow – The flow rate of fuel is constant regardless of the required current. Typically enough fuel is provided for maximum power Constant Stoichiometry – The fuel is provided for 100% fuel utilization at a given current density (λ=1). λ = 1.5 means 50% extra fuel is provided

Constant Flow or Stoichiometry

Concentration Affects Nernst Voltage

Concentration Affects Nernst Voltage

Concentration Affects Rate of Reaction

High porous materials to achieve intimate contact between gas phase Triple Phase Boundary High porous materials to achieve intimate contact between gas phase pores, electrically conducting catalyst and ionic conducting electrolyte -Mechanical Strength -Electrical conductivity -Low corrosion -High porosity -Easily manufactured -High j0

Compact Mixed Reactors (CMR) A mixture of fuel and oxidant flows through a fully porous anode-electrolyte-cathode assembly. CMR cells require selective electrode catalysts (Ru/Se), the need for key enabling materials in direct methanol fuel cell development programmes. www.cmrfuelcells.com

Overall Analysis of Losses

Resistors in Series