High Temperature Fuel Cells Notes

High Temperature Fuel Cells

Introduction

• Advantages of High-Temperature Operation:

  • Faster electrochemical reactions leading to lower activation voltage losses and reduced need for noble metal catalysts.

  • Heat available for hydrogen extraction from readily available fuels like natural gas.

  • Valuable heat source for buildings and industrial processes, making them excellent Combined Heat and Power (CHP) systems.

  • High-temperature exit gases can drive turbines, producing additional electricity through a "bottoming cycle".

• Types of High-Temperature Fuel Cells:

  • Molten Carbonate Fuel Cell (MCFC): Operates around 650°C. Primary challenges involve component degradation over time.

  • Solid Oxide Fuel Cell (SOFC): Offers mechanical simplicity due to its solid-state nature. It's flexible in terms of materials and size, operating between 500-1000°C.

• MCFC and SOFC should be considered as part of an integrated fuel processing and heat generation system.

Common Features

Fuel Reforming

• Steam Reforming:

  • Hydrogen production from hydrocarbons typically involves steam reforming.

  • For methane, the reaction is: $CH<em>4 + H</em>2O \rightarrow 3H_2 + CO$ [8.1]

  • For any hydrocarbon $C<em>xH</em>y$: $(C<em>xH</em>y + xH<em>2O \rightarrow (x + y/2)H</em>2 + xCO)$ [8.2]

  • Steam reforming reactions are endothermic, requiring heat input. High-temperature fuel cells can use exhaust heat for this purpose.

  • Using all the heat from the fuel cell exhaust gases for reforming can lead to high efficiency (typically >50% based on Higher Heating Value - HHV).

• Carbon Monoxide Management:

  • Carbon monoxide produced during steam reforming can poison platinum catalysts used in PEMFCs and PAFCs.

  • The water-gas shift reaction is used to reduce CO content: $CO + H<em>2O \rightarrow CO</em>2 + H_2$ [8.3]

• Desulfurization:

  • Natural gas often contains sulfur compounds, which are catalyst poisons and can deactivate fuel cell electrodes. Therefore, sulfur removal is essential.

Fuel Utilization

• Fuel utilization is crucial when hydrogen is part of a mixture, such as after internal reforming.

• Low hydrogen partial pressure due to high usage significantly lowers cell voltage.

• The effect of pressure and gas concentration on open circuit voltage is described by:

  • $E = E<em>0 + \frac{RT}{2F} \ln(\frac{P</em>{H<em>2} P</em>{O<em>2}^{1/2}}{P</em>{H_2O}})$

  • Change in voltage with hydrogen partial pressure changing from $P<em>1$ to $P</em>2$:

  • $\Delta V = \frac{RT}{2F} \ln(\frac{P<em>2}{P</em>1})$

• As hydrogen is used up, P2 < P1, resulting in a negative $\Delta V$.

• Higher-temperature fuel cells experience a greater voltage drop due to the RT term.

• Oxygen utilization from air also causes a voltage drop as the partial pressure of oxygen decreases.

• Voltage drop can be reduced by using counter-flow, where air and fuel flow in opposite directions, ensuring higher oxygen partial pressure where fuel exits.

• Complete hydrogen consumption is not feasible with reformed fuels containing carbon dioxide or when internal reforming is used. Some hydrogen must pass through for fuel processing or heat generation.

Bottoming Cycles

• Bottoming cycle involves using waste heat from fuel cell exhaust gases to drive a heat engine.

  • Heat can be used in a boiler to produce steam and drive a steam turbine.

  • The entire system, including the fuel cell, can be pressurized to power a gas turbine (GT) with the exit gases.

  • Combining steam and gas turbines with a fuel cell in a triple cycle system is another possibility.

• Combined cycle systems provide an efficient approach to electricity generation.

Molten Carbonate Fuel Cell (MCFC)

How it works

• Electrolyte:

  • A molten mixture of alkali metal carbonates (typically lithium and potassium, or lithium and sodium carbonates).

  • Retained in a ceramic matrix of $LiAlO_2$.

• Operating Temperature:

  • 600-700°C.

  • Carbonates form a highly conductive molten salt with carbonate $(CO_3^{2-})$ ions for ionic conduction.

• Reactions:

  • Carbon dioxide is supplied to the cathode and converted to carbonate ions, facilitating ion transfer.

  • At the anode, carbonate ions convert back into $CO_2$.

• Overall Reaction:

  • $H<em>2 + \frac{1}{2}O</em>2 + CO<em>2(cathode) \rightarrow H</em>2O + CO_2(anode)$ [8.14]

• Nernst Potential:

  • $E = E<em>0 + \frac{RT}{2F} \ln(\frac{P</em>{H<em>2} P</em>{O<em>2}^{1/2}}{P</em>{H<em>2O}}) + \frac{RT}{2F} \ln(\frac{P</em>{CO<em>2(cathode)}}{P</em>{CO_2(anode)}})$ [8.15]

  • If $CO<em>2$ partial pressures are identical at both electrodes, the cell potential depends on the partial pressures of $H</em>2$, $O<em>2$, and $H</em>2O$.

• $CO_2$ Recycling:

  • $CO<em>2$ generated at the anode is typically recycled to the cathode by combusting unused hydrogen or fuel gas into water and $CO</em>2$.

  • The exhaust gas is mixed with fresh air and fed to the cathode inlet.

• Catalysts:

  • Nickel (anode) and nickel oxide (cathode) are adequate catalysts at MCFC temperatures, eliminating the need for noble metals.

• Fuel Flexibility:

  • MCFCs can electrochemically convert carbon monoxide directly and internally reform hydrocarbon fuels.

• Carbon Monoxide as Fuel:

  • Reactions at each electrode if carbon monoxide is used as fuel.

• Electromotive Force (EMF):

  • Calculated similarly to the hydrogen fuel cell.

  • The formula for the "no loss", reversible OCV is: $E = -\frac{\Delta g}{F}$

  • At 650°C, values for hydrogen and carbon monoxide are similar.

• Water-Gas Shift Reaction:

  • If the fuel gas contains both $H_2O$ and CO, the electrochemical oxidation of CO proceeds via the water-gas shift reaction (equation 8.3).

• Internal Reforming:

  • Achieved due to the high operating temperature.

  • Steam is added to the fuel gas before entering the stack, reacting in the presence of a catalyst (reactions 8.1 and 8.2).

  • Heat for the endothermic reforming reactions is supplied by the electrochemical reactions.

• Advantages:

  • Higher overall system efficiencies and greater fuel flexibility.

• Disadvantages:

  • Severe demands on corrosion stability and component lifespan due to high temperatures and the aggressive molten carbonate environment.

Implications of Using a Molten Carbonate Electrolyte

• Electrolyte Management:

  • MCFC relies on a balance in capillary pressures to establish the electrolyte interfacial boundaries in the porous electrodes.

  • Coordination of pore diameters in electrodes and electrolyte matrix establishes electrolyte distribution.

  • Optimum distribution of molten carbonate electrolyte is critical for performance and endurance.

• Undesirable Processes:

  • Corrosion reactions, potential-driven migration, salt creepage, and salt vaporization contribute to the redistribution of molten carbonate.

Cell Components in the MCFC

• Electrolyte:

  • Typically contains 60 wt% carbonate within a 40 wt% $LiAlO_2$ matrix.

  • The γ form of $LiAlO_2$ is the most stable, used in the form of fibers (< 1 μm diameter).

  • Matrices are made using tape-casting methods.

• Tape Casting:

  • Dispersing ceramic materials in a solvent containing binders, plasticizers, and additives.

  • The mixture (“slip”) is cast as a thin film over a smooth surface, with thickness controlled by a blade device.

  • The slip is dried, and organic binders are burnt out at 250-300°C, forming a semi-stiff “green” structure.

• Large-Area Components:

  • Tape casting provides a means of producing large-area components.

• Ohmic Losses:

  • Electrolyte accounts for 70% of ohmic losses.

  • Losses are dependent on electrolyte thickness: $\Delta V = 0.533t$ volts, where t is the thickness in centimeters.

  • Thinner electrolytes (0.25-0.5 mm) reduce ohmic resistance but require balancing with long-term stability.

• Power Density:

  • power density at 650°C is 0.16 W/cm2

• Special Notes:

  • Carbonate absorption into the ceramic matrix (over 450°C) causes significant stack component shrinkage.

  • A reducing gas must be supplied to the anode during heating to maintain nickel in a reduced state.

  • MCFC stacks take a long time to condition.

  • Heating and cooling cycles through the electrolyte melt temperature create stresses, requiring the process to be slow.

  • Anode must be protected from re-oxidation during shutdowns by purging with inert gas.

• Anodes:

  • Made of porous sintered Ni-Cr/Ni-Al alloy, with a thickness of 0.4 to 0.8 mm and porosity between 55-75%.

  • Chromium (10-20%) is added to reduce nickel sintering.

  • Aluminum improves creep resistance and reduces electrolyte loss by forming $LiAlO_2$ within nickel particles.

  • Partial flooding of the anode with molten carbonate acts as a carbonate reservoir.

• Cathodes:

  • Nickel oxide is the current state-of-the-art cathode material.

  • Nickel dissolution in molten carbonates is a major problem.

  • Nickel ions formed in the electrolyte diffuse towards the anode and precipitate out as metallic nickel.

  • Dissolution worsens at high $CO<em>2$ partial pressures because of the reaction: $NiO + CO</em>2 \rightarrow Ni^{2+} + CO_3^{2-}$ [8.16]

  • Using more basic carbonates reduces this problem.

  • The lowest dissolution rates are found for eutectic mixtures 62% $Li<em>2CO</em>3$ + 38% $K<em>2CO</em>3$ and 52% $Li<em>2CO</em>3$ + 48% $Na<em>2CO</em>3$.

  • Addition of alkaline earth oxides (CaO, SrO, and BaO) is also beneficial.

• Minimizing Nickel Dissolution:

  • Using a basic carbonate.

  • Operating at atmospheric pressure and keeping the $CO_2$ partial pressure low.

  • Using a relatively thick electrolyte matrix to increase the $Ni^{2+}$ diffusion path.

• Bipolar Plates:

  • Fabricated from thin sheets of stainless steel with a nickel-coated anode side.

  • Gas-tight sealing is achieved by allowing electrolyte from the matrix to contact the bipolar plate at the cell edge.

  • The bipolar plate is coated with a thin layer of aluminum to prevent corrosion, forming a protective layer of $LiAlO<em>2$ after reacting with $Li</em>2CO_3$.

Internal Reforming

• Catalyst coupling:

  • Coupling the reforming reaction and the anode electrochemical reactions shifts the reforming reaction forward.

• Direct Internal Reforming (DIR):

  • Offers high cell performance advantages compared to Indirect Internal Reforming (IIR).

  • Hydrogen (major product) is consumed directly by the electrochemical reaction.

• Metal Catalyst incorporation:

  • MCFC stack requires a supported metal catalyst for internal reforming.

  • conventional low surface area porous nickel anode has insufficient catalytic activity to support the steam reforming reaction at 650°C.

• Catalyst Pointers:

  • Eliminates the cost of an external reformer and system efficiency is improved, but at the expense of a potentially more complex cell configuration.

• Key Requirements for MCFC Reforming Catalysts:

  • Sustained activity to achieve desired cell performance and lifespan. Optimization of reforming catalyst activity is important to ensure that such temperature variations are kept to a minimum, to reduce thermal stress, and thereby contribute towards a long stack life.

  • Resistance to poisons in the fuel. raw hydrocarbon fuels contain impurities (e.g. sulfur compounds) that are harmful for both the MCFC anode and the reforming catalyst.

  • Resistance to alkali/carbonate poisoning. catalysts are located close to the anode, there is a risk of catalyst degradation through reaction with carbonate or alkali from the electrolyte.

• Poisoning of DIR MCFC:

  • Occurs principal routes: creep of liquid molten carbonate along the metallic cell components and transport in the gas phase in the form of alkali hydroxyl species.

  • Inserting a protective porous shield between the anode and the catalyst is one solution

Performance of MCFCs

• Operating Range:

  • MCFCs operate in the range of 100 to 200 mA/cm2 at 750 to 900 mV per cell.

• Cathode Polarization:

  • Significant cathode polarization occurs in the MCFC.

• Operating Pressure:

  • Benefits of pressurized operation are significant only up to about 5 bar.

  • Above 5 bar, system design constraints introduce disadvantages.

  • From the Nernst equation, $\Delta V = \frac{RT}{4F} \ln(\frac{P<em>2}{P</em>1})$

  • Generally uneconomical to pressurize MCFC systems of less than 1 MW.

• Differential Pressure:

  • The pressure difference between the anode and cathode should be kept low to reduce gas crossover risk.

• Temperature Influence:

  • Thermodynamic calculations indicate that reversible potential decreases with increasing temperature.

  • The cathode polarization dominates the influence of temperature under real operating conditions.

  • Above 650°C, the effect is very slight (0.25 mV per °C).

  • Optimum operating temperature is generally regarded as 650°C due to increased electrolyte evaporation and material corrosion at higher temperatures.

Practical MCFC systems

• MTU Friedrichshafen