C.6: Electrochemistry, rechargeable batteries and fuel cells

What is a voltaic cell?

- A cell consisting of two different anodes dipped into an electrolyte. - Involve redox reactions in which the anode releases electrons as it is oxidized and the cathode gains electrons as it is reduced. A cell runs out when at least of the chemicals involved in these reactions has been used up.

What is a fuel cell?

- A cell that uses a continuous supply of reactants from an external source. - Uses the reaction between a fuel (ex: hydrogen/methanol) and an oxidizing agent (ex: oxygen) in the presence of an electrolyte to produce electrical energy directly. A reaction equivalent to a combustion reaction, although it releases electrical rather than heat energy. - Electrodes: typically made of carbon & incorporate a metal catalyst (nickel). Separated by a porous matrix saturated with an electrolyte in an alkaline (ex: KOH) or acidic solution (ex: phosphoric acid H3PO4) which permits the flow of e- (electric current). Reactions are different at the anode and cathode, depending on of the matrix is saturated with an alkaline or acidic solution, however the overall redox reaction is the same. Ex: Hydrogen-oxygen fuel cell - hydrogen fuel cells in hydrogen powered buses or in the space shuttle

What is a concentration cell?

- A cell with two half-cells using the same chemicals, but with different concentrations of electrolyte. - The half-equation involving the more negative (less positive) electrode potential is reversed to give the overall equation. - The concentrations of ions decrease and increase (in the two half-cells) until they reach equilibrium - when the concentrations are equal and there is no overall cell potential - The standard electrode potential for the overall reaction is 0.00 V because the two half-cells are the same species. - The cell potential can be worked out directly from the Nernst equation.

What is a battery?

- A portable electrochemical device that produces electricity. Made up of one or more voltaic cells connected in series.

What is a proton exchange membrane (PEM) fuel cell?

- A type of fuel cell used in hydrogen-powered vehicles. - Hydrogen passes through a porous carbon anode that contains a platinum catalyst and is oxidized to form hydrogen ions (protons) and electrons (which travel through the external circuit). The hydrogen ions travel through the PEM to the cathode (containing a platinum catalyst) where they combine with oxygen molecules and electrons to form water. Includes the same reaction as that in the phosphoric acid fuel cell. - Rely on external sources of a fuel and oxidizing agent, depleted during use and require constant replenishing (different from primary cells that contain stored chemical energy within them). - Expensive since platinum catalysts are used

How do microbial fuel cells work?

- Bacteria attach themselves to the anode, oxidizing organic matter present in, say, waste water, producing carbon dioxide, protons (H+ ions) and electrons in the absence of oxygen (anaerobic conditions). The electrons produced are transferred from the bacteria to the electrode. The electrons travel through the external circuit to the cathode, while the protons pass through a PEM to the cathode compartment, combining with oxygen and electrons to form water. The reaction is equivalent to the combustion of glucose. - Represents a possible sustainable source of electrical energy, produced from the oxidation of carbohydrates and other substrates in waste water. - Geobacter bacteria (ex: Geobacter sulfurreducens) used in some fuel cells to reduce ethanoate ions (CH3COO-) under anaerobic conditions. Overall reaction equivalent to the complete combustion of ethanoic acid.

What environmental problems are batteries associated with?

- Batteries can contain heavy metals and should be recycled rather than dumped in landfill sites, which can lead to pollution of soil and water with heavy metals.

What are the similarities between fuel cells and rechargeable batteries?

- Both generate electrical energy from chemical energy - Both composed of an anode, cathode and an electrolyte - Both generate a current based on the separation of reduction and oxidation reactions and the flow of electrons through an external circuit.

What are secondary cells?

- Cells that can be recharged using mains electricity (a rechargeable battery). Include reversible reactions, reversed by connecting them to an electricity supply. - Ex: lead-acid batteries used in car engines and lithium-ion batters used in laptops.

What are primary cells?

- Cells that cannot usually be recharged using electricity -> the reaction is non-reversible. - A very expensive source of electrical energy compared to electricity. - Used because of their convenience and portability - Ex: zinc-carbon batteries (used in flashlights) and alkaline batteries

What is a lead-acid battery?

- Consists of lead anodes and cathodes made from lead covered with a layer of lead (IV) oxide (PbO2) in a concentrated sulfuric acid electrolyte. - Large and heavy = unsuitable where they have to be carried around - Used in emergency source of power or car engines -> six electrical cells in series. As the battery discharges (anode = negative electrode, cathode = positive electrode, charging = the reverse), lead at the anode is oxidized releasing electrons, while the lead (IV) oxide at the cathode is reduced. The product of both reactions is solid lead (II) sulfate, which accumulates on the electrodes within the battery. - When a battery loses its charge, the lead (II) sulfate can be converted back to lead and lead (IV) oxide by applying an external electrical source (car = the alternator). -

What are the differences between fuel cells and rechargeable batteries?

- Fuel cells require an external source of chemical energy (fuel), but rechargeable batteries have their chemical energy source within them. - Fuel cells never run out so long as there is a constant supply of fuel from an external source; rechargeable batteries run out and then have to be recharged by connecting them to an electricity supply which reverses the reactions. - Rechargeable batteries are far cheaper than fuel cells. - Fuel cells are capable of generating a far greater quantity of electricity than rechargeable batteries as fuel is supplied constantly. - Fuel cells are non-polluting -> only produce water. Rechargeable batteries may contain toxic metals (Cd, Pb) difficult to dispose of. - No byproduct of rechargeable batteries, while fuel cells produce drinkable water. - Rechargeable batters can only be recharged so many time. Fuel cells have better longevity.

What limits the maximum current that can be obtained from a cell?

- Internal resistance Arises because of the finite amount of time that it takes for ions to diffuse to the electrodes in the cell, limiting the number of electrons that can be removed from the electrode per second, and hence the current.

What can the term "electrochemical cell" refer to?

- It can refer to either voltaic (Galvanic) or electrolytic cells

What is a reformed methanol fuel cell?

- Methanol is reformed to hydrogen and carbon dioxide using water. Hydrogen is then used as the fuel for the fuel cell. Reactions are the same as in the PEM fuel cell.

What does the voltage of a cell depend on?

- Primarily on the materials used to make the cell. However, the nature and concentration of the electrolyte can also affect the voltage.

What does PEM stand for?

- Proton exchange membrane OR a polymer electrolyte membrane

What are rechargeable batteries?

- Secondary cells -> a portable type of electrochemical cell that generates a current via electrically reversible reactions

What are Nickel-cadmium batteries?

- Small rechargeable batteries (AAA to D) used for powering small electronic devices (clocks, calculators, etc.) - Generate smaller voltages than lead-acid batteries, around 1.2 M. - Use a nickel oxide hydroxide [NiO(OH)] cathode and a metallic cadmium anode separated by a potassium hydroxide electrolyte.

What do all cells include?

- The conversion of chemical energy to electrical energy

What is partial pressure?

- The pressure that one of the gases in a mixture of gases would exert if it were in the container by itself (the gases contribution to the total pressure of the mixture). - Calculated relative to a standard state of 1 bar (100 kPa) and are inserted into the expression for the reaction quotient (as a concentration).

What affects the total amount of electrical energy (work) that can be obtained from a cell?

- The quantities of substances used in a cell. More reactants = initially a greater total amount of energy.

What is current?

- The rate of flow of charge.

What are the advantages of electric vehicles over gasoline-powered vehicles?

- They do not produce carbon dioxide when operating. However, carbon dioxide might be produced when the charging electricity is generated or when producing hydrogen for a fuel cell.

What can the Nerst equation be used for?

- To calculate the electrode potentials of half-cells or cells under non-standard conditions E = Ecircle - (RT/nF)lnQ E: electrode potential Ecircle: standard electrode potential R: gas constant, 8.31 J/K/mol T: Temperature (K) n: number of electrons transferred F: Faraday constant, 96,500 C/mol Q: the reaction quotient. Found by dividing the product concentration by the reactant concentration. In a heterogeneous reaction, the concentration of a pure solid (or pure liquid, ex: H2O) is taken as 1. -> Changing the concentration does not have much effect on an electrode, unless the change in concentration is very large. - Two approaches to working out a cell potential under non-standard conditions. 1) Work out each individual cell potential under the non-standard conditions and then to combine them. 2) Work out the standard cell potential first and then to use the Nernst equation to work out the cell potential under non-standard conditions. - The Nerst equation can also be used to work out solution concentrations

What are the two main possibilities for powering vehicles by electricity?

- To use rechargeable batteries (lithium-ion battery) -> can be plugged in and recharged. - To use fuel cells and a hydrogen storage tank, having to be refilled. When deciding between the two, the two important factors are the mass of the battery/fuel cell and the volume of space it occupies. Fuel cells: hydrogen must be stored under pressure to reduce its volume and the fuel tank must be reinforced (= heavy) to withstand high pressures. Also safety to consider in the use of hydrogen. However, the advantage of using fuel cells is that a larger fuel tank, not adding a large amount of mass (given hydrogen is very light), can be added (higher energy-to-mass ratio). With rechargeable batteries, extra rechargeable batteries must be added contributing significantly to the overall mass of the vehicle and taking up space. Energy density of fuel cells similar to that of lithium-ion batteries.

What is a solution to the difficult of storing hydrogen fuel in hydrogen fuel cells?

- Use a liquid fuel (methanol) A direct methanol fuel cell works similar to the PEM fuel cells. The anode contains a platinum-ruthenium catalyst, the catalyst at the cathode is platinum and there is a PEM between the electrodes. Methanol solution and oxygen are fed into the fuel cell. A methanol fuel call can also be constructed using an alkaline electrolyte. - Direct methanol fuel cells are being developed for use in laptops and mobile phones. The fuel cell is built into the device & and a methanol cartridge is plugged into to provide the fuel and disposed when used up. Allows laptops and phones to be used away from the electricity grid.

What is a lithium-ion battery? And how does it work?

- Used as rechargeable cells in mobile telephones, laptops and high-energy usage portable electronics. - Generate voltages around 3-4V (higher than NiCad batteries). - Anode made of carbon with lothium atoms inserted in the lattice. Cathode made of a metal oxide (ex: manganese (IV) oxide OR cobalt (IV) oxide with Li+ ions inserted in the lattice). Electrolyte: a complex lithium salt (ex: lithium hexafluorophosphate (LiPF6) dissolved in an organic solvent). - During discharge, lithium atoms are oxidized at the negative electrode and electrons are released to the external circuit. The lithium ions move through the electrolyte to the cathode, where they become inserted into the lattice of the transition metal oxide. Cobalt ions are reduced at the positive electrode. For every lithium ion present in LixCoO2, the oxidation number of a cobalt ion must be reduced by 1, from +4 to +3. - During charging, the reverse reactions occur. Lithium ions move from the positive to negative electrode . - Lighter than lead-acid storage batteries

pH

10 to the power of the -pH

How is the thermodynamic efficiency of a fuel cell calculated?

= delta G (free energy change for the cell reaction - maximum amount of work that can be obtained from a cell) / delta H (enthalpy change for the cell reaction) - delta G is calculated via the equation: delta g = -nFE - delta H is simply the standard enthalpy change of combustion of hydrogen - 82.9% for a hydrogen fuel cell. Not 100% because the reaction involves a decrease in entropy - energy is needed to cause the redistribution of the available energy among the various energy states of the products which are more ordered. - 96.5% theoretical efficiency for the direct methanol fuel cell. Higher than the hydrogen fuel cell because the reaction involves a smaller decrease in entropy (smaller change in the number of moles of gas). - If a fuel cell reaction involves an increase in entropy, the theoretical efficiency will be greater than 1.

What are the advantages and disadvantages of a hydrogen-oxygen fuel cell?

Advantage - They do not produce any pollution because the only products are water, electricity and a small amount of heat Disadvantage - The production of hydrogen from the electrolysis of water (why is this bad?????) - The hydrogen must be stored on board in some way and the oxygen must come from purified air or an oxygen tank (in a vehicle)

How does the voltage of fuel cells, lead-acid batteries, nickel-cadmium batteries and lithium-ion batteries compare?

Fuel cells - 0.6-0.8 V, but depends on the type of fuel cell. Used in a stack with multiple fuel cells connected in series. Lead-acid batteries - 2V per cell. Usually used as a battery with six cells in series Nickel-cadmium batteries - 1.2 V. Lithium-ion batteries - 3.7 V.

How do the sizes of fuel cells, lead-acid batteries, nickel-cadmium batteries and lithium-ion batteries compare?

Fuel cells Can be made in a variety of sizes. Fuel cell stacks: for buses = large and heavy. Small fuel cells: for laptops. Some fuel cells have a very high power-to-mass ratio and high power-to-volume ratio. Lead-acid batteries - Large and heavy. Low power-to-mass ratio and power-to-volume ratio. Nickel cadmium batteries - Small and light (portable devices). Intermediate power-to-mass ratio and power-to-volume ratio. Lithium-ion batteries - Small and light (portable devices) - Produce the greatest amount of power per unit mass or unit volume of the rechargeable batteries