Galvanic Cells - AOS 1

Oxidation rules

* H: +1 (non-metals), -1 (metals)
* O: -2
* F: -1
* Halogens (group 7): -1, +1 with oxygen

Conjugate pairs

Reducing agent = conjugate oxidising agent

Oxidising agent = conjugate reducing agent

• strong reducing agents have weak conjugate oxidising agents

Balancing equations under alkaline conditions

1\. Balance all elements except hydrogen and oxygen

2\. Balance hydroxide ions by adding OH-

Galvanic cell

An electrochemical cell and a device in which chemical energy is converted to electrical energy (e.g: spontaneous redox)

* galvanic cell = spontaneous
* electrolytic cell = non-spontaneous
* exothermic reaction process
* occurs in all fuel cells

Chemical → electrical

When two half cells are in contact

Chemical → heat

When two half directly mix with each other

If a cell contains only non-solid materials or if an electrode is aq then…

an inert electrode (e.g: platinum or graphite) is used

purpose of salt bridge

neutralises any potential charge build up (anions to anode, cations to cathode)

Spontaneous reactions only occur when…

strongest oxidant (left) is ABOVE the strongest reductant (right)

cell potential difference

E0 (oxidant) top → E0 (reductant) bottom

• cell voltage must be positive

• if negative, need an external power source

primary cell

• galvanic cells

• non-rechargeable

• spontaneous

• products slowly migrate away from electrodes or consumed by side reactions preventing recharge

• go flat because reactants get used up → voltage or current drops

• has to go through an external cell with two half-cells separated

• salt bridge

energy transformation:

- during the discharge cycle, products build up around the electrodes and slow down or even stop the discharging process (polarisation)

- when the cell is allowed to rest, the accumulated products move away from electrodes, allowing for further redox reactions

e.g: daniel cell, alkaline cell

secondary cell

• electrical energy is converted to chemical energy

• non-spontaneous

• galvanic cell in discharge (using = galvanic), electrolytic cell on recharge

• rechargeable battery

• reaction can be forced backwards to recharge cell (anode +, cathode -)

energy transformation:

- when it discharges, it acts as a galvanic cell, converting chemical energy to electrical energy

- when it recharges, it acts as a electrolytic cell. Electrical energy is transformed into chemical energy

e.g: lithium cell, lead acid battery

rate of deterioration of a battery increases as…

temperature increases

fuel cells

A type of galvanic cell that converts chemical energy into electricity and requires a constant supply of reactants to be added

• exothermic reaction

• cannot be recharged

• e.g: hydrogen fuel cell

• 40-60% efficient

• do not have direct contact between oxidant and reductants

  safety issues with hydrogen:

  - flammable and highly combustible, low density which means that it rises rapidly and will quickly disperse in fire

fuel cells characteristics

• hydrogen fuel cell only produces water = sustainable

• electrodes must be porous and conducting

• increasing amount of electrical energy produced can improve efficiency (e.g: increasing surface area or using a catalyst as electrode) → anode = platinum / cathode = nickel powder

• porous electrodes are more efficient at moving charges and reactants

• larger surface areas = more amount of electrical energy

advantages of hydrogen fuel cells

• converts chemical → electrical energy

• sustainable, doesn’t produce greenhouse gases

• more efficiency → requires less energy conversions

• high energy density of H2 means less fuel needs to be carried

disadvantages of hydrogen fuel cells

* expensive
* needs a constant supply of fuel
* storage can be an issue as H2 is FLAMMABLE/EXPLOSIVE

advantages and disadvantages of generic fuel cells

advantages

- convert chemical energy directly to electrical energy → more efficient than the series of energy conversions that take place in power stations that burn fossil fuels

- generate electricity as long as fuel is supplied (does not need to be recharged/replaced)

disadvantages

- expensive compared to conventional fuels, platinum catalyst for example is non-renewable

Galvanic vs Fuel Cells - structure

**Galvanic:** anode, cathode, salt bridge, seperate

**Fuel:** anode, cathode, porous electrode, charges are carried in electrode

Galvanic vs Fuel Cells - efficiency

**Galvanic:** 60-90%

**Fuel:** 40-60%

Galvanic vs Fuel Cells - applications

**Galvanic:** primary cells, small devices, watches

**Fuel:** large vehicles, cars, buses

name the species that crosses the membrane to enable fuel cell operation

protons (H+)

one way to minimise environmental impacts of non-hydrogen fuel cells

sourcing fuel from a renewable/sustainable source so CO2 released can be balanced out

functions of a membrane (hydrogen fuel cell)

• keep the O2 and H2 gas separated

• prevent spontaneous reactions between products

hydrogen gas fuel cells equations (alkaline)

Oxidation (anode) : H2 + 2OH- → H2O + 2e- (hydrogen is oxidised)

Reduction (cathode): O2 + 2H2O + 4e- → 4OH- (oxygen is reduced)

energy transformations

discharge:

- oxidation occurs at anode (-)

- reduction occurs at cathode (+)
recharge (swap charges):
- oxidation occurs at anode (+)

- reduction occurs at cathode (-)

Primary/Secondary Cells vs Fuel Cells*

Definition

- Primary or secondary cell is a type of galvanic cell. These galvanic cells are electrochemical cells that convert chemical energy from spontanteous redox reactiosn into electrical energy

- A fuel cell is a galvanic cell that converts the chemical energy of a fuel into electrical energy. Air or oxygen are supplied continuously

Function

- A primary cell is a source of portable electrical energy

- Fuel cell is a continuous source of high electrical current for both portable and fixed applications

Features

- Fuels cells have reactants that are contained within the cell. Thus, can produce power for only a limited time until their reactants are depleted

battery life

• loss of active materials (reactants and products)

  • products detach from electrodes

  • side reactions

• impurities in cell materials

• corrosion or failure of internal components

• temperature: batteries sensitive to heat, low temps help to prevent self-discharge but rate of reaction will fall at low temps

Similarities & Differences - Fuel Cells & Seconadry Cells

differences

- fuel cells are continuously supplied with fuel and oxygen

- secondary cells can only produce power up until their reactants are depleted

similarities

- both convert chemical energy into electrical energy

- anode (oxidation) and cathodes (reduction)

why do electrodes have to be porous

- porous electrodes maximise surface area and increases the rate of reactions at the anode and cathode (allows reactants to come into contact with electrolyte)

- this results in a higher current because of the higher rate of reaction

- the cell voltage, however, is independent of reaction rate and will remain the same

inert electrodes examples

• platinum, gold, graphite(carbon), and rhodium

fuel cells - electrodes & electrolytes

- must be conductive

- must be porous to allow the reactants to come into contact with the electrolyte

- larger surface area = higher amount of energy

- catalyst (anode = platinum / cathode = nickel powder)

fuel cell design (hydrogen-oxygen)

- two seperate reactant compartments and an electrolyte solutions

- separated from the electrolyte by porous electrodes

anode: hydrogen

cathode: oxygen

industrial applications

molten electrolytes

- higher energy expenditure, greater wear on cell

- expensive, lots of energy tp keep ions

aqueous electrolytes

- water will undergo oxidation and reduction and so may react preferentially to the cations and anions present from salt

- cheaper

electrorefining

used for purifying metals

- metal containing impurities is used as the anode

- pure metal is placed at the cathode

- any metals (impurities) that are stronger reductants will be oxidised at the anode

- any wear instead fall on the bottom of cell (anode mud)

electroplating

process by which a thin layer of metal is deposited onto a metal electrode (electrolysis)

- prevents corrosion

process:

- object plated is attached to the negative terminal (cathode), it is then placed in an electrolyte solution containing pure ions of the metal that forms the plating

- anode will form the plating onto cathode

- power supply removed electrons from positive electrode (anode) and gives them to cathode (negative electrode)

alkaline fuel cell

explain why the two half-cells are seperated in fuel cells

• The half-cells need to be separated so that electrons can travel between the two half-cells in connecting wires (through the load) and the electrical energy harnessed.

• If there was no separation of the two half-cells, oxidation and reduction would occur together and electrons would not move through the external circuit.

state the feature that causes the overall movement of H+ ions across the membrane

• At the cathode the [H⁺] decreases and at the anode the [H⁺] increases. The H⁺ ions move from the higher concentration through the membrane to the lower concentration.

a high concentration in the presence of water means that

it will be the strongest oxidant/reductant

factors that limit the life of a fuel cell

- side reactions at the electrodes

- significant temperature change

- build-up of gases around electrode

characteristics and functions of cells

electrolyte: carry current between the electrodes by the movement of cations (+ ions) towards the cathode and anions (– ions) towards the anode

electrode: transport produced electrons from one half-cell to another, which produce an electrical charge

membrane: keep the O2 and H2 gas separated prevent spontaneous reactions between products

safety considerations of hydrogen fuel cells

- controlling possible ignition sources since hydrogen is highly flammable and explosive in the presence of a spark in air

- following appropriate hazardous materials guidelines

- safe storage/location of hydrogen containers

what temperatures is suitable for batteries

room temperatures (non-extreme)

advantages of batteries

- efficient, less waste of energy/fewer products released to atmosphere

- oxygen not requires