Electrochemistry is the relationship between chemical relations and electricity (voltage and current). Electricity refers to the movement of electrons. In particular, the electrochemical process is any conversion between chemical and electrical energy. It is known that all electrochemical processes involve oxidation-reduction reactions or redox reactions; reactions that involve both the loss of electrons (oxidation) and the gain of electrons (reduction). This process can be seen in an electrochemical cell, the device which conducts the conversion between electrical and chemical energy.
There are two main types of cells, voltaic/galvanic (electrochemical) cells and electrolytic cells, both of which differ by the nature of the initiation of the redox reactions that occur in the cell (as shown in diagram — below). In a voltaic cell, electrical energy is produced by the flow of electrons from the anode to the cathode which results in a spontaneous redox reaction. Whereas, in an electrolytic cell, the flow of electrons from the cathode to anode which produces the electrical energy is initiated by an outside energy source, resulting in a nonspontaneous redox reaction.
Electrochemical processes are used widely in applications such as dry cells, fuel cells and lead storage batteries. In particular, the 12-V lead-acid battery is comprised of a group of six voltaic cells connected together (as shown in diagram — below), each cell producing approximately 2 V. Each voltaic cell, consisting of lead grids, is divided into two half-cells, one in which oxidation occurs and the other in which reduction occurs. Both half-cells are connected by wire. In a typical half-cell, a strip of metal is immersed in the electrolyte (solution containing the metal ions). The conductor in the circuit which carries electrons to and from the electrolyte is known as the electrode. The electrode at which oxidation occurs is known as the anode. In a lead-acid battery, the anode (-) consists of spongy lead (Pb). The electrode at which reduction occurs in known as the cathode (+), which, in a lead-acid battery is comprised of lead oxide (PbO2). Since the same electrolyte, sulfuric acid (H2SO4), is used in both half-cells, a salt bridge is not needed to maintain electrical neutrality by allowing ions to pass from one half-cell to the other.
The half-reactions and overall spontaneous redox reaction that occur in each half-cell when the lead-acid battery discharges are as shown below:
Process in oxidation half-cell: Pb_((s))+ 〖〖SO_4〗^(2-)〗_((aq))→ 〖PbSO_4〗_((s))+2e^-
Process in reduction half-cell: 〖PbO_2〗_((s))+ 〖4H^+〗_((aq) )+ 〖〖SO_4〗^(2-)〗_((aq))+2e^-→ 〖PbSO_4〗_((s))+2〖H_2 O〗_((l))
Overall Reaction: 〖Pb〗_((s))+ 〖PbO_2〗_((s))+2〖〖H_2 SO〗_4〗_((aq))→ 〖2Pb〖SO〗_4〗_((s))+2〖H_2 O〗_((l))
When the lead-acid battery discharges, it conducts electrical energy which is needed to start a car. Initially, solid lead (Pb_((s))) from the anode is oxidised to form a lead ion (〖Pb^(2+)〗_((aq))) which combines with the sulfate ion (〖〖SO_4〗^(2-)〗_((aq))) to form a deposit of lead sulfate (〖PbSO_4〗_((s))). The 2 electrons that are lost from solid lead are transmitted through the wire to the cathode where the lead ion (〖Pb^(4+)〗_((aq))) from solid lead oxide gains them and is reduced to become 〖Pb^(2+)〗_((aq)). This lead ion (〖Pb^(2+)〗_((aq))) combines with the sulfate ion (〖〖SO_4〗^(2-)〗_((aq))) from another sulfuric acid molecule (〖〖H_2 SO〗_4〗_((aq))) to form a deposit of lead sulfate (〖PbSO_4〗_((s))). The two oxide ions (〖O^(2-)〗_((aq))) combine with the 2 hydrogen ions (〖H^+〗_((aq))) from both sulfuric acid molecules (〖〖H_2 SO〗_4〗_((aq))) to make 2 water molecules (4〖H^+〗_((aq))+2〖O^(2-)〗_((aq)) → 2〖H_2 O〗_((l))). When the lead-acid battery is recharged, the electrochemical cell acts as an electrolytic cell and the process is reversed. The process of discharging and recharging occurs simultaneously in the vehicle in which the alternator acts as the outside energy source.
Fuel cells, like lead-acid batteries, are voltaic cells that convert chemical energy into electrical energy. However, the fuel cell involves the reactions of hydrogen gas and oxygen gas as opposed to lead and lead oxide (as shown in diagram — below). The three main compartments of the hydrogen fuel cell are divided by the two electrodes (typically made of carbon). A thin membrane exists between the cathode and anode acting as a salt bridge so that only hydrogen ions may pass through (and not electrons). The cell is designed so that electrical energy can be continually produced through the continual oxidation of the fuel substance, hydrogen gas (H2 gas).
The half-reactions and overall spontaneous redox reaction that occur in the hydrogen fuel cell are as shown below:
Process in oxidation half-cell: 〖 2H_2〗_((g))→ 〖4H^+〗_((aq))+4e^-
Process in reduction half-cell: 〖〖 O〗_2〗_((g))+ 〖4H^+〗_((aq))+4e^-→ 2〖H_2 O〗_((g))
Overall Reaction: 〖2H_2〗_((g))+ 〖O_2〗_((g))→ 2〖H_2 O〗_((g))
The fuel (2〖H_2〗_((g))) flows into the anode where both hydrogen diatomic atoms lose two electrons each and thus are oxidised. The 4 electrons flow through the wire to the cathode while the four hydrogen ions (〖4H^+〗_((aq))) travel through the membrane into the cathode. Simultaneously, oxygen gas (〖O_2〗_((g))) flows in to the cathode compartment (connected to the wire) where both oxygen diatomic atoms gain two electrons each and thus are reduced. The 2 oxide ions (〖O^(2-)〗_((aq))) then combine with the 4 hydrogen ions (〖H^+〗_((aq))) to make 2 water molecules (4〖H^+〗_((aq))+2〖O^(2-)〗_((aq)) → 2〖H_2 O〗_((l))).
One disadvantage of lead-acid batteries is its danger to worker safety, especially through informal recycling processes in developing countries (as shown in diagram — below).
For example, during the recycling process, when the battery is broken, sulfuric acid can easily leak into the soil or enter ground and surface water systems that are used for bathing and drinking. Sulfuric acid (H2SO4) is a highly corrosive, strong, mineral acid. Its corrosiveness on other materials living tissues can be mainly ascribed to its strong acidic nature, and, if concentrated, strong dehydrating and oxidizing properties. Sulfuric acid at a high concentration can cause very serious damage upon contact not only through chemical burns via hydrolysis but also through secondary thermal burns via dehydration. It can lead to permanent blindness if splashed onto eyes and irreversible damage if swallowed. Safety precautions must be strictly observed when handling it.
As well, lead, a significant component of the lead-acid battery is known to be highly toxic, poisoning organs through blood flow. Specifically, lead is known for its affinity for sulfhydryl groups and other organic ligands in proteins. It can also mimic other essential metals such as zinc, iron and especially calcium enabling it to disrupt enzyme systems dependant on these ions, accounting for many of its toxic effects. For example, in July 2011, one third of employees at a Taiwanese-owned battery plan were founds with Blood Lead Levels between 28-48 ug/dL (concentrations below 5 ug/dL can have adverse neurological and cardiovascular consequences).
After the recycling process of batteries, pieces of broken batteries that are left on the floor can affect workers through dermal contact. The temperatures used for refining lead can exceed 1000℃ which create large amounts of lead fume. If the furnace is not under negative pressure or the plant has inadequate ventilation/emission controls, these fumes will be inhaled by the workers. Hence, the metal can enter into the respiratory and circulatory systems causing acute lead poisoning to occur. Chronic poisoning can occur from low amounts of lead entering the body through the lungs or mouth or lead accumulating in the bones over a long period of time. This can cause fatigue, impaired hearing, headaches, loss of appetite and sleep disturbance. This is typically followed by intense pain in the abdomen known as lead colic. In the long term and in extreme cases, lead poisoning can impair physical growth and cause kidney damage, convulsions, coma, delirium and possibly death. Approximately 1 million people are affected by lead pollution through the manufacturing of lead-acid batteries.
Hydrogen fuel-cells, on the other hand, are not nearly as dangerous. They have been proven to be as safe or even safer than other fuels such as gasoline or natural gas. As with any other fuel, hydrogen gas does have a few unique properties such as its ability to leak easily and ignite at very low temperatures. Nevertheless, when handled carefully, the hydrogen fuel-cell is a much safer alternative to the lead-acid battery in terms of worker safety.
Lead-acid batteries also cause significant environmental damage that is much more severe than their hydrogen fuel cell counterparts. Instead of being recycled, 500 000 car batteries are discarded in landfills annually in Australia. That equates to approximately 44-70% of lead from lead-acid batteries being released into the environment as waste. Smelting in the manufacturing of lead-acid batteries emits ash as well as lead particles which contaminates the land surrounding the recycling plant. The manufacturing and transportation of batteries emits exhaust and other pollutants into the atmosphere which increases the Earth’s temperature, hence, contributing to the greenhouse effect. Hence, lead contamination is classified as one of the top, heavy metal pollutants. Data from the census indicates that 900 metric tons of lead were discharged into the environment in 2007. This is particularly evident in the high particulate pollution caused by lead-acid batteries. The lead is spread out through the air contaminating soil, water, food and the atmosphere. In Australia, there is an estimated 15 mg of lead per cubic metre (averaged over an 8-hour time period). There are also 50-400 mg of lead in 1 kilogram of residential soil (Jennings, 2013). Lead and sulfuric acid seep into the soil and contaminate crops and ground water, affecting the quality of drinking water. When entering the water system, sulfuric acid can also contribute to acid rain, which, in turn, can endanger soil, animal life, aquatic life, plant life, trees and water bodies.
The hydrogen fuel-cell, however, is known to be environmentally clean and reliable. The oxidation of hydrogen ensures that the only by-products are water and heat energy. This is evident in diagram — in which the 0% renewable, natural-gas-reformed hydrogen provides a 53% reduction in fuel production pathway emissions from the 2017 gasoline baseline. Hydrogen can be extracted through the electrolysis of water in which an electric current is used to stimulate the non-spontaneous separation of hydrogen and oxygen from water. This electric current can be derived from minimal to zero-emission sources such as solar, wind, nuclear and fossil energy with advanced emission controls and carbon sequestration. Hence, unlike the factory manufacturing of lead-acid batteries, this method of extracting hydrogen is environmentally clean and beneficial.
One advantage of the lead-acid battery is its high energy efficiency in comparison to hydrogen fuel cells. The hydrogen fuel cell is known for its low energy efficiency. Its extremely low energy efficiency ranges from 30% to 40% compared to the lead-acid battery with an 80% to 85% energy efficiency. This is mainly due to hydrogen’s low energy density per volume (energy per unit volume).
As shown in diagram —, hydrogen gas has a volumetric density of less than 5 MJ/L compared to Gasoline with a volumetric density of 32 MJ/L. Hence, large volumes or high-pressure conditions are needed to store an appreciable amount of hydrogen to satisfy the accepted operating range. Specifically, onboard hydrogen storage capacities of 5-13 kg are required to meet the full range of light-duty vehicle platforms. At present, only compressed and liquid hydrogen storage systems can meet these volumetric density targets. Nevertheless, despite the advantage of storing more fuel and reducing delivery frequency, liquefaction requires a substantial amount of electrical energy (30% of its energy value). As well, a 33% decrease in tank volume by increasing compressed hydrogen pressures from 350 bar to 700 bar is also detrimental due to the substantial amount of electrical energy needed. Hence, the energy consumed by the compressors that liquify/compress hydrogen reduces the overall efficiency of the vehicle. Another factor of the fuel cell’s low energy efficiency is the by-product of heat energy that is produced during the reaction in the fuel cell (as shown earlier). This lost energy therefore reduces the amount of energy directed towards the functioning of the vehicle, reducing the overall energy efficiency.
Lead-acid batteries, on the other hand, are much more energy efficient. While energy is lost in heat generation and in gassing due to the decomposition of water in the electrolyte during the recharging phase, this loss of heat and electrical energy is only responsible for approximately 20% of the total energy loss. Sulfation, the accumulation of crystalline lead sulfate sediments, also minimally reduces the electrical efficiency. Nevertheless, lead-acid batteries still have an overall energy efficiency of around 80%, which is approximately 40% greater than that of hydrogen fuel cells (as evident in diagram — below).