# $H_2|O_2$ fuel cells * Fuel cells are electrochemical cells that convert chemical potential energy of a fuel into electricity through redox reactions. * While in batteries the chemical energy comes with metal cells that are ready to be used up, fuel cells need a constant supply of fuel and oxygen in order to produce a steady current. * The most commonly used fuel cell is the proton-exchange membrane fuel cell (PEMFC) which transforms chemical energy liberated in the electrochemical reaction of $H_2$ with $O_2$ to electrical energy. ## How it works * $H_2$ enters the cell at the anode where it is then catalytically separated into free $H^+$ and $e^-$ ions using finely divided platinum powder as a heterogenous catalyst. $$H_2 \rightarrow 2H^++2e^-, \;\;\; E^{°}=0V$$ * $H^+$ then permeate through the proton-exchange membrane (a semipermeable membrane that lets through only protons) to the cathode side. * $e^-$ remain behind and flow through an external load circuit, thus creating a voltage difference between the anode and cathode. * At the cathode, $O2$ enters the cell and combines with $e^-$ from the external circuit and $H^+$ which have travelled through the proton-exchange membrane to form $H_2O$ $$\frac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O, \;\;\; E^{°}=1.2291V$$ * The overall reaction: $$H_2 + \frac{1}{2}O_2 \rightarrow H_2O, \;\;\; E^{°}=1.2291V$$ ## Uses of PEMFCs * PEMFCs are regarded as best suited for fuel cell vehicles and small stationary applications. * They have various advantages: * They deliver a high power density (power generated per unit of weight of the cell) leading to compact size and light weight. * Due to the thermal properties of the proton-exchange membrane, they can operate at relatively low temperatures (at around 60–80 °C) allowing them to warm up quickly, start up quickly, and respond to load changes rapidly. * Since water is the only liquid within the cell, their is low corrosion potential in the cell. * Disadvantages include: * They require strong platinum catalysts as the electrocatalytic activity of other, more common catalysts at these low operating temperatures is to low to enable the heterolytic fission of hydrogen: platinum is very expensive — much effort is put into developing more cost efficient catalysts. * The efficiency of the platinum catalyst is strongly decreased by $CO$ gas if present in the cell, and there is around 1% of $CO$ gas remaining in $H_2$ gas produced by reforming hydrocarbons — expensive fuel processing is thus required to reduce CO content, increasing overall system costs. --- ## Sources * https://fuelcellsworks.com/knowledge/technologies/pemfc/ * https://en.wikipedia.org/wiki/Proton-exchange_membrane_fuel_cell * https://www.sciencedirect.com/topics/chemistry/proton-exchange-membrane-fuel-cells * https://www.sigmaaldrich.com/materials-science/renewable-alternative-energy/pem-fuel-cells.html --- # Nickel metal hydride (NiMH) batteries * The NiMH battery a type of reachargeable battery. * It was developed during the the 1970s and 1980s as a replacement for the previsouly succesfull sealed nickel-cadmium battery. * Before the development of Lithium-Ion batteries, NiMH battery packs were used in portable electronic applications such as in laptops, cellular communication devices, and other consumer electronic devices. --- ## How it works * The positive electrode is made from nickel oxy-hydroxide. * The negative electrode is made from an hydrogen-absorbing alloy which at high density levels absorbs the releasing hydrogen. * The positive and negative electrodes are separated by a thin sheet of paper which prevents shorting between the electrodes and separates the different electrode reactions while still allowing the diffusion of ions to permit current flow throughout its structure. * The reactions are: $$cathode: \;H_2O + M + e^− ⇌ MH + OH^−,\;E°=0.49V$$ $$anode: \;Ni(OH)_2 + OH^− ⇌ NiO(OH) + H_2O + e^−,\;E°=\; –0.83V $$ $$overall: \;Ni(OH)_2 + M ⇌ NiO(OH) + MH, \;V°=1.35V$$ * ![](https://i.imgur.com/5YnxBkn.png) * Overall structure: ![](https://i.imgur.com/QWNzEOG.png) * NiMH use an alkaline electrolyte, usually potassium hydroxide, whose concentration does decrease over charge-discharge cycles. ## Advantages of NiMH batteries: * Their structure allows flexible cell sizes. * They are completely maintenance free. * They are environmentally acceptable and can be easily recycled * They have a very high power density * They have a long life span: over 1000 cycles at 100% depth of discharge and over 1,000,000 cycles at 10% depth of discharge. * They have a wide range of operating temperatures: -30 to +70 °C. ## Disadvantages * NiMH batteries are amongst the hardest batteries to charge accurately: * Overcharge cannot be controlled as for example in Lithium batteries: there is no maximum charge voltage which can be set. * Parallel charging is difficult as the cells can vary largely in resistance and thus may each require a different amount of current. * End-of-charge is difficult to detect and thus slight overcharge is probable. * Their lifetime is limited to 5 years ## Sources * https://sci-hub.st/https://ieeexplore.ieee.org/document/7113594 * https://data.energizer.com/pdfs/nickelmetalhydride_appman.pdf * https://www.sciencedirect.com/topics/engineering/nickel-metal-hydride * https://www.sciencedirect.com/topics/engineering/nickel-metal-hydride-battery --- # Lithium battery * Lithium batteries are primary batteries that have metallic lithium as an anode. * Primary refers to cells that cannot be recharged and hence are discarded once used. * The most common type of lithium cell uses metallic lithium as anode and manganese dioxide as cathode, with a salt of lithium dissolved in an organic solvent. * ![](https://i.imgur.com/IvJSc30.png) * Generally, Lithium cells can be made from various insertion electrodes, common ones include: * Carbon monofluoride | Lithium tetrafluoroborate in propylene * Thionyl chloride | Lithium tetrachloroaluminate in thionyl chloride * Sulfur dioxide on teflon-bonded carbon | Lithium bromide in sulfur dioxide --- ## Advantages * High charge density thus a longer life span. * Can provide extremely high currents and fast discharging times. * Choice of insertion electrodes allows the operating voltage of the battery to be varied due to different electrochemical reactions. --- ## Disadvantages * Risk of explosion due to fast dischargin ability, hence there are regulations in carrying Lithium batteries during air travel. * High manifacturing cost due to the rarity of Lithium metal.