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Topic Review
Proton Exchange Membrane
A proton exchange membrane (PEM) is a semipermeable polymer electrolyte that facilitates the transport of protons while acting as an electronic insulator and gas separator in electrochemical devices [1]. Typically constructed from perfluorosulfonic acid polymers, the membrane contains sulfonic acid groups that enable proton conduction through water-mediated mechanisms, requiring controlled hydration to maintain ionic conductivity [2]. In fuel cell and electrolyzer assemblies, the PEM is positioned between the anode and cathode, allowing protons to migrate between the electrodes while preventing the direct mixing of fuel and oxidant or of product gases [3]. The membrane must simultaneously satisfy requirements for high proton conductivity, low electronic conductivity, chemical stability under oxidizing and reducing conditions, and mechanical integrity across hydration and temperature cycles [4]. PEMs are distinguished from other electrolyte types—such as alkaline membranes or solid oxides—by their polymer-based architecture and by the proton, rather than hydroxyl or oxide ion, as the charge carrier [5].
  • 23
  • 16 Sep 2026
Topic Review
Internal Combustion Engine
An internal combustion engine (ICE) is a heat engine in which the oxidation of a fuel–air mixture occurs inside the engine itself, within one or more cylinders, and the resulting high-temperature, high-pressure gaseous products act directly on a piston, rotor, or nozzle to produce mechanical work [1]. Energy is released by rapid combustion—either spark-ignited burning of a premixed charge or compression ignition of injected fuel—that converts the chemical enthalpy of the fuel into cylinder pressure, which is then transformed through a reciprocating or rotating mechanism into shaft work [2]. Reciprocating ICEs are classified by thermodynamic cycle as four-stroke or two-stroke devices and by ignition mode as spark-ignition (Otto) or compression-ignition (Diesel) engines, with air-standard cycles serving as idealized reference models of the real process [3]. The engine converts the expansion of hot gas against a moving boundary, transferring work through a crankshaft or turbine, and exhausts the products of combustion to the environment [2]. It is distinguished from external combustion engines, in which combustion takes place outside the working cylinder, by the internal conversion of chemical fuel energy to mechanical work [1].
  • 17
  • 16 Sep 2026
Topic Review
CO2 Utilization
CO2 utilization is the conversion of captured carbon dioxide into products that have economic value, so that the gas becomes a feedstock rather than a waste to be disposed of. It is distinct from carbon capture and storage in that the carbon is retained in the economic cycle, and distinct from simple reuse in that the molecule is chemically transformed; whether a given route reduces emissions at all depends on the energy source, the concentration of the feed and the lifetime of the product, since most routes are net consumers of energy. Electrochemical reduction is one of the more flexible options because the product is selected by the catalyst and the potential, and the reaction has been operated both in batch and continuously [1]. Solid sorbents that both capture and activate the molecule shorten the route, natural porous nanorods being one material that does both [2]. Coupling capture and conversion in a single process avoids the energy penalty of releasing the gas again, and integrated schemes convert the captured carbon directly to methane with high selectivity [3]. Conversion to organic molecules follows the same logic. Combining capture with thermal catalytic conversion, including from dilute sources, air among them, is now an active area [5]. Scale and the cost of hydrogen remain the dominant constraints.
  • 10
  • 23 Sep 2026
Topic Review
Molten Salt Electrochemistry
Molten salt electrochemistry is the branch of electrochemistry concerned with electrode and interfacial processes that occur in ionic media consisting of molten inorganic salts—alkali halides, cryolite, chlorides, fluorides, or oxide-containing melts—at elevated temperatures typically ranging from several hundred to above one thousand degrees Celsius [1]. Because the solvent is itself an ionic liquid, the electrolyte possesses exceptionally high electrical conductivity, a wide electrochemical window, and the ability to dissolve stable oxides, sulfides, and refractory compounds that are inaccessible to aqueous electrochemistry. At the working electrode, dissolved metal cations or oxygen anions undergo reduction or oxidation under an applied potential, so that the field embraces electrowinning, electrorefining, electroplating, and the direct electrolytic reduction of solid oxide feedstocks to metal [2]. The reactions are governed by the thermodynamics of the melt, the speciation and activity of dissolved ions, mass transport through the viscous salt, and the nature of the electrode–melt interface. The field therefore forms the electrochemical foundation of the Hall–Héroult aluminum process and of high-temperature metal extraction, recycling, and materials synthesis in which the molten salt serves simultaneously as electrolyte, reactant, and reaction medium [3].
  • 9
  • 26 Sep 2026
Topic Review
Nitrogen Reduction Reaction
Nitrogen reduction reaction is the conversion of dinitrogen (N2) into ammonia or related nitrogen-containing products, most often studied as a six-proton, six-electron process driven electrochemically or photocatalytically at ambient temperature and pressure [1]. Because the N2 triple bond is among the strongest in small molecules, and because the adsorbed N2H, NH2 and NH3 intermediates bind either too weakly or too strongly on most metal surfaces, the attainable rate is constrained by the scaling relations that link their adsorption energies [2]. Rates and Faradaic efficiencies are therefore usually low, and in aqueous electrolytes most of the applied current is consumed by hydrogen evolution rather than by ammonia formation [1]. Reported ammonia yields for metal, carbon-based and single-atom catalysts span several orders of magnitude, and the electrolyte, cell geometry and applied potential shift the measured value substantially; alkali-metal and nitrate or nitrite impurities can also contribute to the detected ammonia, so quantitative control experiments are required [3]. Catalysts examined to date range from transition metals and their nitrides to metal-free carbon materials, and no single material has yet combined high selectivity, high current density and long-term stability [4]. Standardised procedures for ammonia quantification have accordingly become part of how the field reports performance, including isotope labelling with 15N2 and rigorous blank measurements [5].
  • 8
  • 22 Sep 2026
Topic Review
Electrochemical CO2 Reduction
Electrochemical CO2 reduction is the cathodic conversion of carbon dioxide into carbon monoxide, formate, hydrocarbons or alcohols, driven by electricity at an electrode surface whose structure decides which intermediates are stabilised. Selectivity rather than activity is the central problem, because the competing hydrogen evolution reaction shares the potential window and often the active site, and catalyst design therefore has to be considered together with mass transport [1]. Reactor configuration matters as much as catalyst identity: gas-diffusion electrodes, electrolyte composition and cell geometry set the local concentration at the interface and hence which pathway dominates [2]. Bismuth, tin and indium cathodes give formate with high faradaic efficiency because their surfaces favour the adsorbed precursor over carbon monoxide [3]. Bimetallic oxides shift selectivity through interface sites rather than through either component alone [4], while molecular catalysts, metalloporphyrins among them, offer defined sites whose mechanism can be computed directly [5]. Comparing reports across cells stays difficult because local pH and transport are rarely documented together.
  • 8
  • 15 Oct 2026
Topic Review
Anti-freezing
Anti-freezing behaviour is the ability of a material or a solution to remain functional below the freezing point of water, and in soft materials it is usually achieved with a cryoprotectant or a mixed solvent. Adding a salt, a polyol, or an organic solvent lowers the freezing point and disrupts the growth of ice crystals [1]. An organohydrogel replaces part of the water with glycerol or ethylene glycol, which lowers the freezing point and keeps the ionic conductivity usable at sub-zero temperatures [2]. This matters for flexible sensors and for batteries that must operate outdoors in winter [3]. A second route is to introduce an antifreeze protein or a zwitterionic group that inhibits ice nucleation rather than depressing the melting point [4]. The added solvent changes the mechanical properties and can evaporate over time, so the stability of the effect over repeated freeze-thaw cycles has to be demonstrated [5].
  • 6
  • 19 Oct 2026
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