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Oxygen evolution
Oxygen evolution
Oxygen evolution
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Oxygen evolution is the chemical process of generating diatomic oxygen (O2) by a chemical reaction, usually from water, the most abundant oxide compound in the universe. Oxygen evolution on Earth is effected by biotic oxygenic photosynthesis, photodissociation, hydroelectrolysis, and thermal decomposition of various oxides and oxyacids. When relatively pure oxygen is required industrially, it is isolated by distilling liquefied air.[1]

Natural oxygen evolution is essential to the biological process of all complex life on Earth, as aerobic respiration has become the most important biochemical process of eukaryotic thermodynamics since eukaryotes evolved through symbiogenesis during the Proterozoic eon, and such consumption can only continue if oxygen is cyclically replenished by photosynthesis. The various oxygenation events during Earth's history had not only influenced changes in Earth's biosphere, but also significantly altered the atmospheric chemistry. The transition of Earth's atmosphere from an anoxic prebiotic reducing atmosphere high in methane and hydrogen sulfide to an oxidative atmosphere of which free nitrogen and oxygen make up 99% of the mole fractions, had led to major climate changes and caused numerous icehouse phenomena and global glaciations.

In industries, oxygen evolution reaction (OER) is a limiting factor in the process of generating molecular oxygen through chemical reactions such as water splitting and electrolysis, and improved OER electrocatalysis is the key to the advancement of a number of renewable energy technologies such as solar fuels, regenerative fuel cells and metal–air batteries.

Oxygen evolution in nature

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Main article: Oxygen evolving complex

Photosynthetic oxygen evolution is the fundamental process by which oxygen is generated in the earth's biosphere. The reaction is part of the light-dependent reactions of photosynthesis in cyanobacteria and the chloroplasts of green algae and plants. It utilizes the energy of light to split a water molecule into its protons and electrons for photosynthesis. Free oxygen, generated as a by-product of this reaction, is released into the atmosphere.[2][3]

Water oxidation is catalyzed by a manganese-containing cofactor contained in photosystem II, known as the oxygen-evolving complex (OEC) or the water-splitting complex. Manganese is an important cofactor, and calcium and chloride are also required for the reaction to occur.[4] The stoichiometry of this reaction is as follows:

2H2O ⟶ 4e− + 4H+ + O2

The protons are released into the thylakoid lumen, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for adenosine triphosphate (ATP) synthesis via photophosphorylation and the coupling of the absorption of light energy and the oxidation of water for the creation of chemical energy during photosynthesis.[5]

History of discovery

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It was not until the end of the 18th century that Joseph Priestley accidentally discovered the ability of plants to "restore" air that had been "injured" by the burning of a candle. He followed up on the experiment by showing that air "restored" by vegetation was "not at all inconvenient to a mouse." He was later awarded a medal for his discoveries that "...no vegetable grows in vain... but cleanses and purifies our atmosphere." Priestley's experiments were further evaluated by Jan Ingenhousz, a Dutch physician, who then showed that the "restoration" of air only worked while in the presence of light and green plant parts.[4]

Water electrolysis

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Together with hydrogen (H2), oxygen is evolved by the electrolysis of water. The point of water electrolysis is to store energy in the form of hydrogen gas, a clean-burning fuel. The "oxygen evolution reaction (OER) is the major bottleneck [to water electrolysis] due to the sluggish kinetics of this four-electron transfer reaction."[6] All practical catalysts are heterogeneous.

Diagram showing the overall chemical equation.

Electrons (e−) are transferred from the cathode to protons to form hydrogen gas. The half reaction, balanced with acid, is:

2 H+ + 2e− → H2

At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and releasing electrons to the anode to complete the circuit:

2 H2O → O2 + 4 H+ + 4e−

Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen:

Overall reaction:
2 H2O → 2 H2 + O2

Chemical oxygen generation

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Although some metal oxides eventually release O2 when heated, these conversions generally require high temperatures. A few compounds release O2 at mild temperatures. Chemical oxygen generators consist of chemical compounds that release O2 when stimulated, usually by heat. They are used in submarines and commercial aircraft to provide emergency oxygen. Oxygen is generated by the high-temperature decomposition of sodium chlorate:[1]

2 NaClO3 → 2 NaCl + 3 O2

Potassium permanganate also releases oxygen upon heating, but the yield is modest:

2 KMnO4 → MnO2 + K2MnO4 + O2

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Oxygen evolution

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from Grokipedia
Oxygen evolution is the chemical process by which molecular oxygen (O₂) is generated, primarily through the oxidation of (H₂O), releasing O₂ along with protons and electrons. This fundamental reaction underpins life-sustaining biological processes like and enables key technologies in renewable energy production, such as water electrolysis for hydrogen generation. It can also occur via purely chemical means, such as the decomposition of oxygen-rich compounds in controlled environments. In biological systems, oxygen evolution is a cornerstone of oxygenic photosynthesis, carried out by , , and . It takes place within the (OEC)—a manganese-calcium (Mn₄Ca) cluster in (PSII) of chloroplasts—where two water molecules are oxidized in a light-driven, four-electron process: 2H₂O → O₂ + 4H⁺ + 4e⁻. This mechanism, which cycles through five intermediate states (S-states) activated by absorption, supplies electrons to the photosynthetic , enabling the reduction of NADP⁺ to NADPH and the generation of ATP for fixation into sugars. The evolution of this process around 3.4 to 2.9 billion years ago transformed Earth's atmosphere, enabling aerobic respiration and the rise of complex . In electrochemical applications, oxygen evolution manifests as the oxygen evolution reaction (OER), the rate-limiting anodic in for clean fuel production. The OER follows the same net equation as in (2H₂O → O₂ + 4H⁺ + 4e⁻ in acidic media or 4OH⁻ → O₂ + 2H₂O + 4e⁻ in alkaline media) but requires an due to its kinetically sluggish, multi-step nature involving proton-coupled electron transfers and unstable intermediates like *OH, *O, *OOH, and *OO. Efficient catalysts are crucial to overcome these barriers; in acidic electrolyzers, rare iridium-based oxides provide stability but at high cost, while alkaline systems favor earth-abundant nickel- and cobalt-based oxides, though durability remains a challenge. Advances in OER electrocatalysis are vital for scaling up (PEM) and alkaline electrolyzers to store intermittent as . Beyond and , oxygen evolution occurs through reactions for practical oxygen supply. A prominent example is the of (NaClO₃) in oxygen candles or chlorate generators, where heat (often initiated by a primer) drives the exothermic reaction 2NaClO₃ → 2NaCl + 3O₂, producing breathable oxygen for emergency use in , , and . Catalysts like iron powder lower the required temperature from ~600°C, enhancing safety and efficiency in these self-contained devices. Such methods provide a reliable, non-electrical alternative for isolated environments, distinct from large-scale industrial cryogenic .

Fundamentals

Definition and Reaction

Oxygen evolution refers to the chemical process of generating molecular oxygen (O₂) through the oxidation of (H₂O), releasing O₂ along with protons and electrons. In the context of electrochemical , it corresponds to the anodic . In acidic media, this process follows the : 2H2O→O2+4H++4e−2\mathrm{H_2O} \rightarrow \mathrm{O_2} + 4\mathrm{H^+} + 4\mathrm{e^-} In alkaline media, the reaction is: 4OH−→O2+2H2O+4e−4\mathrm{OH^-} \rightarrow \mathrm{O_2} + 2\mathrm{H_2O} + 4\mathrm{e^-} The oxygen evolution reaction (OER) specifically refers to this process as an electrochemical oxidation, distinguishing it from non-electrochemical methods of oxygen production, such as chemical decomposition. The process of oxygen evolution occurs in both natural systems, such as oxygenic photosynthesis, and artificial systems, like water electrolysis, where it is known as the OER. Under standard conditions at pH 0, the theoretical reversible potential for the OER is 1.23 V versus the reversible hydrogen electrode (RHE).

Thermodynamics and Kinetics

The oxygen evolution reaction (OER) is thermodynamically governed by the standard reversible potential of 1.23 versus the (SHE) for the 2H2O→O2+4H++4e−2H_2O \rightarrow O_2 + 4H^+ + 4e^- in acidic media, derived from the free energy change of oxidation. In the context of overall , combining the OER with the (HER) yields a theoretical cell voltage of 2.46 , though practical systems require higher voltages due to additional energetic losses. The (η\eta), defined as η=Eapplied−Etheoretical\eta = E_\text{applied} - E_\text{theoretical}, quantifies these losses and typically ranges from 300 to 400 mV at a of 10 mA cm−2^{-2} for benchmark catalysts like IrO2_2, reflecting the inherent inefficiency of the process. Kinetic challenges arise primarily from the multi-step, four-electron transfer nature of the OER, which imposes high activation barriers and results in intrinsically slow reaction rates compared to two-electron processes like the HER. These barriers are exacerbated by pH-dependent mechanisms: in acidic media, proton-coupled electron transfers dominate, while alkaline conditions favor hydroxide-mediated pathways, often leading to lower overpotentials in basic electrolytes due to altered intermediate stabilities. Pourbaix diagrams illustrate these effects by delineating the stability regions of , where potentials above the oxygen evolution line (approximately 1.23 V at pH 0, sloping with -59 mV/) favor O2_2 formation over stability, guiding the operational windows for OER to avoid excessive side reactions like . The , η=[a+b](/page/ListofFrenchcomposers)log⁡(j)\eta = [a + b](/page/List_of_French_composers) \log(j), where jj is the , aa is a constant, and [b](/page/ListofFrenchcomposers)[b](/page/List_of_French_composers) is the Tafel slope, provides a framework for quantifying OER kinetics, with typical slopes around 60 mV per decade indicating rate-limiting steps involving single transfers after initial proton-coupled processes. energies for OER, often exceeding 50 kJ mol−1^{-1} on common catalysts, underscore the need for thermal or catalytic assistance to overcome these barriers, though exact values vary with material and conditions.

Biological Oxygen Evolution

Oxygenic Photosynthesis

Oxygenic photosynthesis is the primary biological process responsible for oxygen evolution on , occurring in the membranes of chloroplasts in plants and algae, as well as in . In this light-dependent reaction, (PSII) absorbs photons to drive the oxidation of water molecules, releasing oxygen gas, protons, and electrons. The electrons generated replenish the oxidized primary donor in PSII and flow through the photosynthetic to ultimately reduce NADP⁺ to NADPH, while the protons contribute to the proton gradient used for ATP synthesis; this provides the reducing power and energy needed for CO₂ fixation in the Calvin-Benson-Bassham cycle. At the heart of this process lies the (OEC), a hetero-multinuclear Mn₄CaO₅ cluster anchored within the PSII reaction center by residues from the D1 and CP43 proteins. The OEC catalyzes the challenging four-electron oxidation of two molecules (2H₂O → O₂ + 4H⁺ + 4e⁻), accumulating oxidizing equivalents through a series of light-induced steps to overcome the kinetic barriers of O-O bond formation. High-resolution crystal structures have revealed the cluster's distorted chair-like configuration, with the Mn ions in oxidation states ranging from Mn(III)/Mn(IV) in the dark-stable S₁ state, and bound ligands that serve as substrates or proton shuttles. The mechanism proceeds via the Kok cycle, a five-state sequence (S₀ to S₄) first proposed by Bessel Kok, where each absorption advances the OEC by one S-state through events, often mediated by the redox-active residue Yz. The cycle accumulates four oxidizing equivalents, with the transient S₄ state triggering O-O bond formation—likely via nucleophilic attack by a Ca-bound on an oxo radical on Mn₄ or through oxo-oxo coupling—followed by rapid O₂ release and relaxation back to S₀. This rhythmic progression ensures efficient charge separation while minimizing damaging reactive intermediates. The emergence of oxygenic photosynthesis around 3.4 to 2.9 billion years ago, driven by ancient , marked a pivotal evolutionary milestone, culminating in the around 2.4 billion years ago that transformed Earth's anaerobic atmosphere into an oxygen-rich one, paving the way for aerobic respiration and the diversification of complex multicellular life. Under optimal conditions, the for oxygen evolution approaches 0.125 (one O₂ per eight photons absorbed by PSII), reflecting near-unity charge separation but thermodynamic losses in the multi-step process. However, this is modulated by environmental factors: high light intensity can induce by damaging the OEC, while low CO₂ levels limit downstream electron acceptance, reducing overall rates and increasing to protect the system.

Historical Discovery

The discovery of oxygen evolution began with experiments in the late that revealed ' ability to produce a gas essential for respiration and . In 1771, conducted pivotal experiments in , placing a sprig of mint in a sealed containing "air infected with animal respiration" or air depleted by a burning . After exposure to , the air was restored, allowing a to breathe comfortably or a flame to reignite, which Priestley termed "dephlogisticated air"—later identified as oxygen. These findings demonstrated that green release oxygen under light conditions, challenging prevailing phlogiston theories of and air quality. Building on Priestley's work, , a Dutch physician, refined the understanding in 1779 by showing that oxygen release occurs exclusively from the parts of and requires . Using aquatic like , Ingenhousz observed gas bubbles forming only on illuminated leaves, with no production in darkness or from non- tissues, establishing as indispensable for the process. This specificity linked oxygen evolution to what would later be termed . In the early 1800s, Swiss scientists Jean Senebier and Nicolas-Théodore de Saussure advanced quantification: Senebier demonstrated in 1782 that absorb fixed air () in to release oxygen, while de Saussure in 1804 measured that the volume of oxygen evolved equals the consumed, providing early stoichiometric evidence for the reaction. These insights shifted views from early theories suggesting absorbed oxygen to a mechanism where fixation drives oxygen production in . The 20th century brought isotopic confirmation of oxygen's source. In 1941, Samuel Ruben and Martin Kamen at the used the heavy ¹⁸O to label and separately in algal suspensions, revealing that evolved oxygen derived from , not CO₂, via analysis. This resolved a long-standing and confirmed photolysis as central to oxygen evolution. By the 1970s, the (OEC) within was identified as a manganese cluster essential for oxidation, through spectroscopic studies including that detected the metal's changes during oxygen release. These milestones solidified the biochemical basis of oxygen evolution in natural systems.

Artificial Oxygen Evolution

Electrochemical Water Splitting

Electrochemical water splitting involves the , where electrical drives the decomposition of water into and oxygen gases, with oxygen evolution occurring at the as the oxygen evolution reaction (OER). This is coupled with the (HER) at the , requiring an input of typically from renewable sources to produce . The overall reaction is 2H₂O → 2H₂ + O₂, with the OER half-reaction (2H₂O → O₂ + 4H⁺ + 4e⁻ in acidic media or equivalent in alkaline) demanding higher overpotentials due to its sluggish kinetics compared to HER. The minimum theoretical voltage is approximately 1.23 V under standard conditions, though practical systems operate at 1.6–2.0 V to account for overpotentials and resistances. Alkaline electrolysis, the most mature technology, employs a liquid potassium hydroxide (KOH) electrolyte, typically 25–40% concentration, with nickel-based electrodes for both anode and cathode. It operates at moderate temperatures of 60–80°C, enabling current densities up to 0.5 A/cm² in commercial systems, though advanced designs can exceed this. This configuration benefits from the use of abundant, non-precious materials, making it cost-effective for large-scale deployment. Proton exchange membrane (PEM) electrolysis utilizes a solid polymer electrolyte membrane, such as , which conducts protons in an acidic environment. At the anode, iridium or ruthenium oxides serve as catalysts due to their stability in acidic conditions, achieving high current densities of 1–2 A/cm² with efficiencies often exceeding 70% based on higher heating value. The system's compact design and rapid response make it suitable for fluctuating inputs. Anion exchange membrane (AEM) represents an emerging hybrid approach, combining the benefits of alkaline conditions with a solid anion-conducting . It allows the use of non-precious metal catalysts, such as or cobalt-based materials, potentially reducing costs while operating in alkaline media. Current research focuses on improving durability and conductivity to approach the performance of established technologies. On an industrial scale, water electrolysis systems have scaled to megawatt capacities, with examples including 1 MW PEM stacks from manufacturers like Nel Hydrogen, used for on-site production. These stacks integrate multiple cells to achieve outputs of several kilograms of per hour, supporting applications in and .

Chemical Oxygen Production

Chemical oxygen production encompasses non-electrochemical methods that generate molecular oxygen (O₂) through or catalytic of chemical precursors, offering portable and reliable alternatives for confined environments where electrical power is unavailable or impractical. These processes typically involve exothermic reactions initiated by or catalysts, producing oxygen for or without relying on external inputs like voltage. Unlike electrolytic methods, which require sustained and achieve efficiencies around 50-70% based on losses, chemical approaches prioritize simplicity and storability, though they are often single-use and limited by reactant mass. One prominent method is the of (NaClO₃), commonly employed in candles for emergency oxygen supply. The reaction proceeds as 2 NaClO₃ → 2 NaCl + 3 O₂, occurring at temperatures of 400-500°C, where the chlorate decomposes exothermically to yield approximately 0.42 kg of O₂ per kg of NaClO₃ (practical yield). These devices incorporate iron powder as a catalyst and binder materials like to initiate and sustain the reaction, ensuring steady oxygen release over 20-60 minutes depending on candle size. candles are integral to life-support systems in submarines, spacecraft, and , providing high-purity oxygen (>98%) in sealed, self-contained units that activate via a percussion igniter. Another established technique involves the catalytic decomposition of (H₂O₂), represented by 2 H₂O₂ → 2 H₂O + O₂, which liberates 0.47 kg of O₂ per kg of pure H₂O₂ and is highly exothermic (ΔH = -98 kJ/mol). This process is accelerated by catalysts such as (MnO₂) for industrial-scale generation or enzymes like for controlled, low-temperature applications. In medical contexts, stabilized H₂O₂ solutions decompose via to supply supplemental oxygen in portable concentrators or wound treatments, ensuring biocompatibility and rapid response. For propulsion, (HTP, >85% concentration) decomposes in monopropellant thrusters, generating and oxygen for attitude control in rockets and torpedoes, as demonstrated in historical systems like the German V-2 . Metal oxide thermolysis represents an advanced chemical route, particularly in two-step cycles for solar-driven oxygen production. For instance, copper(II) oxide (CuO) undergoes thermal reduction at elevated temperatures (>800°C): 4 CuO → 2 Cu₂O + O₂, releasing oxygen while forming the reduced oxide, which can later react with water or hydrogen in a regeneration step (e.g., Cu₂O + H₂ → 2 Cu + H₂O) to close the cycle without net consumption of the oxide. This method is harnessed in solar thermochemical water-splitting prototypes, where concentrated solar heat drives the decomposition, offering a pathway for renewable oxygen and hydrogen co-production with efficiencies up to 5-8% in lab-scale reactors. Copper-based systems are favored for their reversible redox behavior and moderate operating temperatures compared to ferrites or ceria. Safety is paramount in these processes due to their exothermic nature and potential for runaway reactions. Sodium chlorate decomposition generates intense heat (up to 600°C), necessitating robust to prevent fires or explosions from molten residues, while impurities like chlorides can trigger premature ignition if moisture levels exceed 0.1%. Hydrogen peroxide handling requires stabilization against contaminants (e.g., metals) that catalyze uncontrolled , leading to pressure buildup in storage vessels; concentrations above 50% demand inert materials and below 30°C to mitigate rates exceeding 1% per year. Metal cycles involve high temperatures, requiring inert atmospheres to avoid side reactions with air, and impurity control in synthesis to maintain cycle stability over thousands of loops. Overall, rigorous and , including gloves and eye shields, are essential to manage corrosivity and oxidation risks. Historically, chemical oxygen production gained prominence during , when Japanese forces developed sodium chlorate-based generators for high-altitude pilots to combat hypoxia in operating above 10,000 feet, marking an early shift from gaseous oxygen bottles to compact, heat-activated systems. These innovations laid the groundwork for postwar applications in naval and contexts, emphasizing reliability in oxygen-scarce scenarios.

Photoelectrochemical Processes

Photoelectrochemical processes for oxygen evolution involve the use of materials to harness for driving the oxidation reaction, producing oxygen and protons. In a typical setup, an n-type photoanode, such as (TiO₂) or (BiVO₄), is immersed in an aqueous and illuminated with . Upon absorption, photons excite electrons from the valence band to the conduction band, generating electron-hole pairs; the photogenerated holes migrate to the photoanode surface to oxidize molecules into O₂, while electrons flow to a counter to reduce protons or another species. This configuration enables bias-free or low-bias operation under , distinguishing it from purely electrochemical methods by directly coupling absorption to charge separation and . For efficient oxygen evolution, the semiconductor's band edges must straddle the water redox potentials: the valence band edge should be more positive than +1.23 V versus the reversible hydrogen electrode (RHE) to provide sufficient for the oxygen evolution reaction (OER), while the conduction band edge needs to be more negative than 0 V versus RHE to support hydrogen evolution at the . Materials like TiO₂ ( ~3.2 eV) are thermodynamically suitable but limited to light absorption due to their wide , whereas BiVO₄ ( 2.4–2.6 eV) better utilizes while meeting these alignment criteria. dynamics, including separation and transport, are critical, as recombination losses at surfaces or bulk defects can severely limit densities, often requiring protective layers or co-catalysts like IrO₂ or CoOₓ to enhance OER kinetics. To overcome voltage limitations in single photoanode systems, tandem cell architectures integrate photovoltaic absorbers with photoelectrochemical components, reducing or eliminating external bias. For instance, perovskite-silicon tandem configurations combine a wide-band-gap perovskite top cell with a silicon bottom cell, providing a photovoltage exceeding 2 V to drive both OER and hydrogen evolution reaction (HER) unassisted; recent implementations have achieved solar-to-hydrogen (STH) efficiencies up to 20.8% in monolithic stacked designs. Z-scheme configurations further enhance efficiency by employing two distinct semiconductors—one optimized for OER (e.g., BiVO₄ modified with CoOₓ) and another for HER (e.g., Sm₂Ti₂O₅S₂ with Pt)—connected via a solid-state electron mediator like reduced graphene oxide, allowing independent optimization of each half-reaction and achieving an apparent quantum yield of 7.0% at 420 nm for overall water splitting. These approaches mimic aspects of natural photosynthesis by spatially separating oxidative and reductive processes but rely on inorganic materials for scalability. Recent advances up to 2025 emphasize doping strategies to extend visible light absorption and improve charge transport in photoanodes. In BiVO₄, (Mo) or (W) doping introduces donor states that enhance and densities up to 2.5 mA/cm² at 1.23 V versus RHE, while (Li) doping at 5 at.% increases water oxidation performance by 20% through reduced recombination. For TiO₂, (N) or (H) doping narrows the band gap, enabling visible-light response without compromising OER . These modifications, often combined with nanostructuring, have pushed STH efficiencies toward practical thresholds, though stability under operational conditions remains a key challenge.

Mechanisms and Catalysts

Reaction Pathways

The oxygen evolution reaction (OER) proceeds through distinct mechanistic pathways, primarily the adsorbate evolution mechanism (AEM) and the lattice oxygen mechanism (LOM), each involving sequential proton-coupled electron transfers (PCET) to form O₂ from . In the AEM, oxygen-containing adsorbates form stepwise on active metal sites of the catalyst surface, starting with the adsorption of *OH from oxidation, followed by its dehydrogenation to *O, and subsequent nucleophilic attack by or to yield *OOH, culminating in O₂ release. The O-O bond formation in AEM can occur via a peroxide-like path (*O + H₂O → *OOH) or an oxyl path (*OH + *O → *OOH), with the transition from *O to *OOH often serving as the rate-determining step due to its high energy barrier. The LOM, in contrast, incorporates oxygen atoms from the catalyst's oxide lattice into the O₂ product, enabling a more direct coupling pathway that activates lattice O to form *O or *OO species, often accompanied by surface amorphization or reconstruction to replenish the lattice. This mechanism enhances activity by circumventing some AEM limitations but can reduce selectivity in acidic media due to potential metal dissolution from lattice participation. While AEM relies predominantly on stepwise PCET events where proton transfer precedes or follows , concerted PCET—simultaneous proton and electron movement—has been implicated in certain O-O coupling steps, particularly in LOM variants. pH significantly influences pathway dominance, with AEM prevailing in alkaline conditions where abundant OH⁻ facilitates adsorbate formation, whereas LOM becomes more prominent in acidic environments, leveraging lattice oxygen to mitigate proton scarcity effects on kinetics. Spectroscopic techniques provide direct evidence for these intermediates; for instance, (XAS) and on IrO₂ reveal dynamic *O and *OOH species, supporting Walden-type mechanisms involving concerted PCET and configuration-specific rate limits during OER. Theoretical frameworks further elucidate pathway-activity links through scaling relations, where volcano plots correlate OER overpotential with the binding energies of key intermediates like *O and *OH, predicting optimal catalysts near the plot apex for AEM-dominated processes. These relations, derived from , highlight a universal ~1.6 eV scaling slope between *OH and *OOH energies, constraining AEM efficiency and motivating LOM exploration to decouple such limitations.

Catalyst Materials

The (OEC) in (PSII) features the Mn₄CaO₅ cluster as a natural benchmark for efficient water oxidation, operating at low overpotentials under physiological conditions. This cubane-like structure, with four ions bridged by oxygen atoms and stabilized by a calcium , cycles through S-states to facilitate four-electron oxygen evolution while minimizing oxidative damage. The Ca²⁺ plays a critical role in self-repair mechanisms, enabling rapid reconstitution of the cluster by coordinating water molecules and preventing photodamage during turnover. Synthetic catalysts aim to replicate or surpass this efficiency, with oxides like IrO₂ and RuO₂ serving as state-of-the-art benchmarks for acidic oxygen evolution reaction (OER). These rutile-structured materials exhibit high intrinsic activity due to favorable adsorption energies for OER intermediates, achieving overpotentials as low as ~250 mV at 10 mA/cm² in acid for optimized nanostructures. However, their scarcity and high cost—iridium being rarer than —limit scalability for industrial applications. Recent advances in 2025 have focused on single-atom Ir catalysts, which maximize atomic efficiency by anchoring isolated Ir sites on supports like metal oxides or carbons, reducing loading while maintaining or enhancing activity through optimized coordination environments. Non-precious alternatives dominate alkaline OER, where NiFe (oxy)hydroxides emerge as leading catalysts, often forming layered structures that deliver overpotentials below 300 mV at 10 mA/cm². The synergy between Ni and Fe sites promotes steps, with Fe incorporation tuning the electronic structure for faster kinetics. Spinel Co₃O₄ catalysts offer robust performance primarily in alkaline media, leveraging octahedral Co³⁺ sites as primary active centers for O-O bond formation, though modifications are needed for acidic conditions. Perovskite structures, such as Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O₃₋δ (BSCF), provide high activity through mixed-valence Co/Fe sites and flexible lattice oxygen participation, achieving superior turnover frequencies compared to pure oxides. As of 2025, phosphides (TMPs) and sulfides have gained traction for (AEM) electrolyzers, where their precursors reconstruct to form active (oxy) layers under alkaline conditions. in TMPs, such as NiFeP or CoP, enhances activity by exposing high-valence metal sites and creating defective interfaces that lower energy barriers for water oxidation. These materials offer cost-effective scalability, with overpotentials around 250-300 mV in AEM setups, bridging the gap toward practical . Emerging trends in 2025 include high-entropy TMPs and AI-optimized designs for further improving stability and activity. Design principles for OER catalysts emphasize electronic structure optimization, including d-band center tuning to balance intermediate binding strengths. Shifting the d-band center downward relative to the weakens oxygen adsorption, reducing overpotentials in oxides like perovskites and spinels. Oxygen vacancy engineering further refines performance by increasing and conductivity; for instance, vacancy-rich surfaces in Co₃O₄ or perovskites facilitate lattice oxygen involvement, boosting turnover rates by up to an order of a magnitude. These strategies, informed by , guide the development of durable, earth-abundant catalysts.

Applications and Challenges

Role in Renewable Energy

Oxygen evolution reaction (OER) plays a pivotal role in production through , where it is coupled with the (HER) to generate gas that can power fuel cells for clean energy applications. This process electrolyzes into and oxygen using , enabling the production of carbon-free fuels essential for decarbonizing sectors like transportation and industry. For instance, the European Union's plan targets 10 million tons of domestic production annually by 2030 to support and net-zero goals. Integration of OER with sources enhances the viability of production, such as through solar-driven photoelectrochemical (PEC) systems that directly harness to drive . In PEC setups, OER occurs at photoanodes, converting into chemical fuels like without intermediate conversion, thereby improving overall efficiency. Similarly, wind-powered utilizes offshore or onshore wind farms to supply for OER and HER, producing that can be stored and used to balance grid intermittency while generating synthetic fuels. These integrations are crucial for scaling up renewable to replace fossil-based energy carriers. Beyond terrestrial applications, OER enables oxygen generation in closed-loop systems, such as NASA's Oxygen Generation Assembly (OGA) on the (ISS), which electrolyzes to produce breathable oxygen for astronauts. The OGA has operated reliably since 2006, recovering up to 90% of from crew metabolic byproducts to sustain long-duration missions. Economically, advancements in OER catalysts are key to reducing green hydrogen production costs, which stood at approximately $5 per kilogram in 2020 and are projected to fall below $2 per kilogram by 2030 through lower overpotentials and scalable electrolysis. Efficient OER electrocatalysts minimize energy losses, directly contributing to these cost declines by enhancing electrolysis efficiency. In 2025, pilot projects like the Hydrogen Offshore Production (HOP2) initiative demonstrate progress with proton exchange membrane (PEM) electrolyzers integrated into offshore wind farms, aiming for gigawatt-scale deployment to produce cost-competitive green hydrogen at sea. Additionally, efforts such as the H2SHIFT project are advancing AEM technologies for offshore wind-powered electrolysis, further optimizing renewable integration.

Stability and Efficiency Issues

One persistent challenge in oxygen evolution reaction (OER) is the inherent required, with even benchmark catalysts like IrO₂ exhibiting a minimum of approximately 300 mV at a of 10 mA cm⁻², reflecting the sluggish four-electron transfer kinetics. This scales unfavorably with increasing , often exceeding 400 mV at industrially relevant levels above 500 mA cm⁻², due to mass transport limitations and ohmic losses that hinder scalability for large-scale electrolyzers. Catalyst degradation further exacerbates efficiency losses, particularly through dissolution in acidic media where iridium-based catalysts suffer significant Ir loss, reducing density over time. In alkaline environments, catalysts undergo , such as Ni oxides transforming into oxyhydroxides, which can initially boost activity but leads to structural instability. Additionally, the lattice oxygen mechanism (LOM) promotes amorphization by mobilizing lattice oxygen, resulting in vacancy formation and accelerated , as observed in various transition metal oxides. As of 2025, key challenges include ensuring durability under intermittent operation driven by fluctuations, where rapid cycling accelerates degradation in NiCoOₓ catalysts, as demonstrated in simulated solar-driven tests showing accelerated voltage rise. Impurity poisoning, such as Fe contaminants in alkaline electrolytes, further compromises stability by altering active sites on Ni-based catalysts, necessitating purification strategies for real-world deployment. Efficiency is quantified through metrics like Faradaic efficiency, which must exceed 95% to minimize side reactions and ensure practical viability, often verified via during OER operation. Turnover frequency (TOF) targets above 1000 h⁻¹ per are essential for high-throughput applications, though many catalysts fall short due to deactivation. Mitigation strategies focus on alloying, such as Ru-Ir oxides, which balance activity and dissolution resistance by tuning oxophilicity and electronic structure. Protective layers, including Ni foam supports for non-precious metal catalysts, enhance mechanical integrity and prevent detachment under high current densities, as seen in NiFe systems maintaining performance over extended cycles.

References

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