Highly Efficient Electrocatalysis and Mechanistic Investigation of Intermediate IrOx(OH)y Nanoparticle Films for Water Oxidation

Binuclear Metal Phthalocyanines with Enhanced Activity in the Oxygen Evolution Reaction: A First-Principles Study

The rational design of efficient catalysts for the electrochemical oxygen evolution reaction (OER) critically relies on a comprehensive understanding of the reaction mechanisms. Herein, the alkaline OER on planar mononuclear metal phthalocyanines (MPc, where M = Mn, Co, Fe, and Ni) and binuclear metal phthalocyanines (bi-MPc) is studied using density functional theory (DFT) methods. Both FePc and bi-CoPc exhibit enhanced stability and OER activity, with the energy required for the leaching of central metal being as high as 2.28 and 2.45 eV and the overpotentials of the OER being 0.48 and 0.57 V, respectively. Through electronic structure analysis, it is found that, in the OER process of bi-MPc, the large macrocyclic ligand and metal ions not bonding with the intermediate can serve as hole reservoirs. Intermediate species are further stabilized by the dispersal of a positive charge, reducing the free energy. These findings underscore the significance of macrocyclic ligands in the rate-determining step of the OER catalyst.

Recent Advances in Iridium-based Electrocatalysts for Acidic Electrolyte Oxidation

Ongoing research to develop advanced electrocatalysts for the oxygen evolution reaction (OER) is needed to address demand for efficient energy conversion and carbon-free energy sources. In the OER process, acidic electrolytes have higher proton concentration and faster response than alkaline ones, but their harsh strongly acidic environment requires catalysts with greater corrosion and oxidation resistance. At present, iridium oxide (IrO2 ) with its strong stability and excellent catalytic performance is the catalyst of choice for the anode side of commercial PEM electrolysis cells. However, the scarcity and high cost of iridium (Ir) and the unsatisfactory activity of IrO2 hinder industrial scale application and the sustainable development of acidic OER catalytic technology. This highlights the importance of further research on acidic Ir-based OER catalysts. In this review, recent advances in Ir-based acidic OER electrocatalysts are summarized, including fundamental understanding of the acidic OER mechanism, recent insights into the stability of acidic OER catalysts, highly efficient Ir-based electrocatalysts, and common strategies for optimizing Ir-based catalysts. The future challenges and prospects of developing highly effective Ir-based catalysts are also discussed.

A 2D Nano-architecture (NPSMC@Ir-Ru@rGO) Derived from Graphene Enfolded Polyphosphazene Nanospheres Decorated Ir-Ru Metals (PZS@Ir-Ru@GO) towards Bifunctional Water Splitting

A leap-forward approach has been successfully devised to synthesize a novel hierarchical binary metal modified heteroatom doped 2D micro-/mesporous carbon-graphene nanostructure (NPSMC@Ir-Ru@rGO) for overall water splitting application. To investigate the role of decorating metals, different electrolcatalysts like NPSMC, NPSMC@rGO, NPSMC@Ir@rGO, and NPSMC@Ru@rGO were also synthesized and structural changes were compared and investigated by physiochemical techniques. All of the samples have shown electrocatalytic activities attributed to the presence of heteroatom (N, P, S) doped micro-/mesoporous carbonaceous matrix, amorphous carbon in the coexistence of graphitic lattice carbons, presence of active metal NPs (Ir and/-or Ru), an even distribution of active sites, and graphene 2D interconnected channels to promote electron transfer ability, respectively. However, the Ir-Ru metal codeped nanocatalyst (NPCMS@Ir-Ru@rGO) is proved to be an excellent electrocatalyst based on the synergistic role of Ir-Ru metals that necessitates the low overpotentials of 181 mV and 318 mV to convey a current density of 10 mA cm-2 towards the electroctalytic application of HER and OER, respectively. Furthermore, exhibiting the corresponding Tafel slopes (132 and 70 mV dec-1 ) in an alkaline medium. This work is anticipated to open up new avenues for the development of promising electrocatalysts based on active metals modified heteroatom doped carbon nanomaterials for energy applications.

Ligand effect on switching the rate-determining step of water oxidation in atomically precise metal nanoclusters

The ligand effects of atomically precise metal nanoclusters on electrocatalysis kinetics have been rarely revealed. Herein, we employ atomically precise Au₂₅ nanoclusters with different ligands (i.e., para-mercaptobenzoic acid, 6-mercaptohexanoic acid, and homocysteine) as paradigm electrocatalysts to demonstrate oxygen evolution reaction rate-determining step switching through ligand engineering. Au₂₅ nanoclusters capped by para-mercaptobenzoic acid exhibit a better performance with nearly 4 times higher than that of Au₂₅ NCs capped by other two ligands. We deduce that para-mercaptobenzoic acid with a stronger electron-withdrawing ability establishes more partial positive charges on Au(I) (i.e., active sites) for facilitating feasible adsorption of OH^- in alkaline media. X-ray photo-electron spectroscopy and theoretical study indicate a profound electron transfer from Au(I) to para-mercaptobenzoic acid. The Tafel slope and in situ Raman spectroscopy suggest different ligands trigger different rate-determining step for these Au₂₅ nanoclusters. The mechanistic insights reported here can add to the acceptance of atomically precise metal nanoclusters as effective electrocatalysts.

IrO2-Stablized La2IrO6 perovskite nanotubes via corner-shared interconnections as highly-efficient oxygen evolution electrocatalysts

One-dimensional nanotube heterostructures with IrO₂-stabilized La₂IrO₆ is obtained by an electrospinning approach. The La₂IrO₆/IrO₂ catalyst exhibits superior catalytic activity and strong stability for the oxygen evolution reaction. The synergistic cooperation between the two types of Ir as the active sites in La₂IrO₆/IrO₂ is demonstrated by in situ Raman spectrum and DFT calculation.

Supported Iridium-based Oxygen Evolution Reaction Electrocatalysts - Recent Developments

The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

The Influence of Electrode Thickness on the Structure and Water Splitting Performance of Iridium Oxide Nanostructured Films

For a safe environment, humanity should be oriented towards renewable energy technology. Water splitting (WS), utilizing a photoelectrode with suitable thickness, morphology, and conductivity, is essential for efficient hydrogen production. In this report, iridium oxide (IrO_x) films of high conductivity were spin-cast on glass substrates. FE-SEM showed that the films are of nanorod morphology and different thicknesses. UV-Vis spectra indicated that the absorption and reflectance of the films depend on their thickness. The optical band gap (E_g) was increased from 2.925 eV to 3.07 eV by varying the spin speed (SS) of the substrates in a range of 1.5 × 10³-4.5 × 10³ rpm. It was clear from the micro-Raman spectra that the films were amorphous. The E_g vibrational mode of Ir-O stretching was red-shifted from 563 cm^-1 (for the rutile IrO₂ single crystal) to 553 cm^-1. The IrO_x films were used to develop photoelectrochemical (PEC) hydrogen production catalysts in 0.5M of sodium sulfite heptahydrate Na₂SO₃·7H₂O (2-electrode system), which exhibits higher hydrogen evaluation (HE) reaction activity, which is proportional to the thickness and absorbance of the used IrO_x photocathode, as it showed an incident photon-to-current efficiency (IPCE%) of 7.069% at 390 nm and -1 V. Photocurrent density (Jph = 2.38 mA/cm² at -1 V vs. platinum) and PEC hydrogen generation rate (83.68 mmol/ h cm² at 1 V) are the best characteristics of the best electrode (the thickest and most absorbent IrO_x photocathode). At -1 V and 500 nm, the absorbed photon-to-current conversion efficiency (APCE%) was 7.84%. Electrode stability, thermodynamic factors, solar-to-hydrogen conversion efficiency (STH), and electrochemical impedance spectroscopies (EISs) were also studied.

Understanding the factors governing the water oxidation reaction pathway of mononuclear and binuclear cobalt phthalocyanine catalysts

The rational design of efficient catalysts for electrochemical water oxidation highly depends on the understanding of reaction pathways, which still remains a challenge. Herein, mononuclear and binuclear cobalt phthalocyanine (mono-CoPc and bi-CoPc) with a well-defined molecular structure are selected as model electrocatalysts to study the water oxidation mechanism. We found that bi-CoPc on a carbon support (bi-CoPc/carbon) shows an overpotential of 357 mV at 10 mA cm⁻², much lower than that of mono-CoPc/carbon (>450 mV). Kinetic analysis reveals that the rate-determining step (RDS) of the oxygen evolution reaction (OER) over both electrocatalysts is a nucleophilic attack process involving a hydroxy anion (OH⁻). However, the substrate nucleophilically attacked by OH⁻ for bi-CoPc is the phthalocyanine cation-radical species (Co^II–Pc–Pc˙⁺–Co^II–OH) that is formed from the oxidation of the phthalocyanine ring, while cobalt oxidized species (Pc–Co^III–OH) is involved in mono-CoPc as evidenced by the operando UV-vis spectroelectrochemistry technique. DFT calculations show that the reaction barrier for the nucleophilic attack of OH⁻ on Co^II–Pc–Pc˙⁺–Co^II–OH is 1.67 eV, lower than that of mono-CoPc with Pc–Co^III–OH nucleophilically attacked by OH⁻ (1.78 eV). The good agreement between the experimental and theoretical results suggests that bi-CoPc can effectively stabilize the accumulated oxidative charges in the phthalocyanine ring, and is thus bestowed with a higher OER performance.

bi-CoPc can stabilize accumulated oxidative charges in phthalocyanine ring, which leads to the OER proceeding through a nucleophilic attack of OH^- on the phthalocyanine cation-radical species that is formed from the oxidation of phthalocyanine ring.

Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles

The luminophore Ru(bpy)₂(dcbpy)²⁺ (bpy=2,2’‐bipyridine; dcbpy=4,4’‐dicarboxy‐2,2’‐bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru‐decorated chitosan fibers (NS‐RuCh), with a 20 : 1 ratio between chitosan repeating units and Ru^II chromophores. The properties of the Ru^II compound are unperturbed by the chitosan structure, with NS‐RuCh exhibiting the typical metal‐to‐ligand charge‐transfer (MLCT) absorption and emission bands of Ru^II complexes. When crosslinks are made in the presence of IrO₂ nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO₂⊂NS‐RuCh system, in which both Ru^II photosensitizers and IrO₂ water oxidation catalysts are within the nanofiber structures. NS‐RuCh and IrO₂⊂NS‐RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy‐dispersive X‐ray analysis, which indicated a 2 : 1 ratio between Ru^II chromophores and IrO₂ species. Photochemical water oxidation has been investigated by using IrO₂⊂NS‐RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O₂ with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)₃²⁺ and IrO₂ nanoparticles (0.05) or with respect to that obtained when using NS‐RuCh and “free” IrO₂ nanoparticles (0.10). A fast hole‐scavenging process (rate constant, 7×10⁴ s⁻¹) involving the oxidized photosensitizer and the IrO₂ catalyst within the IrO₂⊂NS‐RuCh system is behind the improved photochemical quantum yield of IrO₂⊂NS‐RuCh.

One-Compartment InGaN Nanowire Fuel Cell in the Light and Dark Operating Modes

A one-compartment H₂O₂ photofuel cell (PFC) with a photoanode based on InGaN nanowires (NWs) is introduced for the first time. The electrocatalytic and photoelectrocatalytic properties of the InGaN NWs are studied in detail by cyclic voltammetry, current versus time measurements, photovoltage measurements, and electrochemical impedance spectroscopy. In parallel, IrO _x (OH) _y as the co-catalyst on the InGaN NWs is evaluated to boost the catalytic activity in the dark and light. For the PFC, Ag is the best as the cathode among Ag, Pt, and glassy carbon. The PFC operates in the dark as a conventional fuel cell (FC) and under illumination with 25% increased electrical power generation at room temperature. Such dual operation is unique, combining FC and PFC technologies for the most flexible use.

Advanced Wound Dressing for Real-Time pH Monitoring

The rapid evolution of wearable technologies is giving rise to a strong push for textile chemical sensors design targeting the real-time collection of vital parameters for improved healthcare. Among the most promising applications, monitoring of nonhealing wounds is a scarcely explored medical field that still lacks quantitative tools for the management of the healing process. In this work, a smart bandage is developed for the real-time monitoring of wound pH, which has been reported to correlate with the healing stages, thus potentially giving direct access to the wound status without disturbing the wound bed. The fully textile device is realized by integrating a sensing layer, including the two-terminal pH sensor made of a semiconducting polymer and iridium oxide particles, and an absorbent layer ensuring the delivery of a continuous wound exudate flow across the sensor area. The two-terminal sensor exhibits a reversible response with a sensitivity of (59 ± 4) μA pH-1 in the medically relevant pH range for wound monitoring (pH 6-9), and its performance is not substantially affected either by the presence of the most common chemical interferents or by temperature gradients from 22 to 40 °C. Thanks to the robust sensing mechanism based on potentiometric transduction and the simple device geometry, the fully assembled smart bandage was successfully validated in flow analysis using synthetic wound exudate.

Hydrous cobalt-iridium oxide two-dimensional nanoframes: insights into activity and stability of bimetallic acidic oxygen evolution electrocatalysts

Acidic oxygen evolution reaction (OER) electrocatalysts that have high activity, extended durability, and lower costs are needed to further the development and wide-scale adoption of proton-exchange membrane electrolyzers. In this work, we report hydrous cobalt-iridium oxide two-dimensional (2D) nanoframes exhibit higher oxygen evolution activity and similar stability compared with commercial IrO₂; however, the bimetallic Co-Ir catalyst undergoes a significantly different degradation process compared with the monometallic IrO₂ catalyst. The bimetallic Co-Ir 2D nanoframes consist of interconnected Co-Ir alloy domains within an unsupported, carbon-free, porous nanostructure that allows three-dimensional molecular access to the catalytically active surface sites. After electrochemical conditioning within the OER potential range, the predominately bimetallic alloy surface transforms to an oxide/hydroxide surface. Oxygen evolution activities determined using a rotating disk electrode configuration show that the hydrous Co-Ir oxide nanoframes provide 17 times higher OER mass activity and 18 times higher specific activity compared to commercial IrO₂. The higher OER activities of the hydrous Co-Ir nanoframes are attributed to the presence of highly active surface iridium hydroxide groups. The accelerated durability testing of IrO₂ resulted in lowering of the specific activity and partial dissolution of Ir. In contrast, the durability testing of hydrous Co-Ir oxide nanoframes resulted in the combination of a higher Ir dissolution rate, an increase in the relative contribution of surface iridium hydroxide groups and an increase in specific activity. The understanding of the differences in degradation processes between bimetallic and monometallic catalysts furthers our ability to design high activity and stability acidic OER electrocatalysts.

Immobilization of Ir(OH)3 Nanoparticles in Mesospaces of Al-SiO2 Nanoparticles Assembly to Enhance Stability for Photocatalytic Water Oxidation

Iridium hydroxide (Ir(OH)3) nanoparticles exhibiting high catalytic activity for water oxidation were immobilized inside mesospaces of a silica-nanoparticles assembly (SiO2NPA) to suppress catalytic deactivation due to agglomeration. The Ir(OH)3 nanoparticles immobilized in SiO2NPA (Ir(OH)3/SiO2NPA) catalyzed water oxidation by visible light irradiation of a solution containing persulfate ion (S2O82−) and tris(2,2′-bipyridine)ruthenium(II) ion ([RuII(bpy)3]2+) as a sacrificial electron acceptor and a photosensitizer, respectively. The yield of oxygen (O2) based on the used amount of S2O82− was maintained over 80% for four repetitive runs using Ir(OH)3/SiO2NPA prepared by the co-accumulation method, although the yield decreased for the reaction system using Ir(OH)3/SiO2NPA prepared by the equilibrium adsorption method or Ir(OH)3 nanoparticles without SiO2NPA support under the same reaction conditions. Immobilization of Ir(OH)3 nanoparticles in Al3+-doped SiO2NPA (Al-SiO2NPA) results in further enhancement of the catalytic stability with the yield of more than 95% at the fourth run of the repetitive experiments.

Boosting Neutral Water Oxidation through Surface Oxygen Modulation

Developing efficient electrocatalysts for oxygen evolution reaction (OER) in pH-neutral electrolyte is crucial for microbial electrolysis cells and electrochemical CO₂ reduction. Unfortunately, the OER kinetics in neutral electrolyte is sluggish due to the low concentration of adsorbed reactants, with overpotentials of neutral OER at present much higher than that in acidic or alkaline electrolyte. Here, hydrated metal cations (Ca²⁺ ) are sought to be incorporated into the state-of-the-art Ru-Ir binary oxide to tailor the surface oxygen environments (lattice-oxygen and adsorbed oxygen species) for efficient neutral OER. Using a sol-gel method, ternary Ru-Ir-Ca oxides are synthesized in atomically homogenous manner, and the obtained catalyst on glassy carbon electrode achieves 10 mA cm^-2 at a low overpotential of 250 mV, with no degradation for 200 h of operation. In situ X-ray absorption spectroscopy, in situ ¹⁸ O isotope-labeling differential electrochemical mass spectrometry, and ¹⁸ O isotope-labeling secondary ion mass spectroscopy studies are carried out. The results reveal that incorporation of Ca²⁺ can enhance the covalency of metal-oxygen bonds and the electrophilic nature of surface metal-bonded oxygen sites; and simultaneously facilitate the adsorption of water molecules on catalyst surface, which accelerates the lattice-oxygen-involved reaction, thus improving the overall OER performance of RuIrCaO_x catalyst.

An iridium-based nanocomposite prepared from an iridium complex with a hydrocarbon-based ligand

For the first time, a chlorobis(cyclooctene)iridium(i) dimer with only a simple hydrocarbon-based ligand is investigated as a heterogeneous catalyst for the oxygen-evolution reaction in the presence of cerium(iv) ammonium nitrate.

Water Oxidation Catalysts for Artificial Photosynthesis

Water oxidation is the primary reaction of both natural and artificial photosynthesis. Developing active and robust water oxidation catalysts (WOCs) is the key to constructing efficient artificial photosynthesis systems, but it is still facing enormous challenges in both fundamental and applied aspects. Here, the recent developments in molecular catalysts and heterogeneous nanoparticle catalysts are reviewed with special emphasis on biomimetic catalysts and the integration of WOCs into artificial photosystems. The highly efficient artificial photosynthesis depends largely on active WOCs integrated into light harvesting materials via rational interface engineering based on in-depth understanding of charge dynamics and the reaction mechanism.

Self-Supported Hierarchical IrO2@NiO Nanoflake Arrays as an Efficient and Durable Catalyst for Electrochemical Oxygen Evolution

Although traditional IrO₂ nanoparticles loaded on a carbon support (IrO₂@C) have been taken as a benchmark catalyst for the oxygen evolution reaction (OER), their catalytic efficiency, operation stability, and IrO₂ utilization are far from satisfactory due to the inferior powdery structure and inevitable corrosion of both IrO₂ and C under the oxidizing potentials. Here, a rational design of a self-supported hierarchical nanocomposite, composed of IrO₂@NiO nanoparticle-built porous nanoflake arrays vertically growing on nickel foam, is proposed, which is demonstrated as a versatile strategy to achieve improved OER activity, remarkable long-term stability, and significantly reduced loading of IrO₂ (0.62 atom %). Impressively, the resultant catalyst drives a steady OER current density of 10 mA cm^-2, requiring 278 mV overpotential in 1.0 M KOH electrolyte for 25 h and outmaneuvring commercial IrO₂@C with much higher mass loading. Further electrochemical investigation and mechanism analysis disclose that the greatly improved electrocatalytic activity stems from the advantageous hierarchical structure and the synergistic effect between IrO₂ and underlying potential-induced NiOOH, whereas the outstanding durability is attributed to the unique role of NiO in preventing IrO₂ dissolution.

Electrocatalytic oxidation of water at a polyoxometalate nanoparticle modified gold electrode

Polyoxometalate nanoparticles, [H₃PMo₁₂O₄₀]NPs, modified gold electrode showed excellent electrocatalytic activity towards water oxidation reaction at an overpotential of 350 mV with a current density of 1.7 mA cm⁻² in neutral pH medium.

Robust noble metal-based electrocatalysts for oxygen evolution reaction

The oxygen evolution reaction (OER) is a kinetically sluggish anodic reaction and requires a large overpotential to deliver appreciable current. Despite the fact that non-precious metal-based alkaline water electrocatalysts are receiving increased attention, noble metal-based electrocatalysts (NMEs) applied in proton exchange membrane water electrolyzers still have advantageous features of larger current and power densities with lower stack cost. Engineering NMEs for OER catalysis with high efficiency, durability and utilization rate is of vital importance in promoting the development of cost-effective renewable energy production and conversion devices. In this tutorial review, we covered the recent progress in the composition and structure optimization of NMEs for OER including Ir- and Ru-based oxides and alloys, and noble-metals beyond Ir and Ru with a variety of morphologies. To shed light on the fundamental science and mechanisms behind composition/structure-performance relationships and activity-stability relationships, integrated experimental and theoretical studies were pursued for illuminating the metal-support interaction, size effect, heteroatom doping effect, phase transformation, degradation processes and single-atom catalysis. Finally, the challenges and outlook are provided for guiding the rational engineering of OER electrocatalysts for applications in renewable energy-related devices.

Enhanced photocatalytic oxygen evolution activity by formation of Ir@IrOx(OH)y core-shell heterostructure

Developing efficient catalysts to accelerate the rate of oxygen evolution reaction (OER) is critical for photocatalytic water-splitting. In this work, metallic Ir, IrO_x(OH)_y, and core-shell Ir@IrO_x(OH)_y were synthesized and employed as OER catalysts for photocatalytic water oxidation. It was found that the Ir@IrO_x(OH)_y core-shell heterostructure catalyst showed the best photocatalytic performance among these three catalysts, with the oxygen evolution rate as high as 59.63 mmol g^-1 h^-1. Detailed investigations revealed that the excellent photocatalytic activity of Ir@IrO_x(OH)_y could be attributed to both the outstanding intrinsic activity of IrO_x(OH)_y shell and the efficient electron transfer between the photosensitizer and catalyst.

Benchmarking Water Oxidation Catalysts Based on Iridium Complexes: Clues and Doubts on the Nature of Active Species

Water oxidation (WO) is a central reaction in the photo- and electro-synthesis of fuels. Iridium complexes have been successfully exploited as water oxidation catalysts (WOCs) with remarkable performances. Herein, we report a systematic study aimed at benchmarking well-known Ir WOCs, when NaIO₄ is used to drive the reaction. In particular, the following complexes were studied: cis-[Ir(ppy)₂ (H₂ O)₂ ]OTf (ppy=2-phenylpyridine) (1), [Cp*Ir(H₂ O)₃ ]NO₃ (Cp*=1,2,3,4,5-pentamethyl-cyclopentadienyl anion) (2), [Cp*Ir(bzpy)Cl] (bzpy=2-benzoylpyridine) (3), [Cp*IrCl₂ (Me₂ -NHC)] (NHC=N-heterocyclic carbene) (4), [Cp*Ir(pyalk)Cl] (pyalk=2-pyridine-isopropanoate) (5), [Cp*Ir(pic)NO₃ ] (pic=2-pyridine-carboxylate) (6), [Cp*Ir{(P(O)(OH)₂ }₃ ]Na (7), and mer-[IrCl₃ (pic)(HOMe)]K (8). Their reactivity was compared with that of IrCl₃ ⋅n H₂ O (9) and [Ir(OH)₆ ]^2- (10). Most measurements were performed in phosphate buffer (0.2 m), in which 2, 4, 5, 6, 7, and 10 showed very high activity (yield close to 100 %, turnover frequency up to 554 min^-1 with 10, the highest ever observed for a WO-driven by NaIO₄ ). The found order of activity is: 10>2≈4>6>5>7>1>9>3>8. Clues concerning the molecular nature of the active species were obtained, whereas its exact nature remains doubtfully.

Stability limits of tin-based electrocatalyst supports

Tin-based oxides are attractive catalyst support materials considered for application in fuel cells and electrolysers. If properly doped, these oxides are relatively good conductors, assuring that ohmic drop in real applications is minimal. Corrosion of dopants, however, will lead to severe performance deterioration. The present work aims to investigate the potential dependent dissolution rates of indium tin oxide (ITO), fluorine doped tin oxide (FTO) and antimony doped tin oxide (ATO) in the broad potential window ranging from −0.6 to 3.2 V_RHE in 0.1 M H₂SO₄ electrolyte. It is shown that in the cathodic part of the studied potential window all oxides dissolve during the electrochemical reduction of the oxide – cathodic dissolution. In case an oxidation potential is applied to the reduced electrode, metal oxidation is accompanied with additional dissolution – anodic dissolution. Additional dissolution is observed during the oxygen evolution reaction. FTO withstands anodic conditions best, while little and strong dissolution is observed for ATO and ITO, respectively. In discussion of possible corrosion mechanisms, obtained dissolution onset potentials are correlated with existing thermodynamic data.

Photo-catalyzed surface hydrolysis of iridium(iii) ions on semiconductors: a facile method for the preparation of semiconductor/IrOx composite photoanodes toward oxygen evolution reaction

Surface hydrolysis of Ir³⁺ induced by photo-generated holes on n-type semiconductors was developed to prepare semiconductor/IrO_x composites for water splitting.

Growth of One-Dimensional RuO2 Nanowires on g-Carbon Nitride: An Active and Stable Bifunctional Electrocatalyst for Hydrogen and Oxygen Evolution Reactions at All pH Values

Development of highly efficient and durable bifunctional electrocatalyst for hydrogen and oxygen evolution reactions (HER and OER) is essential for efficient solar fuel generation. The commercial RuO₂ or RuO₂-based catalysts are highly active toward OER, but their poor stability under different operating conditions is the main obstacle for their commercialization. Herein, we report growth of one-dimensional highly crystalline RuO₂ nanowires on carbon nitride (1D-RuO₂-CN_x) for their applications in HER and OER at all pH values. The 1D-RuO₂-CN_x, as an OER catalyst, exhibits a low onset overpotential of ∼200 mV in both acidic and basic media, whereas Tafel slopes are 52 and 56 mV/dec in acidic and basic media, respectively. This catalyst requires a low overpotential of 250 and 260 mV to drive the current density of 10 mA cm^-2 in acidic and basic media, respectively. The mass activity of 1D-RuO₂-CN_x catalyst is 352 mA mg^-1, which is ∼14 times higher than that of commercial RuO₂. Most importantly, the 1D-RuO₂-CN_x catalyst has remarkably higher stability compared to commercial RuO₂. This catalyst also exhibits superior HER activity with a current density of 10 mAcm^-2 at ∼93 and 95 mV in acidic and basic media. The HER Tafel slopes of this catalyst are 40 mV/dec in acidic condition and 70 mV/dec in basic condition. The HER activity of this catalyst is slightly lower than Pt/C in acidic media, whereas in basic media it is comparable or even better than that of Pt/C at higher overpotentials. The HER stability of this catalyst is also better than that of Pt/C in all pH solutions. This superior catalytic activity of 1D-RuO₂-CN_x composite can be attributed to catalyst-support interaction, enhanced mass and electron transport, one-dimensional morphology, and highly crystalline rutile RuO₂ structure.