Energy Barrier Of Rate-determining Step Research Articles

Self-Powered Electrochemical CO2 Conversion Enabled by a Multifunctional Carbon-Based Electrocatalyst and a Rechargeable Zn-Air Battery.

Multifunctional electrocatalysts are required for diverse clean energy-related technologies (e.g., electrochemical CO2 reduction reaction (CO2RR) and metal-air batteries). Herein, a nitrogen and fluorine co-doped carbon nanotube (NFCNT) is reported to simultaneously achieve multifunctional catalytic activities for CO2RR, oxygen reduction reaction (ORR), and oxygen evolution reaction (OER). Theoretical calculations reveal that the superior multifunctional catalytic activities of NFCNT are attributed to the synergistic effect of nitrogen and fluorine co-doping to induce charge redistribution and decrease the energy barrier of rate-determining step for different electrocatalytic reactions. Furthermore, the rechargeable Zn-air battery (ZAB) with NFCNT electrode delivers a high peak power density of 230mWcm-2 and superior durability over 100 cycles, outperforming the ZAB with Pt/C+RuO2 based electrodes. More importantly, a self-driven CO2 electrolysis unit powered by the as-assembled ZABs is developed, which achieves 80% CO Faraday efficiency and 60% total energy efficiency. This work provides a new insight into the exploration of highly efficient multifunctional carbon-based electrocatalysts for novel energy-related applications.

KIr4O8 Nanowires with Rich Hydroxyl Promote Oxygen Evolution Reaction in Proton Exchange Membrane Water Electrolyzer.

The sluggish kinetics for anodic oxygen evolution reaction (OER) and insufficient catalytic performance over the corresponding Ir-based catalysts are still enormous challenges in proton exchange membrane water electrolyzer (PEMWE). Herein, it is reported that KIr4O8 nanowires anode catalyst with more exposed active sites and rich hydroxyl achieves a current density of 1.0 A cm-2 at 1.68V and possesses excellent catalytic stability with 1230h in PEMWE. Combining in situ Raman spectroscopy and differential electrochemical mass spectroscopy results, the modified adsorbate evolution mechanism is proposed, wherein the rich hydroxyl in the inherent structure of KIr4O8 nanowires directly participates in the catalytic process for favoring the OER. Density functional theory calculation results further suggest that the enhanced proximity between Ir (d) and O (p) band center in KIr4O8 can strengthen the covalence of Ir-O, facilitate the electron transfer between adsorbents and active sites, and decrease the energy barrier of rate-determining step from OH* to O* during the OER.

Planar Chlorination Engineering: A Strategy of Completely Breaking the Geometric Symmetry of Fe-N4 Site for Boosting Oxygen Electroreduction.

Introducing asymmetric elements and breaking the geometric symmetry of traditional metal-N4 site for boosting oxygen reduction reaction (ORR) are meaningful and challenging. Herein, the planar chlorination engineering of Fe-N4 site is first proposed for remarkably improving the ORR activity. The Fe-N4/CNCl catalyst with broken symmetry exhibits a half-wave potential (E1/2) of 0.917V versus RHE, 49 and 72mV higher than those of traditional Fe-N4/CN and commercial 20 wt% Pt/C catalysts. The Fe-N4/CNCl catalyst also has excellent stability for 25000 cycles and good methanol tolerance ability. For Zn-air battery test, the Fe-N4/CNCl catalyst has the maximum power density of 228mW cm-2 and outstanding stability during 150h charge-discharge test, as the promising substitute of Pt-based catalysts in energy storage and conversion devices. The density functional theory calculation demonstrates that the adjacent C─Cl bond effectively breaks the symmetry of Fe-N4 site, downward shifts the d-band center of Fe, facilitates the reduction and release of OH*, and remarkably lowers the energy barrier of rate-determining step. This work reveals the enormous potential of planar chlorination engineering for boosting the ORR activity of traditional metal-N4 site by thoroughly breaking their geometric symmetry.

Single-Pd-Site Catalyst Induced by Different Dimensional Nitrogen of N-Doping Carbon for Efficient Hydroaminocarbonylation of Alkynes.

The unsaturated amides are traditionally synthesized by acylation of carboxylic acids or hydration of nitrile compounds but are rarely investigated by hydroaminocarbonylation of alkynes using heterogeneous single-metal-site catalysts (HSMSCs). Herein, single-Pd-site catalysts supported on N-doping carbon (NC) with different nitrogen dimensions inherited from corresponding metal-organic-framework precursors are successfully synthesized. 2D NC-supported single-Pd-site (Pd1/NC-2D) exhibited the best performance with near 100% selectivity and 76% yield of acrylamide for acetylene hydroaminocarbonylation with better stability, superior to those of Pd1/NC-3D, single-metal-site/nanoparticle coexisting catalyst, and nanoparticle catalyst. The coordination environment and molecular evolution of the single-Pd-site during the process of acetylene hydroaminocarbonylation on Pd1/NC-2D are detailly illuminated by various characterizations and density functional theoretical calculations (DFT). DFT also showed the energy barrier of rate-determining step on Pd1/NC-2D is lower than that of Pd1/NC-3D. Furthermore, Pd1/NC-2D catalyst illustrated the general applicability of the hydroaminocarbonylation for various alkynes.

Embedded metal inducing electron accumulation of multi-mesoporous pyridinic-N doped carbon for enhancing *COOH adsorption in electrochemical CO2 reduction

Nitrogen-doped carbon (NC) for carbon dioxide reduction reaction (CO2RR) are promising candidates. However, its selectivity and activity are limited by the undeveloped pore and limited electronic effect. Herein, we propose a strategy of three birds with one stone to utilize the embedded nickel for inducing electron-rich pyridinic-NC with multimodal porous structure based on the hard template assisted sacrificial template method. Nickel acts as sacrificial agent and inducer to facilitate the formation of mesoporous and reactive nitrogen species for capturing and activating reactants, respectively. Besides, we find that nickel as electron donor endows the NC with electron accumulation for participating actively in PCET reaction. The resultant Ni0.87-NC-1-AE exhibits an eminent FECO of 97.6 % with excellent activity and stability. In-situ Raman spectroscopy and the projected crystal orbital Hamilton populations (pCOHP) confirm the accumulation of *COOH and the stable adsorption of *COOH on the surface. What’s more, the well-designed experiments and DFT calculations accurately locate pyridinic-N as the active site, where the aggregated electrons donated by nickel significantly reduce the energy barrier of rate-determining step and promote the desorption of CO. This work provides an instructive perspective to construct advanced NC electrocatalysts with high mass transfer efficiency and high catalytic activity.

Accelerated Dynamic Reconstruction in Metal-Organic Frameworks with Ligand Defects for Selective Electrooxidation of Amines to Azos Coupling with Hydrogen Production.

Electrosynthesis coupled hydrogen production (ESHP) mostly involves catalyst reconstruction in aqueous phase, but accurately identifying and controlling the process is still a challenge. Herein, we modulated the electronic structure and exposed unsaturated sites of metal-organic frameworks (MOFs) via ligand defect to promote the reconstruction of catalyst for azo electrosynthesis (ESA) coupled with hydrogen production overall reaction. The monolayer Ni-MOFs achieved 89.8 % Faraday efficiency and 90.8 % selectivity for the electrooxidation of 1-methyl-1H-pyrazol-3-amine (Pyr-NH2) to azo, and an 18.5-fold increase in H2 production compared to overall water splitting. Operando X-ray absorption fine spectroscopy (XAFS) and various in situ spectroscopy confirm that the ligand defect promotes the potential dependent dynamic reconstruction of Ni(OH)2 and NiOOH, and the reabsorption of ligand significantly lowers the energy barrier of rate-determining step (*Pyr-NH to *Pyr-N). This work provides theoretical guidance for modulation of electrocatalyst reconstruction to achieve highly selective ESHP.

Sub-2nm IrRuNiMoCo High-Entropy Alloy with Iridium-Rich Medium-Entropy Oxide Shell to Boost Acidic Oxygen Evolution.

Ensuring high catalytic activity and durability at low iridium(Ir)usage is still a big challenge for the development of electrocatalysts toward oxygen evolution reaction (OER) in proton exchange membrane water electrolysis (PEMWE). Here, a rapid liquid-reduction combined with surface galvanic replacement strategy is reported to synthesize the sub 2nm high-entropy alloy (HEA) nanoparticles featured with Ir-rich IrRuNiMo medium-entropy oxide shell (Ir-MEO) and a IrRuCoNiMo HEA core (HEA@Ir-MEO). Advanced spectroscopies reveal that the Ir-rich MEO shell inhibits the severe structural evolution of transition metals upon the OER, thus guaranteeing the structural stability. In situ differential electrochemical mass spectrometry, activation energy analysis and theoretical calculations unveil that the OER on HEA@Ir-MEO follows an adsorbate evolution mechanism pathway, where the energy barrier of rate-determining step is substantially lowered. The optimized catalyst delivers the excellent performance (1.85V/3.0 A cm-2@80°C), long-term stability (>500 h@1.0 Acm-2), and low energy consumption (3.98kWh Nm-3 H2 @1.0 A cm-2) in PEMWE with low Ir usage of ≈0.4 mg cm-2, realizing the dramatical reduction of hydrogen (H2) production cost to 0.88 dollarper kg (H2).

Internal electric field modulation by copper vacancy concentration of cuprous sulfide nanosheets for enhanced selective CO2 photoreduction

Although the internal electric field (IEF) of photocatalysts is acknowledged as a potent driving force for photocharge separation, modulating the IEF intensity to achieve enhanced photocatalytic performances remains a challenge. Herein, cuprous sulfide nanosheets with different Cu vacancy concentration were employed to study IEF modulation and corresponding direct charge transfer. Among the samples, Cu1.8S nanosheets possessed intensified IEF intensity compared with those of Cu2S and Cu1.95S nanosheets, suggesting that an enhanced IEF intensity could be achieved by introducing more Cu vacancies. This intensified IEF of Cu1.8S nanosheets induced numerous photogenerated electrons to migrate to its surface, and the dissociative electrons were then captured by Cu vacancies, resulting in efficient charge separation spatially. In addition, the Cu vacancies on Cu1.8S nanosheets accumulated electrons as active sites to lower the energy barrier of rate-determining step of CO2 photoreduction, leading to the selective conversion of CO2 to CO. Herein, the manipulation of IEF intensity through Cu vacancy concentration regulation of cuprous sulfide photocatalysts for efficient charge separation has been discussed, providing a scientific strategy to rationally improve photocatalytic performances for solar energy conversion.

Readily available, biocompatible sodium citrate catalyst for efficient glycerol carbonate production through transesterification of glycerol and ethylene carbonate

As a by-product in the biodiesel industry, surplus glycerol (Gly) has attracted considerable concern. The transesterification of Gly is one of the most promising reaction pathways to convert Gly to a value-added chemical such as glycerol carbonate (GC). Currently, development of catalysts for the transesterification with high activity is underway. Herein, a readily available biocompatible sodium citrate (Na3-citrate) catalyst was applied for efficient production of GC through transesterification of Gly and ethylene carbonate (EC). The effects of reaction parameters such as reaction time, EC to Gly molar ratio, reaction temperature, and catalyst loading on the reaction were investigated. Under optimal conditions, 0.01 wt% of Na3-citrate showed high catalytic activities with a Gly conversion of > 99 %, a GC yield of > 99 %, and a high turnover frequency of 16,925 gGC⋅g−1Cat⋅h−1. Furthermore, the reaction kinetic model for transesterification was established, and the activation energy of the Na3-citrate catalyzed transesterification was calculated as 51.3 kJ·mol−1. In addition, density functional theory calculations were used to unveil mechanism of the Na3-citrate catalyzed transesterification. It revealed that citrate anion of Na3-citrate could activate hydroxy of Gly via strong hydrogen bond interaction, thereby efficiently promoting transesterification by reducing energy barrier of rate-determining step.

Designed synthesis N-doped sulfonated bifunctional catalyst for efficient 5-hydroxymethylfurfural production: A pyridinic N-triggered proton-shift mechanism at basic sites

Considering the essential role of acidic/basic sites in efficient 5-hydroxymentylfurfural (5-HMF) production from cellulose, the superiority of acid-base bifunctional catalysts was unfolded. However, the lack of in-depth research on catalytic mechanism of active sites, especially the basic sites, greatly hinders its chemical utilization. Herein, N-doped sulfonated bifunctional catalysts were rationally designed. The synthesized NCS-1H catalyst exhibited outstanding activity and recyclability. Favorable 5-HMF yield of 50.0 % with 62.9 % of selectivity was obtained. Spin polarized density function theory (DFT) analysis revealed a pyridinic N-triggered proton-shift mechanism of glucose isomerization. Catalyzed by pyridinic N, the proton of glucose on C2 was shifted to O5, leading to the cleavage of C1-O1 bond. As a result, an enol intermediate was formed and converted to fructose. Compared to pyrrolic N and graphitic N, pyridinic N significantly reduced the reaction energy barrier of rate-determining step to 0.79 eV, i.e., C1-O1 bond cleavage.

Regulation engineering of lignin-derived N-doped carbon-supported FeNi alloy particles towards efficient electrocatalytic oxygen evolution

The electrocatalytic hydrogen production technology using green energy can reduce the dependence on traditional fossil fuels and the net emission of carbon dioxide. However, the high overpotential and low efficiency of the oxygen evolution reaction (OER) on the anode limit the overall water splitting. Here, carboxylic acid ligands with coordination function were designed and synthesized by modifying the surface functional groups of lignin. Bimetallic functionally-doped lignin-derived carbon materials (Fe4Ni4@NC) were synthesized by continuous ion layer adsorption and pyrolysis process using Fe and Ni transition metals as source and template. The catalyst requires only 214 mV of overpotential to produce 10 mA·cm−2, which has much faster catalytic kinetics. Theoretical calculation shows that the energy barrier of rate-determining step in OER is reduced by appropriate feed ratio and nano-size. The facile strategy opens up an economical and efficient solution for designing OER electrocatalysts with high catalytic activity and excellent stability.

Electronic and Geometric Effects Endow PtRh Jagged Nanowires with Superior Ethanol Oxidation Catalysis.

Complete ethanol oxidation reaction (EOR) in C1 pathway with 12 transferred electrons is highly desirable yet challenging in direct ethanol fuel cells. Herein, PtRh jagged nanowires synthesized via a simple wet-chemical approach exhibit exceptional EOR mass activity of 1.63 A mgPt-1 and specific activity of 4.07mA cm-2 , 3.62-fold and 4.28-folds increments relative to Pt/C, respectively. High proportions of 69.33% and 73.42% of initial activity are also retained after chronoamperometric test (80000 s) and 1500 consecutive potential cycles, respectively. More importantly, it is found that PtRh jagged nanowires possess superb anti-CO poisoning capability. Combining X-ray absorption spectroscopy, X-ray photoelectron spectroscopy as well as density functional theory calculations unveil that the remarkable catalytic activity and CO tolerance stem from both the Rh-induced electronic effect and geometric effect (manifested by shortened Pt─Pt bond length and shrinkage of lattice constants), which facilitates EOR catalysis in C1 pathway and improves reaction kinetics by reducing energy barriers of rate-determining steps (such as *CO → *COOH). The C1 pathway efficiency of PtRh jagged nanowires is further verified by the high intensity of CO2 relative to CH3 COOH/CH3 CHO in infrared reflection absorption spectroscopy.

Solar-Driven Reforming of Methane and Nitrogen to Methanol and Ammonium on Iron-Modified Zeolite under Ambient Conditions in Water.

Artificial photosynthesis from selective methane oxidation or nitrogen reduction to value-added chemicals provides a promising pathway for the sustainable chemical industry, while still remaining a great challenge due to the extreme difficulty in C-H and N≡N bond cleavage under ambient conditions. Catalysts that can cocatalyze these two reactions simultaneously are rarely reported. Here, Fe-ZSM-5 with highly dispersed extra-framework Fe-oxo species enables efficient and selective photocatalytic conversion of methane and nitrogen to coproduce methanol and ammonia using H2O as the redox reagent under ambient conditions. The optimized Fe-ZSM-5 photocatalyst achieves up to 0.88 mol/molFe·h of methanol products with 97% selectivity. Meanwhile, the productivity of ammonia is 0.61 mol/molFe·h. In situ EPR and DRIFT studies disclose that water serves as a redox reagent to provide hydroxyl radicals for methane oxidation and protons for nitrogen hydrogenation. Quantum chemical calculations revealed that Fe-oxo species play a significant role in the coactivation of methane and nitrogen molecules, which lowers the energy barriers of rate-determining steps for methanol and ammonia generation.

Flower-like CoP coated NiMoO4 nanorods as self-supported core-shell heterojunction electrode can facilitate oxygen evolution reaction at a low overpotential

Transition metal phosphates, as the most promising catalyst for oxygen evolution reaction (OER), have attracted much attention in recent years. In this work, CoP nanosheets are electrodeposited on NiMoO4 nanorods to form a self-supported core-shell structure electrode with CoP-NiMoO4 heterojunctions. Density functional theory (DFT) calculations demonstrate CoP-12@NiMoO4/NF electrode can reduce reaction energy barrier of rate-determining step (RDS) from 8.743 eV to 2.519 eV. In 1 M KOH, the electrode shows a low overpotential of 165 mV (10 mA cm−2), a low Tafel slope of 45 mV dec−1 and the significant stability. Compared with other non-noble metal OER electrocatalysts, the prepared CoP-12@NiMoO4/NF electrode exhibits better electrocatalytic activity, which can provide a new choice of electrocatalysts for future practical applications.

Construction of Zn-doped RuO2 nanowires for efficient and stable water oxidation in acidic media

Oxygen evolution reaction catalysts capable of working efficiently in acidic media are highly demanded for the commercialization of proton exchange membrane water electrolysis. Herein, we report a Zn-doped RuO2 nanowire array electrocatalyst with outstanding catalytic performance for the oxygen evolution reaction under acidic conditions. Overpotentials as low as 173, 304, and 373 mV are achieved at 10, 500, and 1000 mA cm−2, respectively, with robust stability reaching to 1000 h at 10 mA cm−2. Experimental and theoretical investigations establish a clear synergistic effect of Zn dopants and oxygen vacancies on regulating the binding configurations of oxygenated adsorbates on the active centers, which then enables an alternative Ru−Zn dual-site oxide path of the reaction. Due to the change of reaction pathways, the energy barrier of rate-determining step is reduced, and the over-oxidation of Ru active sites is alleviated. As a result, the catalytic activity and stability are significantly enhanced.

Tensile stress enhanced dynamic oxygen vacancies on interlayer stretched Bi2O2CO3 as pivotal knobs to promote sustainable selective CO2 photoreduction

Surface oxygen vacancies (OVs) with abundant localized electrons on bismuth-oxygen based photocatalysts are proved to have the ability to capture and activate CO2. However, the surface OVs are easily filled with oxygen-containing species and destroyed, losing their effects as active sites and hindering the subsequent CO2 photoreduction. For realistic and sustainable CO2 photoreduction, constructing sustainable and stable surface OVs as active sites on photocatalysts is essential. This work shows the synthesis of interlayer stretched Bi2O2CO3 ultrathin nanosheets with tensile stress, which are beneficial to continuously generating light-induced dynamic OVs. With sufficient active sites, excellent, stable, and selective photoreduction of CO2 to CO under simulated solar light is achieved. The light-induced OVs can reduce the energy barrier of rate-determining step, resulting in the 100% product selectivity. The results presented herein demonstrate the effect of dynamic OVs induced by interlayer tensile strain on catalysts for the enhanced selective CO2 photoreduction process.

Tuning the atomic configuration environment of MnN4 sites by Co cooperation for efficient oxygen reduction

Carbon-based N-coordinated Mn (Mn-Nx/C) single-atom electrocatalysts are considered as one of the most desirable non-precious oxygen reduction reaction (ORR) candidates due to their insignificant Fenton reactivity, high abundance, and intriguing electrocatalytic performance. However, current Mn-Nx/C single-atom electrocatalysts suffer from high overpotentials because of their low intrinsic activity and unsatisfactory chemical stability. Herein, through an in-situ polymerization-assisted pyrolysis, the Co as a second metal is introduced into the Mn-Nx/C system to construct Co, Mn-Nx dual-metallic sites, which atomically disperse in N-doped 1D carbon nanorods, denoted as Co, Mn-N/CNR and hereafter. Using electron microscopy and X-ray absorption spectroscopy (XAS) techniques, we verify the uniform dispersion of CoN4 and MnN4 atomic sites and confirm the effect of Co doping on the MnN4 electronic structure. Density functional theory (DFT) calculations further elucidate that the energy barrier of rate-determining step (*OH desorption) decreases over the 2 N-bridged MnCoN6 moieties related to the pure MnN4. This work provides an effective strategy to modulate the local coordination environment and electronic structure of MnN4 active sites for improving their ORR activity and stability.

Reaction selectivity-regulation via interfacial reconstruction for preventing hazardous slime generation: Driving mechanism of Pb-based anode with oxygen vacancy-rich MnO2

Limited selectivity in coexisted anodic reactions on lead-based anodes leads to severe lead-dissolution, anodic slime-generation and high energy-consumption. Herein, anodic reaction selectivity was regulated by phase-controlled MnO2 (MnO2-PC) on lead-based anodes which was prepared under a lower voltage and a higher Mn2+ concentration. Compared with α-MnO2 formed under normal conditions, composite phase of γ-MnO2 and ε-MnO2 was obtained from MnO2-PC with specific crystal planes exposure and rich oxygen vacancy defects enabling higher oxygen evolution reaction selectivity (OER, overpotential reduced by ∼99 mV). Meanwhile, reaction of lead corrosion and manganese oxidation, as the main reasons for generation of hazardous lead-containing slime, were synergistically suppressed by 86.8% and 91.7%, respectively. Density functional theory (DFT) calculations further clarified that the change in coordinative environment of Mn and O brought lower energy barrier of rate-determining step of OER and stronger electron delocalization ability of MnO2, beneficial for promoting the catalysis of OER. MnO2-PC also displayed excellent inhibition effects on the generation and accumulation of Mn3+, an important precursor of MnO2 slime, and changed the selectivity in the oxidative path of Mn2+. The long-term electrolysis tests proved the stable performance of Pb–Ag/MnO2-PC. This study provides an efficient, low-cost, high economic benefits and environmental-friendly strategy for designing target functional materials towards reaction selectivity regulation via phase-control and microstructure reconstruction.

PO6 geometric configuration unit enhanced electrocatalytic performance of Co3O4 in acidic oxygen evolution

It is challenging to develop high-efficient and stable nonprecious metal-based electrocatalyst for oxygen evolution reaction (OER) in acid for proton exchange membrane (PEM) water splitting. Herein, P atoms were introduced into the lattice of spinel Co3O4 (P-Co3O4) to replace with octahedral coordinated Co3+ via a hydrothermal process following a thermal treatment. The formation of PO6 geometric configuration unit in Co3O4 can trigger electron rearrangement around Co ions, which resulted in the high-active Co2+ site on the surface, significantly decreasing the energy barrier of rate-determining step for OER. Moreover, the weaker covalency of the Co 3d-O 2p bond and higher formation energy of oxygen vacancy around Co2+ in P-Co3O4 inhibited the participation of lattice oxygen during OER process, enabling that P-Co3O4 can work stably in acidic media. The obtained P-Co3O4 afforded satisfying stability over 30 h in a PEM electrolysis device with an overpotential of 400 mV@10 mA/cm2 in 0.1 M HClO4.

Charge redistribution in FeOOH nanoarray by ecological oxygen-reduction deposition for boosting electrocatalytic water oxidation

The exploration for economic and ecological strategies to fulfill macro-synthesis of highly efficient nano-electrocatalysts has always been a key concern for electrocatalytic energy conversion field. Herein, a room-temperature oxygen-reduction deposition method was proposed to achieve the scaled-up preparation of Ni incorporated polycrystalline FeOOH (Ni-FeOOH) nanosheets on Fe metal. Due to the reduction potential difference between Fe metal and O2 in the atmosphere, the O2 reduction would thermodynamically occur in the designed Ni2+ liquid layer covered on Fe metal. Therefore, Ni-FeOOH nanosheets would be in-situ generated on the surface within a short cycle of 20 min. Furthermore, the Ni heteroatom induced the charge redistribution in FeOOH, where the average valence of both Fe and Ni cations was shifted to high states. As a result, the decreased Fermi level and increased electron acceptor significantly reduced the energy barrier of rate-determining step (by ∼1.4 eV). Finally, polycrystalline Ni-FeOOH shows a low overpotential of 239 mV at 10 mA/cm2, a low Tafel slope of 33 mV dec-1, and a dramatically increased TOF value (32.6 times higher than pristine sample), which even surpass some mainstream electrocatalysts consisting of noble-metal composition. This work provides an in-depth insight on Ni-Fe interaction mechanism, as well as a highly economic and ecological strategy to design and synthesize scaled-up FeOOH based electrocatalysts for water oxidation.