High Water Oxidation Activity Research Articles

Nickel-Iron-Modified 2D Metal-Organic Framework as a Tunable Precatalyst for Electrochemical Water Oxidation.

Mixed-metal metal-organic framework (MOF)-based water oxidation precatalysts have aroused a great deal of attention due to their remarkable catalytic performance. Yet, despite significant advancement in this field, there is still a need to design new MOF platforms that allow simple and systematic control over the final catalyst's metal composition. Here, we show that a Zr-BTB 2D-MOF could be used to construct a series of Ni-Fe-based oxide hydroxide water oxidation precatalysts with diverse Ni-Fe compositions. In situ Raman spectroscopy characterization revealed that the MOF precatalysts could be electrochemically converted to the active catalysts (NiFeOOH). In turn, it was found that the highest water oxidation activity was obtained with a catalyst containing a 47:53 Ni:Fe molar ratio. Additionally, the obtained catalyst is also active toward electrochemical methanol oxidation, exhibiting high selectivity toward the formation of formic acid. Hence, these results could pave the way for the development of efficient electrocatalytic materials for a variety of oxidative reactions.

Observation of Support-Dependent Water Oxidation Kinetics on Molecularly-Derived Heterogeneous Ir-Oxide Catalysts

Heterogeneous water oxidation catalysis is central to the development of renewable energy technologies. Understanding the material properties that determine water oxidation activity is critically important for the design of efficient and durable water oxidation catalysts. The electronic structure and surface chemistry of the catalyst influence the accumulation of oxidative charge (holes) on catalytically active sites. Hole distribution dynamics and molecular structure of the active sites are generally accepted to jointly determine activity. However, these properties are typically inherent to materials; it has been difficult to independently tune them on most heterogeneous catalysts, hindering the development of an understanding of their relative influence. Herein, heterogenized dinuclear Ir catalysts (Ir-DHC) supported on different oxides were employed to overcome this challenge. It was found that at low temperatures (288-298 K), the photocatalytic activities of Ir-DHC on indium tin oxide (ITO) and CeO2 supports are comparable within the studied range of fluences (62-151 mW/cm2). By contrast, at higher temperatures (310-323 K), Ir-DHC on ITO exhibits a ∼100% higher water oxidation activity than on CeO2. The divergent activities under these conditions were attributed to the distinct abilities of the supporting substrates to redistribute surface holes. Specifically, ITO is relatively efficient in distributing photogenerated holes between Ir-DHC active sites, whereas CeO2 exhibits sluggish hole redistribution because of its reducibility, thereby limiting hole accumulation at Ir-DHC active sites. The temperature dependence was explained by a temperature induced switch in the rate-determining step of the reaction. These findings highlight the critical role of the supporting substrate in determining the turnover at active sites of heterogeneous catalysts.

Atomically dispersed Ir catalysts exhibit support-dependent water oxidation kinetics during photocatalysis.

Heterogeneous water oxidation catalysis is central to the development of renewable energy technologies. Recent research has suggested that the reaction mechanisms are sensitive to the hole density at the active sites. However, these previous results were obtained on catalysts of different materials featuring distinct active sites, making it difficult to discriminate between competing explanations. Here, a comparison study based on heterogenized dinuclear Ir catalysts (Ir-DHC), which feature the same type of active site on different supports, is reported. The prototypical reaction was water oxidation triggered by pulsed irradiation of suspensions containing a light sensitizer, Ru(bpy)32+, and a sacrificial electron scavenger, S2O82-. It was found that at relatively low temperatures (288-298 K), the water oxidation activities of Ir-DHC on indium tin oxide (ITO) and CeO2 supports were comparable within the studied range of fluences (62-151 mW cm-2). By contrast, at higher temperatures (310-323 K), Ir-DHC on ITO exhibited a ca. 100% higher water oxidation activity than on CeO2. The divergent activities were attributed to the distinct abilities of the supporting substrates in redistributing holes. The differences were only apparent at relatively high temperatures when hole redistribution to the active site became a limiting factor. These findings highlight the critical role of the supporting substrate in determining the turnover at active sites of heterogeneous catalysts.

Coordination modulation of iridium single-atom catalyst maximizing water oxidation activity

Single-atom catalysts (SACs) have attracted tremendous research interests in various energy-related fields because of their high activity, selectivity and 100% atom utilization. However, it is still a challenge to enhance the intrinsic and specific activity of SACs. Herein, we present an approach to fabricate a high surface distribution density of iridium (Ir) SAC on nickel-iron sulfide nanosheet arrays substrate (Ir1/NFS), which delivers a high water oxidation activity. The Ir1/NFS catalyst offers a low overpotential of ~170 mV at a current density of 10 mA cm−2 and a high turnover frequency of 9.85 s−1 at an overpotential of 300 mV in 1.0 M KOH solution. At the same time, the Ir1/NFS catalyst exhibits a high stability performance, reaching a lifespan up to 350 hours at a current density of 100 mA cm−2. First-principles calculations reveal that the electronic structures of Ir atoms are significantly regulated by the sulfide substrate, endowing an energetically favorable reaction pathway. This work represents a promising strategy to fabricate high surface distribution density single-atom catalysts with high activity and durability for electrochemical water splitting.

Structure‐Activity Relationship in a Cobalt Aluminate Nanoparticle Cocatalyst with a Graphitic Carbon Nitride Photocatalyst for Visible‐Light Water Oxidation

AbstractDevelopment of a visible‐light‐driven water oxidation system utilizing a photocatalyst that consists only of earth‐abundant elements is a challenge in artificial photosynthesis. Here the structure of nanoparticulate cobalt aluminate spinel (CoAl2O4) dispersed on a graphitic carbon nitride (g‐C3N4) photocatalyst was studied by electron microscopy and X‐ray absorption fine‐structure spectroscopy. The CoAl2O4 nanoparticle, which is a good cocatalyst for visible‐light water oxidation on the g‐C3N4 photocatalyst, was loaded onto the photocatalyst by a reverse micelle method using Co(NO3)2 ⋅ 6H2O and Al(NO)3 ⋅ 9H2O as precursors, followed by heating at elevated temperatures in air. The O2 evolution activity was enhanced with increasing the heating temperature, reaching a plateau at 573–673 K, beyond which it decreased. Physicochemical analyses indicated that the formation of CoAl2O4 spinel nanoparticles with a minimal extent of aggregation was the key factor to obtain high water oxidation activity.

Mono-/Multinuclear Water Oxidation Catalysts.

Water splitting, in which water molecules can be transformed into hydrogen and oxygen, is an appealing energy conversion and transformation strategy to address the environmental and energy crisis. The oxygen evolution reaction (OER) is dynamically slow, which limits energy conversion efficiency during the water-splitting process and requires high-efficiency water oxidation catalysts (WOCs) to overcome the OER energy barrier. It is generally accepted that multinuclear WOCs possess superior OER performances, as demonstrated by the CaMn4 O5 cluster in photosystem II (PSII), which can catalyze the OER efficiently with a very low overpotential. Inspired by the CaMn4 O5 cluster in PSII, some multinuclear WOCs were synthesized that could catalyze water oxidation. In addition, some mononuclear molecular WOCs also show high water oxidation activity. However, it cannot be excluded that the high activity arises from the formation of dimeric species. Recently, some mononuclear heterogeneous WOCs showed a high water oxidation activity, which testified that mononuclear active sites with suitable coordination surroundings could also catalyze water oxidation efficiently. This Review focuses on recent progress in the development of mono-/multinuclear homo- and heterogeneous catalysts for water oxidation. The active sites and possible catalytic mechanisms for water oxidation on the mono-/multinuclear WOCs are provided.

Electronically-Coupled Phase Boundaries in α-Fe2O3/Fe3O4 Nanocomposite Photoanodes for Enhanced Water Oxidation

Photoelectrochemical (PEC) water splitting reactions are promising for sustainable hydrogen production from renewable sources. We report here, the preparation of α-Fe2O3/Fe3O4 composite films via a single-step chemical vapor deposition of [Fe(OtBu)3]2 and their use as efficient photoanode materials in PEC setups. Film thickness and phase segregation was controlled by varying the deposition time and corroborated through cross-section Raman spectroscopy and scanning electron microscopy. The highest water oxidation activity (0.48 mA/cm2 at 1.23 V vs RHE) using intermittent AM 1.5 G (100 mW/cm2) standard illumination was found for hybrid films with a thickness of 11 μm. This phenomenon is attributed to an improved electron transport resulting from a higher magnetite content toward the substrate interface and an increased light absorption due to the hematite layer mainly located at the top surface of the film. The observed high efficiency of α-Fe2O3/Fe3O4 nanocomposite photoanodes is attributed to the close pr...

Impact of nanoparticle size and lattice oxygen on water oxidation on NiFeOxHy

NiFeOxHy are the most active catalysts for oxygen evolution in a base. For this reason, they are used widely in alkaline electrolysers. Several open questions remain as to the reason for their exceptionally high catalytic activity. Here we use a model system of mass-selected NiFe nanoparticles and isotope labelling experiments to show that oxygen evolution in 1 M KOH does not proceed via lattice exchange. We complement our activity measurements with electrochemistry–mass spectrometry, taken under operando conditions, and transmission electron microscopy and low-energy ion-scattering spectroscopy, taken ex situ. Together with the trends in particle size, the isotope results indicate that oxygen evolution is limited to the near-surface region. Using the surface area of the particles, we determined that the turnover frequency was 6.2 ± 1.6 s−1 at an overpotential of 0.3 V, which is, to the best of our knowledge, the highest reported for oxygen evolution in alkaline solution. The reason for the high water-oxidation activity of Ni(Fe)OxHy catalysts in alkaline electrolyte is not yet well understood. Now, Chorkendorff and co-workers report that oxygen evolution is limited to the near-surface region by measuring the activity trends of mass-selected NiFe nanoparticles.

Efficient Solar‐Driven Water Oxidation over Perovskite‐Type BaNbO2N Photoanodes Absorbing Visible Light up to 740 nm

AbstractPhotoelectrochemical water splitting using semiconductors absorbing a wide range of visible light is a potentially attractive means of harvesting large portions of the solar spectrum. However, this is also very challenging because narrowing the semiconductor band gap lowers the driving force for photoreactions. Herein, a highly active perovskite BaNbO2N exhibiting photoexcitation up to 740 nm for water oxidation is reported. The synthesis route, consisting of moderate nitridation and subsequent annealing in inert Ar flow, enhances the crystallinity of the BaNbO2N surface without inducing the reduction of the Nb species. As a result, a particulate BaNbO2N photoanode exhibits a photocurrent of 5.2 mA cm−2 at 1.23 VRHE under simulated solar irradiation, which is the highest yet reported for an oxynitride responsive at wavelengths above 600 nm. Suppressing the reduction of B‐site cations during the synthesis of perovskite AB(O,N)3, which otherwise results in surface defects or impurities, is critical for achieving high water oxidation activity.

Facile Integration between Si and Catalyst for High-Performance Photoanodes by a Multifunctional Bridging Layer.

Designing high-quality interfaces is crucial for high-performance photoelectrochemical (PEC) water-splitting devices. Here, we demonstrate a facile integration between polycrystalline n+p-Si and NiFe-layered double hydroxide (LDH) nanosheet array by a partially activated Ni (Ni/NiOx) bridging layer for the excellent PEC water oxidation. In this model system, the thermally deposited Ni interlayer protects Si against corrosion and makes good contact with Si, and NiOx has a high capacity of hole accumulation and strong bonding with the electrodeposited NiFe-LDH due to the similarity in material composition and structure, facilitating transfer of accumulated holes to the catalyst. In addition, the back illumination configuration makes NiFe-LDH sufficiently thick for more catalytically active sites without compromising Si light absorption. This earth-abundant multicomponent photoanode affords the PEC performance with an onset potential of ∼0.78 V versus reversible hydrogen electrode (RHE), a photocurrent density of ∼37 mA cm-2 at 1.23 V versus RHE, and retains good stability in 1.0 M KOH, the highest water oxidation activity so far reported for the crystalline Si-based photoanodes. This bridging layer strategy is efficient and simple to smooth charge transfer and make robust contact at the semiconductor/electrocatalyst interface in the solar water-splitting systems.

Efficient Electrochemical Water Oxidation and Oxidative Degradation of Rhodamine B: A Comparative Study Using High‐Purity Birnessites Containing Li+, Na+ or K+ Ions

AbstractHigh‐purity birnessites, containing Na+, K+ or Li+ in the interlayer were prepared respectively with complexing agent EDTA at room temperature. The structural features and chemical compositions were characterized by X‐ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, Brunauer‐Emmett‐Teller analysis and field emission scanning electron microscopy. The electrocatalytic properties of these materials against oxygen evolution reaction (OER) in alkaline media were studied. Cyclic voltammetry and linear sweep voltammetry curves showed that Li‐Bir had the highest water oxidation activity. Meanwhile, Li‐Bir performed stably for 4000 s. For birnessites containing different alkali metals (K+, Na+ or Li+), the organic dye rhodamine B degradation rates were significantly different in acidic media. At pH 1, the degradation rate of Li‐Bir was obviously higher than those of K‐Bir and Na‐Bir. The surface area of Li‐Bir (107.282 cm2 g−1) is much larger than those of Na‐Bir (35.623) and K‐Bir (11.900), indicating that specific surface area was one of the main factors affecting the OER activity and degradation efficiency of birnessite.

Core-Shell NiFe-LDH@NiFe-Bi Nanoarray: In Situ Electrochemical Surface Derivation Preparation toward Efficient Water Oxidation Electrocatalysis in near-Neutral Media.

The corrosion issue with acidic and alkaline water electrolyzers can be avoided by developing water oxidation catalysts performing efficiently under benign conditions. In this Letter, we report that a NiFe-borate layer can be generated on a NiFe-layered double hydroxide nanosheet array hydrothermally grown on carbon cloth via an in situ electrochemical surface derivation process in potassium borate (K-Bi) solution. The resulting 3D NiFe-LDH@NiFe-Bi nanoarray (NiFe-LDH@NiFe-Bi/CC) demonstrates high activity for water oxidation, demanding overpotentials of 444 and 363 mV to achieve 10 mA cm-2 in 0.1 and 0.5 M K-Bi (pH: 9.2), respectively, rivaling the performances of most reported non-noble-metal catalysts in near-neutral media. Notably, this electrode also shows strong electrochemical durability with a high turnover frequency of 0.54 mol O2 s-1 at overpotential of 600 mV. All these features promise its use as an efficient earth-abundant catalyst material for water oxidation under eco-friendly conditions.

Promoting charge carrier utilization by integrating layered double hydroxide nanosheet arrays with porous BiVO4 photoanode for efficient photoelectrochemical water splitting

The increasing exploration of renewable and clean power sources have driven the development of highly active materials for photoelectrochemical (PEC) water splitting. However, it is still a great challenge to enhance the charge utilization. Herein, we report a facile method to fabricate composite photoanode with porous BiVO4 film as the photon absorber and layered double hydroxide (LDH) nanosheet arrays as the oxygen-evolution cocatalysts (OECs). The as-prepared BiVO4/NiFe-LDH photoanode shows an excellent performance for PEC water splitting benefitting from the synergistic effect of the superior charge separation efficiency facilitated by porous BiVO4 film and the excellent water oxidation activity resulting from LDH nanosheet arrays. A photocurrent density is 4.02 mA cm−2 at 1.23 V vs. the reversible hydrogen electrode (RHE). Furthermore, the O2 evolution rate at the surface of BiVO4/NiFe-LDH photoanode is as high as 29.6 μmol h−1 cm−2 and the high activity for water oxidation is maintained for over 30 h. Impressively, the performance of the as-fabricated composite photoanode for PEC water splitting can be further enhanced through incorporating a certain amount of Co2+ cation into NiFe-LDH as OEC. The photocurrent density is achieved up to 4.45 mA cm−2 at 1.23 V vs. RHE. This value is the highest yet reported for un-doped BiVO4-based photoanodes so far.

Mn doped porous cobalt nitride nanowires with high activity for water oxidation under both alkaline and neutral conditions.

We report the successful synthesis of Mn doped CoN nanowires on a carbon fiber cloth substrate (CFC) by a two-step strategy, which can be used as an efficient and stable OER electrocatalyst under both alkaline and neutral conditions. Benefiting from enhanced electronic interaction, high active area and electrical conductivity, the Mn doped CoN nanowires/CFC exhibits enhanced kinetics and improved cycling stability for the OER under both alkaline and neutral conditions. To achieve a current density of 10 mA cm-2, the Mn doped CoN nanowires/CFC requires an overpotential of 265 mV in alkaline solution and 285 mV in neutral solution.

A cobalt-borate nanosheet array: an efficient and durable non-noble-metal electrocatalyst for water oxidation at near neutral pH

As a durable catalyst electrode, a cobalt-borate nanosheet array on a Ti mesh shows high activity for water oxidation in 0.1 M K-Bi (pH: 9.2), achieving a geometrical catalytic current density of 10 mA cm−2 at an overpotential of 469 mV.

Cobalt–borate nanowire array as a high-performance catalyst for oxygen evolution reaction in near-neutral media

As a durable catalyst, cobalt–borate nanowire array on Ti mesh exhibits high activity for water oxidation in near-neutral media, thereby achieving a geometrical catalytic current density of 10 mA cm−2 at an overpotential of 420 mV.

Core-shell CoFe2O4@Co-Fe-Bi nanoarray: a surface-amorphization water oxidation catalyst operating at near-neutral pH.

The exploration of high-performance and earth-abundant water oxidation catalysts operating under mild conditions is highly attractive and challenging. In this communication, core-shell CoFe2O4@Co-Fe-Bi nanoarray on carbon cloth (CoFe2O4@Co-Fe-Bi/CC) was successfully fabricated by in situ surface amorphization of CoFe2O4 nanoarray on CC (CoFe2O4/CC). As a 3D water oxidation electrode, CoFe2O4@Co-Fe-Bi/CC shows outstanding activity with an overpotential of 460 mV to drive a geometrical catalytic current density of 10 mA cm-2 in 0.1 M potassium borate (pH 9.2). Notably, it also demonstrates superior long-term durability for at least 20 h with 96% Faradic efficiency. Density functional theory calculations indicate that the conversion from OOH* to O2 is the rate-limiting step and the high water oxidation activity of CoFe2O4@Co-Fe-Bi/CC is associated with the lower free energy of 1.84 eV on a Co-Fe-Bi shell.

Scalable Synthesis of Efficient Water Oxidation Catalysts: Insights into the Activity of Flame-Made Manganese Oxide Nanocrystals.

Chemical energy storage by water splitting is a promising solution for the utilization of renewable energy in numerous currently impracticable needs, such as transportation and high temperature processing. Here, the synthesis of efficient ultra-fine Mn3O4 water oxidation catalysts with tunable specific surface area is demonstrated by a scalable one-step flame-synthesis process. The water oxidation performance of these flame-made structures is compared with pure Mn2O3 and Mn5O8, obtained by post-calcination of as-prepared Mn3O4 (115 m(2) g(-1)), and commercial iso-structural polymorphs, probing the effect of the manganese oxidation state and synthetic route. The structural properties of the manganese oxide nanoparticles were investigated by XRD, FTIR, high-resolution TEM, and XPS. It is found that these flame-made nanostructures have substantially higher activity, reaching up to 350 % higher surface-specific turnover frequency (0.07 μmolO2 m(-2) s(-1)) than commercial nanocrystals (0.02 μmolO2 m(-2) s(-1)), and production of up to 0.33 mmolO2 molMn (-1) s(-1). Electrochemical characterization confirmed the high water oxidation activity of these catalysts with an initial current density of 10 mA cm(-2) achieved with overpotentials between 0.35 and 0.50 V in 1 m NaOH electrolyte.

CoOOH Nanosheets with High Mass Activity for Water Oxidation

AbstractEndowing transition‐metal oxide electrocatalysts with high water oxidation activity is greatly desired for production of clean and sustainable chemical fuels. Here, we present an atomically thin cobalt oxyhydroxide (γ‐CoOOH) nanosheet as an efficient electrocatalyst for water oxidation. The 1.4 nm thick γ‐CoOOH nanosheet electrocatalyst can effectively oxidize water with extraordinarily large mass activities of 66.6 A g−1, 20 times higher than that of γ‐CoOOH bulk and 2.4 times higher than that of the benchmarking IrO2 electrocatalyst. Experimental characterizations and first‐principles calculations provide solid evidence to the half‐metallic nature of the as‐prepared nanosheets with local structure distortion of the surface CoO6−x octahedron. This greatly enhances the electrophilicity of H2O and facilitates the interfacial electron transfer between Co ions and adsorbed ‐OOH species to form O2, resulting in the high electrocatalytic activity of layered CoOOH for water oxidation.

CoOOH Nanosheets with High Mass Activity for Water Oxidation.

Endowing transition-metal oxide electrocatalysts with high water oxidation activity is greatly desired for production of clean and sustainable chemical fuels. Here, we present an atomically thin cobalt oxyhydroxide (γ-CoOOH) nanosheet as an efficient electrocatalyst for water oxidation. The 1.4 nm thick γ-CoOOH nanosheet electrocatalyst can effectively oxidize water with extraordinarily large mass activities of 66.6 A g(-1), 20 times higher than that of γ-CoOOH bulk and 2.4 times higher than that of the benchmarking IrO2 electrocatalyst. Experimental characterizations and first-principles calculations provide solid evidence to the half-metallic nature of the as-prepared nanosheets with local structure distortion of the surface CoO(6-x) octahedron. This greatly enhances the electrophilicity of H2O and facilitates the interfacial electron transfer between Co ions and adsorbed -OOH species to form O2, resulting in the high electrocatalytic activity of layered CoOOH for water oxidation.