Sustained Water Oxidation Research Articles

Regulation of Oxide Pathway Mechanism for Sustainable Acidic Water Oxidation.

The advancement of acid-stable oxygen evolution reaction (OER) electrocatalysts is crucial for efficient hydrogen production through proton exchange membrane (PEM) water electrolysis. Unfortunately, the activity of electrocatalysts is constrained by a linear scaling relationship in the adsorbed evolution mechanism, while the lattice-oxygen-mediated mechanism undermines stability. Here, we propose a heterogeneous dual-site oxide pathway mechanism (OPM) that avoids these limitations through direct dioxygen radical coupling. A combination of Lewis acid (Cr) and Ru to form solid solution oxides (CrxRu1-xO2) promotes OH adsorption and shortens the dual-site distance, which facilitates the formation of *O radical and promotes the coupling of dioxygen radical, thereby altering the OER mechanism to a Cr-Ru dual-site OPM. The Cr0.6Ru0.4O2 catalyst demonstrates a lower overpotential than that of RuO2 and maintains stable operation for over 350 h in a PEM water electrolyzer at 300 mA cm-2. This mechanism regulation strategy paves the way for an optimal catalytic pathway, essential for large-scale green hydrogen production.

Z-scheme S-modified Zn0.2Cd0.8S/α-Fe2O3 heterostructure for enhanced photoelectrochemical water oxidation

A Z-type heterojunction system is proven to be an efficient strategy to promote the separation of photo-generated carriers and maintain strong redox capacity for an enhanced photoanode. Herein, we synthesized a S-modified Fe2O3/Zn0.2Cd0.8S (Fe/ZCS) photoanode by one-pot hydrothermal treatment of FeOOH/Zn0.2Cd0.8S precursor film, followed by vapor deposition treatment. The as-synthesized S-modified Fe/ZCS displayed enhanced PEC water oxidation performance with a photocurrent density (2.76 mA cm−2) of about 2.63 times that of a pure phase Fe2O3 and 2.05 times that of Fe/ZCS at 1.23 VRHE. After 8 h of continuous operation, the photocurrent density retention rate was still 96.4 % demonstrating its satisfactory stability. Benefiting from the morphology of fine nanorods, the fabricated Z-scheme heterostructure exhibited strong oxidation–reduction ability and the well-distributed Fe(III)-SO42- bonds played key roles as the electron capture centers. The fabricated Z-scheme photoanode exhibited prospective applications in sustainable photoelectrochemical water oxidation.

Active Sites Engineered Bimetallic Iron–Vanadium Oxide (FeVOx) Thin Film Electrocatalyst for Efficient and Sustainable Water Oxidation

Active Sites Engineered Bimetallic Iron–Vanadium Oxide (FeVO_<i>x</i>) Thin Film Electrocatalyst for Efficient and Sustainable Water Oxidation

Dual-site segmentally synergistic catalysis mechanism: boosting CoFeSx nanocluster for sustainable water oxidation

Efficient oxygen evolution reaction electrocatalysts are essential for sustainable clean energy conversion. However, catalytic materials followed the conventional adsorbate evolution mechanism (AEM) with the inherent scaling relationship between key oxygen intermediates *OOH and *OH, or the lattice-oxygen-mediated mechanism (LOM) with the possible lattice oxygen migration and structural reconstruction, which are not favorable to the balance between high activity and stability. Herein, we propose an unconventional Co-Fe dual-site segmentally synergistic mechanism (DSSM) for single-domain ferromagnetic catalyst CoFeSx nanoclusters on carbon nanotubes (CNT) (CFS-ACs/CNT), which can effectively break the scaling relationship without sacrificing stability. Co3+ (L.S, t2g6eg0) supplies the strongest OH* adsorption energy, while Fe3+ (M.S, t2g4eg1) exposes strong O* adsorption. These dual-sites synergistically produce of Co-O-O-Fe intermediates, thereby accelerating the release of triplet-state oxygen ( ↑ O = O ↑ ). As predicted, the prepared CFS-ACs/CNT catalyst exhibits less overpotential than that of commercial IrO2, as well as approximately 633 h of stability without significant potential loss.

Boosting Efficient and Sustainable Alkaline Water Oxidation on a W-CoOOH-TT Pair-Sites Catalyst Synthesized via Topochemical Transformation.

The development of facile methods for constructing highly active, cost-effective catalysts that meet ampere-level current density and durability requirements for an oxygen evolution reaction is crucial. Herein, a general topochemical transformation strategy is posited: M-Co9S8 single-atom catalysts (SACs) are directly converted into M-CoOOH-TT (M = W, Mo, Mn, V) pair-sites catalysts under the role of incorporating of atomically dispersed high-valence metals modulators through potential cycling. Furthermore, in situ X-ray absorption fine structure spectroscopy is used to track the dynamic topochemical transformation process at the atomic level. The W-Co9S8 breaks through the low overpotential of 160mV at 10mAcm-2. A series of pair-site catalysts exhibit a large current density of approaching 1760mAcm-2 at 1.68V vs reversible hydrogen electrode (RHE) in alkaline water oxidation and achieve a ≈240-fold enhancement in the normalized intrinsic activity compare to that reported CoOOH, and sustainable stability of 1000h. Moreover, the O─O bond formation is confirmed via a two-site mechanism, supported by in situ synchrotron radiation infrared and density functional theory (DFT) simulations, which breaks the limit of adsorption-energy scaling relationship on conventional single-site.

Optimizing the active interface structure of MnO2 to achieve sustainable water oxidation in an acidic medium

Developing cost-effective electrocatalysts for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) water electrolysis is crucial to achieving large-scale hydrogen production through water electrolysis.

Direct Z-scheme construction of C2N/Mg(OH)2 heterojunction and first-principles investigation of photocatalytic water splitting

Efficient water-splitting photocatalysts are pivotal in enhancing solar energy utilization to its maximum potential. Using first-principles calculations, the current study systematically explores the heterojunction's electron, carrier transfer, and optical properties. The observations indicate that the C2N/Mg(OH)2 heterojunction manifests a staggered band configuration, culminating in the accomplishment of a Z-type charge transfer paradigm. This mechanism promotes the recombination of e-h pairs within the C2N/Mg(OH)2 heterojunction, leading to the prolonged retention of photogenerated electrons in the Mg(OH)2 layer and photogenerated holes in the C2N layer. Consequently, this enables a sustained water reduction and oxidation reaction. The C2N/Mg(OH)2 heterojunction exhibits oxidation-reduction potentials that cross those of aqueous solutions under acidic (pH = 0) and neutral (pH = 7) conditions, thereby facilitating the photocatalytic process of water decomposition. Moreover, the optical absorption coefficient of the heterojunction surpasses that of a single two-dimensional material. Therefore, the direct Z-type C2N/Mg(OH)2 heterojunction holds promising potential as an exceptional photocatalyst for water decomposition.

Breaking the Ru-O-Ru Symmetry of a RuO2 Catalyst for Sustainable Acidic Water Oxidation.

Proton exchange membrane water electrolysis is a highly promising hydrogen production technique for sustainable energy supply, however, achieving a highly active and durable catalyst for acidic water oxidation still remains a formidable challenge. Herein, we propose a local microenvironment regulation strategy for precisely tuning In-RuO2 /graphene (In-RuO2 /G) catalyst with intrinsic electrochemical activity and stability to boost acidic water oxidation. The In-RuO2 /G displays robust acid oxygen evolution reaction performance with a mass activity of 671 A gcat -1 at 1.5 V, an overpotential of 187 mV at 10 mA cm-2 , and long-lasting stability of 350 h at 100 mA cm-2 , which arises from the asymmetric Ru-O-In local structure interactions. Further, it is unraveled theoretically that the asymmetric Ru-O-In structure breaks the thermodynamic activity limit of the traditional adsorption evolution mechanism which significantly weakens the formation energy barrier of OOH*, thus inducing a new rate-determining step of OH* absorption. Therefore, this strategy showcases the immense potential for constructing high-performance acidic catalysts for water electrolyzers.

Breaking the Ru−O−Ru Symmetry of a RuO2 Catalyst for Sustainable Acidic Water Oxidation

AbstractProton exchange membrane water electrolysis is a highly promising hydrogen production technique for sustainable energy supply, however, achieving a highly active and durable catalyst for acidic water oxidation still remains a formidable challenge. Herein, we propose a local microenvironment regulation strategy for precisely tuning In−RuO2/graphene (In−RuO2/G) catalyst with intrinsic electrochemical activity and stability to boost acidic water oxidation. The In−RuO2/G displays robust acid oxygen evolution reaction performance with a mass activity of 671 A gcat−1 at 1.5 V, an overpotential of 187 mV at 10 mA cm−2, and long‐lasting stability of 350 h at 100 mA cm−2, which arises from the asymmetric Ru−O−In local structure interactions. Further, it is unraveled theoretically that the asymmetric Ru−O−In structure breaks the thermodynamic activity limit of the traditional adsorption evolution mechanism which significantly weakens the formation energy barrier of OOH*, thus inducing a new rate‐determining step of OH* absorption. Therefore, this strategy showcases the immense potential for constructing high‐performance acidic catalysts for water electrolyzers.

Thin alumina-coated polyoxometalates on gold-titania half-shell array photoanode for sustainable plasmonic water oxidation

In solar water splitting, the poor chemical and mechanical durability of photoanode materials under oxidative environments has been raised as a crucial issue. Despite promising water oxidation activity, the stable immobilization of polyoxometalates (POMs) onto photoanodes is very challenging. Here we report sustainable photoelectrochemical water oxidation through the deposition of catalytic POMs onto a plasmonic Au/TiO2 photoanode, followed by the atomic layer deposition of a thin Al2O3 layer. Vacuum deposition techniques were used with polystyrene nanospheres as sacrificial templates to fabricate an Au/TiO2 half-shell array as a photoanode with strong plasmonic absorption and excellent stability in aqueous media. The thin Al2O3 layer serves as a protective layer for [Co4(H2O)2(PW9O34)2]10- (Co4POMs) attached to Au/TiO2 half-shells functionalized with cysteamines. The Al2O3 layer increased the photocurrent and delayed current attenuation by preventing the dissociation of Co4POMs from the surface during the water oxidation. A thicker Al2O3 layer exhibited a higher protection effect but reduced the catalytic activity of Co4POMs in the early stage of water oxidation due to the limited exposure of Co4POMs to electrolytes. Furthermore, regardless of the POMs, the Al2O3 layer also passivated the TiO2 surface, improving electron transfer through the POMs/Au half-shell structure. This work suggests that the catalyst/plasmonic photoelectrode with a thin protection layer is a promising alternative to conventional semiconductor-based photoelectrodes for sustainable and efficient water oxidation.

Co−Co Dinuclear Active Sites Dispersed on Zirconium‐doped Heterostructured Co9S8/Co3O4 for High‐current‐density and Durable Acidic Oxygen Evolution

AbstractDeveloping cost‐effective and sustainable acidic water oxidation catalysts requires significant advances in material design and in‐depth mechanism understanding for proton exchange membrane water electrolysis. Herein, we developed a single atom regulatory strategy to construct Co−Co dinuclear active sites (DASs) catalysts that atomically dispersed zirconium doped Co9S8/Co3O4 heterostructure. The X‐ray absorption fine structure elucidated the incorporation of Zr greatly facilitated the generation of Co−Co DASs layer with stretching of cobalt oxygen bond and S−Co−O heterogeneous grain boundaries interfaces, engineering attractive activity of significantly reduced overpotential of 75 mV at 10 mA cm−2, a breakthrough of 500 mA cm−2 high current density, and water splitting stability of 500 hours in acid, making it one of the best‐performing acid‐stable OER non‐noble metal materials. The optimized catalyst with interatomic Co−Co distance (ca. 2.80 Å) followed oxo‐oxo coupling mechanism that involved obvious oxygen bridges on dinuclear Co sites (1,090 cm−1), confirmed by in situ SR‐FTIR, XAFS and theoretical simulations. Furthermore, a major breakthrough of 120,000 mA g−1 high mass current density using the first reported noble metal‐free cobalt anode catalyst of Co−Co DASs/ZCC in PEM‐WE at 2.14 V was recorded.

Co-Co Dinuclear Active Sites Dispersed on Zirconium-doped Heterostructured Co9 S8 /Co3 O4 for High-current-density and Durable Acidic Oxygen Evolution.

Developing cost-effective and sustainable acidic water oxidation catalysts requires significant advances in material design and in-depth mechanism understanding for proton exchange membrane water electrolysis. Herein, we developed a single atom regulatory strategy to construct Co-Co dinuclear active sites (DASs) catalysts that atomically dispersed zirconium doped Co9 S8 /Co3 O4 heterostructure. The X-ray absorption fine structure elucidated the incorporation of Zr greatly facilitated the generation of Co-Co DASs layer with stretching of cobalt oxygen bond and S-Co-O heterogeneous grain boundaries interfaces, engineering attractive activity of significantly reduced overpotential of 75 mV at 10 mA cm-2 , a breakthrough of 500 mA cm-2 high current density, and water splitting stability of 500 hours in acid, making it one of the best-performing acid-stable OER non-noble metal materials. The optimized catalyst with interatomic Co-Co distance (ca. 2.80 Å) followed oxo-oxo coupling mechanism that involved obvious oxygen bridges on dinuclear Co sites (1,090 cm-1 ), confirmed by in situ SR-FTIR, XAFS and theoretical simulations. Furthermore, a major breakthrough of 120,000 mA g-1 high mass current density using the first reported noble metal-free cobalt anode catalyst of Co-Co DASs/ZCC in PEM-WE at 2.14 V was recorded.

Electrochemical CO2 Reduction to Methyl Formate/Formic Acid in a Dual Non-Aqueous/Aqueous Flow Cell Electrolyzer

Electrochemical conversion of CO2 has been very promising in neutralizing existing carbon in the atmosphere as well as creating a pathway to produce various fuels and chemicals. The use of a CO2 flow electrolyzer makes it possible to achieve high faradaic efficiencies at high current densities from pure CO2 or dilute flue gas feeds. CO2 solubility in methanol is higher than in aqueous solutions and with acidic methanol as a solvent, it is possible to generate a unique product, methyl formate. This work describes the use of mixed propylene carbonate/methanol solutions as a solvent for CO2 reduction using a gas diffusion electrode and an aqueous anolyte and optimization of a high mass flux CO2 electrolyzer to produce methyl formate using a Pb-catalyst at higher current densities. pH-dependent product distribution is also described. The partial current densities varied depending on the cell design; cells where charge passes through more solvent result in lower current density because methanol has a higher solution resistance than aqueous solutions. The methanolic catholyte is paired with an aqueous anolyte to promote a sustainable water oxidation half-reaction.

Optimizing the Spatial Density of Single Co Sites via Molecular Spacing for Facilitating Sustainable Water Oxidation.

Advances in single-atom (-site) catalysts (SACs) provide a new solution of atomic economy and accuracy for designing efficient electrocatalysts. In addition to a precise local coordination environment, controllable spatial active structure and tolerance under harsh operating conditions remain great challenges in the development of SACs. Here, we show a series of molecule-spaced SACs (msSACs) using different acid anhydrides to regulate the spatial density of discrete metal phthalocyanines with single Co sites, which significantly improve the effective active-site numbers and mass transfer, enabling one of the msSACs connected by pyromellitic dianhydride to exhibit an outstanding mass activity of (1.63 ± 0.01) × 105 A·g-1 and TOFbulk of 27.66 ± 1.59 s-1 at 1.58 V (vs RHE) and long-term durability at an ultrahigh current density of 2.0 A·cm-2 under industrial conditions for oxygen evolution reaction. This study demonstrates that the accessible spatial density of single atom sites can be another important parameter to enhance the overall performance of catalysts.

Rapid synthesis of nickel–iron-oxide (NiFeOx) solid-solution nano-rods thin films on nickel foam as advanced electrocatalyst for sustained water oxidation

The development of earth-rich, noble-metal-free, and highly electroactive catalysts to accelerate the oxygen evolution reaction (OER) is a formidable challenge for the establishment of water-splitting technologies. Here, a solid-solution nickel-iron oxide (NiFeOx) electrocatalyst is readily grown on a Ni-foam (NF) substrate by a rapid and economical aerosol-assisted chemical vapor deposition process. In particular, the NiFeOx thin film fabricated in 40 min acquires a distinctive nanorod structure and proves to be stable and efficient for OER in alkaline solutions. It is shown that the catalyst needs a low over-potential of 226 mV to reach the typical current density of 10 mA/cm2, and it can approach a remarkable current density level of 1000 mA/cm2 by taking a slightly higher over-potential of 139 mV. The small Tafel slope of 64 mv.dec−1 and splendid electrochemical stability of 40 h at high current densities outperform many known FeNi-based anodes as well as commercial IrO2 and RuO2. The unique structure of the thin film offers many electroactive sites and a high surface area in combination with an improved electrical conductivity of NF, which is believed to play an imperative role in the excellent activity of the catalyst. The cost-effective and simple strategy to fabricate NiFeOx nano-fibrous is very attractive for the development of electrocatalysts for water splitting.

Auto-selective reaction mechanism on Al-substituted ZnFe2O4 spinel electrode and sustainable water oxidation by oxygen vacancy transition

This study aims to optimize the oxygen evolution reaction (OER) by maximizing OH– adsorption on the mixed spinel ZFAO surface, in which Al3+ is substituted for Fe3+ in the ZnFe2O4 (ZFO) spinel structure composed of inexpensive transition metals. The substituted Al3+ ions simultaneously occupy the tetrahedral and octahedral sites of Fe in the ZFO spinel lattice. As the amount of substituted Al3+ increases, the amount of OH– adsorbed on the crystal surface increases significantly, and the OER activity is maximized at the ZFA0.75O/NF electrode, in which 0.75 M of Al is substituted. The overpotential is greatly reduced to 270 mV at 10 mA cm−2, and the Tafel slope is very low at 79 mA dec−1. These results were constant even after 10 d, which means that the charge transfer reaction proceeds rapidly between the ZFAO spinel catalyst and aqueous solution during the OER without side reactions. In particular, this study confirms that the main OER active sites of the ZFAO catalyst are the Al-tetrahedral and Fe-octahedral sites, and oxygen through the lattice vacancies rapidly transfers, maintaining a stable spinel structure. This study also reveals that the OER selectively pursues two mechanisms: an adsorbate evolution mechanism (AEM) on the Al-tetrahedral site and a lattice oxygen participation mechanism (LOPM) on the Fe-octahedral site. This study contributes to making the catalyst cost-effective, which has been an obstacle to the commercialization of water electrolysis technology, and securing its long-term durability, thereby opening the door to the mass production of clean hydrogen energy.

Single Phase Trimetallic Spinel CoCrxRh2‐xO4 Nanofibers for Highly Efficient Oxygen Evolution Reaction under Freshwater Mimicking Seawater Conditions

AbstractElectrochemical water splitting is a promising pathway for sustainable oxygen production in terms of energy conversion. Seawater electrolysis, especially, is a sustainable approach to carbon‐neutral energy conversion without reliance on freshwater; however, extreme corrosion of anodic electrode caused by highly corrosive Cl− is a main challenge of seawater oxidation. To address this issue, herein, nanofibers of trimetallic spinel CoCrxRh2‐xO4 with various composition ratios are prepared for highly sustained water oxidation electrocatalysis. Among a series of CoCrxRh2‐xO4, CoCr0.7Rh1.3O4 nanofibers exhibit excellent electrocatalytic activity for oxygen evolution reaction (OER): the highest mass activity, the lowest overpotential at 10 mA cm−2 and the smallest Tafel slope with robust long‐term stability under alkaline electrolyte. In addition, CoCr0.7Rh1.3O4 nanofibers deliver better OER performances in simulated seawater than a commercial benchmark catalyst (IrO2 nanoparticles), demonstrating that feasibility of alkaline seawater electrolysis with CoCr0.7Rh1.3O4 nanofibers as an OER electrocatalyst.

Immobilizing Low‐Cost Metal Nitrides in Electrochemically Reconstructed Platinum Group Metal (PGM)‐Free Oxy‐(Hydroxides) Surface for Exceptional OER Kinetics in Anion Exchange Membrane Water Electrolysis (Adv. Energy Mater. 6/2023)

Oxygen Evolution Reaction In article number 2203401, Youngkook Kwon, and co-workers demonstrate an energy-efficient, low-cost oxygen-evolving electrode by immobilizing Ni3N particles on an electrochemically reconstructed V-NiFeOOH surface for sustainable water oxidation in anion exchange membrane water electrolyzers.

Modulating WO3 Crystal Orientation to Suppress Hydroxyl Radicals for Sustainable Solar Water Oxidation

Tungsten trioxide (WO3) is one of the promising semiconductors suitable for photoelectrochemical water oxidation, but its hydroxyl radical (•OH)-induced intrinsic performance degradation remains unclarified. Here, we demonstrate that quenching-treated WO3 with preferred {021} facets shows a highly improved Faradaic efficiency (from 57 to 95%) and its performance stability is more than 36 h relative to that of nontreated WO3 with less than 1-h stability. Using electron paramagnetic resonance (EPR), we find that the •OH could be highly suppressed on the treated WO3 photoanode, while abundant •OH is generated on the nontreated WO3. In situ ultraviolet–visible (UV–Vis) spectroscopy is used to track the presence of surface W–O–O–W intermediates on the treated WO3, suggesting the favorable formation of O–O and thus better oxygen evolution Faradaic efficiency, while the nontreated WO3 favors the formation of •OH, which accumulates on the WO3 surface, thus changing the photoanode/electrolyte interfacial properties and poisoning the oxygen evolution process. This work provides an intrinsic understanding of the degradation of the WO3 photoanode under acidic and neutral conditions.

Facile deposition of FeNi/Ni hybrid nanoflower electrocatalysts for effective and sustained water oxidation.

Bimetallic iron-nickel (FeNi) compounds are widely studied materials for the oxygen evolution reaction (OER) owing to their high electrocatalytic performance and low cost. In this work, we produced thin films of the FeNi alloy on nickel foam (NF) by using an aerosol-assisted chemical deposition (AACVD) method and examined their OER catalytic activity. The hybrid FeNi/Ni catalysts obtained after 1 and 2 h of AACVD deposition show improved charge transfer and kinetics for the OER due to the strong interface between the FeNi alloy and Ni support. The FeNi/Ni-2h catalyst has higher catalytic activity than the FeNi/Ni-1h catalyst because of its nanoflower morphology that provides a large surface area and numerous active sites for the OER. Therefore, the FeNi/Ni-2h catalyst exhibits low overpotentials of 300 and 340 mV at 50 and 500 mA cm-2 respectively, and excellent stability over 100 h, and ∼0% loss after 5000 cycles in 1 M KOH electrolyte. Furthermore, this catalyst has a small Tafel slope, low charge transfer resistance and high current exchange density and thus surpasses the benchmark IrO2 catalyst. The easy, simple, and scalable AACVD method is an effective way to develop thin film electrocatalysts with high activity and stability.