Stable Water Oxidation Catalysts Research Articles

(Invited) Acid Stable Water Oxidation Catalysts for PEM Water Electrolysis

Water electrolysis is a promising method to produce hydrogen from renewable electricity. However, the development of 3d metal catalysts for the oxygen evolution reaction (OER) is a critical challenge, especially in the case of polymer electrolyte membrane (PEM) electrolyzers, which are considered to be one of the most efficient electrolysis systems. The scarcity of iridium required for these systems prevents their widespread deployment at the global terawatt scale, and therefore the development of alternative catalysts has been highlighted as a real bottleneck for water electrolysis.In this talk, I will report on how the incorporation of Mn into the Co3O4 spinel lattice can result in a two order of magnitude increase in stability without compromising activity (1, 2). I will also report on how the lifetime of Mn-based catalysts in acid can be extended by modulating the lattice oxygen structure (3). Finally, I will discuss the performance of Mn-based OER catalysis under the actual conditions of PEM water electrolysis (3).(1) A. Li, S. Kong, C. Guo, H. Ooka, K. Adachi, D. Hashizume, Q. Jiang, H. Han, J. Xiao, R. Nakamura, Enhancing the stability of cobalt spinel oxide towards sustainable oxygen evolution in acid, Nat. Catal., 2022, 5, 109-118, DOI: 10.1038/s41929-021-00732-9(2) A. Li, H. Ooka, N. Bonnet, T. Hayashi, Y. Sun, Q. Jiang, C. Li, H. Han, R. Nakamura, Stable potential windows for long-term electrocatalysis by manganese oxides under acidic conditions, Angew. Chem., Int. Ed., 2019, 58, 5054-5058. DOI: 10.1002/anie.201813361(3) S. Kong, et al (under review)

Constructing the oxygen diffusion paths for promoting the stability of acidic water oxidation catalysts

Anchoring Ir onto metal-oxide support can effectively address the scarcity and insufficient activity problems of Ir-based acidic water oxidation catalysts by triggering the lattice-oxygen-mediated mechanism (LOM). However, the stability of these catalysts is usually sacrificed due to support overoxidation and reconstruction during the LOM pathway. Here, we propose constructing oxygen diffusion paths at a mixed-metal oxide support (ZrOx and TaOx) interface for a stable LOM. The new synthesized Ir-ZrTaOx catalyst exhibits two orders higher mass activity than IrO2 and retains 1,000-h electrolysis. It demonstrates low degradation (4.7 μV h−1) during the 200-h operation in a proton-exchange membrane (PEM) electrolyzer using only 1/4 Ir loading. Operando spectroscopic evidence combined with theory studies suggests that the oxygen diffusion path at the ZrOx/TaOx interface inhibits the overoxidation and detrimental reconstruction of TaOx. The strategy of stabilizing the LOM will benefit the future design of stable and efficient water oxidation catalysts.

Regeneration and Degradation in a Biomimetic Polyoxometalate Water Oxidation Catalyst.

Complete understanding of catalytic cycles is required to advance the design of water oxidation catalysts, but it is difficult to attain, due to the complex factors governing their reactivity and stability. In this study, we investigate the regeneration and degradation pathways of the highly active biomimetic water oxidation catalyst [Mn3+ 2Mn4+ 2V4O17(OAc)3]3-, thereby completing its catalytic cycle. Beginning with the deactivated species [Mn3+ 4V4O17(OAc)2]4- left over after O2 evolution, we scrutinize a network of reaction intermediates belonging to two alternative water oxidation cycles. We find that catalyst regeneration to the activated species [Mn4+ 4V4O17(OAc)2(OH)(H2O)]- proceeds via oxidation of each Mn center, with one water ligand being bound during the first oxidation step and a second water ligand being bound and deprotonated during the final oxidation step. ΔΔG values for this last oxidation are consistent with previous experimental results, while regeneration within an alternative catalytic cycle was found to be thermodynamically unfavorable. Extensive in silico sampling of catalyst structures also revealed two degradation processes: cubane opening and ligand dissociation, both of which have low barriers at highly reduced states of the catalyst due to the presence of Jahn-Teller effects. These mechanistic insights are expected to spur the development of more efficient and stable Mn cubane water oxidation catalysts.

Engineering a molecular ruthenium catalyst into three-dimensional metal covalent organic frameworks for efficient water oxidation.

The water oxidation reaction plays an important role in clean energy conversion, utilization, and storage, but mimicking the oxygen-evolving complex of photosystem II for designing active and stable water oxidation catalysts (WOCs) is still an appealing challenge. Here, we innovatively engineered a molecular ruthenium WOC as a metal complex building unit to construct a series of three-dimensional metal covalent organic frameworks (3D MCOFs) for realizing efficient oxidation catalysis. The resultant MCOFs possessed rare 3D interlocking structures with inclined interpenetration of two-dimensional covalent rhombic nets, and the Ru sites were periodically arranged in the crystalline porous frameworks. Impressively, these MCOFs showed excellent performance towards water oxidation (the O2 evolution rate is as high as 2830 nmol g-1 s-1) via the water nucleophilic attack pathway. Besides, the MCOFs were also reactive for oxidizing organic substrates. This work highlights the potential of MCOFs as a designable platform in integrating molecular catalysts for various applications.

Stabilization and activation of polyoxometalate over poly(vinyl butylimidazolium) cations towards electrocatalytic water oxidation in alkaline media.

Sandwich polyoxometalate [WCo3(H2O)2(CoW9O34)2]12- (Co-WCo3) heterogenized with poly(vinyl butyl imidazolium) cations (PVIM+) acts as a stable electrochemical water oxidation catalyst in 1 M KOH with 0.28 V@10 mA cm-2 overpotential and TOF of 6.16 s-1. Various in situ spectroelectrochemical studies and control experiments provide the details of the catalytic activities and long-term stability of the catalyst.

Immobilization of a [CoIIICoII(H2O)W11O39]7- Polyoxoanion for the Photocatalytic Oxygen Evolution Reaction.

The ongoing transition to renewable energy sources and the implementation of artificial photosynthetic setups call for an efficient and stable water oxidation catalyst (WOC). Here, we heterogenize a molecular all-inorganic [CoIIICoII(H2O)W11O39]7– ({CoIIICoIIW11}) Keggin-type polyoxometalate (POM) onto a model TiO2 surface, employing a 3-aminopropyltriethoxysilane (APTES) linker to form a novel heterogeneous photosystem for light-driven water oxidation. The {CoIIICoIIW11}-APTES-TiO2 hybrid is characterized using a set of spectroscopic and microscopic techniques to reveal the POM integrity and dispersion to elucidate the POM/APTES and APTES/TiO2 binding modes as well as to visualize the attachment of individual clusters. We conduct photocatalytic studies under heterogeneous and homogeneous conditions and show that {CoIIICoIIW11}-APTES-TiO2 performs as an active light-driven WOC, wherein {CoIIICoIIW11} acts as a stable co-catalyst for water oxidation. In contrast to the homogeneous WOC performance of this POM, the heterogenized photosystem yields a constant WOC rate for at least 10 h without any apparent deactivation, demonstrating that TiO2 not only stabilizes the POM but also acts as a photosensitizer. Complementary studies using photoluminescence (PL) emission spectroscopy elucidate the charge transfer mechanism and enhanced WOC activity. The {CoIIICoIIW11}-APTES-TiO2 photocatalyst serves as a prime example of a hybrid homogeneous–heterogeneous photosystem that combines the advantages of solid-state absorbers and well-defined molecular co-catalysts, which will be of interest to both scientific communities and applications in photoelectrocatalysis and CO2 reduction.

Iron Cobaltite (FeCo2O4) Nanocatalysts for Water-Oxidation: Effects of Annealing Temperature on Catalytic Properties

The development of efficient, stable, and non-precious metal water oxidation catalysts (WOCs) is a matter of importance for sustainable energy research. In this work, iron cobaltite (FeCo2O4) nanoparticles were prepared by the coprecipitation method, and we present the effect of heat treatment (250, 350, 450, 650 and 900 °C) on the catalytic properties. Catalytic activity tests of FeCo2O4 nanocatalysts were performed in the presence of ammonium cerium(IV) nitrate (CAN), and the formation of oxygen was followed using a Clark-type oxygen electrode. The samples were characterized by infrared (IR), thermogravimetric analysis (TGA), powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and their surface areas were determined by the Brunauer, Emmett, and Teller (BET) method. Fourier transform infrared (FTIR) data confirm a metal-oxygen bond at the octahedral and tetrahedral sites. XRDs data were characteristic of spinel-like cubic materials. The XPS results confirmed the presence of trivalent and divalent cobalt and iron ions in the samples and showed that the non-heated sample has a greater amount of cobalt on the nanoparticles’ surface than those heated to 900 °C. The surface area decreased from 92.00 m2 g-1 for the material that was unannealed to 2.00 m2 g-1 for the sample annealed at 900 °C. The unannealed nanomaterials showed an oxygen production of 790 mmol s−1 g−1. This was 790 times greater than the oxygen production from nanomaterials heated to 900 °C. Although the surface structure of nanomaterials is unclear, the amount of surface cobalt appears to have implications for catalytic activity. Optimization of superficial cobalt content may be key to improving catalytic activity.

Iron-N-heterocyclic carbene complexes as efficient electrocatalysts for water oxidation under acidic conditions.

The development of stable, Earth-abundant, and high-activity molecular water oxidation catalysts under acidic and neutral conditions remains a great challenge. Here, the use of N-heterocyclic carbene (NHC)-based iron(III) complex 1 {[phenyl(tris(3-methylimidazol-1-ylidene))borate]2Fe(III)}+ as a catalyst for water oxidation under acidic and neutral conditions was investigated. Two iron(II) carbene complexes, 2 {[2,6-bis(3-methylimidazolium-1-yl)pyridine]2Fe}2+ and 3 {[2,6-bis(3-methylimidazolium-1-yl)pyridine-4-carboxylic acid]2Fe}2+, were also used for comparison. A series of experiments demonstrate that complex 1 has excellent performance in terms of both catalytic activity and stability. In addition, the faradaic efficiency and turnover frequency (TOF) reach 95.0% and 2.8 s-1, respectively. An overpotential of ca. 490 mV is obtained at pH 1.5. Density functional theory (DFT) calculations indicate that dehydrogenation is the potential-determining step (PDS) in water oxidation. Complex 1 has a lower free energy barrier in this process than 2 and 3. High-valent Fe species are further proven in 1 by spectroelectrochemical measurements, which are crucial in promoting water oxidation. This study is expected to contribute to the development of homogeneous water oxidation catalysis under acidic and neutral conditions.

Enhanced electrochemical activity of Co3O4/Co9S8 heterostructure catalyst for water splitting

The dearth of efficient, robust, and economical electrocatalysts for water oxidation is dubiously the key obstacle for renewable energy devices, so synthesis of efficient, and cost-effective metal-based water oxidation catalysts is vital. Herein, Co3O4, Co9S8 catalysts and their heterostructure Co3O4/Co9S8 were synthesized and evaluated as water oxidation electrocatalysts. The characterization of Co3O4, Co9S8, and Co3O4/Co9S8 electrocatalysts was performed using Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray diffraction techniques. The heterostructure Co3O4/Co9S8 (1.46 V) exhibited water oxidation electrocatalysis at extremely low onset potential compared to Co3O4 (1.58 V), and Co9S8 (1.48 V) catalysts. A 281 mV overpotential required to attain a current density of 50 mA cm−2 in alkaline solution (1 M KOH), outperforming most of Co-based benchmark electrocatalysts. Further, the Co3O4/Co9S8 heterojunction demonstrated catalytic activity with small Tafel slope of 37 mV dec−1. The finding of electrochemical studies involving controlled potential electrolysis and long-term stability are projected to steer the future advancement in constructing efficient, economical, stable, and earth-abundant metal-based water oxidation catalysts.

Stable Water Oxidation Catalysts Based on in-situ Electrochemical Transition of Nickel Phosphate

Stable Water Oxidation Catalysts Based on in-situ Electrochemical Transition of Nickel Phosphate

Phosphate-Templated Encapsulation of a {CoII 4 O4 } Cubane in Germanotungstates as Carbon-Free Homogeneous Water Oxidation Photocatalysts.

The ever‐growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO4 3− templated synthesis of three abundant‐metal‐based germanotungstate (GT) clusters Na15[Ge4PCo4(H2O)2W24O94] ⋅ 38H2O (Co4), Na2.5K17.5[Ge3PCo9(OH)5(H2O)4W30O115] ⋅ 45H2O (Co9), Na6K16[Ge4P4Co20(OH)14(H2O)18W36O150] ⋅ 61H2O (Co20) with non‐, quasi‐, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT‐scaffolds were active molecular WOCs, with Co9 and Co20 outperforming the well‐known Na10[Co4(H2O)2(PW9O34)2] {Co4P2W18} by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s−1 Co9 currently represents the fastest Co‐GT‐based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co9 was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co4.

Electrochemical Activation of Atomic Layer-Deposited Cobalt Phosphate Electrocatalysts for Water Oxidation.

The development of efficient and stable earth-abundant water oxidation catalysts is vital for economically feasible water-splitting systems. Cobalt phosphate (CoPi)-based catalysts belong to the relevant class of nonprecious electrocatalysts studied for the oxygen evolution reaction (OER). In this work, an in-depth investigation of the electrochemical activation of CoPi-based electrocatalysts by cyclic voltammetry (CV) is presented. Atomic layer deposition (ALD) is adopted because it enables the synthesis of CoPi films with cobalt-to-phosphorous ratios between 1.4 and 1.9. It is shown that the pristine chemical composition of the CoPi film strongly influences its OER activity in the early stages of the activation process as well as after prolonged exposure to the electrolyte. The best performing CoPi catalyst, displaying a current density of 3.9 mA cm–2 at 1.8 V versus reversible hydrogen electrode and a Tafel slope of 155 mV/dec at pH 8.0, is selected for an in-depth study of the evolution of its electrochemical properties, chemical composition, and electrochemical active surface area (ECSA) during the activation process. Upon the increase of the number of CV cycles, the OER performance increases, in parallel with the development of a noncatalytic wave in the CV scan, which points out to the reversible oxidation of Co2+ species to Co3+ species. X-ray photoelectron spectroscopy and Rutherford backscattering measurements indicate that phosphorous progressively leaches out the CoPi film bulk upon prolonged exposure to the electrolyte. In parallel, the ECSA of the films increases by up to a factor of 40, depending on the initial stoichiometry. The ECSA of the activated CoPi films shows a universal linear correlation with the OER activity for the whole range of CoPi chemical composition. It can be concluded that the adoption of ALD in CoPi-based electrocatalysis enables, next to the well-established control over film growth and properties, to disclose the mechanisms behind the CoPi electrocatalyst activation.

Rational Design of an Iridium-Tungsten Composite with an Iridium-Rich Surface for Acidic Water Oxidation.

Developing highly active and stable water oxidation catalysts with reduced cost in acidic media plays a critical role in clean energy technologies such as fuel cells and electrolyzers. Precious iridium-based oxides are still the only oxygen evolution reaction (OER) catalysts with reasonable activity and stability in acid. Herein, we design iridium-tungsten composites with a metallic tungsten-rich core and an iridium-rich surface by the sol-gel method followed by hydrogen reduction. The thus obtained iridium-tungsten catalyst shows much higher intrinsic water oxidation activity (100 mA/mgIr at an overpotential of 290 mV) and stability (100 h at 10 mA/cm2geom) together with reduced iridium content (33 wt % only) as compared with pure iridium oxide. An operando method using H2O2 as a probe molecule is developed to determine the relative adsorption strength of the reaction intermediates (*OH and *OOH) in the OER process. Detailed characterization shows that the tungsten-incorporated surface not only modulates the adsorption energy of oxygen intermediates on iridium but also enhances the stability of iridium species in acid, while the metallic tungsten core exhibits high electrical conductivity, all of which collectively give rise to the much enhanced catalytic performance of iridium-tungsten composite in acidic water oxidation. A single-membrane electrode assembly is further prepared to demonstrate the advantages and potential application of iridium-tungsten composite in practical proton exchange membrane electrolyzers.

Homogeneous Electrochemical Water Oxidation at Neutral pH by Water-Soluble NiII Complexes Bearing Redox Non-innocent Tetraamido Macrocyclic Ligands.

Water oxidation is the bottleneck reaction in artificial photosynthesis. Exploring highly active and stable molecular water oxidation catalysts (WOCs) is still a great challenge. In this study, a water-soluble NiII complex bearing a redox non-innocent tetraamido macrocyclic ligand (TAML) is found to be an efficient electrocatalyst for water oxidation in neutral potassium phosphate buffer. Controlled-potential electrolysis experiments show that it can sustain at a steady current of approximately 0.2 mA cm-2 for >7 h at 1.75 V versus normal hydrogen electrode (NHE) without the formation of NiOx . Electrochemical and spectroelectrochemical tests show that the redox-active ligand, as well as HPO4 2- in the buffer, participate in the catalytic cycle. More importantly, catalytically active intermediate [NiIII (TAML2- )-O. ] is formed via several proton-coupled electron transfer processes and reacts with H2 O with the assistance of base to release molecular oxygen. Thus, the employment of redox non-innocent ligands is a useful strategy for designing effective molecular WOCs.

In-situ transformation of Co(OH)2 into NH4CoPO4•H2O on Co foil: 3D self-supported electrocatalyst with asymmetric local atomic and electronic structure for enhanced oxygen evolution reaction

Development of high efficient and stable water oxidation catalysts is essential for the realization of industrial water-splitting systems. Herein, a novel approach involving an in-situ transformation of Co(OH)2 nanosheets into NH4CoPO4•H2O nanoplates on Co foil is reported. As a 3D self-supported oxygen revolution reaction (OER) electrocatalyst, the as-prepared NH4CoPO4•H2O/Co exhibits remarkable catalytic activity and exceptional stability. Specifically, it can deliver a current density of 10 mA cm−2 at a quite low overpotential of 254 mV with a small Tafel slope of 64.4 mV dec−1 in alkaline electrolyte. Through experimental study and theoretical analysis, the excellent OER performance can be attributed to enriched exposed active sites, favorable electron/proton transfer and mass transport, and its unique asymmetric local atomic and electronic structure. Thus, this present research not only provides a practicable in-situ transformation strategy to design 3D self-supported electrocatalysts, but also enlightens a new way of developing transition-metal phosphates for efficient and stable water oxidation at atomic level.

A Dinuclear Copper Complex Featuring a Flexible Linker as Water Oxidation Catalyst with an Activity Far Superior to Its Mononuclear Counterpart.

The development of highly active, stable, and inexpensive molecular water oxidation catalysts (WOCs) is important for eventually realizing artificial photosynthesis. The dinuclear earth-abundant molecular WOCs are of great interest as the latent synergy of two adjacent metals of the molecule could enhance the activity of the catalyst for water oxidation, just as the synergistic catalysis effect of metal ions in many metalloenzymes. Herein we report the dinuclear copper complex [L1Cu2(μ-OH)](BF4)3 (1, L1 = N,N'-dimethyl-N,N'-bis{2-[bis(2-pyridinylmethyl)amino]ethyl}ethane-1,2-diamine) with a flexible linker and its mononuclear counterpart [L2Cu(OH2)](BF4)2(2, L2 = N,N-dimethyl-N',N'-bis(2-pyridylmethyl)ethane-1,2-diamine) as WOCs. X-ray diffraction analysis showed that in the crystal structure of 1 there is an extraneous water molecule located very close to the bridged O atom, resembling the proposed structure of the transition state for the O-O bond formation. Comparative studies on the performances of 1 and 2 for electrochemical water oxidation at pH 12 manifested that 1 displayed a much higher activity and better stability than that of 2. The kcat1 of 144 s-1 for 1 is on a par with those of the state-of-the-art earth-abundant molecular WOCs reported to date under similar test conditions. Experimental studies and DFT calculations suggest that the water oxidation catalyzed by 1 proceeds via a unimolecular two-site mechanism with a much lower energy barrier for the O-O bond formation step compared to that for 2.

Vertically stacked bilayer heterostructure CoFe2O4@Ni3S2 on a 3D nickel foam as a high-performance electrocatalyst for the oxygen evolution reaction

The as-obtained CoFe2O4@Ni3S2/NF can serve as an active and stable water oxidation catalyst under electrochemical reaction conditions.

Preparative History vs Driving Force in Water Oxidation Catalysis: Parameter Space Studies of Cobalt Spinels.

The development of efficient, stable, and economic water oxidation catalysts (WOCs) is a forefront topic of sustainable energy research. We newly present a comprehensive three-step approach to systematically investigate challenging relationships among preparative history, properties, and performance in heterogeneous WOCs. To this end, we studied (1) the influence of the preparative method on the material properties and (2) their correlation with the performance as (3) a function of the catalytic test method. Spinel-type Co3O4 was selected as a clear-cut model WOC and synthesized via nine different preparative routes. In search of the key material properties for high catalytic performance, these cobalt oxide samples were characterized with a wide range of analytical methods, including X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, Raman spectroscopy, BET surface area analysis, and transmission electron microscopy. Next, the corresponding catalytic water oxidation activities were assessed with the three most widely applied protocols to date, namely, photocatalytic, electrocatalytic, and chemical oxidation. The activity of the Co3O4 samples was found to clearly depend on the applied test method. Increasing surface area and disorder as well as a decrease in oxidation states arising from low synthesis temperatures were identified as key parameters for high chemical oxidation activity. Surprisingly, no obvious property–performance correlations were found for photocatalytic water oxidation. In sharp contrast, all samples showed similar activity in electrochemical water oxidation. The substantial performance differences between the applied protocols demonstrate that control and comprehensive understanding of the preparative history are crucial for establishing reliable structure–performance relationships in WOC design.

A Novel Water Oxidation Catalyst Made By the Facile Deposition of Ruthenium on Graphene Oxide

As populations grow worldwide, there is an ever-increasing need for sustainable and renewable energy sources to keep up with the high demand of energy. One promising technology is electrochemical water-splitting, which creates hydrogen and oxygen at the cathode and anode, respectively. Developing viable, stable catalysts for water splitting is critical since this reaction is truly a green, sustainable method to generate hydrogen fuel. However, more active electrocatalysts for the water oxidation reaction (WOR) are needed to improve the viability of this technology. Currently, these catalysts are most often composed of rare and expensive earth metals such as iridium. More sustainable materials such as those based on ruthenium have been recently investigated as alternatives to the rare metal systems. Ruthenium-based materials are an ideal choice as Ru offers multiple available oxidation states to help facilitate the water-splitting reaction and possibly lower overpotentials[1]. However, many ruthenium-based materials are prone to stability issues in acidic media at high over potentials. In addition, sluggish kinetics are often observed [2]. In this work we synthesized graphene oxide, via a simple and low-cost method, and utilized it as a support for the Ru oxide nanocatalyst[3]. We used graphene oxide due it high mechanical strength, favorable electronic properties, and the ease at which it can be doped with metals[4]. Ruthenium oxide nanoparticles were deposited on the surface of the graphene oxide at a 5wt% metal loading by a facile method. The materials were examined electrochemically in acidic media for the water oxidation reaction, where we observed low overpotentials and high stability during prolonged testing. This simple preparation method could be used to efficiently synthesize active and highly stable water-oxidation catalysts that are more cost-effective. This novel ruthenium stabilized on graphene catalyst could aid in the future development of more sustainable materials for the future of renewable energy technology.

Considerations for the scaling-up of water splitting catalysts

A wealth of candidates are being investigated to improve the catalysts found in acidic and alkaline electrolysers. However, attention should be focused on developing stable water oxidation catalysts with improved intrinsic activity — not only increased geometric activity — alongside best practice for data collection.