Journal Articles
View Journal Publications by Patzke Research Group
2024
Y. Zhao, W. Wan, R. Erni, L. Pan, G. R. Patzke,
Operando spectroscopic monitoring of metal chalcogenides for overall water splitting: New views of active species and sites,
Angewandte Chemie International Edition, e202400048
Metal-based chalcogenides exhibit great promise for overall water splitting, yet their intrinsic catalytic reaction mechanisms remain to be fully understood. In this work, we employed operando X-ray absorption (XAS) and in situ Raman spectroscopy to elucidate the structure–activity relationships of low-crystalline cobalt sulfide (L−CoS) catalysts toward overall water splitting. The operando results for L−CoS catalyzing the alkaline hydrogen evolution reaction (HER) demonstrate that the cobalt centers in the bulk are predominantly coordinated by sulfur atoms, which undergo a kinetic structural rearrangement to generate metallic cobalt in S−Co−Co−S moieties as the true catalytically active species. In comparison, during the acidic HER, L−CoS undergoes local structural optimization of Co centers, and H2 production proceeds with adsorption/desorption of key intermediates atop the Co−S−Co configurations. Further operando characterizations highlight the crucial formation of high-valent Co4+ species in L−CoS for the alkaline oxygen evolution reaction (OER), and the formation of such active species was found to be far more facile than in crystalline Co3O4 and Co-LDH references. These insights offer a clear picture of the complexity of active species and site formation in different media, and demonstrate how their restructuring influences the catalytic activity.
W. Wan, Y. Zhao, J. Meng, C. S. Allen, Y. Zhou, G. R. Patzke,
Tailoring C─ N Containing Compounds into Carbon Nanomaterials with Tunable Morphologies for Electrocatalytic Applications,
Small 20 (7), 2304663
Carbon materials with unique sp2-hybridization are extensively researched for catalytic applications due to their excellent conductivity and tunable physicochemical properties. However, the development of economic approaches to tailoring carbon materials into desired morphologies remains a challenge. Herein, a convenient “bottom-up” strategy by pyrolysis of graphitic carbon nitride (g-C3N4) (or other carbon/nitrogen (C, N)-enriched compounds) together with selected metal salts and molecules is reported for the construction of different carbon-based catalysts with tunable morphologies, including carbon nano-balls, carbon nanotubes, nitrogen/sulfur (S, N) doped-carbon nanosheets, and single-atom catalysts, supported by carbon layers. The catalysts are systematically investigated through various microscopic, spectroscopic, and diffraction methods and they demonstrate promising and broad applications in electrocatalysis such as in the oxygen reduction reaction and water splitting. Mechanistic monitoring of the synthesis process through online thermogravimetric-gas chromatography-mass spectrometry measurements indicates that the release of C─N-related moieties, such as dicyan, plays a key role in the growth of carbon products. This enables to successfully predict other widely available precursor compounds beyond g-C3N4 such as caffeine, melamine, and urea. This work develops a novel and economic strategy to generate morphologically diverse carbon-based catalysts and provides new, essential insights into the growth mechanism of carbon nanomaterials syntheses.
2023
Y. Zhao, D. P. Adiyeri Saseendran, C. A. Triana, G. R. Patzke, (Invited)
Understanding and Optimization of Versatile Molecular and Coordination Polymer-Based 3d Transition Metal Oxygen Evolution Reaction Catalysts,
Electrochemical Society Meeting Abstracts 2023, MA2023-02, 2823-2823
Abstract
The demanding multi-electron transfer process renders the oxygen evolution reaction (OER) a bottleneck for achieving efficient clean hydrogen generation via water electrolysis.[1] Over the past decades, two main categories of catalysts, namely, homogeneous molecular and heterogeneous catalysts, have been implemented for the OER. However, due to the sluggish reaction kinetics and the aggressive reaction media of the OER, the structural integrity of both homogeneous molecular and heterogeneous catalysts faces the dramatic challenges. This calls for a thorough understanding and close monitoring of OER catalysts under their operando reaction conditions. Over the last years, we have been combining a variety of in situ/operando spectroscopy approaches with computational studies toward the comprehensive understanding of our designed catalysts at the atomic level. With this information in hand, we established a full identification of catalytically active species and sites for several systems, some of which are discussed here. First, inspired by nature's {Mn4CaOx} OER complexes, we recently reported on the design of a tetramer Cu-bipyridyl complex for the OER.[2] Structural characterizations demonstrated a new defect-cubane structure of our designed complex, [Cu4(pyalk)4(OAc)4](ClO4)(HNEt3). We found that this Cu-bipyridyl complex can further undergo structural transformations into two unique complexes under different solution conditions, namely Cu-dimer and Cu-monomers, as revealed by in situ UV-vis and ERP characterizations as well as electrospray ionization mass spectrometry. Specifically, the Cu-monomers can be only formed in presence of carbonate buffer (pH 10.5). Otherwise, the structural transformation into a Cu-dimer complex [Cu2(pyalk)2(OAc)2(H2O)] is dominant under solution conditions. Furthermore, electrochemical characterizations revealed an overpotential of 960 mV to reach a current density of 0.1 mA/cm2 of our designed Cu-dimer catalysts, which is comparable with significant Cu-based OER electrocatalysts. To gain in-depth insights into their conversion processes, postcatalytic characterizations of Cu-based molecular catalysts were carried out based on X-ray photoelectron/absorption (XAS/XPS) spectroscopy appraoches. The results showed that nanosized Cu-oxide-based species were formed in situ in Cu-based molecular catalysts after the OER. Our study highlights the crucial role of the structural integrity of molecular catalysts in solutions for their efficient design. In parallel, we explored the structural transformations of heterogeneous electrocatalysts during the OER. As a typical example, we developed a cost-effective and high-performance NiFe-based coordination polymer (referred to as NiFe-CP) as OER electrocatalyst, which is being investigated as the best-known bimetallic combination for the OER.[3] A central element of our study is the monitoring of true catalytically active species. Results from spectroscopic characterizations revealed a kinetic restructuring of NiFe-CPs into NiFe (oxy)hydroxides during the OER. To further improve the OER activity, we introduced a facile NaBH4-assisted reduction strategy to prepare low-crystalline reduced NiFe-CP (denoted as R-NiFe-CP) OER electrocatalysts with rich structural deficiencies. These catalysts can maintain a very low overpotential of 225 mV at 10 mA/cm2 for over 120 h without any performance decline, outperforming many recent reported bimetallic OER electrocatalysts. As revealed by XAS characterizations and density functional theory (DFT) calculations, engineering of structural deficiencies not only tunes the local electronic structure but also optimizes the rate-determining step towards facilitated OH- adsorption. Noteworthy, the true OER active sites of R-NiFe-CPs originate from the in situ reconstructed Ni-O-Fe motifs. However, fundamental questions, as to (a) the role of engineered structural deficiencies in the generation of active species and (b) facilitating the formation of catalytically active dual oxygen-bridged moieties, need to be answered. Combination of time-resolved operando XAS monitoring and DFT calculations enables the tracking and understanding of the kinetic changes of active species and sites under the operando reaction conditions. We found that the OER of R-NiFe-CPs relies on the in situ formation of crucial high-valent NiIV-O-FeIVO moieties.[4] Furthermore, an anionic engineering strategy through heteroatom sulfur incorporation was carried out to obtain S-R-NiFe-CP showing faster OER kinetics. Importantly, engineered sulfur content promotes the generation of catalytically active S-NiIVO-FeIVO motifs prior to the OER. This offers a lower onset potential to trigger the OER of S-R-NiFe-CPs compared to sulfur-free R-NiFe-CPs. Moreover, our results also suggest a dual-site mechanism pathway of S-R-NiFe-CPs during the OER, in which the O-O bond formed atop the S-NiIVO-FeIVO moieties. Such an anionic modulation strategy for promoting the formation of catalytically active structural moieties and for optimizing the OER kinetics opens an avenue to optimize a wide range of heterogeneous catalysts for the OER.H. Chen, J. Li, L. Meng, S. Bae, R. Erni, D. F. Abbott, S. Li, C. A. Triana, V. Mougel, G. R. Patzke,
Modulating Carrier Kinetics in BiVO4 Photoanodes through Molecular Co4O4 Cubane Layers,
Advanced Functional Materials 2023, 33, 2307862
Understanding the role and immobilization of molecular catalysts on photoelectrodes is essential to use their full potential for efficient solar fuel generation. Here, a CoII4O4 cubane with proven catalytic performance and an active H2O─Co2(OR)2─OH2 edge-site moiety is immobilized on BiVO4 photoanodes through a versatile layer-by-layer assembly strategy. This delivers a photocurrent of 3.3 mA cm−2 at 1.23 VRHE and prolonged stability. Tuning the thickness of the Co4O4 layer has remarkable effects on photocurrents, dynamic open circuit potentials, and charge carrier behavior. Comprehensive-time and frequency-dependent perturbation techniques are employed to investigate carrier kinetics in transient and pseudo-steady-state operando conditions. It is revealed that the Co4O4 layer can prolong carrier lifetime, unblock kinetic limitations at the interface by suppressing recombination, and enhance charge transfer. Additionally, its flexible roles are identified as passivation/hole trapping/catalytic layer at respective lower/moderate/higher potentials. These competing functions are under dynamic equilibrium, which fundamentally defines the observed photocurrent trends.
Y. Zhao, D. P. Adiyeri Saseendran, C. Huang, C. A. Triana, W. R. Marks, H. Chen, H. Zhao, G. R. Patzke,
Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design,
Chemical Reviews 2023, 123, 6257-6358
The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst–electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.
D. P. A. Saseendran, J. W. A. Fischer, L. Müller, D. F. Abbott, V. Mougel, G. Jeschke, C. A. Triana, G. R. Patzke,
Copper(II) defect-cubane water oxidation electrocatalysts: from molecular tetramers to oxidic nanostructures,
Chemical Communications 2023, 59, 5866-5869
We report on the synthesis and spectroscopic evidence for a sequence of structural transformations of a new defect-cubane type copper complex, [Cu4(pyalk)4(OAc)4](ClO4)(HNEt3), which acts as a pre-catalyst for water oxidation. In situ and post-catalytic studies showed that the tetrameric complex undergoes a structural transformation into dimeric and monomeric species, induced by water molecules and carbonate anions, respectively. Further, the observed electrocatalytic water oxidation activity has been confirmed to arise from in situ-generated Cu(II) oxidic nanostructures at the electrode interface.
2022
J.M. Naik, B. Bulfin, C.A. Triana, D.C. Stoian, G.R. Patzke,
Cation-Deficient Ce-Substituted Perovskite Oxides with Dual-Redox Active Sites for Thermochemical Applications,
ACS Applied Materials & Interfaces 2023, 15, 806–817
Identifying thermodynamically favorable and stable non-stoichiometric metal oxides is of crucial importance for solar thermochemical (STC) fuel production via two-step redox cycles. The performance of a non-stoichiometric metal oxide depends on its thermodynamic properties, oxygen exchange capacity, and its phase stability under high-temperature redox cycling conditions. Perovskite oxides (ABO3−δ) are being considered as attractive alternatives to the state-of-the-art ceria (CeO2−δ) due to their high thermodynamic and structural tunability. However, perovskite oxides often exhibit low entropy change compared to ceria, as they generally have one only redox active site, leading to lower mass-specific fuel yields. Herein, we investigate cation-deficient Ce-substituted perovskite oxides as a new class of potential redox materials combining the advantages of perovskites and ceria. We newly synthesized the (CexSr1–x)0.95Ti0.5Mn0.5O3−δ (x = 0, 0.10, 0.15, and 0.20; CSTM) series, with dual-redox active sites comprising Ce (at the A-site) and Mn (at the B-site). By introducing a cation deficiency (∼5%), CSTM perovskite oxides with both phase purity (x ≤ 0.15) and high-temperature structural stability under STC redox cycling conditions are obtained. Thermodynamic properties are evaluated by measuring oxygen non-stoichiometry in the temperature range T = 700–1400 °C and the oxygen partial pressure range pO2 = 1–10–4 bar. The results demonstrate that CSTM perovskite oxides exhibit a composition-dependent simultaneous increase of enthalpy and entropy change with increasing Ce-substitution. (Ce0.20Sr0.80)0.95Ti0.5Mn0.5O3−δ (CSTM20) showed a combination of large entropy change of ∼141 J (mol-O)−1 K–1 and moderate enthalpy change of ∼238 kJ (mol-O)−1, thereby creating favorable conditions for thermochemical H2O splitting. Furthermore, the oxidation states and local coordination environment around Mn, Ce, and Ti sites in the pristine and reduced CSTM samples were extensively studied using X-ray absorption spectroscopy. The results confirmed that both Ce (at the A-site) and Mn (at the B-site) centers undergo simultaneous reduction during thermochemical redox cycling.
Y. Zhao, W. Wan, N. Dongfang, C.A. Triana, L. Douls, C. Huang, R. Erni, M. Iannuzzi, G.R. Patzke,
Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation,
ACS Nano 2022, 16, 15318-15327
In-depth insights into the structure–activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived Od-R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni4+ and Fe4+ species, which generate oxygen-bridged NiIV-O-FeIV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free Od-R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-NiIV-O-FeIV-O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.
S.E. Balaghi, A.S. Sologubenko, S. Mehrabani, G.R. Patzke, M.M. Najafpour,
Molecular Mechanism of Copper-based Catalytic Reaction of Water Oxidation. In-situ Electrochemical Liquid Phase Transmission Electron Microscopy Study,
Microscopy and Microanalysis 2022, 28, 138-140
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S.E. Balaghi, S. Heidari, M. Benamara, H. Beyzavi, G.R. Patzke,
Fluoride etched Ni-based electrodes as economic oxygen evolution electrocatalysts,
International Journal of Hydrogen Energy 2022, 47, 1613-1623
Electrochemical water splitting is a promising technology for eco-friendly energy storage. However, the design principles for highly active, robust, and noble metal-free electrocatalysts for industrial-scale hydrogen production remain controversial. Oxygen-free compounds containing anionic species with a very high oxidation potential, such as fluorides, have emerged as high-performance targets for thermodynamically stable oxygen evolution reaction (OER) catalysts. They can further be designed to fit the key criteria of high electrical conductivity and stability. Herein, we present a facile and scalable etching method for constructing fluoride doped metallic nickel-based anodes from industrial Ni foam sources with high application potential for large-scale hydrogen production setups. The fluoride-etched Ni-catalysts were investigated with a wide range of techniques, such as powder X-ray diffraction (PXRD), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Optimized catalysts displayed a promising overpotential of 220 mV for the OER at a current density of 60 mA cm−2, which is competitive with noble metal-based reference catalysts, such as iridium oxide. Electrochemical impedance spectroscopy (EIS) studies demonstrated that etching of the electrode surface in fluoride medium leads to a drastic decrease of Rct. The corresponding decreased resistivity towards electrochemical OER on the electrode surface gives rise to the notably enhanced performance, with a minimum of synthetic effort.
2021
W. Wan, Y. Zhao, S. Wei, C. A. Triana, J. Li, A. Arcifa, C. S. Allen, R. Cao, G. R. Patzke
Mechanistic insight into the active centers of single/dual-atom Ni/Fe-based oxygen electrocatalysts,
Nature Communications 12 (1), 5589
Single-atom catalysts with maximum metal utilization efficiency show great potential for sustainable catalytic applications and fundamental mechanistic studies. We here provide a convenient molecular tailoring strategy based on graphitic carbon nitride as support for the rational design of single-site and dual-site single-atom catalysts. Catalysts with single Fe sites exhibit impressive oxygen reduction reaction activity with a half-wave potential of 0.89 V vs. RHE. We find that the single Ni sites are favorable to promote the key structural reconstruction into bridging Ni-O-Fe bonds in dual-site NiFe SAC. Meanwhile, the newly formed Ni-O-Fe bonds create spin channels for electron transfer, resulting in a significant improvement of the oxygen evolution reaction activity with an overpotential of 270 mV at 10 mA cm−2. We further reveal that the water oxidation reaction follows a dual-site pathway through the deprotonation of *OH at both Ni and Fe sites, leading to the formation of bridging O2 atop the Ni-O-Fe sites.
L. Reith, C. A. Triana, F. Pazoki, M. Amiri, M. Nyman, G. R. Patzke,
Unraveling nanoscale cobalt oxide catalysts for the oxygen evolution reaction: maximum performance, minimum effort,
Journal of the American Chemical Society 143 (37), 15022-15038
The oxygen evolution reaction (OER) is a key bottleneck step of artificial photosynthesis and an essential topic in renewable energy research. Therefore, stable, efficient, and economical water oxidation catalysts (WOCs) are in high demand and cobalt-based nanomaterials are promising targets. Herein, we tackle two key open questions after decades of research into cobalt-assisted visible-light-driven water oxidation: What makes simple cobalt-based precipitates so highly active—and to what extent do we need Co-WOC design? Hence, we started from Co(NO3)2 to generate a precursor precipitate, which transforms into a highly active WOC during the photocatalytic process with a [Ru(bpy)3]2+/S2O82–/borate buffer standard assay that outperforms state of the art cobalt catalysts. The structural transformations of these nanosized Co catalysts were monitored with a wide range of characterization techniques. The results reveal that the precipitated catalyst does not fully change into an amorphous CoOx material but develops some crystalline features. The transition from the precipitate into a disordered Co3O4 material proceeds within ca. 1 min, followed by further transformation into highly active disordered CoOOH within the first 10 min. Furthermore, under noncatalytic conditions, the precursor directly transforms into CoOOH. Moreover, fast precipitation and isolation afford a highly active precatalyst with an exceptional O2 yield of 91% for water oxidation with the visible-light-driven [Ru(bpy)3]2+/S2O82– assay, which outperforms a wide range of carefully designed Co-containing WOCs. We thus demonstrate that high-performance cobalt-based OER catalysts indeed emerge effortlessly from a self-optimization process favoring the formation of Co(III) centers in all-octahedral environments. This paves the way to new low-maintenance flow chemistry OER processes.
J. Li, H. Chen, C. A. Triana, G. R. Patzke
Hematite photoanodes for water oxidation: electronic transitions, carrier dynamics, and surface energetics,
Angewandte Chemie 133 (34), 18528-18544
We review the current understanding of charge carriers in model hematite photoanodes at different stages. The origin of charge carriers is discussed based on the electronic structure and absorption features, highlighting the controversial assignment of the electronic transitions near the absorption edge. Next, the dynamic evolution of charge carriers is analyzed both on the ultrafast and on the surface reaction timescales, with special emphasis on the arguable spectroscopic assignment of electrons/holes and their kinetics. Further, the competitive charge transfer centers at the solid–liquid interface are reviewed, and the chemical nature of relevant surface states is updated. Finally, an overview on the function of widely employed surface cocatalysts is given to illustrate the complex influence of physiochemical modifications on the charge carrier dynamics. The understanding of charge carriers from their origin all the way to their interfacial transfer is vital for the future of photoanode design.
Z. Abdi, S. E. Balaghi, A. S. Sologubenko, M. G. Willinger, M. Vandichel, J.-R. Shen,
S. I. Allakhverdiev, G. R. Patzke, M. M. Najafpour
Understanding the Dynamics of Molecular Water Oxidation Catalysts with Liquid-Phase Transmission Electron Microscopy: The Case of Vitamin B12,
ACS Sustainable Chemistry & Engineering 9 (28), 9494-9505
Cobalt compounds are intensely explored as efficient catalysts for the oxygen evolution reaction (OER). Since vitamin B12 is a soluble cobalt compound with high enzymatic activity, evaluating its OER activity is of relevance for biomimetic catalyst research. In this work, the temporal evolution of a homogenous vitamin B12 catalyst in the early stages of OER was investigated by an advanced combination of in situ electrochemical liquid transmission electron microscopy (EC-LTEM), in situ UV–vis spectroelectrochemistry, and extended X-ray absorption fine structure (EXAFS) methods. For the first time, we provided direct evidence of diffuse layer dynamics on the working electrode interface. The results suggested that the formation of cobalt oxyphosphate nanoparticles on the working electrode interface and in the presence of phosphate buffer is the initial stage of the catalytic pathway. Computational results confirmed that the ligand oxidation pathway could occur at potentials below the OER thermodynamic barrier (1.23 V vs reversible hydrogen electrode (RHE)), which leads to a Co ion leaching into the electrolyte. This study showed that investigation of the apparent molecular mechanisms of OER with metal complexes requires careful analyses. We illustrate the high precision and sensitivity of EC-LTEM under operational conditions to monitor heterogeneous catalysts generated during OER and to evaluate their actual roles in the catalytic process.
H. Chen, J. Li, W. Yang, S. E. Balaghi, C. A. Triana, C. K. Mavrokefalos, G. R. Patzke
The Role of Surface States on Reduced TiO2@BiVO4 Photoanodes: Enhanced Water Oxidation Performance through Improved Charge Transfer,
ACS Catalysis 11 (13), 7637-7646
The efficient transfer of photogenerated carriers and improved stability against corrosion are essential to maximize the performance of photoanodes. Herein, a reduced catalytic layer formed on a TiO2 protected BiVO4 (R-TiO2@BiVO4) photoanode has been prepared for progress on both fronts. Specifically, R-TiO2@BiVO4 photoanodes at pH 8 displayed a high photocurrent of 2.1 mA cm–2 at 1.23 VRHE and a more negative onset potential of 234 mVRHE compared to pristine BiVO4. We here discovered two surface states on BiVO4 photoanodes through photoelectrochemical impedance studies. In contrast, only one of them, located at higher potential, appeared on oxygen-vacancy-rich R-TiO2@BiVO4 photoanodes. For BiVO4 photoanodes, the first surface state (SS1) is located near the onset potential (∼0.45 VRHE), while the second surface state (SS2) sits near the water oxidation potential (∼1.05 VRHE). However, SS1 at lower energetics, which originated from water oxidation intermediates with slow kinetics, is passivated in R-TiO2@BiVO4 photoanodes. In contrast, the hole densities in SS2 at higher energetics were greatly enhanced in R-TiO2@BiVO4 photoanodes, due to the increased accumulation of intermediates with fast water oxidation kinetics. Therefore, SS2 is proposed as a reaction center, which is related to the amount and occupancy of oxygen vacancies. Additionally, surface recombination centers in BiVO4 photoanodes are passivated by TiO2, which prevents electron trapping into the irreversible surface conversion of VO2+ to VO2+. These observations provide fundamental understanding of the role of surface states and of the function of oxygen vacancies in BiVO4 photoanodes. Our study offers detailed insight into key strategies for optimal photoelectrochemical performance through surface property tuning.
W. Wan, Y. Zhao, C. A. Triana, J. Li, G. R. Patzke
Noble-Metal-Free Nanostructured and Graphene Supported Electrocatalysts for Water Splitting,
Electrochemical Society Meeting Abstracts 239, 1223-1223
Clean hydrogen production through water splitting is a direct and promising sustainable energy pathway to access and store non-fossil chemical fuels. Synthetic design strategies to access low cost, robust and high performance noble metal-free electrocatalysts are vital for forthcoming water splitting applications. To accomplish this goal, the nanoscale design of multifunctional catalysts based on cost-effective graphene supports, hollow nano-architectures and well-coordinated hybrid organic-inorganic structures has been attracting increasing research interest because of the flexible tuning options of their electronic states, chemical composition and local coordination environments.
Over the last years, we have been implementing systematic investigations based on synthetic design, spectroscopy approaches and computational modeling toward the understanding of the mechanism of action in a series of novel, robust and efficient bi-functional electrocatalysts.
Recently, we explored synergistic effects of NiFe alloys encapsulated into N-doped carbon nanotubes (N-CNTs) supported on reduced graphene (rGO) nanosheets for overall water splitting.[1] This so-called NiFe-N-CNT-rGO electrocatalyst was synthesized through a direct and flexible strategy using g-C3N4 as a precursor, where thermally reduced NiFe nanoparticles act as surface nucleation sites for C species to assist N-CNT growth through an unconventional reduction-nucleation-growth mechanism. The catalyst has a low overpotential of 270 mV for oxygen evolution reaction (OER), being superior in performance to a wide range of reported noble-metal-free catalysts. As revealed from density functional theory (DFT) calculations, N-doping and charge transfer at the CNT walls tune the free energy in the electronic structure toward adsorption of intermediates, which greatly enhanced catalytic performance. This synthetic strategy not only leads to excellent performance and opens up new avenues for other carbon-supported alloy catalysts, but its scalability and easy applicability also render it quite promising for large-scale water splitting applications.
In parallel, we explored molecular strategies to conjugate single active sites of metal-phthalocyanine molecules MPc (M=Ni, Co, Fe) on GO.[2] We aim for a thorough understanding of the role of the local coordination environment around the single metal site in the synergistic mechanisms and the catalytic performance. With an advanced combination of ADF (annular dark field)-STEM, operando X-ray absorption spectroscopy and DFT calculations, we observed changes in the electronic and local structure environment of MPc-GO under operando electrochemical OER conditions. Single active metal Ni, Co, Fe sites attain high valence states due to chemisorbed OH− species upon catalytic activation, while the local orders suffer from structural distortion. These changes in the electronic and structural properties lead to the formation of HO-M-N4 moieties with high OER reactivity under operando conditions. These results provide new insight into the fundamental understanding of structure-performance relationships for single active site catalysts.
Likewise, we studied hollow transition metal sulfide nanoboxes synthesized through anion exchange processes[3] between S2- and CN- species that lead to the formation of Prussian blue (PB), M-S@PB (M=Co, Fe, Ni) heterostructures. We found that the high performance of those nanostructures arises from in situ formation of active Co-Fe oxide/hydroxide species during OER, attaining a superior performance compared to noble-metal-catalysts such as RuO2, with a low overpotential of 286 mV and high durability over longer operational periods. The flexible tuning options of the electronic, chemical and local-atomic coordination of those nanostructures render them promising hybrid catalysts for dual OER and HER over an extended pH range.
We have further explored new soft templating strategies for the first 1D cobalt coordination polymer electrocatalyst with bio-inspired features, and we applied state-of-the-art analytical techniques and computational modeling to monitor its formation in situ and to unravel its structure.[4] The highly durable 1D-cobalt coordination polymer catalyst implements strategies to transfer key motifs of inorganic materials into a hydrogen-bonded ligand environment, namely the embedment of the essential {H2O-Co2(OH)2-OH2} edge-site motif of cobalt oxide catalysts into a flexible and robust organic matrix. While this unconventional catalyst shows exceptional performance in commercial alkaline electrolyzers and organic transformations, its flexible structure offers new pathways toward the rational design of mixed molecular-solid disordered catalysts.
S. E. Balaghi, S. Mehrabani, Y. Mousazade, R. Bagheri, A. S. Sologubenko, Z. Song, G. R. Patzke, M. M. Najafpour
Mechanistic Understanding of Water Oxidation in the Presence of a Copper Complex by In Situ Electrochemical Liquid Transmission Electron Microscopy,
ACS Applied Materials & Interfaces 13 (17), 19927-19937
The design of molecular oxygen-evolution reaction (OER) catalysts requires fundamental mechanistic studies on their widely unknown mechanisms of action. To this end, copper complexes keep attracting interest as good catalysts for the OER, and metal complexes with TMC (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) stand out as active OER catalysts. A mononuclear copper complex, [Cu(TMC)(H2O)](NO3)2 (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), combined both key features and was previously reported to be one of the most active copper-complex-based catalysts for electrocatalytic OER in neutral aqueous solutions. However, the functionalities and mechanisms of the catalyst are still not fully understood and need to be clarified with advanced analytical studies to enable further informed molecular catalyst design on a larger scale. Herein, the role of nanosized Cu oxide particles, ions, or clusters in the electrochemical OER with a mononuclear copper(II) complex with TMC was investigated by operando methods, including in situ vis-spectroelectrochemistry, in situ electrochemical liquid transmission electron microscopy (EC-LTEM), and extended X-ray absorption fine structure (EXAFS) analysis. These combined experiments showed that Cu oxide-based nanoparticles, rather than a molecular structure, are formed at a significantly lower potential than required for OER and are candidates for being the true OER catalysts. Our results indicate that for the OER in the presence of a homogeneous metal complex-based (pre)catalyst, careful analyses and new in situ protocols for ruling out the participation of metal oxides or clusters are critical for catalyst development. This approach could be a roadmap for progress in the field of sustainable catalysis via informed molecular catalyst design. Our combined approach of in situ TEM monitoring and a wide range of complementary spectroscopic techniques will open up new perspectives to track the transformation pathways and true active species for a wide range of molecular catalysts.
J. M. Naik, C. Ritter, B. Bulfin, A. Steinfeld, R. Erni, G. R. Patzke
Reversible Phase Transformations in Novel Ce‐Substituted Perovskite Oxide Composites for Solar Thermochemical Redox Splitting of CO2,
Advanced Energy Materials 11 (16), 2003532
Thermochemical splitting of CO2 and H2O via two-step metal oxide redox cycles offers a promising approach to produce solar fuels. Perovskite-type oxides with the general formula ABO3 have recently gained attention as an attractive redox material alternative to the state-of-the-art ceria, due to their high structural and thermodynamic tunability. A novel Ce-substituted lanthanum strontium manganite perovskite-oxide composite, La3+0.48Sr2+0.52(Ce4+0.06Mn3+0.79)O2.55 (LSC25M75) is introduced, aiming to bridge the gap between ceria and perovskite oxide-based materials by overcoming their individual thermodynamic constraints. Thermochemical CO2 splitting redox cyclability of LSC25M75 evaluated with a thermogravimetric analyzer and an infrared furnace reactor over 100 consecutive redox cycles demonstrates a twofold higher conversion extent to CO than one of the best Mn-based perovskite oxides, La0.60Sr0.40MnO3. Based on complementary in situ high temperature neutron, synchrotron X-ray, and electron diffraction experiments, unprecedented structural and mechanistic insight is obtained into thermochemical perovskite oxide materials. A novel CO2 splitting reaction mechanism is presented, involving reversible temperature induced phase transitions from the n = 1 Ruddlesden–Popper phase (Sr1.10La0.64Ce0.26)MnO3.88 (I4/mmm, K2NiF4-type) at reduction temperature (1350 °C) to the n = 2 Ruddlesden–Popper phase (Sr2.60La0.22Ce0.18)Mn2O6.6 (I4/mmm, Sr3Ti2O7-type) at re-oxidation temperature (1000 °C) after the CO2 splitting step.
S. Heidari, S. E. Balaghi, A. S. Sologubenko, G. R. Patzke
Economic manganese-oxide-based anodes for efficient water oxidation: rapid synthesis and in situ transmission electron microscopy monitoring,
ACS Catalysis 11 (5), 2511-2523
Earth-abundant, environmentally friendly, and low-cost manganese oxide materials are promising resources for water oxidation catalysts in clean solar fuel applications. We here introduce a convenient and economic method for manufacturing stable and highly efficient manganese-oxide-based anodes for electrochemical water oxidation under neutral conditions. The electrodes were fabricated through thermal decomposition of acidic KMnO4 solution. The phase transitions of the manganese oxide film during calcination and thermal decomposition of KMnO4 were monitored with in situ heating transmission electron microscopy (TEM), in situ heating scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM/EDX), and in situ heating powder X-ray diffraction (PXRD). In-depth monitoring of formation pathways and phase transformations by in situ techniques under high temperatures shed light upon the fabrication of efficient manganese oxides for energy conversion applications. After parameter optimizations, the best-performing manganese oxide catalyst was applied for water electrolysis for 100 h with a stable current density of 1.0 mA/cm2 at an overpotential of 490 mV in neutral pH. Post operando characterizations of key oxide film properties showed no significant changes. The readily commercially available precursor enables a simple and rapid fabrication method, and the promising stability and high performance of the herein developed electrodes render them quite promising for technological water splitting systems.
J. Li, W. Wan, C. A. Triana, H. Chen, Y. Zhao, C. K. Mavrokefalos, G. R. Patzke
Reaction kinetics and interplay of two different surface states on hematite photoanodes for water oxidation,
Nature Communications 12 (1), 255
Understanding the function of surface states on photoanodes is crucial for unraveling the underlying reaction mechanisms of water oxidation. For hematite photoanodes, only one type of surface states with higher oxidative energy (S1) has been proposed and verified as reaction intermediate, while the other surface state located at lower potentials (S2) was assigned to inactive or recombination sites. Through employing rate law analyses and systematical (photo)electrochemical characterizations, here we show that S2 is an active reaction intermediate for water oxidation as well. Furthermore, we demonstrate that the reaction kinetics and dynamic interactions of both S1 and S2 depend significantly on operational parameters, such as illumination intensity, nature of the electrolyte, and applied potential. These insights into the individual reaction kinetics and the interplay of both surface states are decisive for designing efficient photoanodes.
R. Güttinger, G. Wiprächtiger, O. Blacque, G. R. Patzke
Co/Ni-polyoxotungstate photocatalysts as precursor materials for electrocatalytic water oxidation,
RSC Advances 11 (19), 11425-11436
An open-core cobalt polyoxometalate (POM) [(A-α-SiW9O34)Co4(OH)3(CH3COO)3]8− Co(1) and its isostructural Co/Ni-analogue [(A-α-SiW9O34)Co1.5Ni2.5(OH)3(CH3COO)3]8− CoNi(2) were synthesized and investigated for their photocatalytic and electrocatalytic performance. Co(1) shows high photocatalytic O2 yields, which are competitive with leading POM water oxidation catalysts (WOCs). Furthermore, Co(1) and CoNi(2) were employed as well-defined precursors for heterogeneous WOCs. Annealing at various temperatures afforded amorphous and crystalline CoWO4- and Co1.5Ni2.5WO4-related nanoparticles. CoWO4-related particles formed at 300 °C showed substantial electrocatalytic improvements and were superior to reference materials obtained from co-precipitation/annealing routes. Interestingly, no synergistic interactions between cobalt and nickel centers were observed for the mixed-metal POM precursor and the resulting tungstate catalysts. This stands in sharp contrast to a wide range of studies on various heterogeneous catalyst types which were notably improved through Co/Ni substitution. The results clearly demonstrate that readily accessible POMs are promising precursors for the convenient and low-temperature synthesis of amorphous heterogeneous water oxidation catalysts with enhanced performance compared to conventional approaches. This paves the way to tailoring polyoxometalates as molecular precursors with tuneable transition metal cores for high performance heterogeneous electrocatalysts. Our results furthermore illustrate the key influence of the synthetic history on the performance of oxide catalysts and highlight the dependence of synergistic metal interactions on the structural environment.
J. Li, C. A. Triana, W. Wan, D. P. Adiyeri Saseendran, Y. Zhao, S. E. Balaghi, S. Heidari, G. R. Patzke
Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives,
Chemical Society Reviews 50 (4), 2444-2485
The development of reliable water oxidation catalysts (WOCs) is essential for implementing artificial photosynthesis on a large technological scale. WOC research has evolved into two major branches, namely molecular and heterogeneous catalysts. Manifold design principles and plenty of mechanistic insights have been developed in these individual fields after decades of investigations. Over the past years, a growing need for knowledge transfer between both sides has emerged in order to expedite the development and optimization of next-generation WOCs. In this review, we first provide selected recent highlights in the area of molecular WOCs with different nuclearities, together with current mechanistic insight. WOCs offering molecular integrity under operational conditions are ideal platforms for elucidating reaction mechanisms and well-defined structure–function correlations at the atomic level. Next, recent mechanistic advances and design strategies for heterogeneous WOCs are illustrated for representative examples, together with a discussion of their inherent limitations in mechanistic studies. Finally, illustrative cases of knowledge transfer between molecular and heterogeneous WOCs are discussed to highlight the advantages of combining the best of both catalyst types. For the sake of conciseness, this review focuses primarily on WOCs based on the first-row transition metals, which are attracting increasing attention for both fundamental studies and economic applications.
2020
W. Wan, C. A. Triana, J. Lan, J. Li, C. S. Allen, Y. Zhao, M. Iannuzzi, G. R. Patzke
Bifunctional single atom electrocatalysts: coordination–performance correlations and reaction pathways,
ACS Nano 14 (10), 13279-13293
Single atom catalysts (SACs) are ideal model systems in catalysis research. Here we employ SACs to address the fundamental catalytic challenge of generating well-defined active metal centers to elucidate their interactions with coordinating atoms, which define their catalytic performance. We introduce a soft-landing molecular strategy for tailored SACs based on metal phthalocyanines (MPcs, M = Ni, Co, Fe) on graphene oxide (GO) layers to generate well-defined model targets for mechanistic studies. The formation of electronic channels through π–π conjugation with the graphene sheets enhances the MPc-GO performance in both oxygen evolution and reduction reactions (OER and ORR). Density functional theory (DFT) calculations unravel that the outstanding ORR activity of FePc-GO among the series is due to the high affinity of Fe atoms toward O2 species. Operando X-ray absorption spectroscopy and DFT studies demonstrate that the OER performance of the catalysts relates to thermodynamic or kinetic control at low- or high-potential ranges, respectively. We furthermore provide evidence that the participation of ligating N and C atoms around the metal centers provides a wider selection of active OER sites for both NiPc-GO and CoPc-GO. Our strategy promotes the understanding of coordination–activity relationships of high-performance SACs and their optimization for different processes through tailored combinations of metal centers and suitable ligand environments.
Y. Zhao, W. Wan, Y. Chen, R. Erni, C. A. Triana, J. Li, C. K. Mavrokefalos, Y. Zhou, G. R. Patzke
Understanding and optimizing ultra‐thin coordination polymer derivatives with high oxygen evolution performance,
Advanced Energy Materials 10 (37), 2002228
Engineering low-crystalline and ultra-thin nanostructures into coordination polymer assemblies is a promising strategy to design efficient electrocatalysts for energy conversion and storage. However, the rational utilization of coordination polymers (CPs) or their derivatives as electrocatalysts has been hindered by a lack of insight into their underlying catalytic mechanisms. Herein, a convenient approach is presented where a series of Ni10-xFex-CPs (0 ≤ x ≤ 5) is first synthesized, followed by the introduction of abundant structural deficiencies using a facile reductive method (R-Ni10-xFex-CPs). The representative low-crystalline R-Ni8Fe2-CPs (R-NiFe-CPs) with a thickness of sub-2 nm display promising oxygen evolution reaction (OER) performance with a very low overpotential of 225 mV at 10 mA cm−2 and high long-term durability over 120 h. Comprehensive investigations including X-ray absorption spectroscopy, density functional theory, and mass diffusion theory reveal strong synergistic effects of structural deficiencies on the OER activity. A super-Nernstian pH-dependence of 85.15 mV pH−1 suggests that the catalytic OER mechanism of R-NiFe-CPs involved a decoupled proton-electron transfer (PT/ET) pathway, leading to notably higher OER activity compared to the concerted coupled proton-electron transfer pathway. New insights into the catalytic reaction mechanisms of CP-related materials open up new approaches to expedite the design of efficient electrocatalysts.
K. Lienau, C. A. Triana, L. Reith, S. Siol, G. R. Patzke
Microwave-Hydrothermal Tuning of Spinel-Type Co3O4 Water Oxidation Catalysts,
Frontiers in Chemistry 8, 473
Water oxidation is the bottleneck reaction for overall water splitting as a direct and promising strategy toward clean fuels. However, the development of robust and affordable heterogeneous water oxidation catalysts remains challenging, especially with respect to the wide parameter space of synthesis and resulting material properties. Oxide catalysts performance in particular has been shown to depend on both synthetic routes and applied catalytic test methods. We here focus on spinel-type Co3O4 as a representative case for an in-depth study of the influence of rather subtle synthetic parameter variations on the catalytic performance. To this end, a series of Co3O4 samples was prepared via time-saving and tunable microwave-hydrothermal synthesis, while systematically varying a single parameter at a time. The resulting spinel-type catalysts were characterized with respect to key materials properties, including crystallinity, oxidation state and surface area using a wide range of analytical methods, such as PXRD, Raman/IR, XAS and XPS spectroscopy. Their water oxidation activity in electrocatalytic and chemical oxidation setups was then compared and correlated with the obtained catalyst properties. Both water oxidation methods displayed related trends concerning favorable synthetic parameters, namely higher activity for lower synthesis temperatures, lower precursor concentrations, addition of hydrogen peroxide and shorter ramping and reaction times, respectively. In addition to the surface area, structural features such as disorder were found to be influential for the water oxidation activity. The results prove that synthetic parameter screening is essential for optimal catalytic performance, given the complexity of the underlying performance-properties relationships.
S. E. Balaghi, C. A. Triana, G. R. Patzke
Molybdenum-doped manganese oxide as a highly efficient and economical water oxidation catalyst,
ACS Catalysis 10 (3), 2074-2087
The development of efficient and noble-metal-free electrocatalysts for the challenging oxygen evolution reaction (OER) is crucial for sustainable energy solutions. In this work, a facile co-precipitation method, followed by thermal postsynthetic treatment in N2/air, was developed to synthesize molybdenum-doped α-Mn2O3 materials (Mn2O3:1.72%Mo, Mn2O3:2.64%Mo, Mn2O3:32.23%Mo, and Mn2O3:49.67%Mo) as low-cost water-oxidizing electrocatalysts. Powder X-ray diffraction (PXRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM) investigations showed the presence of strong distortions in the molybdenum-doped α-Mn2O3 host lattice (Mn2O3:2.64%Mo) and an average oxidation state of Mn2.8+. Several test assays demonstrated that these structural features significantly promote the OER activity. Mn2O3:2.64%Mo was found to exhibit very good activity among the series in cerium ammonium nitrate (CAN)-assisted water oxidation with a maximum turnover frequency (TOF) of 585 μmol O2 m–2 h–1, which is a 15-fold improvement of the pure α-Mn2O3 activity and higher than the value of the previously reported benchmark Mn-based catalyst, birnessite. The optimized catalyst (Mn2O3:2.64%Mo) excelled through a low onset potential (300 mV) and a promising overpotential of 570 mV for OER at a current density of 10 mA cm–2, which is only 20 mV above that of the noble metal benchmark RuO2 electrode and competitive with that of the most active Mn-based OER catalysts reported to date. Electrochemical impedance spectroscopy (EIS) studies demonstrated that the catalytically active surface area of Mn2O3:2.64%Mo is much higher than that of α-Mn2O3 for the OER at the applied potential. In addition, stability during 30 h without degradation was achieved, which exceeds that of a wide range of current noble-metal-free electrocatalysts. Our study provides a facile and effective approach for the preparation of economical and high-performance manganese-based electrocatalysts for water oxidation.
B. Bulfin, J. Vieten, S. Richter, J. M. Naik, G. R. Patzke, M. Roeb, C. Sattler, A. Steinfeld
Isothermal relaxation kinetics for the reduction and oxidation of SrFeO 3 based perovskites,
Physical Chemistry Chemical Physics 22 (4), 2466-2474
The perovskite oxide SrFeO3 has favourable redox properties for oxygen exchange applications, including oxygen separation and oxygen production chemical looping cycles. For such applications, lower temperature operation can improve the energy demand and feasibility of the process, but can also lead to kinetic limitations. Here we investigate the oxidation and reduction reaction kinetics of SrFeO3 in the temperature range 450–750 K. Isothermal relaxation techniques are used to observe the reaction rates across this temperature range, using a thermogravimetric analysis system. Experimental data are analysed according to an isoconversional method and fit with a simple power law model to extract activation energies. The apparent activation energy of oxidation and reduction was found to be 92 ± 16 and 144 ± 17 kJ mol−1 respectively. Comparison of oxidation and reduction kinetics together with considerations of particle size indicate that the oxidation reaction rate may be limited by diffusion in the bulk, while the reduction reaction rate is limited by the surface reaction. Furthermore, we also investigated the mixed perovskite Sr0.93Ca0.07Fe0.9Co0.1O3, which exhibited a 4-fold increase in the oxidation rate.
D. Bleiner, M. Trottmann, R. Müller, L. Rush, I. Kuznezov, A. Cabas-Vidani, Y. Romanyuk, A. Tiwari, G. R. Patzke, C. S. Menoni, J. J. Rocca
3D chemical mapping of thin films by means of x-ray laser microanalysis,
X-Ray Lasers 2018: Proceedings of the 16th International Conference on X-Ray Lasers 16
A capillary discharge X-ray laser is a tabletop coherent source, which has matured for application in microanalysis. Soft X-ray pulses were delivered onto functional materials to induce photo-ablation in the nanoscale. Such samplings were measured inline by means of mass spectrometry, and thus perform depth profile analysis. This paper discusses the complementarity or this technique to contending microanalytical techniques, to have a direct methodology for space resolved in situ thin film characterization.
L. Fagiolari, F. Zaccaria, F. Costantino, R. Vivani, C. K. Mavrokefalos, G. R. Patzke, A. Macchioni
Ir-and Ru-doped layered double hydroxides as affordable heterogeneous catalysts for electrochemical water oxidation,
Dalton Transactions 49 (8), 2468-2476
Three M-doped LDHs (M = noble metal active site, LDH = layered double hydroxides; Ir-1, Ir-ZnAl; Ru, Ru-ZnAl; Ir-2, Ir-MgAl), containing small amounts of M (ca. 2 mol% and even <1 mol% for Ru and Ir, respectively), were prepared by following simple and established synthetic procedures. Their characterization indicates that M atoms are effectively incorporated into the brucite-like layers of LDH, without phase segregation. The resulting materials catalyse electrochemical water oxidation (WO), when immobilized in carbon paste electrodes, with performances that exceed those of the benchmark system IrO2, as probed by linear sweep voltammetry (LSV). Some of these catalysts undergo continuous activation upon chronoamperometric and chronopotentiometric treatments over several hours. The crystalline structure of all of them is preserved during electrocatalytic experiments, and no significant leaching of noble metal in solution is detected. The results herein reported highlight the remarkable potential of these doped M-LDHs and confirm that dispersing Ir and Ru centers in layered and cheap inorganic materials results in easily accessible metal centers, providing highly active catalysts, while minimizing the utilization of noble metals.
2019
L. Reith, K. Lienau, C. A. Triana, S. Siol, G. R. Patzke
Preparative history vs driving force in water oxidation catalysis: parameter space studies of cobalt spinels,
ACS Omega 4 (13), 15444-15456
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.
L. Ni, R. Güttinger, C. A. Triana, B. Spingler, K. K. Baldridge, G. R. Patzke
Pathways towards true catalysts: computational modelling and structural transformations of Zn-polyoxotungstates,
Dalton Transactions 48 (35), 13293-13304
Current catalysis undergoes a paradigm shift from molecular and heterogeneous realms towards new dynamic catalyst concepts. This calls for innovative strategies to understand the essential catalytic motifs and true catalysts emerging from oxidative transformation processes. Polyoxometalate (POM) clusters offer an inexhaustible reservoir for new noble metal-free catalysts and excellent model systems whose structure–activity relationships and mechanisms remain to be explored. Here, we first introduce a new {ZnnNa6−n(B-α-SbW9O33)2} (n = 3–6) catalyst family with remarkable tuning options of the Zn-based core structure and high activity in H2O2-assisted catalytic alcohol oxidation as a representative reaction. Next, high level solution-based computational modelling of the intermediates and transition states was carried out for [Zn6Cl6(SbW9O33)2]12− as a representative well-defined case. The results indicate a radical-based oxidation process with the involvement of tungsten and adjacent zinc metal centers. The {ZnnNa6−n(B-α-SbW9O33)2} series indeed efficiently catalyses alcohol oxidation via peroxotungstate intermediates, in agreement with strong spectroscopic support and other experimental evidence for the radical mechanism. Finally, the high performance of [Zn6Cl6(SbW9O33)2]12− was traced back to its transformation into a highly active and robust disordered Zn/W-POM catalyst. The atomic short-range structure of this resting pre-catalyst was elucidated by RMC modelling of the experimental W-L3 and Zn-K edge EXAFS spectra and supported with further analytical methods. We demonstrate that computational identification of the reactive sites combined with the analytical tracking of their dynamic transformations provides essential input to expedite cluster-based molecular catalyst design.
J. Li, W. Wan, C. A. Triana, Z. Novotny, J. Osterwalder, R. Erni, G. R. Patzke
Dynamic role of cluster cocatalysts on molecular photoanodes for water oxidation,
Journal of the American Chemical Society 141 (32), 12839-12848
While loading of cocatalysts is one of the most widely investigated strategies to promote the efficiency of photoelectrodes, the understanding of their functionality remains controversial. We established new hybrid molecular photoanodes with cobalt-based molecular cubane cocatalysts on hematite as a model system. Photoelectrochemical and rate law analyses revealed an interesting functionality transition of the {Co(II)4O4}-type cocatalysts. Their role changed from predominant hole reservoirs to catalytic centers upon modulation of the applied bias. Kinetic analysis of the photoelectrochemical processes indicated that this observed transition arises from the dynamic equilibria of photogenerated surface charge carriers. Most importantly, we confirmed this functional transition of the cocatalysts and the related kinetic properties for several cobalt-based molecular and heterogeneous catalysts, indicating wide applicability of the derived trends. Additionally, complementary analytical characterizations show that a transformation of the applied molecular species occurs at higher applied bias, pointing to a dynamic interplay connecting molecular and heterogeneous catalysis. Our insights promote the essential understanding of efficient (molecular) cocatalyzed photoelectrode systems to design tailor-made hybrid devices for a wide range of catalytic applications.
F. Song, K. Al-Ameed, M. Schilling, T. Fox, S. Luber, G. R. Patzke
Mechanistically driven control over cubane oxo cluster catalysts,
Journal of the American Chemical Society 141 (22), 8846-8857
Predictive and mechanistically driven access to polynuclear oxo clusters and related materials remains a grand challenge of inorganic chemistry. We here introduce a novel strategy for synthetic control over highly sought-after transition metal {M4O4} cubanes. They attract interest as molecular water oxidation catalysts that combine features of both heterogeneous oxide catalysts and nature’s cuboidal {CaMn4O5} center of photosystem II. For the first time, we demonstrate the outstanding structure-directing effect of straightforward inorganic counteranions in solution on the self-assembly of oxo clusters. We introduce a selective counteranion toolbox for the controlled assembly of di(2-pyridyl) ketone (dpk) with M(OAc)2 (M = Co, Ni) precursors into different cubane types. Perchlorate anions provide selective access to type 2 cubanes with the characteristic {H2O-M2(OR)2-OH2} edge-site, such as [Co4(dpy-C{OH}O)4(OAc)2(H2O)2](ClO4)2. Type 1 cubanes with separated polar faces [Co4(dpy-C{OH}O)4(L2)4]n+ (L2 = OAc, Cl, or OAc and H2O) can be tuned with a wide range of other counteranions. The combination of these counteranion sets with Ni(OAc)2 as precursor selectively produces type 2 Co/Ni-mixed or {Ni4O4} cubanes. Systematic mechanistic experiments in combination with computational studies provide strong evidence for type 2 cubane formation through reaction of the key dimeric building block [M2(dpy-C{OH}O)2(H2O)4]2+ with monomers, such as [Co(dpy-C{OH}O)(OAc)(H2O)3]. Furthermore, both experiments and DFT calculations support an energetically favorable type 1 cubane formation pathway via direct head-to-head combination of two [Co2(dpy-C{OH}O)2(OAc)2(H2O)2] dimers. Finally, the visible-light-driven water oxidation activity of type 1 and 2 cubanes with tuned ligand environments was assessed. We pave the way to efficient design concepts in coordination chemistry through ionic control over cluster assembly pathways. Our comprehensive strategy demonstrates how retrosynthetic analyses can be implemented with readily available assembly directing counteranions to provide rapid access to tuned molecular materials.
C. K. Mavrokefalos, G. R. Patzke
Water oxidation catalysts: the quest for new oxide-based materials,
Inorganics 7 (3), 29
The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have entered the energy market and have paved the way to replace fossil-based energy sources, in the long term. In particular, electrochemical solar-to-hydrogen technologies have attracted a lot of interest—not only in academia, but also in industry. Solar water splitting (artificial photosynthesis) is one of the most active areas in contemporary materials and catalysis research. The development of low-cost, efficient, and stable water oxidation catalysts (WOCs) remains crucial for artificial photosynthesis applications, because WOCs still represent a major economical and efficient bottleneck. In the following, we summarize recent advances in water oxidation catalysts development, with selected examples from 2016 onwards. This condensed survey demonstrates that the ongoing quest for new materials and informed catalyst design is a dynamic and rapidly developing research area.
F. O. von Rohr, A. Ryser, H. Ji, K. Stolze, J. Tao, J. J. Frick, G. R. Patzke, R. J. Cava
The h‐SbxWO3+2x Oxygen Excess Antimony Tungsten Bronze,
Chemistry – A European Journal 25 (8), 2082-2088
We describe the previously unreported oxygen excess hexagonal antimony tungsten bronze with composition Sb0.5W3O10, in the following denoted as h-SbxWO3+2x with x=0.167, to demonstrate its analogy to classical AxWO3 tungsten bronzes. This compound forms in a relatively narrow temperature range between 580 °C