Thin porous nanosheets of NiFe layered-double hydroxides toward a highly efficient electrocatalyst for water oxidation

NiFe layered double hydroxides (NiFe-LDH) are low cost and earth abundant electrocatalysts for oxygen evolution reaction (OER). Herein, the NiFe-LDH nanosheets induced by ZIF-67 (NiFe-LDH/ZIF-67) were prepared via a coupling method of a self-sacrificing template method and a co-precipitation method. Atomic force microscope and field emission transmission electron microscopy analysis indicate that NiFe-LDH/ZIF-67 consist of thin porous nanosheets. X-ray photoelectron spectra analysis show that NiFe-LDH/ZIF-67 contains more oxygen vacancies than pristine NiFe-LDH. The overpotential of NiFe-LDH/ZIF-67 is 222 mV at 10 mA cm⁻² for OER, lower than that of ZIF-67, pristine NiFe-LDH, and the commercial RuO₂, indicating that its high electrocatalytic activity for OER. The high electrocatalytic activity of NiFe-LDH/ZIF-67 may be attributed to its thin porous nanosheet structure, which is derived from the structural influence of ZIF-67 and the coupling effect of a self-sacrificing template method and a co-precipitation method.     

Cobalt nanoparticles embedded nitrogen doped carbon,preparation from alkali deprotonation assisted ZIF-67 and its electrocatalytic performance in oxygen evolution reaction

The development of safe, cost-effective and efficient electrocatalyst is significant for oxygen evolution reaction (OER). Herein, cobalt nanoparticles embedded-nitrogen doped carbon composite (Co-NC) is prepared by pyrolysis of a Co based zeolitic imidazolate framework (ZIF-67) precursor, which is synthesized by KOH assisted deprotonation of 2-methylimidazole ligand and coordination with Co(Ac)₂ in a safe ethanol medium. The alkali deprotonation and employment of Co(Ac)₂ reactant obviously enhance the yield, phase purity and structural stability of the afforded ZIF-67 precursor (denoted as K-ZIF-67-Ac) and further the OER catalytic efficiency of the resultant Co-NC composite. The typical Co-NC catalyst prepared by carbonization of K-ZIF-67-Ac at 800 °C in N₂ atmosphere, denoted as K-ZIF-67-Ac-800, displays obviously lower overpotential (350 mV) at current density of 50 mA cm⁻², higher OER catalytic kinetics and robust durability over other ZIF-67 derived Co-NC catalysts. The current work paves a feasible avenue to prepare efficient Co-NC OER catalyst from alkali deprotonation assisted ZIF precursor.     

Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties

Hollow nanostructures with their intriguing structural features are attractive for efficient energy conversion and storage technology. Herein we report the designed construction of novel CoS₂ nanoboxes by employing the Co-based zeolitic imidazolate (ZIF-67) nanocubes (NCs) as the starting template. Delicate manipulation of the template-engaged reaction time between thioacetamide (TAA) and ZIF-67 NCs leads to the formation of hollow CoS₂ nanostructures. As a result of the unique nanobox structure, the optimized CoS₂ nanoboxes (NBs) have an enlarged electrolyte-accessible surface, abundant mass diffusion pathways, and high structural integrity, which afford the current density of 10 mA cm⁻² at a low overpotential of 290 mV for oxygen evolution reaction (OER) in 1 M KOH solution. Remarkably, these CoS₂ NBs could also display excellent long-term stability for more than 40 h electrochemical test in an alkaline medium, showing a class of advanced electrocatalysts for OER and beyond.     

Nickel foam-supported Fe,Ni-Polyporphyrin microparticles: Efficient bifunctional catalysts for overall water splitting in alkaline media

Developing highly active, stable and sustainable electrocatalysts for overall water splitting is of great importance to generate renewable H₂ for fuel cells. Herein, we report the synthesis of electrocatalytically active, nickel foam-supported, spherical core-shell Fe-poly(tetraphenylporphyrin)/Ni-poly(tetraphenylporphyrin) microparticles (FeTPP@NiTPP/NF). We also show that FeTPP@NiTPP/NF exhibits efficient bifunctional electrocatalytic properties toward both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Electrochemical tests in KOH solution (1 M) reveal that FeTPP@NiTPP/NF electrocatalyzes the OER with 100 mA cm⁻² at an overpotential of 302 mV and the HER with 10 mA cm⁻² at an overpotential of 170 mV. Notably also, its catalytic performance for OER is better than that of RuO₂, the benchmark OER catalyst. Although its catalytic activity for HER is slightly lower than that of Pt/C (the benchmark HER electrocatalyst), it shows greater stability than the latter during the reaction. The material also exhibits electrocatalytic activity for overall water splitting reaction at a current density of 10 mA cm⁻² with a cell voltage of 1.58 V, along with a good recovery property. Additionally, the work demonstrates a new synthetic strategy to an efficient, noble metal-free-coordinated covalent organic framework (COF)-based, bifunctional electrocatalyst for water splitting.     

Tailoring Cr doped NiFeP nanoparticles embedded in 3D cellulose nanofibrils carbon architecture for efficient oxygen evolution reaction

Oxygen evolution reaction (OER) is considered the bottleneck that restricting the pace of electrocatalytic hydrogen production. Modulating structure and heterogeneous doping are essential approaches to effectively promote the electrocatalytic efficiency and stability. Herein, three-dimensional (3D) porous Cr doped NiFeP nanoparticles encapsulated in cellulose nanofibrils (CNF) carbon architecture (Cr–NiFeP/NC) with high-efficiency and durable OER performance was constructed. CNF played crucial role on the construction of 3D porous framework and promoting the OER performance significantly. Benefiting from the 3D porous structure, high specific surface area and exposed abundant active sites, the Cr–NiFeP/NC electrocatalyst displayed excellent OER performance, which the overpotential to deliver the current density of 10 mA cm⁻² was only 249 mV with a Tafel slope of 51.2 mV dec⁻¹ in 1.0 M KOH, outperforming the RuO₂ and other reported electrocatalysts remarkably. In addition, the Cr–NiFeP/NC electrocatalyst exhibited outstanding stability, which the overpotential was only increased by 2.5% after 48 h chronopotential measurement to deliver a current density of 10 mA cm⁻² with stable morphology and structure. This work demonstrated an integrated strategy of Cr doping and 3D porous structure modulating employed CNF as skeleton for the efficient and durable OER performance, providing a spark for hydrogen production by water splitting.     

Mesoporous nickel-sulfide/nickel/N-doped carbon as HER and OER bifunctional electrocatalyst for water electrolysis

The development of non-precious metal-based highly active bi-functional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is critical factor for making water electrolysis a viable process for large-scale industrial applications. In this study, bi-functional water splitting electrocatalysts in the form of nickel-sulfide/nickel nanoparticles integrated into a three-dimensional N-doped porous carbon matrix, are prepared using NaCl as a porous structure-forming template. Microstructures of the catalytic materials are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and N₂ adsorption-desorption analysis. The most active catalyst synthesized in this study exhibits a low HER overpotential of 70 mV at 10 mA cm⁻² and a low Tafel slope of 45 mV dec⁻¹. In OER, the optimized sample performs better than a state-of-the-art RuO₂ catalyst and produces an overpotential of 337 mV at 10 mA cm⁻², lower than that of RuO₂. The newly obtained materials are also used as HER/OER electrocatalysts in a specially assembled two-electrode water splitting cell. The cell demonstrates high activity and good stability in overall water splitting.     

MoS2/NiFeS2 heterostructure as a highly efficient electrocatalyst for overall water splitting at high current densities

Searching for efficient, stable and low-cost nonprecious catalysts for oxygen and hydrogen evolution reactions (OER and HER) is highly desired in overall water splitting (OWS). Herein, presented is a nickel foam (NF)-supported MoS₂/NiFeS₂ heterostructure, as an efficient electrocatalyst for OER, HER and OWS. The MoS₂/NiFeS₂/NF catalyst achieves a 500 mA cm⁻² current density at a small overpotential of 303 mV for OER, and 228 mV for HER. Assembled as an electrolyzer for OWS, such a MoS₂/NiFeS₂/NF heterostructure catalyst shows a quite low cell voltage (≈1.79 V) at 500 mA cm⁻², which is among the best values of current non-noble metal electrocatalysts. Even at the extremely large current density of 1000 mA cm⁻², the MoS₂/NiFeS₂/NF catalyst presents low overpotentials of 314 and 253 mV for OER and HER, respectively. Furthermore, MoS₂/NiFeS₂/NF shows a ceaseless durability over 25 h with almost no change in the cell voltage. The superior catalytic activity and stability at large current densities (>500 mA cm⁻²) far exceed the benchmark RuO₂ and Pt/C catalysts. This work sheds a new light on the development of highly active and stable nonprecious electrocatalysts for industrial water electrolysis.     

Embedding Co2P nanoparticles into co-doped carbon hollow polyhedron as a bifunctional electrocatalyst for efficient overall water splitting

The reasonable design and construction of non-precious metal electrocatalysts with low cost and high performance is critical for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, a facile polymerization-pyrolysis method is proposed to encapsulate Co₂P nanoparticles in co-doped hollow carbon shell by using ZIF-67 and P-containing polymers as precursor. The unique construction not only effectively prevents nanoparticles from detaching, showing good stability after long-term testing, but also provides abundant active sites, large surface areas and large pore volumes, enabling the electrolyte and electrode material to full contact. As expected, the Co₂P/NPSC-800 performs superior HER performance with low overpotential of 173 mV at 10 mA cm⁻² and excellent stability of 88% retention for 35 h and OER performance with low overpotential of 320 mV at 10 mA cm⁻², which endows Co₂P/NPSC-800 with good catalytic activity in overall water splitting. Furthermore, density functional theory (DFT) calculations reveal that the metallic property and the decreased reaction barriers of Co₂P can promote the catalytic reactions. This work offers an effective route in synthesizing other transition metal phosphides with high catalytic properties.     

Surface engineering of 2D MOF to construct high-density Co9S8 nanoparticles supported on nitrogen doped carbon for efficient oxygen evolution reaction

Transition metal sulfides and their hybrids are promising alternative to precious metal catalyst for the oxygen evolution reaction (OER). Herein, the high-density Co₉S₈ nanoparticles (NPs) embedded in N-doped carbon has been prepared by using surface-engineered zeolitic imidazolate framework-9 (ZIF-9) nanosheets as precursor. The surface of ZIF-9 was modified with TAA, which is able to create chemical barrier and prevents metal from aggregation in the subsequent pyrolysis, thus making small Co₉S₈ NPs densely anchored on carbon layers. Arising from the unique structure, Co₉S₈@NC affords an optimized electronic structure and rich effective reactive sites for OER. As expected, Co₉S₈@NC exhibits small overpotential of 264 mV at 10 mA cm⁻², low Tafel slope of 68.4 mV dec⁻¹, and superior stability for alkaline OER (0.1 M KOH). The electrolysis cell, which was equipped with Co₉S₈@NC cathode and Pt/C anode, shows low water splitting voltage of 1.58 V at 10 mA cm⁻² in 1.0 M KOH. This work employs an efficacious surface engineering strategy to design metal sulfide-based electrocatalysts for enhancing OER performance.     

Electrodeposition of NiFe layered double hydroxide on Ni3S2 nanosheets for efficient electrocatalytic water oxidation

The oxygen evolution reaction (OER) plays a vital role in various energy conversion applications. Up to now, the highly efficient OER catalysts are mostly based on noble metals, such as Ir- and Ru-based catalysts. Thus, it is extremely urgent to explore the non-precious electrocatalysts with great OER performance. Herein, a simple electrodeposition combined with hydrothermal method is applied to synthesize a non-precious OER catalyst with a three-dimensional (3D) core-shell like structure and excellent OER performance. In our work, NiFe layered double hydroxide (LDH) was electrodeposited on Ni₃S₂ nanosheets on nickel foam (NF), which exhibits a better performance compared with RuO₂, and a low overpotential of 200 mV is needed to reach the current density of 10 mA/cm² in 1 M KOH. Notably, the Ni₃S₂/NiFe LDH only need an overpotential of 273 mV to reach the current density of 200 mA/cm².     

Spatially confined growth of ultrathin NiFe layered double hydroxide nanosheets within carbon nanofibers network for highly efficient water oxidation

The rational design of catalysts with low cost, high efficient and robust stability toward oxygen evolution reaction (OER) is greatly desired but remains a formidable challenge. In this work, a one-pot, spatially confined strategy was reported to fabricate ultrathin NiFe layered double hydroxide (NiFe-LDH) nanosheets interconnected by ultrafine, strong carbon nanofibers (CNFs) network. The as-fabricated NiFe-LDH/CNFs catalyst exhibits enhanced OER catalytic activity in terms of low overpotential of 230 mV to obtain an OER current density of 10 mA cm^?2 and very small Tafel slope of 34 mV dec^?1, outperforming pure NiFe-LDH nanosheets assembly, commercial RuO₂, and most non-noble metal catalysts ever reported. It also delivers an excellent structural and electrocatalytic stability upon the long-term OER operation at a large current of 30 mA cm^?2 for 40 h. Furthermore, the cell assembled by using NiFe-LDH/CNFs and commercial Pt/C as anode (+) and cathode (?) ((+)NiFe-LDH/CNFs||Pt/C(?)) only requires a potential of 1.50 V to deliver the water splitting current of 10 mA cm^?2, 130 mV lower than that of (+)RuO₂||Pt/C(?) couple, demonstrating great potential for applications in cost-efficient water splitting devices.     

Structure engineering of amorphous P–CoS hollow electrocatalysts for promoted oxygen evolution reaction

Oxygen evolution reaction (OER) is a key process involved in many energy-related conversion systems. An ideal OER electrocatalyst should possess rich active sites and optimal binding strength with oxygen-containing intermediates. Although numerous endeavors have been devoted to the modification and optimization of transition-metal-based OER electrocatalysts, they are still operated with sluggish kinetics. Herein, an ion-exchange approach is proposed to realize the structure engineering of amorphous P–CoS hollow nanomaterials by utilizing the ZIF-67 nanocubes as the precursors. The precise structure control of the amorphous hollow nanostructure contributes to the large exposure of surface active sites. Moreover, the introduction of phosphorus greatly modifies the electronic structure of CoS₂, which is thus favorable for optimizing the binding energies of oxygenated species. Furthermore, the incorporation of phosphorus may also induce the formation of surface defects to regulate the local electronic structure and surface environment. As a result of this, such P–CoS hollow nanocatalysts display remarkable electrocatalytic activity and durability towards OER, which require an overpotential of 283 mV to afford a current density of 10 mA cm^?2, outperforming commercial RuO₂ catalyst.     

ZIF67@MoO3 NSs@NF composite electrocatalysts reinforced by chemical bonds and oxygen vacancy for efficient oxygen evolution reaction and overall water-splitting

A kind of composite electrocatalysts with the structure of MoO₃ nanosheets coated by ZIF67 nanocrystals and grown on the nickel foam substrate (ZIF67@MoO₃ NSs@NF) is prepared and mainly used as the electrode for oxygen evolution reaction (OER) and overall water splitting. The excellent electrocatalytic activity of ZIF67@MoO₃ NSs@NF are demonstrated. It can use the overpotential (?) of 178 mV and 386 mV respectively to drive 10 mA cm^?2 and 50 mA cm^?2. It is also observed that the ZIF67@MoO₃ NSs@NF electrode has the highest initial current density (45.7 mA cm^?2) at 1.618 V and can maintain more than 90% of the initial current density after 20,000 s. The ZIF67@MoO₃ NSs@NF electrode also shows the small HER overpotential of 135 mV at 10 mA cm^?2. Furthermore, the voltage of ZIF67@MoO₃ NSs@NF as a bifunctional overall water splitting catalysts is 1.58 V at 10 mA cm^?2, which is superior to another noble metal electric catalyst combination RuO₂/NF(+)//Pt–C/NF(?). And the ZIF67@MoO₃ NSs@NF(+)//ZIF67@MoO₃ NSs@NF(?) combination can maintain more than 90% of the initial current density after 65,000 s at 1.58 V. The main reason is the composite interface of MoO₃ NSs and ZIF67 phases with Co–O bonds, C–O–Mo bonds and oxygen vacancies defects facilitates the increase of the active sites and efficient electron transfer rate.     

Co0.85Se on three-dimensional hierarchical porous graphene-like carbon for highly effective oxygen evolution reaction

Designing an efficient, cheap and abundant catalyst for oxygen evolution reaction (OER) is crucial for the development of sustainable energy sources. A novel catalyst which could be a promising candidate for such electrocatalysts is described. Co_0.85Se supported on three-dimensional hierarchical porous graphene-like carbon (HPG) exhibits outstanding catalytic performances for OER in alkaline medium. It is found that the onset overpotential is 311 mV on the Co_0.85Se/HPG electrode, which is more 28 and 41 mV negative than that on the Co/HPG and Co₃O₄/HPG electrodes. What's more, the value of Tafel slope is 61.7 mV dec⁻¹ and the overpotential at the current density of 10 mA cm⁻² is 385 mV on this electrode. The Co_0.85Se/HPG of this work is an appealing electrocatalyst for OER in basic electrolyte.     

Synergistically modulating the electronic structure of Cr-doped FeNi LDH nanoarrays by O-vacancy and coupling of MXene for enhanced oxygen evolution reaction

Water splitting is an environmentally friendly method of hydrogen generation. However, it is severely limited by the slow anodic oxygen evolution reaction (OER). Iron-nickel layered double hydroxides (FeNi LDH) are promising electrocatalysts for OER, but their intrinsically low electrical conductivity and activity limit the practical applications. Herein, chromium-doped FeNi LDH nanoarrays in situ vertically grown on the surface of the Ti₃C₂T_x MXene (Cr-FeNi LDH/MXene) are successfully synthesized. Remarkably, the robust interaction and electrical coupling between Cr–FeNi LDH and MXene, as well as conspicuous charge transfer and the oxygen vacancies optimizing the adsorption free energy of intermediates, equip the nanocomposites with brilliant catalytic activity and stability toward OER. Thus, the optimized Cr–FeNi LDH/MXene shows a considerable boost in the OER, which affords low overpotential (232 mV at 10 mA cm^?2) and excellent durability. This work offers a new path to designing highly efficient and earth-abundant catalysts for water splitting and beyond.     

Construction of ZIF-67/MIL-88(Fe,Ni) catalysts as a novel platform for efficient overall water splitting

Constructing bifunctional non-precious metal electrocatalysts is necessary for effective overall water splitting (OWS), but challenging. Herein, a novel hybrid nanostructure of ZIF-67/MIL-88(Fe, Ni), denoted as Co-M-Fe/Ni(x) (x represents the mass of ZIF-67), was successfully synthesized by hydrothermal and in-situ growth method, and showed a highly efficient and stable bifunctionality of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolyte. The Co-M-Fe/Ni(150) exhibited excellent OER performance with a low overpotential of 269 mV and 149 mV @ 10 mA cm^?2 for OER and HER in 1 mol L^?1 KOH, respectively. With Co-M-Fe/Ni(150) as cathode and anode, the integrated OWS device had achieved low potential of 1.52 V @ 10 mA cm^?2, exhibiting its excellent performance of OWS. Based on the results of experiments, ZIF-67 and MIL-88(Fe, Ni), as metal-organic frameworks (MOFs), which have a large specific surface area, uniform distribution of porous structures facilitates charge transmission, promoting the penetration of electrolytes, and improves electron transfer rate. The mechanism of the superior electrocatalytic performance of Co-M-Fe/Ni(150) may be attributed to the synergy of ZIF-67 and MIL-88(Fe, Ni). This work provides guidance for the rational design or optimization of non-noble composites for energy conversion.     

Facile modified polyol synthesis of FeCo nanoparticles with oxyhydroxide surface layer as efficient oxygen evolution reaction electrocatalysts

The oxygen evolution reaction (OER) at anode requires high overpotential and is still challenging. The metallic core-oxyhydroxide layer structure is an efficient method to lower an overpotential. We synthesized Fe rich FeCo core-Co rich FeCo oxyhydroxide layer with a different particle size of 173 nm, 225 nm, and 387 nm (FeCo 173, 225, 387) through a difference in the reduction rate of Fe/Co precursors using facile modified polyol synthesis. To investigate the effect of conductivity, CoFe₂O₄ nanoparticles of 80–130 nm were synthesized. Among samples, FeCo 173 showed remarkable catalytic performance of 316 mV at a current density of 10 mA/cm² in 0.1 M KOH compared to RuO₂ (408 mV), FeCo 225 (323 mV), FeCo 387 (334 mV), CoFe₂O₄ (382 mV). Moreover, FeCo 173 showed good stability for 60,000 s while RuO₂ showed a gradual increase in overpotential to maintain 10 mA/cm² after 15,000 s in chronopotentiometry. The excellent performance was attributed to Fe-rich metallic core, a small amount of Fe doping into CoOOH, and the synergic effect between the active site of Co rich FeCoOOH and conductive Fe rich metallic core. Following this result, it shows that the use of such FeCo electrodes has advantages in the production of hydrogen via electrochemical water oxidation.     

A partial sulfidation approach that significantly enhance the activity of FeCo layered double hydroxide for oxygen evolution reaction

We report a partial sulfidation approach that effectively boosts the OER activity of FeCo-layered double hydroxides (LDH). It is found that the mild sulfurized FeCo-LDH nanosheets using Na₂S converted a portion of their surface metal-hydroxide bonds to metal-sulfur (-hydrosulfide) bonds without significantly altering their crystal structure. The sulfidation degree is controlled by Na₂S concentration for obtaining a moderately surface electronic configurations. Benefits from the regulated electronic configurations, the sulfurized FeCo-LDH nanosheets only require an overpotential of 281 mV to produce oxygen at 10 mA cm⁻² and their Tafel slope is 51.8 mV dec⁻¹, which are both lower than the 348 mV and 72.7 mV dec⁻¹ of pristine FeCo-LDH nanosheets. The sulfurized catalysts have sustained 12 h of operation without notable activity loss. This work can provide new insights into understanding the roles of metal-sulfur bonds for OER and offer an attractive strategy to design low cost but efficient OER catalysts.     

Ni3S2-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

In situ construction of heterostructure NiSe–NiO nanoarrays with rich oxygen vacancy on MXene for efficient oxygen evolution

Developing high-efficiency, non-noble, earth-available electrocatalysts for the oxygen evolution reaction (OER) is vital for electrochemical energy conversion, but it is still challenging. Herein, we ingeniously designed a partial selenization method to construct NiSe–NiO heterostructure grown in situ on Ta₄C₃T_x MXene (denoted as NiSe–NiO/Ta₄C₃T_x MXene). NiSe–NiO/Ta₄C₃T_x MXene's plethora of heterointerfaces provides a wealth of active sites, fast charge and mass transfer, and favorable adsorption energies for OER intermediates, all of which contribute synergistically to the oxidation of alkaline water. As expected, taking advantage of the strong chemical and electron synergistic effects of NiSe and NiO, the synthesized NiSe–NiO/Ta₄C₃T_x MXene exhibits excellent activity for OER with a low overpotential of 255 mV at 10 mA cm⁻², a small Tafel slope of 47.4 mV dec⁻¹, as well as excellent long-term stability, exceeding that of its competitors. This study offers a novel synthetic route toward developing high-performance OER electrocatalysts for renewable energy conversion/storage systems and beyond by optimizing the catalysts' composition and architecture.