Noble Metal-free Electrocatalysts Research Articles

Synergistic Catalytic Sites in High-Entropy Metal Hydroxide Organic Framework for Oxygen Evolution Reaction.

The integration of multiple elements in a high-entropy state is crucial in the design of high-performance, durable electrocatalysts. High-entropy metal hydroxide organic frameworks (HE-MHOFs) are synthesized under mild solvothermal conditions. This novel crystalline metal-organic framework (MOF) features a random, homogeneous distribution of cations within high-entropy hydroxide layers. HE-MHOF exhibits excellent electrocatalytic performance for the oxygen evolution reaction (OER), reaching a current density of 100mA cm-2 at ≈1.64 VRHE, and demonstrates remarkable durability, maintaining a current density of 10mA cm-2 for over 100 h. Notably, HE-MHOF outperforms precious metal-based electrocatalysts despite containing only ≈60% OER active metals. Ab initio calculations and operando X-ray absorption spectroscopy (XAS) demonstrate that the high-entropy catalyst contains active sites that facilitate a multifaceted OER mechanism. This study highlights the benefits of high-entropy MOFs in developing noble metal-free electrocatalysts, reducing reliance on precious metals, lowering metal loading (especially for Ni, Co, and Mn), and ultimately reducing costs for sustainable water electrolysis technologies.

Unveiling the Structural Effects in Hybrid Copper Phosphonate Frameworks for Selective Electrocatalytic CO2 Reduction Reaction.

Electrochemical CO2 reduction holds tremendous promise for transforming carbon dioxide into several value-added energy feedstocks and utilizing renewable energy sources. Herein, we have developed two novel copper-based organophosphonates for selective electrocatalytic conversion of CO2 to CH3OH conversion. The two-dimensional layer structure of Cu3[(Hhedp)2(C4H4N2)].2H2O (I) and the three-dimensional Cu3[(H3hedp)2(C4H4N2)4(SO4)].2H2O (II) have been isolated as single crystals via a hydrothermal strategy. Compound I consists of Cu2+ oxidation states exclusively, while compound II has Cu1+ oxidation states in a network wherein a Cu2+-phosphonate template is embedded inside the framework. Depending on mixed valent oxidation states, compound II exhibits high selectivity compared to compound I for the electrocatalytic reduction of CO2 to CH3OH (C1) as the primary product and CH3COOH (C2) as the secondary product. Notably, product selectivity is enhanced as the Faradaic efficiency (FE) of the competing hydrogen evolution reaction (HER) is significantly reduced in compound II relative to that of I, particularly at higher applied reduction potentials. The optimal ratio of Cu1+ active sites in compound II plays a pivotal role in enhancing methanol selectivity, stabilizing critical intermediates, and maintaining ideal reduction potentials as a noble-metal free electrocatalyst. Moreover, the optical band gap and the Mott-Schottky measurements further suggest the title Cu-phosphonate materials could be promising and effective photocatalysts.

Theoretical Investigation of Single-, Double-, and Triple- p-block Metals Anchored on g-CN Monolayer for Oxygen Electrocatalysis.

The design and development of highly active non-noble metal electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are crucial for metal-air batteries. In this work, the electrocatalytic performance of different p-block metal (PM = Sn, Sb, Pb and Bi) atoms embedded in the g-CN monolayer (PMx@g-CN, x = 1-3) for the OER and ORR was systematically investigated by density functional theory (DFT). The strong interaction between PMx atoms and g-CN substrates indicates the good stability of PMx@g-CN catalysts. Among all the designed catalysts, Bi3@g-CN is found to be a promising bifunctional electrocatalyst for both the OER and ORR with the calculated overpotential ηOER and ηORR of 0.23 and 0.25 V, respectively. With the atomic active sites of PMx increasing from x = 1 to 3, the OER and ORR catalytic activity is enhanced. The correlations between the overpotentials of the OER/ORR and Bader charge values of the anchored PMx atoms of the catalysts were established. Our findings contribute to searching for noble metal-free bifunctional electrocatalysts and shed light on the rational design of atomic active sites on electrocatalysts.

Doping-mediated interface engineered Ni3S2: Economically viable electrochemical methanol-mediated water splitting

Electrochemical water splitting is the front runner to meet the global green hydrogen demand. The development of a noble metal-free earth-abundant electrocatalyst is desired to rationalize an economically viable electrochemical water-splitting system. The sluggish anodic oxygen evolution reaction (OER) is the bottleneck of the electrochemical water-splitting system. Herein, to improve OER kinetics, group VIB elements such as chromium (Cr), molybdenum (Mo), and tungsten (W) doped nickel sulfides (Ni3S2) nanosheets (NSs) are synthesized by two-step hydrothermal method to modulate the intrinsic electrocatalytic activity of the pure Ni3S2. Among all the doped samples, the Cr-doped Ni3S2 catalyst exhibited excellent OER with an ultra-low overpotential of 248 mV and MOR with a potential of 1.36 V @ 10 mA cm−2 vs RHE achieving a lower Tafel slope of 95.5 mV/dec and 46.4 mV/dec and outstanding durability with negligible degradation respectively. Furthermore, the Pt/C||Cr-Ni3S2 (1.48 V) system outperforms commercial Pt/C||RuO2 (1.53 V @ 10 mA cm−2) in a two-electrode configuration. The methanol-mediated hybrid water splitting system only warranted a cell voltage of 1.38 V to attain 10 mA cm−2 for the Pt/C||Cr-Ni3S2 system in the presence of 0.1 M methanol (MeOH). The methanol oxidation reaction (MOR) mediated the hybrid water electrolyzer system demonstrates reduced operation cell voltage while additionally electro-synthesizing formate as a value-added chemical. The overall work is motivated to rationalize the importance of doping-mediated interface-engineered catalyst synthesis to improve the electrocatalytic efficiency of the hybrid water electrolyzer.

Manganese doped tailored cobalt sulfide as an accelerated catalyst for oxygen evolution reaction

Developing an efficient, robust, and noble metal-free electrocatalyst that can catalyse oxygen evolution reactions (OER) remains a significant challenge. CoS2, a representative of pyrite form transition metal dichalcogenides, has recently been identified as an economical catalyst. Here, an incredibly quick and scalable technique for novel catalysts synthesized with the use of the microwave method was introduced. Manganese-doped cobalt sulphide (Mn-CoS2) showed outstanding OER with a very low overpotential of 227 mV at 10 mA cm−2. Exposure of surface atoms resulted in high electrochemical activity, where the defects facilitated charge and mass transfer along the nanostructure, allowing surface dependent electrochemical reactions to be performed more efficiently. The electronic properties of pristine and transition-metal-doped CoS2 structures were also investigated using density functional theory (DFT). To better understand transition metal’s dependent impact on crystal structure, orbital electronic participation, charge density, and charge transformation in both pristine and Mn-dopedCoS2 frameworks were calculated and analysized. Our synthesis approach is primarily commercial and extensible, overcoming synthesis challenge of transition metal sulphide nanostructures with prime quality and implying a potential for commercial uses.

Facile synthesis of spherical-shaped CeC2–TiO2 nanocomposite electrocatalyst with boosted alkaline OER

Developing noble metal-free multifunctional electrocatalysts to catalyze water oxidation is vital for future renewable energy systems. Herein, through several defect modifications, the CeC2–TiO2 nanocomposite on the strength of enriched oxygen vacancies was successfully fabricated by facile hydrothermal method on SS (stainless steel) substrate, attributed to the conductive ability for fast electron transportation. A well-defined phase growth, vibrational bonding of metal atoms, morphological determination, and elemental composition for synthesized heterostructured electrocatalysts were revealed by XRD, FTIR, TEM, and EDX, respectively. XPS has complied to understand the moderate binding energies of CeC2–TiO2 for OER intermediates, which signifies the strong interactions between electronic states of Ce, C, Ti, and O atoms. With the plentiful catalytic active sites, the resultant CeC2–TiO2 entails low overpotentials of only 169 mV and 108.8 mV/dec Tafel slope to afford 10 mA cm−2 for OER in 1.0 M KOH are tested through cyclic and linear sweep voltammogram measurements, thereby exhibit promising activity in alkaline water electrolysis. EIS measurements revealed a decreased polarization resistance for the composite dominated by the small semicircle diameter, corresponding to the adsorption of intermediates. Finally, observed results strongly recommended that the CeC2–TiO2 catalyst exhibited superior OER performance due to excellent electrical chemical coupling between CeC2 and TiO2.

Disentangling the catalytic origin in defect engineered 2D NiCoMoS@Ni(CN)2 core-shell heterostructure for energy-saving hydrazine-assisted water oxidation

The major hindrance to efficient electrocatalytic hydrogen generation from water electrolysis is the sluggish kinetics with corresponding large overvoltage of oxygen evolution reaction. Herein, we report a defective 2D NiCoMoS@Ni(CN)2 core-shell heterostructure derived from Hofmann-type MOF as an efficient and durable high-performance noble metal-free electrocatalyst for hydrazine oxidation reaction (HzOR) in alkaline media. The sluggish oxygen evolution reaction was replaced with a more thermodynamically favourable HzOR, enabling energy-saving electrochemical hydrogen production with 2D NiCoMoS@Ni(CN)2 acting as a bifunctional electrocatalyst for anodic HzOR and cathodic hydrogen generation. Operating at room temperature, the two-electrode electrolyzer delivers 100 mA cm−2 from a cell voltage of just 257 mV, with strong long-term electrochemical durability and nearly 100% Faradaic efficiency for hydrogen evolution in 1.0 M KOH aqueous solution with 0.5 M hydrazine. The density functional theory (DFT) was employed to investigate the origin of catalytic performance and showed that high vacancy creation within the heterointerface endowed NiCoMoS@Ni(CN)2 with favourable functionalities for excellent catalytic performance.

Synergistic enhancement of Zn-air battery performance via integration of Ni-doped cobalt sulfide nanoparticles within N, S-doped carbon matrix

Exploring precious metal-free bifunctional electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is essential for the practical application of rechargeable Zn-air battery (ZAB). Herein, Ni-doped Co9S8 nanoparticles embedded in a defect-rich N, S co-doped carbon matrix (d-NixCo9-xS8@NSC) are synthesized via a facile pyrolysis and acid treatment process. The introduction of abundant defects in both the carbon matrix and metal sulfide provides numerous active sites and significantly enhances the electrocatalytic performances for both the ORR and OER. d-NixCo9-xS8@NSC exhibits a superior half-wave potential of 0.841 V vs. RHE for the ORR and delivers a low overpotential of 0.329 V at 10 mA cm−2 for the OER. Additionally, Zn-air secondary battery using d-NixCo9-xS8@NSC as the air cathode displays a higher specific capacity of 734 mAh gZn−1 and a peak power density of 148.03 mW cm−2 compared to those of state-of-the-art Pt/C-RuO2 (673 mAh gZn−1 and 136.9 mW cm−2, respectively). These findings underscore the potential of d-NixCo9-xS8@NSC as a high-performance electrocatalyst for secondary ZABs, offering new perspectives on the design of efficient noble metal-free electrocatalysts for future energy storage and conversion applications.

Metal Doping Regulates Electrocatalysts Restructuring During Oxygen Evolution Reaction.

High-efficiency and low-cost catalysts for oxygen evolution reaction (OER) are critical for electrochemical water splitting to generate hydrogen, which is a clean fuel for sustainable energy conversion and storage. Among the emerging OER catalysts, transition metal dichalcogenides have exhibited superior activity compared to commercial standards such as RuO2, but inferior stability due to uncontrolled restructuring with OER. In this study, we create bimetallic sulfide catalysts by adapting the atomic ratio of Ni and Co in CoxNi1-xSy electrocatalysts to investigate the intricate restructuring processes. Surface-sensitive X-ray photoelectron spectroscopy and bulk-sensitive X-ray absorption spectroscopy confirmed the favorable restructuring of transition metal sulfide material following OER processes. Our results indicate that a small amount of Ni substitution can reshape the Co local electronic structure, which regulates the restructuring process to optimize the balance between OER activity and stability. This work represents a significant advancement in the development of efficient and noble metal-free OER electrocatalysts through a doping-regulated restructuring approach.

Theoretical and experimental investigations of vanadium pentoxide–based electrocatalysts for the hydrogen evolution reaction in alkaline media

A key approach towards better realization of intermittent renewable energy resources, namely, solar and wind, is green electrochemical hydrogen production from water electrolysis. In recent years, there have been increasing efforts aimed at developing noble metal-free electrocatalysts that are earth-abundant and electroactive towards hydrogen evolution reaction (HER) in alkaline electrolytes, wherein an initial water dissociation step is followed by a two-electron transfer cathodic reaction. Although relatively earth-abundant, vanadium-based electrocatalysts have been sparsely reported due to subpar electroactivity and kinetics towards water electrolysis in general and alkaline electrolysis in specific. Herein, we investigate the fine-tuning of orthorhombic V2O5-based electrocatalysts as candidates for HER through a scalable two-step sol–gel calcination procedure. Briefly, surface-induced anionic oxygen deficiencies and cationic dopants are synergistically studied experimentally and theoretically. To that end, first-principle facet-dependent density function theory (DFT) calculations were conducted and revealed that the coupling of certain dopants on V2O5 and co-induction of oxygen vacancies can enhance the catalytic HER performance by the creation of new electronic states near the Fermi level (EF), enhancing conductivity, and modulating surface binding of adsorbed protons, respectively. This was reflected experimentally through kinetically non-ideal alkaline electrochemical HER using Zn0.4V1.6O5 whereby − 194 mV of overpotential was required to attain − 10 mA/cm2 of current density, as opposed to pristine V2O5 which required 32% higher overpotential requirement at the same conditions. The disclosed work can be extended to other intrinsically sluggish transition metal (TM)–based oxides via the presented systematic tuning of surface and bulk microenvironment modulation.Graphical

Acidic Oxygen Evolution Reaction: Fundamental Understanding and Electrocatalysts Design.

Water electrolysis driven by "green electricity" is an ideal technology to realize energy conversion and store renewable energy into hydrogen. With the development of proton exchange membrane (PEM), water electrolysis in acidic media suitable for many situations with an outstanding advantage of high gas purity has attracted significant attention. Compared with hydrogen evolution reaction (HER) in water electrolysis, oxygen evolution reaction (OER) is a kinetic sluggish process that needs a higher overpotential. Especially in acidic media, OER process poses higher requirements for the electrocatalysts, such as high efficiency, high stability and low costs. This review focuses on the acidic OER electrocatalysis, reaction mechanisms, and critical parameters used to evaluate performance. Especially the modification strategies applied in the design and construction of new-type electrocatalysts are also summarized. The characteristics of traditional noble metal-based electrocatalysts and the noble metal-free electrocatalysts developed in recent decades are compared and discussed. Finally, the current challenges for the most promising acidic OER electrocatalysts are presented, together with a perspective for future water electrolysis.

Hollow-structured cobalt sulfide electrocatalyst for alkaline oxygen evolution reaction: Rational tuning of electronic structure using iron and fluorine dual-doping strategy

Utilizing renewable electricity for water electrolysis offers a promising way for generating high-purity hydrogen gases while mitigating the emission of environmental pollutants. To realize the water electrolysis, it is necessary to develop highly active and precious metal-free electrocatalyst for oxygen evolution reaction (OER) which incurs significant overpotential due to its complicated four-electron transfer mechanism. Hence, we propose a facile preparation method for hollow-structured Fe and F dual-doped CoS2 nanosphere (Fe-CoS2-F) as an efficient OER electrocatalyst. The uniform hollow and porous structure of Fe-CoS2-F enlarge the specific surface area and increase the number of exposed active sites. Furthermore, the Fe and F dual-dopants synergistically contributed to the adjustment of electronic structure, thereby promoting the adsorption/desorption of oxygen-containing reaction intermediates on active sites during the alkaline OER procedure. As a result, the prepared Fe-CoS2-F exhibits outstanding OER activity, characterized by a low overpotential of 298 mV to achieve a current density of 10 mA cm−2 and a Tafel slope as small as 46.0 mV dec−1. Based on computational theoretical calculations, the introduction of the dual-dopants into CoS2 structure reduce the excessively strong adsorption energy of reaction intermediate in the rate determining step, leading to effectively promoted electrocatalytic cycle for OER in alkaline environment. This study presents an effective strategy for preparing noble metal-free OER electrocatalysts with promising potential for large-scale industrial water electrolysis.

Macromolecular crosslinked poly(aryl piperidinium)-based anion exchange membranes with enhanced ion conduction for water electrolysis

Anion exchange membranes (AEMs) are essential in AEM water electrolysis, providing a cost-effective alternative to proton exchange membranes through the use of noble metal-free electrocatalysts. However, the “trade-off” between conductivity and dimensional stability in AEMs still hinders the development of high-performance water electrolysis. Herein, partially functionalized poly(phenylene oxide) (QPPO) is combined with poly(aryl piperidinium) (QPAP) to fabricate novel crosslinked AEMs (C-QPAP-x-QPPO), addressing the conductivity-stability issue. The crosslinked C-QPAP-x-QPPO AEMs show lower water swelling compared to their non-crosslinked QPAP membrane, mainly due to increased chain entanglements from the crosslinking process. C-QPAP-2-QPPO AEM exhibits a high conductivity of 139.4 mS cm−1 at 80 °C, attributed to the well-defined microphase separation structure. In addition, C-QPAP-2-QPPO AEM demonstrates exceptional alkaline durability, retaining over 86% conductivity and showing slight degradation after 2400 h in 1 M KOH at 60 °C. A water electrolyzer using the C-QPAP-2-QPPO achieves a maximum current density of 1440 mA cm−2 at 80 °C (2.0 V). These findings demonstrate the potential applications of crosslinked C-QPAP-x-QPPO AEMs in electrochemical devices.

Large-area, flexible bimetallic phosphorus-based electrodes for prolong-stable industrial grade overall seawater splitting

The economical and efficient preparation of highly stable electrodes for hydrogen from water splitting is one of the current challenges. Herein, a flexible and durable bifunctional nickel–cobalt-phosphide electrode is constructed in situ on hydrophilicity filter paper (NiCoP@HP) via a one-step mild electroless plating method for industrial-scale water splitting. The bimetallic synergy of Ni-Co facilitates efficient electron transfer, while the electronegativity of phosphide contributes to high intrinsic activity, and the flexible paper substrate allow the electrode to be folded and bent. The NiCoP@HP electrodes exhibit outstanding performance for the hydrogen/oxygen evolution reaction (HER/OER), with low overpotentials only 43 mV and 164 mV at a current density of 10 mA cm−2 in simulated seawater (1.0 M KOH + 0.5 M NaCl). Importantly, the NiCoP@HP electrode demonstrates long-term stability, operating for over 1440 h at a current density of 500 mA cm−2 in 1.0 M KOH + 0.5 M NaCl and 1.0 M KOH + Seawater, respectively. This universally applicable method allows the preparation of a range of highly efficient catalytic electrodes (Fe, Mo, W, etc.). This work provides a simple, scalable, and versatile approach for the in situ construction of noble metal-free, highly efficient, and cost-effective bifunctional electrocatalysts, opening new possibilities for industrial-scale water splitting.

Constructing Chainmail-Structured CoP/C Nanospheres as Highly Active Anodic Electrocatalysts for Oxygen Evolution Reaction.

Constructing highly active and noble metal-free electrocatalysts is significant for the anodic oxygen evolution reaction (OER). Herein, uniform carbon-coated CoP nanospheres (CoP/C) are developed by a direct impregnation coupling phosphorization approach. Importantly, CoP/C only takes a small overpotential of 230 mV at the current density of 10 mA cm-2 and displays a Tafel slope of 56.87 mV dec-1. Furthermore, the intrinsic activity of CoP/C is 21.44 times better than that of commercial RuO2 under an overpotential of 260 mV. In situ Raman spectroscopy studies revealed that a large number of generated Co-O and Co-OH species could facilitate the *OH adsorption, effectively accelerating the reaction kinetics. Meanwhile, the carbon shell with a large number of mesoporous pores acts as the chainmail of CoP, which could improve the active surface area of the catalyst and prevent the Co sites from oxidative dissolution. This work provides a facile and effective reference for the development of highly active and stable OER catalysts.

Hybrid high-performance oxygen reduction reaction Fe–N–C electrocatalyst for anion exchange membrane fuel cells

Atomically dispersed active sites and hierarchical porosity in Fe–N–C catalyst materials are essential for efficient oxygen reduction reaction (ORR) electrocatalysis and effective mass transport, respectively. However, the majority of these catalysts fail to deliver outstanding electrocatalytic activity and stumble when used as cathode materials in fuel cells. Herein, we developed a catalyst with partially transformed metallic Fe into Fe-Nx sites embedded in the carbonaceous matrix, and its excellent electrocatalytic performance is attributed to the hybrid nanoparticles and built-in Fe-Nx site structures. The optimized Fe@Fe–N–C catalyst was tested for ORR performance both ex-situ using the rotating (ring)-disk electrode technique and in an anion exchange membrane fuel cell (AEMFC). The Fe@Fe–N–C electrocatalyst exhibited remarkably enhanced ORR activity in alkaline media with a half-wave potential of 0.822 V vs. reversible hydrogen electrode (RHE) and showed the maximum power density (Pmax) of 242 mW cm−2 in an AEMFC test, surpassing the peak power density obtained from the Pt/C cathode catalyst (Pmax = 220 mW cm−2) under H2–O2 conditions. Therefore, the study provides a promising prospect for designing improved, cost-effective, and noble metal-free electrocatalysts for achieving high performance in AEMFCs.

Facile one-step solid-state synthesis of CuO nanoparticles finely decorated over carbon sheets for improved OER activity

Renewable energy storage and conversion in electrochemical fuel cells require highly efficient and cost-effective noble metal free electrocatalysts for oxygen evolution reaction (OER). Downscaling of a material to nanoscale morphology can lead to enhancement in its surface activities affecting its physical and chemical properties which can well serve the purpose. In lieu to this, it could be one of the first reports to tune the morphology of CuO particles to 10–25 nm finely trapped inside non-graphitic carbon sheets using a novel single step solid state reaction. XRD, SEM, TEM, XPS, and Raman analyses confirm the structural and morphological attributes. The synergistic effect from carbon and copper oxide resulted in the attainment of best-in-class electrode material for oxygen evolution reaction (OER). The as-prepared CuO/C shows an excellent OER activity with a low overpotential of 454 mV (vs. Ag/AgCl) at a current density of 10 mA cm−2 with a low Tafel slope of 74.9 mV dec−1. The amount of carbon is optimized to get optimal performance from the CuO nanopowder.

Heterostructure Boosts a Noble-metal-free Oxygen-evolving Electrocatalyst in Acid

Developing noble metal-free electrocatalysts (NMFEs) for oxygen evolution reaction (OER) is tremendously challenging in acid. Despite extensive research efforts, few reported NMFEs can compete with Ru/Ir oxides for acidic OER....

Recent advances in noble metal-free electrocatalysts to achieve efficient alkaline water splitting

Electrochemical water splitting is one of the promising approaches for generating hydrogen.

Improving hydrogen production activity of bifunctional CuCoFe-layered double hydroxides for electrocatalytic water splitting: Effects of valence state evolution and oxygen vacancies

The development of noble metal-free electrocatalysts is crucial for efficient water splitting via both hydrogen (HER) and oxygen evolution reaction (OER) in alkaline electrolyte. In this study, bifunctional CuCoFe layered double hydroxides grown in-situ on Ni foam (NF) have been prepared to achieve high-efficient water splitting. The CuCoFe LDHs need a low overpotential of 49 mV for HER and 255 mV for OER to deliver a current density of 10 mA·cm−2, which is superior to many reported LDH-based electrocatalysts. On the one hand, the Cu2+ ions induce distortions of structure units, which promotes the formation of oxygen vacancies and facilitates electron migration as well as charge transfer. On the other hand, excessive addition of Cu2+ ions can lead to a reduction in active sites due to the acceleration of Co2+ oxidation during preparation. This work presents an opportunity to regulate the structure of LDH-based electrocatalysts by the new approach of Cu-doping, resulting in enhanced control over valence state evolution and oxygen vacancies.