Oxygen Evolution Reaction Potential Research Articles

One-pot synthesis of graphene nanosheets‑nickel cobalt LDHs nanocomposite for electrocatalysis of oxygen evolution reaction

Oxygen evolution reaction (OER) is known as a bottleneck for the overall water-splitting process due to its slow kinetics. In this regard, constructing a high-performance, cost-effective electrode material may act as a game changer. Here, a 2D/2D nanocomposite has prepared out of graphene nanosheets and nickel‑cobalt layered double hydroxide (NiCo LDH) by a simple solvothermal method. The 2D flower-like structures of NiCo LDH attached to graphene nanosheets have observed in field-emission scanning (FE-SEM) and transmission (TEM) electron microscopy images. The X-ray diffraction (XRD) patterns have revealed the formation of Ni(OH)2 and Co(OH)2 lattices along with hexagonal graphene crystals. X-ray energy dispersive spectroscopy (EDS) has noted the 2.2:1 ratio of nickel to cobalt in the graphene/NiCo LDH nanocomposite. X-ray elemental maps have shown the uniform distribution of elements in the sample, and the corresponding functional groups have observed in the Fourier-transform infrared (FTIR) and Raman spectra. X-ray photoelectron spectroscopy (XPS) has determined the coexistence of divalent and trivalent metals in the nanocomposite. The N2 adsorption-desorption study has shown signs of slit-shaped ion-accessible micro and mesopores with high specific surface area for the nanocomposite. It has benefited the ion diffusion process during the exposure of the modified electrode to the electrolyte. The graphene/NiCo LDH nanocomposite has provided the onset potential of 1.56 V, overpotential of 338 mV at 10 mA cm−2, Tafel slope of 69 mV dec−1, charge-transfer resistance (Rct) of 27 Ω, double layer capacitance (Cdl) of 24 μF, electrochemically surface area (ECSA) of 0.6 cm2, and roughness factor (RF) of 20. The electrode has maintained 98.3 % of its initial signal after 10 h continuous measurement at OER potential. This electrocatalytic activity refers to the sheet-like morphology, effective hydroxide ion transfer through interlayer spaces, enhanced conductivity, and high chemical stability achieved by a constructive synergy between graphene nanosheets and NiCo LDH.

Pd-Co₃O₄/Acetylene Black Nanocomposite as an Efficient and Robust Bifunctional ORR/OER Electrocatalyst for Rechargeable Zinc-Air Batteries

Developing cost-effective and high-performing bifunctional electrocatalysts is pivotal for advancing rechargeable zinc-air batteries (ZABs). In this study, a Pd-Co₃O₄ loaded on acetylene black (Pd-Co₃O₄/C) electrocatalyst was developed with excellent properties for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) using reduced H₂PdCl₄ to modify Co₃O₄/C. The experimental findings indicate that the Pd-Co₃O₄/C electrocatalyst exhibits a favorable half-wave potential for ORR at 0.91 V, a feasible potential for OER at 1.48 V (measured at 10 mA/cm2), also minor Tafel slopes for both ORR (44.65 mV/dec) and OER (58.41 mV/dec). Additionally, it evidences a power density of 186 mW/cm2 in ZABs and good durability during galvanostatic charge-discharge cycling. The impressive performance of Pd-Co₃O₄/C can be ascribed primarily to the beneficial synergistic effect resulting from the composition of Co₃O₄ and Pd embedded within graphite carbon. This research lays the groundwork for the expansion of additional cost-effective and highly effective electrocatalysts in the coming years.

Fleeting-Active-Site-Thrust Oxygen Evolution Reaction by Iron Cations from the Electrolyte.

Oxygen evolution reaction (OER) is key to sustainable energy and environmental engineering, thus necessitating rational design of high-performing electrocatalysts that requires understanding the structure-performance relationship with a possible dynamic nature under working conditions. Herein, we uncover a novel type of OER mechanisms thrust by the fleeting active sites (FASs) dynamically formed on Ni-based layered double hydroxides (Ni-LDHs) by Fe cations from the electrolyte under OER potentials. We employ grand-canonical ensemble methods and microkinetic modeling to elucidate the potential-dependent structures of FASs on Ni-LDHs and demonstrate that the fleeting-active-site-thrust (FAST) mechanism delivers superior OER activity via the FAST intramolecular oxygen coupling pathway, which also suppresses the lattice oxygen mechanism, leading to improved operando stability of Ni-LDHs. We further reveal that introducing only trace-level loadings (10-100 ppm) of FASs on Ni-LDHs can significantly boost and govern the catalytic performance for OER. This underscores the crucial importance of considering the novel FAST mechanism in OER and also suggests the electrolyte as a key part of the structure-performance relationship as well as an effective design strategy via engineering the electrolyte.

Interfacial Acid‐Like Microenvironment and Orbital Modulating Strategy toward Efficient Hydrogen Evolution in Neutral High‐Salinity Wastewater/Seawater

Electrochemical high‐salinity wastewater splitting is a promising technology for green hydrogen (H2) production. However, the kinetics of hydrogen evolution reaction (HER) in neutral media is slow, and the high theoretical potential of oxygen evolution reaction leads to large energy losses. Herein, an iron‐based electrocoagulation‐coupled hydrogen production integrated system (IEHPS) is constructed, which is realized by coupling low‐potential anodic iron oxidation reaction with cathodic HER. The non‐noble metal HxWO3‐Ni catalyst is synthesized by fabricating a proton sponge HxWO3 to achieve an interfacial acid‐like microenvironment and doping it with Ni heteroatom to modulate the 4d orbital of W, thereby weakening the adsorption strength of the W site toward hydrogen. Consequently, the HxWO3 Ni demonstrates remarkable performance characteristics, boasting a mere 131 mV overpotential at 10 mA cm−2 and Tafel slope of 44 mV dec− in neutral media. Operating at an applied voltage of 1.5 V, the IEHPS exhibits a high hydrogen production rate of 235 mL g−1 min−1 in seawater. It achieves nearly complete removal of contaminants like rhodamine B and heavy metal ions within a rapid 8–20 min, with an energy consumption of only 3.7 kWh Nm−3. This study provides a promising pathway for efficient and energy‐saving production of high‐purity hydrogen and effective treatment of high‐salinity wastewater.

Ni–Cu Bimetallic Oxides with Tailored Morphology for High‐Performance Alkaline Water Oxidation

AbstractWater oxidation, a crucial step in electrochemical water splitting, is often hindered by its complex kinetics. In our pursuit of designing efficient electrocatalysts, we have found that morphological modifications during synthesis are powerful tools. By manipulating various parameters, such as reaction time and temperature, we have tuned the morphology of the catalysts, leading to a significant enhancement in the oxygen evolution reaction (OER) rate. Our thorough research involves using a simple hydrothermal method for synthesizing bimetallic Ni–Cu oxides, resulting in the emergence of different morphologies by varying reaction times from 4 to 24 h. The OER performance in an alkaline medium, using a 1 M KOH solution, has been meticulously investigated, and our results have unveiled the highest activity for Ni–Cu fabricated with a growth time of 24 h. Ni–Cu oxide 24 exhibits a remarkably low onset potential for OER at 222 mV with a Tafel slope of 373 mV/dec. The catalyst boasts the lowest Rct value of 2.788 ohm, ensuring faster charge transfer rates and stability up to 1000 cyclic voltammetry (CV) cycles, thereby supporting its electrochemical applications for water splitting.

A facile synthesis of N-doped carbon encapsulated multimetallic carbonitride as a robust electrocatalyst for oxygen evolution reaction

Electrocatalytic water splitting is a promising solution for generating clean hydrogen. Transition metal compounds are among the most extensively investigated catalysts developed to date for water oxidation in alkaline media, a process also known as the oxygen evolution reaction (OER). However, the application of these catalysts was constrained by insufficient stability arising from surface oxidation and metal dissolution under high OER potential. In this work, we developed a facile approach using urea-based gel as the precursor of preparing a series of multimetallic carbonitride particles which were encapsulated by N-doped carbon (NC). In particular, (MoCoFeNiZr)CN@NC core–shell structure delivered a low overpotential of 246 mV at a current density of 10 mA cm−2 in 1 M KOH during OER. Importantly, operando differential electrochemical mass spectrometry (DEMS), together with multiple microscopic and spectroscopic analyses, indicated that the NC shells effectively maintained the crystalline stability of carbonitride via suppressing the surface reconstruction during catalysis. The highly graphitic NC also demonstrates excellent stability against oxidation. This work shows a promising strategy of stabilizing electrocatalyst at high anodic potential, paving the way for the development of robust electrode materials for energy conversion.

Boosting OER activity of Fe-N Moieties via Ni induced d-Orbital Tailoring for durable rechargeable Zn-Air batteries

Fe-N-C-based catalysts are considered the most promising Pt candidates due to their high oxygen reduction reaction (ORR) activity and low cost, whereas their oxygen evolution reaction (OER) performance is extremely poor due to the sluggish O-O coupling process, which hampers the practical application of rechargeable zinc-air batteries. Here, we in situ infiltrate Ni and Fe ions into metal triazolidine (MET)-derived N-doped porous carbon via a microporous confinement-pyrolysis strategy to enhance the OER activity of the Fe-N sites. The introduction of Ni induced the electronic delocalization of Fe atoms at the Fe-N center, which lowered the adsorption energy barriers for the decisive velocity step (O*→OOH*), thus accelerating the O-O bond coupling and OER kinetics. In addition, the preferential formation of NiOOH at high OER potentials ensures the catalytic stability of zinc-air batteries by trapping Fe ions escaping from carbon corrosion to form FeOOH. The optimized catalyst (FeNi-NC) has an overpotential of only 351 mV at a current density of 50 mA cm−2 under alkaline conditions. More importantly, the assembled ZABs can be stably charged and discharged for more than 500 h at 10 mA cm−2, demonstrating excellent battery charging and discharging stability. This work shows the great potential of strong electronic interactions between polymetals to improve Fe-N-C catalysts.

Unlocking OER potential: Tailoring layered double perovskites through self-reconstruction

The sluggish rate of the anode oxygen evolution reaction (OER) is a major bottleneck for green hydrogen production, making a room-temperature alkaline water electrolyser critical to unlocking the full potential of this technology. Using a perovskite oxide as an electrochemical matrix and forming a self-assembled oxyhydroxide (OOH) layer on the surface via a self-reconstruction activation process (SRAP) is a promising strategy to enhance OER catalytic activity. However, the mechanistic details and long-term impact of SRAP, especially its relationship to catalytic activity, oxygen collection efficiency, and catalyst dissolution, are not fully understood. To address this knowledge gap, we used layered double perovskite (LDP) PrBa0·75Ca0·25Co1·5Fe0·5O5+δ(PBCCF) and PrBa0·75Ca0·25Co1·5Fe0·4Ni0·1O5+δ (PBCCFN) as model electrochemical catalysts and then triggered SRAP. Subsequently, chronoamperometry (CA), rotating ring disk electrode (RRDE) and in-situ electrochemical quartz crystal microbalance (EQCM) were used to study the effect of the Ni-doping on PBCCF SRAP in terms of catalytic activity, oxygen collection efficiency and catalyst stability. The results showed that the OER catalytic current of the LDP PBCCF and PBCCFN was enhanced via SRAP and the oxygen collection efficiency can be maintained, suggesting that the increase in catalytic current is attributed to OER catalytic activity increase rather than electrochemical catalyst dissolution. Additionally, the effect of the catalytic activity enhancement was more significant in PBCCFN. More importantly, the stability of the LDP PBCCF and PBCCFN after SRAP are higher than simple perovskite Ba0·5Sr0·5Co0·8Fe0·2O3-δ (BSCF) because of the increase in the redeposition rate. The observations suggest that using LDP PBCCF and PBCCFN as matrices followed by triggering SRAP can provide valuable insights for designing LDP based OER electrocatalysts.

Enhanced Oxygen‐Reaction Electrocatalysis and Corrosion Resistance of CoCrFeNi Thin Films by Tuned Microstructure and Surface Oxidation

Oxygen electrocatalysts play a key role in renewable and fossil‐free energy production. Bifunctional catalysts active for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) allow use of the same material system for both energy production (ORR) and fuel generation (OER). However, optimizing the performance of bifunctional catalysts requires in depth understanding of the catalyst structure, its surface chemistry in terms of active sites and the underlying catalytic mechanism. Here, the catalytic performance of CoCrFeNi thin films is investigated, synthesized using high‐power impulse magnetron sputtering, as bifunctional oxygen electrocatalysts. The film crystal structure and morphology, and thereby the catalytic performance, can be tuned by the ion acceleration (bias) to the substrate. To further enhance the catalytic activity, anodization is used to electrochemically modify the films, forming a thicker oxide layer enriched in Co and Ni cations which significantly improves the ORR performance. Anodization improves the catalyst stability during OER, with an OER potential of 1.45 V versus the reversible hydrogen electrode (RHE) at 10 mA cm−2 for more than 24 h. While the corrosion resistance is high both before and after anodization, in terms of catalytic activity the anodized films outperformed the as‐deposited ones. This makes anodized films excellent electrocatalyst candidates in corrosive alkaline environments such as fuel cells and electrolyzers.

High-throughput screening of efficient graphdiyne supported transition metal single atom toward water electrolysis and oxygen reduction

Efficient single-atom electrocatalysts show great potential for application in the field of renewable energy. In this work, dispersion-corrected density functional theory (DFT) calculations based on high-throughput screening were used to identify effective graphdiyne (GDY) supported single-atom electrocatalysts (SAECs) for water electrolysis and the oxygen reduction reaction (ORR). The solvation effect on the electrocatalytic performance of the catalysts was carefully discussed. A general design principle was established to evaluate the activities of TM@GDY SAECs for ORR and oxygen evolution reaction (OER). ΔGOH* could serve as the sole descriptor for the onset potential of OER and ORR. Additionally, a universal structural descriptor φ, which is strongly related to ΔGOH*, was identified. This descriptor only includes intrinsic features such as the number of electrons in d orbitals and elemental electronegativity. Consequently, a structure-activity volcano was plotted. Combined with the electronic properties calculations, the mechanism behind the relationship between ΔGOH* and descriptor φ was deeply analyzed. Among the investigated candidates, TM@GDY (TM = Ni, Pt, Pd, Cu, Co, Rh, and Ag) are potential bifunctional electrocatalysts for OER and ORR. Three non-noble metal-based SAECs (Ni@GDY, Co@GDY, and Cu@GDY) and two noble metal-based SAECs (Rh@GDY and Ag@GDY) are potential trifunctional electrocatalysts for OER, ORR, and the hydrogen evolution reaction.

Clustered VCoCOx Nanosheets Anchored on MXene–Ti3C2@NF as a Superior Bifunctional Electrocatalyst for Alkaline Water Splitting

Ti3C2, one typical MXene, has great potential to be coupled with various transition metals. Herein, a novel and effective catalyst is developed by synergistically loading VCoCOx onto Ti3C2‐modified nickel foam (VCoCOx–Ti3C2@NF). Field emission scanning electron microscope and high‐resolution transmission electron microscopy are employed to characterize the morphology and structure. As expected, the catalyst optimized by response surface methodology attains overpotentials of 290 and 64 mV and Tafel slopes of 82 and 79 mV dec−1 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. By using the bifunctional VCoCOx–Ti3C2@NF catalyst, the water splitting current density achieves 10 mA cm−2 in 1.0 mol L−1 KOH electrolyte at cell voltage of 1.52 V, comparable with the noble metal electrolyzer Pt@C@NF||RuO2@NF (1.57 V). Furthermore, the resulting catalyst exhibits excellent cycling durability after 120 h of continuous catalysis, which retains 103.8% and 105.4% of potential (V vs reversible hydrogen electrode) for OER and HER, respectively. Density functional theory calculation reveals that the Gibbs free energy barriers for the OER and HER intermediates are reduced due to the integration of VCoCOx with Ti3C2@NF. The fabricated VCoCOx–Ti3C2@NF catalyst is a promising electrochemical material for clean energy production.

Factorial Optimization of CoCuFe-LDH/Graphene Ternary Composites as Electrocatalysts for Water Splitting.

The layered double hydroxides (LDHs) have demonstrated significant potential as non-noble-metal electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Their unique compositional and structural properties contribute to their efficiency and stability as catalysts. In this study, CoCuFe-LDH composites were grown on graphene (G) via a cost-effective and straightforward one-step hydrothermal process. A 2-level full-factorial model was employed to determine the impact of Co (1.5, 3, and 4.5 mmol) and graphene (10, 30, and 50 mg) concentrations on the onset potential of OER and HER, which were the chosen response variables. OER and HER activity variabilities were assessed in triplicate using Co[3]Cu[3]Fe[3]-LDH/G[30] (central point), which were determined at 0.01% and 0.02%, respectively. Statistical analyses demonstrated that Co[4.5]Cu[3]Fe[3]-LDH/G[10] and Co[1.5]Cu[3]Fe[3]-LDH/G[10] showed the lowest onset potential at 1.52 V and -0.32 V (V vs RHE) for the OER and HER, respectively, suggesting that a high cobalt concentration enhances OER performance, while optimal HER catalysis was achieved with lower cobalt concentrations. Moreover, the trimetallic composites exhibited good stability with negligible loss of catalytic activity over 24 h.

Transition metal doped into layered double hydroxide as efficient electrocatalysts for oxygen evolution reaction: A DFT study

The doping of transition metal (TM) into layered double hydroxide (LDH) is a widely used method for developing efficient and durable electrocatalysts for the oxygen evolution reaction (OER). In this study, a series of TM-doped bimetallic LDH (TMII/TMIII@M3N-LDH(110)) models were constructed and their OER properties were systematically evaluated using density functional theory (DFT) calculations. Furthermore, the overall volcano plot was constructed, from which seven potential OER catalysts were screened out. Among them, PtIII@Co3Fe-LDH(110) and RuII@Co3Fe-LDH(110) were identified as the most promising candidates, exhibiting excellent OER performance with a theoretical overpotential (ηOER) of 0.20 V, and were analyzed for the doping effect of TM by electronic structure. The results demonstrated that TM doped into M3N-LDH(110) plays a crucial role in the OER process. These theoretical studies not only pave the way for further exploration of efficient OER catalysts but also illustrate the potential of trimetallic LDH as high-performance catalysts.

Role of active redox sites and charge transport resistance at reaction potentials in spinel ferrites for improved oxygen evolution reaction

Fe and Ni oxide-based systems are often investigated as electrocatalysts for the oxygen evolution reaction (OER) in water electrolysis and the improvement in OER activity is explained by two main reasons, (i) improving bulk electrical conductivity and (ii) synergic effect between Ni and Fe. Among the two factors, the identification of the dominant factor for OER is very important for catalyst engineering in the right direction. In order to have a better understanding, spinel-structured ZnFe2O4 nanoparticles have been synthesized and its bulk conductivity is studied by progressively varying Ni doping. The conductivity of the synthesised compounds follows the order of ZnFe2O4 > Ni0.3Zn0.7Fe2O4 > Ni0.5Zn0.5Fe2O4 > Ni0.7Zn0.3Fe2O4 > NiFe2O4. The oxygen evolution studies reveal that NiFe2O4 is highly active with an onset potential of 1.57 V versus reversible hydrogen electrode (RHE) and Tafel slope of 102 mV/dec. With the increase in Ni doping, the structure changes from spinel to inverse spinel as confirmed by X-ray diffraction and Raman spectra. Further, the contribution from Ni redox sites in addition to Fe ion sites and FexNi1−xOOH species formation, causes improved OER performance. This study uniquely proves that the OER activity majorly relies on redox/active metal sites, FexNi1−xOOH species and charge transfer resistance at OER potential rather than the bulk electrical conductivity of spinel ferrites. This conclusion is supported by voltammetry and impedance studies of five different spinel ferrites.

Influence of Oxygen Vacancies in La0.4Sr0.6FeO3-δ Perovskite Oxide Nanoparticles for the Oxygen Evolution Reaction

Iron incorporation is one of the key factors to generate highly active NiFe(OH)x electrocatalysts for the oxygen evolution reaction (OER). However, it remains controversial whether Fe(IV) is present during OER and how the oxygen vacancies in Fe(IV)-containing metal oxides affect their activities. In this work, we develop a facile approach to control oxygen vacancy content, via an ozone treatment under ambient pressure during cooling for the synthesis of La1-x Sr x FeO3-δ (0 ≤ x ≤ 1, 0 ≤ δ ≤ 0.5x) perovskite oxides - an important class of energy-related materials that contain a rare Fe(IV) oxidation state. We have initially synthesized a series of La1-x Sr x FeO3-δ compounds using a polymerized complex method. The concentration of oxygen vacancies and Fe(IV) are determined by redox titration, and the crystal structures are derived by analyzing X-ray diffraction patterns using Rietveld refinement. Significant amounts of oxygen vacancies are found in the as-synthesized compounds with x ≥ 0.8: La0.2Sr0.8FeO3-δ (δ = 0.066) and SrFeO3-δ (δ = 0.195). A subsequent ambient-pressure ozone treatment approach is able to substantially reduce the amount of oxygen vacancies in these compounds to achieve levels near the ideal oxygen stoichiometry of 3 for La0.2Sr0.8FeO3-δ (δ = 0.006) and SrFeO3-δ (δ = 0.021). The oxygenation/deoxygenation kinetics can be tuned by the cooling rate after annealing. As the oxygen vacancy concentration decreases, the structure of SrFeO3-δ evolves from orthorhombic to cubic, demonstrating that the crystal structures in metal oxides can be highly sensitive to the number of oxygen vacancies. We have further conducted a systematic evaluation of the OER activity and stability of La1-x Sr x FeO3-δ perovskites in 1 M KOH. Their initial OER activity first increases with increasing Sr content (from x = 0 to 0.8), and then decreases when the Sr content is increased to 1. Their stability, as evaluated by monitoring element leaching from the electrodes show that La does not leach at a detectable rate, but Sr and Fe leach substantially. The leaching of Sr occurs at similar rates under open circuit potential (OCP) and OER potential, suggesting a nonelectrochemical dissolution process. The leaching of Fe is, however, strongly dependent on the electrode potential. More Fe leaching is observed at the OER potential than OCP. Additionally, the electrode with higher initial OER activity leaches more Fe. These results indicate that OER facilitates the dissolution of Fe from the electrode. The leaching of Fe, in turn, is considered responsible for the activity loss of La1- x Sr x FeO3- δ during OER. This study brings new insight into the degradation mechanism of La1- x Sr x FeO3- δ and related metal oxides during electro-oxidation processes.

Dynamic Oxygen Collection Efficiency: Unraveling the Effects of Perovskites on Carbon Corrosion As Composite Catalysts for Oxygen Electrode

Introduction The composite catalyst comprising perovskite and carbon has been extensively studied as a catalyst for the oxygen evolution reaction (OER) at the oxygen electrode in alkaline media. However, the competitive corrosion of carbon under harsh OER potential necessitates additional caution when determining OER activity from the overall oxidation current. The use of a rotating ring-disk electrode (RRDE) is acknowledged for its ability to quantitatively determine the faradaic efficiency of the OER. [1] However, for practical applications, the generation of oxygen bubbles can cause unpredictable disturbance, leading to the erroneous collection efficiency. In this study, we aim to establish a dynamic current-response collection efficiency during the generation of oxygen bubbles. This was achieved by modelling with an Au/Au RRDE, well-known for its electrochemical stability under harsh conditions. In addition, using this calibrated collection efficiency, we investigate the influence of perovskite catalysts on the degradation behaviors of carbon materials at OER potentials in alkaline media. Materials and electrochemical measurements Measurements of oxygen gas collection efficiency were conducted using a modeled RRDE comprising an Au disk electrode and an Au ring electrode. The RRDE served as the working electrode, with a Pt wire as the counter electrode, a reversible hydrogen electrode (RHE) as the reference electrode, and a solution of Ar-saturated 1 mol dm-3 KOH as the electrolyte. The disk electrode underwent a 5-minute chronopotentiometry with varied oxidation currents, while the ring electrode underwent chronoamperometry at 0.3 V (vs. RHE). The collection efficiency was calculated from the ring current attributed to the oxygen 2-electron reduction reaction.Two kinds of perovskite catalysts, namely Sr2Co0.8Fe0.2O3Cl[2] (hereafter SCFOC) and LaCoO3 (hereafter LCO), and two carbonaceous materials, namely graphitized carbon black (TOKABLACK#3845, hereafter TB) and ketjen black (EC-600JD, hereafter KB), were used to evaluate the influence of perovskite on the carbon degradation behavior. Four composite catalysts, each comprising perovskite and carbon with a weight ratio of 5:1, were loaded on a disk electrode of the glassy carbon disk/Au ring RRDE. A cell analogous to the aforementioned design was assembled. Electrochemical measurements were conducted through chronoamperometry, employing various potentials at the disk electrode, and maintaining a potential of 0.3 V vs. RHE at the ring electrode, for a duration of 5 minutes. Results and discussion Figure 1 illustrates a three-dimensional representation of the oxygen gas collection efficiency of the Au/Au RRDE as two functions of disk current density and measurement time. It was evident that the oxygen gas collection efficiency varied significantly from the theoretical value (25.6 %) and was dependent on the disk current density and measurement time. The constrained solubility of oxygen in aqueous solution and the hydrophobic nature of the PTFE between the ring and the disk might cause a decrease in the collection efficiency due to the partial retention of oxygen bubbles on the electrode as the oxygen generation current increased and the measurement time elapsed. This suggests that, unlike typical solution reactions, the dynamic determination of the oxygen gas collection efficiency is essential, considering current density and measurement time.Figure 2 illustrates the computation of faradaic oxygen efficiency, utilizing dynamic oxygen collection efficiency. The composite catalyst composed of TB and SCFOC was taken as an illustrative example. The variation in collection efficiency, synchronized with the shape of disk current density, was employed in determining the faradaic oxygen efficiency. This dynamic collection efficiency yielded a comparatively smooth faradaic oxygen efficiency.Utilizing the calibrated faradaic oxygen efficiency, we proceeded to assess the capacity of carbon degradation by integrating the current density of the carbon oxidation reaction (COR) calculated by subtracting the OER current from the overall current. As depicted in Figure 3, the COR capacity as a function of the potential suggested that the components of the composite catalysts had distinct impacts on carbon degradation. Specifically, SCFOC appears to enhance degradation, whereas LCO exhibits a comparatively less pronounced effect.A more comprehensive discussion of these findings will be presented at the upcoming meeting.[1] I.S. Filimonenkov et al., Electrochim. Acta 321, 134657 (2019).[2] Y. Miyahara et al., Chem. Mater. 32, 8195 (2020). Figure 1

Probing Mixed Phase Metallic Ni/Amorphous Nickel Phosphide Nanocatalysts during Oxygen Evolution Reaction Using Operando X-Ray Absorption Spectroscopy

Metal phosphide-containing nanomaterials have demonstrated remarkable efficacy as non-precious metal-based catalysts for the alkaline oxygen evolution reaction (OER). While it is widely acknowledged that various OER catalysts undergo structural reconstruction under applied OER potentials, monitoring these changes is challenging because they are often rapid and require instrumentation not typically found in the typical lab setting. In this study, we utilize operando X-ray absorption spectroscopy (XAS) to determine the structural reconstruction of nickel-based metal phosphide catalysts during OER. We have developed the synthesis of nickel-based phosphide nanoparticles with a range of phosphorus incorporation. These nanoparticles are composed of metallic nickel and amorphous nickel phosphide phases as determined by XRD, TEM and XAS. At low levels of P incorporation, from 2% to 15%, the synthesis results in the mixture of metallic nickel and amorphous phosphide phases. At higher levels of phosphorus incorporation, from 20% to 30%, amorphous nickel phosphide is the predominant phase. Electrochemical characterization of this nickel phosphide series for OER revealed that the catalysts with 20% phosphorus incorporation exhibited superior OER activity. The phosphides with lower phosphorus content, such as 2%, had smaller Ni2+/Ni3+ redox peaks which has been shown to suppress the conversion to Ni(OH)2, leading to lowered OER activity. Increased amorphous nickel phosphide phase led to greater activity, suggesting that the active component in the catalyst comes from the amorphous nickel phosphide phase. Meanwhile, higher phosphorus content (20-30%) where the amorphous phosphide is the predominate phase resulted in larger redox peaks, signifying greater conversion to Ni(OH)2. Operando XAS will be applied to investigate these metallic nickel/nickel phosphide nanocatalysts to elucidate their structural reconstruction during OER

A Literature Survey of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) are a new class of oxygen evolution reaction (OER) electrocatalysts, which have become a relevant topic for electrocatalytic energy conversion and storage research. This is because of their earth abundance, high electrical conductivity, and resultant high OER performance. Furthermore, some X-ide electrocatalysts demonstrate various degrees of oxidation resistance under OER potentials due to their differences in chemical composition, crystal structure, and morphology. Interestingly, there are three possible post-OER electrocatalyst models: a fully oxidized oxide/(oxy)hydroxide material, a partially oxidized core@shell structure, and an unoxidized material.1 In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Recently, several topical reviews have been published. However, typically these literature reviews have provided limited conclusions targeted at only certain X-ides. Additionally, certain reviews omit key points, especially regarding the “catalytically active species” in X-ide OER electrocatalysts.In this work, we provide a comprehensive summary of (i) the experimental parameters (e.g., substrates, electrocatalyst loading amounts, electrocatalyst shapes, geometric overpotentials, Tafel slopes, etc.) and (ii) the details of the electrochemical stability tests and following post analyses used in all the X-ide OER electrocatalyst publications from 2013 (the birth of this research area) to 2022.2 In particular, based on the reported post-OER analytical results, we attempt to exhaustively classify all the X-ide electrocatalysts into four groups: no oxidation, partial oxidation, full oxidation, and unknown, which is relevant to the nature of the “catalytically active species” in X-ide OER electrocatalysts. After investigating the simpler X-ide systems, transition metal-based polyanion compounds/composites are also surveyed. Finally, we introduce the reported novel and special analytical techniques employed for observing X-ide reconstruction (i.e., oxidation and other phase transformations) during OER testing, which can be useful in selecting the analytical technique of maximum utility for the researchers’ purpose. Additionally, for each material sub-group (e.g., borides, carbides, nitrides, phosphides, sulfides, selenides, and tellurides), we also point out further challenges to be addressed and propose further questions yet to be answered, which may provide clues pointing to future research tasks. References B. R. Wygant, K. Kawashima, and C. B. Mullins, ACS Energy Lett., 3, 2956–2966 (2018).K. Kawashima et al., Chem. Rev., 123, 12795–13208 (2023).

Optimal Design of a Binder-Free Manganese/Cobalt Bilayer Bifunctional Catalyst for Rechargeable Zinc–Air Batteries

We have developed a bilayer film comprising cobalt oxyhydroxide (CoOOH) underlayer and manganese dioxide (MnO2) upper layer, which are active toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), respectively. The bilayer bifunctional catalyst is synthesized by electrodepositing cobalt hydroxide (Co(OH)2) on a porous carbon paper (CP) and subsequently immersing the obtained Co(OH)2/CP in a potassium permanganate (KMnO4) solution without binders or conductive additives. Specifically, electron transfer between the already-deposited Co(OH)2 and MnO4 – proceeded in the solution, yielding MnO2, until all the Co ions become trivalent, after which self-terminates. The proposed method only allows for the construction of the minimum required bifunctional catalyst only at the reaction site of the gas-diffusion electrode, i.e., at the so-called three-phase interface, thus remarkably increasing catalyst utilization while improving reactant and product diffusions. The developed catalyst shows stable MnO2/CoOOH cycles at |20| mA cm–2 with a minimal difference (0.764 V) between the OER and ORR potentials, reflecting the structural advantage of the proposed catalyst. This work proposes efficient bifunctional catalysts having spatially separated OER/ORR reactive sites that can be synthesized via the simple and scalable electrochemical method, which does not require the skill and optimization of binder and electron-conducting additives.

Perovskite nanostructure anchored on reduced graphene (rGO) nanosheets as an efficient electrocatalyst for oxygen evolution reaction (OER)

Energy is the basic need of this modern era and water splitting is the best known renewable source of energy. Designing an effective, high performing and durable electrocatalyst has become a serious endeavour to enhance water-splitting efficiency. For this purpose, a hydrothermal method was used to produce a non-toxic, environmentally friendly and cost-effective rGO/ZnSnO3, a composite material to improve water oxidation. For this, various analytical approaches were used to investigate the structural, textural, compositional, thermal and morphological features of the reported materials. The electrochemical properties of the rGO/ZnSnO3 nanohybrid was also analyzed by a three-electrode setup in a 1 M potassium hydroxide (KOH) and the resulting nanohybrid exhibited exceptionally small overpotential (212 mV) at an ideal current density (j) of 10 mA/cm2. The larger electrochemical surface area (ECSA) value 506.25 cm2, minimum charge transfer resistance (Rct) 0.18 Ω and remarkable stability around 20 h revealed that the produced composite material exhibited excellent potential for OER. Further examination revealed a significantly low Tafel value (37 mV/dec), suggesting that the rGO/ZnSnO3 nanohybrid possesses improved electrocatalytic efficiency and fast reaction kinetics. The nanohybrid mentioned above (rGO/ZnSnO3) shows significant potential for electrolysis of water and other electrochemical processes attributed to its considerable surface area, various active sites, exceptional stability, rapid electron mobility, low resistivity and favourable electrical conductivity.