Oxygen Evolution Process Research Articles

Solution-processed Sb2O3: A promising electrode material for high-performance supercapacitors and electrocatalysts

Easy accessibility, wide range of redox states, affordable cost, and simple synthesis methods of metal oxides have gained a significant interest in the domains of electrochemical supercapacitors and electrocatalysts. The successive ionic layer adsorption and reaction (SILAR) chemical synthesis approach has been introduced to preparing nanocrystalline antimony oxide (Sb2O3) for both energy storage and water splitting applications. The Sb2O3 electrode demonstrates a remarkable specific capacitance of 3011 F/g at a current density of 2 A/g in a 1 M solution of potassium hydroxide (KOH). The current design of the Sb2O3//Bi2O3 (bismuth oxide) asymmetric electrochemical supercapacitor device has endowed a specific capacitance of 391 F/g, a maximum energy density of 78.2 Wh/Kg at a power density of 1000 W/Kg, and an outstanding capacitance retention of 77 % even after 5000 redox cycles. The Sb2O3 electrode demonstrates outstanding performance in the oxygen evolution process at a current density of 10 mA cm2, with an overpotential of 206 mV. The Sb2O3 as an electrocatalyst also exhibits exceptional results in the hydrogen evolution reaction with an overpotential of 124 mV/dec. Both of these electrochemical processes are distinguished by the evolution of oxygen which is capable of facilitating the process of water splitting to obtain clean energy for day by day growing population.

Pioneering molecularly-level Fe sites immobilized on graphene quantum dots as a key activity descriptor in achieving highly efficient oxygen evolution reaction

The emergence of efficient electrocatalysts to facilitate the thermodynamically demanding and kinetically challenging oxygen evolution process (OER), which includes the coordinated transfer of four protons and four electrons, remains an issue. The effective transfer of energy encourages molecular-level control over key redox transitions involving small-molecule substrates (O2, and H2O), at or near electrode surfaces. While many molecular catalysts have been shown highly active for OER, immobilizing those onto the heterogenous solid matrices is challenging. Herein, graphene quantum dots (GQDs), a carbonaceous sub-class, was used as the solid matrices to conjugate the molecular catalyst onto its surface owing to its high surface area and electronic conductivity. Through the coordinated environment of the conjugated material, the molecular Fe sites on the GQDs surface exhibited outstanding OER features, achieving an ultra-low overpotential of 223 mV and 241 mV at a current density of 50 and 100 and mA/cm2, respectively, and excellent stability over 24 h. Our results marked a pioneering step in that through conjugation, these molecular sites were well dispersed at the GQDs matrices and were intrinsically active for OER.

Solution blow spun Co3O4 nanofibers as electrocatalyst for oxygen evolution processes

In this study, Co3O4 nanofibers were produced using the solution blow spinning (SBS) method. The investigation focused on studying their structure, morphology, and electrocatalytic properties in relation to the oxygen evolution reaction (OER) in alkaline solution (KOH). Calcination at 600 °C resulted in Co3O4 fibers without the presence of secondary phases (NF600), while calcined fibers at 400 °C showed a Co3O4–CoO composite phase (NF400), confirmed by XRD. The size of the crystallites, determined through Rietveld refinement analysis of XRD patterns, ranged from 42.6 nm to 56.2 nm for the calcined fibers at 400 °C and 600 °C, respectively, confirming the nanometric scale of the cobalt oxide fibers. Morphological analyses confirmed the production of rough nanofibers with monomodal distribution, with mean diameter of 385 nm for NF600 and 485 nm for NF400. The EDS confirms the presence of the chemical elements of cobalt and oxygen, estimating their abundance in the sample. The XPS analysis showed that the concentration of oxygen vacancies and the presence of species with higher oxidation states (Co3+) were beneficial to improve OER. Nanofibers calcined at 600 °C showed an overpotential of 330 mV at 10 mA cm−2, more efficient Tafel kinetics and low resistance to charge transfer (0.37 Ω) when compared to nanofibers calcined at 400 °C (338 mV). This result demonstrates the competitive catalytic activity of cobalt oxide nanofibers produced by SBS compared to other systems mentioned in the literature. Therefore, this work offers a new perspective on the development of electrocatalysts based on the structure and properties of cobalt oxide electrodes by the SBS method.

Nanoneedles like FeP engineered on Ni-Foam as an effective catalyst towards overall alkaline freshwater, urea, and seawater splitting

The slow kinetics of electrochemical urea and water oxidation processes at electrode surfaces, which can be recognised as promising resources for pollution-free hydrogen energy production, motivate the scientific community to design and fabricate low-cost, high-efficiency electrocatalysts. Because the performance of electrocatalysts is dependent on their structural, morphological, and electronic properties, herein, the tuning of these properties of Fe3O4@Ni-Foam via phosphorylation and sulfurization, yielding iron phosphide (FeP@Ni-Foam) and iron sulphide (FeS@Ni-Foam), with nanoneedles (along with microstructures) and nanoflower-like morphologies, respectively, is investigated. Among all the prepared samples, FeP is found to be the most effective electrocatalyst for the oxygen evolution process (OER), the urea oxidation reaction (UOR), and seawater electrolysis. The overpotentials observed for OER, UOR, and seawater splitting are significantly reduced when using FeP as compared to other materials, with values of 207 mV, 133 mV, and 287.1 mV, respectively, at a current density of 10 mA/cm2. The enhanced catalytic activity of FeP over FeS and Fe3O4 is attributed to morphological changes, improved electronic conductivity, and exceptional endurance. The theoretical studies reveal that FeP has a better density of states over the fermi level than FeS and Fe3O4, lowering the energy barriers for the OER and UOR processes and demonstrating significant catalytic activity towards these processes. This work demonstrates that phosphorylation and sulfurization treatments can alter morphologies and electrical characteristics, hence improving the catalytic activity of the materials.

Perovskite-type samarium iron oxide adorned on reduced graphene oxide composite: A high-performance electrocatalyst for OER

The oxygen evolution process (OER), an essential and necessary reaction in water splitting to produce clean hydrogen fuel were investigated using a number of electrocatalysts. Non-noble metal catalysts are increasingly being used in place of noble metal catalysts because of their unique qualities, which include affordability, environmental friendliness and electrocatalytic activity. In this work, SmFeO3/rGO composite is prepared for an anion exchange reaction, which is used as catalyst on nickel foam in 1.00 M KOH electrolyte for OER. The fabricated electrocatalysts were confirmed by X-ray powder diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray (EDX) spectroscopy and scanning electron microscopy (SEM). The BET determined surface area of SmFeO3/rGO composite is 51 cm2/g which is better for OER activity. The distinct structure of the nanocomposite displays increased surface area, increasing active sites with plentiful potential for charge transfer and prolonging the material's life. The electrochemical results represented that composite had lower overpotential (180 mV) with Tafel slope (34 mV/dec), displaying higher charge transfer during OER showing efficient electrocatalytic activity and high reliability of 55h. The reasons for lower overpotential and lower Tafel slope values were increased surface area and enhanced number of active spots in composite and also due to presence of non-kinetic effects that alter the Tafel slope values. The electrocatalyst may be able to facilitate the oxidation reaction based on all of these qualities. As a consequence, created composite reacts with outstanding efficacy for the OER process and energy conversion applications.

Engineering active sites on binary metal selenide heterointerface catalyst to boost urea electrooxidation

Electrocatalytic urea oxidation reaction (UOR) with a low thermodynamic potential is a perfect substitute for anodic oxygen evolution process (OER) in the effective generation of hydrogen. However, because of the slow kinetics of UOR, its potential application for commercial use remains untapped. Nickel-based materials may be an option for urea oxidation reaction but the heavily filled d orbital prevents enhanced adsorption and activity. Here, taking into account the adsorption-energy scaling limitations, Co3+ with partially filled d orbital is introduced into nickel selenide to form a heterointerface catalyst (2D CoSe2/Ni0.85Se) that enhances UOR. As anticipated, the 2D CoSe2/Ni0.85Se electrode displays a low potential of 1.33 V to achieve 100 mA cm−2 for UOR while retaining strong durability for 300 h. Further, 2D CoSe2/Ni0.85Se catalyst is used as an anode in an anion exchange membrane flow electrolyzer, which achieved an industrial-level voltage of 1.91 V at 1 A cm−2 and robust durability.

Fast and In-Depth Reconstruction of Two-Dimension Cobalt-Based Zeolitic Imidazolate Framework in Glucose Oxidation Processes.

Currently, metal-organic frameworks (MOFs) have emerged as viable candidates for enduring electrode materials in nonenzyme glucose sensing. However, given the inherent water susceptibility of MOFs and their complete self-reconstruction during the process of electrochemical oxygen evolution in alkaline conditions, we are motivated to explore the truth of MOFs catalyzing glucose oxidation. In this work, we fabricated a two-dimensional cobalt-based zeolitic imidazolate framework (ZIF-L) as the electrode material for catalyzing glucose oxidation in alkaline conditions. Our explorations revealed that while the initial glucose catalytic response varied among ZIF-L samples with differing thicknesses, the ultimate steady-state catalytic performance remained largely consistent. This phenomenon arose from the transformation of ZIF-L with distinct thicknesses into CoOOH with uniform morphological and structural characteristics during the glucose catalysis process. And in situ Raman spectroscopy elucidated the sustained equilibrium within the glucose catalytic system, wherein the dynamic interconversion between CoOOH and Co(OH)2 governs the overall process. This study contributes to an enhanced understanding of the glucose catalytic mechanism aspects of nonenzymatic glucose sensor electrode materials, offering insights that serve as inspiration for the development of advanced glucose electrode materials.

Microzone-Acidification-Driven Degradation Mechanism of the NiFe-Based Anode in Seawater Electrolysis.

The anode stability is critical for efficient and reliable seawater electrolyzers. Herein, a NiFe-based film catalyst was prepared by anodic oxidation to serve as a model electrode, which exhibited a satisfactory oxygen evolution performance in simulated alkaline seawater (1 M KOH + 0.5 M NaCl) with an overpotential of 348 mV at 100 mA cm-2 and a long-term stability of over 100 h. After that, the effects of the current density and bulk pH of the electrolyte on its stability were evaluated. It was found that the electrode stability was sensitive to electrolysis conditions, failing at 20 mA cm-2 in 0.1 M KOH + 0.5 M NaCl but over 500 mA cm-2 in 0.5 M KOH + 0.5 M NaCl. The electrode dissolved, and some precipitates immediately formed at the region very close to the electrode surface during the electrolysis. This can be ascribed to the pH difference between the electrode/electrolyte interface and the bulk electrolyte under anodic polarization. In other words, the microzone acidification accelerates the corrosion of the electrode by Cl-, thus affecting the electrode stability. The operational performances of the electrode under different electrolysis conditions were classified to further analyze the degradation behavior, which resulted in three regions corresponding to the stable oxygen evolution, violent dissolution-precipitation, and complete passivation processes, respectively. Thereby increasing the bulk pH could alleviate the microzone acidification and improve the stability of the anode at high current densities. Overall, this study provides new insights into understanding the degradation mechanism of NiFe-based catalysts and offers electrolyte engineering strategies for the application of anodes.

A short review on graphene derivatives towards photoelectrochemical water splitting

Graphene oxide is vital in photoelectrochemical (PEC) water splitting, serving as an essential photoanode material. Its semiconducting nature allows for the generation of photocurrents, promoting water oxidation at the anode and contributing to hydrogen production efficiency. Additionally, graphene is a two-dimensional carbon allotrope that has quickly emerged as a highly promising material in PEC water splitting, potentially transforming renewable energy and sustainable hydrogen generation. Graphene improves PEC water-splitting efficiency by facilitating efficient charge transport, rapid electron transfer, and effective redox reactions at the electrode-electrolyte interface. It possesses high electrical conductivity, a large specific surface area, and excellent charge carrier mobility. Its unique band structure enables efficient light absorption across a broad spectrum, including visible light, resulting in better light-to-electricity conversion. Furthermore, the inherent catalytic activity of graphene speeds up the oxygen evolution process (OER), increasing water oxidation and aiding hydrogen gas production.

Enhanced oxygen evolution and urea oxidation reaction using a nanosheet-structured NiO@P-doped carbon composite as an anode catalyst

Study of the oxygen evolution and urea oxidation process for producing hydrogen using alkaline electrolytes of NiO@p-doped carbon electrocatalyst.

Progress in MXene‐based catalysts for oxygen evolution reaction

AbstractElectrochemical water splitting for hydrogen generation is considered one of the most promising strategies for reducing the use of fossil fuels and storing renewable electricity in hydrogen fuel. However, the anodic oxygen evolution process remains a bottleneck due to the remarkably high overpotential of about 300 mV to achieve a current density of 10 mA cm−2. The key to solving this dilemma is the development of highly efficient catalysts with minimized overpotential, long‐term stability, and low cost. As a new 2D material, MXene has emerged as an intriguing material for future energy conversion technology due to its benefits, including superior conductivity, excellent hydrophilic properties, high surface area, versatile chemical composition, and ease of processing, which make it a potential constituent of the oxygen evolution catalyst layer. This review aims to summarize and discuss the recent development of oxygen evolution catalysts using MXene as a component, emphasizing the synthesis and synergistic effect of MXene‐based composite catalysts. Based on the discussions summarized in this review, we also provide future research directions regarding electronic interaction, stability, and structural evolution of MXene‐based oxygen evolution catalysts. We believe that a broader and deeper research in this area could accelerate the discovery of efficient catalysts for electrochemical oxygen evolution.

Fabrication of zirconium ditelluride/graphene oxide nanocomposite array for oxygen evolution and hydrogen evolution reactions

Future commercial energy conversion and storage requisitions may involve using two-dimensional (2D) materials that retain their structure and are feasible for water electrolysis. In this study, we advance in tuning the electronic arrangement of 2D materials like ZrTe2/graphene oxide for electrocatalytic energy conversions such as oxygen evolution reaction (OER) as well as hydrogen evolution reaction (HER). The fabricated materials characterize via X-rays diffraction analysis (XRD), Scanning electron microscopy (SEM), and Brunauer Emmett Teller (BET) adsorption-desorption isotherm to confirm their structure, shape, size and porosity. Subsequently, the fabricated nanocomposite employs for the electrochemical characterization, and the resultant composite shows a meager overpotential of 208 mV with a minimal Tafel slope of 31 mV/dec at a current density of the 10 mA/cm2 for the oxygen evolution process and represents the overpotential of 242 mV@-10 mA/cm2 for hydrogen evolution process. The fabricated ZrTe2/GO nanocomposite also shows the high stability of 40 h compared to the individual materials. This strategy provides a novel approach to designing tellurium-based nanomaterials for various applications.

(Invited) Proton Surface Exchange Kinetics and Stability of Thin-Film Triple Conducting Oxides for Protonic Ceramic Electrolysis Cells

Protonic ceramic electrolysis cells (PCECs) have been attracting attention due to their ability to operate in the intermediate temperature (300 - 500 ˚C) sweet-spot. These temperatures enable fast kinetics, inexpensive materials (no precious metals; stainless interconnects), and lower operating voltage for producing green hydrogen, without the accelerated degradation that may be observed at higher temperatures. The steam electrodes often support triple conduction and thereby host the proton exchange / oxygen evolution process at the surface, where steam is split. Steam electrode materials development is at a relatively early stage, with open questions around the relationships among bulk composition, evolving surface chemistry, kinetic quantities, and degradation mechanisms. We are investigating the proton exchange surface coefficient (k) and its evolution over time on model thin film electrodes in order to ultimately develop highly active and long-term stable steam electrodes for PCECs.ABO3 perovskites with Ba on the A-site and nominally multivalent ions on the B-site are candidates for steam electrodes given the varied compositions in this class that can transport oxide ions, protons, and electronic species, and the lowering of hydration enthalpies by the basicity introduced by Ba. However, very few measurements of k exist for these materials, and the evolution of surface chemistry and thus k is poorly understood.In this work, we fabricated and compared two perovskite triple conducting oxides by growing geometrically well-defined films of Ba(Pr,Y)O3-x (BPY) and Ba(Co,Fe,Zr,Y)O3-x (BCFZY) with pulsed laser deposition. We examined the proton surface exchange coefficients (k-values) using simultaneous in-situ optical transmission relaxation and electrical conductivity relaxation measurements upon switching pH2O at a temperature of 400 ˚C, and continued these switches as a function of time to assess stability. To our knowledge, this is the first use of an optical transmission method to probe elevated temperature steam splitting kinetics, and we interpret the optical (and electrical) changes as indicative of redox during the hydrogenation process. The k-values of BPY thin films were approximately ten times higher than those of the BCFZY thin films, and BPY also retained higher k-values over time. The bulk and surface chemical compositions of BPY and BCFZY thin films before and after relaxation measurements were compared using Rutherford backscattering spectrometry, angle-resolved X-ray photoelectron spectroscopy, and scanning transmission electron microscopy with spectroscopy. Pronounced Ba enrichment and Si contamination (from the quartz tube) were evident at the surface of BPY by both S/TEM-EDS and XPS, whereas for BCFZY these surface chemical features were only detectable by XPS and absent in S/TEM-EDS. Implications for materials design of steam electrodes will be discussed, considering efficiency and lifetime.

Electrochemical Scanning Probe Microscopy Evaluation of Li-Ion Battery Cathode Degradation through in Situ Oxygen Evolution Measurement and Interfacial Property Characterization

Degradation processes involving oxygen evolution significantly limit the performance, reversibility, and capacity of high energy density Li-ion battery (LiB) cathodes. In addition, oxygen evolution processes create hazardous conditions for LiB operation. Despite the importance of these processes, characterization of evolved O2 with sufficient spatial and temporal versatility is hampered by the complex setups required to perform electrochemical mass spectrometry, the gold standard for evaluating this process. Recently, we introduced a scanning electrochemical microscopy (SECM) approach that enables in-situ, real-time (i.e., few ms temporal resolution), chemically selective detection of O2.[1] Measurements were accomplished by positioning an SECM probe (i.e., an Au ultramicroelectrode) near operating cathode surfaces for detecting O2 via the oxygen reduction reaction (ORR). The superior signal-to-noise of this method enabled the observation of two peaks corresponding to a steady-state evolution at high voltage and a transient O2 evolution at low voltage on materials such as lithium cobalt oxide (LCO) and NMC electrodes.Here, we expand the capabilities of our technique by incorporating correlated measurements aimed at understanding how oxygen evolution profiles are related to surface electrochemical and mechanical properties. Specifically, we will focus on the use of feedback-mode SECM and the use of reactive mediators to evaluate the changes in the rates of electron transfer measured on materials undergoing O2 and after structural degradation. Furthermore, the use of electrochemical AFM allows us to measure the mechanical properties of these interfaces, in an attempt to tie the structural changes and the evolution of oxidizing species with the formation of cathode-electrolyte interfaces. Altogether, these multimodal SECM and AFM methods elucidate the manifold chemical, structural, and performance changes underpinning LiB cathode operation.[1] Mishra, A.; Sarbapalli, D.; Hossain, M.S.; Gossage, Z.T.; Li, Z.; Urban, A.; Rodríguez-López, J. Highly Sensitive Detection and Mapping of Incipient and Steady-State Oxygen Evolution from Operating Li-Ion Battery Cathodes via Scanning Electrochemical Microscopy. J. Electrochem. Soc., 2022, 169, 086501. Figure 1

(Keynote) Competition of Oxygen and Chlorine Evolution on Oxide Surfaces – a Soft X-Ray and Dems Approach to Identification of Active Site

Anodic gas evolving reactions, e.g. oxygen evolution and chlorine evolution, are of fundamental importance both in energy storage as well as in electrolytic production of chemicals. Both reactions are of electrocatalytic nature, proceed in the same potential range and are catalyzed by the same types of electrode materials. While the oxygen evolution is thermodynamically favored the two electron chlorine evolution is kinetically preferred, namely at acid and neutral pH. This kinetic preference is industrially explored in chlor-alkali processes as well in chlorate electrolysis. The growing interest in electrolytic hydrogen production fueled by the popularity of the hydrogen as the likely replacement of fossil fuels brings up the question of the tolerance of the industrial anodes to presence of chlorides in the water intended for electrolytic hydrogen production. Such a tolerance can be best achieved by steering the selectivity of the anode between oxygen and chlorine evolution aiming eventually at the possibility of the sea water use in hydrogen producing water electrolysis. So far existing attempts at control the anode selectivity in presence of chlorides explore two principal approaches – pH control and surface engineering. While the pH related approach is rationalized by different pH sensitivity of each of the competing reaction the surface engineering related activities so far rely on trial and error approach. Qualitatively the selectivity may be controlled by either selectively promoting/suppressing each of the contributing reactions. While the literature lists attempts when the overall selectivity towards OER is apparently achieved by selective suppression of CER [1] more recent reports suggest that the actual selectivity is in fact controlled by the ability of the catalysts to support a lattice oxygen activating mechanism of the oxygen evolution process [2]. While these assignments are based on correlation of selectivity measurements with local structure and DFT modelling and a direct evidence of such a behavior is so far missing. This paper will document here the capability of the real-time and operando soft X-ray absorption spectroscopy [3] to provide this missing evidence. Anodic behavior of two type of ruthenium oxide electrodes in chloride free and chloride containing solutions will be documented by comparison of the nature and potential dependent population of the oxygen related intermediates formed at anode surface in the potential range of oxygen and chlorine evolution in acid media (pH=1) and combined with DEMS data to identify the active sites for each contributing electrode reaction.

Decoupled hydrogen and oxygen evolution for efficient water splitting by using nickel hydride batteries

Cost-effective alkaline water splitting represents a promising and sustainable approach for hydrogen production. However, the prevention of hydrogen/oxygen mixing and the efficient use of unstable renewable energy are challenging issues. Herein, we propose a novel solution involving the integration of a nickel-metal hydride (Ni-MH) battery as a redox mediator and affordable a NiFe LDHs–NiFe alloy gradient hybrid bifunctional catalysts as gas evolution electrodes, effectively decoupling the hydrogen and oxygen evolution processes during water splitting. Then, the hydrogen evolution reaction (HER) cell using hydrogen storage alloy and catalytic electrode only requires 0.28 V and the oxygen evolution reaction (OER) cell using nickel hydroxide and catalytic electrode only requires 0.25 V to reach a high current density of 100 mA cm−2. Furthermore, a NiOOH-Zn cell is introduced as an alternative, replacing the OER reaction with Zn. This NiOOH-Zn cell can be discharged without further energy input. Moreover, such a decoupled water electrolysis system can seamlessly integrate with a Ni-MH battery, providing the flexibility to efficiently convert renewable energy sources into both hydrogen and electricity. This integrated system maximizes the utilization of renewable energy, making a significant contribution to sustainable energy production.

Electrochemical activation of CoMo LDH nanosheets driving surface reconstruction for enhanced hydrogen evolution

Surface reconstruction emerging in the electrochemical oxygen evolution process is regarded to be significantly important for the activated catalysis efficiency, however, which is rarely recognized for hydrogen evolution reaction (HER). Here, the electrochemical activation of CoMo layered double hydroxide (CoMo-LDH) nanosheet arrays is demonstrated to significantly activate the electrochemical HER property. The catalysis performance improvement originates from the surface transformation of CoMo-LDH to Co(OH)2/CoOOH, as well as the dissolution of Mo and its adsorption on the catalyst surface in K2Mo3O10 phase. Based on the combined benefits of optimized electronic structure, increased catalysis sites, and promoted charge transfer, the overpotentials at catalysis current densities of 10 and 100 mA cm−2 are as low as 113 and 202 mV. This study demonstrates the electrochemical activation strategy to drive the surface reconstruction of bimetal hydroxides for enhanced HER performance, inspiring the research interests on the reconstruction and activation enhancement behaviors of HER electrocatalysts.

Solvent-assisted phase modification of Ni[sbnd]P material to boost electrocatalytic water splitting and charge storage capacity

Due to their redox features, Fe/Ni-phosphides and phosphates could be utilized as multi-functional electrode materials for hydrogen and oxygen evolution processes (HER and OER) and supercapacitors, however, regulation in their geometry and electrical structure for further performance improvement is still challenging. Herein, we developed a solvent-assisted approach to modify the NiP material's phase, growing Ni2P, Ni12P5, and Ni(PO3)2 cross-linked nanosheets on Ni-foam using the same precursor and different solvents. When compared to Ni2P and Ni12P5, the Ni(PO3)2 sample showed outstanding activity and endurance towards HER (134 mV overpotential@10 mA/cm2 and 112 mV/dec Tafel value) and OER (342 mV overpotential@10 mA/cm2 and 78 mV/dec Tafel value), respectively. Moreover, the specific capacitance of the Ni(PO3)2 electrode was found to be 4366 (at 10 mV/s) and 7582 mF/cm2 (at 1 mA/cm2). Theoretical calculations showed that the density of states and its distribution across the Fermi level increased in the order of Ni2P < Ni12P5 < Ni(PO3)2 which improved their conductivity and electronic properties, benefiting electrocatalytic HER and OER activity and the charge-storing capacity of Ni(PO3)2.

Water Flow-Driven Coupling Process of Anodic Oxygen Evolution and Cathodic Oxygen Activation for Water Decontamination and Prevention of Chlorinated Byproducts.

Electrochemical advanced oxidation process (EAOP) is a promising technology for decentralized water decontamination but is subject to parasitic anodic oxygen evolution and formation of toxic chlorinated byproducts in the presence of Cl-. To address this issue, we developed a novel electrolytic process by water flow-driven coupling of anodic oxygen evolution reaction (OER) and cathodic molecular oxygen activation (MOA). When water flows from anode to cathode, O2 produced from OER is carried by water through convection, followed by being activated by atomic hydrogen (H*) on Pd cathode to produce •OH. The water flow-driven OER/MOA process enables the anode to be polarized at low potential (1.7 V vs SHE) that is lower than that of conventional EAOP whose •OH is produced from direct water oxidation (>2.3 V vs SHE). At a flow rate of 30 mL min-1, the process could achieve 94.8% removal of 2,4-dichlorophenol (2,4-DCP) and 71.5% removal of chemical oxygen demand (COD) within 45 min at an anode potential of 1.7 V vs SHE and cathode potential of -0.5 V vs SHE. To achieve the comparable 2,4-DCP removal performance, 4.3-fold higher energy consumption was needed for the conventional EAOP with titanium suboxide anode (anode potential of 2.9 V vs SHE), but current efficiency declined by 3.5 folds. Unlike conventional EAOP, chlorate and perchlorate were not detected in the OER/MOA process, because low anode potential <2.0 V vs SHE was thermodynamically unfavorable for the formation of chlorinated byproducts by anodic oxidation, indicated by theoretical calculations and experimental data. This study provides a proof-in-concept demonstration of water flow-driven OER/MOA process, representing a paradigm shift of electrochemical technology for water decontamination and prevention of chlorinated byproducts, making electrochemical water decontamination more efficient, more economic, and more sustainable.

Switching the Oxygen Evolution Mechanism on Atomically Dispersed Ru for Enhanced Acidic Reaction Kinetics.

Designing stable single-atom electrocatalysts with lower energy barriers is urgent for the acidic oxygen evolution reaction. In particular, the atomic catalysts are highly dependent on the kinetically sluggish acid-base mechanism, limiting the reaction paths of intermediates. Herein, we successfully manipulate the steric localization of Ru single atoms at the Co3O4 surface to improve acidic oxygen evolution by precise control of the anchor sites. The delicate structure design can switch the reaction mechanism from the lattice oxygen mechanism (LOM) to the optimized adsorbate evolution mechanism (AEM). In particular, Ru atoms embedded into cation vacancies reveal an optimized mechanism that activates the proton donor-acceptor function (PDAM), demonstrating a new single-atom catalytic pathway to circumvent the classic scaling relationship. Steric interactions with intermediates at the anchored Ru-O-Co interface played a primary role in optimizing the intermediates' conformation and reducing the energy barrier. As a comparison, Ru atoms confined to the surface sites exhibit a lattice oxygen mechanism for the oxygen evolution process. As a result, the delicate atom control of the spatial position presents a 100-fold increase in mass activity from 36.96 A gRu(ads)-1 to 4012.11 A gRu(anc)-1 at 1.50 V. These findings offer new insights into the precise control of single-atom catalytic behavior.