Low-cost Electrocatalysts For Oxygen Evolution Reaction Research Articles

Recent strategies for improving the catalytic activity of ultrathin transition metal sulfide nanosheets toward the oxygen evolution reaction

Hydrogen is considered as one of the clean secondary energy sources required in industry and daily life. Electrocatalytic hydrogen production is believed to be the most reliable method for converting renewable energy into chemical energy. Two-dimensional (2D) transition metal disulfides (TMDs), as an emerging electrocatalysts, have been widely investigated for oxygen evolution reaction (OER) in water splitting. Herein, this review summarizes the recent progress and the strategies of ultrathin TMDs for improving the OER electrocatalytic activities. The basic catalytic mechanism of OER is first introduced, and then the catalytic characteristics of 2D TMDs are presented. Importantly, this review specifically concentrates on strategies to improve catalytic activity such as phase engineering, defect/vacancy engineering, heterostructure, doping effect, and catalyst support material. It can be concluded that these regulatory strategies can largely enhance the electrocatalytic performance by increasing the active site exposure and improving the electrocatalytic kinetics. In addition, the challenges and perspectives of 2D TMDs catalysts are presented, which offers valuable guidance in designing efficient and low-cost OER electrocatalysts for future energy applications. We hope that this review will inspire future studies and bridge the gap between strategy design and practical application.

Low-Voltage Hydrogen Production via Hydrogen Peroxide Oxidation Facilitated by Oxo Ligand Axially Coordinated to Cobalt in Phthalocyanine Moiety

The global energy sector is shifting from fossil-based systems of energy production and consumption to renewable-based ones like wind and solar energy. Hydrogen has emerged as one of the energy carriers for the renewable-based economy, storing electrical energy generated from the renewables and being used as a fuel to generate power. Water electrolysis, powered by renewable energies, is one of the most feasible green hydrogen production technologies. At least 1.23 V and typically > 2 V is required between hydrogen and oxygen evolution reactions (HER and OER) for splitting water. OER is notorious for its slow kinetics so that the OER determines hydrogen production rate. Therefore, it has been emphasized to develop high-performance and low-cost OER electrocatalysts for reducing overpotentials required for electrolyzing water. Even if up-to-date upgraded electrocatalysts have decreased the kinetic overpotentials, it is clearly evident that it is impossible to split water molecules at the cell voltage less than 1.23 V. To decrease the potential required for water electrolysis, water oxidation reaction to drive OER was suggested to be replaced by oxidation of organic molecules or nitrogen hydrides such as formic acid and hydrazine. However, even if the more negative E o of the organic molecules allowed the open-circuit potential difference between cathodic and anodic processes to be reduced, it sounds self-contradictory to employ carbon-based organic molecules and therefore produce carbon dioxide for producing green hydrogen.Suggesting hydrogen peroxide as an electrolysis medium for hydrogen production and therefore as a hydrogen carrier, in this work, we present a cobalt phthalocyanine having electron-poor CoN4 (+δ) in its phthalocyanine moiety as an electrocatalyst for hydrogen peroxide oxidation reaction (HPOR), demonstrating that the electrocatalyst guaranteed high hydrogen production rate by hydrogen peroxide splitting. The electron deficiency of cobalt allowed CoN4 to have the highly HPOR-active monovalent oxidation state and facilitate HPOR at small overpotentials range around the onset potential. The strong interaction between the electron-deficient cobalt and oxygen of peroxide adsorbates in Co-OOH- encouraged an axially coordinated cobalt oxo complex (O=CoN4) to form, the O=CoN4 facilitating the HPOR efficiently at high overpotentials. Low-voltage oxygen evolution reaction (OER) guaranteeing low-voltage hydrogen production was successfully demonstrated in the presence of the metal-oxo complex having electron-deficient CoN4. Hydrogen production by 391 mA cm-2 at 1 V and 870 mA cm-2 at 1.5 V was obtained.Also, we evaluated the techno-economic benefit of hydrogen peroxide as a hydrogen carrier by comparing hydrogen peroxide with other hydrogen carriers. Here, even though water (H2O) can be considered as a carrier, about 61 kWh/kg of energy is required for converting transported H2O to H2 again. Thus, in this case, if renewable energy can be supplied from the demand site, H2 can be produced directly on-site from the demand site without having to transport the H2 produced from another renewable energy complex. If renewable power cannot be supplied from the demand site, all the required energy for water electrolysis should be supplied with existing fossil fuel-based energy, it is like losing the eco-friendly significance of H2 energy, and considering energy consumption, using other carriers will be more economical and environmental. In this situation, H2O2 can act as a hydrogen carrier as a hydrogen/oxygen simultaneous receptor. When generated hydrogen is stored and transported, high cost arises. To lower the cost, many countries are paying attention to LOHC (=liquid organic hydrogen carrier) and ammonia which are materials to storage hydrogen energy. For LOHC, methylcyclohexane 6.1 wt.% and 47 kg H2/m3, dibenzyltoluene 6.2 wt.% and 57 kg H2/m3, ethylcarbazole 5.8 wt. % and 57 kg H2/m3, with a hydrogen storage capacity of 47 - 57 kg H2/m3. For ammonia, it has a hydrogen storage capacity of 17.6 wt. %, and 108 kg H2/m3. In the case of H2O2, it has 5.9 wt.% and 85.3 kg H2/m3, which belongs to the high hydrogen storage capacity compared to other hydrogen storage materials. To evaluate the economic analysis, we conducted to identify that H2O2 can be utilized for green H2 supply by comparing diverse hydrogen storage and transportation pathways. Furthermore, the scenario analysis was performed for CGH2, LH2, NH3, toluene-based LOHC, and H2O2. Figure 1

Construction of Flower-like FeCo2O4 Nanosheets on Ni Foam as Efficient Electrocatalyst for Oxygen Evolution Reaction

Developing efficient transition metal oxide electrodes is essential to energy conversion and storage. In this work, flower-Like FeCo2O4 nanosheets supported on Ni foam were synthesized by facile hydrothermal and calcination treatment. Various temperatures influence the morphologies and oxygen evolution reaction activities. Especially, FeCo2O4/NF-120 °C catalysts showed the best oxygen evolution reaction (OER) activity due to the fact that 3D Ni foam provided good conductive substrate-forming FeCo2O4 nanosheets, which enhanced the electrochemical stability and facilitated the transport of electrolyte and release of oxygen. In addition, the synergistic effect between Fe and Co also enhanced active sites and promoted the OER catalytic performance. The flower-like FeCo2O4/Ni electrodes showed a low overpotential of 124 and 339 mV at the current density of 10 and 50 mA cm−2 for OER, respectively. Also, they displayed a low tafel slope of 43.78 mV dec−1 and good stability in alkaline electrolyte. This research could promote the design of low-cost electrocatalysts for OER.

Hierarchical Ni/Co-hydroxides on NiCo2O4 for boosting the electrocatalytic activity of the oxygen evolution reaction

Herein, hierarchical Ni/Co-layered double hydroxide decorated NiCo2O4 electrocatalyst (NiCo-LDH@NCO) on nickel foam (NF) was synthesized using a facile hydrothermal method with electrochemical reconstruction. By converting the already existing NCO surface to hierarchical NiCo-LDH, a larger surface area and high electrical conductivity are obtained. Both structure and morphology of fabricated electrocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM) techniques. The electrochemical reconstruction process is accompanied by changes in the morphology of the NCO that significantly increase accessibility to the electrolyte and active surface area. Therefore, the as-obtained electrocatalyst exhibits excellent oxygen evolution reaction (OER) activity and long-term stability for 35 h continuous reaction, superior-performing the reported other OER catalysts. In addition, this work opens a new path for the development of highly efficient and innovative low-cost OER electrocatalysts able to replace Pt-based metals for energy conversion devices.

Facile synthesis of Co-Ni layered double hydroxides nanosheets wrapped on a prism-like metal-organic framework for efficient oxygen evolution reaction

The construction of low-cost oxygen evolution reaction (OER) electrocatalysts with high activity and good durability is a considerable challenge for facilitating the efficient utilization of green energy. Herein, the prism-like materials of institute lavoisier frameworks-88 (MIL-88) was first synthesized by a hydrothermal method. Then, Co-Ni layered double hydroxides (CoNi-LDHs) nanosheets were directly wrapped on the MIL-88 surface by electrodeposition to form core-shell MIL-88@CoNi-LDHs composites. Due to the distinct structure and synergistic effect between the MIL-88 core and CoNi-LDHs shell, it was found that MIL-88@CoNi-LDHs had outstanding OER activity with a small Tafel slope (45.55 mV dec-1), low overpotential (314 mV) at 10 mA cm−2, and superior durability. This study provides a prospective pathway to exploit highly efficient low-cost electrocatalysts for OER.

CoMo layered double hydroxide equipped with carbon nanotubes for electrocatalytic oxygen evolution reaction

To carry out effective resource reforming of sustainable electricity, hydrogen production by electrochemical water splitting provides an eco-friendly and economical way. Nevertheless, the oxygen evolution reaction (OER) at the anode is limited by the slow reaction process, which hinders the large-scale development and application of electrolysis technology. In this work, we present an electrocatalyst with superior OER performance, which attributed to the abundant active sites and good electronic conductivity. The two-dimensional CoMo Layered Double Hydroxide nanosheets are synthesized and deposited on conductive carbon nanotubes (CoMo LDH/CNTs), and then hybrid composites show better catalytic performance than their undecorated counterpart under identical conditions. Specifically, CoMo LDH/CNTs exhibit the low overpotential of 268 mV to obtain 10 mA cm−2 and satisfactory stability (more than 40 h). We emphasize that this hybridization strategy with a conductive supporting framework could design more abundant and low-cost OER electrocatalysts to minimize electrical energy consumption, thereby achieving efficient conversion between energy sources.

Boosting catalyst activity with high valency metal species through Fe doping on normal spinel NiCr2O4 for superior water oxidation

Designing and tailoring valence states of transition metal has been highlighted as a potential technology to develop efficient electrocatalysts for water splitting because high valence metal sites enable to accelerate reaction kinetics for oxygen evolution reaction (OER). In this study, we designed and engineered spinel nickel–chromium oxide (NiFexCr2-xO4, 0 ≤ x ≤ 0.6) with high-valence metal sites and porous structure consisting of interconnected small nanoparticles using an in-situ Fe heteroatom doping synthesis route. The Fe-doped NiFexCr2-xO4 series (x = 0.2, 0.4, 0.6) catalysts exhibit superior OER activities compared to the pristine NiCr2O4 and benchmark IrO2 catalysts. Moreover, it is observed that for x = 0.2, the NiFexCr2-xO4 catalyst exhibited the best OER performance with a low overpotential of 187 mV at 10 mA cm−2, a high current density of 172 mA cm−2 at 0.28 V, and electrochemical long-term stability (decreasing only by ∼3 %) after a 200 h test. We further demonstrate that the remarkably improved catalytic performance of NiFexCr2-xO4 is attributed to the increased high-valence metal species (Ni3+ and Cr6+) and the highly oxidative oxygen species (O22−/O−), which are highly active to accelerate catalytic reactions, as well as the improved charge transfer kinetics and the porous structural feature by rationally doped Fe. Thus, it is believed that this research provides a novel strategy for developing efficient, robust, and low-cost OER electrocatalysts.

Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation.

In-depth insights into the structure-activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived Od-R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni4+ and Fe4+ species, which generate oxygen-bridged NiIV-O-FeIV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free Od-R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-NiIV-O-FeIV-O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.

3D-printed NiFe-layered double hydroxide pyramid electrodes for enhanced electrocatalytic oxygen evolution reaction

Electrochemical water splitting has been considered one of the most promising methods of hydrogen production, which does not cause environmental pollution or greenhouse gas emissions. Oxygen evolution reaction (OER) is a significant step for highly efficient water splitting because OER involves the four electron transfer, overcoming the associated energy barrier that demands a potential greater than that required by hydrogen evolution reaction. Therefore, an OER electrocatalyst with large surface area and high conductivity is needed to increase the OER activity. In this work, we demonstrated an effective strategy to produce a highly active three-dimensional (3D)-printed NiFe-layered double hydroxide (LDH) pyramid electrode for OER using a three-step method, which involves direct-ink-writing of a graphene pyramid array and electrodeposition of a copper conducive layer and NiFe-LDH electrocatalyst layer on printed pyramids. The 3D pyramid structures with NiFe-LDH electrocatalyst layers increased the surface area and the active sites of the electrode and improved the OER activity. The overpotential (η) and exchange current density (i0) of the NiFe-LDH pyramid electrode were further improved compared to that of the NiFe-LDH deposited Cu (NiFe-LDH/Cu) foil electrode with the same base area. The 3D-printed NiFe-LDH electrode also exhibited excellent durability without potential decay for 60 h. Our 3D printing strategy provides an effective approach for the fabrication of highly active, stable, and low-cost OER electrocatalyst electrodes.

Defective high-entropy rocksalt oxide with enhanced metal‒oxygen covalency for electrocatalytic oxygen evolution

High-entropy materials are emerging electrocatalysts by integrating five or more elements into one single crystallographic phase to optimize the electronic structures and geometric environments. Here, a rocksalt-type high-entropy oxide Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O (HEO) is developed as an electrocatalyst towards the oxygen evolution reaction (OER). The obtained HEO features abundant cation and oxygen vacancies originating from the lattice mismatch of neighboring metal ions, together with enlarged Co/Ni‒O covalency due to the introduction of less electronegative Mg and Zn. As a result, the HEO exhibits superior intrinsic OER activities, delivering a turnover frequency (TOF) 15 and 84 folds that of CoO and NiO at 1.65 V, respectively. This study provides a mechanistic understanding of the enhanced OER on HEO and demonstrates the potential of high-entropy strategy in developing efficient oxygen electrocatalysts by elaborately incorporating low-cost elements with lower electronegativity. A rocksalt-type high-entropy oxide Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O featuring enhanced Co/Ni–O covalency and enriched metal and oxygen vacancies was prepared as high-activity and low-cost OER electrocatalyst.

Construction of N and Fe co-doped CoO/CoxN interface for excellent OER performance

Water electrolysis is a promising strategy to produce green hydrogen. However, this process is hampered by the high cost of precious electrode and the sluggish kinetics of oxygen evolution reaction (OER). Thus, it is desirable for developing an efficient and low-cost OER electrocatalyst. Herein, N and Fe co-doped CoO nanoparticles (CoxFeyO-N), rich of O vacancies and the interface of CoO/CoxN, were successfully prepared via the annealing of Fe doped cobalt-based coordination polymers in an NH3 atmosphere. The ratio of Co2+ and Fe3+ was systematically varied, and Co0.89Fe0.11O-N possesses the highest OER activity in 1.0 M KOH. Furthermore, through growth of the resultant Co0.89Fe0.11O-N onto nickel foam, the corresponding OER activity can be further enhanced with the overpotential of 360 mV at current density of 50 mA cm−2. The superior OER activity of Co0.89Fe11O-N originates from the presence of oxygen vacancies and CoO/CoxN interface. During the reaction, CoO-N was partially oxidized into Co3O4, which accompanies CoO to co-catalyze OER. The study provides new insights for the preparation and understanding the active sites for Co-based catalysts for OER.

N- doped CoP nanoparticles embedded in electrospun N-doped porous carbon nanofiber as high-efficiency oxygen evolution electrocatalysts

The development of high-efficiency and low-cost oxygen evolution reaction (OER) electrocatalysts has attracted increasing research attention. Herein, we report a novel nanocomposite of N-doped CoP nanoparticles embedded in porous carbon nanofibers (N–CoP@NPCNFs) synthesized using zeolitic imidazolate framework-67 (ZIF-67)@Polyacrylonitrile (PAN) nanofibers (ZIF-67@PNFs) as the precursor through carbonization, oxidation and phosphidation with N-doping. The fabricated N1–CoP@NPCNFs-900 shows outstanding electrocatalytic properties for OER due to a synergistic effect between the carbon nanofibers and the N-doped CoP nanoparticles. It exhibits a low overpotential of only 266 mV at 10 mA cm−2, a small Tafel slope of 70 mV dec−1 and excellent durability for OER in 1.0 M KOH. Therefore, the obtained N1–CoP@NPCNFs-900 is promising for application as a high-efficiency and low-cost OER electrocatalyst.

MOF derived multi-metal oxides anchored N, P-doped carbon matrix as efficient and durable electrocatalyst for oxygen evolution reaction

Because the oxygen evolution reaction (OER) process is a rate-determining step for water splitting, it is extremely significant to rationally design and explore highly efficient and durable as well as the low-cost OER electrocatalyst to promote electrocatalytic water splitting. Based on this consideration, herein, for the first time, multi-metal oxides MoO2/Co2Mo3O8/Fe2Mo3O8 (MCF) compactly anchored N, P-doped carbon matrix (CNT and amorphous carbon derived from cobalt hexacyanoferrate) (MCF/NPCCNT) is fabricated by simply pyrolyzing composite precursor consisted of phosphomolybdic acid, cobalt hexacyanoferrate and CNT under the nitrogen atmospheres. The as-prepared MCF/NPCCNT-40 hybrid catalyst delivers a high OER electrochemical activity with a smaller overpotential of 292 mV at the current density of 10 mA cm−2 and a lower Tafel slope of 45.9 mV dec-1, as well as favorable cycle stability in 1.0 M KOH solution, markedly improving the charge transfer efficiency in catalytic processes and preventing the corrosion of the metallic compounds. In addition, the MCF/NPCCNT-40 hybrid catalyst exhibits large mass density of 323.2 A g−1 at a fixed potential of 1.7 V, which is the highest mass activity towards OER process compared to the other reference samples. The design idea and synthetic strategy of this work can be expanded to design and fabricate other novel, highly effective and low-cost multi-metal oxide anchored N, P-doped carbon matrix based electrocatalysts with greatly improved OER performance.

Hollow FeP/Fe3O4 Hybrid Nanoparticles on Carbon Nanotubes as Efficient Electrocatalysts for the Oxygen Evolution Reaction.

We develop a method to prepare hollow FeP/Fe3O4 hybrid nanoparticles supported on carbon nanotubes (CNTs), which could be used as highly active and efficient electrocatalysts. The Fe@Fe3O4/CNT hybrids were first synthesized by annealing the CNTs adsorbed with Fe(NO3)3, followed by controlled phosphorization treatment. They exhibit an outstanding catalytic activity for oxygen evolution reaction (OER) with a low overpotential of 229 mV at a current density of 10 mA cm-2, a high turnover frequency value of 0.35 s-1 at an overpotential of 300 mV, and an ultralow Tafel slope of 27.6 mV dec-1, which is much better than that of FeP/Fe3O4, FeP/CNTs, Fe3O4/CNTs, and the commercial RuO2 electrocatalyst. More importantly, the Tafel slope is much lower than most non-noble metal-based OER electrocatalysts reported in the previous literature studies as far as we know. The excellent OER performance benefits from the electron transfer from Fe3O4 to FeP confirmed by X-ray photoelectron spectroscopy, together with good conductivity of CNTs. This hybrid structure supported on conductive CNTs may offer an efficient method to design earth-abundant and low-cost electrocatalysts for OER in practical applications.

Transition metal carbides in electrocatalytic oxygen evolution reaction

Oxygen evolution reaction (OER) is admitted to an important half reaction in water splitting for sustainable hydrogen production. The sluggish four-electron process is known to be the bottleneck for enhancing the efficiency of OER. In this regard, tremendous efforts have been devoted to developing effective catalysts for OER. In addition to Ir- or Ru-based oxides taken as the benchmark, transition metal carbides have attracted ever-increasing interest due to the high activity and stability as low-cost OER electrocatalysts. In this review, the transition metal carbides for water oxidation electrocatalysis concerning design strategies and synthesis are briefly summarized. Some typical applications for various carbides are also highlighted. Besides, the development trends and outlook are also discussed.

Ternary boron-, phosphorus- and oxygen-doped amorphous nickel nanoalloys for enhanced activity towards the oxygen evolution reaction

The oxygen evolution reaction (OER) is a key reaction in hydrogen production through water splitting and in fuel cells. Nickel nanoalloys are potential low-cost OER electrocatalysts, but their catalytic activity and stability need to be improved. The doping of nickel catalysts with anionic elements combined with partial amorphization can improve their electrocatalytic activity. In this study, we report a ternary B-, P- and O-doped amorphous nickel nanoalloy (BPO-Ni) as an effective electrocatalyst for the OER. Compared with binary boron–oxygen-doped nickel (BO-Ni) and phosphorus–oxygen-doped nickel (PO-Ni), ternary BPO-Ni has higher OER catalytic activity and better durability under alkaline conditions. Our results reveal that the high catalytic activity of the B-, P- and O-doped Ni nanoalloy is derived from the synergistic effects between the ternary heteroatoms and the Ni atoms, thereby increasing their catalytic activity. This work produces a new non-metal doped amorphous nickel nanoalloy structure, and offers a way of studying novel inexpensive and efficient OER catalysts.

Copper nickel alloy nanorods textured nanoparticles for oxygen evolution reaction

Facile and low-cost synthesis routes of highly competent catalyst materials for applications in oxygen evolution reaction are imperative. In this work, we synthesised bimetallic alloy crystals using nickel nitrate as a metal ions source and trimesic acid as an organic linker. Further annealing at an elevated temperature under a nitrogen atmosphere yielded alloy nanorods textured nanoparticles. Numerous characterization techniques were used, revealing the bimetallic nature of the synthesised product. The product was a highly efficient and stable material for oxygen evolution reaction (OER) in an alkaline medium. In 1.0 M KOH solution, the copper nickel alloy with graphitic carbon (Cu3.8Ni@C) catalyst showed remarkable OER activity, obtaining an overpotential of 233 mV at a current density of 10 mA cm−2 with stability up to 18 h under continuous electrolysis. Consequently, we believe that this work could contribute insights into low-cost electrocatalysts for OER.

Promoting Electrocatalytic Oxygen Evolution over Transition-Metal Phosphide-Based Nanocomposites via Architectural and Electronic Engineering.

Exploring noble-metal-free advanced electrocatalysts for oxygen evolution reaction (OER) is of great significance for development of sustainable energy systems. Rationally engineering chemical composition, electronic structure, and micro/nanoarchitecture of these catalysts represent a promising way to achieve their desired performance. Herein, we propose a general synthetic approach to prepare a series of porous nanosheets of bi- and tritransition-metal phosphides with homogeneous compositions supported by reduced graphene oxide (the resultant samples are termed as rGO/MaMb-P for simplicity, including rGO/CoFe-P, rGO/NiFe-P, rGO/CoNi-P, and rGO/CoNiFe-P). The resultant sheet-on-sheet hierarchical nanocomposites exhibit outstanding OER performance with low overpotentials, small Tafel slopes, and long durability in alkaline electrolyte, which compete favorably with the state-of-the-art OER catalysts reported to date. The remarkable electrocatalytic performances originate from their hierarchical structures and tailorable compositions with optimized electronic structure modulation and synergistically electroactive sites, together with large active surface area and smooth mass/charge transports. This synthetic route could serve as a facile and controllable process to simultaneously realize the architectural manipulation, compositional regulation, and electronic modulation for low-cost OER electrocatalysts based on earth-abundant metal elements in practical water electrolysis and other energy applications.

In situ iron coating on nanocatalysts for efficient and durable oxygen evolution reaction

The oxygen evolution reaction (OER) is essential to many important energy conversion and storage systems. To achieve high-performance and low-cost OER electrocatalysts, great effort has been devoted to the development of noble-metal-free OER nanocatalysts. While nanostructured materials have been shown effective at improving the catalytic activity, the stability of the nanocatalyst has yet to be significantly improved. Furthermore, nanocatalyst syntheses are typically complex, limiting their practical application. Herein, we design a noble-metal-free nanocatalyst which has a CoFePx core and an in situ Fe coating layer to enhance the material's catalytic activity and durability for OER. We utilize the vapor pressure difference of Fe and Co at high temperature to synthesize the CoFePx-Fe core-shell nanoparticles via a one-step high-temperature shock technique. The high-temperature method strongly anchors the CoFePx nanoparticles onto a carbon substrate. The thin Fe shell layer (~2 nm) coated on the CoFePx greatly enhances the durability of the CoFePx catalyst without hampering its activity due to the geometric effect. This nanocatalyst with an in situ Fe shell demonstrates a successful nanocoating approach for fabricating catalytically active protection layers on catalysts with improved activity and durability.

Zirconium-Regulation-Induced Bifunctionality in 3D Cobalt-Iron Oxide Nanosheets for Overall Water Splitting.

The design of high-efficiency non-noble bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is paramount for water splitting technologies and associated renewable energy systems. Spinel-structured oxides with rich redox properties can serve as alternative low-cost OER electrocatalysts but with poor HER performance. Here, zirconium regulation in 3D CoFe2 O4 (CoFeZr oxides) nanosheets on nickel foam, as a novel strategy inducing bifunctionality toward OER and HER for overall water splitting, is reported. It is found that the incorporation of Zr into CoFe2 O4 can tune the nanosheet morphology and electronic structure around the Co and Fe sites for optimizing adsorption energies, thus effectively enhancing the intrinsic activity of active sites. The as-synthesized 3D CoFeZr oxide nanosheet exhibits high OER activity with small overpotential, low Tafel slope, and good stability. Moreover, it shows unprecedented HER activity with a small overpotential of 104 mV at 10 mA cm-2 in alkaline media, which is better than ever reported counterparts. When employing the CoFeZr oxides nanosheets as both anode and cathode catalysts for overall water splitting, a current density of 10 mA cm-2 is achieved at the cell voltage of 1.63 V in 1.0 m KOH.