Optimization of Oxygen Evolution Reaction with Electroless Deposited Ni-P Catalytic Nanocoating

Development of a free-standing flexible electrode for efficient overall water-splitting performance via electroless deposition of iron-nickel-cobalt on polyacrylonitrile-based carbon cloth

Developing cost-effective and highly efficient electrodes for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is crucial, particularly for alkaline OER and HER. Herein, electroless plating of iron-nickel-cobalt (Fe-Ni-Co) electrocatalyst on a flexible polyacrylonitrile (PAN)-based carbon cloth (CC) was performed to synthesize a free-standing catalytic electrode for bifunctional electrocatalysts. The three-dimensional porous backbone of PAN-based CC and the synergistic effect between Fe-Ni-Co and corresponding hydroxides result in outstanding OER and HER activities, that is low overpotentials of 230 and 150 mV at 10 mA cm-2 and Tafel slopes of 63 and 57.1 mV dec-1, respectively. Additionally, a complete alkaline electrolyzer was constructed using Fe-Ni-Co plated PAN-based CC as both the cathode and anode. When operated at 10 mA cm-2, this setup enabled efficient water splitting at a low potential of 1.57 V. This study provides new opportunities for the development of advanced Fe-Ni-Co electrocatalyst with superior bifunctional OER and HER performance and cost-effectiveness that are suitable for integration into PAN-based CC via electroless plating.

Nanocomposites based on Cu2O coated silver nanowire networks for high-performance oxygen evolution reaction

The development of highly active, low-cost, and robust electrocatalysts for the oxygen evolution reaction (OER) is a crucial endeavor for the clean and economically viable production of hydrogen via electrochemical water splitting. Herein, cuprous oxide (Cu2O) thin films are deposited on silver nanowire (AgNW) networks by atmospheric-pressure spatial atomic layer deposition (AP-SALD). AgNW@Cu2O nanocomposites supported on conductive copper electrodes exhibited superior OER activity as compared to bare copper substrate and bare AgNWs. Moreover, a relationship between Cu2O thickness and OER activity was established. Notably, the most effective catalyst (AgNW@50nm-thick Cu2O) demonstrated very high OER activity with a low overpotential of 409 mV to deliver a current density of 10 mA cm-2 (η 10), a Tafel slope of 47 mV dec-1, a turnover frequency (TOF) of 4.2 s-1 at 350 mV, and good durability in alkaline media (1 M KOH). This highlights the potential of AgNWs as a powerful platform for the formation of highly efficient copper oxide catalysts towards OER. This work provides a foundation for the development of nanostructured Cu-based electrocatalysts for future clean energy conversion and storage systems.

Novel High-Entropy FeCoNiMoZn-Layered Hydroxide as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

The exploration of catalysts for the oxygen evolution reaction (OER) with high activity and acceptable price is essential for water splitting to hydrogen generation. High-entropy materials (HEMs) have aroused increasing interest in the field of electrocatalysis due to their unusual physicochemical properties. In this work, we reported a novel FeCoNiMoZn-OH high entropy hydroxide (HEH)/nickel foam (NF) synthesized by a facile pulsed electrochemical deposition method at room temperature. The FeCoNiMoZn-OH HEH displays a 3D porous nanosheet morphology and polycrystalline structure, which exhibits extraordinary OER activity in alkaline media, including much lower overpotential (248 mV at 10 mA cm-2) and Tafel slope (30 mV dec-1). Furthermore, FeCoNiMoZn-OH HEH demonstrates excellent OER catalytic stability. The enhanced catalytic performance of the FeCoNiMoZn-OH HEH primarily contributed to the porous morphology and the positive synergistic effect between Mo and Zn. This work provides a novel insight into the design of HEMs in catalytic application.

Active-site-enriched dendritic crystal Co/Fe-doped Ni3S2 electrocatalysts for efficient oxygen evolution reaction

The electrochemical decomposition of water plays a critical role in green and sustainable energy. However, the development of efficient and low-cost non-noble metal catalysts to overcome the high potential of the anodic oxygen evolution reaction (OER) is still challenging. In this work, electrocatalysts with high OER activity were obtained by doping Co/Fe bimetals into Ni₃S₂ (CF-NS) via a simple single-step hydrothermal method by adjusting the doping ratio of bimetals. A series of characterization studies revealed that the introduction of a Co/Fe co-dopant increased the number of active sites and improved the electroconductibility, while optimizing the electronic structure of Ni₃S₂. Meanwhile, Fe-induced high valence Ni contributed to the production of an OER active phase NiOOH. The unique dendritic crystal morphology facilitated the disclosure of the active sites and the expansion of mass transfer channels. The optimized sample required a low overpotential of 146 mV to obtain a current density of 10 mA cm^-2 in 1.0 M KOH solution. The optimized sample also operated stably for at least 86 h. In sum, the proposed method looks very promising for designing efficient, stable, and low-cost non-precious metal catalysts with high conductivity and multiple active sites, useful for future synthesis of transition metal sulfide catalysts.

Chemical Transformation Induced Core-Shell Ni2P@Fe2P Heterostructures toward Efficient Electrocatalytic Oxygen Evolution

The oxygen evolution reaction (OER) is a crucial reaction in water splitting, metal-air batteries, and other electrochemical conversion technologies. Rationally designed catalysts with rich active sites and high intrinsic activity have been considered as a hopeful strategy to address the sluggish kinetics for OER. However, constructing such active sites in non-noble catalysts still faces grand challenges. To this end, we fabricate a Ni₂P@Fe₂P core-shell structure with outperforming performance toward OER via chemical transformation of rationally designed Ni-MOF hybrid nanosheets. Specifically, the Ni-MOF nanosheets and their supported Fe-based nanomaterials were in situ transformed into porous Ni₂P@Fe₂P core-shell nanosheets composed of Ni₂P and Fe₂P nanodomains in homogenous dispersion via a phosphorization process. When employed as the OER electrocatalyst, the Ni₂P@Fe₂P core-shell nanosheets exhibits excellent OER performance, with a low overpotential of 238/247 mV to drive 50/100 mA cm^-2, a small Tafel slope of 32.91 mV dec^-1, as well as outstanding durability, which could be mainly ascribed to the strong electronic interaction between Ni₂P and Fe₂P nanodomains stabilizing more Ni and Fe atoms with higher valence. These high-valence metal sites promote the generation of high-active Ni/FeOOH to enhance OER activity.

Facile Synthesis of Sulfate-Intercalated CoFe LDH Nanosheets Derived from Two-Dimensional ZIF-9(III) for Promoted Oxygen Evolution Reaction

Layered double hydroxide (LDH) has emerged as a promising electrocatalyst; however, the synthetic method usually requires high temperature and high pressure, and sulfate-intercalated LDH is rarely reported. Herein, the sulfate-intercalated CoFe LDH nanosheets were successfully fabricated at ambient temperature via a facile strategy, using two-dimensional ZIF-9(III) as a template and FeSO4 as both etchant and iron source. When the as-prepared sulfate-intercalated CoFe LDH acts as an electrocatalyst, it presents superior electrocatalytic performance for the oxygen evolution reaction (OER), requiring low overpotential (η@10 mA cm−2 = 218 mV) with a small Tafel slope of 59.9 mV dec−1 in 1.0 M KOH, which compares favorably with commercial RuO2 and most reported transition-metal electrocatalysts. The high catalytic activity of CoFe LDH might be ascribed to the large interlayer space distance originating from special SO42− ions and the strong synergistic effects between Fe and Co. This work provides a novel and feasible approach to designing highly efficient electrocatalysts based on advanced LDH materials for OER.

Vertically growing nanowall-like N-doped NiP/NF electrocatalysts for the oxygen evolution reaction

Developing low-cost, high-performance and corrosion-resistant catalysts for water splitting is anticipated, but it will also be a big challenge. In this study, nanowall-like N-Ni₅P₄/Ni₂P/NF (N-NiP/NF) was synthesized by a simple two-step method involving hydrothermal treatment and phosphorylation. The catalyst has good catalytic activity for the OER, and only 160 mV is required to achieve a current density of 10 mA cm^-2 in 1 M KOH, which is even better than RuO₂, with good corrosion resistance. In addition, N-Co₂P/Ni₂P/NF (N-CoP/NF) was synthesized by the same method with good electrocatalytic properties and good conductivity towards the HER. N-NiP/NF was used as the anode and N-CoP/NF was used as the cathode to form the N-NiP//N-CoP double electrode system, which showed excellent electrolytic performance for water splitting, requiring only 1.48 V to reach 10 mA cm^-2. This is mainly due to the strong electronegativity of N that makes the N doping induce the electron transfer process, which results in a high catalytic activity of the adjacent transition metal atoms and thus promotes the electrolysis of water, as well as the unique vertical nanowall-like structure, which gives the material a large surface area and accessibility to active sites, facilitating the adsorption of water molecules and catalytic reactions. In addition, the unique structure favors the diffusion of water molecules and the release of gaseous products, ensuring close contact between the catalyst and the electroactive material. This simple non-metallic N doping strategy provides a new way to produce efficient non-precious metal catalysts.

Interface Synergistic Effect from Hierarchically Porous Cu(OH)2@FCN MOF/CF Nanosheet Arrays Boosting Electrocatalytic Oxygen Evolution

The electrolysis of water is an efficient and environmentally friendly technology for large-scale hydrogen production. However, the oxygen evolution reaction (OER) involves a multi-electron–proton coupling transfer step that limits the efficiency of water splitting. Therefore, there is an urgent need to develop electrocatalysts with expected activity and stability to accelerate the kinetics of the oxygen evolution reaction. In this paper, hierarchically porous Cu(OH)2@(Fe, Co, Ni)MOF/CF nanosheet (denoted as Cu(OH)2@FCN MOF/CF) arrays were successfully prepared by the hydrothermally induced in situ growth of FCN MOF nanosheets using modified Cu(OH)2 nanowires as carriers; herein, the tuned active species of metal ligands in the FCN MOF composition structure are used as the main catalytic reaction size in the OER. The synergistic effect of a unique porous structure and the active metal-ligand species in the MOF render the catalyst a large electrochemically active surface area and more active species. Then, the active material is fully contacted with the electrolyte to expose more electrochemically active sites, thus greatly improving the electrocatalytic activity and durability of the OER. Specifically, the Cu(OH)2@FCN MOF/CF delivers a minimum overpotential of 290 mV and low Tafel slope of 96.15 mV·dec−1 at 10 mA·cm−2 as well as ultra-long cycling stability. The resulted OER performance is superior to most reported MOF-based electrocatalysts. This novel structural design not only provides a new strategy for the facile preparation of low-cost and high-efficiency OER electrocatalysts but also paves an avenue for the development of other MOF-based composite electrocatalysts with excellent electrocatalytic performances.

Synergetic Effects of Mixed-Metal Polyoxometalates@Carbon-Based Composites as Electrocatalysts for the Oxygen Reduction and the Oxygen Evolution Reactions

The smart choice of polyoxometalates (POMs) and the design of POM@carbon-based composites are promising tools for producing active electrocatalysts for both the oxygen reduction (ORR) and the oxygen evolution reactions (OER). Hence, herein, we report the preparation, characterization and application of three composites based on doped, multi-walled carbon nanotubes (MWCNT_N6) and three different POMs (Na12[(FeOH2)2Fe2(As2W15O56)2]·54H2O, Na12[(NiOH2)2Ni2(As2W15O56)2]·54H2O and Na14[(FeOH2)2Ni2(As2W15O56)2]·55H2O) as ORR and OER electrocatalysts in alkaline medium (pH = 13). Overall, the three POM@MWCNT_N6 composites showed good ORR performance with onset potentials between 0.80 and 0.81 V vs. RHE and diffusion-limiting current densities ranging from −3.19 to −3.66 mA cm−2. Fe4@MWCNT_N6 and Fe2Ni2@MWCNT_N6 also showed good stability after 12 h (84% and 80% of initial current). The number of electrons transferred per O2 molecule was close to three, suggesting a mixed regime. Moreover, the Fe2Ni2@MWCNT_N6 presented remarkable OER performance with an overpotential of 0.36 V vs. RHE (for j = 10 mA cm−2), a jmax close to 135 mA cm−2 and fast kinetics with a Tafel slope of 45 mV dec−1. More importantly, this electrocatalyst outperformed not only most POM@carbon-based composites reported so far but also the state-of-the-art RuO2 electrocatalyst. Thus, this work represents a step forward towards bifunctional electrocatalysts using less expensive materials.