Template assisted hydrothermal synthesis of CoSnO3 hollow microspheres for electrocatalytic oxygen evolution reaction

The practical complications suffered by the most recognized electrochemical energy systems, such as, water-electrolyzers and metal-air batteries reside in the half-cell oxygen evolution reaction. To resolve this problem, continuous colossal efforts are required to develop the active, affordable and sustainable electrocatalysts. Shape-tailoring of the catalysts, constructed from non-noble metals is one of the emerging strategies to augment the activity of the material toward electrochemical reactions. In the present work, we demonstrate the template-assisted hydrothermal synthesis of hierarchical CoSnO₃ hollow microspheres, constructible from the wafer-thin sheets of CoSnO₃. The hierarchical CoSnO₃ hollow microspheres possess a high specific surface area of 153.59 m²/g, and mesoporous configuration, which are the essential pre-requisites of an electrochemical system. In addition to this, the proposed CoSnO₃ hollow microspheres possess adequate electroactive surface area (793.5 cm²) and happens to be a suitable candidate for driving the oxygen evolution reaction with a low overpotential of 282 mV and Tafel slope of 96.5 mV/dec in alkaline medium. The higher turnover frequency (0.0045 s⁻¹), high specific and mass activities (2.195 mA/cm²_EASA and 28.752 mA/mg, respectively) were observed for CoSnO₃ hollow spheres. Furthermore, the chronoamperometric measurement reveals a good stability of CoSnO₃ hollow microspheres in alkaline condition, satisfying the fundamental demand of an energy system.     

Ni/NiO@rGO as an efficient bifunctional electrocatalyst for enhanced overall water splitting reactions

Herein, we fabricated bifunctional, noble metal-free, highly efficient nickel/nickel oxide on reduced graphene oxide (Ni/NiO@rGO) by chemical synthesis approach for electrochemical water splitting reaction. Its structural and morphological characterization using thermogravimetric analysis (TGA), transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), energy dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD) represents, Ni/NiO@rGO is having Ni/NiO NPs ∼10 nm (±2 nm) on graphene oxide with face-centered cubic (FCC) crystal structure. Moreover, the presence of Ni/NiO (2.26%), O (6.56%), N (0.74%) and C (90.44%) from EDAX analysis further confirms the formation of Ni/NiO@rGO and it also supported by FTIR studies. This nanocatalyst is examined further for electrocatalytic water splitting reactions (HER and OER). It demonstrated low overpotential 582 mV to achieve current density at 10 mA cm⁻² and smaller Tafel slope of 63 mV dec⁻¹ obtained in 0.5 M H₂SO₄ towards HER. Also, at the other end at onset potential of 1.6 V vs. RHE towards OER. It demonstrated low overpotential 480 mV to achieve current density at 10 mA cm⁻² and smaller Tafel slope of 41 mV dec⁻¹ in 0.5 M KOH towards OER observed. Hydrogen fuel is eco-friendly to the environment and noteworthy performance of earth-saving reactions.     

Synthesis of high crystalline nickel‐iron hydrotalcite‐like compound as an efficient electrocatalyst for oxygen evolution reaction

The nickel‐iron hydroxide‐like catalyst for oxygen evolution reaction (OER) is prepared by an improved coprecipitation method. The crystallization degree of hydrotalcite‐like compound is high, and the lamellar structure is homogeneous with no agglomeration, which helps to build efficient mass‐transfer layer channel of OH^? ions. The NiFe layered double hydroxide (LDH)/carbon nanotubes (CNTs) electrode shows good performance and stability for OER. The potential of NiFe LDH/CNTs electrode is only 0.592 V (vs HgO/Hg) at 200 mA·cm^?2 in 6 mol·L^?1 potassium hydroxide (KOH) electrolyte, which shows excellent catalytic activity for OER. The NiFe LDH/CNTs electrode works continuously for 620 hours at 200 mA·cm^?2, with the groove voltage only rises 0.1 V.     

The ?NHx Group Induced Formation of 3D α‐Co(OH)2 Curly Nanosheet Aggregates as Efficient Oxygen Evolution Electrocatalysts

Recently, the curly structure attracts researchers' attention due to the strain effect, electronic effect, and improved surface area, which exhibits enhanced electrocatalytic activity. However, the synthesis of metastable curved structures is very difficult. Herein, a simple room temperature coprecipitation method is proposed to synthesize 3D cobalt (Co) hydroxide (α‐Co(OH)₂) electrocatalysts that consist of curly 2D nanosheets. The formation process of curly nanosheets is elaborated systematically and the results demonstrate that the ? NH_x group has great effect on the formation of curly structure. Combining the advantage of 2D curly nanosheet and 3D aggregate structure, the as‐prepared α‐Co(OH)₂ curly nanosheet aggregates show the best water oxidation activity with an overpotential of 269 mV at j = 10 mA cm^?2 in 1.0 m KOH. The electrocatalytic process studies demonstrate that the formation of Co^IV?O species is the rate‐determining step. Theoretical calculations further confirm the beneficial effect of the bent structure on the conductivity, the adsorption of OH^? and the formation of OOH* species.     

A General Method to Ultrathin Bimetal‐MOF Nanosheets Arrays via In Situ Transformation of Layered Double Hydroxides Arrays

Structure engineering of ultrathin metal–organic framework (MOF) nanosheets to self‐supporting and well‐aligned MOF superstructures is highly desired for diverse applications, especially important for electrocatalysis. In this work, a facile layered double hydroxides in situ transformation strategy is developed to synthesize ultrathin bimetal‐MOF nanosheets (BMNSs) arrays on conductive substrates. This approach is versatile, and applicable to obtain various BMNSs or even trimetal‐MOF nanosheets arrays on different substrates. As a proof of concept application, the obtained ultrathin NiCo‐BDC BMNSs array exhibits an excellent catalytic activity toward the oxygen evolution reaction with an overpotential of only 230 mV to reach a current density of 10 mA cm^?2 in 1 m KOH. The present work demonstrates a strategy to prepare ultrathin bimetal‐MOF nanosheets arrays, which might open an avenue for various promising applications of MOF materials.     

2D Free‐Standing Nitrogen‐Doped Ni‐Ni3S2@Carbon Nanoplates Derived from Metal–Organic Frameworks for Enhanced Oxygen Evolution Reaction

2D metal–organic frameworks (2D MOFs) are promising templates for the fabrication of carbon supported 2D metal/metal sulfide nanocomposites. Herein, controllable synthesis of a newly developed 2D Ni‐based MOF nanoplates in well‐defined rectangle morphology is first realized via a pyridine‐assisted bottom‐up solvothermal treatment of NiSO₄ and 4,4′‐bipyridine. The thickness of the MOF nanoplates can be controlled to below 20 nm, while the lateral size can be tuned in a wide range with different amounts of pyridine. Subsequent pyrolysis treatment converts the MOF nanoplates into 2D free‐standing nitrogen‐doped Ni‐Ni₃S₂@carbon nanoplates. The obtained Ni‐Ni₃S₂ nanoparticles encapsulated in the N‐doped carbon matrix exhibits high electrocatalytic activity in oxygen evolution reaction. A low overpotential of 284.7 mV at a current density of 10 mA cm^?2 is achieved in alkaline solution, which is among the best reported performance of substrate‐free nickel sulfides based nanomaterials.     

Surface Reconstruction of Covellite CuS Nanocrystals for Enhanced OER Catalytic Performance in Alkaline Solution

Oxygen evolution reaction (OER) is one of the important half-reactions in energy conversion equipment such as water-spitting devices, rechargeable metal-air batteries, and so on. It is beneficial to develop efficient and low-cost catalysts that understand the reaction mechanism of OER and analyze the reconstruction phenomenon of transition metal sulfide. Interestingly, copper sulfide and cuprous sulfide with the same components possess different reconstruction behaviors due to their different metal ion valence states and different atomic arrangement modes. Because of a unique atomic arrangement sequence and certain cationic defects, the reconstruction phenomenon of CuS nanomaterials are that S²⁻ is firstly oxidized to SO₄²⁻ and then Cu^x+ is converted into CuO via Cu(OH)₂. In addition, the specific “modified hourglass structure” of CuS with excellent conductivity is easier to produce intermediates. Compared with Cu₂S, CuS exhibits excellent OER activity with a lower overpotential of 192 mV at 10 mA cm⁻² and remarkable electrochemical stability in 1.0 m KOH for 120 h. Herein, this study elucidates the reconstruction modes of CuS and Cu₂S in the OER process and reveals that CuS has a stronger Cu S bond and a faster electronic transmission efficiency due to “modified hourglass structure,” resulting in faster reconstruction of CuS than Cu₂S.     

Efficient overall water splitting using nickel boride-based electrocatalysts

Currently there is tremendous interest in the discovery of low cost and efficient electrocatalysts for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). In this work, iron-doped nickel boride (Fe_xNi_1-xB) and nickel boride (NiB) were successfully grown on 3D self-supporting graphene (SSG) electrodes via a one-step reduction approach. The Fe_0.2Ni_0.8B/SSG electrode required a very low overpotential of only 263 mV for OER (the best OER activity achieved to date for a metal boride). NiB/SSG showed modest OER performance but excellent HER activity. A water electrolyzer comprising Fe_0.2Ni_0.8B/SSG and NiB/SSG delivered a current density of 10 mA cm⁻² at a voltage of only 1.62 V. Further, the Fe_0.2Ni_0.8B/SSG and NiB/SSG catalysts showed excellent stability with no deactivation observed over 14 h of testing. Results demonstrate that nickel boride-based electrocatalysts are promising lost cost alternatives to precious metal-based electrocatalysts for OER, HER and overall water splitting.     

Anti-disturbance control of oxygen feeding for vehicular fuel cell driven by feedback linearization model predictive control-based cascade scheme

The accurate control of automotive fuel cell oxygen excess ratio (OER) is necessary to improve system efficiency and service life. To this end, an anti-disturbance control driven by a feedback linearization model predictive control (MPC)-based cascade scheme is proposed. It considers strong nonlinear coupling and disturbance injection of fuel cell oxygen supply. A six-order nonlinear fuel cell oxygen feeding model is presented. It is further formulated using an extended state observer to rapidly reconstruct the OER, to overcome the slow response and interference errors of sensor measurements. In the proposed cascade control, the outer loop is the anti-disturbance control which is used to realize the optimized OER tracking and the inner loop via the feedback linearization to linearize the oxygen feeding behaviors conducts MPC to regulate the air compressor output mass flow. The feedback linearization demonstrates a robust tracking performance of nonlinear outputs, and the integral absolute error of anti-disturbance control is 0.3021 lower than that of PI control under a custom test condition. Finally, the numerical validation on a hybrid driving cycle indicates that the proposed cascade control can regulate the fuel cell OER with an average absolute error of 0.02313 in the high air compressor operation efficiency zone.     

Construction of hierarchical Prussian Blue Analogue phosphide anchored on Ni2P@MoOx nanosheet spheres for efficient overall water splitting

In response to the energy crisis, molybdenum-based catalyst has been proposed as a high-performance electrocatalytic material due to its low price and excellent HER performance. However, in contrast with its excellent HER performance, its poor OER performance often limits practical application as a high-performance overall water splitting catalyst. In this study, Prussian blue analogue (PBA) is grown in-situ on molybdenum-based nanosheet spheres by a simple and ingenious method and then subjected to phosphorization. The resulting composite catalyst exhibits highly efficient overall water splitting performance, overpotentials at current densities of 10 mA cm⁻² and 100 mA cm⁻² for the HER and OER are −61 mV and 268 mV, respectively. Moreover, an alkaline electrolyzer makes up by the catalyst both as positive and negative can reach a cell voltage 1.494 V at 10 mA cm⁻² for the overall water splitting. This method has provided a new strategy to effective combine PBA and molybdenum-based catalyst.