Electrochemical construction of S-doped MnOx/Mn integrated film on carbon paper in a choline chloride based deep eutectic solvent for enhanced electrochemical water oxidation

The rational design and synthesis of highly efficient, precious metal-free electrocatalysts in the promotion of electrochemical water oxidation is essential for the sustainable use of renewable energy. Herein, an integrated, S-doped MnO_x/Mn film developed on carbon paper (CP) formed by a two-step transformation approach that includes template-free electrodeposition and in-situ electrochemical oxidation is employed as an efficient oxygen evolution reaction (OER) electrocatalyst. The doping of S and in-situ electro-oxidation lead to a tailored electronic structure and phase composition, which endow the composite electrode with a significantly enhanced OER catalytic performance. This S-doped integrated electrode perfectly combines highly active MnO_x outer layers with conductive metallic Mn inner layers to offer abundant catalytic interfaces and electronic paths, leading to favorable reaction kinetics. As a result, the optimal S-doped MnO_x/Mn/CP offers a low overpotential of 435 mV at 10 mA cm^?2 with a Tafel slope of 89.97 mV dec^?1 as well as improved stability. Impressively, a highly intrinsic catalytic activity with a turnover frequency of 0.0112 s^?1 is obtained for S-doped MnO_x/Mn/CP, which is 4.2-fold higher than that of MnO_x/Mn/CP. S doping plays a critical role in stabilizing the Mn^IIIO_x active phase and is ultimately responsible for the enhanced catalytic performance. This work provides an integrated design concept and S doping approach suitable for boosting the OER catalytic performance of MnO_x.     

Electrocatalytic performance of Ni modified MnOx/C composites toward oxygen reduction reaction and their application in Zn–air battery

A series of Ni modified MnO_x/C composites were synthesized by introducing NaBH₄ to MnO₂/C aqueous suspension containing Ni(NO₃)₂. The physical properties and the activity of the composites toward the oxygen reduction reaction (ORR) were investigated via transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and the electrochemical techniques. The results show that the higher activity of the composites toward the ORR is correlated with the higher content of MnOOH species transformed from Mn(II) on the surface of the composite. The main nickel species in the composites is Ni(OH)₂, while Ni(OH)₂ shows little activity toward the ORR. However, introducing Ni(OH)₂ with proper amount into the MnO_x/C improves the distribution of the active material MnO_x, which contributes to a surface with more MnOOH. The optimal composite is of the Ni/Mn atomic ratio of 1:2 and the MnO_x loading of 28 wt.%. The maximum power density of the zinc–air battery with the optimized Ni modified MnO_x/C as the cathode catalyst reaches up to 122 mW cm⁻², much higher than the one with the MnO_x/C as the air cathode catalyst (89 mW cm⁻²), and slightly higher than those with the Pd/C and Pt/C as the cathode catalysts.     

Iridium-titanium oxides for efficient oxygen evolution reaction in acidic media

Active and highly stable oxygen evolution reaction (OER) electrocatalyst for PEM-based water electrolysis are currently in high demand. Herein, we report a rutile iridium-titanium oxide solid solution (IrTiO_x) through a facile one-step annealing of a Ti-based metal-organic framework precursor. The composite exhibits excellent OER activity and stability in acidic media, with a low overpotential of 296 mV at 10 mA cm⁻² while the OER activity was retained during a 100-h galvanostatic stability test at a constant current of 10 mA cm⁻² in 0.5 M H₂SO₄, outperforming the state-of-the-art IrO₂-based electrocatalysts. We further demonstrate the structure evolution of iridium-titanium oxide during OER operations. In contrast to the initial uniform distribution of Ir and Ti over the entire architecture, after OER stability test, a hollow morphology is formed, in which the particle surface is covered with an IrO_x-rich layer and entire particle becomes hollow. We ascribe the structure evolution to the Ir/Ti leaching and redeposition during the OER operations. We propose that the structure evolution of iridium-titanium oxide during the electrochemical process is responsible for the high OER activity and stability of IrTiO_x.     

Roles of Pb and MnOx in PtPb/MnOx-CNTs catalyst for methanol electro-oxidation

Design of novel nano-scale catalysts with high activity and low cost for methanol oxidation reaction is crucial for the development of direct methanol fuel cell. In this study, MnO_x, Pt and Pb were forced to precipitate successively on the surface of carbon nanotubes for fabricating a PtPb/MnO_x-CNTs catalyst. Physical characterizations indicated that there existed a mass of Mn (IV, Ⅴ), Pb (Ⅱ) and Pt (0) species, and partial alloying between Pt and Pb in this catalyst. Methanol oxidation reaction with this novel composite exhibited over 3 times higher specific activity (140.9 mA cm⁻²) and somewhat lower onset potential (−0.1 V vs. Hg/Hg₂SO₄) than the values on Pt/CNTs (44.2 mA cm⁻² and 0 V, respectively). Fundamental understanding in reaction mechanisms enabled us to reveal the distinguishing functions between Pb and MnO_x in methanol oxidation processes. The addition of Pb resulted in the enhanced intrinsic activity towards electro-oxidation of residual intermediate species, while dehydrogenation in methanol oxidation processes was obviously improved by using MnO_x-CNTs as a support.     

Pronounced effect of phosphidization on the performance of CoOx encapsulated N-doped carbon nanotubes towards oxygen evolution reaction

Research on water splitting reaction is on priority to explore an alternative source of energy with little to no carbon emissions. Among the two half reactions of electrochemical water splitting, oxygen evolution reaction (OER) is highly desirable yet challenging to prepare a cost effective and viable electrocatalyst to boost the OER activity. Herein, we have prepared a novel electrocatalyst, CoO_x-CoP/N-CNTs, by the phosphidization of cobalt oxides (CoO_x) encapsulated N- doped carbon nanotubes (CoO_x/N-CNTs). The CoO_x/N-CNTs composite is derived via pyrolysis of cobalt based zeolitic imidazole framework (ZIF-12) at 950 °C under argon atmosphere. The CoO_x/N-CNTs is phosphidized at various temperatures ranging from 320 °C to 400 °C. The optimized temperature to attain the best catalytic activity is 380 °C. The phosphatized material, CoO_x-CoP/N-CNTs, shows superior performance towards OER with an overpotential of 250 mV @ 20 mAcm^?2 vs 532 mV @ 20 mAcm^?2 of un-phosphidized material, CoO_x/N-CNTs, which shows significant effect of phosphidization. The maximum current density of 160 mAcm^?2 in 1 M KOH solution is achieved.     

Manganese molybdate and its Fe-substituted products as new efficient electrocatalysts for oxygen evolution in alkaline solutions

Pure and Fe-substituted manganese molybdates with nominal compositional formula Fe_xMn_1−xMoO₄ (x = 0, 0.25, 0.50 and 0.75) have been prepared by a co-precipitation method at pH ≈ 2 and characterized by FT-IR, XRD, XPS, TEM, electrochemical impedance and polarization techniques. The oxygen evolution reaction (OER) study reveals that Fe substitutions from 0.25 to 0.75 mol for Mn increase the apparent electrocatalytic activity of the base oxide showing maximum with 0.5 mol Fe. At E = 0.60 V vs. Hg/HgO in 1 M KOH at 298 K, the apparent activity of the base oxide increased ∼58 times. It is observed that on Fe addition, the Tafel slope decreases from ∼60 to ∼35 mV, however, the order of the OER with respect to OH⁻ concentration as observed ∼2 on the base oxide (i.e. MnMoO₄) does not change.     

Manganese oxide with different morphology as efficient electrocatalyst for oxygen evolution reaction

Three-dimensional (3D) manganese oxides consisted of tetragonal phase Mn₃O₄ and α-MnO₂ with different morphology have been directly grown vertically on Ti foil by a simple electrochemical method without any template and used as the catalysts for oxygen evolution reaction (OER). The results show that manganese oxides with different morphology show high activity and good stability for OER and the manganese oxide (MnO_x) nanowire arrays obtained at 70 °C show higher activity and better stability than MnO_x with cotton wool structure and MnO_x nanosheet arrays.     

Iron-doped cobalt nitride nanoparticles (Fe–Co3N): An efficient electrocatalyst for water oxidation

The exploration of efficient, low-cost and earth-abundant oxygen-evolution reaction (OER) electrocatalysts and the understanding of the intrinsic mechanism are important to advance the clean energy conversion technique based on electrochemical water oxidation. In this work, Fe-doping Co₃N catalysts were successfully synthesized by a simple nitridation reaction of the Co_3-xFe_xO₄ precursor. This material exhibited a low overpotential of 294 mV at a current density of 10 mA cm^?2, and a small Tafel slope of 49 mV dec^?1 in 1 M KOH solution, superior to the performance of Co₃N and IrO₂. As revealed by the spectroscopic and electrochemical analyses, the enhanced OER performance mainly originates from the electronic modulation induced by the incorporation of Fe into Co₃N, benefitting the formation of CoOOH as active surface species and thus facilitating the OER process. These findings also demonstrate the introduction of heterogeneous element is a simple and effective strategy to regulate the OER property of the cobalt nitrides (Co₃N) catalysts.     

Phytic acid-derived Co2-xNixP2O7-C/RGO and its superior OER electrocatalytic performance

A phytic acid-derived Co_2-xNi_xP₂O₇-C/RGO composite was designed and facilely synthesized, in which phytic acid acted as both a phosphoric source and carbon source. Both carbon derived from phytic acid and reduced graphene oxide (RGO) in composite, enhanced the conductivity and thus improve its electrocatalytical capability. As-synthesized Co_1.22Ni_0.78P₂O₇-C/RGO composite exhibited excellent oxygen evolution reaction (OER) catalytic performances: At the current density of 10 mA cm⁻², only a low overpotential of 283 mV and a small Tafel slope of 51 mV dec⁻¹ were observed. Good OER catalytic performance was retained even after 10 h continuously running at a constant voltage, which is even comparable to those of first-rate noble metal catalyst RuO₂. In addition, the performances of Co_2-xNi_xP₂O₇-C/RGO catalysts were also strongly dependent on Ni content.     

Ni/Ce co-doping δ-MnO2 nanosheets with oxygen vacancy for enhanced electrocatalytic oxygen evolution reaction

The design and manufacture of strongly engaged, low-cost, and resilient oxygen evolution reaction (OER) electrocatalysts is the most challenging task in electrochemical hydrolysis. Herein, Ce and Ni co-doped MnO₂ (NiCe/MnO₂) nanosheets (NSs) with oxygen vacancy (V_O) and abundant active sites have been prepared in one step employing a defect strategy. The co-doping of Ce/Ni on the one hand reduced the catalyst particle size and increased the specific surface area, which promoted the exposure of more active sites. On the other hand, heteroatom doping altered the species the crystalline surface, stimulating the formation of Vo and thus activating the catalyst performance simultaneously. The OER performance of NiCe/MnO₂ NSs was significantly enhanced over the pure δ-MnO₂, with an overpotential of 170 mV (10 mA cm^?2), which was verified by density functional theory. This work shows a straightforward and practical method for making non-precious metal electrocatalysts with high electrochemical hydrolysis performance.     

Large-area manganese oxide nanorod arrays as efficient electrocatalyst for oxygen evolution reaction

Large-area manganese oxide nanorod arrays (MnO₂ NRAs) have been directly grown vertically on Ti foil with a uniform length and diameter by a simple electrochemical method without any templates. The deposition temperature is one of the most important parameters for formation MnO₂ NRAs and at 25 °C no MnO₂ NRAs can be obtained. The results show that MnO₂ has high activity and good stability for oxygen evolution reaction (OER) and the structure of nanorod arrays pronounced enhances MnO₂ activity. The onset potential of MnO₂ NRAs is lower than that of Pt foil and lower 401 mV than that of MnO₂ film, indicating that the structure of MnO₂ NRAs shows an easy OER for water split. The MnO₂ NRAs may be of great potential in electrochemical water split.     

Cu/MnOx–CeO2 and Ni/MnOx–CeO2 catalysts for the water–gas shift reaction: Metal–support interaction

Monometallic copper and nickel catalysts supported on cerium-manganese mixed oxides are prepared, characterized and evaluated for the Water–Gas Shift (WGS) reaction. Active metal loading of 2.5 wt% and 7.5 wt% are used to impregnate MnO_x–CeO₂ supports with 30% and 50% Mn:Ce molar ratio. The structure of the samples strongly depends on both the active metal employed and the manganese content in the mixed support. For both Cu and Ni samples, the best catalytic behavior is found in samples supported on the MnO_x–CeO₂ oxides with 30% Mn:Ce molar ratio, as a result of the presence of Cu_xMn_yO₄ spinel-type phases in the case of copper catalysts and the presence of a NiMnO₃ mixed oxide with defect ilmenite structure in the case of nickel catalysts.     

Electrochemical induction of Mn(III) in the structure of Mn(IV) oxide: Toward a new approach for water splitting

Oxygen-evolution reaction (OER) through water-oxidation reaction is an essential reaction for water splitting, which provides electrons for hydrogen production. Mn oxides are among the commonly investigated types of metal oxides as OER catalysts. Recent studies showed that Mn(III) ions in the structure of Mn(IV) oxide have an essential role in OER. Previous works have only focused on adding different materials to induce Mn(III) ions in the structure of Mn(IV). Indeed, Mn(III) ions could be induced in the Mn(IV)-oxide structure in the presence of organic compounds, reductants, fluoride, chloride, and gold nanoparticles. However, a challenging issue in the field is using a stable and perdurable force during OER to induce Mn(III) ions in the structure of Mn(IV). Herein, the effects of inducing Mn(III) ions on OER by different potentials on the surface of the prepared Mn(IV) oxide on fluorine-doped tin oxide were investigated in the phosphate buffer at pH 11, 7, and 3. Mn(IV) oxide under OER/Mn(III) induction shows no decrease in the current density, but for the same sample under OER alone, a decrease (15%) is observed in the current density after only 1000 s. At a lower inducing potential, a decrease for OER is observed, which corresponds to Mn(IV)/(III) reduction to Mn(II), and Mn(II) leaking effect. This investigation sheds new light on OER in the presence of Mn oxide and is a promising, low-cost, and environmentally friendly strategy, which results in an increase in the yield of OER.     

Dysprosium doped copper oxide (Cu1-xDyxO) nanoparticles enabled bifunctional electrode for overall water splitting

The production of hydrogen, a favourable alternative to an unsustainable fossil fuel remains as a significant hurdle with the pertaining challenge in the design of proficient, highly productive and sustainable electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, the dysprosium (Dy) doped copper oxide (Cu_1-xDy_xO) nanoparticles were synthesized via solution combustion technique and utilized as a non-noble metal based bi-functional electrocatalyst for overall water splitting. Due to the improved surface to volume ratio and conductivity, the optimized Cu_1-xDy_xO (x = 0.01, 0.02) electrocatalysts exhibited impressive HER and OER performance respectively in 1 M KOH delivering a current density of 10 mAcm^?2 at a potential of ?0.18 V vs RHE for HER and 1.53 V vs RHE for OER. Moreover, the Dy doped CuO electrocatalyst used as a bi-functional catalyst for overall water splitting achieved a potential of 1.56 V at a current density 10 mAcm^?2 and relatively high current density of 66 mAcm^?2 at a peak potential of 2 V. A long term stability of 24 h was achieved for a cell voltage of 2.2 V at a constant current density of 30 mAcm^?2 with only 10% of the initial current loss. This showcases the accumulative opportunity of dysprosium as a dopant in CuO nanoparticles for fabricating a highly effective and low-cost bi-functional electrocatalyst for overall water splitting.     

N/C doped nano-size IrO2 catalyst of high activity and stability in proton exchange membrane water electrolysis

It is highly desirable to synthesize and deploy low-cost and highly efficient catalysts for the oxygen evolution reaction (OER) to catalyze water splitting. We show that N/C doped amorphous iridium oxide combines the benefits of nano-size (approximately 2 nm), which results in exposure to large active surface areas and features of oxygen defects, which make for an electronic structure suitable for the OER. Systematic studies indicate that the OER activity of the iridium oxide catalyst is accelerated by the effect of the structure and chemical state of the iridium element. Remarkably, the N/C doped amorphous iridium oxide catalyst shows a lower cell voltage of 1.774 V at 1.5 A cm⁻², compared with IrO₂ (1.847 V at 1.5 A cm⁻²), and it can maintain such a high current density for over 200 h without noticeable performance deterioration. This work provides a promising method for the improving OER electrocatalysts and the construction of an efficient and stable PEM water cracking system.     

Crystalline nickel sulfide integrated with amorphous cobalt sulfide as an efficient bifunctional electrocatalyst for water splitting

The development of highly efficient and low-cost electrocatalysts is critical to the mass production of hydrogen from water splitting. Herein, a facile yet effective method was developed to synthesize bimetallic sulfides Ni₃S₂/CoS_x, which were aimed for use as the electrocatalysts in both HER and OER. Encouragingly, the Ni₃S₂/CoS_x demonstrated a low overpotential of 110 mV for HER at a current density of 10 mA·cm^?2. It was discovered that the surface of Ni₃S₂/CoS_x during OER process would undergo an in-situ oxidation to form MOOH (M = Co, Ni), that is, MOOH/Ni₃S₂/CoS_x were the real functioning species in catalysis, which had an excellent OER activity and a low overpotential of 226 mV. Additionally, the assembled electrolyzer required only a low cell voltage of 1.53 V to achieve a current density of 10 mA·cm^?2 in a 1 M KOH solution, and its performance was stable. Overall, this work provided a promising strategy for the facile fabrication of low-cost amorphous electrocatalysts, which is expected to promote the progress of overall water splitting.     

Thin-film iron-oxide nanobeads as bifunctional electrocatalyst for high activity overall water splitting

Developing only Fe derived bifunctional overall water splitting electrocatalyst both for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) while performing at low onset overpotential and with high catalytic stability is a rare instance. We present here the first demonstration of unique iron-oxide nanobeads (FeO_x-NBs) based electrocatalyst executing both OER and HER with high activity. Thin-film electrocatalytic FeO_x-NBs assembly is surface grown via simple spray coating (SC). The unique SC/FeO_x-NBs propels OER initiating water oxidation just at 1.49 V_RHE (η = 260 mV) that is the lowest observable onset potential for OER on simple Fe-oxide based catalytic films reported so far. Catalyst also reveals decently high HER activity and competent overall water splitting performance in the FeO_x-NBs two-electrode system as well. Catalyst also presents stable kinetics, with promising high electrochemically active surface area (ECSA) of 1765 cm², notable Tafel slopes of just 54 mV dec^1? (OER) and 85 mV dec^1? (HER), high exchange current density of 1.10 mA cm^2? (OER), 0.58 mA cm^2? (HER) and TOF of 74.29s^1?@1.58V_RHE, 262s^1?@1.62V_RHE (OER) and 82.5s^1?@-0.45V_RHE, 681s^1?@-0.56V_RHE (HER).     

ZIF-67-derived Mn doped Co9S8 supported on N-Enriched porous carbon polyhedron as an efficient electrocatalyst for oxygen evolution reaction

Rational design of efficient oxygen evolution reaction (OER) electrocatalysts plays a significant role in various applications like water splitting and metal-air batteries. Simultaneous modulation of geometric and electronic structure is a promising strategy for boosting the electrocatalytic active of OER catalysts. Herein, a novel type of Mn doped Co₉S₈ supported on N-enriched porous carbon polyhedron composite material (Mn–Co₉S₈/NC) is constructed via absorption-pyrolysis-sulfurization treatment of Zeolitic-imidazolate frameworks (ZIF-67). ZIF-67 derived N-enriched porous carbon polyhedron serves as the porous skeleton for anchoring numerous Co₉S₈ nanoparticles. The results confirm that the incorporation of Mn in Co₉S₈/NC can improve the degree of graphitization compared with Co₉S₈/NC, implying the enhancement of the conductivity. Meanwhile, the incorporation of Mn can lead to electronic modulation of Co species to bump up the intrinsic activity of active site in Mn–Co₉S₈/NC. Due to the synergistic effect of Mn, Co₉S₈ and porous carbon structure, the specific surface area and electronic structure are optimized, endowing the maximum utilization of active sites. The Mn–Co₉S₈/NC electrocatalyst exhibits superior OER activity with the overpotential of 286 mV at current density of 10 mA cm⁻² in 1.0 M KOH electrolyte. This work provides prospective insights into the synergistic coupling of geometric and electronic structure of Metal-Organic Frameworks (MOFs) material for efficient electrocatalysts.     

Microwave-assisted growth of spherical core-shell NiFe LDH@CuxO nanostructures for electrocatalytic water oxidation reaction

The high energy demand for electrochemical water splitting arises from sluggish oxygen evolution reaction (OER) kinetics. In this regard, Layered double hydroxide (LDH) has been introduced as an outstanding catalyst for the OER due to its exceptional physiochemical and 2D infrastructure properties. Herein, we report the design and synthesiss of core-shell nanostructured electrocatalyst by rationally decorating vertically oriented NiFe LDH ultrathin nanosheets on Cu_xO support (NiFe LDH@Cu_xO) via microwave-assisted hydrothermal reaction. For OER, the NiFe LDH@Cu_xO core-shell nanostructured catalyst demonstrated promising electrocatalytic performance, requiring only 1.43 V onset potential and 270 mV overpotential at 10 mA cm^?2. The NiFe LDH@Cu_xO also outperformed pristine NiFe LDH and iridium oxide (IrO₂) in terms of electrocatalytic activity, durability, and Faradaic efficiency. The fabricated NiFe-LDH@Cu_xO electrocatalyst with outer shell NiFe-LDH ultrathin nanosheets provides numerous exposed active sites, benefits electrolyte diffusion and oxygen gas releasing and also reduces the interfacial charge transfer resistance to enhance OER activity. Furthermore, exclusive core-shell 3D infra-structure effectively prevents NiFe-LDH nanosheets agglomeration and restacking, enhancing electrochemical stability.