Electrochemical study of La0.6Sr0.4Co0.8Fe0.2O3 during oxygen evolution reaction

In this paper, oxygen evolution reaction (OER) mechanism in La_0.6Sr_0.4Co_0.8Fe_0.2O₃ was investigated in KOH solution by electrochemical impedance spectroscopy (EIS) and voltammetric measurements. The Tafel slopes and reaction orders evaluated in this paper are consistent with the B. O’Grady’s Path for oxygen evolution on oxides. The activation energy for OER in La_0.6Sr_0.4Co_0.8Fe_0.2O₃ was 28.3 kJ mol⁻¹. The obtained apparent porosity of La_0.6Sr_0.4Co_0.8Fe_0.2O₃ electrode is 48% and the roughness factor is around 1.6 × 10⁴. The polarization resistance of La_0.6Sr_0.4Co_0.8Fe_0.2O₃ is much low compared with other similar oxides. This can be due the high roughness and high porosity in addition to the low active energy for the process.     

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.     

Nanofiber-based Sm0.5Sr0.5Co0.2Fe0.8O3-δ/N-MWCNT composites as an efficient bifunctional electrocatalyst towards OER/ORR

It is of significance to search non-noble metal OER/ORR catalysts with perfect performance. The introduction of carbon into perovskite can significantly enhance oxygen electrocatalytic activity. Herein, nanofiber-based Sm_0.5Sr_0.5Co_0.2Fe_0.8O_3-δ/rGO (SSCF28/rGO) and Sm_0.5Sr_0.5Co_0.2Fe_0.8O_3-δ/N-MWCNT (SSCF28/N-MWCNT) hybrids with various mass ratios were synthesized successfully by a facile ultrasonic mixing method and their oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) properties were compared and studied. In 0.1 M KOH, SSCF28/N-MWCNT = 1.3 with optimal mass ratio shows better OER/OER bifuntional catalytic activity than SSCF28/rGO = 2:1. After 1000 CV cycles, SSCF28/N-MWCNT = 1.3 remains stable. Compared to SSCF28/rGO = 2:1, SSCF28/N-MWCNT = 1:3 shows promising practical applicability in metal-air batteries. The excellent OER/ORR activity of SSCF28/N-MWCNT = 1:3 can be attributed to the component optimization of perovskite and carbon and the synergistic effect between nanofiber-structured SSCF28 and N-functionalized MWCNT (N-MWCNT).     

Efficient electrocatalytic oxygen evolution by Fe3C nanosheets perpendicularly grown on 3D Ni foams

The development of efficient and earth-abundant catalyst for the oxygen evolution reaction (OER) is an important challenge for the renewable energy research community. Here, we report a facile method to design a nanosheet-like structure of Fe₃C, perpendicularly grown on Ni foam (NF) as a catalyst for OER by hydrothermal reaction and annealing treatment at a low temperature. The synthesized Fe₃C has good conductivity, small charge transfer resistance (R_ct), high active site density, and excellent stability, which make it highly efficient OER catalyst. Fe₃C/NF has a low overpotential of 262 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 49 mV dec⁻¹. Remarkably, Fe₃C/NF also has good electrochemical stability, which helps maintain its activity in 1.0 M KOH solution for at least 100 h. Therefore, the simple method to prepare perpendicularly grown Fe₃C on a NF substrate can enhance the OER properties, which make Fe₃C/NF a promising material for replacing noble metal-based catalysts for OER applications.     

One-pot solvothermal synthesis of Co2P nanoparticles: An efficient HER and OER electrocatalysts

Development of an inexpensive electrocatalyst for hydrogen evolution (HER) and oxygen evolution reactions (OER) receives much traction recently. Herein, we report a facile one-pot ethyleneglycol (EG) mediated solvothermal synthesis of orthorhombic Co₂P with particle size ~20–30 nm as an efficient HER and OER catalysts. Synthesis parameters like various solvents, temperatures, precursors ratios, and reaction time influences the formation of phase pure Co₂P. Investigation of Co₂P as an electrocatalyst for HER in acidic (0.5 M H₂SO₄) and alkaline medium (1.0 M KOH), furnishes low overpotential of 178 mV and 190 mV, respectively to achieve a 10 mA cm^?2 current density with a long term stability and durability. As an OER catalyst in 1.0 M KOH, Co₂P shows an overpotential of 364 mV at 10 mA cm^?2 current density. Investigation of Co₂P NP by XPS analysis after OER stability test under alkaline medium confirms the formation of amorphous cobalt oxyhydroxide (CoOOH) as an intermediate during OER process.     

PdCoNi nanoparticles supported on nitrogen-doped porous carbon nanosheets for room temperature dehydrogenation of formic acid

The development of cost-effective heterogeneous catalysts for the dehydrogenation of formic acid (FA) is the key challenge for the commercialization of FA as a hydrogen-storage medium. Herein, PdCoNi nanoparticles (NPs) with different element ratios supported on N-doped carbon nanosheets (N-CN) were designed, which exhibit excellent catalytic dehydrogenation performance for FA. Compared with PdCoNi NPs loaded on the carbon nanosheets (CN), the introduction of pyrrolic N to CN induces the formation of ultrafine, monodispersed and amorphous Pd_0.6Co_0.2Ni_0.2 NPs with a size of 1.60 nm, which significantly increases the number of active sites and the instant contact between FA and catalysts. The as-prepared Pd_0.6Co_0.2Ni_0.2/N-CN catalyst shows more than 99% conversion and 100% H₂ selectivity at room temperature, with a record-high initial turnover frequency (TOF_initial) of 1249.0 h⁻¹ among non-noble containing Pd-based catalysts, which demonstrates the high potential of Pd_0.6Co_0.2Ni_0.2/N-CN as a practical catalyst for the hydrogen generation from FA.     

One-pot synthesis of Fe2O3/C by urea combustion method as an efficient electrocatalyst for oxygen evolution reaction

The hematite type Fe₂O₃/C catalyst for oxygen evolution reaction (OER) was prepared using a facile urea combustion method. The Fe₂O₃/C electrode showed excellent electrocatalytic ability towards OER in alkaline medium. The phase and morphology of the product were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The sintering temperature is 600 °C, and the calcination time is 3 h, which is the best preparation process condition. The particles were irregular spherical with the size of 10–30 nm and dispersed uniformly. The current density of the Fe₂O₃/C electrode was 147 mA cm⁻² at 0.6 V (vs. HgO/Hg) in 6 mol L⁻¹ KOH electrolyte at room temperature.     

Nanorod-like NiFe metal-organic frameworks for oxygen evolution in alkaline seawater media

Searching for highly active and durable oxygen evolution reaction (OER) electrocatalysts is the key to break through the bottleneck of overall water splitting. Here, we prepare Ni_xFe-BDC (H₂BDC = terephthalic acid) nanorods with different Ni/Fe ratios by a facile solvothermal method for the OER. The optimal Ni₃Fe-BDC exhibits a low overpotential (η₁₀) of 265 mV and a Tafel slope of 90 mV·dec⁻¹ in 1 M KOH. Moreover, it shows a low η₁₀ of 280 mV and excellent stability in the mixture of 1 M NaCl and 1 M KOH, which has strong corrosion resistance to Cl⁻ anions. The role of Fe³⁺ not only increases the charge transfer rate of Ni₃Fe-BDC, but also affects the specific surface area of the catalyst with high electrochemical activity. Kinetic studies show that both Fe and Ni sites act as active centers, which catalyze synergistically to reduce the reaction kinetic energy barrier. Characterization results of the used Ni₃Fe-BDC reveal that the in situ formed rod-like Ni₃FeOOH is the active site for the OER.     

NiFe2O4–Ni3S2 nanorod array/Ni foam composite catalyst indirectly controlled by Fe3+ immersion for an efficient oxygen evolution reaction

A NiFe alloy was designed on nickel foam (NF) as a precursor using cathodic electrodeposition. NiFe₂O₄–Ni₃S₂ nanorods (NRs) composite catalysts were prepared by Fe³⁺ impregnation and further hydrothermal sulfuration methods. NiFe₂O₄–Ni₃S₂ nanosheets (NSs) were also prepared by direct hydrothermal sulfuration of the NiFe alloy for comparison. Compared to the dense NS structure of the NiFe₂O₄–Ni₃S₂ NSs/NF, the NiFe₂O₄–Ni₃S₂ NRs/NF showed better oxygen evolution performance due to its unique weed-like NR array structure composed of additional oxygen evolution reaction (OER) active sites, with a strong electron interaction for Ni and Fe and the active sulfide synergistic effect with oxides. Therefore, Driving a current density of 10 mA cm^?2 only requires an overpotential of 189 mV and the catalyst could provide 100 mA cm^?2 continuously and be constant for more than 80 h in 1.0 M KOH. This experiment indicated that Fe³⁺ immersion had an indirect regulating effect on the morphological growth of the catalyst, which provided a novel concept for designing better OER catalysts.     

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.     

Bifunctional iron doped CuS catalysts towards highly efficient overall water electrolysis in the alkaline electrolyte

Efficient catalysts towards overall water electrolysis in alkaline electrolytes were highly desirable for the hydrogen production technology. The surface electronic states of copper in CuS nanocrystal catalysts were modified by iron doping through a simple wet-chemical method. The iron-doped CuS catalysts displayed drastically enhanced catalytic activities for overall water electrolysis in the strong alkaline electrolyte of 1 M KOH after a simple cyclic voltammetry activation. The optimized catalytic performance for overall water electrolysis was achieved in the CuFe_0.6S_1.6 catalyst, which exhibited a low overpotential of ?237 mV for HER and 302 mV for OER to reach 10 mA cm^?2. The high activities for overall water electrolysis in CuFe_0.6S_1.6 were induced by the enhanced charge transfer from Cu to S via iron doping, which not only modified the surface electronic state of copper but also enhanced charge transfer during the electrochemical reactions. Moreover, the catalysts displayed satisfying stability for over 20 h at a high current density of 300 mA cm^?2 for both HER and OER, showing great potential for industrial water electrolysis.     

Electrodeposition of NiFe layered double hydroxide on Ni3S2 nanosheets for efficient electrocatalytic water oxidation

The oxygen evolution reaction (OER) plays a vital role in various energy conversion applications. Up to now, the highly efficient OER catalysts are mostly based on noble metals, such as Ir- and Ru-based catalysts. Thus, it is extremely urgent to explore the non-precious electrocatalysts with great OER performance. Herein, a simple electrodeposition combined with hydrothermal method is applied to synthesize a non-precious OER catalyst with a three-dimensional (3D) core-shell like structure and excellent OER performance. In our work, NiFe layered double hydroxide (LDH) was electrodeposited on Ni₃S₂ nanosheets on nickel foam (NF), which exhibits a better performance compared with RuO₂, and a low overpotential of 200 mV is needed to reach the current density of 10 mA/cm² in 1 M KOH. Notably, the Ni₃S₂/NiFe LDH only need an overpotential of 273 mV to reach the current density of 200 mA/cm².     

Electrochemical synthesis of Li-doped NiFeCo oxides for efficient catalysis of the oxygen evolution reaction in an alkaline environment

Oxygen evolution reaction (OER) is an essential reaction for overall electrochemical water splitting. In this present study, we adopt a facile electrochemical deposition method to synthesize the Li-doped NiFeCo oxides for OER in an alkaline medium. The scanning electron microscopy, X-ray diffraction, Brunauer-Emmet-Teller method and X-ray photo-electron spectroscopy provides the information of morphology, structure, specific surface area and electronic state of the electrocatalysts respectively. Investigates the electrochemical properties by the thin-film technique on a rotating disk electrode and in a single-cell laboratory water electrolyzer connects with electrochemical impedance spectroscopy. Among the catalysts under investigation, Ni_0·9Fe_0·1Co_1·975Li_0·025O₄ exhibits the highest activity towards oxygen evolution reaction, and explains the activity by the oxygen binding energy; such knowledge can be helped to develop better catalyst. We achieve onset over potential 220 mV and receive 10 mA cm^?2 current density at over potential 301 mV with Tafel slope 62 mV dec^?1 in 1 M KOH solution. The results are similar to recently published catalysts in the literature. In water electrolyzer, the Ni_0·9Fe_0·1Co_1·975Li_0·025O₄ modified nickel foam anode exhibits a current density of 143 mA cm^?2 at a cell voltage of 1.85 V in 10 wt% KOH and a temperature of 50 °C.     

In situ growth of NiCoFe-layered double hydroxide through etching Ni foam matrix for highly enhanced oxygen evolution reaction

It is of great significance to develop the nonprecious metal oxide electrocatalysts toward oxygen evolution reaction (OER) for water splitting. Herein we report an in-situ growth of the ternary NiCoFe-layered double hydroxide nanosheets on surface etching nickel foam (NiCoFe LDHs/NF) without adding any nickel precursor. In this method, etching Ni matrix by Fe³⁺ not only provides the slowly released nickel ions, but also intensifies the Fe–Ni interaction between the directly grown active species and Ni foam. Therefore the composition, electronic structure, and morphology of the electrocatalysts can be easily regulated only by adjusting Co²⁺:Fe³⁺ ratio in the precursor solution. The obtained NiCo₁Fe₁ LDH/NF, which is formed in 1:1 Co²⁺:Fe³⁺ solution, has highest content of Ni³⁺ and Co³⁺ active sites and the largest electrochemical active area. It exhibits an outstanding OER performance with a small overpotential of 231 mV at 10 mA cm^?2 and excellent durability at 50 mA cm^?2 in 1.0 M KOH solution.     

Fe-doped Co9S8@CoO aerogel with core-shell nanostructures for boosted oxygen evolution reaction

The efficiency and stability of electrocatalysts for oxygen evolution reaction (OER) generally depends on the intrinsic catalytic activity and extrinsic active sites. To intrinsically and extrinsically improve OER performance, herein, we present a novel OER catalyst of Fe-doped Co₉S₈@CoO aerogel with core-shell nanostructures via structural/electronic modulation strategy. The structural modulation is realized by 3D porous aerogel and core-shell nanoarchitecture which provides rich exposed active sites and guarantees effective ion diffusion and O₂ release, and the amorphous shell layer of CoO can effectively prevent the corrosion of Co₉S₈ by the electrolyte during OER process. The electronic modulation is realized by Fe heteroatom doping for Co₉S₈ and oxygen vacancies in CoO layer was caused by partial oxidation process, which facilities the transfer of electrons during OER process. These merits make Fe-doped Co₉S₈@CoO an efficient and stable OER catalyst: it requires only a low overpotential of 296 mV to obtain 10 mA cm^?2 and it delivers a low Tafel slope of 65 mV dec^?1 with good stability, which are even better than those of commercial RuO₂. This work presents an effective structural/electronic modulation strategy to develop efficient and stable OER catalysts for water splitting.     

An efficient tri-metallic anodic electrocatalyst for urea electro-oxidation

Tri-metallic MnNiFe alloy nanoparticles with four different Mn:Ni:Fe weight ratios (0.5:2.0:0.5, 0.5:1.0:0.5, 1.0:1.0:1.0, and 2.0:0.5:2.0) on reduced graphene oxide (rGO) supports were synthesized using a one-pot hydrothermal method. The as-prepared catalysts were characterized by X-ray diffraction, inductively coupled plasma-mass spectroscopy, Brunauer-Emmett-Teller analysis, scanning electron microscopy, and transmission electron microscopy, and their catalytic activities were measured by cyclic voltammetry and chronoamperometry. In urea electro-oxidation, the Mn_0.5Ni_2.0Fe_0.5/rGO catalyst exhibited superior electrocatalytic activity compared to Ni/rGO and commercial Ni/C. The Mn_0.5Ni_2.0Fe_0.5/rGO catalyst exhibited a mass activity of 1753.97 mA mg⁻¹_Ni, along with an onset potential of 0.34 V (vs. Ag/AgCl) in 1.0 M KOH and 0.33 M urea solution, which is ~4.2 times and 9.8 times higher than those of Ni/rGO and commercial Ni/C, respectively. Furthermore, a single cell comprising of Mn_0.5Ni_2.0Fe_0.5/rGO catalyst exhibited a peak power density of 30.08 mW cm⁻² in 0.33 M urea and 1.0 M KOH at 50 °C.     

Investigation of La0.6Sr0.4Co1-xNixO3-δ (x=0, 0.2, 0.4, 0.6, 0.8) catalysts on solid oxide fuel cells anode for biogas dry reforming

The introduction of catalyst on anode of solid oxide fuel cell (SOFC) has been an effective way to alleviate the carbon deposition when utilizing biogas as the fuel. A series of La_0.6Sr_0.4Co_1-xNi_xO_3-δ (x = 0, 0.2, 0.4, 0.6, 0.8) oxides are synthesized by sol-gel method and used as catalysts precursors for biogas dry reforming. The phase structure of La_0.6Sr_0.4Co_1-xNi_xO_3-δ oxides before and after reduction are characterized by X-ray diffraction (XRD). The texture properties, carbon deposition, CH₄ and CO₂ conversion rate of La_0.6Sr_0.4Co_1-xNi_xO_3-δ catalysts are evaluated and compared. The peak power density of 739 mW cm^?2 is obtained by a commercial SOFC with La_0.6Sr_0.4Co_0.4Ni_0.6O_3-δ catalyst at 850 °C when using a mixture of CH₄: CO₂ = 2:1 as fuel. This shows a great improvement from the cell without catalyst for internal dry reforming, which is attributed to the formation of NiCo alloy active species after reduction in H₂ atmosphere. The results indicate the benefits of inhibiting the carbon deposition on Ni-based anode through introducing the La_0.6Sr_0.4Co_0.4Ni_0.6O_3-δ catalyst precursor. Additionally, the dry reforming technology will also help to convert part of the exhaust heat into chemical energy and improve the efficiency of SOFC system with biogas fuel.     

NiCo-BDC derived Co3+ enriched NiCoxOy/NF nanosheets for oxygen evolution reaction

In recent years, transition metal catalysts due to their low price, excellent conductivity and high activity have been extensively studied in OER. It is worth noting that Co³⁺ plays a vital role in the process of oxygen evolution catalysis, because Co³⁺ ions are considered to be active sites. The development of catalysts with high Co³⁺ content has important application prospects for improving water oxidation efficiency. This paper uses a simple ligand-assisted synthesis method to promote the transformation of two-dimensional layered double hydroxides (LDHs) into two-dimensional metal organic frameworks (MOFs). MOF-OH/NF are quickly and easily prepared by electrooxidation, which exhibit superior OER properties, with a low overpotential of 260 mV and 420 mV for the oxygen evolution reaction (OER) at 100 mA cm^?2 and 1000 mA cm^?2. Moreover, the MOF-OH/NF has good stability in 1.0 M KOH electrolyte solution. This study provides a new idea for the design and synthesis of novel composite electrocatalysts.     

SrTi0.1CoxFe0.9-xO3-δ Perovskites for enhanced oxygen evolution reaction activity

We developed a series of Fe doping in Co-based perovskites SrTi_0.1Co_xFe_0.9-xO_3-δ (x = 0.5, 0.6, 0.7, 0.9) to investigate their OER activity and stability in alkaline media. Among all the samples, SrTi_0·1Co_0·5Fe_0·4O_3-δ (donated as STCF-154) shows wonderful OER activity with an overpotential of 0.37 V, a current density of 33.65 mA cm⁻² at 1.71 V, and a Tafel slope of 94.82 mV dec⁻¹. Besides, the potential of STCF-154 remained nearly unchanged for at least 8 h at a fixed current density of 10 mA cm⁻²_disk on GC electrode. The improved activity and stability are likely originating from the highly oxidative oxygen species O₂²⁻/O⁻ formed in STCf-154, which can easily migrate from bulk STCF-154 and “spillover” to the surface of the catalyst during OER process. The Fe doping in Co-based perovskites had synergetically enhanced activity and can be considered as a good candidate for the OER in alkaline solution.     

Effect of anion exchange ionomer content on electrode performance in AEM water electrolysis

Anion exchange membrane water electrolysis (AEMWE) has acquired substantial consideration as a cost-effective hydrogen production technology. The anion ionomer content in the catalyst layers during hydrogen and oxygen evolution reaction (HER and OER) is of ultimate significance. Herein, an in-situ half-cell analysis with reference electrodes was carried out for simultaneous potential measurements and identification of the influence of the anion exchange ionomer (AEI) content on anode and cathode performance. The measured half-cell potentials proved the influence of AEI content on the catalytic activity of HER and OER, which was supported by the rotating disk electrode (RDE) measurements. Cathode overpotential of Ni/C was not negligible and more affected by the AEI content than anode with the optimized AEI content of 10 wt% while NiO anode OER overpotential was independent of the AEI content. For the same AEI content, PGM catalysts showed higher electroactivity than Ni-based catalysts for HER and OER and the cathode catalyst's intrinsic activity is of high importance in the AEM electrolysis operation. Post-mortem analysis by SEM mapping of both AEI and catalyst distributions on the electrode surface showed the effect of AEI loading on the catalyst morphology, which could be related to the electrode performance.