Hollow NH2-MIL-101@TA derived electrocatalyst for enhanced oxygen reduction reaction and oxygen evolution reaction

Non-precious metal-based electrocatalysts with excellent activity and stability are highly desired for the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a tannic acid (TA) etching strategy is used to inhibit the metal aggregation and achieve muti-metal doping. The hollow NH₂-MIL-101@TA derived Fe–N–C catalyst exhibits superior ORR catalytic activity with an E_1/2 of 0.872 V and a maximum output power density of 123.4 mW cm⁻² in Zn-air battery. Since TA can easily chelate with metal ions, Fe/Co–N–C and Fe/Ni–N–C are also synthesized. Fe/Ni–N–C manifests exceptional bifunctional activity with an E_j = 10 of 1.67 V and a potential gap of 0.833 V between E_j = 10 and E_1/2 in alkaline electrolyte, which is 45 mV smaller than Pt/C–IrO₂. The improvement of ORR and OER performance of the catalysts via the simple TA etching and chelation method provides a novel strategy for the design and synthesis of efficient electrocatalysts.     

Facile electrodeposited amorphous Co–Mo–Fe electrocatalysts for oxygen evolution reaction

A new type of superior activity and highly cost-effective amorphous electrocatalyst Co–Mo–Fe on nickel foam (NF) supports is prepared by facile one-step rapid electrodeposition. The amorphous electrocatalyst Co–Mo–Fe/NF shows excellent oxygen evolution reaction (OER) performance, with a small overpotential of 218 mV at 10 mA cm^?2 current density in 1 M KOH. It only needs overpotential of 252 mV at 50 mA cm^?2 current density in 1 M KOH, and the Tafel slope is 45 mV dec^?1. The results show that the doping of Fe significantly improves the oxygen evolution capacity of the Co–Mo–Fe system. The synergistic effect of the three metals and the doping of the third metal iron make the oxygen evolution active sites of the whole system increase significantly. This provides a feasible direction for the oxygen evolution reaction of cobalt transition metal.     

Electrodeposition of Mo-doped NiFex nanospheres on 3D graphene fibers for efficient overall alkaline water splitting

Developing efficient bifunctional catalysts for hydrogen and oxygen evolution reactions (HER/OER) has attracted great interest in hydrogen production from water splitting. In this work, a novel material of Mo-doped NiFe_x nanospheres on 3D graphene fibers (Mo-NiFe_x/3DGFs) has been successfully fabricated through a simple and cheap one-step electrodeposition method. The Mo-NiFe_x/3DGFs possessed ultra-high conductivity and specific surface area, greatly benefiting to electrocatalytic hydrolysis activity. And it was found that Fe element could obviously promote OER process, while Mo doping facilitated both OER and HER reactions. We proved that there existed synergetic roles between Fe and Mo element, which could realize the control of the electronic structure and optimize the adsorption/desorption of intermediates. And electrochemical tests showed that the Mo–Ni/3DGFs exhibited a relatively smaller overpotential of 109.9 mV for HER, while the Mo-NiFe_x/3DGFs presented better OER performance with an overpotential of 240.8 mV at the current density of 100 mA cm^-2 in 1.0 M KOH. Finally, a system for overall water splitting constructed by Mo–Ni/3DGFs||Mo–NiFe_0.68/3DGFs electrodes has a low cell voltage of 1.52 V at 10 mA cm^?2 and long-term stability, exceeding most of literature results. Our findings provide insight into possibilities for the simple synthesis of high-performance and cheap catalysts, and laid the foundation for the practical application of transition metal catalysts.     

Autologous growth of Fe-doped Ni(OH)2 nanosheets with low overpotential for oxygen evolution reaction

Highly active and durable electrocatalysts for oxygen evolution reaction (OER) play a vital role in water splitting. Despite numerous efforts, the strategies to prepare durable and effective electrocatalysts via scalable methods still remain a great challenge. In this work, we fabricated Fe-doped Ni(OH)₂ ultrathin nanosheets (Fe–Ni–OH/Ni) via autologous growing of Ni(OH)₂ from Ni foam, and in situ electrochemical-assisted doping Fe into Ni(OH)₂. Benefiting from the unique structure with large surface areas and strong coupling effects between Fe and Ni, the optimal Fe–Ni–OH electrodes exhibit remarkable catalytic performance toward OER, which requires an overpotential of 220 mV to achieve a current density of 10 mA cm⁻² with a Tafel slope of 48.3 mV dec⁻¹. The Fe–Ni–OH electrodes also possess high stability even under a high current density of 500 mA cm⁻² for 600 h with an ultralow overpotential of 290 mV. Using Ni–Fe–OH electrodes as both anode and cathode for overall water splitting, only a small overpotential of 1.57 V is required to reach a current density of 10 mA cm⁻². Moreover, the high catalytic performance and scalable preparation method can meet the emergency needs for the practical application.     

N-doped bamboo-like CNTs combined with CoFe–CoFe2O4 as a highly efficient electrocatalyst towards oxygen evolution

In this work, we developed a novel hybrid with N-doped bamboo-like carbon nanotubes combined with the CoFe alloy and its oxide (denoted as CoFe–CoFe₂O₄/N-CNTs) via an extremely facile one-step pyrolysis method. When applied as an electrocatalyst towards OER in 1.0 M KOH solution, CoFe–CoFe₂O₄/N-CNTs exhibits a small overpotential of 334 mV to afford a current density of 10 mA cm⁻² associated with a low Tafel slope of 80 mV dec⁻¹ and long-term stability. The remarkable OER electrocatalytic performance of CoFe–CoFe₂O₄/N-CNTs is mainly attributed to the synergistic effect between CoFe and CoFe₂O₄ as well as the increased active sites resulting from the introduction of N-CNTs. More impressively, the introduction of Fe can significantly cut down the usage of relatively toxic and expensive Co accompanied with the dramatic enhancement of OER electrocatalytic performance originating from the robust synergistic effect between Co and Fe.     

Interface engineering and heterometal doping Co–Mo/FeS for oxygen evolution reaction

The sluggish kinetics of the oxygen evolution reaction (OER) limits the development of water electrolysis technology and the long-term efficiency of hydrogen energy production. In addition, it is important to evaluate the reconstruction performance of OER catalysts for actual water electrolysis. We created a self-supported electrode with FeS film coated Fe foam as a substrate, ordered resoluble molybdate (MoO₄²⁻) anions in interlayers, and Co-doped as a catalytically active phase for the OER. The catalyst is capable of electrochemical self-reconstruction (ECSR). With the dissolution of molybdate and sulfur ions, the catalyst surface cobalt iron oxide (CoFe₂O₄) forms an active amorphous FeCoOOH, which is favorable for alkaline OER. We realized the introduction of new active sites in the catalyst reconstruction process. Finally, the composite CoFeO_x catalyst increased the specific surface area, promoted bubble transport, and enhanced electron mass transfer. The synergistic coupling effect of the catalyst makes it have excellent OER activity and stability. Remarkably, Co–Mo/FeS nanosheets afforded an electrocatalytic OER with a current density of 100 mA cm⁻² at a low overpotential of 321 mV. These discoveries open up new opportunities for the application of doping and template-directed surface reconfiguration, which holds promise as an effective electrocatalyst for the OER.     

Iron-doped metal-organic framework with enhanced oxygen evolution reaction activity for overall water splitting

Water electrolysis is an energy conversion technology to provide green and clean hydrogen energy. Developing a high-efficient and durable electrocatalyst is a critical material for water electrolysis. Therefore, we synthesize a series of iron-doped metal-organic frameworks (MOFs) by a facile one-pot hydrothermal method. In the conventional three-electrode-cell, the Co/Fe (1:1)-MOF catalyst exhibits an overpotential of 317 mV at a current density of 10 mA cm⁻² in the oxygen evolution reaction (OER). Furthermore, the electrolysis performance of Co/Fe (1:1)-MOF catalyst is further evaluated in a home-made anion-exchange-membrane water electrolysis cell. With the Co/Fe (1:1)-MOF as the OER catalyst and commercial Pt/C as the hydrogen-evolution-reaction catalyst, the cell presents an overpotential of 490 mV at a large current density of 500 mA cm⁻², which is superior to the benchmark cell with commercial IrO₂ as the OER catalyst in the alkaline media. Theoretical calculation demonstrates that the introduction of Fe dopant into MOFs significantly reduces the binding energy of 1O and 1OOH intermedium during the OER progress. Consequently, the electrocatalytic activity is increased, which is perfectly consistent with the experimental results. This work suggests that the iron-doped MOFs structure significantly improves the electrocatalytic activity and provides a facile strategy to produce hydrogen at a large current density for industrial water electrolysis.     

Co-based MOF-derived Co/CoN/Co2P ternary composite embedded in N- and P-doped carbon as bifunctional nanocatalysts for efficient overall water splitting

In this work, we developed ternary metallic cobalt-cobalt nitride-dicobalt phosphide composite embedded in nitrogen and phosphorus co-doped carbon (Co/CoN/Co₂P-NPC) as bifunctional catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The as-prepared Co/CoN/Co₂P-NPC is achieved by simultaneous annealing and phosphating of a Co–N rich metal-organic frameworks (MOFs) precursor. Compare with the phosphorus-free Co/CoN embedded nitrogen-doped carbon electrocatalyst (Co/CoN-NC), the as-prepared Co/CoN/Co₂P-NPC display superior HER and OER low overpotential of 99 mV and 272 mV at current density of 10 mA cm⁻². When Co/CoN/Co₂P-NPC electrocatalyst is use as bifunctional catalysts in overall alkaline water splitting, it exhibit excellent behaviour with 10 mA cm⁻² current at overall cell potential of 1.60 V. The excellent performance of Co/CoN/Co₂P-NPC electrocatalyst is attributed to the phosphating process that could further enhance synergistic effect, create stronger electronic interactions, and form efficient dual heteroatom doping to optimize the interfacial adhesion within the electrocatalyst. This present work will create more opportunities for the development of new, promising and more active sites electrocatalysts for alkaline electrolysis.     

Promotional effects of trace Ni on its dual-functional electrocatalysis of Co/N-doped carbon nanotube catalysts for ORR and OER

It is still a great challenge for developing efficient dual-functional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalysts are critical to enhance the efficiency of metal-air cells and fuel cells. In this study, a one-pot vapor deposition method was used to realize the synchronously dope of N and Ni (trace) into Co/C to form Co–Ni (trace)/N-doped carbon nanotubes (Co–Ni (trace)/NCNTs). An interesting result is that injecting dicyandiamide (DCD) into Ni foam as a precursor led to the in situ formation of NCNTs, with synchronous doping of trace Ni into Co species. The cooperative effects of the Co–Ni (trace) and N-doped carbon nanotubes resulted in superior dual-functional electrocatalytic performance of Co–Ni (trace)/NCNTs for the ORR (half-wave potential E_1/2 vs. RHE: 0.83 V, electron transfer number n: 3.97) and OER (overpotential vs. RHE: 337 mV at 10 mA cm^?2, Tafel slope: 94.0 mV dec^?1). Moreover, the Co–Ni (trace)/NCNTs catalyst showed excellent stability during 20,000 s of durability testing for both ORR and OER. This study provides a feasible strategy for designing efficient nonnoble metal-catalysts for renewable energy conversion devices.     

Development of Ni–Fe based ternary metal hydroxides as highly efficient oxygen evolution catalysts in AEM water electrolysis for hydrogen production

A number of mixed metal hydroxide oxygen evolution reaction (OER) catalysts i.e. Ni–Fe, Ni–Co, Ni–Cr, Ni–Mo, Ni–Fe–Co, Ni–Fe–Mo and Ni–Fe–Cr were prepared by cathodic electrodeposition and characterised by SEM, TEM, EDS, XPS and micro X-CT. The compositions of selected catalysts were optimised to give lower OER overpotentials in alkaline media. Further optimisation of Ni–Fe based ternary metal hydroxide catalysts such as Ni–Fe–Co and Ni–Fe–Mo was carried out, showing improved performance at high current densities up to 1 A cm⁻² in 1 M NaOH, 333 K. The influence of electrodeposition parameters such as current density, pH, electrodeposition time and temperature on the electrocatalytic performance of ternary Ni–Fe–Co metal hydroxide was further investigated and optimised. The durability of the optimised catalyst was tested at a current density of 0.5 A cm⁻² in an anion exchange membrane (AEM) water electrolyser cell at 4 M NaOH, 333 K, demonstrating stable performance over 3.5 h.     

Facile modified polyol synthesis of FeCo nanoparticles with oxyhydroxide surface layer as efficient oxygen evolution reaction electrocatalysts

The oxygen evolution reaction (OER) at anode requires high overpotential and is still challenging. The metallic core-oxyhydroxide layer structure is an efficient method to lower an overpotential. We synthesized Fe rich FeCo core-Co rich FeCo oxyhydroxide layer with a different particle size of 173 nm, 225 nm, and 387 nm (FeCo 173, 225, 387) through a difference in the reduction rate of Fe/Co precursors using facile modified polyol synthesis. To investigate the effect of conductivity, CoFe₂O₄ nanoparticles of 80–130 nm were synthesized. Among samples, FeCo 173 showed remarkable catalytic performance of 316 mV at a current density of 10 mA/cm² in 0.1 M KOH compared to RuO₂ (408 mV), FeCo 225 (323 mV), FeCo 387 (334 mV), CoFe₂O₄ (382 mV). Moreover, FeCo 173 showed good stability for 60,000 s while RuO₂ showed a gradual increase in overpotential to maintain 10 mA/cm² after 15,000 s in chronopotentiometry. The excellent performance was attributed to Fe-rich metallic core, a small amount of Fe doping into CoOOH, and the synergic effect between the active site of Co rich FeCoOOH and conductive Fe rich metallic core. Following this result, it shows that the use of such FeCo electrodes has advantages in the production of hydrogen via electrochemical water oxidation.     

Ce and MoS2 dual-doped cobalt aluminum layered double hydroxides for enhanced oxygen evolution reaction

Element doping has become an important means of designing and developing efficient and stable Oxygen Evolution Reaction (OER). In this work, a new type of CoAl LDH doped with Ce and MoS₂ was designed and prepared. The catalyst first prepared Ce-doped CoAl LDH (Ce–CoAl LDH) by co-precipitation and one-step hydrothermal method. Then, MoS₂ was introduced between Ce–CoAl LDH layers by hydrothermal method. The double doping of Ce and MoS₂ could significantly improve the electrocatalytic activity of CoAl LDH. At the current density of 10 mA/cm², the potential of 5% Ce–CoAl LDH@MoS₂ is 1.508 V, which is much lower than the 1.733 V of CoAl LDH. And the ECSA of 5% Ce-CoAL LDH@MoS₂ (23.4 mF/cm²) is nearly 8 times that of CoAl LDH (7.9 mF/cm²). Its excellent OER performance benefits from the increase of active sites and the enhancement of conductivity. This work provides a new research idea for doping design of effective OER electrocatalysts.     

Nanostructure Fe–Co–B/bacterial cellulose based carbon nanofibers: An extremely efficient electrocatalyst toward oxygen evolution reaction

It is very crucial to design and prepare environmentally friendly, efficient and sustainable oxygen evolution reaction (OER) catalysts for water splitting reaction. In this work, a series of Fe–Co–B nanosheets with different molar ratio of Fe/Co precursor have been recombined with bacterial cellulose based carbon nanofiber (BCCNF) through simple electroless deposition method at room temperature. It is indicated that, when the Fe/Co molar ratio is 1:3, Fe–Co–B/BCCNF exhibits an excellent catalytic property with overpotential of only 160 mV at 10 mA cm^?2. Furthermore, it displays a prominent electrochemical stability, even over 30 h of OER process under alkaline condition. The strategy of designing 3D nanostructure and constructing metal-boride-based catalyst for OER provides valuable insights into efficient oxygen evolution.     

Partial crystallization of Co–Fe oxyhydroxides towards enhanced oxygen evolution activity

Developing high-performance noble-metal-free electrocatalysts for the oxygen evolution reaction (OER) is of great significance for the large-scale implement of electrochemical water splitting. Here, we demonstrate that cobalt-iron oxyhydroxide (CoFe_0.8(OH)_x) with appropriate crystallization exhibits excellent OER activity with a low overpotential of only 246 mV, which is much lower than that achieved on amorphous one. Kinetic experiments indicate that the OER active sites should be di-μ-oxo bridged Fe–Co, on which lower energy barrier is attained than that on Fe–O and Co–O sites. Partial crystallization is beneficial to the lattice contraction, the formation of Co–O–Fe sites, and the decrease of charge transfer resistance, thus accelerating OER process.     

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.     

Se-doped cobalt oxide nanoparticle as highly-efficient electrocatalyst for oxygen evolution reaction

Electrolysis of water has been one of the most promising approaches for renewable energy resources while the efficient oxygen evolution reaction (OER) remains challenging. Herein, a series of different ratio of Se doped Co₃O₄ nanoparticles XSe-Co₃O₄ are prepared by hydrothermal method and applied as OER electrocatalysts. Se^2? is doped into the Co₃O₄ crystal lattice by substituting of O^2? and a large number of oxygen vacancies are generated, which provides more available activity sites for OER. Se doping increases the surface ratio of Co²⁺/Co³⁺ and accelerates the electron transport that favors OER activity promotion. The optimized doping ratio of 6%Se–Co₃O₄ presents low overpotential of 281 mV at 10 mA cm^?2, as well as a low Tafel slope of 70 mV dec^?1 in 1 M KOH solution, which has great advantages compared to the recently reported Co₃O₄-based OER electrocatalysts. This work provides new ideas for the development of efficient Co₃O₄-based OER electrocatalysts.     

Anion-modulation in CoMoO4 electrocatalyst for urea-assisted energy-saving hydrogen production

The anode oxygen evolution reaction (OER) is a delayed half-reaction of water splitting that requires a relatively high overpotential. Therefore, a more easily oxidized urea oxidation reaction (UOR) has been implemented to replace OER. Co–Mo-based bimetallic oxides have been recognized as interesting candidates for electrocatalytic water splitting due to their unique d electron configurations, but the low conductivity and limited active sites still hinder their development. Herein, we demonstrated that anion-modulation in CoMoO₄ nanoplates as coupled hydrogen evolution reaction (HER) and UOR for convenient and efficient urea-assisted hydrogen-production system are demonstrated. The findings of the experiments show that nitrogen doping and phosphorus doping exhibit excellent activity toward alkaline HER and UOR, respectively. As a result, the N–CoMoO₄ and P–CoMoO₄ electrode exhibit low potentials of ?0.062 V and 1.251 V (vs. RHE) to reach a current of 10 mA cm^?2 for HER and UOR. The full urea electrolysis is driven by N–CoMoO₄||P–CoMoO₄ executes stably for 24 h at a low potential of 1.41 V. This is a unique anion-modulation method in electrocatalysts to combine hydrogen generation and sewage treatment, which could pave the way for the creation of long-term energy conversion systems.     

Constructing Ag modified NiCoSe2 hybrid nanosheet on 3D-nickel foam as a robust oxygen evolution reaction electrocatalyst

Transition metal selenides are promising oxygen evolution reaction (OER) catalysts, and element doping is a strategy to further enhance their catalytic performance. However, the choice of doping elements and methods needs further study. In this work for cobalt selenide, Ni is has been selected for isomorphic doping to NiCoSe₂ with silver doped in bulk to obtain Ag–NiCoSe₂ nanosheet on NF. The addition of nickel can form a eutectic phase with cobalt to obtain an effective synergistic catalytic effect. The addition of Ag can regulate the adsorption energy of the active site and conductivity. The as-prepared Ag–NiCoSe₂/NF has good catalytic performance for OER, and a current density of 100 mA cm^?2 can be achieved with only 270 mV. Its catalytic ability is far superior to that of non-Ag-doped NiCoSe₂, which is due to the fact that the sheet structure provides more active sites, and the metal doping modulates the electronic configuration, which effectively improves the intrinsic activity. Moreover, Ag–NiCoSe₂/NF can maintain a long-term high-efficiency catalytic process for more than 20 h at a current density of 100 mA cm^?2. Therefore, this work proves that metal doping is of great significance for improving the catalytic activity of transition metal selenides.     

Carbon nanorod supported metal alloy nanocubes using polydopamine as location reagent for water splitting

Transition metal catalysts were supposed to be the most likely substitute for commercial noble metal catalysts, and the development of highly active and long-term catalyst for water splitting are the future trend. Herein, Ni rectangular nitrogen doped carbon nanorods@Fe–Co nanocubes (Ni-CNRs@Fe–Co cubes) were fabricated via a facile template-free method. This simple strategy not only realizes the structure tailoring, but also achieves high-quality nitrogen-doping. Specifically, nickel dimethylglyoxime [Ni(dmg)₂] with rectangular rodlike structure was firstly synthesized by solution method, then metal-organic frameworks Fe–Co nanocube with different contents were loaded on rectangular carbon nanorods with polydopamine as the locating and the connecting agent, and finally Ni-CNRs@xFe-Co cubes were obtained by a one-step calcination. A series of electrochemical tests were researched on materials with different metal contents in the 1 M KOH solution. The Ni-CNRs@Fe–Co cubes show excellent electrocatalytic activity in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). For HER and OER, the Tafel slopes were 83.3 mV dec⁻¹ and 71 mV dec⁻¹, the onset potential were −167 mV and 1.62 V, and reached the current densities of 10 mA cm⁻², the overpotential just needed 196 mV and 433 mV, respectively. This novel synthetic strategy will provide a template-free way for cheap electrocatalysts of non-precious metal for OER and HER.     

The Mo as electron trapper and donor to modulate the electron of CoP2 in phosphating process

To prepare bifunctional electrocatalyst towards HER and OER is extremely important for promoting the development of electrochemical water splitting technology. Herein, the element doping method is employed to tune the electron environment of cobalt phosphide (CoP). The Mo-doped CoP supported on carbon cloth (CC) is constructed by solvothermal and annealing method. The effect of Mo on the electron modulation of CoP during different phosphating time was studied carefully. It is noted that the Mo play an important role in tuning the electron state of Co and P elements which can trap the electron and was reduced to low valence, then transfer the electron to Co and P. With increasing the phosphating time, the electron transfer phenomenon between Mo and CoP is obvious. Benefiting from the electron engineering of Mo, Co and P as well as thin and wrinkle sheets structure, the optimal electrocatalyst only requires 39 mV and 251 mV to deliver 10 mA cm⁻² for HER and OER, respectively. Also, as for the whole water splitting performance, it delivers 10 mA cm⁻² at cell voltage of 1.56 V. Importantly, Faraday efficiency of the optimal catalyst achieves 99.9% for HER due to the tuned electron state of Co and P, high ECSA and low R_ct.