Recent innovations of silk-derived electrocatalysts for hydrogen evolution reaction,oxygen evolution reaction and oxygen reduction reaction

Numerous researches have proved that heteroatom-doping, especially N-doping, is able to enhance the electrocatalytic performance for carbon materials toward water electrolysis and oxygen reduction reaction. Hence, the production of N-doped carbon materials from cheap and earth-abundant precursor resources such as biomass materials has great potential in the application of these fields. Among various biomass precursors, silk, a N-rich protein, is an ideal candidate for the synthesis of N-doped carbon. Meanwhile, without addition of chemical N-rich precursors during preparation, N-doped carbon derived from silk is clean and easy to synthesize but possesses superb performance. Silk derived catalysts can exhibit overpotential of 61 mV @ 10 mA/cm² (49 mV @ 10 mA/cm² of commercial 20%Pt/C for comparison) and Tafel slope of 89 mV/dec towards hydrogen evolution reaction. To date, great progress has been made in the application of silk-derived catalysts to electrocatalysts, but there exists few summary work. Herein, we summarized recent progress on silk-derived electrocatalysts, focusing on their preparation process and their application on water electrolysis and oxygen reduction reactions. This review provides consolidated accounts of silk-derived catalysts and highlights ideas of their preparation and their performance.     

Second diffusion of Co particles during MoS2 incorporated in N-doped carbon nanotubes towards superior electrochemical activity

Transition metal endowed carbon nanotubes are widely used in electrochemical catalytic reactions. The distribution of transition metal affected their performance. Herein, Co particles were firstly embedded in N-doped carbon nanotubes. During the deposition of molybdenum disulfide (MoS₂), the size of Co particles was drastically decreased. This phenomenon is ascribed to the reaction between Co and S²⁻ ions. Finally, MoS₂/Co/N-doped carbon heterostructures formed. Such heterostructures exhibited excellent activity for electrocatalytic water splitting. It effectively improves the electrocatalytic hydrogen evolution ability of the carbon nanotubes with an over potential of 468 mV at 50 mA/cm² current density. In addition, the formation mechanism of the Mo–N–C interface coupling structure has also been proposed. This unique structure facilitates further design research on carbon materials. It is worth mentioning that nitrogen-doped bamboo-like carbon nanotubes have good oxygen reduction reaction activity, and the half-wave potential can reach 792 mV, the onset potential is 931 mV, and the ultimate diffusion current density is 5.3 mV/cm², which is close to the expensive commercial Pt/C electrodes.     

High active and ultra-stable bifunctional FeNi/CNT electrocatalyst for overall water splitting

Efficient non-noble metal catalysts are desirable to greatly improve the efficiency of anodic oxygen evolution and cathodic hydrogen evolution reactions. Herein, iron-nickel/carbon nanotube composites are synthesized as efficient bifunctional electrocatalysts for water splitting. The catalyst is homogeneously distributed, while the formation of iron-nickel alloy is confirmed. Because of the synergism of iron and copper and the contribution of carbon nanotubes, the Fe–Ni/CNT electrocatalyst shows excellent oxygen evolution reaction performance with the overpotential of 221 mV at 10 mA cm^?2 and maintains stable at 0.48 V for 150 h. It expedites overall water splitting at 10 mA cm^?2 with 1.50 V and show excellent stability at 20 mA cm^?2 for 65 h, providing great potential for large-scale applications.     

Template-free synthesis of biomass-derived hierarchically mesoporous carbon with ultra-small FeNi nanoparticles for oxygen evolution reaction

Highly active and stable catalysts towards electrochemical oxygen evolution reaction are crucial for efficient water splitting and sustainable hydrogen generation. Here we report a carbon supported FeNi catalyst synthesized from an in situ freeze-drying method (fd-FeNi/C) with a commonly seen biomass, jasminum mesnyi flower. The fd-FeNi/C exhibits a N-doped hierarchically mesoporous carbon structure decorated with small FeNi nanoparticles with a small diameter of ∼ 4 nm. Electrochemical measurements show excellent catalytic performance in 1 M KOH solution with an overpotential of 301 mV at the current density of 10 mA cm⁻². The value is 41 mV lower than that of the commercial IrO₂/C. A small Tafel slope (64.5 mV dec⁻¹) and high stability are also recorded. This work provides a facile, scalable, and template-free approach to convert biomass into highly active electrochemical catalysts, which shows great potential for future applications.     

Bio-derived nanoporous activated carbon sheets as electrocatalyst for enhanced electrochemical water splitting

Designing highly efficient and durable metal-free electro-catalysts replacing the precious (non)noble metals is crucial to the future hydrogen economy and various renewable energy conversion and storage devices. Herein, we report an efficient low-cost nanoporous activated carbon sheets (NACS) with hierarchical pore architecture from Indian Ooty Varkey (IOV) food waste for oxygen evolution (OER) and hydrogen evolution reactions (HER) by following “waste to wealth creation” strategy. Characterization of NACS carbo-catalyst reveals the presence of pyridinic-nitrogen inherited by self-doping of N from the biomass with high BET surface area (1478.0 m² g^-1). As an electrocatalyst in alkaline medium, it exhibits low-onset potential (1.36 V vs. RHE), an overpotential (η₁₀) of 0.34 V at 10.0 mA cm⁻² with a small Tafel value (43 mV dec⁻¹), and good stability towards OER compared to Pt or Ir commercial catalysts. Tested as HER catalyst, it displays an impressive HER activity with a low-onset potential of −0.085 V (vs. RHE), and overpotential (η₁₀) of 0.38 V at 10.0 mA cm⁻² with a small Tafel slope of 85 mV dec⁻¹.     

Carbon supported nickel phosphide as efficient electrocatalyst for hydrogen and oxygen evolution reactions

Hydrogen production through water splitting is an efficient and green technology for fulfilling future energy demands. Carbon nanotubes (CNT) supported Ni₂P has been synthesized through a simpler hydrothermal method. Ni₂P/CNT has been employed as efficient electrocatalysts for hydrogen and oxygen evolution reactions in acidic and alkaline media respectively. The electrocatalyst has exhibited low overpotential of 137 and 360 mV for hydrogen and oxygen evolution reactions respectively at 10 mA cm^?2. Lower Tafel slopes, improved electrochemical active surface area, enhanced stability have also been observed. Advantages of carbon support in terms of activity and stability have been described by comparing with unsupported electrocatalyst.     

Three-dimensionally ordered macroporous FeP self-supported structure for high-efficiency hydrogen evolution reaction

Developing noble metal-free hydrogen evolution catalysts with excellent performance is of great significance but still remains a considerable challenge for electrocatalytic water splitting. This paper reports the synthesis of three-dimensionally ordered macroporous iron phosphide self-supported structure on carbon cloth (3DOM-FeP/CC) through an electrostatic self-assembly method along with heat-treatment and phosphidation. The resultant 3DOM-FeP/CC catalyst combined the advantages of the interconnected ordered macropores, the self-supported structure and the highly-conductive carbon substrate, which endowed the catalyst with substantial accessible active sites, abundant mass transport channels for electrolyte/products, and convenient electron transfer pathways. As a result, the 3DOM-FeP/CC catalyst exhibited outstanding catalytic activity for the hydrogen evolution reaction in acid electrolyte, providing a current density of 10 mA cm⁻² at an overpotential of 68 mV with a small Tafel slope of 42 mV dec⁻¹. This research provides a unique strategy to fabricate low-cost and high-efficiency catalysts for generating hydrogen from water.     

N-doped graphene quantum dots incorporated cobalt ferrite/graphitic carbon nitride ternary composite for electrochemical overall water splitting

Multicomponent electrocatalysts containing carbon supports play a crucial role in influencing the hydrogen and oxygen evolution reactions which enhance the total water splitting. Herein, we report a ternary composite with cobalt ferrite, graphitic carbon nitride, and N-doped graphene quantum dots prepared via hydrothermal technique. The purity of the samples is established by carrying out various characterization methods. The intrinsic characteristics of the obtained materials are investigated by employing electrocatalytic processes in an alkaline media toward hydrogen and oxygen evolution reactions. Cobalt ferrite/graphitic carbon nitride/N doped graphene quantum dots electrocatalyst demonstrates a very low overpotential towards hydrogen evolution reaction of 287 mV at a constant 10 mA cm^?2 current density in 1.0 M KOH. Tafel slope and R_ct values generated are 94 mV dec^?1 and 0.86 cm², respectively. Oxygen evolution reaction studies reveal an overpotential of 445 mV at 10 mA cm^?2 with a Tafel slope of 69 mV dec^?1. Finally, the cell potential needed for the cobalt ferrite/graphitic carbon nitride/N doped graphene quantum dots electrode to achieve 10 mA cm^?2 in total water splitting is only 2.0 V while displaying long-term stability.     

Construction of Co-decorated 3D nitrogen doped-carbon nanotube/Ti3C2Tx-MXene as efficient hydrogen evolution electrocatalyst

Rational design of transition metal catalysts with robust and durable electrocatalytic activity for hydrogen evolution reactions (HER) is extremely important for renewable energy conversion and storage, as well as water splitting. Heteroatom doping has emerged as a feasible strategy for enhancing electrocatalytic activity. Here, cobalt nanoparticles (Co-NPs) were coated with nitrogen-doped carbon nanotubes (NCNTs) prepared via an in situ growth on accordion-like Ti₃C₂T_x-MXene (Co-NCNT/Ti₃C₂T_x). Such an intriguing structure showed great features: abundant anchoring sites for NCNT in situ growth, intimate integration of Co-NPs and NCNTs, high-speed electron transfer between 1D NCNTs and 2D Ti₃C₂T_x-MXenes, and a large number of effective catalytic active sites. This Co-NCNT/Ti₃C₂T_x hybrid catalyst was demonstrated to possess excellent HER performance with low overpotential (η₁₀, 190 mV), small Tafel slope (78.4 mV dec⁻¹), large electrochemically active surface area, and good long-term stability, thus outperforming many reported electrocatalysts. The present strategy provided a facile route for the design of transition metal HER catalysts with NCNT and MXene.     

Electrodeposited Ni-Co-Sn alloy as a highly efficient electrocatalyst for water splitting

Water splitting to produce hydrogen and oxygen is considered as a feasible solution to solve the current energy crisis. It is highly desirable to develop inexpensive and efficient electrocatalyst for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this paper, nanostructured Ni-Co-Sn alloys were electrodeposited on copper foil and the excellent electrocatalytic performances for both HER and OER in alkaline media were achieved. The optimized Ni-Co-Sn electrode shows a low onset overpotential of −18 mV and a small Tafel slope of 63 mV/dec for the HER, comparable to many state-of-the-art non-precious metal HER catalysts. For the OER, it produces an overpotential of 270 mV (1.50 V vs. RHE) at current density of 10 mA/cm², which is better than that of the commercial Ir/C catalyst. In addition to high electrocatalytic activities, it exhibits good stability for both HER and OER. This is the first report that Ni-Co-Sn is served as a cost-effective and highly efficient bifunctional catalyst for water splitting and it will be of great practical value.     

Ruthenium quantum dots supported on carbon nanofibers as an efficient electrocatalyst for hydrogen evolution reaction

The hydrogen evolution reaction (HER) is a key step for producing hydrogen by water electrolysis, and an economical, facile and environment friendly method of fabricating catalysts for HER is urgent and essential. In this work, we design a high efficient and stable HER catalyst though a simple adsorption and pyrolysis method. The fabricated catalyst presents ruthenium (Ru) quantum dots (QDs) uniformly distributes on the carbon nanofibers (CNF) with a three dimensional (3D) networks structure (Ru@CNF). By means of quantum size effect of Ru QDs and the 3D networks structure of the carbon nanofibers, the former is beneficial to provide more catalytic active sites and the latter is in favour of electron transport. The sample Ru@CNF exhibits a low overpotential of 20 mV at a current density of 10 mA cm⁻² and Tafel slope of 31 mV dec⁻¹ in 1 M KOH, which is better than that of Pt/C (28 mV and 36 mV dec⁻¹), and most of reported Ru-based and transition metal catalysts. Furthermore, it exhibits robust stability when testing at an overpotential of 75 mV for 24 h. Therefore, this work provides a low-cost, simple and feasible method for fabricating HER catalyst, which possesses commercial application prospect in the field of producing hydrogen by water electrolysis.     

Structural engineering of hierarchically hetestructured Mo2C/Co conformally embedded in carbon for efficient water splitting

Developing low-cost and high efficient electrocatalysts for both oxygen and hydrogen evolution reaction in an alkaline electrolyte toward overall water splitting is still a significant challenge. Here, a novel hierarchically heterostructured catalyst composed of ultrasmall Mo₂C and metallic Co nanoparticles confined within a carbon layer is produced by a facile phase separation strategy. During thermal reduction of CoMoO₄ nanosheets in CO ambient, in-situ generated nanoscale Co and ultrafine Mo₂C conformally encapsulated in a conductive carbon layer. In addition, some carbon nanotubes catalyzed by Co nanoparticles vertically grew on its surface, creating 3D interconnected electron channels. More importantly, the integrated C@Mo₂C/Co nanosheets assembled into the hierarchical architecture, providing abundant active surface and retaining the structural integrity. Benefiting from such unique structure, the constructed hierarchical heterostructure shows low overpotentials of 280 mV and 145 mV to reach a current density of 10 mA cm⁻² for OER and HER in an alkaline electrolyte. Furthermore, the symmetrical electrolyzer assembled with catalyst exhibits a small cell voltage of 1.67 V at 10 mA cm⁻² in addition to outstanding durability, demonstrating the great potential as a high efficient bifunctional electrocatalyst for overall water splitting.     

Bifunctional electrocatalysts for water splitting from a bimetallic (V doped-NixFey) Metal–Organic framework MOF@Graphene oxide composite

An ongoing challenge still lies in the exploration of proficient electrocatalysts from earth-abundant non-precious metals instead of noble metal-based catalysts for clean hydrogen energy through large-Scale electrochemical water splitting. However, developing a non-precious transition metals based, stable electrocatalyst for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is important challenge for modern energy conversion technology. In this report Vanadium doped bimetallic nickel-iron nanoarray, fabricated by carbon supported architecture through carbonization process for electrochemical water splitting. Three types of catalysts were prepared in different molar ratio of Ni/Fe. The electrocatalytic performance demonstrated that the catalyst with equal mole ratio (0.06:0.06) of Ni/Fe possess high catalytic activity for both OER and HER in alkaline and acidic medium. Besides, our findings revealed that the doping of vanadium could play a strong synergetic effect with Ni/Fe, which provide a small overpotential of 90 mV and 210 mV at 10 mA cm^?2 for HER and OER respectively compared to the other two catalyst counterparts. Also, the catalyst with 1:1 (Ni/Fe) molar ratio showed a high current density of 208 mA cm^?2 for HER at 0.5 M H₂SO₄ and 579 mA cm^?2 for OER at 1 M KOH solution, the both current densities are much higher than the other two catalysts (different Ni/Fe ratio). In addition, the presented catalysts showed extremely good durability, reflecting in more than 20 h of consistent Chronoamprometry study at fixed overpotential η = 250 mV without any visible voltage elevation. Similarly, the (Ni/Fe) equal ratio catalyst showed better corrosion potential 0.209 V vs Ag/AgCl and lower current density 0.594 × 10^?12 A cm^?2 in high alkaline medium. The V-doping, MOF/GO surface defects are significantly increased the corrosion potential of the V-Ni_xFe_y-MOF/GO electrocatalyst. Besides, the water electrolyzed products were analysed by gas chromatography to get clear insights on the formed H₂ and O₂ products.     

In-situ construction and activities of La0·8Sr1·2Fe0·5Ni0·5O4+δ@CNTs hybrids as bifunctional oxygen catalysts

Perovskite oxides are proved to be promising oxygen bifunctional catalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Constructing a perovskite oxide/carbon composite with intimate interaction at the interface is of great importance for conductivity and bifunctional oxygen activities. In this work, combining “exsolution effect” of perovskite oxide at high temperature and reducing atmosphere, carbon nanotubes (CNTs) are in-situ synthesized on the surface of a La_0·8Sr_1·2Fe_0·5Ni_0·5O_4+δ (LSFN) perovskite oxide with K₂NiF₄ structure via a simple chemical vapor deposition (CVD) method. Physical characterizations show that CNTs are twining around the surface of LSFN particles with strong interaction. Under the function of synergistic effect between LSFN and CNTs, more mobile oxygen species, improved surface electronic structure and optimized charge distribution and transformation are obtained. Finally, the as-prepared LSFN@CNTs composites exhibit superior oxygen electrocatalytic performances in alkaline solution, with an ORR overpotential of 761 mV at ?1.0 mA cm^?2, a small OER overpotential of 314 mV at 10 mA cm^?2 and an enhanced cycling stability of >3000 cycles, which outperforms commercial IrO₂ catalyst and published perovskite oxide based bifunctional oxygen catalysts.     

Bifunctional nanoporous ruthenium-nickel alloy nanowire electrocatalysts towards oxygen/hydrogen evolution reaction

A class of ruthenium-nickel alloy catalysts featured with nanoporous nanowires (NPNWs) were synthesized by a strategy combining rapid solidification with two-step dealloying. RuNi NPNWs exhibit excellent electrocatalytic activity and stability for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in which the RuNi-2500 NPNWs catalyst shows an OER overpotential of 327 mV to deliver a current density of 10 mA cm^?2 and the RuNi-0 NPNWs catalyst requires the overpotential of 69 mV at 10 mA cm^?2 showing the best HER activity in alkaline media. Moreover, the RuNi-1500 NPNWs catalyst was used as the bifunctional electrocatalyst in a two-electrode alkaline electrolyzer for water splitting, which exhibits a low cell voltage of 1.553 V and a long-term stability of 24 h at 10 mA cm^?2, demonstrating that the RuNi NPNWs catalysts can be considered as promising bifunctional alkaline electrocatalysts.     

Electrodeposition of the manganese-doped nickel-phosphorus catalyst with enhanced hydrogen evolution reaction activity and durability

Hydrogen technology is widely considered a novel clean energy source, and electrolysis is an effective method for hydrogen evolution. Therefore, efficient hydrogen evolution reaction (HER) catalysts are urgently needed to replace precious metal catalysts and meet ecological and environmental protection standards. Herein, Ni–Mn–P electrocatalysts are synthesized using facile electrodeposition technology. The influence of the Mn addition on the catalytic behavior is studied by the comprehensive analysis of catalytic performance and morphology of the catalysts. Among them, the Ni–Mn–P0.01 catalyst exhibits small coral-like structures, greatly improving the adsorption and desorption of hydrogen ions and reducing the overpotential hydrogen evolution. Consequently, overpotential at 10 mA cm^?2 electric current density is 113 mV, and the value of the Tafel slope achieves 74 mV/dec. Furthermore, the Ni–Mn–P catalyst shows long-time (20 h) stability at current densities of 10 and 60 mA/cm². The results confirm that the synergistic effect of Ni, Mn, and P accelerates the electrochemical reaction. Meanwhile, the addition of manganese element can change the micromorphology of the catalyst, thereby exposing more active sites to participate in the reaction, enhancing water ionization, improving the catalytic performance. This study opens a new way toward improving the activity of the catalyst by adjusting Mn concentration during the electrodeposition process.     

Bimetallic Fe–Co chalcogenophosphates as highly efficient bifunctional electrocatalysts for overall water splitting

Fabrication of multicomponent materials is the most effective strategy to develop high-performance multifunctional catalysts. In this work, a series of bimetallic Fe–Co chalcogenophosphates were facilely prepared and used as bifunctional water electrolysis catalysts. The results have shown that the obtained catalysts showed high performances for hydrogen and oxygen evolution reactions, and overall water splitting. For the optimum catalyst, only 260 and 365 mV of overpotential for HER and OER, and 1.59 V of cell voltage for water splitting was needed respectively in 1 M KOH when 10 mA cm^?2 of current density was reached. High stability and Faraday efficiency were also obtained, and the obtained results confirm that the catalyst is competitive in application in water electrolysis.     

Polyoxometalate@ZIF-67 derived carbon-based catalyst for efficient electrochemical overall seawater splitting and oxygen reduction

Electrochemical water splitting, metal-air batteries and fuel cells are the representatives of energy storage/conversion technology, which can solve the shortage of traditional fossil energy and environmental issues. However, under neutral conditions, especially in natural seawater, there is still a lack of catalysts with efficient catalytic activity and stability, which hinders the development of hydrogen production. We herein designed a facile strategy to fabricate an efficient and stable multifunctional carbon-based electrocatalyst, PMA@ZIF-67-C-AT, by carbonization and acid treatment of H₃PMo₁₂O₄₀@ZIF-67 precursor. It not only performs good catalytic activity for oxygen evolution in 0.2 M phosphate buffer solution (PBS) and actual seawater, but also exhibits hydrogen evolution overpotential of 650 mV (j = 3 mA/cm²) in PBS and even as low as 570 mV (j = 10 mA/cm²) in seawater. Besides, it also displays excellent catalytic performance for oxygen reduction (E_1/2 at 0.83 V and the Tafel slope of 63 mV/dec). By investigating the composition and morphology of the materials, it was found that acid treatment changed the active components and carbon matrix of the catalysts, thus affecting the catalytic performance.     

Fe-doped NiCo2S4 catalyst derived from ZIF – 67 towards efficient hydrogen evolution reaction

The development of economical, efficient and stable non-noble metal catalysts plays a key role in electrocatalytic hydrogen evolution. NiCo₂S₄ has been proved to be an efficient non-noble catalyst, to further improve its electrocatalytic performance is a meaningful work. In this paper, the effects of Fe doping on electrochemical performance of NiCo₂S₄ is investigated. The Fe-doped NiCo₂S₄ catalyst is prepared by a facile solvothermal method with metal-organic-framework (MOF, ZIF-67) as template, and it exhibits an improved hydrogen evolution reaction (HER) performance with an overpotential of 181 mV at 10 mA cm^?2, a Tafel slope of 125 mV dec^?1 compared with that of NiCo₂S₄ (252 mV overpotential and 149 mV dec^?1 Tafel slope). The combination of improved conductivity, mesopores architecture retained with the ZIF-67 template, which result the reduced internal resistance, enhanced charge transportation as well as large electrochemical double-layer capacitance. This work provides an effective and synergistic strategy for fabricating NiCo₂S₄-based catalysts toward electrochemical water splitting.