Electrochemical carbon dioxide (CO2) reduction can in principle convert carbon emissions to fuels and value-added chemicals, such as hydrocarbons and alcohols, using renewable energy, but the ...efficiency of the process is limited by its sluggish kinetics1,2. Molecular catalysts have well defined active sites and accurately tailorable structures that allow mechanism-based performance optimization, and transition-metal complexes have been extensively explored in this regard. However, these catalysts generally lack the ability to promote CO2 reduction beyond the two-electron process to generate more valuable products1,3. Here we show that when immobilized on carbon nanotubes, cobalt phthalocyanine-used previously to reduce CO2 to primarily CO-catalyses the six-electron reduction of CO2 to methanol with appreciable activity and selectivity. We find that the conversion, which proceeds via a distinct domino process with CO as an intermediate, generates methanol with a Faradaic efficiency higher than 40 per cent and a partial current density greater than 10 milliamperes per square centimetre at -0.94 volts with respect to the reversible hydrogen electrode in a near-neutral electrolyte. The catalytic activity decreases over time owing to the detrimental reduction of the phthalocyanine ligand, which can be suppressed by appending electron-donating amino substituents to the phthalocyanine ring. The improved molecule-based electrocatalyst converts CO2 to methanol with considerable activity and selectivity and with stable performance over at least 12 hours.
Electrochemical systems, such as fuel cell and water splitting devices, represent some of the most efficient and environmentally friendly technologies for energy conversion and storage. ...Electrocatalysts play key roles in the chemical processes but often limit the performance of the entire systems due to insufficient activity, lifetime, or high cost. It has been a long-standing challenge to develop efficient and durable electrocatalysts at low cost. In this Perspective, we present our recent efforts in developing strongly coupled inorganic/nanocarbon hybrid materials to improve the electrocatalytic activities and stability of inorganic metal oxides, hydroxides, sulfides, and metal–nitrogen complexes. The hybrid materials are synthesized by direct nucleation, growth, and anchoring of inorganic nanomaterials on the functional groups of oxidized nanocarbon substrates including graphene and carbon nanotubes. This approach affords strong chemical attachment and electrical coupling between the electrocatalytic nanoparticles and nanocarbon, leading to nonprecious metal-based electrocatalysts with improved activity and durability for the oxygen reduction reaction for fuel cells and chlor-alkali catalysis, oxygen evolution reaction, and hydrogen evolution reaction. X-ray absorption near-edge structure and scanning transmission electron microscopy are employed to characterize the hybrids materials and reveal the coupling effects between inorganic nanomaterials and nanocarbon substrates. Z-contrast imaging and electron energy loss spectroscopy at single atom level are performed to investigate the nature of catalytic sites on ultrathin graphene sheets. Nanocarbon-based hybrid materials may present new opportunities for the development of electrocatalysts meeting the requirements of activity, durability, and cost for large-scale electrochemical applications.
A hybrid electrocatalyst for the oxygen reduction reaction (ORR), consisting of Co1−xS nanoparticles directly nucleated and grown on sheets of reduced graphene oxide (RGO; see SEM image), was ...prepared by a mild solution‐phase reaction followed by an annealing step. The Co1−xS–RGO hybrid has the highest catalytic performance of all cobalt chalcogenide based ORR catalysts, as revealed inter alia by measurements with a rotating‐disk electrode (see picture; RHE = reversible hydrogen electrode).
Ni(OH)2 nanocrystals grown on graphene sheets with various degrees of oxidation are investigated as electrochemical pseudocapacitor materials for potential energy storage applications. ...Single-crystalline Ni(OH)2 hexagonal nanoplates directly grown on lightly oxidized, electrically conducting graphene sheets (GS) exhibit a high specific capacitance of ∼1335 F/g at a charge and discharge current density of 2.8 A/g and ∼953 F/g at 45.7 A/g with excellent cycling ability. The high specific capacitance and remarkable rate capability are promising for applications in supercapacitors with both high energy and power densities. A simple physical mixture of pre-synthesized Ni(OH)2 nanoplates and graphene sheets shows lower specific capacitance, highlighting the importance of direct growth of nanomaterials on graphene to impart intimate interactions and efficient charge transport between the active nanomaterials and the conducting graphene network. Single-crystalline Ni(OH)2 nanoplates directly grown on graphene sheets also significantly outperform small Ni(OH)2 nanoparticles grown on heavily oxidized, electrically insulating graphite oxide (GO), suggesting that the electrochemical performance of these composites is dependent on the quality of graphene substrates and the morphology and crystallinity of the nanomaterials grown on top. These results suggest the importance of rational design and synthesis of graphene-based nanocomposite materials for high-performance energy applications.
Developing earth‐abundant, active, and stable electrocatalysts for water splitting is a vital but challenging step for realizing efficient conversion and storage of sustainable energy. Here, a ...multiscale structure‐engineering approach to construct iron (Fe) doped cobalt monophosphide (CoP) hybrids for efficient electrocatalysis of water splitting is reported. A two‐step method is developed to synthesize CoP nanosheets with uniform Fe doping and hybridization with carbon nanotubes (CNTs). The nanostructuring, uniform doping, and hybridization with CNT afford efficient electrocatalysts comparable to Pt/C for hydrogen evolution reactions in acidic, neutral, and alkaline electrolytes. It is found that the Fe doping level has different effects on catalytic activities in different electrolytes. Furthermore, after in situ oxidization/hydrolysis of the phosphides to corresponding oxyhydroxides, the hybrid electrocatalysts exhibit better performances than the benchmark commercial Ir/C for catalyzing the oxygen evolution reaction. A two‐electrode alkaline water electrolyzer constructed with these hybrid electrocatalysts can afford a current density of 10 mA cm−2 at a voltage of 1.5 V.
Iron‐doped cobalt monophosphide nanosheet/carbon nanotube hybrids are constructed as electrocatalysts for the hydrogen evolution reaction. They can also be in situ electrochemically transformed into high‐performance electrocatalysts for the oxygen evolution reaction. An alkaline water electrolyzer constructed with the best catalysts among these hybrids could afford a current density of 10 mA cm−2 for overall water splitting at a voltage of 1.50 V.
Molybdenum disulfide (MoS2)‐based materials have been recently identified as promising electrocatalysts for hydrogen evolution reaction (HER). However, little work has been done to improve the ...catalytic performance of MoS2 toward HER in alkaline electrolytes, which is more suitable for water splitting in large‐scale applications. Here, it is reported that the hybridization of 0D nickel hydr(oxy)oxide nanoparticles with 2D metallic MoS2 nanosheets can significantly enhance the HER activities in alkaline and neutral electrolytes. Impressively, the optimized hybrid catalyst can drive a cathodic current density of 10 mA cm−2 at an overpotential of ≈73 mV for HER in 1 m KOH, about 185 mV smaller than the original MoS2. The improved HER activity is attributed to a bifunctional mechanism adopted in these hybrid catalysts, in which nickel hydr(oxy)oxide promotes the water adsorption and dissociation to supply protons for subsequent reactions occurred on MoS2 to generate H2.
1T‐MoS2 nanosheets hybridized with nickel hydr(oxy)oxide nanoparticles can synergistically facilitate the hydrogen evolution reaction (HER) in alkaline and neutral electrolytes through a bifunctional mechanism. The optimized hybrid catalyst can drive a cathodic current density of 10 mA cm−2 at an overpotential of ≈73 mV for HER in 1 m KOH, about 185 mV smaller than the original MoS2.
Metal phthalocyanines (MePcs) have been considered as promising catalysts for CO
2
reduction electrocatalysis due to high turnover frequency and structural tunability. However, their performance is ...often limited by low current density and the performance of some systems is controversial. Here, we report a carbon nanotube (CNT) hybridization approach to study the electrocatalytic performance of MePcs (Me = Co, Fe and Mn). MePc molecules are anchored on CNTs to form the hybrid materials without noticeable molecular aggregations. The MePc/CNT hybrids show higher activities and better stabilities than their molecular counterparts. FePc/CNT is slightly less active than CoPc/CNT, but it could deliver higher Faradaic efficiencies for CO production at low overpotentials. In contrast, the catalytic performance of MePc molecules directly loaded on substrate is hindered by molecular aggregation, especially for FePc and MnPc. Our results suggest that carbon nanotube hybridization is an efficient approach to construct advanced MePc electrocatalysts and to understand their catalytic performance.
Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen ...electrode catalysts with high activity at low cost remains a great challenge. Here, we report a hybrid material consisting of Co₃O₄ nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although Co₃O₄ or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen doping of graphene. The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co₃O₄ and graphene.
Advanced materials for electrocatalytic and photoelectrochemical water splitting are central to the area of renewable energy. In this work, we developed a selective solvothermal synthesis of MoS2 ...nanoparticles on reduced graphene oxide (RGO) sheets suspended in solution. The resulting MoS2/RGO hybrid material possessed nanoscopic few-layer MoS2 structures with an abundance of exposed edges stacked onto graphene, in strong contrast to large aggregated MoS2 particles grown freely in solution without GO. The MoS2/RGO hybrid exhibited superior electrocatalytic activity in the hydrogen evolution reaction (HER) relative to other MoS2 catalysts. A Tafel slope of ∼41 mV/decade was measured for MoS2 catalysts in the HER for the first time; this exceeds by far the activity of previous MoS2 catalysts and results from the abundance of catalytic edge sites on the MoS2 nanoparticles and the excellent electrical coupling to the underlying graphene network. The ∼41 mV/decade Tafel slope suggested the Volmer–Heyrovsky mechanism for the MoS2-catalyzed HER, with electrochemical desorption of hydrogen as the rate-limiting step.
Developing active, stable, and low-cost electrocatalysts which can promote the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in the same electrolyte is undoubtedly a vital ...progress toward a hydrogen economy. Herein, we report that such electrocatalysts can be easily prepared by pyrolyzing a precursor composed of nickel and iron salts with urea under inert atmospheres without any post-treatments. The obtained products are composed of metallic nickel–iron alloy nanoparticles either encapsulated in or dispersed on nitrogen-doped bamboo-like carbon nanotubes (CNTs). This simple synthesis route could simultaneously realize nanostructuring, doping, and hybridizing with nanocarbon, which have been demonstrated as efficient strategies to optimize the catalytic activity of an electrocatalyst. The in situ formed hybrid catalysts exhibit good catalytic performances for both OER and HER under alkaline conditions, and the doping content of iron significantly affects the activities. When the best electrocatalyst is loaded on nickel foam with a loading of 2 mg cm–2, a symmetric two-electrode cell can execute overall water splitting at a current density of 10 mA cm–2 with only 1.58 V and shows negligible degradation after 24 h of operation. The excellent electrocatalytic activity and facile preparation method enable this hybrid electrocatalyst to be a promising candidate for future large-scale applications in water splitting.
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