Xiumin Li

Nanostructured catalysts for electrochemical water splitting: current state and prospects

Hydrogen is an ideal candidate for the replacement of fossil fuels in the future due to zero emission of carbonaceous species during its utilization. Water electrolysis is a dependable link of primary renewable energy and stable hydrogen energy. In this work, the fundamentals of water electrolysis, current popular electrocatalysts developed for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) in liquid electrolyte water electrolysis are reviewed. The main HER catalysts include noble metals, non-noble metals and composites, noble metal-free alloys, metal carbides, chalcogenides, phosphides and metal-free materials while the OER catalysts are focused on efficient Co-based, Ni-based materials and layered double hydroxide (LDH) materials. The strategies to improve catalytic activity, long-term durability and endurance to electrochemical erosion are introduced. The main challenges and future prospects for the further development of electrodes for water electrolysis are discussed. It is expected to give guidance for the development of novel low-cost nanostructured electrocatalysts for electrochemical water splitting.

Homogeneous nanosheet Co3O4 film prepared by novel unipolar pulse electro-deposition method for electrochemical water splitting

A unipolar pulse electro-deposition (UPED) method is used to prepare nanosheet Co3O4 film on a carbon rod substrate in aqueous solution and compared with other conventional methods, i.e., the cyclic voltammetry (CV) method, potentiostatic method (PM) and pulse potentiostatic method (PPM). Co3O4 films prepared by the UPED method show a more uniform structure on carbon rod and higher catalytic activity than those by other methods, and oxygen evolution reaction over it with an overpotential of 275 ± 2.3 mV results in a current density of 10 mA cm−2 in 1.0 mol L−1 KOH. Such an excellent performance should be attributed to its highly porous nanosheet structure with honeycomb-like morphology.

Cobalt hydroxide [Co(OH)2] loaded carbon fiber flexible electrode for high performance supercapacitor

Cobalt hydroxide nanoflakes are uniformly loaded on flexible carbon fiber (CF) paper, and provide good electrical connectivity to the current collector for use as supercapacitors. The unique porous nanostructure offers low ion diffusion and charge transfer resistance in the electrode. The effect of loading mass on electrochemical properties is investigated. The electrode with a mass loading of 2.5 mg cm−2 shows the maximum specific capacitance of 386.5 F g−1 at a current density of 1 mA cm−2. Also the same electrode provides a good rate capability with energy and power densities of 133.5 W h kg−1 and 1769 W kg−1, respectively even at a higher current density of 10 mA cm−2. The electrode reveals a cyclic stability of 92% over 2000 cycles. This kind of flexible, lightweight electrode could be effectively utilized for flexible supercapacitor fabrication, especially for wearable electronics.

A novel electroactive λ-MnO2/PPy/PSS core–shell nanorod coated electrode for selective recovery of lithium ions at low concentration

A novel electroactive Li+ ion-imprinted hybrid film consisting of λ-MnO2/PPy/PSS core–shell nanorods is successfully fabricated on an electrode by using the unipolar pulse electrodeposition (UPED) technique. When the electrode is applied for selective electrochemical extraction of low concentrations of Li+ ions from aqueous solutions via an electrochemically switched ion exchange (ESIX) process, the Li+ ion adsorption capacity reaches 35.2 mg g−1 with an adsorption equilibrium time of less than 2 h. The excellent ion separation performance of this hybrid film should be attributed to its low ion transfer resistance due to its porous structure and the high electric driving force during the ESIX process. In particular, owing to the unique Li+ ion imprinted vacant sites in the crystal structure of spinel λ-MnO2 nanorods, the selectivity factor for Li+/Na+ reaches 46.0 with a molar ratio of 1 : 1. It is expected that this λ-MnO2/PPy/PSS hybrid film can be applied as a promising electroactive material for effective separation of Li+ ions from seawater.

Embedded structure catalyst: a new perspective from noble metal supported on molybdenum carbide

An embedded structure of noble metal (Pt) in situ supported on molybdenum carbide was found for the first time, which hindered Pt sintering at high temperature, and promoted the interaction between Pt and molybdenum carbide. This structure exhibited an excellent and stable catalytic activity for water gas shift reaction at low temperature.

Facile fabrication of CuO microcube@Fe–Co3O4 nanosheet array as a high-performance electrocatalyst for the oxygen evolution reaction

A “top-down” tactic was offered to synthesize highly efficient electrocatalysts for the oxygen evolution reaction (OER), in which a CuO microcube supported Fe doped Co3O4 nanosheet composite was obtained using a facile unipolar pulse electrodeposition (UPED) method combined with a thermal treatment method. Specifically, an Fe doped Co(OH)2 shell was electrodeposited on the electrode with a Cu particle core simultaneously. Interestingly, it is found that Fe–Co(OH)2 nanosheets grew around the Cu particle core. As a result, a novel electrocatalyst with the Cu cube core and the Fe–Co(OH)2 nanosheet shell was successfully formed in one step. After subsequent thermal oxidation treatment, the CuO microcube@Fe–Co3O4 nanosheet core–shell composite was obtained on the electrode. Benefiting from the one-step electrodeposition, the composite electrode with a smart 3D hierarchical structure as well as the intrinsic activity of Fe–Co3O4 nanosheets showed a remarkably enhanced catalytic performance for OER, which revealed a low overpotential of 232 mV at 10 mA cm−2 current density.

Silver-doped molybdenum carbide catalyst with high activity for electrochemical water splitting

A hybrid catalyst composed of silver (Ag) doped wire-like molybdenum carbide (MoxCy) with pure β-phase and carbon nanotubes (CNTs) was coated well on a carbon rod electrode for the hydrogen evolution reaction (HER). The effects of Ag loading amount and carbonization temperature on the crystal form of MoxCy were investigated in detail. It is found that the MoxCy crystal form can be tuned by adjusting the preparation conditions, and nanostructured wire-like Mo2C with pure β-phase was obtained at a temperature over 750 °C. Ag/MoxCy composite nanomaterials were investigated by X-ray diffraction, UV/vis spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy and Brunauer-Emmett-Teller surface area analysis. The hybrid catalyst was further deposited on the carbon nanotube (CNT) modified carbon rod substrate. Due to the high surface area and 3D porous network-like microstructure, the Ag/Mo2C/CNTs hybrid electrode showed enhanced catalytic performance when comparing with the corresponding pure one. Particularly, for the Ag-doped Mo2C/CNTs hybrid electrode with an optimum 1 Ag : 5 Mo molar ratio of the precursors, a current density of 10 mA cm-2 was obtained by applying an overpotential of 142 mV in 0.5 mol L-1 H2SO4 solution. It is expected that such a hybrid electrode can be widely applied for effective electrolysis of water to produce hydrogen.

Mn doped CoP nanoparticle clusters: an efficient electrocatalyst for hydrogen evolution reaction

Since metal cations in layered double hydroxide materials (LDHs) can be substituted partially by M3+ ions, LDHs can be applied as ideal precursors for the preparation of foreign metal doped phosphides. In this work, a Mn doped cobalt phosphide (CoP) electrocatalyst coated electrode was prepared from CoMn LDH precursors by a unipolar pulse electro-deposition (UPED) method combined with a thermal phosphorization process. Experimental results indicated that the Mn-CoP coated electrode needed a much lower overpotential than the original CoP coated one in the same current density for hydrogen evolution reaction (HER) in water splitting. The CoP coated electrode required an overpotential of 127 mV to support 10 mA cm−2 current density in 0.5 M H2SO4 solution, and in contrast, the Mn-CoP coated one required only 108 mV overpotential to support the same current density. Furthermore, in the 1.0 M KOH solution, the Mn-CoP coated electrode revealed better performance, and the overpotential at 10 mA cm−2 current density was reduced to as low as 95 mV.