Xiangye Liu

Niobium Nitride Nb4N5 as a New High-Performance Electrode Material for Supercapacitors

Supercapacitors suffer either from low capacitance for carbon or derivate electrodes or from poor electrical conductivity and electrochemical stability for metal oxide or conducting polymer electrodes. Transition metal nitrides possess fair electrical conductivity but superior chemical stability, which may be desirable candidates for supercapacitors. Herein, niobium nitride, Nb4N5, is explored to be an excellent capacitive material for the first time. An areal capacitance of 225.8 mF cm−2, with a reasonable rate capability (60.8% retention from 0.5 to 10 mA cm−2) and cycling stability (70.9% retention after 2000 cycles), is achieved in Nb4N5 nanochannels electrode with prominent electrical conductivity and electrochemical activity. Faradaic pseudocapacitance is confirmed by the mechanistic studies, deriving from the proton incorporation/chemisorption reaction owing to the copious +5 valence Nb ions in Nb4N5. Moreover, this Nb4N5 nanochannels electrode with an ultrathin carbon coating exhibits nearly 100% capacitance retention after 2000 CV cycles, which is an excellent cycling stability for metal nitride materials. Thus, the Nb4N5 nanochannels are qualified for a candidate for supercapacitors and other energy storage applications.

Rational composition and structural design of in situ grown nickel-based electrocatalysts for efficient water electrolysis

Earth-abundant and highly efficient electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are desired for water-splitting to produce hydrogen. Some nickel-based materials are usually used in water-alkaline electrolysis, but their composition and structure are still not optimized. In this work, porous aligned flake arrays of Ni-embedded NiO (Ni/NiO) and single-crystalline NiFe layered double hydroxide (LDH) are proposed to be HER and OER electrocatalysts to produce H2 and O2, respectively. The former catalyst, fabricated by non-contact Al-reduction of nickel hydroxide precursors, showed high HER activity, approaching that of commercial Pt/C. The latter catalyst, prepared by the fluorinion-assisted hydrothermal method, possessed higher activity for the OER than the well-known RuO2. The water-alkaline electrolyser assembled by the arrays of Ni/NiO and NiFe LDH in 1 M NaOH exhibits an ultra-small cell voltage of 1.52 V at a current density of 20 mA cm−2 at room temperature, as well as good long-term stabilities. These high performances of our nickel-based arrays result from their improved charge transfer and mass transport, and faster kinetics of catalytic reactions. So the arrays of Ni/NiO and NiFe LDH are promising in the application of water-splitting devices.

A Robust and Conductive Black Tin Oxide Nanostructure Makes Efficient Lithium-Ion Batteries Possible

SnO2 -based lithium-ion batteries have low cost and high energy density, but their capacity fades rapidly during lithiation/delithiation due to phase aggregation and cracking. These problems can be mitigated by using highly conducting black SnO2-x , which homogenizes the redox reactions and stabilizes fine, fracture-resistant Sn precipitates in the Li2 O matrix. Such fine Sn precipitates and their ample contact with Li2 O proliferate the reversible Sn → Li x Sn → Sn → SnO2 /SnO2-x cycle during charging/discharging. SnO2-x electrode has a reversible capacity of 1340 mAh g-1 and retains 590 mAh g-1 after 100 cycles. The addition of highly conductive, well-dispersed reduced graphene oxide further stabilizes and improves its performance, allowing 950 mAh g-1 remaining after 100 cycles at 0.2 A g-1 with 700 mAh g-1 at 2.0 A g-1 . Conductivity-directed microstructure development may offer a new approach to form advanced electrodes.

Organic–inorganic halide perovskite based solar cells – revolutionary progress in photovoltaics

Photovoltaic technology has been presented with a great opportunity for development, owing to the recent and unprecedented rapid development of a new-type of solar cell based on organic–inorganic halide perovskites. Their power conversion efficiency (η) has surpassed 19% since the first perovskite-based solar cell (η = 3.8%) was reported in 2009. Moreover, this performance seems to be still far from fully optimized because of its versatile fabrication techniques and device configurations. In this review, the history of perovskites for photovoltaic applications and the landmark achievements to date are briefly outlined. Focusing on these new halide perovskite solar absorbers, the crystal structure, electronic structure, and intrinsic physical properties are systematically described, in an attempt to unravel the origins of superior solar cell performance. To meet the requirements of high-efficiency photovoltaics, the unique solar perovskite absorbers and electron and hole transport materials are discussed, as well as some unanswered questions and challenges facing their further development and commercialization.

Rational design of cobalt–chromium layered double hydroxide as a highly efficient electrocatalyst for water oxidation

The design of a high performance, stable and cost-effective electrocatalyst for oxygen evolution is crucial for H2 production from electrochemical water splitting. Here, as a dual-functional-site catalyst, cobalt–chromium layered double hydroxide (CoCr LDH) nanosheets are designed and synthesized, where Co2+ is the catalytically active site and Cr3+ is the charge transfer site. OER investigation of CoCr LDH is conducted for the first time. The CoCr LDH nanosheets have a high specific surface area of 151.78 m2 g−1 and exhibit outstanding OER activities, among the best of Co-based candidates. Accordingly, our catalyst affords a low onset potential of 1.47 V (vs. reversible hydrogen electrode, RHE) and a stable current density of 22.8 mA cm−2 at 1.61 V (vs. RHE) for 12 h. The Tafel slope of CoCr LDH is 81.0 mV dec−1, smaller than that of state-of-the-art RuO2 (90.1 mV dec−1). Therefore, the CoCr LDH nanosheets are promising OER catalysts.

An electron injection promoted highly efficient electrocatalyst of FeNi3@GR@Fe-NiOOH for oxygen evolution and rechargeable metal–air batteries

Efficient catalysts for oxygen evolution reactions (OERs) are a key renewable energy technology for fuel cells, metal–air batteries and water splitting, but few non-precious oxygen electrode catalysts with high activity have been discovered. Here, we propose a general strategy based on electron injection to manipulate the work function of electrocatalysts to obtain an extraordinary performance beyond precious catalysts. Based on the density functional theory calculation, the NiOOH/Ni hybrid reveals the smallest overpotential compared to NiOOH. A novel hybrid catalyst is designed to grow Fe-doped NiOOH on graphene-encapsulated FeNi3 nanodots (FeNi3@GR@Fe-NiOOH). Accordingly, the catalyst exhibits excellent OER activity and superior durability, affording a low onset potential of 1.45 V vs. reversible hydrogen electrode (RHE) and a stable current density of 11.0 mA cm−2 at 1.6 V (vs. RHE) for over 12 h. The achieved turnover frequency of 1.16 s−1 at an overpotential of 300 mV is the best performance among the reported similar catalysts, and even better than that of the state-of-the-art noble-metal catalysts (RuO2 and IrO2). The high electrocatalytic efficiency and robust durability are essential conditions for a superior air electrode material for Zn–air batteries. Our catalyst cycled stably for 360 cycles at 1 mA cm−2 in 20 h with no obvious attenuation over 100 cycles for 100 h.

Ti3+-Promoted High Oxygen-Reduction Activity of Pd Nanodots Supported by Black Titania Nanobelts

One-dimensional nanocrystals favoring efficient charge transfer have attracted enormous attentions, and conductive nanobelts of black titania with a unique band structure and high electrical conductivity would be interestingly used in electrocatalysis. Here, Pd nanodots supported by two kinds of black titania, the oxygen-deficient titania (TiO2-x) and nitrogen-doped titania (TiO2-x:N), were synthesized as efficient composite catalysts for oxygen-reduction reaction (ORR). These composite catalysts show improved catalytic activity with lower overpotential and higher limited current, compared to the Pd nanodots supported on the white titania (Pd/TiO2). The improved activity is attributed to the relatively high conductivity of black titania nanobelts for efficient charge transfer (CT) between Ti3+ species and Pd nanodots. The CT process enhances the strong metal-support interaction (SMSI) between Pd and TiO2, which lowers the absorption energy of O2 on Pd and makes it more suitable for oxygen reduction. Because of the stronger interaction between Pd and support, the Pd/TiO2-x:N also shows excellent durability and immunity to methanol poisoning.

Hierarchical Ni/NiTiO3 derived from NiTi LDHs: a bifunctional electrocatalyst for overall water splitting

Developing economical and stable bifunctional electrocatalysts for overall water splitting is of enormous importance for sustainable energy systems. Here, Ni/NiTiO3 with a villiform structure assembled by nanosheets is presented as an efficient bifunctional electrocatalyst. The Ni/NiTiO3 with a molar ratio of 15 : 1 (Ni : Ti) manifests remarkable catalytic performance for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in alkaline solutions, with onset overpotentials of 270 mV for the OER and ∼50 mV for the HER. Large amounts of Ni nanoparticles inset in the NiTiO3 nanosheets possess a high specific surface of 28.4 m2 g−1 which is far greater than that of bare Ni (7.7 m2 g−1). The results of long-term OER operation confirm that the introduction of Ti4+ is favorable for enhanced stability. When the catalyst is employed as both the cathode and anode for overall water splitting, it displays an onset potential of 1.55 V in 1 M KOH solution which can rival that of the integrated performance of Pt/C and RuO2. Hence, our Ni/NiTiO3 electrocatalysts have enormous potential for realistic large-scale water splitting.

Co nanoparticles embedded in a 3D CoO matrix for electrocatalytic hydrogen evolution

Earth-abundant and highly efficient electrocatalysts for the hydrogen evolution reaction (HER) are desired for hydrogen production from water-splitting. Here, Co nanoparticles were embedded in the 3D CoO matrix via a template-free method, including cobalt hydroxy-carbonate nanowire arrays grown on Ni foam and the following non-contact Al-reduction process. The as-prepared 3D hierarchical structured Co/CoO nanowires possess good charge transfer and mass transport properties, and a synergistic effect at the Co/CoO interface can hugely facilitate the HER kinetics. A suitable balance between Co and CoO in the catalyst is crucial for high catalytic activity. And the optimal Co/CoO array exhibited outstanding HER activities in 1 M NaOH, achieving nearly zero onset potential, and a current density of 100 mA cm−2 with a small overpotential of 167 mV. They also showed good long-term stabilities. This hybrid Co/CoO nanowire array is a promising material for large-scale hydrogen production from water-splitting.

In situ grown Nb4N5 nanocrystal on nitrogen-doped graphene as a novel anode for lithium ion battery

The metal-rich niobium nitride of Nb4N5 has higher conductivity than Nb3N5 and a higher theoretical specific capacity than NbN. To rationally design a metal-rich anode material, Nb4N5 nanocrystals coated by nitrogen-doped graphene (N-G) have been successfully synthesized by a facile in situ ice bathing method with subsequent annealing in NH3. The use of these as an anode material is reported for the first time. The discharge capacity is 487 mA h g−1 at the current density of 0.1 A g−1 (0.0819 mA cm−2) after 200 cycles and the high rate discharge capacity is 125 mA h g−1 at a current density of 5 A g−1 (4.0926 mA cm−2). Specially, the discharge capacity is still enhanced after 200 cycles at 0.1 A g−1 (0.0819 mA cm−2). The Nb4N5/N-G hybrid could be a promising anode material for LIBs with a high rate performance and long cycle life.