Xuqiang Ji

Bimetallic Nickel-Substituted Cobalt-Borate Nanowire Array: An Earth-Abundant Water Oxidation Electrocatalyst with Superior Activity and Durability at Near Neutral pH

There is an urgent demand to develop earth-abundant electrocatalysts for efficient and durable water oxidation under mild conditions. A nickel-substituted cobalt-borate nanowire array is developed on carbon cloth (Ni-Co-Bi/CC) via oxidative polarization of NiCo2 S4 nanoarray in potassium borate (K-Bi). As a bimetallic electrocatalyst for water oxidation, such Ni-Co-Bi/CC is superior in catalytic activity and durability in 0.1 m K-Bi (pH: 9.2), with a turnover frequency of 0.33 mol O2 s-1 at the overpotential of 500 mV and nearly 100% Faradaic efficiency. To drive a geometrical catalytic current density of 10 mA cm-2 , it only needs overpotential of 388 mV, 34 mV less than that for Co-Bi/CC, outperforming reported non-noble-metal catalysts operating under benign conditions. Notably, its activity is maintained over 80 000 s. Density functional theory calculations suggest that the O* to OOH* conversion is the rate-determining step and Ni substitution decreases the free energy on Co-Bi from 2.092 to 1.986 eV.

Electrochemical Ammonia Synthesis via Nitrogen Reduction Reaction on a MoS2 Catalyst: Theoretical and Experimental Studies

The discovery of stable and noble-metal-free catalysts toward efficient electrochemical reduction of nitrogen (N2 ) to ammonia (NH3 ) is highly desired and significantly critical for the earth nitrogen cycle. Here, based on the theoretical predictions, MoS2 is first utilized to catalyze the N2 reduction reaction (NRR) under room temperature and atmospheric pressure. Electrochemical tests reveal that such catalyst achieves a high Faradaic efficiency (1.17%) and NH3 yield (8.08 × 10-11 mol s-1 cm-1 ) at -0.5 V versus reversible hydrogen electrode in 0.1 m Na2 SO4 . Even in acidic conditions, where strong hydrogen evolution reaction occurs, MoS2 is still active for the NRR. This work represents an important addition to the growing family of transition-metal-based catalysts with advanced performance in NRR.

Co(OH)2 Nanoparticle‐Encapsulating Conductive Nanowires Array: Room‐Temperature Electrochemical Preparation for High‐Performance Water Oxidation Electrocatalysis

It is highly desired but still remains challenging to design and develop a Co-based nanoparticle-encapsulated conductive nanoarray at room temperature for high-performance water oxidation electrocatalysis. Here, it is reported that room-temperature anodization of a Co(TCNQ)2 (TCNQ = tetracyanoquinodimethane) nanowire array on copper foam at alkaline pH leads to in situ electrochemcial oxidation of TCNQ- into water-insoluable TCNQ nanoarray embedding Co(OH)2 nanoparticles. Such Co(OH)2 -TCNQ/CF shows superior catalytic activity for water oxidation and demands only a low overpotential of 276 mV to drive a geometrical current density of 25 mA cm-2 in 1.0 m KOH. Notably, it also demonstrates strong long-term electrochemical durability with its activity being retrained for at least 25 h, a high turnover frequency of 0.97 s-1 at an overpotential of 450 mV and 100% Faradic efficiency. This study provides an exciting new method for the rational design and development of a conductive TCNQ-based nanoarray as an interesting 3D material for advanced electrochemical applications.

MoO3 nanosheets for efficient electrocatalytic N2 fixation to NH3

The synthesis of NH3 heavily depends on the energy-intensive Haber–Bosch process with a large amount of greenhouse gas emission. Electrochemical reduction offers a carbon-neutral process to convert N2 to NH3 at ambient conditions, but requires efficient and stable catalysts for the N2 reduction reaction. Mo-dependent nitrogenases and synthetic molecular complexes have attracted increasing attention for N2 fixation; however, less attention has been paid to Mo-based nanocatalysts for electrochemical N2 conversion to NH3. Herein, we report that MoO3 nanosheets act as an efficient non-noble-metal catalyst for electrochemical N2 fixation to NH3 with excellent selectivity at room temperature and atmospheric pressure. In 0.1 M HCl, this catalyst exhibits remarkable NRR activity with an NH3 yield of 4.80 × 10−10 mol s−1 cm−2 (29.43 μg h−1 mgcat.−1) and a faradaic efficiency of 1.9%. Moreover, this catalyst also shows high electrochemical stability and durability. Density functional theory calculations reveal that the outermost Mo atoms serve as the active sites for effective N2 adsorption.

Remarkable enhancement of the alkaline oxygen evolution reaction activity of NiCo2O4 by an amorphous borate shell

It is highly desirable but still a challenging task to find a simple, fast and straightforward method to greatly improve the alkaline oxygen evolution reaction (OER) performance of a NiCo2O4 catalyst. In this communication, we demonstrate that developing an amorphous borate shell on a NiCo2O4 surface can boost its OER activity in alkaline media. As a 3D catalyst electrode, a NiCo2O4@Ni–Co–B nanoarray on carbon cloth needs an overpotential of only 270 mV to achieve a geometrical catalytic current density of 10 mA cm−2 in 1.0 M KOH, which is 100 mV less than that for a NiCo2O4 nanoarray. Notably, this electrode also demonstrates strong electrochemical durability, maintaining its activity for at least 100 h. The superior activity of NiCo2O4@Ni–Co–B is attributed to the amorphous Ni–Co–B shell on NiCo2O4 favoring the in situ electrochemical generation of more active species during water oxidation.

Benzoate Anion-Intercalated Layered Cobalt Hydroxide Nanoarray: An Efficient Electrocatalyst for the Oxygen Evolution Reaction

Efficient oxygen evolution reaction (OER) catalysts are highly desired to improve the overall efficiency of electrochemical water splitting. We develop a benzoate anion-intercalated layered cobalt hydroxide nanobelt array on nickel foam (benzoate-Co(OH)2 /NF) through a one-pot hydrothermal process. As a 3 D electrode, benzoate-Co(OH)2 /NF with an expanded interlayer spacing (14.72 Å) drives a high OER catalytic current density of 50 mA cm-2 at an overpotential of 291 mV, outperforming its carbonate anion-intercalated counterpart with a lower interlayer spacing of 8.81 Å (337 mV overpotential at 50 mA cm-2 ). Moreover, this benzoate-Co(OH)2 /NF can maintain its catalytic activity for 21 h.

WO3 Nanoarray: An Efficient Electrochemical Oxygen Evolution Catalyst Electrode Operating in Alkaline Solution

It is fascinating to design and synthesize high-efficiency and noble-metal-free alkaline oxygen evolution reaction (OER) electrocatalysts. In this Communication, we describe the one-step hydrothermal synthesis of a WO3 nanoarray directly grown on conductive carbon cloth (WO3/CC) for efficient water oxidation in 1.0 M KOH. As a monolithically integrated array catalyst, WO3/CC exhibits superior OER activity demanding overpotential as low as 280 mV to afford a benchmarking catalytic current density of 10 mA cm-2. It is worth noting that WO3/CC also possesses strong electrochemical durability with 95% Faradaic yields.

High-performance artificial nitrogen fixation at ambient conditions using a metal-free electrocatalyst

Conversion of naturally abundant nitrogen to ammonia is a key (bio)chemical process to sustain life and represents a major challenge in chemistry and biology. Electrochemical reduction is emerging as a sustainable strategy for artificial nitrogen fixation at ambient conditions by tackling the hydrogen- and energy-intensive operations of the Haber–Bosch process. However, it is severely challenged by nitrogen activation and requires efficient catalysts for the nitrogen reduction reaction. Here we report that a boron carbide nanosheet acts as a metal-free catalyst for high-performance electrochemical nitrogen-to-ammonia fixation at ambient conditions. The catalyst can achieve a high ammonia yield of 26.57 μg h–1 mg–1cat. and a fairly high Faradaic efficiency of 15.95% at –0.75 V versus reversible hydrogen electrode, placing it among the most active aqueous-based nitrogen reduction reaction electrocatalysts. Notably, it also shows high electrochemical stability and excellent selectivity. The catalytic mechanism is assessed using density functional theory calculations.Electrochemical reduction of nitrogen is a promising route to industrial-scale nitrogen fixation at ambient conditions, but is challenged by activation of inert nitrogen. Here the authors report a metal-free catalyst that selectively reduces nitrogen to ammonia with high efficiency and stability.

Nickel-carbonate nanowire array: An efficient and durable electrocatalyst for water oxidation under nearly neutral conditions

It is highly attractive but still remains a great challenge to develop an efficient electrocatalyst for oxygen evolution reaction under nearly neutral conditions. In this work, we report the transformation of Ni3S2 nanowire array on nickel foam into the amorphous nickel carbonate nanowire array on nickel foam (NiCO3/NF). The resulting NiCO3/NF shows high electrocatalytic activity towards water oxidation and affords current density of 50 mA∙cm‒2 at overpotential of 395 mV in 1.0 mol∙L‒1 KHCO3. Moreover, this NiCO3/NF is also durable with a long-term electrochemical durability of 60 h. This catalyst electrode achieves a high turnover frequency of 0.21 mol O2∙s‒1 at the overpotential of 500 mV.