Min Ma

NiCoP Nanoarray: A Superior Pseudocapacitor Electrode with High Areal Capacitance

High-performance supercapacitors require the design and development of electrode materials with high conductivity and a large electrolyte-accessible surface area. Here, the use of a conductive NiCoP nanoarray on nickel foam (NiCoP/NF) as a superior pseudocapacitor electrode is demonstrated. This 3D electrode exhibits high areal capacitances of 9.2 and 5.97 F cm-2 at current densities of 2 and 50 mA cm-2 , respectively, with good rate capability and cycling stability. The asymmetric supercapacitor (ASC) device assembled using NiCoP/NF as positive electrode and active carbon as negative electrode delivers a high energy density of 1.16 mWh cm-2 at a power density of 1.6 mW cm-2 with 72 % retention of its initial specific capacitance after 2000 cycles at 50 mA cm-2 . The practical use is further demonstrated with two such ASC devices in series to light six LED indicators and also to drive an alkaline water electro- lyzer using NiCoP/NF as both cathode and anode for hydrogen production.

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.

Core–Shell-Structured NiS2@Ni-Bi Nanoarray for Efficient Water Oxidation at Near-Neutral pH

The development of high‐performance and cost‐effective water oxidation catalysts operating under mild conditions is attractive for the conversion of electricity into chemical fuels. In this communication, we report the in situ electrochemical development of a Ni‐Bi layer (5–8 nm thick) on a NiS2 nanoarray supported on carbon cloth (i.e., NiS2@Ni‐Bi/CC) for efficient water oxidation in 0.1 m potassium borate electrolyte at pH 9.2. As a durable 3 D catalyst electrode, core–shell‐structured NiS2@Ni‐Bi/CC shows excellent catalytic activity for water oxidation and approaches a geometrical catalytic current density of 20 mA cm−2 at an overpotential of 486 mV, with long‐term electrochemical stability for at least 24 h with a high turnover frequency of 0.28 mol O2 s−1 at an overpotential of 500 mV and nearly 100 % Faradic efficiency for oxygen evolution.

Three-Dimensional Nickel–Borate Nanosheets Array for Efficient Oxygen Evolution at Near-Neutral pH

Nickel-borate nanosheets array on titanium mesh (Ni-Bi NA/TM) was derived from NiSe2 nanosheets array on titanium mesh (NiSe2 NA/TM) by electrochemical transformation. As a three-dimensional electrode, Ni-Bi NA/TM exhibited high catalytic activity toward the oxygen evolution reaction and required a low overpotential of 430 mV at 10 mA cm-2 in 0.1 m potassium borate (pH 9.2), with outstanding long-term stability and high turnover frequency.

A nickel–borate–phosphate nanoarray for efficient and durable water oxidation under benign conditions

The development of earth-abundant electrocatalysts for efficient water oxidation under moderate conditions is highly desired but still a big challenge. In this communication, we demonstrate the topotactic conversion of a nickel phosphide nanoarray on carbon cloth into a nickel–borate–phosphate nanoarray (Ni–Bi–Pi/CC) by oxidative polarization in potassium borate water. When used as a 3D water oxidation catalyst, such Ni–Bi–Pi/CC shows high activity with a geometrical catalytic current density of 10 mA cm−2 at an overpotential of only 440 mV in 0.1 M K–Bi, rivaling the performances of the reported Ni-containing catalysts operated under benign conditions. Notably, this electrode also demonstrates strong long-term electrochemical durability with 100% Faradaic efficiency for oxygen evolution. All these features promise its use as an attractive low-cost catalyst electrode in water-splitting devices for mass production of hydrogen fuels under environmentally friendly conditions.

Co-based nanowire films as complementary hydrogen- and oxygen-evolving electrocatalysts in neutral electrolyte

Designing and developing non-noble-metal electrocatalysts for efficient full water splitting at neutral pH is highly desired but still remains a huge challenge. In this communication, we report the development of homologous Co-based nanowire films, Co2N nanowire film on a Ti mesh (Co2N/TM) and Co-phosphate nanowire film on a Ti mesh (Co–Pi/TM), as complementary catalysts for stable electrochemical hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in neutral electrolyte. Co2N/TM shows remarkable HER activity with the need of an overpotential of 290 mV to drive 10 mA cm−2 in 1.0 M phosphate-buffered saline (PBS), and Co–Pi/TM is superior in OER activity affording 10 mA cm−2 at overpotentials of only 430 and 300 mV in 0.1 and 1.0 M PBS, respectively. The two-electrode water electrolyzer using Co2N/TM as a cathode and Co–Pi/TM as an anode affords a water-splitting current density of 10 mA cm−2 at a cell voltage of 1.78 V with 100% Faradaic efficiency in 1.0 M PBS, promising the practical uses of such homologous Co-based catalyst materials in technological devices.

Homologous Catalysts Based on Fe-Doped CoP Nanoarray toward High-Performance Full Water Splitting under Benign Conditions

The design and development of earth-abundant electrocatalysts for efficient full water splitting under mild conditions are highly desired, yet remain a challenging task. A homologous Fe-doped Co-based nanoarray incorporating complementary catalysts is shown to effect high-performance and durable water splitting in near-neutral media. Iron-doped cobalt phosphate borate nanoarray on carbon cloth (Fe-Co-Pi-Bi/CC) derived from iron-doped cobalt phosphide on CC (Fe-CoP/CC) through oxidative polarization behaves as a highly active bimetallic electrocatalyst for water oxidation with an overpotential of 382 mV to afford a catalytic current density of 10 mA cm-2 in 0.1 m potassium borate (K-Bi, pH 9.2). Fe-CoP/CC is also highly active for the hydrogen evolution reaction, capable of driving 10 mA cm-2 at an overpotential of only 175 mV in 0.1 m K-Bi. A two-electrode water electrolyzer incorporating Fe-Co-Pi-Bi/CC as anode and Fe-CoP/CC as cathode achieves 10 mA cm-2 water-splitting current at a cell voltage of 1.95 V with strong long-term electrochemical durability.

Highly efficient and durable water oxidation in a near-neutral carbonate electrolyte electrocatalyzed by a core–shell structured NiO@Ni–Ci nanosheet array

The development of cost-effective and high-performance water oxidation electrocatalysts operating in carbonate (Ci) electrolytes is highly desired but still remains a great challenge. In this communication, we report the in situ electrochemical development of an amorphous Ni–Ci layer on a NiO nanosheet array supported on carbon cloth (NiO@Ni–Ci/CC) as a 3D catalyst electrode for water oxidation in 1.0 M KHCO3 (pH: 8.3). To drive a geometrical catalytic current density of 15 mA cm−2, such core–shell structured NiO@Ni–Ci/CC only demands an overpotential of 387 mV, with its catalytic activity being maintained for at least 25 h. And at an overpotential of 500 mV, this catalyst electrode achieves a high turnover frequency of 0.18 mol O2 per s.

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.

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.