Electron-Donating Conjugation Effect Modulated Zn2+ Reduction Reaction for Separator-Free Aqueous Zinc Batteries.

Zinc-based aqueous batteries (ZABs) are attracting extensive attention due to the low cost, high capacity, and environmental benignity of the zinc anode. However, their application is still hindered by the undesired zinc dendrites. Despite Zn-surface modification being promising in relieving dendrites, a thick separator (i.e. glass fiber, 250-700 µm) is still required to resist the dendrite puncture, which limits volumetric energy density of battery. Here, we pivot from the traditional interphase plus extra separator categories, proposing an all-in-one ligand buffer layer (ca. 20 µm) to effectively modulate the Zn2+ transfer and deposition behaviors proved by in situ electrochemical digital holography. Experimental characterizations and density functional theory simulations further reveal that the catechol groups in the buffer layer can accelerate the Zn2+ reduction reaction (ZRR) through the electron-donating p-π conjugation effect, decreasing the negative charge in the coordination environment. Without extra separators, the elaborated system endows low polarization below 28.2 mV, long lifespan of 4950 h at 5 mA cm-2 in symmetric batteries, and an unprecedented volumetric energy density of 99.2 Wh L-1 based on the whole pouch cells. The concomitantly "separator-free" and "dendrite-free" conjugation effect with an accelerated ZRR process could foster the progression of metallic anodes and benefit energetic aqueous batteries.

Electric-Field-Assisted Alkaline Hydrolysis of Metal-Organic Framework Bulk into Highly Porous Hydroxide for Energy Storage and Electrocatalysis.

Metal-organic frameworks (MOFs) have attracted tremendous attention in the field of supercapacitors and electrocatalysis due to their open metal sites and high surface area. However, their inherent instability and poor electrical conductivity lead to limited electrochemical performance. Herein, we have employed a new and simple strategy for converting MOF bulk into porous Zn-Co hydroxide composites with the assistance of electric fields with different cycles. This method can alter the migration behavior of charged molecules/ions and improve the nucleation rate of hydroxide, thus adjusting the morphology of derivatives. As a supercapacitor electrode, the optimal material of Zn0.3Co0.7(OH)2 with an electric-field application time of 1200 cycles shows excellent electrochemical performance with a high specific capacity of 981.2 C g-1 at 1 A g-1. Additionally, the fabricated asymmetric supercapacitor exhibits an energy density of 42.5 Wh kg-1 at a power density of 750.0 W kg-1 and a remarkable cycling stability (99% after 11,000 cycles). Simultaneously, the as-prepared Zn0.3Co0.7(OH)2 with an electric-field application time of 1200 cycles delivers prominent OER performances, which can exhibit low overpotentials of 300 and 326 mV at 50 and 100 mA cm-2, respectively, and shows a small Tafel slope of 31.5 mV dec-1. This study represents a new strategy for the synthesis of economical and efficient electrode materials for supercapacitors and OER electrocatalysts and offers a novel way for the mild preparation of nanoderivatives from MOFs.

Electrochemical Anion-Exchanged synthesis of porous Ni/Co hydroxide nanosheets for Ultrahigh-Capacitance supercapacitors.

The commonly reported calcination strategy usually requires high temperature to crack the metal-organic frameworks (MOFs) particles, which often lead to uncontrollable growth of nanomaterials. Here, for the first time, we utilize an electrochemical anion-exchanged method to control the hydrolysis of MOFs and synthesize porous Ni/Co hydroxide nanosheets. After the electrochemical anion-exchange, the organic ligands of MOFs nanosheets can be recycled and reused. Applying an electric field to the MOFs bulk in alkaline solution can accelerate the nucleation rate of hydroxide and change the migration behavior of charged ions/molecules, which can tailor the microstructure of derivatives and improve deep charge and discharge capability of the electrodes. As a result, the hydroxide with the optimized Ni:Co molar ratio of 7:3 and electric-field application time of 1000 cycles [Ni0.7Co0.3(OH)2-1000c] provides much better electrochemical properties than the materials synthesized without electric-field assistance: a high specific capacitance of 2115C g-1 (4230F g-1). A hybrid supercapacitor with the Ni0.7Co0.3(OH)2-1000c electrode shows a high energy density of 74.7 Wh kg-1, an improved power density (5,990.6 W kg-1), and an excellent cyclic stability (8,000 cycles). This study not only provides a novel strategy for the preparation of low-cost, deep-discharge electrodes for supercapacitors, but also proposes an unconventional method for mild synthesizing MOFs materials into porous nanoscale derivatives with tailored micromorphology.

Core-shell nanostructured Zn-Co-O@CoS arrays for high-performance hybrid supercapacitors.

Although zinc oxide (ZnO) with wide distribution is one of the most attractive energy storage materials, the low electronic conductivity and insufficient active sites of bulk ZnO increase the internal resistance and reduce the capacity of electrodes for supercapacitors. Herein, CoS nanosheets are coated on the surface of heterostructured ZnO/Co3O4 nanowires to synthesize a core-shell Zn-Co-O@CoS electrode by a three-step method. The built-in electric field formed between ZnO and Co3O4 can enhance the conductivity of the composite electrode. The coating of amorphous CoS can also provide sufficient active sites and improve the chemical stability of ZnO/Co3O4 nanowires. As a result, the as-prepared Zn-Co-O@CoS electrode delivers a high specific capacity of 1190 C g-1, which is 7 times higher than that of the pristine ZnO electrode. Besides, a hybrid supercapacitor (HSC) with the Zn-Co-O@CoS electrode exhibits a high energy density of 56.8 W h kg-1 at a power density of 771.6 W kg-1. Furthermore, we assembled a solar-charging power system by combining the HSC and monocrystalline silicon plates to prove the practicability of the device, which can power a toy electric fan successfully. This study provides an effective idea and strategy for preparing Zn-based supercapacitor electrodes with low cost and deep discharge.

Rational design of flower-like Co-Zn LDH@Co(H2PO4)2 heterojunctions as advanced electrode materials for supercapacitors.

Layered double hydroxides (LDHs) with high theoretical specific capacity have been considered as one of the most promising candidates for high-performance supercapacitors. However, the low electronic conductivity and insufficient active sites hinder the further large-scale application of bulk LDHs. Here, we successfully synthesized heterostructured Co-Zn LDH@Co(H2PO4)2 nanoflowers by a simple hydrothermal method. As the amount of Co(H2PO4)2 in the whole heterostructure increases, the nanosheets steadily evolve into nanoflowers with a high surface area, providing more electrochemically active sites. Moreover, the built-in electric field formed between Co-Zn LDH and Co(H2PO4)2 improves the conductivity of the composite electrode. As a result, the as-prepared Co-Zn LDH@Co(H2PO4)2 shows a high specific capacity of 919 C g-1 at a current density of 1 A g-1. A hybrid supercapacitor (HSC) with activated carbon (AC) as the negative electrode and Co-Zn LDH@Co(H2PO4)2 as the positive electrode delivers an energy density of 30.4 W h kg-1 at a power density of 400 W kg-1, and 95.3% of the initial capacity is retained after 5000 cycles. This study provides a novel synthesis strategy for constructing heterojunctions to enhance the energy storage properties of LDH-based materials.