Poly(Dibenzothiophene-Terphenyl Piperidinium) for High-Performance Anion Exchange Membrane Water Electrolysis.

The anion exchange membrane water electrolysis is widely regarded as the next-generation technology for producing green hydrogen. The OH- conductivity of the anion exchange membrane plays a key role in the practical implementation of this device. Here, we present a series of Z-S-x membranes with dibenzothiophene groups. These membranes contain sulfur-enhanced hydrogen bond networks that link surrounding surface site hopping regions, forming continuous OH- conducting highways. Z-S-20 has a high through-plane OH- conductivity of 182±28 mS cm-1 and ultralong stability of 2650 h in KOH solution at 80 °C. Based on rational design, we achieved a high PGM-free alkaline water electrolysis performance of 7.12 A cm-2 at 2.0 V in a flow cell and demonstrated durability of 650 h at 2 A cm-2 at 40 °C with a cell voltage increase of 0.65 mV/h.

Stable Anion Exchange Membrane Bearing Quinuclidinium for High-performance Water Electrolysis.

Anion exchange membranes (AEMs) are core components in anion exchange membrane water electrolyzers (AEM-WEs). However, the stability of functional quaternary ammonium cations, especially under high temperatures and harsh alkaline conditions, seriously affects their performance and durability. Herein, we synthesized a 1-methyl-3,3-diphenylquinuclidinium molecular building unit. Density functional theory (DFT) calculations and accelerated aging analysis indicated that the quinine ring structure was exceedingly stable, and the SN2 degradation mechanism dominated. Through acid-catalyzed Friedel-Crafts polymerization, a series of branched poly(aryl-quinuclidinium) (PAQ-x) AEMs with controllable molecular weight and adjustable ion exchange capacity (IEC) were prepared. The stable quinine structure in PAQ-x was verified and retained in the ex situ alkaline stability. Furthermore, the branched polymer structure reduces the swelling rate and water uptake to achieve a tradeoff between dimensional stability and ionic conductivity, significantly improving the membrane's overall performance. Importantly, PAQ-5 was used in non-noble metal-based AEM-WE, achieving a high current density of 8 A cm-2 at 2 V and excellent stability over 2446 h in a gradient constant current test. Based on the excellent alkaline stability of this diaryl-quinuclidinium group, it can be further considered as a multifunctional building unit to create multi-topological polymers for energy conversion devices used in alkaline environments.

Highly Efficient Biomass Upgrading by a Ni-Cu Electrocatalyst Featuring Passivation of Water Oxidation Activity.

Electricity-driven organo-oxidations have shown an increasing potential recently. However, oxygen evolution reaction (OER) is the primary competitive reaction, especially under high current densities, which leads to low Faradaic efficiency (FE) of the product and catalyst detachment from the electrode. Here, we report a bimetallic Ni-Cu electrocatalyst supported on Ni foam (Ni-Cu/NF) to passivate the OER process while the oxidation of 5-hydroxymethylfurfural (HMF) is significantly enhanced. A current density of 1000 mA cm-2 can be achieved at 1.50 V vs. reversible hydrogen electrode, and both FE and yield keep close to 100 % over a wide range of potentials. Both experimental results and theoretical calculations reveal that Cu doping impedes the OH* deprotonation to O* and hereby OER process is greatly passivated. Those instructive results provide a new approach to realizing highly efficient biomass upgrading by regulating the OER activity.

A high-performance watermelon skin ion-solvating membrane for electrochemical CO2 reduction.

Ion-solvating membranes have been gaining increasing attention as core components of electrochemical energy conversion and storage devices. However, the development of ion-solvating membranes with low ion resistance and high ion selectivity still poses challenges. In order to propose an effective strategy for high-performance ion-solvating membranes, this study conducted a comprehensive investigation on watermelon skin membranes through a combination of experimental research and molecular dynamics simulation. The micropores and continuous hydrogen-bonding networks constructed by the synergistic effect of cellulose fiber and pectin enable the hypodermis of watermelon skin membranes to have a high ion conductivity of 282.3 mS cm-1 (room temperature, saturated with 1 M KOH). The negatively charged groups and hydroxyl groups on the microporous channels increase the formate penetration resistance of watermelon skin membranes in contrast to commercially available membranes, and this is crucial for CO2 electroreduction. Therefore, the confinement of proton donors and negatively charged groups within three-dimensional microporous polymers gives inspiration for the design of high-performance ion-solvating membranes.

Homogeneous Electrochemical Water Oxidation at Neutral pH by Water-Soluble NiII Complexes Bearing Redox Non-innocent Tetraamido Macrocyclic Ligands.

Water oxidation is the bottleneck reaction in artificial photosynthesis. Exploring highly active and stable molecular water oxidation catalysts (WOCs) is still a great challenge. In this study, a water-soluble NiII complex bearing a redox non-innocent tetraamido macrocyclic ligand (TAML) is found to be an efficient electrocatalyst for water oxidation in neutral potassium phosphate buffer. Controlled-potential electrolysis experiments show that it can sustain at a steady current of approximately 0.2 mA cm-2 for >7 h at 1.75 V versus normal hydrogen electrode (NHE) without the formation of NiOx . Electrochemical and spectroelectrochemical tests show that the redox-active ligand, as well as HPO4 2- in the buffer, participate in the catalytic cycle. More importantly, catalytically active intermediate [NiIII (TAML2- )-O. ] is formed via several proton-coupled electron transfer processes and reacts with H2 O with the assistance of base to release molecular oxygen. Thus, the employment of redox non-innocent ligands is a useful strategy for designing effective molecular WOCs.

Copper-based homogeneous and heterogeneous catalysts for electrochemical water oxidation.

Water oxidation is currently believed to be the bottleneck in the field of electrochemical water splitting and artificial photosynthesis. Enormous efforts have been devoted toward the exploration of water oxidation catalysts (WOCs), including homogeneous and heterogeneous catalysts. Recently, Cu-based WOCs have been widely developed because of their high abundance, low cost, and biological relevance. However, to the best of our knowledge, no review has been made so far on such types of catalysts. Thus, we have summarized the recent progress made in the development of homogeneous and heterogeneous Cu-based WOCs for electrochemical catalysis. Furthermore, the evaluations of catalytic activity, stability, and mechanism of these catalysts are carefully concluded and highlighted. We believe that this review can summarize the current progress in the field of Cu-based electrochemical WOCs and help in the design of more efficient and stable WOCs.

An organic polymer CuPPc-derived copper oxide as a highly efficient electrocatalyst for water oxidation.

Herein, we report a novel CuPPc (copper polymeric phthalocyanine)/CF (copper foam) nanoflake material, as precatalyst for the generation of an excellent water oxidation catalyst (WOC). Under optimized conditions, the CuPPc-derived Cu oxide affords a current density of 10 mA cm-2 under an overpotential (η) of 287 mV and sustains for at least 50 h in 1.0 M KOH. The strategy presented here is favorable to develop the electrocatalysts for water splitting.

Urchin-Like Cobalt-Copper (Hydr)oxides as an Efficient Water Oxidation Electrocatalyst.

The development of efficient and low-cost oxygen evolution reaction (OER) catalysts is essential for the generation of clean hydrogen energy from water splitting. Herein, a novel hierarchical urchin-like cobalt-copper (hydr)oxide in situ grown on copper foam (CoCuOx Hy (S)/CF) was synthesized through the electrochemical transformation of cobalt-copper sulfides (Co9 S8 -Cu1.81 S) via anodization process. This CoCuOx Hy (S)/CF anode exhibited a low overpotential (η) of 274 mV at a current density of 100 mA cm-2 with a robust durability over a period of 40 h when operated at 10 mA cm-2 . Further investigations imply that the unique nanowires aggregated urchin-like structure of CoCuOx Hy (S) derived from the in situ anion exchange process could facilitate the exposure of active sites and accelerate electron transfer. More importantly, the incorporation of copper resulted in an electronic delocalization around the cobalt species, which contributed to reach a high-valent catalytically active cobalt species and further improved the OER performance.

Hierarchical CoS2/Ni3S2/CoNiOx nanorods with favorable stability at 1 A cm-2 for electrocatalytic water oxidation.

Herein, we have reported an easily synthesized CoS2/Ni3S2/CoNiOx water oxidation catalyst with excellent catalytic activity and superior durability. The as-prepared catalyst required overpotential (η) as low as 256 mV to exhibit a current density of 10 mA cm-2 in 1.0 M KOH. Remarkably, it sustained a current density of 1 A cm-2 for one week in 30% KOH solution with only 25 mV increment of η. Thus, it is a hopeful candidate as a highly-effective water oxidation electrode in practical applications.

Facile Synthesis of a Ternary Metal Hydroxide with Acid Treatment as an Effective and Durable Electrocatalyst in Water Oxidation.

Developing low-cost, effective, and durable oxygen-evolution catalysts (OECs) is still a huge challenge owing to the uphill thermodynamic reaction and the involvement of a four-electron and four-proton kinetics process. Herein, a facilely prepared NiCoFe(OH)x /NiOOH/NF electrode affords current densities of 10 and 100 mA cm-2 at overpotentials of 223 and 254 mV, respectively, and a Tafel slope as low as 33.5 mV dec-1 in 1 m KOH, which is superior to NiCoFe(OH)x /NF electrodes electrodeposited with a traditional method. This electrode also displays surprisingly high durability at a current density of 50 mA cm-2 for over 50 hours under alkaline conditions. Component analysis and an electrochemical study revealed that the catalytic activity enhancement of this newly prepared electrode is mainly attributed to the formation of the electrically conductive NiOOH interlayer.

Hierarchically Structured FeNiOx Hy Electrocatalyst Formed by In†Situ Transformation of Metal Phosphate for Efficient Oxygen Evolution Reaction.

A simple and low-cost fabrication method is needed to obtain effective and robust heterogeneous catalysts for the oxygen evolution reaction (OER). In this study, an electrocatalyst FeNiOx Hy with a hierarchical structure is synthesized on nickel foam by a simple fabrication method through anion exchange from a metal phosphate to a metal hydroxide. The as-fabricated FeNiOx Hy electrode requires overpotentials of 206 and 234 mV to deliver current densities of 10 and 50 mA cm-2 , respectively. The catalytic performance of FeNiOx Hy is superior to that of most previously reported FeNi-based catalysts, including NiFe layered double hydroxide. The catalyst also shows good long-term durability at a current density of 50 mA cm-2 over 50 h with no activity decay under 1 m KOH. By comparison to the directly electrodeposited FeNi hydroxide in morphology and electrochemical properties, the improved activity of the catalyst could be mainly attributed to an enhancement of its intrinsic activity, which was caused by the anion exchange of phosphate to (oxy)hydroxide. Further studies by cyclic voltammetry indicated a stronger interaction between Ni and Fe from the negative shift of the oxidation peak of Ni2+ /Ni3+ in comparison with reported FeNiOx Hy , which promoted the generation of active Ni3+ species more easily. This work may provide a new approach to the simple preparation of effective and robust OER catalysts by anion exchange.

Ligand-Controlled Electrodeposition of Highly Intrinsically Active and Optically Transparent NiFeOx Hy Film as a Water Oxidation Electrocatalyst.

A highly intrinsically active and optically transparent NiFeOx Hy water oxidation catalyst was prepared by electrodeposition of [Ni(C12 -tpen)](ClO4 )2 complex (Ni-C12 ). This NiFeOx Hy film has a current density of 10 mA cm-2 with an overpotential (η) of only 298 mV at nanomolar concentration and the current density of 10 mA cm-2 remains constant over 22 h in 1 m KOH. The extremely high turnover frequency of 0.51 s-1 was obtained with η of 300 mV. More importantly, such outstanding activity and transparency (optical loss <0.5 %) of the NiFeOx Hy film are attributed to a ligand effect of the dodecyl substituent in Ni-C12 , which enables its future application in solar water splitting.