Controlled Construction of Core-Shell Structured Prussian Blue Analogues towards Enhanced Oxygen Reduction.

Metal-organic frameworks-based electrocatalysts have been developed as highly desirable and promising candidates for catalyzing oxygen reduction reaction (ORR), which, however, usually need to be prepared at elevated temperatures and may suffer from the framework collapse in water environments, largely preventing its industrial application. Herein, this work demonstrates a facile low-temperature ion exchange method to synthesize Mn and Fe co-loaded Prussian blue analogues possessing core-shell structured frameworks and favorable water-tolerance. Among the catalysts prepared, the optimal HMPB-2.6Mn shows a high ORR electrocatalytic performance featuring a half-wave potential of 0.86 V and zinc-air battery power density of 119 mW cm-2 , as well as negligible degradation up to 60 h, which are comparable to commercial Pt/C. Such an excellent electrocatalytic performance is attributed to the special core-shell-like structure with Mn concentrated in outer shell, and the synergetic interactions between Mn and Fe, endowing HMPB-Mn with outstanding ORR activity and good stability.

Constructing FeS and ZnS Heterojunction on N,S-Codoped Carbon as Robust Electrocatalyst toward Oxygen Reduction Reaction.

Highly active and cost-efficient electrocatalysts for oxygen reduction reaction (ORR) are significant for developing renewable energy conversion devices. Herein, a nanocomposite Fe/ZnS-SNC electrocatalyst with an FeS and ZnS heterojunction on N,S-codoped carbon has been fabricated via a facile one-step sulfonating of the pre-designed Zn- and Fe-organic frameworks. Benefitting from the electron transfer from FeS to adjacent ZnS at the heterointerfaces, the optimized Fe/ZnS-SNC900 catalyst exhibits excellent ORR performances, featuring the half-wave potentials of 0.94 V and 0.81 V in alkaline and acidic media, respectively, which is competitive with the commercial 20 wt.% Pt/C (0.87 and 0.76 V). The flexible Zn-air battery equipping Fe/ZnS-SNC900 affords a higher open-circuit voltage (1.45 V) and power density of 30.2 mW cm-2. Fuel cells assembled with Fe/ZnS-SNC900 as cathodic catalysts deliver a higher power output of 388.3 and 242.8 mW cm-2 in H2-O2 and -air conditions. This work proposes advanced heterostructured ORR electrocatalysts that effectively promote renewable energy conversions.

Adsorption Site Regulations of [W-O]-Doped CoP Boosting the Hydrazine Oxidation-Coupled Hydrogen Evolution at Elevated Current Density.

Hydrazine oxidation reaction (HzOR) assisted hydrogen evolution reaction (HER) offers a feasible path for low power consumption to hydrogen production. Unfortunately however, the total electrooxidation of hydrazine in anode and the dissociation kinetics of water in cathode are critically depend on the interaction between the reaction intermediates and surface of catalysts, which are still challenging due to the totally different catalytic mechanisms. Herein, the [W-O] group with strong adsorption capacity is introduced into CoP nanoflakes to fabricate bifunctional catalyst, which possesses excellent catalytic performances towards both HER (185.60 mV at 1000 mA cm-2) and HzOR (78.99 mV at 10,00 mA cm-2) with the overall electrolyzer potential of 1.634 V lower than that of the water splitting system at 100 mA cm-2. The introduction of [W-O] groups, working as the adsorption sites for H2O dissociation and N2H4 dehydrogenation, leads to the formation of porous structure on CoP nanoflakes and regulates the electronic structure of Co through the linked O in [W-O] group as well, resultantly boosting the hydrogen production and HzOR. Moreover, a proof-of-concept direct hydrazine fuel cell-powered H2 production system has been assembled, realizing H2 evolution at a rate of 3.53 mmol cm-2 h-1 at room temperature without external electricity supply.

WO3 -Assisted Synergetic Effect Catalyzes Efficient and CO-Tolerant Hydrogen Oxidation for PEMFCs.

Developing anode catalysts with substantially enhanced activity for hydrogen oxidation reaction (HOR) and CO tolerance performance is of great importance for the commercial applications of proton exchange membrane fuel cells (PEMFCs). Herein, an excellent CO-tolerant catalyst (Pd-WO3 /C) has been fabricated by loading Pd nanoparticles on WO3 via an immersion-reduction route. A remarkably high power density of 1.33 W cm-2 at 80 °C is obtained by using the optimized 3Pd-WO3 /C as the anode catalyst of PEMFCs, and the moderately reduced power density (73% remained) in CO/H2 mixed gas can quickly recover after removal of CO-contamination from hydrogen fuel, which is not possible by using Pt/C or Pd/C as anode catalyst. The prominent HOR activity of 3Pd-WO3 /C is attributed to the optimized interfacial electron interaction, in which the activated H* adsorbed on Pd species can be effectively transferred to WO3 species through hydrogen spillover effect and then oxidized through the H species insert/output effect during the formation of Hx WO3 in acid electrolyte. More importantly, a novel synergetic catalytic mechanism about excellent CO tolerance is proposed, in which Pd and WO3 respectively absorbs/activates CO and H2 O, thus achieving the CO electrooxidation and re-exposure of Pd active sites for CO-tolerant HOR.

Dual-Site W-O-CoP Catalysts for Active and Selective Nitrate Conversion to Ammonia in a Broad Concentration Window.

Environmentally friendly electrochemical reduction of contaminated nitrate to ammonia (NO3 - RR) is a promising solution for large quantity ammonia (NH3 ) production, which, however, is a complex multi-reaction process involving coordination between different reaction intermediates of nitrate reduction and water decomposition-provided active hydrogen (Hads ) species. Here, a dual-site catalyst of [W-O] group-doped CoP nanosheets (0.6W-O-CoP@NF) has been designed to synergistically catalyze the NO3 - RR and water decomposition, especially the reactions between the intermediates of NO3 - RR and water decomposition-provided Hads species. This catalytic NO3 - RR exhibits an extremely high NH3 yield of 80.92 mg h-1 cm-2 and a Faradaic efficiency (FE) of 95.2% in 1 m KOH containing 0.1 m NO3 - . Significantly, 0.6W-O-CoP@NF presents greatly enhanced NH3 yield and FE in a wide NO3 - concentration ranges of 0.001-0.1 m compared to the reported. The excellent NO3 - RR performance is attributed to a synergistic catalytic effect between [W-O] and CoP active sites, in which the doped [W-O] group promotes the water decomposition to supply abundant Hads , and meanwhile modulates the electronic structure of Co for strengthened adsorption of Hads and the hydrogen (H2 ) release prevention, resultantly facilitating the NO3 - RR. Finally, a Zn-NO3 - battery has been assembled to simultaneously achieve three functions: electricity output, ammonia production, and nitrate treatment in wastewater.

Active site recovery and N-N bond breakage during hydrazine oxidation boosting the electrochemical hydrogen production.

Substituting hydrazine oxidation reaction for oxygen evolution reaction can result in greatly reduced energy consumption for hydrogen production, however, the mechanism and the electrochemical utilization rate of hydrazine oxidation reaction remain ambiguous. Herein, a bimetallic and hetero-structured phosphide catalyst has been fabricated to catalyze both hydrazine oxidation and hydrogen evolution reactions, and a new reaction path of nitrogen-nitrogen single bond breakage has been proposed and confirmed in hydrazine oxidation reaction. The high electro-catalytic performance is attributed to the instantaneous recovery of metal phosphide active site by hydrazine and the lowered energy barrier, which enable the constructed electrolyzer using bimetallic phosphide catalyst at both sides to reach 500 mA cm-2 for hydrogen production at 0.498 V, and offer an enhanced hydrazine electrochemical utilization rate of 93%. Such an electrolyzer can be powered by a bimetallic phosphide anode-equipped direct hydrazine fuel cell, achieving self-powered hydrogen production at a rate of 19.6 mol h-1 m-2.

CoW Bimetallic Carbide Nanocatalysts: Computational Exploration, Confined Disassembly-Assembly Synthesis and Alkaline/Seawater Hydrogen Evolution.

Earth-abundant tungsten carbide exhibits potential hydrogen evolution reaction (HER) catalytic activity owing to its Pt-like d-band electronic structure, which, unfortunately, suffers from the relatively strong tungsten-hydrogen binding, deteriorating its HER performance. Herein, a catalyst design concept of incorporating late transition metal into early transition metal carbide is proposed for regulating the metal-H bonding strength and largely enhancing the HER performance, which is employed to synthesize CoW bi-metallic carbide Co6 W6 C by a "disassembly-assembly" approach in a confined environment. Such synthesized Co6 W6 C nanocatalyst features the optimal Gibbs free energy of *H intermediate and dissociation barrier energy of H2 O molecules as well by taking advantage of the electron complementary effect between Co and W species, which endows the electrocatalyst with excellent HER performance in both alkaline and seawater/alkaline electrolytes featuring especially low overpotentials, elevated current densities, and much-enhanced operation durability in comparison to commercial Pt/C catalyst. Moreover, a proof-of-concept Mg/seawater battery equipped with Co6 W6 C-2-600 as cathode offers a peak power density of 9.1 mW cm-2 and an open-circuit voltage of ≈1.71 V, concurrently realizing hydrogen production and electricity output.

Fe2+ /Fe3+ Cycling for Coupling Self-Powered Hydrogen Evolution and Preparation of Electrode Catalysts.

A novel Zn-Fe flow battery featuring an Fe3+ reduction reaction (Fe3+ RR)-coupled zinc oxidation, and an Fe2+ oxidation reaction (Fe2+ OR)-coupled hydrogen evolution reaction (HER) system as well, was established. This battery is capable of driving two Fe2+ OR-coupled HER systems in series based on the above Fe2+ /Fe3+ cycling, for efficient self-powered hydrogen evolution. Meanwhile, this Fe2+ /Fe3+ cycling enables the preparation of a multifunctional catalyst, Pt-3@SXNS (siloxene nanosheet), by the Fe2+ OR-promoted dispersion of Pt nanoparticles on SXNS; alternatively, this support could be obtained by Fe3+ RR-assisted exfoliation using Fe3+ from the anolyte of Fe2+ OR-coupled HER. The Pt-3@SXNS catalyst exhibits excellent catalytic activities toward Fe3+ RR in the Zn-Fe flow battery, HER, and Fe2+ OR in the electrolyzer, which is attributed to the strong electronic interaction between Pt and Si. This work offers a new strategy for energy storage and low-cost hydrogen production from acidic wastewater.

N-Doped Carbon Electrocatalyst: Marked ORR Activity in Acidic Media without the Contribution from Metal Sites?

Fe-N-C electrocatalysts have been demonstrated to be the most promising substitutes for benchmark Pt/C catalysts for the oxygen reduction reaction (ORR). Herein, we report that N-doped carbon materials with trace amounts of iron (0-0.08 wt. %) show excellent ORR activity and durability comparable and even superior to those of Pt/C in both alkaline and acidic media without significant contribution by the metal sites. Such an N-doped carbon (denoted as N-HPCs) features a hollow and hierarchically porous architecture, and more importantly, a noncovalently bonded N-deficient/N-rich heterostructure providing the active sites for oxygen adsorption and activation owing to the efficient electron transfer between the layers. The primary Zn-air battery using N-HPCs as the cathode delivers a much higher power density of 158 mW cm-2 , and the maximum power density in the H2 -O2 fuel cell reaches 486 mW cm-2 , which is comparable to and even better than those using conventional Fe-N-C catalysts at cathodes.

Construction of CoP2-Mo4P3/NF Heterogeneous Interfacial Electrocatalyst for Boosting Water Splitting.

Developing highly efficient, cost effective and durable bifunctional electrocatalyst remains a key challenge for overall water splitting. Herein, a bifunctional catalyst CoP2-Mo4P3/NF with rich heterointerfaces was successfully prepared by a two-step hydrothermal-phosphorylation method. The synergistic interaction between CoP2 and Mo4P3 heterogeneous interfaces can optimize the electronic structure of active sites, leading to the weak adsorption of H on the Mo sites and the increased redox activity of the Co site, resultantly improving the HER/OER bifunctional catalytic activity. The synthesized CoP2-Mo4P3/NF catalyst exhibits excellent electrocatalytic activity in 1.0 M KOH with low overpotentials of 77.6 and 300.3 at 100 mA cm-2 for HER and OER, respectively. Additionally, the assembled CoP2-Mo4P3/NF||CoP2-Mo4P3/NF electrolyzer delivers a current density of 100 mA cm-2 at a cell voltage of 1.59 V and remains stable for at least 370 h at 110 mA cm-2, indicating the potential application prospective in water splitting.

Hydrogen Evolution/Oxidation Electrocatalysts by the Self-Activation of Amorphous Platinum.

Amorphous nanostructures usually exhibit special and intriguing catalytic activities, and the electrochemical performance can be tuned during operation. Herein, a facile approach of the self-activation of amorphous platinum (A-Pt) nanospheres has been applied to develop a durable and efficient hydrogen electrode catalyst toward both the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR), which was in situ converted to crystalline counterparts and partially oxidized during the electrochemical cycling, leading to the self-activated enhancements of both HER and HOR activities with the decreased overpotential by 5 times and the increased hydrogen oxidation current density by 67%, respectively. Especially, in addition to 12 times higher mass activity compared to benchmark Pt/C, in situ-activated A-Pt also demonstrated a lower HER overpotential even after 20 000 cycles than Pt/C. The significantly improved catalytic performance benefits from the rapid self-reconstruction processes (crystallization and oxidation) of the amorphous Pt during electrochemical cycling. This work shows the intriguing properties of the amorphous nanostructure and provides a new idea for designing an efficient electrocatalyst by phase engineering.

Metal-Nitrogen-Carbon Catalysts of Specifically Coordinated Configurations toward Typical Electrochemical Redox Reactions.

Metal-nitrogen-carbon (M-N-C) material with specifically coordinated configurations is a promising alternative to costly Pt-based catalysts. In the past few years, great progress is made in the studies of M-N-C materials, including the structure modulation and local coordination environment identification via advanced synthetic strategies and characterization techniques, which boost the electrocatalytic performances and deepen the understanding of the underlying fundamentals. In this review, the most recent advances of M-N-C catalysts with specifically coordinated configurations of M-Nx (x = 1-6) are summarized as comprehensively as possible, with an emphasis on the synthetic strategy, characterization techniques, and applications in typical electrocatalytic reactions of the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, CO2 reduction reaction, etc., along with mechanistic exploration by experiments and theoretical calculations. Furthermore, the challenges and potential perspectives for the future development of M-N-C catalysts are discussed.

Strong piezocatalysis in barium titanate/carbon hybrid nanocomposites for dye wastewater decomposition.

In this work, the strong piezocatalysis is found in the two-step hydrothermally-synthesized barium titanate/carbon hybrid nanocomposites and is used for rhodamine B dye decomposition. As the carbon content increases from 0 to 5 wt%, the catalytic performance of hybrid nanocomposites first increases and then slightly decreases. When the carbon content increases to 2 wt%, the barium titanate/carbon hybrid nanocomposites exhibit the optimal piezocatalytic performance, which have the ~75.5% dye decomposition ratio and the ~0.04901 min-1 reaction rate constant after the 40 min vibration stimulation, while that of the pure barium titanate are 48.4% and 0.01942 min-1, respectively. The improvement of piezocatalytic performance in barium titanate/carbon hybrid nanocomposites can be ascribed to the action of carbon's charge transfer which promotes the effective separation of the piezoelectrically-induced electric charges. After three runs recycle utilization tests, the barium titanate/carbon hybrid nanocomposites still exhibit ~70% decomposition ratio of rhodamine B dye. The strong piezocatalytic performance and the good reusability make the barium titanate/carbon hybrid nanocomposites potential in the field of wastewater treatment through utilizing natural vibration energy in future.

Surface reconstruction of cobalt phosphide nanosheets by electrochemical activation for enhanced hydrogen evolution in alkaline solution.

Transition metal phosphides exhibit promising catalytic performance for the hydrogen evolution reaction (HER); however their surface structure evolution during electrochemical operation has rarely been studied. In this work, we investigate the surface reconstruction of CoP nanosheets by an in situ electrochemical activation method. After remodeling, CoP nanosheets experience an irreversible and significant evolution of the morphology and composition, and low-valence Co complexes consisting of Co(OH) x species are formed on the surface of CoP nanosheets, and they largely accelerate the dissociation of water. Benefiting from the synergistic effect of CoP and Co(OH) x , the working electrode shows a remarkably enhanced HER activity of 100 mV at 10 mA cm-2 with a Tafel slope of 76 mV dec-1, which is better than that of most transition metal phosphide catalysts. This work would provide a deep understanding of surface reconstruction and a novel perspective for rational design of high performance transition metal phosphide electrocatalysts for water related electrolysis.

Engineering crystalline CoOOH anchored on an N-doped carbon support as a durable electrocatalyst for the oxygen reduction reaction.

Finding highly active, low-cost and stable electro-catalysts for the oxygen reduction reaction is challenging but crucial for several applications such as fuel cells and metal batteries. Herein, we report a novel electro-catalyst of crystalline CoOOH anchored on an N-doped carbon support through a facile method, and we explore its origin of activity via investigating the correlation between the composition of the catalyst and its electrochemical properties. Nitrogen is successfully incorporated into a carbon support, and CoOOH is desirably anchored on a carbon matrix forming Co-N coordination sites in the catalyst, which is revealed to be responsible for the much enhanced oxygen reduction activity compared to the oxygen reduction activities of the electrocatalysts having either single-doped nitrogen or anchored CoOOH species in a carbon support. Furthermore, it is deduced that the active sites for enhanced oxygen reduction can be attributed to Co-Nx, carbon atoms neighboring the doped nitrogen and even the CoOOH species. The optimized catalyst CoOOH@NC-175/300 shows lower onset potential and half-wave potential and much better cyclic stability compared to those of the state-of-the-art Pt/C catalysts. This novel finding provides a new approach for the rational engineering of low-cost, highly active and stable non-precious ORR electrocatalysts for applications in energy conversion and storage.

Fabrication of a mesoporous Ba0.5Sr0.5Co0.8Fe0.2O3-δ perovskite as a low-cost and efficient catalyst for oxygen reduction.

A precious metal-free cathode catalyst, mesoporous Ba0.5Sr0.5Co0.8Fe0.2O3-δ (m-BSCF), with a well crystallized perovskite framework and a porous structure (pore size of ∼10 nm) has been synthesized by a one-step co-nanocasting method. The obtained mesoporous perovskite m-BSCF demonstrated a much higher oxygen reduction reaction catalytic activity than its macroporous (CS-BSCF of ∼5 µm pore size) and nonporous counterparts (SG-BSCF). The mesoporous structure and oxygen vacancies endowed the obtained perovskite oxide m-BSCF with an approximate 4e- pathway for the ORR comparable to the benchmark 20 wt% Pt/C and a stable electro-catalytic activity with 91% current density being retained after 5000 cycles, and excellent methanol tolerance. The redox couples Co4+/Co3+ and Fe3+/Fe2+ are proposed to accelerate the ORR on active species, i.e., MOOH (M = Co, Fe) groups produced by the substitution species of Co and Fe in the B-site of perovskite m-BSCF, and in the meantime oxygen vacancies generated by the redox couples play a key role in activating the oxygen molecules.

A Redox-anchoring Approach to Well-dispersed MoCx /C Nanocomposite for Efficient Electrocatalytic Hydrogen Evolution.

Here we report a redox-anchoring strategy for synthesizing a non-noble metal carbide (MoCx ) nanocomposite electrocatalyst for water electrolysis in acidic media, using glucose and ammonium heptamolybdate as carbon and Mo precursors, respectively, without the need of gaseous carbon sources such as CH4 . Specifically, the aldehyde groups of glucose are capable of reducing Mo6+ to Mo4+ (MoO2 ), and thus molybdenum species can be well anchored by a redox reaction onto a carbon matrix to prevent the aggregation of MoCx nanoparticles during the following carbonization process. The morphology and chemical composition of the electrocatalysts were well characterized by BSE-SEM, TEM, XRD and XPS. The obtained MoCx -2 sample showed a reasonably high hydrogen evolution reaction (HER) activity and excellent stability in an acidic electrolyte, and its overpotential required for a current density output of 20 mA cm-2 is as low as 193 mV. Such a prominent performance is ascribed to the excellent dispersity and nano-size, and the large reactive surface area of MoCx particles. This work may open a new way to the design and fabrication of other non-noble metal carbide nanocatalysts for various electrochemical applications.

Nitrogen-Doped Carbon Vesicles with Dual Iron-Based Sites for Efficient Oxygen Reduction.

A nitrogen-doped vesicle-like porous carbon with well-integrated dual iron-based catalytic sites was developed through direct pyrolysis of inexpensive and abundant precursors. Benefiting from the mesoporous structures with synchronous construction of Fe-Nx and Fe/Fe3 C@NC sites, the optimized catalyst exhibited outstanding performance for the oxygen reduction reaction (ORR) in alkaline media, even superior to the commercial Pt/C catalyst. Detailed characterizations revealed that Fe/Fe3 C@NC sites can make major catalytic contributions in basic media, whereas the Fe-Nx sites were found to play an indispensable role for ORR in acidic media.

Manganese Oxide Nanorod-Decorated Mesoporous ZSM-5 Composite as a Precious-Metal-Free Electrode Catalyst for Oxygen Reduction.

A precious-metal-free cathode catalyst, MnO2 nanorod-decorated mesoporous ZSM-5 zeolite nanocomposite (MnO2 / m-ZSM-5), has been successfully synthesized by a hydrothermal and electrostatic interaction approach for efficient electrochemical catalysis of the oxygen reduction reaction (ORR). The active MnOOH species, that is, Mn(4+) /Mn(3+) redox couple and Brønsted acid sites on the mesoporous ZSM-5 matrix facilitate an approximately 4 e(-) process for the catalysis of the ORR comparable to commercial 20 wt % Pt/C. Stable electrocatalytic activity with 90 % current retention after 5000 cycles, and more importantly, excellent methanol tolerance is observed. Synergetic catalytic effects between the MnO2 nanorods and the mesoporous ZSM-5 matrix are proposed to account for the high electrochemical catalytic performance.