Noble Metal-based Electrocatalysts Research Articles

Metal-organic aerogel derived hierarchical porous metal-carbon nanocomposites as efficient bifunctional electrocatalysts for overall water splitting

An efficient strategy to construct non-noble metal-base electrocatalysts for water splitting is the direct carbonization of metal–organic aerogel composites. Herein for the first time, a novel tube-like metal–carbon nanocomposite with encapsulated small-size individual Fe, Cr and Ni nanoparticles, is prepared by the carbonization of a FeCr-doped Ni-benzenetricarboxylate aerogel. The slender skeleton of the aerogel, supercritical drying and Cr doping alleviates metal aggregation and facilitates the in-situ growth of carbon tubes. This nanocomposite exhibits remarkably low overpotential of the hydrogen evolution reaction (137 mV) and oxygen evolution reaction (220 mV). Further, the cell voltage could be as low as 1.54 V with the current density of 10 mA cm−2 and illustrates excellent stability under a continuous operation for 50 h. This non-noble metal-base electrocatalyst is comparable to noble metal-based electrocatalysts and the impressive performance is ascribed to the abundant active catalytic sites and short reactant diffusion pathways. This work demonstrates great capability of aerogel derivation in the highly active electrocatalyst design for promising electrochemical applications.

Cu nanoclusters on N-doped carbon nanotubes as efficient electrocatalyst for oxygen reduction reaction

Earth-rich transition metal-based catalysts are considered to be the most promising candidates to alternate noble metal-based electrocatalysts for oxygen reduction reaction (ORR). Nevertheless, the practical applications have been hampered due to their poor electrocatalytic behaviors and stability. Here, we report the facial thesis of Cu nanoclusters supported on N-doped carbon nanotubes by a simple grinding and heat treatment method, which presented a superior electrocatalytic performance than commercial Pt/C. In alkaline media, an onset potential (E0) and half-wave potential (E1/2) of 0.98 V and 0.88 V (RHE) have been achieved, respectively. In addition, the synthesized catalysts present a good tolerance and long-term stability to methanol, and exhibit less than 7% H2O2 formation. It is inferred that the enhanced electrocatalytic behavior could be attributed to the formation of Cu-NX active sites, which are coupled into the nitrogen-rich carbon nanotubes. This low-cost, extremely durable and stable electrocatalyst showed a great potential as an efficient cathode material for a variety of renewable energy conversion technologies, such as fuel cells and metal-air cells.

Void confinement and doping-modulation of IrNi alloy nanoparticles on hollow carbon spheres for efficient hydrogen oxidation/evolution reactions

Developing noble metal-based alloy nanoparticle (NP) electrocatalysts on solid supports with fast kinetics for alkaline hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is of considerable importance but remains a bottleneck for the potential applications of alkaline water electrolyzers and fuel cells. Herein, we design and implement a void confinement-mediated strategy to synthesize homogeneously distributed IrNi alloy NPs (4.23 nm in diameter) using nitrogen-abundant hollow carbon spheres (NHCSs) as the matrices. Attributing to the small nanostructured IrNi alloys, highly porous NHCSs with rich N-dopant, and the composition-tuned synergistic effect between Ir and Ni metals, the Ir2Ni8/NHCSs composite exhibits impressive electrocatalytic performances for HOR and HER, with an exchange current density of 246 A g Ir−1 in 0.1 M KOH and an overpotential of 54 mV at 10 mA cm−2 in 1.0 M KOH, respectively, outperforming the benchmark Pt/C and many reported noble metal-based electrocatalysts. The electrocatalyst also delivers favorable long-term stability for HOR and HER. The present study offers a feasible way to fabricate bifunctional electrocatalysts toward alkaline HOR/HER catalysis.

Development and characterization of NiMoP alloy coating for electrocatalytic hydrogen evolution reaction in alkaline media

The development of reasonable electrocatalysts with high proficiency at low cost is critical for the large-scale production of hydrogen by water splitting. NiMoP alloy coating has been proposed to be a promising alternative to noble metal-based electrocatalysts for water splitting. In this work, we successfully developed molybdenum incorporated NiP coating through chemical reduction method, and by tuning the composition of Mo, Ni, and P, a high charge delocalization is achieved on the surface of the coating. The incorporation of molybdenum into the NiP matrix effectively increases the number of active sites during the hydrogen evolution reaction. The NiMo0.02P coating exhibits a relatively small overpotential of 128 mV at a current density of 10 mA cm−2 during HER. The kinetic parameters were evaluated based on Tafel plots and EIS measurements and the possible mechanism of HER was assumed. The coating exhibit excellent stability with no surface degradation even after long-term electrochemical studies. The NiMoP coating is promising in the respect of its application for large or irregular-shaped electrodes that are required for industrial applications. The synergism between Ni and Mo and the presence of nickel and molybdenum phosphide on the surface of coating results in catalyst with excellent application in HER.

Cu2S Nanoflakes Decorated with NiS Nanoneedles for Enhanced Oxygen Evolution Activity.

Metal sulfides are considered excellent materials for oxygen evolution reaction because of their excellent conductivity and high electrocatalytic activity. In this report, the NiS-Cu2S composites were prepared on copper foam (NiS-Cu2S-CF) using a facile synthetic strategy. The scanning electron microscopy results confirmed that the NiS nanoneedles were successfully grown on Cu2S nanoflakes, greatly increasing the active sites. Particularly, the optimized 15% NiS-Cu2S-CF composite demonstrated excellent oxygen evolution activity with a small overpotential of 308 mV@20 mA cm−2, which is significantly smaller than that of noble metal-based electrocatalysts and other NiS-Cu2S-CF composites. The enhanced oxygen evolution activity is attributed to the unique morphology that can provide ample active sites, rich ion-transfer pathways, and the synergistic effect between NiS and Cu2S, which can boost the electron transfer rate.

Controllable growth of Fe-doped NiS2 on NiFe-carbon nanofibers for boosting oxygen evolution reaction

The construction of high-efficiency and low-cost electrocatalysts toward oxygen evolution reaction (OER) to improve the overall water decomposition performance is a fascinating route to deal with the clean energy application. Herein, Fe-doped NiS2 crystals grown on the surface of carbon nanofibers (CNFs) encapsulated with NiFe alloy nanoparticles ((Ni,Fe)S2/NiFe-CNFs) are fabricated through an electrospinning-calcination-vulcanization process, which has been used as a splendid electrocatalyst for OER. Benefitting from the abundant electrochemical active sites from the incorporation of Fe element in NiS2 and the synergistic effect between NiFe-CNFs and surface sulfides, the obtained (Ni,Fe)S2/NiFe-CNFs catalyst exhibits highly electrochemical activities and satisfactory durability toward OER in an alkaline medium with a low overpotential of only 287 mV at a high current density of 30 mA cm−2, and with a little decline in the current retention after 48 h, suggesting its superior OER performance even compared with some noble metal-based electrocatalysts. Additionally, a two-electrode system conducted by using the (Ni,Fe)S2/NiFe-CNFs and commercial Pt/C as electrodes, only needs a cell voltage of 1.54 V to afford 10 mA cm−2 for overall water splitting, which is even much better than the RuO2||Pt/C electrolyzer. This study offers a promising approach to prepare high-efficiency OER catalysts toward overall water splitting.

Methanol electro-oxidation to formate on iron-substituted lanthanum cobaltite perovskite oxides

Electrochemically producing formate by oxidizing methanol is a promising way to add value to methanol. Noble metal-based electrocatalysts, which have been extensively studied for the methanol oxidation reaction, can catalyze the complete oxidation of methanol to carbon dioxide, but not the mild oxidation to formate. As a result, exploring efficient and earth-abundant electrocatalysts for formate production from methanol is of interest. Herein, we present the electro-oxidation of methanol to formate, catalyzed by iron-substituted lanthanum cobaltite (LaCo1−xFexO3). The Fe/Co ratio in the oxides greatly influences the activity and selectivity. This effect is attributed to the higher affinity of Fe and Co to the two reactants: CH3OH and OH−, respectively. Because a balance between these affinities is favored, LaCo0.5Fe0.5O3 shows the highest formate production rate, at 24.5 mmol ​h−1 goxide−1, and a relatively high Faradaic efficiency of 44.4% in a series of (LaCo1−xFexO3) samples (x ​= ​0.00, 0.25, 0.50, 0.75, 1.00) at 1.6 ​V versus a reversible hydrogen electrode.

3D Urchin-like Hierarchical Black TiO2 Hollow Nanospheres: A Highly Active and Stable Electrocatalyst for Water Oxidation in Alkaline and Neutral Media

Highly proficient, earth-abundant, long-term stable bifunctional electrocatalysts are very much desired for overall water splitting reactions. The present paper describes one such stable bifunctional electrocatalyst, 3D hollow nanostructured hierarchical hydrogenated TiO2 (H-TiO2) prepared by low-temperature treatment. The H-TiO2 electrocatalyst can catalyze oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with a low overpotential of 270 and 128 mV at 10 mA cm–2 in alkaline media. Moreover, it exhibits an unusual catalytic activity in neutral electrolyte as well. The corresponding lower Tafel values indicate faster reaction kinetics on the H-TiO2 surfaces. When H-TiO2 is implemented as both the anode and cathode in a two-electrode system for overall water splitting in alkaline and neutral media, it only needs 1.57 and 2.11 V to reach a current density of 10 mA cm–2 and displays good durability for 12 h. The results suggest that Ti3+ and abundant oxygen vacancies in H-TiO2 favor HER and OER, which might originate from the enhanced electrical conductivity and rapid charge transfer rate due to the controlled interplay of the favoring factors. Thus, the present research work demonstrates a promising strategy for developing a highly efficient, stable, and carbon-free substitute for noble metal-based electrocatalysts for the overall water splitting in alkaline and neutral media.

Metal-free honeycomb-like electrocatalyst with high specific mass activity for accelerated oxygen reduction reaction in both alkaline and acidic media

The fabrication of superior oxygen reduction reaction (ORR) electrocatalysts, a challenging process, is currently focuses on expensive and rare platinum (Pt) group based metals. However, these noble metals-based electrocatalysts often suffer from disadvantages, including poor stability, exorbitant price, and low selectivity. Here, we report a simple strategy to synthesize a porous honeycomb-like carbon-based electrocatalyst codoped with nitrogen (N) and phosphorus (P) without any template. Benefiting from the synergistic impact of N, P co-doping, the as-developed honeycomb-like carbon (N,P-HLC) displays splendid ORR electrochemical activity with a large onset potential of 1.0 V (vs. RHE), and half-wave potential (E1/2) of 0.85 V (vs. RHE). What's more, N,P-HLC displays remarkable ORR performance under the super low electrocatalyst loading capacity of 0.15 mg cm−2, which is highly comparable to benchmark Pt/C (E1/2 = 0.83 V, loading capacity 0.38 mg cm−2). Meanwhile, N,P-HLC also exhibits exceptional ORR activity in the harsh acid conditions. This work introduces a robust, low-cost and superb electrocatalytic materials to replace the noble metal-based ORR electrocatalysts.

Ni(OH)2 microspheres in situ self-grown on ultra-thin layered g-C3N4 as a heterojunction electrocatalyst for oxygen evolution reaction

Oxygen evolution reaction (OER), as a part reaction of the overall water splitting, is deemed as a prospective technology for large-scale energy storage. However, the sluggish kinetics (large overpotential) and the expensive cost of noble metal-based electrocatalysts (RuO2 and IrO2) restrain the widespread usage of OER. Hence, the discovery of non-precious metal-based electrocatalysts with tiny overpotential, satisfactory current density and outstanding stability has become urgent. In this work, we have prepared ultra-thin layered g-C3N4 by a two-step thermal peeling method and synthesized Ni(OH)2/g-C3N4 composite by a simple one-step solvothermal method. Benefiting from the layered g-C3N4 as substrate, the overpotential of Ni(OH)2/g-C3N4 composite was just 240 mV at 10 mA cm−2, which gives an unexpected improvement in the electrochemical performance of the composite samples compared to pure Ni(OH)2 (η = 460 mV). Additionally, the formation of heterojunction effectively reduces the resistance to electron transport in OER, the resistance of Ni(OH)2/g-C3N4 (Rct = 38.8 Ω) is far smaller than those of bare Ni(OH)2 (Rct = 41.6 Ω) and layered g-C3N4 (Rct = 43.6 Ω) at the open circuit potential. Ni(OH)2/g-C3N4 composite samples display outstanding electrochemical stability, maintaining 85% of initial current density for 12 h. Moreover, density functional theory (DFT) demonstrates that ΔG3 of Ni(OH)2/g-C3N4 composite (η = 0.46 V) is much lower than that of bare Ni(OH)2 (η = 0.64 V). These undeniable results demonstrate that Ni(OH)2/g-C3N4 composite materials is promising alternative materials to replace precious metal-based OER electrocatalysts in the field of water splitting.

Trace amount of ceria incorporation by atomic layer deposition in Co/CoOx-embedded N-doped carbon for efficient bifunctional oxygen electrocatalysis: Demonstration and quasi-operando observations

As a promising class of alternatives to noble metal-based oxygen electrocatalysts, hybrids of metal/metal oxide (M/MO) and N-doped carbon have been widely explored. To address the often-insufficient catalytic activity of single M/MO-embedded N–C systems, researchers have introduced a second M/MO, usually via wet processes. In this work, we leverage the unique capability of atomic layer deposition (ALD) to enable the introduction of a trace amount of ceria throughout a Co/CoOx-embedded N-doped carbon nanostructure in a highly uniform and dispersed manner to maximize heterogeneous interfacial areas and thus catalytically active sites. An optimally prepared catalyst achieves an ORR onset potential of 0.95 V (0.1 M KOH) and an OER potential of 1.53 V at 10 mA cm−2 (1 M KOH) and exhibits excellent cyclic durability. Quasi-operando observations reveal the multifaceted roles of the tiny amount of introduced ceria in facilitating electrocatalytic activity and enhancing durability. The ceria activates reactant adsorbates and transfers the activated intermediates to neighboring CoOx and electronically couples with Co and N species for enhanced catalytic activity. A high concentration of trivalent Ce state is readily and continuously restored during ORR/OER for an uninterrupted activation of reactants, also contributing to a highly stable reaction.

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Potential-Induced Spin Changes in Fe/N/C Electrocatalysts Assessed by In Situ X-ray Emission Spectroscopy.

The commercial success of the electrochemical energy conversion technologies required for the decarbonization of the energy sector requires the replacement of the noble metal-based electrocatalysts currently used in (co-)electrolyzers and fuel cells with inexpensive, platinum-group metal-free analogs. Among these, Fe/N/C-type catalysts display promising performances for the reduction of O2 or CO2 , but their insufficient activity and stability jeopardize their implementation in such devices. To circumvent these issues, a better understanding of the local geometric and electronic structure of their catalytic active sites under reaction conditions is needed. Herein we shed light on the electronic structure of the molecular sites in two Fe/N/C catalysts by probing their average spin state with X-ray emission spectroscopy (XES). Chiefly, our in situ XES measurements reveal for the first time the existence of reversible, potential-induced spin state changes in these materials.

Synergies of Fe Single Atoms and Clusters on N-Doped Carbon Electrocatalyst for pH-Universal Oxygen Reduction.

Single atomic metal-N-C materials have attracted immense interest as promising candidates to replace noble metal-based electrocatalysts for the oxygen reduction reaction (ORR). The coordination environment of metal-N-C active centers plays a critical role in determining their catalytic activity and durability, however, attention is focused only on the coordination of metal atoms. Herein, Fe single atoms and clusters co-embedded in N-doped carbon (Fe/NC) that deliver the synergistic enhancement in pH-universal ORR catalysis via the four-electron pathway are reported. Combining a series of experimental and computational analyses, the geometric and electronic structures of catalytic sites in Fe/NC are revealed and the neighboring Fe clusters are shown to weaken the binding energies of the ORR intermediates on Fe-N sites, hence enhancing both catalytic kinetics and thermodynamics. This strategy provides new insights into the understanding of the mechanism of single atom catalysis.

Multimetallic nanostructures for electrocatalytic oxygen evolution reaction in acidic media

In this review, the authors highlighted the recent progress on the noble metal-based multimetallic electrocatalysts toward the oxygen evolution reaction under acidic condition.

Synthesis of noble metal-based intermetallic electrocatalysts by space-confined pyrolysis: Recent progress and future perspective

Noble metal-based intermetallics are promising electrocatalysts for sustainable energy conversion and consumption processes. High-temperature pyrolysis (>500 °C) methods are used to control their crystalline orderings, critical to their electrocatalytic activity and durability. However, the high temperature would cause severe aggregation, resulting in a low catalytic active surface area. Significant research efforts have been devoted to addressing this issue. This short review summarizes recent research progress on synthesizing noble metal-based intermetallic electrocatalysts by space-confined pyrolysis. We focus on three strategies: isolation in pores, coverture by shells, and immobilization by salts. The advantages and existing problems of different methods are highlighted. Last, important issues to be addressed in future research are also discussed. We hope that this article will stimulate future research to develop high-performance intermetallic catalysts for practical applications.

Oxygen-Deficient Cobalt-Based Oxides for Electrocatalytic Water Splitting.

An apparent increased interest has been recently devoted towards the previously untrodden path for anionic point defect engineering of electrocatalytic surfaces. The role of vacancy engineering in improving photo‐ and electrocatalytic activities of transition metal oxides (TMOs) has been widely reported. In particular, oxygen vacancy modulation on electrocatalysts of cobalt‐based TMOs has seen a fresh spike of research work due to the substantial improvements they have shown towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Oxygen vacancy engineering is an effective scheme to quintessentially tune the electronic structure and charge transport, generate secondary active surface phases, and modify the surface adsorption/desorption behavior of reaction intermediates during water splitting. Based on contemporary efforts for inducing oxygen vacancies in a variety of cobalt oxide types, this work addresses facile and environmentally benign synthesis strategies, characterization techniques, and detailed insight into the intrinsic mechanistic modulation of electrocatalysts. It is our foresight that appropriate utilization of the principles discussed herein will aid researchers in rationally designing novel materials that can outperform noble metal‐based electrocatalysts. Ultimately, future electrocatalysis implementation for selective seawater splitting is believed to depend on regulating the surface chemistry of active and stable TMOs.

Melamine-assisted synthesis of ultrafine Mo2C/Mo2N@N-doped carbon nanofibers for enhanced alkaline hydrogen evolution reaction activity

Noble metal-free electrocatalysts with high activity are highly desirable for the large-scale application of hydrogen evolution reaction (HER). Mo2C-based nanomaterials have been proved as a promising alternative to noble metal-based electrocatalysts owing to the Pt-resembled d-band density and optimal intermediates-adsorption properties. However, the aggregation and excessive growth of crystals often occur during their high-temperature synthesis procedure, leading to low catalytic utilization. In this study, the ultrafine Mo2C/Mo2N heterostructure with large surface and interface confined in the N-doped carbon nanofibers (N-CNFs) was obtained by a melamine-assisted method. The synergistic effect of Mo2C/Mo2N heterostructure and plenty active sites exposed on the surface of ultrafine nanocrystals improves the electrocatalytic activity. Meanwhile, the N-CNFs ensure fast charge transfer and high structural stability during reactions. Moreover, the in-situ synthesis method strengthens the interfacial coupling interactions between Mo2C/Mo2N heterostructure and N-CNFs, further enhancing the electronic conductivity and electrocatalytic activity. Owing to these advantages, Mo2C/Mo2N@N-CNFs exhibit excellent HER performance with a low overpotential of 75 mV at a current density of 10 mV cm−2 in alkaline solution, superior to the single phased Mo2C counterpart and recently reported Mo2C/Mo2N based catalysts. This study highlights a new effective strategy to design efficient electrocatalysts via integrating heterostructure, nanostructure and carbon modification.

Robust copper nanocrystal/nitrogen-doped carbon monoliths as carbon monoxide-resistant electrodes for methanol oxidation reaction

Noble metal-based electrocatalysts present high activities for methanol oxidation reaction (MOR), but are limited by their high cost, low stability and poor resistance to carbon monoxide (CO) poisoning. The development of active and stable non-noble metal electrocatalysts for MOR is desired, but remains a challenge. Herein, we report a simple strategy to make copper nanocrystal/nitrogen-doped carbon (Cu/N-C) monoliths, which can serve as active and robust electrodes for MOR. Copper nanocrystals were electrochemically deposited onto a conductive polyaniline hydrogel and calcined to form Cu/N-C monolith, where the active copper nanocrystals are protected by nitrogen-doped carbon. Owing to their extremely high electrical conductivity (1.25 × 105 S cm−1) and mechanical robustness, these Cu/N-C monoliths can be directly used as electrodes for MOR, without using substrates or additives. The optimal Cu/N-C (FT) @500 monolith shows a high MOR activity of 189 mA cm−2 at 0.6 V vs. SCE in alkaline methanol solution, superior to most of reported Cu-based MOR catalysts. Cu/N-C (FT)@500 also presents a better stability than Pt/C catalyst in the long-term MOR test at high current densities. Upon carbon monoxide (CO) poisoning, Cu/N-C (FT)@500 retains 96% of its MOR activity, far exceeding the performance of Pt/C catalyst (61% retention). Owing to its facile synthesis, outstanding activity, high stability and mechanical robustness, Cu/N-C (FT)@500 monolith is promising as a low-cost, efficient and CO-resistant electrocatalyst for MOR.

Evaluation of electrochemical properties for water splitting by NiO nano-cubes synthesized using Olea ferruginea Royle

Low-cost and efficient electro-catalysts can play a key role in electro-catalytic water oxidation reaction. Hereinto, highly stable semiconducting nickel oxide (NiO) nanoparticles were synthesized by a facile, biomimetic and cost effective one pot synthesis route. Nickel (II) acetate tetrahydrate was used as a precursor and medicinal plant Olea ferruginea Royle as a reducing agent. The biosynthesized NiO NPs were scrutinized by UV–Vis absorption spectroscopy UV Vis.), Fourier transform infrared spectroscopy (FTIR), gas chromatography mass spectroscopy (GC–MS), X-ray diffractometer (XRD), energy dispersive X-ray spectrometer (EDX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The average particle size of the NiO NPs was 13 ± 1 nm which was calculated by Scherrer equation, with uniform cubic shape. NiO supported on Ni-foam was investigated as an electro-catalyst for water splitting application. The results revealed an over potential of 0.41 V and Tafel slope of 97 mVdec−1 at 10 mA/cm2 which is comparable to benchmark catalysts. These results suggests that the organic framework derived NiO could be a competitor to the chemically synthesized electrode material that comprises of costly reagents and require advanced synthesis approaches. Thus, in present study we believe that bio-assisted NiO is an efficient electrode material for water splitting studies and could be a sustainable step towards the replacement of noble metal-based electro-catalysts for energy conversion.