Electrocatalytic H2 production from seawater over Co, N-codoped nanocarbons

Growth of MoSe2 nanosheets with small size and expanded spaces of (002) plane on the surfaces of porous N-doped carbon nanotubes for hydrogen production

MoSe₂ nanosheets with small size and expanded spaces between the (002) planes were grown on the surfaces of porous N-doped carbon nanotubes (NCNTs) with much higher HER activity than carbon nanotubes without N dopants. Owing to the synergistic effects between the MoSe₂ nanosheets and the porous NCNT substrate, MoSe₂/NCNTs exhibit superior HER activity to layered metal chalcogenides reported previously.

Significant Enhancement of Water Splitting Activity of N-Carbon Electrocatalyst by Trace Level Co Doping

Replacement of precious metal electrocatalysts with highly active and cost efficient alternatives for complete water splitting at low voltage has attracted a growing attention in recent years. Here, this study reports a carbon-based composite co-doped with nitrogen and trace amount of metallic cobalt (1 at%) as a bifunctional electrocatalyst for water splitting at low overpotential and high current density. An excellent electrochemical activity of the newly developed electrocatalyst originates from its graphitic nanostructure and highly active Co-Nx sites. In the case of carefully optimized sample of this electrocatalyst, 10 mA cm(-2) current density can be achieved for two half reactions in alkaline solutions-hydrogen evolution reaction and oxygen evolution reaction-at low overpotentials of 220 and 350 mV, respectively, which are smaller than those previously reported for nonprecious metal and metal-free counterparts. Based on the spectroscopic and electrochemical investigations, the newly identified Co-Nx sites in the carbon framework are responsible for high electrocatalytic activity of the Co,N-doped carbon. This study indicates that a trace level of the introduced Co into N-doped carbon can significantly enhance its electrocatalytic activity toward water splitting.

Structural Evolution of Co-Based Metal Organic Frameworks in Pyrolysis for Synthesis of Core-Shells on Nanosheets: Co@CoOx@Carbon-rGO Composites for Enhanced Hydrogen Generation Activity

In this article, Co-based metal organic frameworks (MOFs) with two shapes were used as pyrolysis precursor to synthesize multilayer core-shells composites loaded on reduced graphene oxide (rGO) sheets. The core-shell structures were obtained by the formation of cores from metal ions and carbon shells from carbonization of ligands. Controllable oxidation of Co cores to CoOx shells generated multilayer core-shell structures anchored onto the surface of rGO sheets. The N-doped composites were obtained by adding poly vinylpyrrolidone. The multilayer core-shells composites exhibited superior catalytic activity toward hydrogen generation compared to their single layer counterparts. By using the N-doped multilayer composites, high hydrogen generation specific rate of 5560 mL min(-1) gCo(-1) was achieved at room temperature. The rGO sheets in composites improved their structure stability. These catalysts exhibited high stability after used five cycling. This synergistic strategy proposes simple, efficient, and versatile blue-prints for the fabrication of rGO composites from MOFs-based precursors.

Active and Durable Hydrogen Evolution Reaction Catalyst Derived from Pd-Doped Metal-Organic Frameworks

The water electrolysis is of critical importance for sustainable hydrogen production. In this work, a highly efficient and stable PdCo alloy catalyst (PdCo@CN) was synthesized by direct annealing of Pd-doped metal-organic frameworks (MOFs) under N2 atmosphere. In 0.5 M H2SO4 solution, PdCo@CN displays remarkable electrocatalytic performance with overpotential of 80 mV, a Tafel slope of 31 mV dec(-1), and excellent stability of 10 000 cycles. Our studies reveal that noble metal doped MOFs are ideal precursors for preparing highly active alloy electrocatalysts with low content of noble metal.

Active Sites Implanted Carbon Cages in Core-Shell Architecture: Highly Active and Durable Electrocatalyst for Hydrogen Evolution Reaction

Low efficiency and poor stability are two major challenges we encounter in the exploration of non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) in both acidic and alkaline environment. Herein, the hybrid of cobalt encapsulated by N, B codoped ultrathin carbon cages (Co@BCN) is first introduced as a highly active and durable nonprecious metal electrocatalysts for HER, which is constructed by a bottom-up approach using metal organic frameworks (MOFs) as precursor and self-sacrificing template. The optimized catalyst exhibited remarkable electrocatalytic performance for hydrogen production from both both acidic and alkaline media. Stability investigation reveals the overcoating of carbon cages can effectively avoid the corrosion and oxidation of the catalyst under extreme acidic and alkaline environment. Electrochemical active surface area (EASA) evaluation and density functional theory (DFT) calculations revealed that the synergetic effect between the encapsulated cobalt nanoparticle and the N, B codoped carbon shell played the fundamental role in the superior HER catalytic performance.

Nanostructured molybdenum phosphide/N,P dual-doped carbon nanotube composite as electrocatalysts for hydrogen evolution

MoP/N,P dual-doped carbon nanotube composite exhibited excellent activity and long-term stability toward HER both in acidic and alkaline media, superior to most of catalysts reported previously.

Well-dispersed CoS2 nano-octahedra grown on a carbon fibre network as efficient electrocatalysts for hydrogen evolution reaction

Well-dispersed CoS₂ nanocrystals grown on a carbon fibre network have been shown to be efficient electrocatalysts for hydrogen evolution reaction.

Theoretical Insight into the Hydrogen Evolution Activity of Open-Ended Carbon Nanotubes

Carbon nanotubes (CNTs), while inactive by themselves, are often used as a platform in the search of new catalysts for the hydrogen evolution reaction (HER) by introducing metal nanoparticles or other dopants. Here, we examine the HER activity of pristine open-ended CNTs considering both the effects of chirality and hydrogen coverage using electronic structure calculations. The results indicate that the formation of different 5-ring structures at the end of the CNT introduces surface sites that are highly active toward HER, whereas the activity of traditional 6-ring sites is not greatly altered by tube termination. At fixed hydrogen coverage, the enhanced activity of these sites was attributed to valence orbitals residing close to the highest occupied molecular level facilitating electron transfer to protons.

Interconnected Co-Entrapped, N-Doped Carbon Nanotube Film as Active Hydrogen Evolution Cathode over the Whole pH Range

The use of electrocatalysts with low metal content (metal-deficient) or metal free for the hydrogen evolution reaction (HER) can prevent or decrease metal ion release, which reduces environmental impact; development of such catalysts with high activity and durability over the whole pH range is thus highly desired but still remains a huge challenge. Herein, we describe the direct growth of a film consisting of interconnected Co-entrapped, N-doped carbon nanotubes on carbon cloth using chemical vapor deposition from dicyanodiamine using a Co3 O4 nanowire array as catalyst. This integrated architecture is used as a flexible 3D electrode for the electrolytic hydrogen evolution with outstanding catalytic activity and durability in acidic media. Moreover, this electrode is also highly efficient under neutral and basic conditions. It offers us an attractive carbon-based metal-deficient HER catalyst outperforming most transition-metal and all metal-free/deficient catalysts.

Carbon-protected bimetallic carbide nanoparticles for a highly efficient alkaline hydrogen evolution reaction

The hydrogen evolution reaction (HER) is one of the two important half reactions in current water-alkali and chlor-alkali electrolyzers. To make this reaction energy-efficient, development of highly active and durable catalytic materials in an alkaline environment is required. Herein we report the synthesis of carbon-coated cobalt-tungsten carbide nanoparticles that have proven to be efficient noble metal-free electrocatalysts for alkaline HER. The catalyst affords a current density of 10 mA cm(-2) at a low overpotential of 73 mV, which is close to that (33 mV) required by Pt/C to obtain the same current density. In addition, this catalyst operates stably at large current densities (>30 mA cm(-1)) for as long as 18 h, and gives nearly 100% Faradaic yield during alkaline HER. The excellent catalytic performance (activity and stability) of this nanocomposite material is attributed to the cooperative effect between nanosized bimetallic carbide and the carbon protection layer outside the metal carbide. The results presented herein offer the exciting possibility of using carbon-armoured metal carbides for an efficient alkaline HER, although pristine metal carbides are not, generally, chemically stable enough under such strong alkaline conditions.

Carbon-armored Co9S8 nanoparticles as all-pH efficient and durable H2-evolving electrocatalysts

Splitting water to produce hydrogen requires the development of non-noble-metal catalysts that are able to make this reaction feasible and energy efficient. Herein, we show that cobalt pentlandite (Co9S8) nanoparticles can serve as an electrochemically active, noble-metal-free material toward hydrogen evolution reaction, and they work stably in neutral solution (pH 7) but not in acidic (pH 0) and basic (pH 14) media. We, therefore, further present a carbon-armoring strategy to increase the durability and activity of Co9S8 over a wider pH range. In particular, carbon-armored Co9S8 nanoparticles (Co9S8@C) are prepared by direct thermal treatment of a mixture of cobalt nitrate and trithiocyanuric acid at 700 °C in N2 atmosphere. Trithiocyanuric acid functions as both sulfur and carbon sources in the reaction system. The resulting Co9S8@C material operates well with high activity over a broad pH range, from pH 0 to 14, and gives nearly 100% Faradaic yield during hydrogen evolution reaction under acidic (pH 0), neutral (pH 7), and basic (pH 14) media. To the best of our knowledge, this is the first time that a transition-metal chalcogenide material is shown to have all-pH efficient and durable electrocatalytic activity. Identifying Co9S8 as the catalytically active phase and developing carbon-armoring as the improvement strategy are anticipated to give a fresh impetus to rational design of high-performance noble-metal-free water splitting catalysts.

Noble metal-free hydrogen evolution catalysts for water splitting

This review summarizes the recent research efforts toward noble metal-free hydrogen evolution electrocatalysts.