Archive for the ‘Materials’ Category

Copper A3 Coupling using a Switchable Homogeneous/Heterogeneous Catalyst

A MOC, I learned this week, is a metal-organic cage. I was familiar with MOMs, MOFs and MOBs, but MOCs were a new one. A MOM (metal-organic material) is a coordination-driven assembly constructed from metal nodes linked by organic ligands. MOMs encompass both MOFs (metal-organic frameworks) and MOCs (metal-organic cages). A MOF is an extended network with the potential for inner porosity, and a MOC is a discrete metal-ligand cluster. And that’s just about as far down the nomenclature rabbit hole I’m willing to go. If you’re keeping up you’ll realise that I forgot one! A MOB is a crowd of graduate students competing for free coffee at the public seminar.

Dong and co-workers at Shandong Normal University designed and prepared a MOM catalyst constructed from copper(II) nodes and a tripodal ligand consisting of a phenylic wheel functionalised with diketones. In chloroform these two components arrange into discrete MOC assemblies containing two tripodal ligands and three copper ions. The copper ions in the cluster are each coordinated to two diketone moieties (in a acetylacetonate-like fashion) in a quasi-square planar arrangement.

Synthesis of the tripodal ligand functionalised with diketone coordinating moieties.

Synthesis of the tripodal ligand functionalised with diketone coordinating moieties.

An interesting property of the material is that it can switch between the MOC form, soluble in halogenated solvents, and an insoluble MOF that assembles upon addition of 1,4-dioxane. 1,4-Dioxane is both an anti-solvent and a ligand; coordination between copper and 1,4-dioxane binds the discrete MOC cages to each other, arranging them into the extended MOF structure. This behaviour can be exploited to prepare a practical catalyst that combines the benefits of both homogeneous and heterogeneous catalysis, namely that homogeneous catalysts are generally more efficient, selective and easier to study, but heterogeneous catalysis are easier to recover and recycle. What better way to solve this problem than with a catalyst that is homogeneous during the reaction conditions, but heterogeneous when it comes to product separation?

Reversible metal-organic cage MOC(top left)-MOF(top right) metal-organic framework transition mediated by the addition of 1,4-dioxane. Coordination bonds between 1,4-dioxane shown (bottom image).

Reversible MOC(top left)-MOF(top right) transition mediated by the addition of 1,4-dioxane. Coordination bonds between 1,4-dioxane shown (bottom image).

The authors used the A3 coupling reaction to demonstrate this concept in a catalytic reaction. The A3 reaction is a transition metal-catalysed, multi-component coupling reaction between aldehydes, alkynes and amines. The products are propargylamines, practical synthetic intermediates for the synthesis of nitrogen heterocycles. The A3 reaction has been extensively studied and can be effected by a wide range of transition metal catalysts. Its versatility makes it a popular choice as a model catalytic reaction to demonstrate innovative ideas in catalytic design – as the authors have done here.

Coordination-driven assemblies have a unique potential for the synthesis of differentially soluble materials, used by the authors for novel catalytic design. Whether this particular metal-ligand combination can be applied to other copper catalysed reactions remains to be seen, nevertheless the principle offers an innovative approach that augments the range of methods striving to bridge the gap between homogeneous and heterogeneous catalysis.

To find out more please read:

Cu3L2 metal-organic cages for A3-coupling reactions: reversible coordination interaction triggered homogeneous catalysis and heterogeneous recovery

Gong-Jun Chen, Chao-Qun Chen, Xue-Tian Li, Hui-Chao Ma and Yu-Bin Dong.
Chem. Commun., 2018, 54, 11550-11553
DOI: 10.1039/c8cc07208f

About the author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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The Birth of a Semiconducting Metal Organic Framework by Sulfur Coordination

Metal organic frameworks (MOFs) are crystalline nanomaterials composed of metal ions or clusters coordinated with organic ligands. Owing to the versatility of their building blocks, MOFs have multiple functionalities and can serve as gas separators, sensors, catalysts, electrode materials etc. Now the structure diversity of MOFs is further enriched by Wu and coworkers from Soochow University, China. Specifically, the researchers synthesized a semiconducting MOF with tetra-coordinated sulfur units. This breakthrough was recently published in ChemComm.

The uniqueness of the synthesized semiconducting MOF (MCOF-89) is its square-planar tetra-coordinated metal-sulfur structure, which is observed in MOFs for the first time. It was believed that putting a sulfur atom next to a metal node of MOFs was extremely difficult, because of the large discrepancy in bonding energy between metal-sulfur bonds and conventional metal-carboxylate bonds. Incorporating sulfur atoms thereby could undermine the structural stability of MOFs.

The authors addressed this challenge by designing a tetra-coordination environment as illustrated in Figure 1. The four manganese-sulfur bonds effectively reinforced the unstable S coordination. MCOF-89 was synthesized via a solvothermal reaction with Mn(CH3COO)2 and thiourea as the Mn and S sources, respectively.

Figure 1. The structure of MCOF-89. The illustration on the left is a three-dimensional lattice structure (the red, green and yellow balls represent oxygen, manganese and sulfur), and the structure on the right shows the Mn-S square-planar tetra-coordination configuration (M = manganese).

The synthesized S-incorporated MOF is a semiconductor with a bandgap of 2.82 eV. Additionally, this MOF is photoactive and is able to generate a photocurrent of ~1.9 µA/cm2 upon light irradiation.

This work exemplifies how molecular design can lead to the discovery of novel MOFs with extraordinary structures. It could also inspire other synthesis protocols toward various metal-chalcogenide-containing MOFs with unexpected properties.

 

To find out more please read:

A Semiconducting Metal-Chalcogenide–Organic Framework with Square-Planar Tetra-Coordinated Sulfur

Huajun Yang, Min Luo, Zhou Wu, Wei Wang, Chaozhuang Xue and Tao Wu

Chem. Commun., 2018, 54, 11272-11275

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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MOFS, ZMOFS and Automobiles

Mohamed Eddaoudi and co-workers at KAUST have synthesised a porous metal organic framework (MOF) constructed from carboxylic acid-functionalised imidazole linkers coordinated to yttrium and potassium cations. The researchers classified this material as a zeolite-like MOF (ZMOF) due to its topological resemblance to the naturally occurring zeolite mineral analcime.

The material’s architecture, with cylindrical channels and a pore aperture measuring 3.8 x 6.2 Å, promised utility as a molecular sieve, and the authors showed the ZMOF could be used to sort small chain alkanes based on their level of branching. Linear and mono-branched pentanes and butanes were adsorbed by the material for different lengths of time (linear isomers were retained longer than their branched counterparts) allowing kinetic separation, while the di-branched alkane 2,2,4-trimethylpentane was excluded entirely. The authors anticipate that this material could have practical applications in crude oil refining, to upgrade petroleum into more energy-efficient fuels with reduced emissions.

ZMOF zeolite-like metal organic framework crystal structure with analcime (ana) topology showing channels and pore aperture.

ZMOF crystal structure with analcime (ana) topology showing channels and pore aperture.

The petroleum used to power internal combustion engines consists of a mixture of low molecular weight, linear and branched alkanes. The research octane number (RON) is a standard measure of petroleum performance, and indicates how much pressure a fuel can withstand before self-igniting (knocking) in the engine. High compression engines, which are more energy efficient and release less emissions than regular engines, require high RON fuels.

The RON increases with the proportion of branched alkanes, so can be improved by supplementing fuels with branched isomers obtained by catalytic isomerisation. This process generates a mixture of linear and branched alkanes, so the desired products must be isolated via fractional distillation, which is energy intensive. This creates a dilemma: high RON fuels are more energy efficient, but their energy-intensive production reduces the net benefit.

The authors envisaged an energy-efficient strategy for increasing the RON of petroleum fuels: A low RON fuel is pumped into the engine, where it encounters a separation chamber consisting of ZMOF-based membranes. The membrane excludes and redirects di-branched alkanes, which have a very high RON, to the internal combustion engine. The low RON fraction, consisting of mono-branched and linear alkanes, passes through the ZMOF pores to undergo further reforming processes downstream. In other words: low RON fuels go in, but high RON fuels are combusted.

Scheme showing how ZMOF materials could be used to upgrade gasoline by separating alkanes based on their level of branching. zeolite-like metal organic framework petroleum reforming

Scheme showing the RON of common small-chain alkanes and the use of ZMOF membranes in upgrading gasoline by separating alkanes based on their level of branching

In this work the authors show the potential of ZMOFs to maximise the energetic potential and reduce emissions of petroleum based fuels, while also offering a glimpse of the more general strategy of energy-efficient separations of chemically-similar molecules using tailored materials.

To find out more please read:

Upgrading gasoline to high octane number using zeolite-like metal organic framework molecular sieve with ana-topology

M. Infas H. Mohideen, Youssef Belmabkhout, Prashant M. Bhatt, Aleksander Shkurenko, Zhijie Chen, Karim Adil, Mohamed Eddaoudi.
Chem. Commun., 2018, 54, 9414-9417
DOI: 10.1039/c8cc04824j

About the author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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Better Make It A Double

Synthesizing nanomaterials consisting of two-particle pairs, or dimers, is no longer a headache. Hongyu Chen and coworkers from Nanjing Tech University, China recently developed a protocol that can produce gold dimers with a record high yield. This breakthrough is published in Chem. Commun.

Dimers are suitable platforms to study the effects of particle-particle interactions on the electrical and optical properties of the constituent materials. Unfortunately, no conventional synthesis methods to exclusively produce dimers from single particles have been successful. This is because of the uncontrollable particle-aggregation rate that leads to the formation of multi-particle clusters. Therefore, how to couple single particles into dimers without triggering their further aggregation has become a tough nut to crack.

Chen and coworkers found a solution by developing a polymer-assisted method that generates gold dimers with high yield. Firstly, they encapsulated individual gold nanoparticles with polymer shells made of polystyrene-b-poly(acrylic acid). Under optimized conditions, the gold nanoparticles were mostly coupled into dimers (Figure 1), achieving a dimer yield of 65%. This is the highest dimer yield achieved for one-step synthesis methods.

Figure 1. (a) A transmission electron microscopy (TEM) image of the polymer-encapsulated gold single particles. (b) A TEM image and (c) a scanning electron microscopy image of the synthesized gold dimers. All scale bars are 200 nm.

The key to this success is due to three factors: temperature, solvent composition and acid concentration. All these factors can change the strength of the repulsion force among the polymer shells. The force must be meticulously tuned to a level that is weak enough to induce 1-to-1 coupling, but strong enough to prevent 1-to-multiple or multiple-to-multiple aggregation. Through a set of control experiments, the authors identified the optimal conditions to be 60 oC, dimethylformamide/water (v/v)=6:1 and 5 mM of hydrochloric acid.

The method demonstrated herein could be extended to other particles. It may also inspire versatile synthesis strategies towards complex nanostructures with high selectivity.

 

To find out more please read:

Controllable Oligomerization: Defying Step-Growth Kinetics in the Polymerization of Gold Nanoparticles

Xuejun Cheng, Gui Zhao, Yan Lu, Miao Yan, Hong Wang and Hongyu Chen

Chem. Sci., 2018, DOI: 10.1039/C8CC03424A

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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ChemComm poster prize winner at the 2018 European Materials Research Society Spring Meeting

The 2018 European Materials Research Society (EMRS) Spring Meeting was held from the 18th – 22nd June in the Strasbourg Convention Centre in France.

The EMRS Spring Meeting is the society’s major conference and covers all aspects of materials science including energy and environment, biomaterials, semiconductors, nanomaterials, functional materials, and materials processing and characterization. It offers on average 25 topical symposia and is widely recognised as being of the highest international significance, with approximately 2,500 attendees every year.

ChemComm is proud to announce that the ChemComm poster prize was awarded to Dr Manal Alkhamisi from the University of Nottingham (School of Physics and Astronomy) for ‘The Growth and Fluorescence of Phthalocyanine Monolayers and Thin Films on Hexagonal Boron Nitride’. Manal was awarded the prize by ChemComm Associate Editor Steven De Feyter.

Well done Manal!

 

ChemComm Associate Editor Steven De Feyter (left) awarding the poster prize to Dr Manal Alkhamisi (right)

 

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Come for the colour changing crystals, stay for the science

Synthesis of copper bimetallic complexes from imidazolyl ligands, and the solvatochromic materials formed upon crystallization and solvent guest-exchange. The solvatochromic behaviour was quantified with visible-region diffuse reflectance spectra.

Synthesis of copper bimetallic complexes from imidazolyl ligands, and the solvatochromic materials formed upon crystallization and solvent guest-exchange. The solvatochromic behaviour was quantified with visible-region diffuse reflectance spectra.

During the first inorganic chemistry course I took during my undergraduate degree, our professor started the class by passing around some mineral samples, promising us that if we pursued the chemistry of metals we could work with beautifully coloured crystals every day. At the time, colour seemed like such a trite detail amongst the complexity of the subject. Why would you choose a field of study based on something so simple? Well, after a PhD dominated by pale yellow oils, I think I get it now.

Nikolayenko and Barbour at the University of Stellenbosch in South Africa bring us colour! The authors synthesised organometallic copper complexes, which crystallise to form porous single crystals that drastically change colour upon absorption of various solvents. The authors investigated the solvatochromic mechanism using X-ray crystallography, EPR, UV-visible spectroscopy and DFT calculations. Solvatochromic materials are not just made to look pretty; they have potential to be used as sensitive, selective and recyclable sensors to detect solvent vapours with useful applications in industrial process risk management, chemical threat detection and environmental monitoring.

The researchers synthesised a series of complexes comprised of a bidentate ligand with 2-methylimidazolyl groups coordinated to copper(II) ions. The complexes stack to form channels in the crystal, capable of trapping solvent molecules to give different coloured crystals: DMSO and THF-containing crystals are green (λmax = 574 nm and 540 nm, respectively), those containing acetonitrile are red (λmax = 624 nm), and crystals trapping acetone, ether and pentane are yellow (λmax = 588), orange (λmax = 598 nm) and red/brown (λmax = 592 nm), respectively.

The authors revealed a correlation between the size of the solvent guest, coordination geometry of the copper complex, and the ligand field splitting. Small guests such as acetonitrile minimally perturb the metallocyclic framework, preserving a rhombic ligand field geometry (large δxy of g values in the EPR spectrum), small ligand d-orbital splitting and red-shifted optical spectra. Large guests such as THF have the opposite effect, giving ligand field geometries approaching tetragonal (small δxy), large ligand field d-orbital splitting and blue-shifted optical spectra.

By delving into the complexity beneath a seemingly simple phenomenon, Nikolayenko, Barbour and their co-workers have shown using a series of single-crystal complexes that there is nothing simple about colour (and nothing trite about detail).

To find out more please read:

Supramolecular solvatochromism: mechanistic insight from crystallography, spectroscopy, and theory

Varvara I. Nikolayenko, Lisa M. van Wyk, Orde Q. Munro, Leonard J. Barbour.
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c8cc02197j

About the author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

 

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Pt3Ni-Coated Palladium Nano-branches Outperformed Pt in Catalyzing Ethanol Oxidation

Researchers in China recently developed a new Pd-based catalyst that outperformed Pt, the benchmark catalyst for electrochemical oxidation of ethanol. This catalyst, synthesized by a one-pot chemical reduction method, consists of branched Pd nanocrystals coated with thin Pt3Ni shells.

The ethanol oxidation reaction (EOR) is a typical anode reaction that drives the energy output from fuel cells. Due to its intrinsically slow kinetics, the reaction requires proper EOR catalysts to facilitate the oxidation. Pt-based materials are highly active in promoting EOR, but the scarcity of Pt leads to high costs and demands efficient methods to recycle these materials. In addition, the instable catalytic activity of Pt significantly reduces the lifetime of EOR catalysts containing Pt. Clearly, developing inexpensive EOR catalysts with comparable performance to Pt is meaningful for the affordability and durability of fuel cells.

A research team led by Shuifen Xie at Huaqiao University and Shenzhen Research Institute of Xiamen University in China, have demonstrated a one-pot chemical reduction method of a novel EOR catalyst. This catalyst is made of Pt3Ni-coated Pd nanocrystals as shown in Figure 1. There are three main advantages for this catalyst over the benchmark Pt: Firstly, the core material Pd is more affordable. Secondly, the ultrathin Pt-alloy coating, Pt3Ni, contains relatively less Pt and is reported to exhibit high EOR catalytic activity. Thirdly, the little lattice mismatch between Pt and Pd allows seamless integration of the two metals that is beneficial to preserve the structural integrity and ensure excellent durability.

Figure 1. (a) The schematic illustration showing the key steps of the one-pot chemical reduction method. The catalyst is formed via consecutive reduction of Pd2+, Pt2+ and Ni2+ to Pd nano-branches, Pt nanoparticles and Pt3Ni coatings, respectively. (b) A TEM image of a representative morphology of a branched nanocrystal. (c) Elemental mappings depict that Pt and Ni elements exist mainly in the shell while Pd is in the core.

Electrochemical characterizations revealed that the catalytic performance of the Pt3Ni-coated Pd nanocrystals outperformed those of two commercial catalysts: Pt/C (Pt particles supported on activated carbon) and Pd black (a fine powder elemental Pd). Figure 2a compares the linear-sweep voltammograms of the three samples. The synthesized catalyst showed appreciably enhanced oxidation current at potentials beyond 0.4 V vs. RHE. The histograms in Figure 2b clearly display that the mass activity and the specific activity of the synthesized nanocrystals are the highest. The authors ascribed the superior performance to the high surface area (42.50 m2 g-1), the ultrathin Pt3Ni coating with its {111} crystal planes exposed, and the core-shell configuration.

Figure 2. (a) Linear-sweep voltammograms of the synthesized catalyst (Pd@Pt3Ni/C), Pt/C and Pd black. (b) The comparison of mass activity (i.e. oxidation current normalized to the masses of the catalysts) and specific activity (i.e. oxidation current normalized to the areas of the catalysts) of Pd@Pt3Ni/C, Pt/C and Pd black.

This work signifies the feasibility of Pd-based nano-catalysts as alternatives to Pt towards catalyzing EOR. It is also expected to encourage the effort in developing a diverse array of inexpensive and high-performance catalysts for other reactions pertaining to fuel cells, including but not limited to oxidation of fuels other than ethanol and oxygen reduction reactions.

To find out more please read:

One-Pot Synthesis of Pd@Pt3Ni Core-Shell Nanobranches with Ultrathin Pt3Ni{111} Skins for Efficient Ethanol Electrooxidation

Yuanyuan Wang, Wei Wang, Fei Xue, Yong Cheng, Kai Liu, Qiaobao Zhang, Maochang Liu and Shuifen Xie

Chem. Commun., 2018, DOI: 10.1039/c8cc02816h

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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The CO2-Capturing Mechanism of Quaternary Nitrogen-Containing Polymers Revealed Experimentally

A group of scientists from Washington University at St. Louis, USA have disclosed experimentally how CO2 is captured by polymers with quaternary nitrogen cations. Using solid-state nuclear magnetic resonance (NMR), the authors established that CO2 molecules were absorbed as bicarbonate anions (HCO3).

The increasing amount of CO2 has posed a number of concerning environmental issues such as climate change, rising sea level and ocean acidification. Capturing CO2 from the atmosphere is an effective way to lower the CO2 concentration. Recently, a family of polymer absorbents containing quaternary nitrogen functional groups, termed humidity-swing polymers, have been identified as promising absorbents to absorb CO2 directly from air. However, the limited understanding of the chemical mechanism related to their CO2-capturing capability hindered the development of these promising absorbents.

In ChemComm, Yang et al. used solid-state 13C NMR to explore how CO2 molecules were captured and released. Figure 1a presents the NMR spectra of a humidity-swing polymer absorbent itself (top), upon contacting with CO2 (middle) and after releasing CO2 (bottom). The most striking feature is the appearance of an additional sharp peak at a chemical shift of 161 ppm in the middle spectrum, which did not show up in the other two spectra. The authors further studied the shape evolution of the additional peak, with respect to temperature, and concluded that the peak was due to HCO3 anions. Additionally, the authors also identified the presence of hydroxide anions in the absorbent after CO2 was released.

Figure 1. (a) The solid-state 13C NMR spectra of the humidity-swing polymeric absorbent (structure shown in the inset of the middle spectrum) itself (top), upon contacting with CO2 (middle) and after releasing CO2 (bottom). (b) The proposed pathways of how CO2 molecules interact with the quaternary-N anions of the absorbent.

The researchers then proposed the CO2 adsorption-desorption mechanism (illustrated in Figure 1b) based on the experimental results. The storage and release of CO2 depend on the humidity level of the surroundings: When the humidity is low, the polymer absorbs CO2 and forms HCO3 anions; the negative charge of HCO3 is counter-balanced by the neighboring quaternary N cations. When the humidity is increased, HCO3 anions combine with water and decompose to CO2 and hydroxide anions. This proposed pathway does not involve CO32- anions, which differs from the previously-reported mechanisms derived from theoretical simulations.

The published results represent the first set of experimental evidence elucidating how CO2 molecules interact with humidity-swing polymeric absorbents. The acquired mechanistic insight could provide valuable guidelines for the design of CO2 absorbents with ultrahigh absorption capacity.

 

To find out more please read:

Humidity-Swing Mechanism for CO2 Capture from Ambient Air
Hao Yang, Manmilan Singh and Jacob Schaefer
Chem. Commun., 2018, DOI: 10.1039/c8cc02109k

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Ruthenium Currency for a Hydrogen Fuel Economy

A group of researchers at the Chinese Academy of Sciences and Southwest University want us to kick the fossil fuels habit. Their research comes to us from China, a country using roughly one quarter of the world’s yearly energy consumption, and where the finite nature of fossil fuels is a very real threat to energy supply security. Leading in energy use, China also leads the world in electricity production from renewable sources and investment in clean energy projects.

Hydrogen is considered a viable alternative to fossil fuels as it is energy rich, more so than petrol or ethanol at 39 kWh/kg (petrol: 13 kWh/kg, ethanol: 8.2 kWh/kg), and upon combustion emits only water vapour. However, hydrogen is often obtained from fossil fuels, and it will only be a practical option for the world’s future energy needs if it can be produced from a renewable source.

Preparation of the Ru2P/reduced graphene oxide electrocatalyst for the hydrogen evolution reaction

Preparation of the Ru2P/reduced graphene oxide catalyst

To this end, water splitting offers a solution. In a water electrolysis cell, hydrogen is produced at the cathode via the hydrogen evolution reaction (HER, 2H+ + 2e –> H2), and molecular oxygen is produced at the anode (2H2O –> O2 + 4H+ + 4e). It is ideal in theory, but high energy efficiencies are required to make water splitting viable, and this relies on the development of catalytic electrodes to minimize overpotentials required to drive the reaction. Currently, state of the art HER electrocatalysts use platinum, which is expensive and rare. Furthermore, platinum catalysts are most efficient in an acidic electrolyte and proceed 2-3 times slower in alkaline solutions. On the other hand, the best oxygen evolution catalysts perform better in alkaline environments. Using an alkaline electrolyte has overall advantages as it is less corrosive, thus increasing the stability and lifetime of the electrolytic cell.

The authors have developed a HER catalyst, using ruthenium, with overpotentials and current densities superior to Pt/C in both alkaline and acidic conditions.

DFT calculation to probe the hydrogen adsorption energies on the active catalytic surface of the Ru2P on reduced graphene oxide catalyst.

DFT calculation to probe the hydrogen adsorption energies on the active catalytic surface of the Ru2P catalyst. a) and b) front and side views of the calculated Ru2P/reduced graphene oxide surface. c) free energy diagram for the HER with different catalysts.

The electrocatalyst is comprised of small, uniform Ru2P nanoparticles (~2-4 nm) evenly distributed on reduced graphene oxide sheets. The activity of the prepared catalyst (1.0 mg cm-2) for the HER was measured in an acidic medium (0.5 M H2SO4) and the overpotential to achieve a current density of -10 mA cm-2 was -22 mV, superior to Pt/C (-27 mV). In an alkaline environment (1.0 M KOH) catalyst performance was enhanced, with an overpotential of -13 mV (29 mV lower than Pt/C). High Faradaic efficiencies of more than 98% were measured in both acidic and alkaline solutions. Additionally, analysis was undertaken to further understand how the structure and composition of the catalyst influences its activity. Double layer capacitance measurements gave clues about the catalyst surface, while theoretical DFT calculations were used to study H-adsorption energies.

There is no way to avoid the reality that ruthenium is also a rare and costly metal, and for this reason may not hold the key to solving our energy woes. However, of real value are the insights gained from probing the structure function relationship of this highly active catalyst, which may guide the synthesis of rationally-designed catalysts using inexpensive and abundant materials.

To find out more please read:

Ultrasmall Ru2P nanoparticles on graphene: a highly efficient hydrogen evolution reaction electrocatalyst in both acidic and alkaline media

Tingting Liu, Shuo Wang, Qiuju Zhang, Liang Chen, Weihua Hu, Chang Ming Li.
Chem. Commun., 2018, 54, 3343-3346
DOI: 10.1039/c8cc01166d

About the author:

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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In-Situ Electron Paramagnetic Resonance Spectroscopy Revealed the Charge Storage Behavior of Activated Carbon

Recently in Chem. Commun., Wang et al. from University of Manchester and Liverpool John Moores University, U.K. demonstrated that in-situ electron paramagnetic resonance (EPR) spectroscopy was a powerful tool to study the charge storage mechanism of activated carbon.

Activated carbon is a type of microporous carbon used for electrodes of supercapacitors (a family of charge-storage devices similar to batteries). Conventional electrochemical testing techniques (e.g. cyclic voltammetry) are able to evaluate the overall performance of electrode materials but are unable to reveal the charge storage mechanism at the atomic level. Understanding the charge storage mechanism is crucial to guide the design and synthesis of electrode materials with improved performance. During the past decade, the development of numerous in-situ probing techniques has allowed materials researchers to explore the microscopic charge-discharge behaviour of supercapacitor electrodes.

In the published paper, in-situ EPR spectroscopy was used to study the electrochemical properties of activated carbon under different external potentials. EPR is very sensitive to electron spins originating from unpaired electrons that are generated upon charging or discharging electrode materials. This characteristic makes EPR a suitable technique for in-situ studies. To carry out the experiments, the authors designed and constructed a capillary three-electrode testing cell (Figure 1a). This cell was placed in an EPR spectrometer and its activated carbon electrode was connected to an external power source (to apply external potentials to the activated carbon electrode). The authors collected the spectra of the activated carbon electrode at selected applied potentials, an example of which is shown in Figure 1b.

Analysis of the obtained spectra offered important information about how the surface of activated carbon changed at different potentials. Specifically, the authors deconvoluted the signal into two components: the narrow signal and the broad signal corresponding to the blue and red curves in Figure 1b, respectively. The peak intensity of the narrow signal increased drastically when charging the electrode, but remained almost unchanged when altering the testing temperature. This observation suggests that the origin of the narrow peaks was the surface-localized electrons. These localized electrons were likely from the oxidized products (i.e. radicals) of carboxylate and alkoxide groups on the surface of the activated carbon, evolved during the charging process. The broad signal was ascribed to electrons located on aromatic units (e.g. graphene domains) and its intensity was found to be proportional to the number of ions electrically adsorbed on the activated carbon surface.

Figure 1. (a) The structure of the self-built capillary three-electrode cell: CE – counter electrode (Pt wire); RE – reference electrode (Ag/AgCl); WE – working electrode (activated carbon). (b) A typical EPR signal (black) that can be deconvoluted into narrow peaks (blue) and broad peaks (red).

This work highlights EPR spectroscopy as a novel tool for in-situ investigation of the charge-storage mechanism of carbon-based supercapacitor electrodes, and could be potentially extended to study other types of materials. The availability of diverse in-situ techniques is expected to provide more in-depth fundamental understanding that will guide researchers to rationally develop electrodes with optimized performance.

 

To find out more please read:

In-Situ Electrochemical Electron Paramagnetic Resonance Spectroscopy as A Tool to Probe Electric Double Layer Capacitance

Bin Wang, Alistair J. Fielding and Robert A. W. Dryfe

Chem. Commun. 2018, DOI: 10.1039/c8cc00450a

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is an online blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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