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Prolonging the Lifetimes of Dye-Sensitized Solar Cells by Positioning Dyes

19 May 2020

Dye-sensitized solar cells (DSSCs) are electrochemical devices that can convert solar energy into electricity. The critical component of a DSSC is its dye molecules which are covalently adsorbed on the electrodes of the DSSC. These molecules are responsible for light absorption and energy conversion in the device. DSSCs are more economical than commercial Si-based solar cells, but their lifetimes are limited (~6 years vs. 20-30 years of Si-based counterparts).

A research team from Xiamen University, China, recently demonstrated in Chemical Science (DOI: 10.1039/D0SC00588F) that the anchoring stability of the dyes determined the longevity of DSSCs. They specifically studied N719, a Ru-containing dye, adsorbed on three different crystal facets of rutile TiO₂ (electrode). N719 adsorbed on the TiO₂(111) facets was the most stable among all the facets studied.

The researchers adopted surface-enhanced Raman spectroscopy (Fig.1) in their research, the setup of which involved two laser beams. One 405-nm laser excited the dye molecules to initiate energy conversion, and another 638-nm laser collected the Raman scattering signals at the dye/TiO₂ interface. The obtained Raman spectra showed the peaks associated with the vibrations of N719.

Figure 1. The experimental setup. The 405-nm laser excites N719 dye molecules adsorbed on rutile TiO2. The 638-nm laser probed the Raman scattering signals of N719. The Au nanoparticle (yellow sphere) enhances the Raman signal intensity.

The Raman spectra revealed that the adsorption stability of N719 depended on the crystal facet of TiO₂. For TiO₂(001), after illumination for 36 min, the Raman peaks of N719 gradually diminished (Fig. 2a), indicating that the dye molecules were either detached from TiO₂ or decomposed. A similar trend was observed for N719 on TiO₂(110) (Fig. 2b). Mass spectroscopy detected that the electrolytes after 36-min illumination contained N719 molecules missing an S atom. This result indicated that the C=S bond of N719 was broken, leading to the loss of the dye. In contrast, N719 on TiO₂(111) exhibited stable Raman signals during the identical illumination duration (Fig. 2c).

The different stability was ascribed to variation in the dissociation energy of the C=S bond. Density functional theory (DFT) simulation proved that the cleavage of the C=S bond on TiO₂(111) had an energy barrier of 3.5 eV, about 1.0 eV and 1.5 eV higher than those on TiO₂(110) and TiO₂(001), respectively. The higher energy barrier suppresses bond dissociation and stabilizes the adsorption of N719.

Figure 2. Raman spectra of N719 adsorbed on (a) TiO2(001), (b) TiO2(110), and (c) TiO2(111). Spectra were collected with an interval of 4 min. Peaks highlighted in yellow and blue are from TiO2 and N719, respectively. The schemes on the right show the simulated structures of dye-adsorbed (top) and desorbed (bottom) TiO2 facets. Ph–N=C=S represents N719 in simulation. The yellow spheres are S.

This work highlights the importance of dye positioning in promoting long-lasting performance of DSSCs.

For expanded understanding, please read:

In Situ Raman Study of the PhotoInduced Behavior of Dye Molecules on TiO₂(hkl) Single Crystal Surfaces

Sheng-Pei Zhang, Jia-Sheng Lin, Rong-Kun Lin, Petar M. Radjenovic, Wei-Min Yang, Juan Xu, Jin-Chao Dong, Zhi-Lin Yang, Wei Hang, Zhong-Qun Tian, and Jian-Feng Li

Chem. Sci., 2020, DOI: 10.1039/D0SC00588F

Tianyu Liu acknowledges Zacary Croft at Virginia Tech, U.S., for his careful proofreading of this post.

About the blogger:

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

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Seeing is Believing: Is Your Polymerization Working?

07 Apr 2020

If you are a polymer chemist, you may have dreamt of having a pair of eyes that can directly tell whether your polymerization reaction is working or not. Good news! Randall H. Goldsmith of the University of Wisconsin-Madison, U.S. and coworkers have developed an optical technique to monitor the course of a polymerization reaction in real-time. This breakthrough has been published in Chemical Science (DOI: 10.1039/C9SC05559B).

This technique relies on two parameters, fluorescence polarization anisotropy and aggregation-induced emission, to examine molecular weights. The authors specifically studied a well-controlled polymerization of norbornene (NB) into polynorbornene. Besides monomer, they added small amounts of two fluorescent probes, perylene diimide-functionalized norbornene (PDI-NB) and tetraphenylethylene-linked norbornene (TPE-NB), to the polymerization system. These NB derivatives were co-polymerized with NB (Fig.1).

Figure 1. Polymerization of norbornene (NB) catalyzed by a ruthenium-based Grubbs Generation II catalyst (Grubbs Gen II). PDI-NB and TPE-NB are two probes co-polymerized with NB to monitor the growth of polynorbornene.

The technique temporally resolved the evolution of polynorbornene molecular weight. The polymerization incorporated the probe molecules into the backbones of polynorbornene. As polymer chains grew, the rotational freedom of PDI-NB became increasingly limited, giving rise to an enhanced anisotropy signal during the first 200 min of polymerization (Fig. 2a, top panel and Fig. 2b, red curve). Further polymerization brought the incorporated TPE-NB together, triggering the aggregation-induced fluorescence emission of TPE-NB (Fig. 2a, bottom panel and Fig. 2b, blue curve). Importantly, the rise of the emission intensity from TPE-NB immediately followed the saturation of anisotropy signal from PDI-NB, making the two molecules complementary for monitoring the polymer growth in different time scales. Critically, the anisotropy signal intensity of PDI-NB correlated positively with the weight-average molecular weight of polynorbornene (Fig. 2c), demonstrating the capability of this technique to track the progression of polymer growth.

Figure 2. (a) (top) Color-scale images of anisotropy values of PDI-NB and (bottom) emission intensities of TPE-NB over time. Dots are top-view toluene microdroplets where polymerization happens. Scale bar: 250 µm. (b) Time-evolution of average anisotropy values of PDI-NB (red) and aggregation-induced-emission (AIE) intensity of TPE-NB (blue) throughout a polymerization. (c) The correlation between measured anisotropy values and the weight-average molecular weights of polynorbornene. The red dashed line provides a visualization of the trend.

The reported technique is applicable to other ring-opening metathesis polymerizations (ROMPs) involving monomers such as norbornadiene.

For expanded understanding, please read:

Optical Monitoring of Polymerizations in Droplets with High Temporal Dynamic Range

Andrew C. Cavell, Veronica K. Krasecki, Guoping Li, Abhishek Sharma, Hao Sun, Matthew P. Thompson, Christopher J. Forman, Si Yue Guo, Riley J. Hickman, Katherine A. Parrish, Alán Aspuru-Guzik, Leroy Cronin, Nathan C. Gianneschi, and Randall H. Goldsmith

Chem. Sci., 2020, 11, 2647-2656.

The blogger acknowledges Zac Croft at Virginia Tech, U.S., for his careful proofreading of this post.

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How Does Nano-Confinement Boost Oxygen-Reduction Electrocatalytic Activity?

16 Jan 2020

Why do biological enzymes exhibit superior catalytic activity? Well, this is mainly because they can firmly anchor reactants into catalytically active pockets using their three-dimensional structures. Inspired by this nano-confinement effect, scientists have developed “nanozymes,” a family of artificial enzymes.

A group of scientists from the University of New South Wales (Australia), University of Utah (USA), and Ruhr-Universität Bochum (Germany) investigated the interplay between the degree of nano-confinement and oxygen reduction reaction (ORR) activity. They discovered that ORR activity only scaled proportionally to the degree of confinement at low overpotentials (ORR theoretical potential: 1.23 V vs. RHE). The results have been published in Chemical Science (doi: 10.1039/C9SC05611D).

Synthesized by etching Ni from Pt-Ni alloy nanoparticles, metallic Pt nanoparticles possessing channels of different opening sizes served as the nanozymes with different confinement degrees. When the Ni content increased, the channel of the etched nanoparticles widened. Specifically, Pt nanozymes prepared from Pt-Ni nanoparticles with Pt/Ni = 1/1.5 (NZ_small) contained 69% channel openings smaller than 2 nm (Fig. 1a); Whereas those from the precursors with Pt/Ni = 1/2.5 (NZ_medium) and 1/3 (NZ_large) had 52% (Fig. 1b) and 34% (Fig. 1c) <2-nm-wide channels, respectively. Therefore, the nano-confinement degrees of the three nanozymes followed the sequence of NZ_small>NZ_medium>NZ_large.

Figure 1. High-resolution transmission electron microscopy images (left) and channel diameter distribution profiles (right) of (a) NZ_small, (b) NZ_medium, and (c) NZ_large.

The ORR activity of the three nanozymes strongly depended on the magnitude of the overpotential. With the outer surfaces passivated by surfactants, the ORR activities of the nanozymes were only associated with O₂ reduction within their channels. At low overpotentials (Fig. 2, inset), NZ_small had the highest ORR activity among all the catalysts evaluated by the authors, as indicated by its lowest kinetic current. At high overpotentials or low applied biases; however, the ORR activity of NZ_medium and NZ_large rapidly increased (Fig. 2). NZ_medium and NZ_large became more active than NZ_small at potentials lower than 0.82 V (vs. RHE) and 0.80 V (vs. RHE), respectively.

Figure 2. The potential (E)-dependence of kinetic current density (jk). Inset: low-overpotential (high potentials) region. Legends: solid black – NZ_small; solid blue – NZ_medium; solid red – NZ_large; open black – mesoporous Pt nanoparticles without nano-channels. The consistently small absolute j_k of the mesoporous nanoparticles reflected its relatively low ORR activity.

The finite element simulation revealed the underlying mechanism of the experimental results. At low overpotentials, the ORR activity was governed by the kinetics of O₂-reduction. Due to high charge density, the local proton concentration inside the channels of NZ_small was the highest, leading to the fastest reaction and the highest ORR activity. At high overpotentials, the ORR activity became mass-transport limited. Nanozymes with wide channel openings, that is, NZ_medium and NZ_large, allowed a large amount of O₂ to diffuse into the channels, which enhanced O₂ supply and augmented ORR activity.

This work unveiled the potential-dependence of the ORR catalytic activities of porous Pt nanoparticles under different degrees of nano-confinement. This insight could rationalize and further enable the design of nanozymes with tailorable ORR activities.

To find out more, please read:

The Importance of Nanoscale Confinement to Electrocatalytic Performance

Johanna Wordsworth, Tania M. Benedetti, Ali Alinezhad, Richard D. Tilley, Martin A. Edwards, Wolfgang Schuhmann, and J. Justin Gooding

Chem. Sci., 2019, DOI: 10.1039/C9SC05611D

Tianyu Liu acknowledges Zac Croft at Virginia Tech, U.S., for his careful proofreading of this post.

About the blogger:

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

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Can Al3+ Indeed Intercalate Layered Metal Oxides in Aqueous Electrolytes?

30 Sep 2019

Intercalation of multivalent ions, e.g., Al³⁺, has received increasing attention as an energy-boosting strategy for rechargeable batteries. The charge storage process of the wide-spread Li-ion batteries relies on Li⁺ intercalating electrodes with layered structures. Multivalent-ion batteries could accommodate more charges than Li-ion batteries because their ions carry more charge than Li⁺. This concept, however, has now been challenged by a research team led by BenoÎt Limoges and Véronique Balland of Université de Paris, France. Their mechanistic investigation, published in Chemical Science, revealed that Al³⁺ ions were unable to intercalate electrodes in aqueous electrolytes.

The authors selected TiO₂ arrays as the object of their study. These ~1 µm-high arrays were grown using the glancing angle deposition technique. By applying negative potentials to the TiO₂ arrays, cations such as Al³⁺ could diffuse through the inter-array slits and interact with TiO₂ (Figure 1).

Figure 1. Structure of the TiO₂ arrays. (left) Scanning electron micrographs and (right) a cartoon illustrating ion diffusion pathways.

Electrochemistry tests elucidated that the charge-storage process of TiO₂ in an Al³⁺-containing aqueous electrolyte correlated to proton intercalation. This conclusion was mainly based on the nearly identical cyclic voltammograms (Figure 2, top) and capacity vs. potential curves (Figure 2, bottom) of the TiO₂ arrays in both AlCl₃ and acetic acid aqueous electrolytes. Since acetic acid solution had no Al³⁺, the observed charge-storage activity could not be attributed to Al³⁺ intercalation. Instead, the authors argued that protons dissociated either from hydrated Al³⁺ cations, [Al(H₂O)₆]³⁺ or acetic acid must intercalate TiO₂ and result in the observed charge-storage capacities.

Figure 2. (top) Cyclic voltammograms and (bottom) capacity vs. potential curves of TiO₂ in (left) Al³⁺-containing and (right) acetic acid aqueous electrolytes. The electrolytes are 0.3 M KCl with different concentrations of AlCl₃ or acetic acid: 0 M (black), 25 mM (blue), 50 mM (purple), 100 mM (magenta), and 250 mM (red).

The authors believe that the misconception of Al³⁺ intercalation is due to the overlooking of Al³⁺ hydration, which is inevitable when Al³⁺ is present in aqueous electrolytes. Removing the water shell (a prerequisite for ion intercalation) is energy costly for Al³⁺ because of the strong binding force between water molecules and Al³⁺. Additionally, even if Al³⁺ ions intercalate TiO₂, their movement is strongly hindered by Coulombic interactions within the TiO₂ lattice. The immobilized intercalated ions would then block other ions from entering the TiO₂ lattice. Together, both factors prevent Al³⁺ from intercalating into TiO₂.

In summary, this work demonstrates that the charge-storage capacity of TiO₂ in Al³⁺-containing aqueous electrolytes is most probably due to proton intercalation. This conclusion also applies to other multivalent cations, including Zn²⁺ and Mn²⁺, as shown in this work.

To find out more, please read:

On the Unsuspected Role of Multivalent Metal Ions on the Charge Storage of A Metal Oxide Electrode in Mild Aqueous Electrolytes

Yee-Seul Kim, Kenneth D. Harris, BenoÎt Limoges, and Véronique Balland

Chem. Sci., 2019, doi: 10.1039/c9sc02397f

Tianyu Liu acknowledges Zachary L. Croft of Virginia Tech, the U.S., for his constructive comments on this post.

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 the communication of scientific endeavors and cutting-edge research to both the general public and other 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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Making Gyroid Polymer Films to Speed up Proton Conduction

30 Jul 2019

Proton exchange membranes (PEMs) are essential to the functionality of fuel cells. They conduct protons in electrolytes and drive electricity generation by oxidizing fuels. Following the success of Nafion^® –– a family of commercial proton-conductive fluoropolymers –– materials researchers around the globe are developing innovative PEMs with high proton conductivities and affordable prices.

A group of Japanese researchers has recently synthesized self-standing polymer films with a gyroid nanostructure. These films possess two unique characteristics that other PEMs rarely have: a high proton conductivity in the order of 10^-1 S/cm and retention of the conductivity across a wide temperature range (20-120 °C). This finding has been published in Chem. Sci. (doi: 10.1039/C9SC00131J).

The authors used a tailor-made macromolecule, Diene-GZI (Figure 1a), as the building block. It had an amphipathic structure, with one end being a hydrophilic zwitterionic group and another end of a hydrophobic alkyl chain. When mixed with bis(trifluoromethanesulfonyl)imide and water, multiple Diene-GZI molecules could assemble together into a gyroid network –– an infinitely periodic minimal surface (Figure 1b). After the self-assembly, ultra-violet-irradiation-induced polymerization solidified the morphology of the gyroid nanostructure.

Figure 1. (a) The molecular structure of Diene-GZI. (b) Solidification of the self-assembled gyroid via polymerization.

The high proton conductivity of the polymer film originated from its three-dimensional gyroid structure. Since the gyroid surface was densely coated with the hydrophilic zwitterionic chains, the film could readily uptake as high as 15.6 wt.% of water at a relative humidity of 90%. The adsorbed water layers formed a three-dimensional continuous pathway along the gyroid surface, serving as proton-conduction expressways and resulting in a high conductivity in the order of 10^-1 S/cm. Due to the strong binding force between water and the zwitterionic groups, heating the polymer film to 120 °C did not decrease the water content significantly, and thus, the proton conductivity remained high. Additionally, the control films with no gyroid structures were unable to compete with the gyroid film in terms of proton conductivities within the measured temperature range (Figure 2).

Figure 2. The dependence between proton conductivities and temperature. Legends: red solid circles – gyroid film; others – control samples without the gyroid nanostructure.

This work highlights the critical role of rational design of raw materials to augment the proton conductivities of PEMs. The advantage of the gyroid phase in speeding up ion diffusion could also inspire innovative materials in applications demanding ultrafast ion transport, e.g., supercapacitor electrodes.

To find out more, please read:

Gyroid Structured Aqua-Sheets with Sub-Nanometer Thickness Enabling 3D Fast Proton Relay Conduction

Tsubasa Kobayashi, Ya-xin Li, Ayaka Ono, Xiang-bing Zeng, and Takahiro Ichikawa

Chem. Sci., 2019, 10, 6245-6253

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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A Single Nickel Site Reduces Nitrate Ions to Nitrogen Gas

03 Jun 2019

Surface runoff of agricultural or landscape areas with excessive nitrate fertilizers has resulted in increasing nitrate ion concentrations in freshwater. This issue leads to eutrophication and algae blooms that significantly threaten aquatic lives and ecosystems. Eliminating nitrate ions is thus necessary to address the nitrate-borne water pollution. Nature presents a delicate yet complicated process for turning NO₃^– into N₂ by four metalloenzymes. Is it possible to develop an artificial method with just one catalyst to drive the same process?

Lee, Baik, and coworkers at the Institute for Basic Science (IBS) and Korea Advanced Institute of Science and Technology (KAIST) answered yes. The researchers synthesized a square-planar nickel(II)-based complex, (PNP)Ni(ONO₂) (PNP = N[2-P’Pr-4-methyl-C₆H₃]₂), which converted NO₃^– to N₂ in the presence of CO and NO. Ni²⁺ is the only active site. This breakthrough has been published in Chemical Science (DOI: 10.1039/C9SC00717B).

The reaction mechanism involves four successive redox reactions among (PNP)Ni(ONO₂), CO, and NO (Figure 1). The first two steps consecutively transfer two oxygen atoms from (PNP)Ni(ONO₂) to CO at room temperature, yielding two molecules of CO₂ and a Ni-nitrosyl complex, (PNP)Ni(NO). Afterwards, the (PNP)Ni(NO) undergoes a disproportionation reaction with NO, generating (PNP)Ni(NO₂) and N₂O. The as-formed N₂O interacts with the remaining (PNP)Ni(NO) and is eventually reduced to N₂. The yield of N₂ is 46% (based on the amount of N₂O).

Figure 1. The schematic shows the artificial nitrate reduction with a Ni(II)-based complex, (PNP)Ni(ONO₂), as the catalyst. Pr: propyl group.

Besides nitrate reduction, the (PNP)Ni(ONO₂) could act as a potential alternative to the platinum-containing catalysts used in the catalytic converters of gasoline cars, because it transforms hazardous CO and NO into less toxic CO₂ and N₂.

To find out more please read:

One Metal is Enough: A Nickel Complex Reduces Nitrate Anions to Nitrogen Gas

Jinseong Gwak, Seihwan Ahn, Mu-Hyun Baik and Yunho Lee

Chem. Sci., 2019, 10, 4767-4774

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Learning from Nature: A Cu(II)-Porphyrin Complex Produces Oxygen Gas from Water at Ultra-Small Overpotential

25 Feb 2019

Sunlight-assisted water splitting represents a sustainable way to convert solar energy into chemical energy in hydrogen and oxygen gases. Due to its high activation energy, the oxygen evolution reaction (OER) requires large overpotential for initiation. Developing suitable OER catalysts to reduce the overpotential thus becomes instrumental for the feasibility of solar energy harvesting.

Recently, a group of scientists led by Rui Cao from Renmin University of China, and Shaanxi Normal University, China, has developed a water-soluble Cu(II)-porphyrin complex as a high-performance OER catalyst. This breakthrough has been published in Chemical Science (DOI: 10.1039/C8SC04529A).

Inspired by the molecular structure of a natural OER catalyst in the photosynthesis system – photosystem II (PSII), the researchers designed a Cu²⁺-coordination compound with a porphyrin ligand, tetrakis(4-N-methylpyridyl)porphyrin (Figure 1a), which mimics the structure of PSII. This biomimetic Cu²⁺-complex exhibits outstanding catalytic OER activity in a phosphate buffer solution at pH=7.0. The current of the cyclic voltammogram of the Cu²⁺-complex increases sharply (due to O₂ evolution) at an onset potential of 1.13 V vs. normal hydrogen electrode (Figure 1b), corresponding to an OER overpotential of 310 mV. For comparison, the cyclic voltammograms of a blank buffer solution and a CuSO₄-containing buffer solution show no pronounced current enhancement (Figure 1b), indicating the electrolyte itself and the un-coordinated Cu²⁺ cannot generate O₂ within the tested potential range. The 310 mV overpotential is approximately two times smaller than the typical values exhibited by previously reported Cu complexes.

Figure 1. (a) The molecular structure of Cu²⁺-tetrakis(4-N-methylpyridyl)porphyrin complex. (b) Cyclic voltammograms of 1 mM Cu²⁺-tetrakis(4-N-methylpyridyl)porphyrin (red), bare buffer solution (black) and buffer solution containing 1 mM CuSO₄ (green). The electrode is a piece of fluorine-doped tin oxide glass slide.

The authors ascribed the ultra-small OER overpotential to the formation of an oxidized form of the Cu²⁺-porphyrin complex. This oxidized species is generated after the complex loses one electron, and is active for O-O bond formation and subsequent O₂ evolution. The energy barrier of this one-electron-oxidation pathway is expected to be much lower than those of conventional processes involving higher-valent Cu species (e.g., Cu⁴⁺-oxo), which facilitates OER at small overpotential.

With the complete catalytic cycle of water oxidation by the Cu²⁺-porphyrin complex being fully revealed, OER will become more efficient and energy-saving.

To find out more please read:

Low Overpotential Water Oxidation at Neutral pH Catalyzed by A Copper(II) Porphyrin

Yanju Liu, Yongzhen Han, Zongyao Zhang, Wei Zhang, Wenzhen Lai, Yong Wang and Rui Cao

Chem. Sci., 2019, DOI: 10.1039/C8SC04529A

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Carbon Coating Promotes CO2 Reduction on Nickel Surfaces

18 Dec 2018

Reducing CO₂ into fuels such as CO or hydrocarbons is a promising strategy to reduce the net emission of greenhouse gases and mitigate climate change. The CO₂ reduction reaction, however, is an energy-costly process. Therefore, CO₂ reduction catalysts are necessary to enhance the CO₂ conversion efficiency and minimize the overall energy demand.

Researchers are developing high-performance yet inexpensive CO₂ reduction catalysts. Noble metals such as Au, Ag and Pd exhibit high CO₂-to-CO conversion efficiency, but their scarcity restricts their large-scale practicability. Metallic Fe, Co and Ni are active in reducing CO₂ and therefore, have been identified as alternatives to the noble metal catalysts.

Recently in Chem. Sci., a group of scientists led by Zhenyu Sun from Beijing University of Chemical Technology and Yousung Jung from Korea Advanced Institute of Science and Technology (KAIST) pushed the CO₂-reduction activity of Ni metal to a new height. The researchers synthesized carbon-supported Ni nanocrystals via pyrolysis of Ni-based metal organic frameworks (MOFs) in argon. The resultant Ni nanoparticles had an average diameter of ~30 nm and were embedded in N-doped carbon scaffolds (Fig. 1a). Each nanoparticle was uniformly coated with a thin layer of amorphous carbon (Fig. 1b).

Figure 1. (a) The elemental mapping of the carbon-supported Ni nanoparticles. Green dots and blue regions are Ni nanoparticles and carbon matrices, respectively. (b) The scanning transmission electron microscope image showing the thin carbon coating on a Ni nanoparticle surface. (c) The CO2-to-CO conversion efficiencies at different applied potentials. Ni-NC_ATPA@C and Ni-NC_TPA@C are carbon-supported Ni nanoparticles derived from MOFs with 2-amino-terephthalic acid and terephthalic acid organic linkers, respectively. NC_ATPA@C is the Ni-free carbon powder derived from ATPA.

The CO₂-to-CO conversion efficiencies of these Ni-C nanocomposites were the highest among all the reported carbon-supported Ni nanoparticles. The best Ni catalyst achieved a maximal efficiency of ~94% at an overpotential of 0.59 V, while previously reported Ni-C catalysts typically exhibited efficiencies lower than 25%. The significantly improved conversion efficiency was associated with the thin carbon coating. This coating prevented the Ni nanoparticles from directly contacting with aqueous electrolytes, and thus minimized hydrogen evolution reaction, a side reaction that decreased the conversion efficiency.

This work highlights the instrumental role of the surface carbon layers in promoting the CO₂-reduction activity of Ni nanoparticles. The carbon-coating strategy could be extended to other low-cost transition metals, which may lead to a variety of cost-effective CO₂-reduction catalysts.

To find out more please read:

Carbon-Supported Ni Nanoparticles for Efficient CO₂ Electroreduction

Mingwen Jia, Changhyeok Choi, Tai-Sing Wu, Chen Ma, Peng Kang, Hengcong Tao, Qun Fan, Song Hong, Shizhen Liu, Yun-Liang Soo, Yousung Jung, Jieshan Qiu and Zhenyu Sun

Chem. Sci., 2018, 9, 8775-8780

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How Can We Stabilize Bismuth in Potassium-Ion Batteries? Add Salts!

16 Jul 2018

The large-scale manufacturing and widespread demand of consumer electronics in modern societies call for batteries with affordable prices. Potassium-ion batteries are one of the economical alternatives to lithium-ion batteries, as the cost of potassium is much lower than lithium. However, these types of batteries have yet to be commercially available, partly due to the lack of high-performance and stable anode materials. Bismuth (Bi) is a promising anode material for potassium-ion batteries, because of its substantially higher theoretical charge-storage capacity than the conventional ones. Unfortunately, its poor durability severely hinders the applicability.

Now the drawback of Bi has been successfully addressed by increasing the electrolyte concentration. This breakthrough, demonstrated by Chuan-Fu Sun and coworkers from Chinese Academy of Sciences, China, was recently published in Chemical Science.

Sun and coworkers investigated the interplay between the electrolyte concentration and the charge-storage performance stability of Bi nanoparticles (Figure 1a). They identified the optimal concentration of the solute, potassium bis(tri-fluoromethylsulfonyl)imide, to be 5 M. Under this condition, the irreversible electrolyte reduction reaction on the Bi surface experienced the highest resistance. Impeding this unwanted reaction thus elongated the lifespan of Bi electrodes. In addition, the concentrated electrolyte resulted in thin layers of the reduction products being deposited on the Bi surface. This allowed ions in the electrolyte to easily penetrate the surface coatings and interact with the encapsulated Bi nanoparticles, maintaining the intrinsically high capacity of Bi (Figure 1b). Other concentrations either led to rapid battery failure or significantly reduced capacity.

Figure 1. (a) A scanning electron microscopy image of the Bi nanoparticles. (b) The change of the Bi electrode capacity vs. charge-discharge cycle number with different electrolyte concentrations. CE: coulombic efficiency.

This work innovates the design and development of commercially viable potassium-ion batteries. The strategy of increasing the electrolyte concentration could possibly be adapted to solve electrode instability issues associated with other rechargeable batteries.

To find out more please read:

Concentrated Electrolytes Stabilize Bismuth-Potassium Batteries

Ruding Zhang, Jingze Bao, Yu-Huang Wang and Chuan-Fu Sun

Chem. Sci., 2018, DOI: 10.1039/c8sc01848k

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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Welcoming a New Member into the Aluminum Battery Family

20 Jun 2018

Yu and coworkers from The University of Queensland, Australia have introduced an aluminum-selenium (Al-Se) battery as a new member of the rechargeable Al-ion battery family. This battery reported in Chemical Science exhibited a capacity of 178 mAh per gram of Se, high discharging voltage above 1.5 V and satisfactory lifetime.

Al-ion batteries have attracted increasing attention as next-generation energy-storage devices. They are potentially more affordable and safer than Li-ion batteries, due to the natural abundance and the existence of native oxide surface layers of aluminum, respectively. One of the major challenges hindering the wide application of Al-ion batteries is the lack of feasible cathode materials. Previously investigated cathodes have drawbacks of low charge-storage capacity, low discharging voltage, poor electrical conductivity or chemical instability.

Inspired by sulfur, Yu and coworkers selected selenium as a cathode material for Al-ion batteries. Selenium has substantially higher electrical conductivity and lower ionization potential than sulfur, which is expected to improve the energy-storage capacity of batteries. However, a major drawback of selenium is that the oxidation product generated upon charging batteries, Se₂Cl₂, can dissolve quickly in electrolytes and lead to battery failure. Solving this problem would make selenium a promising cathode for Al-ion batteries.

To resolve this issue, the authors introduced a mesoporous carbon named CMK-3, nanorods that are capable of physically adsorbing Se₂Cl₂. The cathode, composed of Se nanowires and CMK-3 nanoparticles, is thus anticipated to improve the lifespan of batteries, as any Se₂Cl₂ that is generated will be confined inside the pores of CMK-3 (Figure 1).

Figure 1. A schematic illustrating the CMK-3’s capability of trapping Se₂Cl₂. The chemical equation below shows how selenium reacts with aluminum during charge and discharge processes.

As expected, the performance of these Al-Se batteries was stable. They retained more than 80% of the initial capacity after 50 consecutive charge-discharge cycles at 100 mA/g (Figure 2a). Additionally, the discharging capacity of the batteries reached 178 mAh per gram of selenium at 100 mA/g, and the discharging potential was above 1.5 V (Figure 2b).

Figure 2. (a) The specific capacity of the Al-Se batteries of each cycle at different current densities. (b) The variation of battery potential with specific capacity of the 2nd, 5th, 10th, and 30th charge-discharge cycles.

These promising Al-Se batteries could encourage future work to continue progress into the development of affordable and durable Al-ion batteries.

To find out more please read:

Rechargeable Aluminum-Selenium Batteries with High Capacity

Xiaodan Huang, Yang Liu, Chao Liu, Jun Zhang, Owen Noonan and Chengzhong Yu

Chem. Sci., 2018, DOI: 10.1039/C8SC01054D

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