Chemical Communications Blog

Electro-oxidizing urea in water has the dual benefit of generating electricity and treating wastewater. In alkaline media, the sluggish kinetics of the urea oxidation reaction, CO(NH₂)₂ + 6OH^– → N₂↑ + CO₂↑ + 5H₂O + 6e^–, due to its transfer of six electrons demand efficient catalysts to speed up this process.

A research team led by Xiujuan Sun and Rui Ding, both at Xiangtan University, China, used a co-precipitation method to synthesize urea-oxidation catalysts. These catalysts comprised of graphene-anchored nanoparticles of metallic Ni, Co, and Mo, as well as their alloys and hydro/oxides (NCM/G) (Figure 1). NCM/G with the optimal composition displayed a mass activity of 140.9 mA cm^-2 mg_cat^-1 and an onset potential of 1.32 V vs. RHE (current density = 10 mA/cm²). Their results are published in Chemical Communications (DOI: 10.1039/D0CC02132F).

Figure 1. Representative (a) transmission electron microscopy image and (b-f) elemental mappings of the synthesized catalyst.

The catalysts exhibited different catalytic activities dependent on their chemical compositions, which were tunable by varying the moles of the Ni, Co, and Mo precursors. Cyclic voltammograms of various NCM/G all showed markedly increased current density at potentials beyond 0.3 V vs. Ag/AgCl in urea-containing aqueous solutions (Figure 2a), marking their catalytic activity for urea oxidation. Among all the tested catalysts, NCM/G with Ni:Co:Mo = 80:10:10 achieved the largest current density at 0.6 V vs. Ag/AgCl, indicating its highest catalytic activity. Additionally, chronoamperometry demonstrated that increasing the Mo content was beneficial for maintaining catalyst stability, as the current density of NCM/G with the lowest amount of Mo (the black curve in Figure 2b) decayed the fastest. Combining the results of cyclic voltammetry and chronoamperometry, the authors deduced that the optimal molar ratio of Ni:Co:Mo was 80:10:10.

Figure 2. (a) Cyclic voltammograms (scan rate: 1 mV/s) and (b) chronoamperometry (potential: 0.5 V vs. Ag/AgCl) profiles of NCM/G with different Ni, Co, and Mo contents. Electrolyte: 1.0 M KOH + 0.33 M urea in water. The molar ratios of Ni:Co:Mo of NCM/G 90505, 811, and 71515 are 90:5:5. 80:10:10, and 70:15:15, respectively.

The optimization of the chemical composition demonstrated in this work can rationalize the development of high-performance, metallic electrocatalysts for urea oxidation.

For expanded understanding, please read:

Trimetallic NiCoMo/Graphene Multifunctional Electrocatalysts with Moderate Structural/Electronic Effects for Highly Efficient Alkaline Urea Oxidation Reaction

Wei Shi, Xiujuan Sun, Rui Ding, Danfeng Ying, Yongfa Huang, Yuxi Huang, Caini Tan, Ziyang Jia, and Enhui Liu

Chem. Commun., 2020, DOI: 10.1039/D0CC02132F

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/.

Lithium–sulfur (Li–S) batteries are rechargeable batteries with elemental sulfur and metallic lithium as the cathode and anode, respectively. These batteries are promising electrochemical energy storage devices because their energy densities are three to five times higher than those of Li-ion batteries. Unfortunately, the practicality of Li–S batteries is hindered by their short lifetimes due to two processes that occur on the Li anode surface: the growth of Li dendrites and the irreversible polysulfide reduction. Adding LiNO₃ into battery electrolytes has proven to be useful to prolong battery lifetimes, but the underlying mechanism is uncertain.

In Chemical Communications (doi: 10.1039/c9cc06504k), Sawangphruk and coworkers from Vidyasirimedhi Institute of Science and Technology, Thailand have offered valuable insights to settle the dispute over the effects of LiNO₃. The researchers performed theoretical reactive molecular dynamics simulations and elucidated two roles of LiNO₃ in Li–S batteries.

The first discovery was that LiNO₃ promoted the formation of smooth, double-layered solid electrolyte interfaces (SEIs) on the Li surface. SEIs are thin layers composed of electrolyte-decomposition products, including Li-containing organic compounds and inorganic salts. By simulating the charge distribution near a Li metal surface, the authors mapped the Li-Li radial pair distribution profiles in three phases (Fig. 1a). The similarity between the profiles of the dense phase (the Li metal) and the nest phase evidenced the presence of an amorphous, Li-containing layer atop the Li metal surface. Beyond this amorphous layer was a liquid-like film with Li element distributed homogenously. This double-layered SEI altered the kinetics of Li deposition onto the Li surface upon charging, resulting in smooth and dense SEIs (Figs. 1b and c) that avoided Li dendrite formation.

Figure 1. (a) Li-Li radial pair distribution functions of the dense phase (Li metal), nest phase (the layer atop Li), and disperse phase (the outermost layer). (b and c) Top-view scanning electron microscopy images of the Li metal surface in (b) LiNO₃-free and (c) LiNO₃-containing electrolytes. Both electrolytes had lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a solute, and 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) as solvents.

Another effect of LiNO₃ was to capture polysulfide compounds. Through their simulations, the authors deduced the reaction pathways involving the electrolyte molecules, LiNO₃ or LiClO₄ additives, and lithium polysulfide compounds (Fig. 2a). The concentration of Li_xNO_y, the reduction products of LiNO₃ when contacted Li metal, in the LiNO₃-containing electrolyte was much higher than those in the additive-free and LiClO₄-containing electrolytes. First-principle calculations proved that the highly electro-negative N and O atoms in Li_xNO_y could capture lithium polysulfides via dipole-dipole interactions. This process reduced the likelihood of polysulfide reduction on Li that passivated anodes.

Figure 2. (a) A scheme of the reaction pathways involving the electrolyte, additive, and polysulfide molecules. (b) Product distributions in electrolytes without additives and with LiNO₃ or LiClO₄.

LiNO₃ elongates the lifetimes of Li–S batteries by forming smooth SEIs to impede Li dendrite formation, while maintaining the reactivity of Li anodes by capturing lithium polysulfides.

To find out more, please read:

Insight into the Effect of Additives Widely Used in Lithium–Sulfur Batteries

Salatan Duangdangchote, Atiweena Krittayavathananon, Nutthaphon Phattharasupakun, Nattanon Joraleechanchai, and Montree Sawangphruk

Chem. Commun., 2019, 55, 13951-13954

Tianyu Liu acknowledges John Elliott of Virginia Tech, the 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. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

Lithium-metal batteries are a family of rechargeable batteries with higher charge-storage capacities than those of lithium-ion batteries. The boosted charge-storage performance of lithium-metal batteries is rooted in its anode material – Li metal, as it possesses an ultrahigh theoretical capacity (3860 mAh/g). However, the growth of dendrites on Li surfaces during charging could short-circuit batteries, cause combustion, and trigger explosions.

A research group led by Feng Li at the Institute of Metal Research, Chinese Academy of Sciences, recently devised a strategy to suppress the notorious Li dendrite growth in lithium-metal batteries. By tuning the composition of the electrolytes, the authors strengthened the coordination between Li⁺ and electrolyte solvents, which slowed the growth of Li dendrites. This work has been published in Chemical Communications (doi: 10.1039/C9CC07092C).

The researchers introduced an electrolyte additive, tetraethylene glycol dimethyl ether (TEGDME), as a coordination ligand to Li⁺. Compared to other components in the electrolyte, i.e., 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL), TEGDME contains more oxygen atoms that can form multiple, robust coordination bonds with Li⁺. Specifically, density functional theory calculations showed that the binding energy between Li⁺ and electrolyte molecules increased by 0.31 eV after introducing TEGDME, reaching an absolute value of 4.93 eV. The enhanced binding force made the separation of Li⁺ from TEGDME (a prerequisite for Li-dendrite growth) energetically consuming and kinetically sluggish (Figure 1). These characteristics could decelerate Li-dendrite formation and elongate battery lifetimes.

Figure 1. Lithium-dendrite growth in different electrolytes: (a) weak coordination with Li+ promotes fast dendrite growth while (b) strong coordination with Li+ decelerates dendrite formation.

To confirm the above idea, the authors assembled lithium batteries with TEGDME+DME+DOL or DME+DOL electrolytes. Cycling stability tests demonstrated that the battery with the TEGDME-added electrolyte survived 60 charge-discharge cycles at a current density of 1C, whereas the capacity of the battery without TEGDME rapidly decayed beyond 30 cycles under identical testing conditions (Figure 2a). Scanning electron microscopy images revealed that the number of rod-shaped Li dendrites on the anode in the TEGDME-added electrolyte (Figure 2c) was less than that in the TEGDME-free electrolyte (Figure 2b), further confirming that the enhanced cycling stability resulted from the Li-dendrite suppressing effect of TEGDME.

Figure 2. (a) Cycling stability performance of lithium-metal batteries with two different electrolytes. The cathode material in both batteries was lithium iron phosphate (LFP). (b and c) SEM images of the Li anode surface after charging in (b) DME+DOL and (c) DME+DOL+TEGDME electrolytes.

This work highlights the importance of tailoring the electrolyte composition for preserving the stability and safety of lithium-metal batteries.

To find out more, please read:

Suppressing Lithium Dendrite Formation by Slowing Its Desolvation Kinetics

Huicong Yang, Lichang Yin, Huifa Shi, Kuang He, Hui-Ming Cheng, and Feng Li

Chem. Commun., 2019, doi: 10.1039/C9CC07092C

Tianyu Liu acknowledges Xiaozhou Yang of Virginia Tech, the 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 as a way to introduce cutting-edge research to broad audiences. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

Solar-driven water reduction is a sustainable method to acquire hydrogen fuel. An indispensable component of this reaction is the photocatalyst which drives spontaneous hydrogen gas evolution from water when illuminated. Hetero-structured materials consisting of two or more catalysts stand out as promising hydrogen evolution catalysts, due to the combined advantages of their constituents (e.g. enhanced light-absorption capability). Unfortunately, the weak adhesion between different components is the Achilles heel of conventional hetero-structured photocatalysts. It impedes electron transport from the photocatalysts to the nearby water molecules, hindering the catalytic activity.

A research group led by Xiao Xiao and Jian-Ping Zou from Nanchang Hangkong University of China has demonstrated a solution to the aforementioned challenge. They firmly connected two photocatalysts – Pt-loaded carbon nitride (CN) and the covalent organic framework CTF-1 – via amide bonds, resulting in a new type of hetero-structured photocatalyst, CN/CTF-1, which exhibited a hydrogen evolution rate approximately 3 times faster than those of conventional hetero-structured photocatalysts made of weakly bound CN and CTF-1.

The researchers adopted a two-step method to synthesize CN/CTF-1. They first reacted CTF-1 sheets with 4-aminobenzoic acid to graft carboxylic groups onto the surfaces of the CTF-1 sheets. A subsequent amide condensation between the amine groups of the CN and the carboxyl groups on the CTF-1 bridged the two components. The amide groups serve as electron transport pathways and facilitate the movement of photo-excited electrons from CTF-1 to CN (Figure 1a) which liberates hydrogen gas.

The covalent amide “bridges” gave CN/CTF-1 a fast hydrogen production rate. Quantitatively, when irradiated with a 300 W Xe lamp at 160 mW/cm², CN/CTF-1 produced ~4 mmol H₂ per gram of CN/CTF-1 after 4 h (0.85 mmol H₂ h^-1 g_catalyst^-1), whereas under identical conditions, weakly adhered CN and CTF-1 sheets as well as a physical mixture of CN and CTF-1 all achieved H₂ evolution rates of ~1 mmol H₂ per gram of photocatalyst (0.30 mmol H₂ h^-1 g_catalyst^-1) (Figure 1b).

Figure 1. (a) (Pt-loaded) CN sheets are covalently bound to CTF-1 sheets via amide bonds. These covalent bonds serve as electron transport “bridges” that facilitate the diffusion of photo-excited electrons from CTF-1 to CN. (b) H2 evolution rates of four photocatalysts: 1 – covalently bound CN/CTF-1; 2 and 3 –  weakly adhered CN and CTF-1; 4 – a physical mixture of CN and CTF-1.

The covalent bonding strategy is applicable to other coupling reactions such as the Friedel-Crafts reaction. This general method could create a new paradigm for designing and synthesizing high-performance hetero-structured photocatalysts.

To find out more please read:

A General Strategy via Chemically Covalent Combination for Constructing Heterostructured Catalysts with Enhanced Photocatalytic Hydrogen Evolution

Gang Zhou, Ling-Ling Zheng, Dengke Wang, Qiu-Ju Xing, Fei Li, Peng Ye, Xiao Xiao, Yan Li, and Jian-Ping Zou

Chem. Commun., 2019, 55, 4150-4153

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/.

Increasing the volumetric energy densities of batteries is essential for improving the durability of portable electronics and the operating ranges of electric vehicles. One way to improve energy density is to enlarge the mass fraction of active materials in battery electrodes; however, the degree of enhancement remains limited. This limitation results from the densification of the electrodes when the mass fraction increases, making electron transport and ion diffusion throughout the electrodes sluggish. These drawbacks lower the utilization efficiency of the overall electrode materials.

A team of scientists from China and the United States has recently addressed the aforementioned challenges. Specifically, they synthesized a 3D cathode of carbon-coated Li₂MnSiO₄ (Li₂MnSiO₄/C) with a structure mimicking a honeycomb filled with bee pupas (Fig. 1). This lithium-ion battery cathode possesses a high mass fraction of 90% (of overall electrode mass) as well as a volumetric energy density as high as 2443 Wh/dm³.

The uniquely structured electrodes were prepared through a hard-template method (Fig. 1). Using polystyrene particles, silica surface coating, and Li₂MnSiO₄ precursor infiltration, the authors synthesized a carbon-coated Li₂MnSiO₄ honeycomb scaffold with each cavity filled with a carbon-coated Li₂MnSiO₄ particle. This architecture differed from previously reported 3D structures, which typically had a large portion of voids, and enabled an ultrahigh active-material mass loading of 90 wt.%. Additionally, the gaps between the scaffold and the particles functioned as ion-diffusion channels, and the carbon coatings served as electron-transport expressways. These characteristics effectively addressed the problem of sluggish ion diffusion and electron transport.

Figure 1. The synthesis procedures of the BPFH-shaped Li2MnSiO4/C electrode. The green particles and yellow scaffold represent polystyrene spheres and the silica coating, respectively.

Due to the facilitated electron transport and ion diffusion, the Li₂MnSiO₄/C electrode with a bee pupa-filled honeycomb (BPFH) structure (Fig. 2a) exhibited an outstanding charge-storage performance. Specifically, it delivered a high volumetric capacity of 643 mAh/cm³ at a current density of 0.1 C, corresponding to a volumetric density of 2443 Wh/dm³. This volumetric capacity was approximately two times higher than that of a Li₂MnSiO₄/C honeycomb lattice without any Li₂MnSiO₄ particles (Fig. 2b). After 100 consecutive charge-discharge cycles, the BPFH-shaped Li₂MnSiO₄/C electrode retained a volumetric capacity of 328 mAh/cm³ (Fig. 2c).

Figure 2. (a and b) Scanning electron microscopy images of (a) the BPFH-shaped Li2MnSiO4/C electrode and (b) the Li2MnSiO4/C scaffold. (c) The capacities and the Coulombic efficiencies of the two electrodes during 100 charge-discharge cycles.

The demonstrated BPFH architecture could be extended to other materials for the synthesis of battery electrodes with both high mass fractions of active materials and outstanding volumetric energy densities.

To find out more please read:

A Bee Pupa-Infilled Honeycomb Structure-Inspired Li₂MnSiO₄ Cathode for High Volumetric Energy Density Secondary Batteries

Jinyun Liu, Xirong Lin, Huigang Zhang, Zihan Shen, Qianqian Lu, Junjie Niu, Jinjin Li and Paul V. Braun

Chem. Commun., 2019, 55, 3582-3585

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/.

Organic compounds with conjugated electron structures are emerging as promising Li-ion battery cathodes due to their high capacity and environmental benignity. To make these cathodes practically feasible, organic electrodes are typically incorporated with metal ions to boost their energy densities. The addition of metal ions, however, usually jeopardizes the structural integrity of the electrodes and shortens battery lifetime.

Recently, three groups of Chinese researchers demonstrated that increasing the electrolyte concentration could effectively prolong the lifespan of metal-incorporated organic cathodes. The researchers studied cuprous tetracyano-quinodimethane (CuTCNQ), a Cu²⁺-containing organic Li-ion battery cathode, and observed its significantly improved cycling stability in a 7 M LiClO₄ electrolyte compared to a 1 M electrolyte. This work was published recently in ChemComm.

CuTCNQ in a typical diluted electrolyte of 1 M LiClO₄ exhibited unsatisfactory stability. Its first-cycle charging capacity reached ~180 mAh/g, but it dropped appreciably to 23 mAh/g after the first discharging process (Figure 1a). Concurrently, the electrolyte turned from clear to yellow (Figure 1b), due to the dissolution of TCNQ. These observations unequivocally showed the rapid disintegration of CuTCNQ in diluted electrolytes.

Figure 1. (a) The first-cycle charge-discharge profile of CuTCNQ in a liquid electrolyte containing ethylene carbonate (EC), propylene carbonate (PC) and 1 M LiClO4 (1 M LiClO4-EC/PC). (b) Photographs showing the electrolyte color before and after the first charge-discharge cycle.

CuTCNQ was found to be more stable in electrolytes with concentrations higher than 1 M. When the LiClO₄ concentration increased to 3 M, 5 M and 7 M, the specific capacities of CuTCNQ retained after 50 consecutive charge-discharge cycles were ~25 mAh/g, ~70 mAh/g, and ~110 mAh/g, respectively (Figure 2a). All of these capacities were higher than that of CuTCNQ in 1 M LiClO₄ after the same cycle number (<10 mAh/g). Additionally, the electrolytes experienced nearly no color change, suggesting little TCNQ was dissolved (Figure 2b).

The elevated stability of CuTCNQ correlates to the formation of Li⁺-ClO₄^– ion pairs in concentrated electrolytes (Figure 2c). With increasing LiClO₄ concentration, Li⁺ and ClO₄^– tend to form ion pairs that coordinate with solvent molecules. Solvent-coordination reduces the number of free solvent molecules that can dissolve TCNQ, thus minimizing the dissolution of TCNQ.

Figure 2. (a) The cycling stability performances of CuTCNQ with electrolytes with different LiClO4 concentrations. (b) Photographs showing the electrolyte color before and after 50 charge-discharge cycles at different LiClO4 concentrations. (c) Li+ and ClO4- form solvent-coordinated ion pairs in super-concentrated electrolytes (e.g., 7 M).

This work provides a facile approach to mitigate the capacity fading of CuTCNQ. The strategy may be extended to stabilize other metal-incorporated organic cathodes in Li-ion batteries.

To find out more please read:

Sustainable Cycling Enabled by A High-Concentration Electrolyte for Lithium-Organic Batteries

Ying Huang, Chun Fang, Wang Zhang, Qingju Liu and Yunhui Huang

Chem. Commun., 2019, 55, 608-611

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/.