2024 Vol. 9, No. 9

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Research Highlight
CO2 electrolysis to formic acid for carbon neutralization
Kezhen Qi, Shu-yuan Liu, Yingjie Zhang, Hui Zhang, Vadim Popkov, Oksana Almjasheva
2024, 9(9): 1333-1335. doi: 10.1016/j.gee.2024.04.011
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To avoid carbonate precipitation for CO2 electrolysis, developing CO2 conversion in an acid electrolyte is viewed as an ultimately challenging technology. In Nature, Xia et al. recently explored a proton-exchange membrane system for reducing CO2 to formic acid with a Pb-PbSO4 composite catalyst derived from waste lead-acid batteries based on the lattice carbon activation mechanism. Up to 93% Faradaic efficiency was realized when formic acid was produced by this technology.
Review articles
Advanced 3D ordered electrodes for PEMFC applications: From structural features and fabrication methods to the controllable design of catalyst layers
Kaili Wang, Tingting Zhou, Zhen Cao, Zhimin Yuan, Hongyan He, Maohong Fan, Zaiyong Jiang
2024, 9(9): 1336-1365. doi: 10.1016/j.gee.2023.11.002
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The catalyst layers (CLs) electrode is the key component of the membrane electrode assembly (MEA) in proton exchange membrane fuel cells (PEMFCs). Conventional electrodes for PEMFCs are composed of carbon-supported, ionomer, and Pt nanoparticles, all immersed together and sprayed with a micron-level thickness of CLs. They have a performance trade-off where increasing the Pt loading leads to higher performance of abundant triple-phase boundary areas but increases the electrode cost. Major challenges must be overcome before realizing its wide commercialization. Literature research revealed that it is impossible to achieve performance and durability targets with only high-performance catalysts, so the controllable design of CLs architecture in MEAs for PEMFCs must now be the top priority to meet industry goals. From this perspective, a 3D ordered electrode circumvents this issue with a support-free architecture and ultrathin thickness while reducing noble metal Pt loadings. Herein, we discuss the motivation in-depth and summarize the necessary CLs structural features for designing ultralow Pt loading electrodes. Critical issues that remain in progress for 3D ordered CLs must be studied and characterized. Furthermore, approaches for 3D ordered CLs architecture electrode development, involving material design, structure optimization, preparation technology, and characterization techniques, are summarized and are expected to be next-generation CLs for PEMFCs. Finally, the review concludes with perspectives on possible research directions of CL architecture to address the significant challenges in the future.
Cell-free biocatalysis coupled with photo-catalysis and electro-catalysis: Efficient CO2-to-chemical conversion
Junzhu Yang, Chi-Kit Sou, Yuan Lu
2024, 9(9): 1366-1383. doi: 10.1016/j.gee.2023.10.002
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The increasing atmospheric carbon dioxide (CO2) concentration has exposed a series of crises in the earth's ecological environment. How to effectively fix and convert carbon dioxide into products with added value has attracted the attention of many researchers. Cell-free enzyme catalytic system coupled with electrical and light have been a promising attempt in the field of biological carbon fixation in recent years. In this review, the research progresses of photoenzyme catalysis, electroenzyme catalysis and photo-electroenzyme catalysis for converting carbon dioxide into chemical products in cell-free systems are systematically summarized. We focus on reviewing and comparing various coupling methods and principles of photoenzyme catalysis and electroenzyme catalysis in cell-free systems, especially the materials used in the construction of the coupling system, and analyze and point out the characteristics and possible problems of different coupling methods. Finally, we discuss the major challenges and prospects of coupling physical signals and cell-free enzymatic catalytic systems in the field of CO2 fixation, suggesting possible strategies to improve the carbon sequestration capacity of such systems.
Advances in selective conversion of carbohydrates into 5-hydroxymethylfurfural
Jie Liang, Jianchun Jiang, Tingting Cai, Chao Liu, Jun Ye, Xianhai Zeng, Kui Wang
2024, 9(9): 1384-1406. doi: 10.1016/j.gee.2023.11.005
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Converting carbohydrates into 5-hydroxymethylfurfural (5-HMF) is an attractive and promising route for value-added utilization of agricultural and forestry biomass resource. As an important platform compound, 5-HMF possesses high active furan structure with hydroxymethyl and aldehyde group for production of various bio-chemicals and materials, meanwhile, which suffer from low stability and poor yield during the industrial biorefinery process. Hence, selective production of 5-HMF with high-yield and low-cost has attracted extensive attention from scientific and industrial researchers. This review sorted and described the latest advanced research on solvent and catalyst system, as well as energy field effect for production of 5-HMF with different feedstock in detail, emphatically discussing the solvent effect and its synergistic effect with other aspects. Besides, the future prospects and challenges for production of 5-HMF from carbohydrates were also presented, which provide a profound insight into industrial 5-HMF process with economic and environmental feature.
Research papers
A novel metal-free porous covalent organic polymer for efficient room-temperature photocatalytic CO2 reduction via dry-reforming of methane
Sheng-Yan Yin, Ziyi Li, Yingcai Hu, Xiao Luo, Jishan Li
2024, 9(9): 1407-1418. doi: 10.1016/j.gee.2023.03.003
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At room temperature, the conversion of greenhouse gases into valuable chemicals using metal-free catalysts for dry reforming of methane (DRM) is quite promising and challenging. Herein, we developed a novel covalent organic porous polymer (TPE-COP) with rapid charge separation of the electron-hole pairs for DRM driven by visible light at room temperature, which can efficiently generate syngas (CO and H2). Both electron donor (tris(4-aminophenyl) amine, TAPA) and acceptor (4,4',4″,4'''-((1 E,1'E,1″E, 1'''E)-(ethene-1,1,2,2-tetrayltetrakis (benzene-4,1-diyl)) tetrakis (ethene-2,1-diyl)) tetrakis (1-(4-formylbenzyl) quinolin-1-ium), TPE-CHO) were existed in TPE-COP, in which the push-pull effect between them promoted the separation of photogenerated electron-hole, thus greatly improving the photocatalytic activity. Density functional theory (DFT) simulation results show that TPE-COP can form charge-separating species under light irradiation, leading to electrons accumulation in TPE-CHO unit and holes in TAPA, and thus efficiently initiating DRM. After 20 h illumination, the photocatalytic results show that the yields reach 1123.6 and 30.8 μmol g-1 for CO and H2, respectively, which are significantly higher than those of TPE-CHO small molecules. This excellent result is mainly due to the increase of specific surface area, the enhancement of light absorption capacity, and the improvement of photoelectron-generating efficiency after the formation of COP. Overall, this work contributes to understanding the advantages of COP materials for photocatalysis and fundamentally pushes metal-free catalysts into the door of DRM field.
Li-promoted C3N4 catalyst for efficient isomerization of glucose into fructose at 50℃ in water
Wang Liu, Yanfei Zhang, Mengya Sun, Xinpeng Zhao, Shenggang Li, Xinqing Chen, Liangshu Zhong, Lingzhao Kong
2024, 9(9): 1419-1426. doi: 10.1016/j.gee.2023.04.005
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Efficient and selective glucose-to-fructose isomerization is a crucial step for production of oxygenated chemicals derived from sugars, which is usually catalyzed by base or Lewis acid heterogeneous catalyst. However, high yield and selectivity of fructose cannot be simultaneously obtained under mild conditions which hamper the scale of application compared with enzymatic catalysis. Herein, a Li-promoted C3N4 catalyst was exploited which afforded an excellent fructose yield (40.3 wt%) and selectivity (99.5%) from glucose in water at 50 ℃, attributed to the formation of stable Li-N bond to strengthen the basic sites of catalysts. Furthermore, the so-formed N6-Li-H2O active site on Li-C3N4 catalyst in aqueous phase changes the local electronic structure and strengthens the deprotonation process during glucose isomerization into fructose. The superior catalytic performance which is comparable to biological pathway suggests promising applications of lithium containing heterogeneous catalyst in biomass refinery.
Furfural residues derived nitrogen-sulfur co-doped sheet-like carbon: An excellent electrode for dual carbon lithium-ion capacitors
Xiaoying Guo, Yan Qiao, Zonglin Yi, Christian Marcus Pedersen, Yingxiong Wang, Xiaodong Tian, Peide Han
2024, 9(9): 1427-1439. doi: 10.1016/j.gee.2023.05.007
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The state-of-the-art lithium-ion capacitors (LICs), consisting of high-capacity battery-type anode and high-rate capacitor-type cathode, can deliver high energy density and large power density when comparing with traditional supercapacitors and lithium-ion batteries, respectively. However, the ion kinetics mismatch between cathode and anode leads to unsatisfied cycling lifetime and anode degradation. Tremendous efforts have been devoted to solving the abovementioned issue. One promising strategy is altering high conductive hard carbon anode with excellent structural stability to match with activated carbon cathode, assembling dual-carbon LIC. In this contribution, one-pot in-situ expansion and heteroatom doping strategy was adopted to prepare sheet-like hard carbon, while activated carbon was obtained involving activation. Ammonium persulfate was used as expanding and doping agent simultaneously. While furfural residues (FR) were served as carbon precursor. The resulting hard carbon (FRNS-HC) and activated carbon (FRNS-AC) show excellent electrochemical performance as negative and positive electrodes in a lithium-ion battery (LIB). To be specific, 374.2 mAh g-1 and 123.1 mAh g-1 can be achieved at 0.1 A g-1 and 5 A g-1 when FRNS-HC was tested as anode. When combined with a highly porous carbon cathode (SBET = 2961 m2 g-1) synthesized from the same precursor, the LIC showed high specific energy of 147.67 Wh kg-1 at approximately 199.93 W kg-1, and outstanding cycling life with negligible capacitance fading over 1000 cycles. This study could lead the way for the development of heteroatom-doped porous carbon nanomaterials applied to Li-based energy storage applications.
Self-separation ionic liquid catalyst for the highly effective conversion of H2S by α,β-unsaturated carboxylate esters under mild conditions
Wenjie Xiong, Xiaomin Zhang, Xingbang Hu, Youting Wu
2024, 9(9): 1440-1448. doi: 10.1016/j.gee.2023.03.001
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The deep-processing utility of pure hydrogen sulfide (H2S) is a significant direction in natural gas chemical industry. Herein, a brand-new strategy of H2S conversion by α,β-unsaturated carboxylate esters into thiols or thioethers using task-specific carboxylate ionic liquids (ILs) as catalyst has been developed, firstly accomplishing the phase separation of product and catalyst without introducing the third component. It can be considered as a cascade reaction in which the product selectivity can be controlled by adjusting the molar ratio of H2S to α,β-unsaturated carboxylate esters. Also, the effects of ILs with different anions and cations, intermittent feeding operations, as well as pressure-time kinetic behaviors on cascade reaction were investigated. Furthermore, the proposed interaction mechanism of H2S conversion using butyl acrylate catalyzed by [Emim][Ac] was revealed by DFT-based theoretical calculation. The approach enables the self-phase separation promotion of catalyst and product and achieves 99% quantitative conversion under mild conditions in the absence of solvent, making the entire process ecologically benign. High-efficiency reaction activity can still be maintained after ten cycles of the catalyst. Therefore, the good results, combined with its simplicity of operation and the high recyclability of the catalyst, make this green method environmentally friendly and cost-effective. It is anticipated that this self-separation method mediated by task-specific ILs will provide a feasible strategy for H2S utilization, which will guide its application on an industrial scale.
Low-temperature graphitization of lignin via Co-assisted electrolysis in molten salt
Shijie Li, Wei-Li Song, Xue Han, Qingqing Cui, Yan-li Zhu, Shuqiang Jiao
2024, 9(9): 1449-1458. doi: 10.1016/j.gee.2023.04.006
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The ever-growing energy demand and environmental issues have stimulated the development of sustainable energy technologies. Herein, an efficient and environmentally friendly electrochemical transformation technology was proposed to prepare highly graphitized carbon materials from an abundant natural resource-lignin (LG). The preparation process mainly includes pyrolytic carbonization of raw LG material and electrochemical conversion of amorphous carbon precursor. Interestingly, with the assistance of Co catalyst, the graphitization degree of the products was significantly improved, in which the mechanism was the removal of heteroatoms in LG and the rearrangement of carbon atoms into graphite lattice. Furthermore, tunable microstructures (nanoflakes) under catalytic effects could also be observed by controlling the electrolytic parameters. Compared with the products CN1 (without catalyst) and CN5 (with 10% catalyst), the specific surface area are 158.957 and 202.246 m2 g-1, respectively. When used as the electrode material for lithium-ion batteries, CN5 delivered a competitive specific capacity of ~350 mAh g-1 (0.5 C) compared with commercial graphite. The strategy proposed in this work provides an effective way to extract value-added graphite materials from lignin and can be extended to the graphitization conversion of any other amorphous carbon precursor materials.
Nitrogen-doping boosts *CO utilization and H2O activation on copper for improving CO2 reduction to C2+ products
Yisen Yang, Zhonghao Tan, Jianling Zhang, Jie Yang, Renjie Zhang, Sha Wang, Yi Song, Zhuizhui Su
2024, 9(9): 1459-1465. doi: 10.1016/j.gee.2023.09.002
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To improve the electrocatalytic transformation of carbon dioxide (CO2) to multi-carbon (C2+) products is of great importance. Here we developed a nitrogen-doped Cu catalyst, by which the maximum C2+ Faradaic efficiency can reach 72.7% in flow-cell system, with the partial current density reaching 0.62 A cm-2. The in situ Raman spectra demonstrate that the *CO adsorption can be strengthened on such a N-doped Cu catalyst, thus promoting the *CO utilization in the subsequent C-C coupling step. Simultaneously, the water activation can be well enhanced by N doping on Cu catalyst. Owing to the synergistic effects, the selectivity and activity for C2+ products over the N-deoped Cu catalyst are much improved.
Piezoelectric-enhanced n-TiO2/BaTiO3/p-TiO2 heterojunction for highly efficient photoelectrocatalysis
Minhua Ai, Zihang Peng, Xidi Li, Faryal Idrees, Xiangwen Zhang, Ji-Jun Zou, Lun Pan
2024, 9(9): 1466-1476. doi: 10.1016/j.gee.2023.12.001
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Charge separation is critical for achieving efficient solar-to-hydrogen conversion, whereas piezoelectric-enhanced photoelectrochemical (PEC) systems can effectively modulate band bending and charge migration. Herein, we design an n-TiO2/BaTiO3/p-TiO2 (TBTm) heterojunction in which the piezoelectric BaTiO3 layer is sandwiched between n-TiO2 and p-TiO2. The built-in electric field of TBTm can provide a strong driving force to accelerate carrier separation and prolong carrier lifetime. Consequently, the TBT3 achieves a prominent photocurrent density, as high as 2.13 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE), which is 2.4- and 1.5-times higher than TiO2 and TiO2-BaTiO3 heterojunction, respectively. Driven by mechanical deformation, the induced dipole polarization can further regulate built-in electric fields, and the piezoelectric photocurrent density of TBT3-800 is 2.84 times higher than TiO2 at 1.23 V vs. RHE due to the construction of piezoelectric-heterostructures. This work provides a piezoelectric polarization strategy for modulating the built-in electric field of heterojunction for PEC system.
Low-temperature chemistry in plasma-driven ammonia oxidative pyrolysis
Mingming Zhang, Qi Chen, Guangzhao Zhou, Jintao Sun, He Lin
2024, 9(9): 1477-1488. doi: 10.1016/j.gee.2023.05.010
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Ammonia is gaining increasing attention as a green alternative fuel for achieving large-scale carbon emission reduction. Despite its potential technical prospects, the harsh ignition conditions and slow flame propagation speed of ammonia pose significant challenges to its application in engines. Non-equilibrium plasma has been identified as a promising method, but current research on plasma-enhanced ammonia combustion is limited and primarily focuses on ignition characteristics revealed by kinetic models. In this study, low-temperature and low-pressure chemistry in plasma-assisted ammonia oxidative pyrolysis is investigated by integrated studies of steady-state GC measurements and mathematical simulation. The detailed kinetic mechanism of NH3 decomposition in plasma-driven Ar/NH3 and Ar/NH3/O2 mixtures has been developed. The numerical model has good agreements with the experimental measurements in NH3/O2 consumption and N2/H2 generation, which demonstrates the rationality of modelling. Based on the modelling results, species density profiles, path flux and sensitivity analysis for the key plasma-produced species such as NH2, NH, H2, OH, H, O, O(1D), O2(a1Δg), O2(b1Σg+), Ar*, H-, Ar+, NH3+, O2- in the discharge and afterglow are analyzed in detail to illustrate the effectiveness of the active species on NH3 excitation and decomposition at low temperature and relatively higher E/N values. The results revealed that NH2, NH, H as well as H2 are primarily generated through the electron collision reactions e + NH3 → e + NH2 + H, e + NH3 → e + NH + H2 and the excited-argon collision reaction Ar* + NH3 + H → Ar + NH2 + 2H, which will then react with highly reactive oxidative species such as O2*, O*, O, OH, and O2 to produce stable products of NOx and H2O. NH3 → NH is found a specific pathway for NH3 consumption with plasma assistance, which further highlights the enhanced kinetic effects.

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