Solar carbon fuel via photoelectrochemistry

A promising strategy to mitigate both energy shortage and global warming is the conversion of CO 2 into chemicals that can be used as fuels (chemical fuels) by utilizing renewable energy sources. Up to date, solar-driven CO 2 reduction has been achieved with photochemical (PC) and photoelectrochemical (PEC) systems or electrochemical cells combined with a photovoltaic system (PV-EC). This study is intended to compare and highlight the state-of-the-art PEC systems for CO 2 reduction and show the limitation factors that still hinder their widespread utilization. The review starts with a description of semiconducting photocatalyst properties and fundamental understanding of PEC CO 2 reduction process. Then, the most significant performance metrics used for evaluation of PEC systems are explained in details. In addition, recent progress in PEC CO 2 reduction systems is summarized and classified in different categories according to the chemical product. Different strategies such as doping, combination of two or more semiconductors, synthesis of nanostructured materials, passivation layers and co-catalysts that enhance light absorption, chemical stability, charge transfer and reduce ohmic losses and overpotentials of photoactive materials are reviewed. Besides the improvement of photocatalysts, research progress on the front of PEC reactor design, combined with the development of advanced modelling tools and characterization techniques are expected to bring PEC CO 2 reduction a step closer to commercialization

Existence of connected and autonomous vehicles in mixed traffic: Impacts on safety and environment

With the growing market penetration of connected and autonomous vehicles (CAVs), the interaction between conventional human-driven vehicles (HDVs) and CAVs will be inevitable. However, the effects of CAVs in mixed traffic streams have not been extensively studied in China. This study aims to quantify the changes in driving characteristics of an HDV while following a CAV compared to following another HDV and investigate the corresponding impact on traffic safety and the environment caused by these changes. Firstly, two scenarios were built on a driving simulation platform. In scenario 1, the driver follows a vehicle programmed to execute the speed profile of the HDV obtained from the Shanghai Naturalistic Driving Study (SH-NDS) project. In scenario 2, the driver follows a vehicle whose speed profile is calibrated according to the Cooperative Adaptive Cruise Control (CACC) follow-along theory. Secondly, the speed, acceleration, and headway of 30 individuals in each following scenario were analyzed. Speed and acceleration volatility (standard deviation, deviation rate) and time-to-collision (TTC) were selected as indexes to assess the safety impact. The emission and fuel consumption models were used to determine the environmental impact after being localized by the parameters. HDVs following CAVs exhibit less driving volatility in speed and acceleration, show remarkable improvements in TTC, consume less fuel, and produce fewer emissions on average. By introducing CAVs into the road traffic system, traffic operation safety and environmental quality will be improved, with a more stable flow status, lower collision risk, and less air pollution.</p

Theoretical Screening of Transition Metal–N₄‑Doped Graphene for Electroreduction of Nitrate

Electrochemical nitrate reduction reaction (NO3RR) is an advantageous conversion technology for nitrate removal and ammonia synthesis. Single-atom catalysts, owing to their utmost metal atom utilization efficiency, are promising electrocatalysts for NO3RR but are rarely investigated in systematic ways. In this study, a theoretical screening is performed on transition metal–N4-doped graphene (TM–N4/C) as active and selective electrocatalysts for NO3RR, where detailed reaction mechanisms and activity origins are explored. Volcano plots of activity trends show that Cu– and Pt–N4/C are highly active for NO3RR following the NH3 and N2 formation pathways, respectively, whose activities can be attributed to the optimal NO and N adsorptions. In addition, a contour plot of selectivity trend shows that Re– and Pt–N4/C are highly selective toward NH3 and N2 formations, respectively. This work provides theoretical insights into the rational design of TM–N4/C catalysts for NO3RR and opportunities for efficient nitrate removal and ammonia synthesis strategies

Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity

We determined the size-dependent specific and mass activities of the oxygen reduction in HClO4 solutions on the Pt particles in the range of 1–5 nm. The maximal mass activity at 2.2 nm is well explained based on density functional theory calculations performed on fully relaxed nanoparticles. The presence of the edge sites is the main reason for the low specific activity in nanoparticles due to very strong oxygen binding energies at these sites. Our results clearly demonstrate that the catalytic activity highly depends on the shape and size of the nanoparticles

Hollow Porous Hierarchical-Structured 0.5Li₂MnO₃·0.5LiMn_0.4Co_0.3Ni_0.3O₂ as a High-Performance Cathode Material for Lithium-Ion Batteries

We report a novel hollow porous hierarchical-architectured 0.5Li₂MnO₃·0.5LiMn_0.4Co_0.3Ni_0.3O₂ (LLO) for lithium-ion batteries (LIBs). The obtained lithium-rich layered oxides possess a large inner cavity, a permeable porous shell, and excellent structural robustness. In LIBs, such unique features are favorable for fast Li⁺ transportation and can provide sufficient contact between active materials and electrolytes, accommodate more Li⁺, and improve the kinetics of the electrochemical reaction. The as-prepared LLO displays an extremely high initial discharge capacity (296.5 mAh g^–1 at 0.2 C), high rate capability (162.6 mAh g^–1 at 10 C), and excellent cycling stability (237.6 mAh g^–1 after 100 cycles at 0.5 C and 153.8 mAh g^–1 after 200 cycles at 10 C). These values are superior to most literature data

Impacts of Perchloric Acid, Nafion, and Alkali Metal Ions on Oxygen Reduction Reaction Kinetics in Acidic and Alkaline Solutions

Fundamental understandings on the impacts induced by anions and cations on oxygen reduction reaction (ORR) are of great interest in designing more efficient catalysts and identifying reasons for discrepancies in activities measured in different protocols. In this study, the specific adsorption of ClO₄^–, Nafion ionomer, and cations on Pt/C, Pd/C, and transition metal, N codoped carbon-based (Me–N–C) catalysts, and their effects on the ORR kinetics were systematically investigated. It was found that ClO₄^– had a negligible impact on the ORR activity of Pt/C possibly due to its weak adsorption. Nafion ionomers, on the other hand, showed a significant poisoning effect on the bulk Pt electrode. Its impact on Pt/C, however, is negligible even with a very high I/C ratio (1.33) in acidic solutions. The three catalysts showed different behaviors in alkaline solutions. The noncovalent interaction between hydrated cations and surface OH groups was found on Pt/C and had an obvious impact on the ORR kinetics. This noncovalent interaction, however, was not observed on Pd/C, which showed the same ORR activity in all three electrolytes (LiOH, NaOH, and KOH). The ORR activity of Me–N–C increased following the order of KOH < NaOH < LiOH. This trend is totally opposite to that of Pt/C. The mechanisms for the material-dependent activity trend in different cation solutions were discussed

Pt Monolayer on Porous Pd−Cu Alloys as Oxygen Reduction Electrocatalysts

We demonstrate the synthesis of a core−shell catalyst consisting of a Pt monolayer as the shell and porous/hollow Pd−Cu alloy nanoparticles as the core. The porous/hollow Pd−Cu nanoparticles were fabricated by selectively dissolving a less noble metal, Cu, using an electrochemical dealloying process. The Pt mass activity for the oxygen reduction reaction of a Pt monolayer deposited on such a porous core is 3.5 times higher than that of a Pt monolayer deposited on bulk Pd nanoparticles and 14 times higher than that of state-of-the-art Pt/C electrocatalysts

Active Sites on Heterogeneous Single-Iron-Atom Electrocatalysts in CO₂ Reduction Reaction

Nitrogen-coordinated single-metal-atom catalysts (Me–N–C) are promising candidates for CO2-to-CO electrocatalytic conversion. The nature of real active sites in this type of electrocatalyst, however, is not clear. In this Letter, we study the specific interactions between the reaction intermediates and a model single-iron-atom catalyst (Fe–N–C) by combining in situ infrared absorption spectroscopy and density functional theory (DFT) calculations. For the first time, we confirm that the Fe centers in Fe–N4 moieties hosted by the complete graphitic layer are poisoned by strongly adsorbed CO and should not be the real active sites for gaseous CO production. Further DFT calculation results suggest that the high CO selectivity and reaction rate may originate from Fe–N4 moieties embedded in a defective graphitic layer that have balanced binding energies of adsorbed COOH and CO species. These findings add significant new insights into the mechanisms of CO2 reduction on carbon-based single-atom electrocatalysts

The Role of Transition Metal and Nitrogen in Metal–N–C Composites for Hydrogen Evolution Reaction at Universal pHs

For the first time, we demonstrated that transition metal and nitrogen codoped carbon nanocomposites synthesized by pyrolysis and heat treatment showed excellent catalytic activity toward hydrogen evolution reaction (HER) in both acidic and alkaline media. The overpotential at 10 mA cm^–2 was 235 mV in a 0.5 M H₂SO₄ solution at a catalyst loading of 0.765 mg cm^–2 for Co–N–C. In a 1 M KOH solution, the overpotential was only slightly increased by 35 mV. The high activity and excellent durability (negligible loss after 1000 cycles in both acidic and alkaline media) make this carbon-based catalyst a promising alternative to noble metals for HER. Electrochemical and density functional theory (DFT) calculation results suggested that transition metals and nitrogen played a critical role in activity enhancement. The active sites for HER might be associated with metal/N/C moieties, which have been also proposed as reaction centers for oxygen reduction reaction