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We examine the performance of a number of single-atom M-N/C electrocatalysts with a common structure in order to deconvolute the activity of the framework N/C support from the metal M-N4 sites in M-N/Cs. The formation of the N/C framework with coordinating nitrogen sites is performed using zinc as a templating agent. After the formation of the electrically conducting carbon-nitrogen metal-coordinating network, we (trans)metalate with different metals producing a range of different catalysts (Fe-N/C, Co-N/C, Ni-N/C, Sn-N/C, Sb-N/C, and Bi-N/C) without the formation of any metal particles. In these materials, the structure of the carbon/nitrogen framework remains unchanged-only the coordinated metal is substituted. We assess the performance of the subsequent catalysts in acid, near-neutral, and alkaline environments toward the oxygen reduction reaction (ORR) and ascribe and quantify the performance to a combination of metal site activity and activity of the carbon/nitrogen framework. The ORR activity of the carbon/nitrogen framework is about 1000-fold higher in alkaline than it is in acid, suggesting a change in mechanism. At 0.80 VRHE, only Fe and Co contribute ORR activity significantly beyond that provided by the carbon/nitrogen framework at all pH values studied. In acid and near-neutral pH values (pH 0.3 and 5.2, respectively), Fe shows a 30-fold improvement and Co shows a 5-fold improvement, whereas in alkaline pH (pH 13), both Fe and Co show a 7-fold improvement beyond the baseline framework activity. The site density of the single metal atom sites is estimated using the nitrite adsorption and stripping method. This method allows us to deconvolute the framework sites and metal-based active sites. The framework site density of catalysts is estimated as 7.8 × 1018 sites g-1. The metal M-N4 site densities in Fe-N/C and Co-N/C are 9.4 × 1018 sites-1 and 4.8 × 1018 sites g-1, respectively.

International audience; Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O$_2$) reduction at the cathode of proton exchange membrane fuel cells (PEMFCs) are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O$_2$ reduction, their controlled synthesis and stability for practical applications remains challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilisation remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, we addressed this issue by coordinating Fe in a highly porous nitrogen doped carbon support (~3295 m$^2$ g$^{-1}$), prepared by pyrolysis of inexpensive 2,4,6triaminopyrimidine and a Mg$^{2+}$ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54×10$^{19}$ sites g$_{FeNC}$$^{-1}$ and a record 52% FeN$_x$ electrochemical utilisation based on in situ nitrite stripping was achieved. The Fe single atoms are characterised pre-and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which were further studied by density functional theory calculations.

The electrochemical CO2 reduction reaction (CO2RR) to value-added chemicals with renewable electricity is a promising method to decarbonise parts of the chemical industry. Recently, single metal atoms in nitrogen-doped carbon (MNC) have emerged as potential electrocatalysts for CO2RR to CO with high activity and faradaic efficiency, although the reaction limitation for CO2RR to CO is unclear. To understand the comparison of intrinsic activity of different MNCs, we synthesized two catalysts through a decoupled two-step synthesis approach of high temperature pyrolysis and low temperature metalation (Fe or Ni). The highly meso-porous structure resulted in the highest reported electrochemical active site utilisation based on in situ nitrite stripping; up to 59±6% for NiNC. Ex-situ X-ray absorption spectroscopy confirmed the penta-coordinated nature of the active sites. The catalysts are amongst the most active in the literature for CO2 reduction to CO. Our density functional theory calculations (DFT) show that their binding to the reaction intermediates approximates to that of Au surfaces. However, we find that the TOFs of the most active catalysts for CO evolution converge, suggesting a fundamental ceiling to the catalytic rates.

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O2) reduction at the cathode of proton exchange membrane fuel cells (PEMFCs) are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O2 reduction, their controlled synthesis and stability for practical applications remains challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilisation remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, we addressed this issue by coordinating Fe in a highly porous nitrogen doped carbon support (~3295 m2 g-1), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine and a Mg2+ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54×10^19 sites gFeNC-1 and a record 52% FeNx electrochemical utilisation based on in situ nitrite stripping was achieved. The Fe single atoms are characterised pre- and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which were further studied by density functional theory calculations.

This work aims at using Computational Fluid Dynamics (CFD) method to establish a gasoline direct injection (GDI) engine nozzle and combustion chamber model to simulate cavitation and flash boiling phenomena and analyze how these phenomena affect the engine performance. FLUENT 15.0 is used to simulate the flow of fuel. The cavitation phenomenon in GDI nozzle was simulated comprehensively, and the influences of parameter values such as inlet pressure and outlet pressure on cavitation were studied in this work. The results show that high injection pressure can promote the occurrence of cavitation and high outlet pressure has an inhibitory effect on cavitation. However, the effect of cavitation on atomization cannot be seen intuitively only through the simulation of the internal nozzle. The two-dimensional inter nozzle model established in this work is a foundation for the establishment of external nozzle model. It can provide specific boundary conditions at nozzle outlet. In addition, a three-dimensional external nozzle model was established to simulate the flash boiling spray in the combustion chamber. Based on the mechanism, it can be found that flash boiling spray improves the atomization quality. After verifying the simulation results with relevant experiments, these models could bring great convenience to the study of cavitation and flash boiling with sufficient reliability in further study.

This work aims to apply Computational Fluid Dynamic (CFD) method to establish a flash evaporation spray cooling (FESC) model to simulate the heating process and find the optimum cooling performance. The heat transfer process during FESC is studied through numerical simulation using commercial code ANSYS FLUENT. The species transport model and the discrete phase model are applied to simulate the multiphase flow and heat transfer process. The turbulence effect is included. The effects of flow rate, nozzle pressure, nozzle angle, and the nozzle orifice size on spray cooling are investigated through analyzing the final surface temperature distributions. This work revealed the mechanism of the heat transfer process in FESC by means of particle tracks and velocity magnitude distribution. The simulated results for the effect of flow rate were compared with other researchers’ previous published experimental results. The comparison shows same trend, which verified the model and the simulation result. The optimum cooling performance is found by analyzing various working conditions. The results show that high flow rate, high nozzle pressure, small nozzle angle and small nozzle orifice can improve the FESC characteristics. The detailed mechanisms of these effects under various working conditions are also discussed. Under giving working conditions, the optimum cooling performance is obtained for the condition where mass flow rate of working fluid is 2L/min, the nozzle pressure is 100MPa, the nozzle angle is 15 degrees and the orifice size of the nozzle is 1mm.

Proton exchange membrane fuel cells are a new energy technology with great potential due to advantages such as high efficiency and no pollution. The structure of the gas flow channels has a profound impact on the overall performance of the fuel cell. Different flow channel geometries have their own advantages and disadvantages, and a good understanding of the influence of these structures on performance can provide a reference for the design and improvement of flow channel geometries in various application contexts. Numerical models can be used as a reasonable and reliable tool to evaluate the influence of operating and structural parameters on cell performance and service time by simulating the transport processes of substances and heat as well as electrochemical reactions inside the fuel cell and can be used for the optimisation of cell design. This paper reviews the recent models of proton exchange membrane fuel cells, summarises and analyses the effect of gas flow channels on fuel cells, and organises and concludes efficient design of flow channel structures to enhance PEMFC performance in terms of the cross-section shape, length, width, number of flow channels, and baffle position.

Under mild conditions of room temperature and pressure, and using either pure oxygen or air, aldehydes are converted using a heterogeneous Fe–N/C catalyst to produce the corresponding organic peroxy acid and acylperoxy radicals, which forms the epoxide from cyclohexene with high yield (91% for isobutyraldehyde in O2). Real-time monitoring of the rate of oxygen consumption and the electrochemical potential of the Fe–N/C catalyst has been used to study the formation of the peroxy acid and subsequent catalytic epoxidation of cyclohexene. Using isobutyraldehyde, it is shown that the aldehyde and the iron-based carbon catalyst (Fe–N/C) are involved in the rate determining step. Addition of a radical scavenger increases the induction time showing that radicals are initiated by the reaction between the aldehyde and the catalyst. Furthermore, UV-vis spectroscopy with 2,2′-azino-di-(3-ethylbenzthiazoline sulfonic acid) (ABTS) proved the in situ formation of peroxy acid. In the presence of cyclohexene, the peroxy acid leads to the corresponding epoxide with high yield. Monitoring the open circuit potential (OCP) and oxygen flow concurrently follows the production of the peroxy acid. The epoxidation reaction can take place only when the increase in open circuit potential is greater than 0.14 V, suggesting an in situ direct link between the relative oxidative strength of the peroxy acid and the likelihood of epoxidation.

Non-precious iron-based catalysts (Fe-NCs) require high active site density (SD) to meet the performance targets as cathode catalysts in proton exchange membrane fuel cells (PEMFCs). SD is generally limited to that achieved at 1-3 wt%(Fe) loading due to the undesired formation of iron-containing nanoparticles at higher loadings. Here we show that by pre-forming a carbon-nitrogen matrix using a sacrificial metal (Zn) in the initial synthesis step and then exchanging iron into this preformed matrix we achieve 7 wt% iron coordinated solely as single atom Fe-N4 sites as identified by 57Fe cryo Mössbauer spectroscopy and X-ray absorption spectroscopy. SD values measured by in situ nitrite stripping and ex situ CO chemisorption methods are 4.7x1019 and 7.8x1019 sitesg-1, with a turnover frequency of 5.4 electrons̭sites-1s-1 at 0.80 V in 0.5M H2SO4 electrolyte. The catalyst delivers excellent PEMFC performance with current densities of 41.3 mAcm-2 at 0.90 ViR-free using H2-O2 (10.6 Ag-1) and 145 mA cm-2 at 0.80 V (199 mAcm-2 at 0.80 ViR-free) using H2-air.

Nitrogen-doped porous carbons containing atomically dispersed iron are prime candidates for substituting platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. These carbon catalysts are classically synthesized via complicated routes involving multiple heat-treatment steps to form the desired Fe-Nx sites. We herein developed a highly active Fesingle bondNsingle bondC catalyst comprising of exclusive Fe-Nx sites by a simplified solid-state synthesis protocol involving only a single heat-treatment. Imidazole is pyrolyzed in the presence of an inorganic salt-melt resulting in highly porous carbon sheets decorated with abundant Fe-Nx centers, which yielded a high density of electrochemically accessible active sites (1.36 × 1019 sites g−1) as determined by the in situ nitrite stripping technique. The optimized catalyst delivered a remarkable ORR activity with a half-wave potential (E1/2) of 0.905 VRHE in alkaline electrolyte surpassing the benchmark Pt catalyst by 55 mV. In acidic electrolyte, an E1/2 of 0.760 VRHE is achieved at a low loading level (0.29 mg cm−2). In PEMFC tests, a current density of 2.3 mA cm−2 is achieved at 0.90 ViR-free under H2–O2 conditions, reflecting high kinetic activity of the optimized catalyst.