Catalytic Layer Research Articles

Effects of operating conditions on the performance uniformity of the proton-exchange membrane fuel cell stack

The performance uniformity of the proton-exchange membrane fuel cell (PEMFC) stack depends sensitively on the operating parameters. In this work, the dispersion of performance across the cells is explored through experiments and simulations, and the effects of operating conditions on the performance uniformity are evaluated using a novel criterion. According to the results, operating conditions such as stoichiometric ratio (SR), back pressure (PB), dew point temperature (TD), and heating temperature (TH) have a great influence on the performance uniformity and determine the content of liquid-saturated water and the uniformity of oxygen distribution at the catalytic layer (CL)/gas diffusion layer (GDL) interface of fuel cell. Further research shows that decreasing the content of liquid-saturated water and increasing the uniformity of oxygen distribution can improve the output performance stability, while increasing the oxygen content can improve the output performance of the PEMFC stack. What's more, when the stoichiometric ratio (SR), back pressure (PB), dew point temperature (TD), and heating temperature (TH) are 2.0, 3.0, 0.7 bar, 0.6 bar, 343 K, 333 K, and 348 K, respectively, the performance uniformity of the PEMFC stack achieves the best.

Modified equations for the performance of oxygen reduction reaction catalyst from rotating disk electrode to membrane electrode assembly

Kinds of electrocatalysts developed for the cathode of Polymer electrolyte membrane fuel cell (PEMFC) show more outstanding performance when loading on rotating disk electrode (RDE) than assembling in the membrane electrode assembly (MEA), and this phenomenon has not be explained quantitatively yet. Here we developed the Butler-Volmer equation further on the base of the detailed disassembly of the elemental reaction processes that occurring on the surface of the cathodic catalyst and in the MEA. These analytical equations quantifiably indicate how the catalyst's structural parameters and electrode's microstructural parameters to influence the performance of catalyst. Further analysis shows there may be more than one order of magnitude for the PEMFC's current density improvement when using the traditional catalyst like Pt/C, after optimizing the structure of catalytic layer. Meanwhile, the increasing hydrogen permeation not only lower the open-circuit voltage but also cause the whole polarization curve moving down, which means an attenuation of PEMFC's lifetime. These equations shown in this work are established on the strict deduction of mathematics and physics and will give the new guidance for evaluating and improving the performance of electrocatalysts when assembling in MEA.

Study of Rh/Ce<sub>0.75</sub>Zr<sub>0.25</sub>O<sub>2 – δ</sub>/θ-Al<sub>2</sub>O<sub>3</sub>/FeCrAl Catalyst Regeneration after Diesel Fuel Autothermal Reforming

A study of soot (coke) formation on the surface of a structured Rh/Ce0.75Zr0.25O2/Al2O3/FeCrAl catalyst during autothermal reforming of diesel fuel into synthesis gas was performed. The SEM studies revealed the formation of fibrous carbon particles of 5–50 µm in size on the catalyst surface. It was found that the process of coke formation occurs on the catalytic coating surface, causes no exfoliation and/or damage of the catalytic layer, and the carbon deposits are readily oxidized during catalyst regeneration by oxygen or water vapor. Intensive oxidation of soot with oxygen begins at a temperature of 450°C; a major part of carbon deposits is oxidized even before the reactor furnace reaches the operating temperature of diesel fuel autothermal reforming (750°C). Water vapor oxidizes carbon deposits as well, but less efficiently than oxygen. The catalyst regeneration with water vapor proceeds actively at a temperature of 750°C that proves the possibility of catalyst self-regeneration in the process of diesel fuel autothermal reforming, which is performed with water excess.

Synthesis of Catalytic Microswimmers Based on Anisotropic Platinum Sorption on Melamine Barbiturate Supramolecular Structures

Herein, a straightforward synthesis approach for the formation of anisotropic microswimmers is proposed as an alternative to a top–down fabrication for asymmetric artificial swimmers. Melamine barbiturate (MBA)–self‐assembled supramolecular crystals are utilized as an asymmetric matrix for selective chemical platinum deposition. Surface functional groups of prevalent crystallographic planes for platinum sorption are presented by acidic sites of the assembly. The formed platinum catalytic layer is separated from the bulk MBA and investigated using X‐Ray photoelectron spectroscopy and scanning electron microscope energy‐dispersive X‐Ray spectroscopy analysis. Both methods confirm platinum adsorption and reduction of platinum ions during the deposition process. The MBA asymmetric swimmers show directional motion in a fuel environment, proved by the mean‐squared displacement study. Previously reported pH‐dependency and encapsulation capacity of MBA opens wide opportunities for functional materials formation.

In situ Hydrothermal Oxidation of Ternary FeCoNi Alloy Electrode for Overall Water Splitting

Exploring noble metal‐free catalyst materials for high efficient electrochemical water splitting to produce hydrogen is strongly desired for renewable energy development. In this article, a novel bifunctional catalytic electrode of insitu‐grown type for alkaline water splitting based on FeCoNi alloy substrate has been successfully prepared via a facile one‐step hydrothermal oxidation route in an alkaline hydrogen peroxide medium. It shows that the matrix alloy with the atom ratio 4:3:3 of Fe:Co:Ni can obtain the best catalytic performance when hydrothermally treated at 180°C for 18 h in the solution containing 1.8 M hydrogen peroxide and 3.6 M sodium hydroxide. The as‐prepared Fe0.4Co0.3Ni0.3‐1.8 electrode exhibits small overpotentials of only 184 and 175 mV at electrolysis current density of 10 mA cm−2 for alkaline OER and HER processes, respectively. The overall water splitting at electrolysis current density of 10 mA cm−2 can be stably delivered at a low cell voltage of 1.62 V. These characteristics including the large specific surface area, the high surface nickel content, the abundant catalyst species, the balanced distribution between bivalent and trivalent metal ions, and the strong binding of in‐situ naturally growed catalytic layer to matrix are responsible for the prominent catalytic performance of the Fe0.4Co0.3Ni0.3‐1.8 electrode, which can act as a possible replacement for expensive noble metal‐based materials.

Investigation the Ni0·9Cu0·1TiO3-δ reforming layer for direct ethanol solid oxide fuel cells

When the solid oxide fuel cell uses ethanol as fuel, it is highly prone to cause carbon deposition and cell failure in the traditional Ni-based ceramic anode. Herein, Ni0·9Cu0·1TiO3-δ catalytic reforming layer is coated on the surface of the traditional Ni-based anode to address this issue. It has been shown that the reforming layer of Ni0·9Cu0·1TiO3-δ decomposes into Ni–Cu alloy and TiO2 under reducing atmosphere, which significantly improves the coking resistance performance of the anode. Cells with Ni0·9Cu0·1TiO3-δ layer have excellent electrocatalytic performance, and the maximum power density can reach 867 mW cm−2 at 800 °C using ethanol as fuel. Moreover, it doesn’t show significant degradation in the 50h stability test, proving that the Ni0·9Cu0·1TiO3-δ reforming layer has good coking resistance performance and electrocatalytic activity when used in direct ethanol solid oxide fuel cells.

Enhancement of CO2RR product formation on Cu-ZnO-based electrodes by varying ink formulation and post-treatment methods

Numerous catalysts have been reported with enhanced performance, e.g. longer lifetime and improved selectivity, for the electrochemical CO2 reduction reaction (CO2RR). Respectively little is, however, known about the influence of the electrode structuring and pre-treatment on this reaction for catalytic layers. Thus, we herein report on the modification of the catalyst environment of a Cu-ZnO-carbon black catalyst by variation of the ink composition and subsequent electrode treatment before performing CO2RR. We furthermore provide insight into the impact of different solvents, ionomer, and additives like pore forming agents used for the ink preparation as well as post-treatment steps in terms of pressing and sintering of the generated electrodes on the CO2RR performance. Although using the same catalyst for all electrodes, remarkable differences in hydrophobicity, surface morphology, and electrochemical performance with respect to stability and product distribution were observed. Our study reveals the critical role of the catalytic layer assembly aside from using proper catalysts. We furthermore show that the parasitic hydrogen formation and flooding behavior can be lowered and C2+ product formation can be enhanced when operating in optimized gas diffusion electrodes.

Influence of the catalyst layer thickness on the determination of the OER activity of Fe3O4@CoFe2O4 core-shell nanoparticles

Transition metal oxides-based catalytic layers often present a complex 3D porous architecture affecting the evaluation of their intrinsic electrocatalytic activity. In this work the oxygen evolution reaction activity of core-shell Fe3O4@CoFe2O4 nanoparticles combining a conductive magnetite core and a catalytically active cobalt ferrite shell was studied at different loading and thickness of the catalytic layer. It was observed that their apparent activity is decreasing and that the Tafel slopes are becoming convex when the loading increases. The activity decay could be attributed to the significant resistance to charge transport in the thick porous catalyst layer. This resistance could be estimated by fitting the electrochemical impedance spectra using the transmission line model. The influence of the layer thickness on the experimental current-potential curves and on their Tafel slopes could be simulated using a simple model based on the Telegrapher's equations. It is concluded that in order to measure accurately the activity and Tafel slopes of an electrocatalyst, thin layers must be used, notably for catalyst layers that are not highly conductive.

Research on Cold Start of Proton-Exchange Membrane Fuel Cells Based on Model Predictive Control

The cold start of fuel cells limits their wide application. Since the water produced by fuel cells takes up more space when it freezes, it may affect the internal structure of the stack, causing collapse and densification of the pores inside the catalytic layer. This paper mainly analyzes the influence of different startup strategies on the stack cold start, focusing on the change in the stack temperature and the ice volume fraction of the catalytic layer. When designing a startup strategy, it is important to focus not only on the optimization of the startup time, but also on the principle of minimizing the damage to the stack. A lumped parameter cold-start model was constructed, which was experimentally verified to have a maximum error of 8.9%. On this basis, a model predictive control (MPC) algorithm was used to control the starting current. The MPC cold-start strategy reached the freezing point at 17 s when the startup temperature was -10 °C, which is faster than other startup strategies. Additionally, the time to ice production was controlled to about 20 s. Compared with the potentiostatic strategy and maximum power strategy, MPC is optimal and still has great potential for further optimization.

Investigation on a Shutdown Control Strategy with Residual Oxygen Rapid Elimination for Proton Exchange Membrane Fuel Cell System

During the shutdown process of the fuel cell system for vehicles, the air entering the anode chamber can form the hydrogen/air interface, accelerating the carbon corrosion of the catalytic layer. According to optimized control strategies, the carbon corrosion of fuel cells can be reduced. Nowadays, the main control strategies include gas purging and the consumption of residual oxygen in the stack by the auxiliary load. However, the oxygen in the fuel cell stack cannot be fully consumed or can cause the single-cell voltage to rise to 0.8 V with an inappropriate discharge current drop rate and auxiliary load resistance value, thus affecting the protective effect of the shutdown strategy. In this work, a shutdown strategy of the fuel cell system is studied. After the experiment, the optimized value of the discharge current drop rate and the auxiliary load resistance were obtained. With the resistance value of 50 Ω and the current drop rate of 7 A/s, the shutdown time of the fuel cell system is 13.5 s and the time of single-cell voltage above 0.82 V in the fuel cell stack is 0.1 s. Thus, the optimized shutdown strategy can reduce the shutdown time.

A Novel Miniature and Selective CMOS Gas Sensor for Gas Mixture Analysis-Part 3: Extending the Chemical Modeling.

This is the third part of the paper presenting a miniature, combustion-type gas sensor (dubbed GMOS) based on a novel thermal sensor (dubbed TMOS). The TMOS is a micromachined CMOS-SOI transistor, which acts as the sensing element and is integrated with a catalytic reaction plate, where ignition of the gas takes place. The first part was focused on the chemical and technological aspects of the sensor. In Part 2, the emphasis was on the physical aspects of the reaction micro-hot plate on which the catalytic layer is deposited. The present study focuses on applying several advanced simulation tools, which extend our understanding of the GMOS performance, as well as pellistor sensors in general. The three main challenges in simulating the performance are: (i) how to define the operating temperature based on the input parameters; (ii) how to measure the dynamics of the temperature increase during cyclic operation at a given duty cycle; (iii) how to model the correlation between the operating temperature and the sensing response. The simulated and analytical models and measured results are shown to be in good agreement.

Flower-Like NiS2 /WS2 Heterojunction as Polysulfide/sulfide Bidirectional Catalytic Layer for High-Performance Lithium-Sulfur Batteries.

The slow sulfur oxidation-reduction kinetics are one of the key factors hindering the widespread use of lithium-sulfur batteries (LSBs). Herein, flower-shaped NiS2 -WS2 heterojunction as the functional intercalation of LSBs is successfully prepared, and effectively improved the reaction kinetics of sulfur. Flower-like nanospheres composed of ultra-thin nanosheets (≤10nm) enhance quickly transfer of mass and charge. Meanwhile, the heterostructures simultaneously serve as an electron receptor and a donor, thereby simultaneously accelerating the bidirectional catalytic activity of reduction and oxidation reactions in the LSBs. In addition, the adsorption experiment, chemical state analysis of elements before and after the reaction and theoretical calculation have effectively verified that NiS2 -WS2 heterojunction nanospheres optimize the adsorption capacity and bidirectional catalytic effect of polysulfides. The results show that the initial discharge capacity of NiS2 -WS2 functional intercalation is as high as 1518.7mAhg-1 at 0.2 C. Even at a high current density of 5 C, it still shows a discharge specific capacity of 615.7mAhg-1 , showing excellent rate performance. More importantly, the capacity is 258.9mAhg-1 after 1500 cycles at 5 C, and the attenuation per cycle is only 0.039%, and the Coulomb efficiency remains above 95%.

Unidirectional domain growth of hexagonal boron nitride thin films

Two-dimensional van der Waals (2D-vdW) layered hexagonal boron nitride (h-BN) has gained tremendous research interest over recent years due to its unconventional domain growth morphology, fascinating properties and application potentials as an excellent dielectric layer for 2D-based nano-electronics. However, the unidirectional domain growth of h-BN thin films directly on insulating substrates remains significantly challenging because of high-bonding anisotropicity and complex growth kinetics than the conventional thin films growth, thus resulting in the formation of randomly oriented domains morphology, and hindering its usefulness in integrated nano-devices. Here, ultra-wide bandgap h-BN thin films are grown directly on low-miscut atomically smooth highly insulating c-plane sapphire substrates (without using any metal catalytic layer) by pulsed laser deposition, showing remarkable unidirectional triangular-shape domains morphology. This unidirectional domain growth is attributed to the step-edge guided nucleation caused by reducing the film-substrate interfacial symmetry and energy, thereby breaking the degeneracy of nucleation sites of random domains, as revealed by the density functional theory (DFT) calculations. Through extensive characterizations, we further demonstrate the excellent single crystal-like functional properties of films. Our findings might pave the way for feasible large-area direct growth of electronic-quality h-BN thin films on insulating substrates for high-performance 2D-electronics, and in addition would be beneficial for hetero engineering of 2D-vdW materials with emergent phenomena.

Heterogenous Iron Oxide Assemblages for Use in Catalytic Ozonation: Reactivity, Kinetics, and Reaction Mechanism.

Heterogeneous catalytic ozonation (HCO) has gained increasing attention as an effective process to remove refractory organic pollutants from industrial effluents. However, widespread application of HCO is still limited due to the typically low efficacy of catalysts used and matrix passivation effects. To this end, we prepared an Al2O3-supported Fe catalyst with high reactivity via a facile urea-based heterogeneous precipitation method. Due to the nonsintering nature of the preparation method, a heterogeneous catalytic layer comprised of γ-FeOOH and α-Fe2O3 is formed on the Al2O3 support (termed NS-Fe-Al2O3). On treatment of a real industrial effluent by HCO, the presence of NS-Fe-Al2O3 increased the removal of organics by ∼100% compared to that achieved with a control catalyst (i.e., α-Fe2O3/Al2O3 or γ-FeOOH/Al2O3) that was prepared by a conventional impregnation and calcination method. Furthermore, our results confirmed that the novel NS-Fe-Al2O3 catalyst demonstrated resistance to the inhibitory effect of high concentration of chloride and sulfate ions usually present in industrial effluent. A mathematical kinetic model was developed that adequately describes the mechanism of HCO process in the presence of NS-Fe-Al2O3. Overall, the results presented here provide valuable guidance for the synthesis of effective and robust catalysts that will facilitate the wider industrial application of HCO.

Study on Purging Strategy of Polymer Electrolyte Membrane Fuel Cell under Different Operation Conditions

The commercial proton exchange membrane fuel cell (PEMFC) system needs to be equipped with the capacity to survive a harsh environment, including sub-freezing temperatures. The cold start of PEMFC brings about great technical challenges, mainly due to the ice blockage in the components, which seriously hinders the multi physical transmission process. A multiscale, two-dimensional model was established to explore the gas purging in PEMFC under different electrochemical reaction intensities. The results indicate that the optimal case is obtained by B3-1 with a power density of 0.796 W cm−2, and the power density increases first and then decreases, followed by stoichiometric flow ratio (ξ) changes. It is worth noting that the water mole fraction in the PEM is closely related to the water concentration gradient. However, the differences in the initial water distribution in porous media have little bearing on the condensed water in the gas channel, and the liquid water in the gas diffusion layer (GDL) is preferably carried away ahead of other porous parts. The results also show that the increase in the purge speed and temperature can remove the excess water on GDL and the catalytic layer in a short time. For a nitrogen-based purge, the operating condition in case B3-1 is shown as the best strategy based on the output performance and economic analysis during the shutdown and purge process.

First principles study of V/Pd interface interactions and their hydrogen absorption properties

Hydrogen permeation through vanadium/palladium (V/Pd) metal composite membranes is an effective and practical method of separating hydrogen from gas mixtures. In order to gain an insight into the relation between the interfacial structure and hydrogen adsorption/diffusion properties of the catalytic Pd layer bonded to the metal membrane, and then improve the ability of the alloy membrane to purify hydrogen, the first principle based on the density functional theory is used to study the hydrogen adsorption/diffusion behavior at the V/Pd metal composite membrane interface. The results show that because the charge density at the V/Pd interface increases with the V/Pd bonding increasing, the dissolution energy of hydrogen atom (H) increases with it approaching to the interface, and it has the highest dissolution energy near the V/Pd interface (0.567 eV). Hydrogen migration energy barrier calculations show that compared with the maximum energy barrier for horizontal diffusion of H along the V/Pd interface (0.64 eV), the H vertical V/Pd interface energy barrier (0.56 eV) is small, thus H tends to migrate vertically V/Pd interface and diffuse from the Pd layer to the V substrate side. As the hydrogen solvation energy of the Pd layer at the V/Pd interface (0.238 eV) is higher than that on the V membrane side (–0.165 eV), H will gather on the V film side of the interface, which is easy to cause hydrogen to be embrittled. Calculations of Pd/Fe doping of the V matrix show that comparing with the undoped energy barrier (0.56 eV), Pd/Fe doping can significantly reduce the maximum energy barrier (0.45 eV/0.54 eV) in the diffusion path of the interface, which is favorable for hydrogen permeation and diffusion. And the doped interface can inhibit the interdiffusion of V layer and catalytic Pd layer to a certain extent, which improves the structural stability of the composite film.

RESEARCH INTO THE EFFECT OF ETCHING PROCESS OF POROUS TITANIUM CURRENT COLLECTORS IN HYDROCHLORIC ACID ON THE PERFORMANCE OF WATER ELECTROLYZERS WITH A PROTON EXCHANGE MEMBRANE

Based on the modern methods of SEM, voltammetry, and gravimetric analysis, samples of current collectors used in electrolyzers with a solid polymer electrolyte, made of porous titanium with various degrees of etching in concentrated hydrochloric acid, were studied. A gravimetric study of the etching process of samples of various sizes from porous titanium showed that throughout the process there is a loss of mass of the sample, with the exception of two short intervals, presumably due to the formation of hydrides (60-80 sec) and oxides (4-5 min), and the samples of smaller volumes were more sensitive to weight loss and weight growth. The use of the SEM method made it possible to reveal that etching over different time intervals changes the structure of porous titanium, reducing the average particle size and increasing their porosity .In this case, the surface inhomogeneity increases due to the formation of cavities in titanium and deterioration of the contact of the current collector surface with the catalytic layer. The consequence of this is the difference in the current-voltage characteristics of electrolysis cells with different duration of etching of the current collectors: there is a decrease in the productivity of electrolysis with an increase in the duration of etching of the current collectors by ~8% for etching more than 1 minute and ~18% for etching more than 4 minutes as compared to the simultaneous immersion of the sample in acid to clean the surface.

High performance flexible thin-film thermoelectric generator heated by methanol catalytic combustion

A flexible thin-film micro combustion-thermoelectric coupled (tf-MTC) device is proposed as a power supply for wireless sensors with small scale. Which is composed of thin-film thermoelectric generator (TEG) and catalytic layer. A simple but effective method is proposed for the integration of catalytic layer and thin-film TEG. With consideration of power generation system miniaturization, methanol is chosen as the fuel of catalytic combustion for its special characteristic of room-temperature self ignition and steady performance during catalytic combustion based on platinum catalyst. The centralized device with high output performance shows potential as a portable power. To make full use of the catalytic combustion energy, the integration method of catalytic layer and the electrode is proposed to form decentralized device, which greatly improves the flexible tf-MTC device output performance. The open voltage increases by nearly 103%, the value reaches 0.65 V at temperature difference 171.2 °C during steady catalytic combustion at flow rate of 1000 mL/min, and the corresponding maximum output power is 13.18 μ W with a load resistance of 8 k Ω . And the response of device to methanol, including temperature and open voltage are recorded to identify the stability of self-ignition characteristic with the fuel of methanol. • Energy system with combination of thin-film TEG and catalytic combustion. • Integration of catalytic layer and electrode of thin-film TEG. • Rapid response of the energy system to methanol-air.

Особенности применения перспективных катализаторов из аморфного сульфида молибдена при создании фотокатодов p-Si и pn-Si для получения водорода в кислотном растворе

The chemical state of MoSx catalytic layers and interfaces with p-type silicon (p-Si) and p-n-junction silicon (pn-Si) at various stages of the preparation and operation of photocathodes has been studied. The SiOx oxide layer formed at the interface noticeably suppressed the photocurrent for p-Si cathodes, but its removal in HF solution provided high photocurrents exceeding the values characteristic of pn-Si cathodes at zero potential. Studies of the energy bands showed that at all stages (after the formation of the photocathode, pre- and post-treatment in acid solutions), the configuration of the energy bands for the p-Si and np-Si photocathodes provided charge transfer for the evolution of hydrogen according to the Z-scheme. The difference in the characteristics of the p-Si and pn-Si photocathodes was due to the possible dependence of the photovoltage in the p-Si surface layer on the state of the interface with the catalyst film, which had no effect on the photovoltage in pn-Si.

Research on SOFC Properties of Low Concentration Gas Fuel and SFM-GDC Reforming

When gas fuel is used in solid oxide fuel cells, methane cracking occurs at high temperature in the Ni-based anode, forming granular carbon deposition in the active site of the anode, and finally completely wrapped Ni metal, greatly losing the electrochemical performance. Therefore, it is very necessary to add a reforming catalytic layer in the anode of the battery. The methane is reformed into CO and H2 fuel before entering the anode, and the place where methane cracking occurs is mostly transferred to the reforming layer, which reduces the catalytic reforming burden of the Ni-based anode, so as to effectively improve the battery performance. In this paper, Sr2Fe1.5Mo0.5O6 with double perovskite structure was synthesized and prepared, which maintained very good stability in high temperature REDOX atmosphere and had high electrical conductivity. The combination of GDC and SFM can increase the reaction site of the three-phase interface of the reforming layer, and also help to improve the thermal expansion matching between the reforming layer and anode. In this paper, the electrochemical performance of straight cell coated with SSFM-GDC reforming layer has been studied. Low concentration gas in coal mine contains methane, oxygen, nitrogen, and water hydrogen sulfide, etc., and its reaction process in solid oxide fuel cells is complicated. Therefore, studying the reaction mechanism of low concentration gas in solid oxide fuel cells has important practical significance for promoting its application.