Fabrication of TiN inverse opal structure and Pt nanoparticles by atomic layer deposition for proton exchange membrane fuel cell

A titanium nitride (TiN) inverse opal structure was fabricated on carbon paper as a support of Pt for application in proton exchange membrane fuel cell (PEMFC). Polystyrene spheres with different diameters were coated on carbon paper by spin coating in multilayers as a template. Titanium dioxide (TiO₂) thin film was then deposited on the template by atomic layer deposition (ALD). The TiN inverse opal structure was fabricated by direct nitridation of TiO₂ in flowing ammonia atmosphere at above 800 °C. Platinum nanoparticles were then deposited uniformly on TiN by ALD. The performances of PEMFC using Pt@TiN@carbon paper composite as electrodes were examined. The homemade electrodes showed at least 13 times higher platinum specific power density than commercial E-Tek electrodes.     

Development and evaluation of nitride coated titanium bipolar plates for PEM fuel cells

In order to improve the conductivity of titanium bipolar plate under the premise of ensuring its corrosion resistance for the proton exchange membrane fuel cell (PEMFC), the nitride coatings are deposited on the surface of titanium bipolar plate via a powder immersion reaction assisted coating (PIRAC) method. Both the scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) show that the dense titanium nitride coatings with the thickness around 1.5–2.5 μm are successfully prepared. Furthermore, the X-ray photoelectron spectroscopy (XPS) results confirm the presence of TiN, TiN_xO_y and TiO₂ phases on the surface of nitride coatings, and the content of these phases is tunable by adjusting the prepared temperatures. Both the microstructure, the thickness and the composition of the nitride coatings could be associated with the corrosion resistance and the interfacial contact resistance of the nitrided samples. We find that the nitrided samples prepared at 1000 °C could be the ideal mixed coating materials with the proper combination of the corrosion resistance and the interfacial contact resistance, which also show the best long-term stability in simulated PEMFC cathode environment.     

Effects of hydrogen content on powder metallurgy characteristic of titanium hydrides

The titanium dihydride (TiH₂) powder metallurgy has been attracted a lot of attention, but TiH₂ powder is difficult to press moulding. In this paper, the titanium hydride powder metallurgy including TiH₂ and unsaturated titanium hydrides (TiH_1.5) was investigated simultaneously compared with pure titanium metal powder metallurgy. The results indicates that the titanium hydride powder metallurgy is accompanied by the deoxidation self-purification effect during dehydrogenation process for both of TiH₂ and TiH_1.5, which have higher sintering density than pure titanium. There are the three stages relative to densification rate, namely the slow, rapid and full densification stages for all of three materials. The compressive yield strengths increase rapidly in the rapid densification stage and are unchangeable almost in the full densification stage after holding 2 h at 1300 °C. The titanium hydride powder metallurgy is helpful to obtain much better mechanical properties than the pure titanium metal powder metallurgy. Here the compressive yield strength of the as-sintered TiH₂ compact with the maximum hydrogen content is the best but has very small difference compared with that of the as-sintered TiH_1.5 compact after full sintering densification.     

Ru-decorated Pt nanoparticles on N-doped multi-walled carbon nanotubes by atomic layer deposition for direct methanol fuel cells

We present atomic layer deposition (ALD) as a new method for the preparation of highly dispersed Ru-decorated Pt nanoparticles for use as catalyst in direct methanol fuel cells (DMFCs). The nanoparticles were deposited onto N-doped multi-walled carbon nanotubes (MWCNTs) at 250 °C using trimethyl(methylcyclopentadienyl)platinum MeCpPtMe₃, bis(ethylcyclopentadienyl)ruthenium Ru(EtCp)₂ and O₂ as the precursors. Catalysts with 5, 10 and 20 ALD Ru cycles grown onto the CNT-supported ALD Pt nanoparticles (150 cycles) were prepared and tested towards the electro-oxidation of CO and methanol, using cyclic voltammetry and chronoamperometry in a three-electrode electrochemical set-up. The catalyst decorated with 5 ALD Ru cycles was of highest activity in both reactions, followed by the ones with 10 and 20 ALD Ru cycles. It is demonstrated that ALD is a promising technique in the field of catalysis as highly dispersed nanoparticles of controlled size and composition can be deposited, with up-scaling prospects.     

Improved performance of dye-sensitized solar cells with TiO2/alumina core-shell formation using atomic layer deposition

Alumina (Al₂O₃) shell formation on TiO₂ core nanoparticles by atomic layer deposition (ALD) is studied to suppress the recombination of charge carriers generated in a dye-sensitized solar cell (DSSC). It is relatively easy to control the shell thickness using the ALD method by controlling the number of cycles. An optimum thickness can be identified, which allows tunneling of the forward current while suppressing recombination. High-resolution TEM measurements show that a uniform Al₂O₃ shell is formed around the TiO₂ core particles and elemental mapping of the porous TiO₂ layer reveals that the Al₂O₃ distribution is uniform throughout the layer. The amount of dye absorption is increased with increase in the shell thickness but electrochemical impedance spectroscopic (EIS) measurement shows a drastic increase in the resistance. With an optimum Al₂O₃ thickness of 2 nm deposited by ALD, a 35% improvement in the cell efficiency (from 6.2 to 8.4%) is achieved.     

Synthesis of the tetragonal titanium dihydride inultradispersed state

The ultradispersed tetragonal titanium dihydride (specific surface of10–55 m²/g, the average size of particles of 15–40 nm), stabilized by 0.1–0.15 atomof nitrogen per molecule of the titanium dihydride, was prepared by an interaction of the titaniumpowder with ammonia under a pressure of 0.7–0.8 MPa in a presence of the ammonium chloride.The influence of a size of the particles of the initial titanium powder (10–100 mcm) and atemperature (200–400°C) on a direction of an interaction and on a state (microcrystalline orultradispersed) of the titanium dihydride was investigated. It has been established that the derivedtitanium dihydride crystallizes into a tetragonal lattice with the constants a_o = 4.468d, c_o = 4.391d and is resistant in an inert atmosphere to 600°C.     

Plasma nitrided titanium as a bipolar plate for proton exchange membrane fuel cell

Plasma nitriding was applied to improve the surface performance of titanium bipolar plate. XRD and SEM results showed a titanium nitride layer was formed after nitridation. In comparison with pure titanium, the interfacial contact resistance of plasma nitrided titanium was reduced to some extent by the nitridation treatment. However, high corrosion current was observed under electrochemical tests in 0.5 M H₂SO₄ + 5 ppm HF. Both the electrical conductivity and corrosion resistance of the surface of plasma nitriding titanium did not reach the level of graphite. Some more improvements are expected in the plasma nitriding process or another surface modification on pure titanium.     

Investigation of Al2O3 diffusion barrier layer fabricated by atomic layer deposition for flexible Cu(In,Ga)Se2 solar cells

The use of Al₂O₃ fabricated by atomic layer deposition (ALD) as a metal diffusion barrier between the stainless steel substrate and the back contact layer in flexible Cu(In,Ga)Se₂ (CIGS) photovoltaic (PV) devices was found to reduce metal ion diffusion from the substrate and reduce the number of defects at the CIGS absorber layer, as determined from the secondary ion mass spectrometry (SIMS) depth profile and quantitative defect analysis using C–V measurements. Cells with Al₂O₃ barrier layers were found to show higher efficiency and uniformity compared to cells with ZnO barrier layers. XRD pattern analysis showed the Al₂O₃ barrier layer's amorphous characteristic which can form a complex diffusion path. In addition, quantum efficiency (QE) analysis of the cells showed that the main advantage of using an Al₂O₃ barrier layer is derived from the increase in the current density due to the decrease in the number of recombination sites resulting from the decrease in the number of defects due to the amorphous nature of the layer. Therefore, cells with an Al₂O₃ barrier layer fabricated by ALD showed better average conversion efficiency and uniformity (11.23 ± 1.86%) compared to cells with a ZnO barrier layer fabricated by sputtering. Ongoing advancements in ALD processes make the use of Al₂O₃ barrier layers promising for obtaining large-scale flexible solar cells.     

Application of dense nano-thin platinum films for low-temperature solid oxide fuel cells by atomic layer deposition

Nano-thin platinum (Pt) films with a dense microstructure for low-temperature solid oxide fuel cells (LT-SOFCs) were fabricated by atomic layer deposition (ALD) and were characterized in terms of their micro-structural properties and electrochemical performance. Pt thin films with a purity level of ∼99% were achieved by controlling the O₂ pulsing time. The agglomeration behavior of the ALD Pt thin films was characterized by the annealing temperature, becoming extremely severe above 550 °C. An LT-SOFC with a 25 nm thick dense ALD Pt cathode layer exhibited a peak power density of ∼110 mW/cm² at 450 °C.     

Deposition of heterojunction of ZnO on hydrogenated TiO2 nanotube arrays by atomic layer deposition for enhanced photoelectrochemical water splitting

The heterojunction of ZnO was deposited on hydrogenated TiO₂ nanotube arrays (H–TiO₂) by atomic layer deposition (ALD) with various cycles. The ZnO was uniformly wrapped with the H–TiO₂ samples and the thickness could be accurately controlled by the cycle numbers of ALD. The higher growth rate ~2.7 Å/cycle was obtained due to the surface amorphous layer, compared with the air-treated samples (A-TiO₂), ~2.3 Å/cycle. When the cycle numbers increased to 200, nanowire arrays appeared. Interestingly, the absorption in the visible light region improved more significantly when ALD ZnO was employed for the H–TiO₂ rather than the A-TiO₂ samples. The H–TiO₂ samples with 42 nm of ALD ZnO exhibited enhanced photoelectrochemical water splitting performances, compared with the A-TiO₂ with 42 nm of ALD ZnO. This was related to the higher degree of the electronic band bending and improved photo-response in the UV and visible light region, resulting from the oxygen vacancies.     

Steam reforming of n-dodecane over mesoporous alumina supported nickel catalysts: Effects of metal-support interaction on nickel catalysts

Developing a highly active and stable Ni-based catalyst is still a challenge for the generation of on-site hydrogen through steam reforming of long-chained hydrocarbons, such as kerosene fuels. Ni nanoparticles (ca. 5 nm) on mesoporous alumina prepared by atomic layer deposition (ALD) were employed in steam reforming of n-dodecane, and exhibited a turnover frequency (TOF) of 477.6 h⁻¹, whereas Ni nanoparticles on commercial alumina support prepared by impregnation method exhibited a TOF of 100 h⁻¹. The high activity of ALD Ni catalysts was ascribed to high reduction degree, as confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and H₂-chemisorption. A deactivation was also observed on the ALD prepared catalysts, which was ascribed to the weak metal-support interaction, as confirmed by H₂ temperature-programmed reduction (TPR). The ALD Ni/Al₂O₃ catalysts were further modified with CeO₂ and they showed enhanced stability with 8% deactivation degree in steam reforming of n-dodecane. Further characterizations of spent catalysts showed that the presence of CeO₂ was favorable for stabilizing Ni nanoparticles by enhancing moderate metal-support interaction, and reducing the formation of coke on the interfaces of NiCeO₂.     

Chromium as reactant for solar thermochemical synthesis of ammonia from steam, nitrogen, and biomass at atmospheric pressure

Ammonia for fertilization plays a crucial role in agriculture. It is an important commodity chemical, and it can serve as a fuel for combustion engines or as a carrier molecule for hydrogen. Global NH₃ production of over 100 million metric tons per year relies almost entirely on natural gas for energy and hydrogen. About 2% of the world’s energy budget is spent to produce NH₃. Experiments towards a solar thermochemical cycle for NH₃ synthesis at near atmospheric pressure using a transition metal reactant and a Fresnel-lens solar furnace are reported here: reacting Cr metal powder with gaseous N₂ to Cr nitride, hydrolyzing Cr nitride powder with steam to NH₃ and Cr₂O₃, and finally reducing Cr₂O₃ powder back to Cr with mixtures of H₂, CO, and N₂. At about 1000 °C it was found that Cr readily fixes N₂ from the gas phase as Cr nitride (4.13 × 10⁻² mol N₂/mol Cr/min, 85 ± 4 mol% of hexagonal Cr₂N after 5.6 min). Cr₂N converts over time to a cubic CrN phase. Corrosion of Cr nitride with steam at 1000 °C and about 1 bar forms Cr₂O₃ and CrO while liberating 53 ± 11 mol% of the nitrogen contained in the solid Cr nitride in 60 min. Of the N liberated, 0.28 ± 0.07 mol% forms the desired NH₃. This results in a yield of 0.15 ± 0.02 mol% NH₃ relative to the N in the nitride (1.07 × 10⁻⁴ mol NH₃/mol Cr/min). Addition of CaO/Ca(OH)₂ powder or quartz wool to provide more reactive sites and promote protonation of N increased the yield of NH₃ only slightly (0.24 ± 0.01 or 0.39 ± 0.03 mol% NH₃ relative to the N in the nitride respectively). The thermochemical cycle is closed by heating Cr₂O₃ to 1200–1600 °C with a reduction yield near the surface of the particles of approximately 82.85 mol% (40 min at 1600 °C) in a gas stream of H₂ and CO (2.7 × 10⁻³ mol Cr/mol Cr₂O₃/min). An unreacted core model was applied to estimate the activation energy of Cr₂O₃ reduction with 128 ± 4 kJ/mol. Cr appears promising to promote nitridation and oxide reduction as a basis for a future custom-designed reactant with high specific surface area enabling sustainable and more scalable NH₃ production from N₂ and H₂O at ambient pressure without natural gas consumption.     

Atomic layer deposited thin film metal oxides for fuel production in a solar cavity reactor

Alumina thin film structures were produced by coating high surface area polymer particles via atomic layer deposition (ALD), using the polymer as a sacrificial template. Burnout of the polymer material left high surface area, high pore volume structures, with 15 nm wall thickness. Further deposition of up to 27 mol% Co and Fe was performed via ALD to produce high surface area CoFe₂O₄ particles for thermochemical water splitting. The ALD particles were thermally cycled in electrically heated lab reactors and on-sun using a concentrated solar, reflective cavity reactor. Surface area measurements of cycled ALD particles showed improved surface area retention as compared to bulk Fe₂O₃ nanopowders. Reaction rates as high as 15.2 and 9.8 μmol/s/g were observed, on-sun, for H₂O and CO₂ splitting respectively. Thermochemical cycling in a concentrated solar cavity reactor showed an order of magnitude increase in solar utilization efficiency between ALD particles and bulk Fe₂O₃ nanopowders.     

Catalytic effect of titanium nitride nanopowder on hydrogen desorption properties of NaAlH4 and its stability in NaAlH4

Single crystalline titanium nitride (TiN) nanopowder is synthesized by a mechano-chemical reaction between titanium chloride (TiCl₃) and lithium nitride (Li₃N) by means of high-energy ball milling. The TiN nanopowder has an average particle size of 6 nm and is introduced into sodium alanate (NaAlH₄) as a catalyst. During hydrogen sorption cycles, TiN-catalyzed NaAlH₄ exhibits a greater hydrogen desorption rate and higher hydrogen capacity than TiCl₃-catalyzed NaAlH₄. Contradicting thermodynamic predictions, in situ X-ray diffraction results reveal that TiN nanopowder remains stable and produces no by-products (e.g., Ti-Al compounds) in the reaction with NaAlH₄ during hydrogen desorption. In situ Raman spectroscopy also confirms the stability of TiN nanopowder in NaAlH₄. This implies that the sustained hydrogen sorption kinetics and hydrogen capacity of TiN-catalyzed NaAlH₄ originate from the structural and chemical stability of TiN nanopowder in NaAlH₄ for the given conditions of the hydrogen cycle test.     

Microwave-assisted synthesis of edge-grafted carbon nitride by triazole rings for efficient photocatalytic hydrogen evolution

Copolymerization of conjugated nitrogen heterocycles within carbon nitride nowadays is proven to be a successful strategy to promote charge separation. Here, a novel edge-grafted carbon nitride nanosheets by triazole rings were synthesized from the copolymerization of dicyandiamide and 3-amino-1, 2, 4-triazole via microwave-assisted heating with improved yield. Based on the building of a feasible donor-acceptor (D-A) system by such edge grafting, the rapid charge transfer pathways were formed. Benefiting from the thin-layer structure of carbon nitride via microwave-assisted heating and abundant π electrons by intramolecular doping, the system showed enhanced visible light absorption from n-π1 electrons transition, facile charge separation and transfer, and then an increased H₂ evolution rate (637.58 μmol h⁻¹ g⁻¹), which is 17.43 times than the triazole modified sample from conventional thermal condensation method. This work enlightens an innovative pathway for the regulation of carbon nitride photocatalysts by conjugated nitrogen heterocycles for efficient solar energy conversion applications.     

A comparative study on the catalytic activities and stabilities of atomic-layered platinum on dispersed Ti0.9Cu0.1N nanoparticles supported by N-doped carbon nanotubes (N-CNTs) and reduced graphene oxide (N-rGO)

In our previous research, titanium-based nitride with high conductivity and superior corrosion resistance were developed as an ideal core material for replacing noble metal to form Pt-based core-shell catalysts by pulse electrodeposition. Meanwhile, the smaller sizes of nitride cores would also be available for pulse electrodeposition by dispersing them on carbon nanotubes (CNT). To achieve a better practice on the preparation of the Pt-based core-shell catalysts, in this work, both nitrogen-doped carbon nanotubes (N-CNT) and reduced graphene oxide (N-rGO) were used to support the copper-doped titanium nitride (Ti_0.9Cu_0.1N) cores. In the course of pulse electrodeposition, their influences as supports on the electronic states of electrodeposited Pt as well as their catalytic activities were compared. The results showed that the Pt preferred to electrodeposit on Ti_0.9Cu_0.1N cores supported by N-CNT and formed a core-shell structure. While with the same electrodeposition process, the Pt was found to be electrodeposited not only on the Ti_0.9Cu_0.1N cores supported by N-rGO with heavy aggregations but also on the N-rGO support. Raman spectroscopy analysis indicated that the higher degree of structural defects on N-rGO, as support, might have contributed to such divergence observation.     

Cobalt oxide on mesoporous carbon nitride for improved photocatalytic hydrogen production under visible light irradiation

Cobalt oxide (Co₃O₄) nanoparticles decorated on mesoporous carbon nitride (Co₃O₄/MCN) nanocomposites for photocatalytic hydrogen evolution were investigated in this work. MCN was prepared using 3-amino-1,2,4-triazole, high nitrogen content, as a single molecular carbon and nitrogen precursor and SiO₂ nanoparticles as the hard template. Complementary characterization techniques were employed to understand the textural and chemical properties of the nanocomposites. The bare MCN showed high photocatalytic activity under visible light irradiation without using any co-catalyst. The photocatalytic activity of Co₃O₄/MCN with a Co₃O₄ mass content of 5 wt % presented two times higher than the bare MCN, which is attributed to the enhanced visible-light harvesting and more efficient charge separation. Mechanistic study shows lower electron-hole recombination rate, higher charge separation efficiency occurs after the formation of p-n type heterojunction.     

Atomic layer deposition assisted Pt-SnO2 hybrid catalysts on nitrogen-doped CNTs with enhanced electrocatalytic activities for low temperature fuel cells

Nitrogen doped carbon nanotubes (CN_x) of a high nitrogen concentration were synthesized directly on carbon paper as the skeleton of a 3D composite electrode. Ultra-fine SnO₂ nanoparticles about 1.5 nm were deposited on CN_x with atomic layer deposition (ALD) technique. Pt nanoparticles from 1.5 to 4 nm were deposited on CN_x/carbon paper and SnO₂/CN_x/carbon paper with ethylene glycol reduction method. Three dimensional Pt/CN_x/carbon paper and Pt-SnO₂/CN_x/carbon paper composite electrodes were obtained, respectively. They were characterized over oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) for low temperature fuel cells. With similar sizes of Pt nanoparticles, the electrochemical surface area (ECSA) of Pt-SnO₂/CN_x/carbon paper is larger than that of Pt/CN_x/carbon paper. Pre-deposited SnO₂ nanoparticles promote the electrocatalytic activity of Pt toward ORR, carbon monoxide (CO) stripping and MOR. The underlying mechanisms for the enhanced activities are discussed.     

Facile one-step synthesis of porous graphene-like g-C3N4 rich in nitrogen vacancies for enhanced H2 production from photocatalytic aqueous-phase reforming of methanol

Exfoliation of bulk graphitic carbon nitride (g-C₃N₄) to single- or few-layered structures is an effective way to improve the photocatalytic performance. However, the synthesis methods for few-layer g-C₃N₄ are relatively complicated and time-consuming, with the bandgap of g-C₃N₄ increasing through quantum size effects, thus hampering effective utilization of visible light. To effectively exfoliate the bulk g-C₃N₄ to single or few-layered structures in a facile way without losing its visible light absorption ability is still a challenge. Herein, porous graphene-like g-C₃N₄ nanosheets with abundant nitrogen vacancies were prepared by facile thermal polymerization of melamine using graphene oxide (GO) as a sacrificial template. The two-dimensional (2D) layer morphology and nitrogen defect structure were proved using AFM, SEM, TEM, EA, XPS and EPR techniques. Compared with the bulk g-C₃N₄, the as-prepared g-C₃N₄ nanosheet possesses a high specific surface area, enhanced absorption ability of visible light, and elevated charge carrier generation and separation efficiency because of the unique structural features. The in situ DRIFT spectrum indicates that the surface nitrogen vacancies also serve as excellent locations for methanol adsorption and activation. Consequently, an excellent photocatalytic activity of hydrogen production from methanol aqueous-phase reforming is obtained, which is about 14 times more productive than the bulk g-C₃N₄.     

Silicon lifetime enhancement by SiNx:H anti-reflective coating deposed by PECVD using SiH4 and N2 reactive gas

Hydrogenated films of silicon nitride SiN_x:H are largely used as antireflective coating as well as passivation layer for industrial crystalline and multicrystalline silicon solar cells. In this work, we present a low cost plasma enhanced chemical vapor deposition (PECVD) of this thin layer by using SiH₄ and N₂ as a reactive gases. A study was carried out on the variation effect of the ratio silane (SiH₄) to nitrogen (N₂) and time deposition on chemical composition, morphologies, reflectivity and carrier lifetime. The thickness was varied, in order to obtain a homogeneous antireflective layer. The Fourier transmission infrared spectroscopy (FTIR) shows the existence of Si–N and Si–H bonds. The morphologies of the sample were studied by Atomic Force Microscopy (AFM). The resulting surface of the SiN_x:H shows low-reflectivity less than 5% in wavelength range 400–1200 nm. As a result, an improvement in minority carrier lifetime has been achieved to about 15 μs.