Surface activation inspires high performance of ultra-thin Pd membrane for hydrogen separation

Development of a P84/ZCC Composite Carbon Membrane for Gas Separation of H2/CO2 and H2/CH4

Hydrogen (H2) has become one of the promising alternative clean energy resources. Membrane technology is a potential method for hydrogen separation or production. This study aims to develop a new carbon membrane for hydrogen separation or production. Moreover, the permeation behavior of H2, CO2, and CH4 through a hollow fiber composite carbon membrane derived from P84 co-polyimide and with incorporation of zeolite composite carbon (ZCC) was also examined. ZCC was synthesized via the impregnation method of sucrose into zeolite-Y pores, followed by carbonization at 800 °C. Thus, this filler has a high surface area, high microporosity, ordered pore structure, and low hydrophilicity. The presence of zeolites in ZCC is predicted to increase certain gases' affinity for the membrane. Various heating rates (1-5 °C/min) were applied during pyrolysis to understand the effect of the heating rate on the pore structure and H2/CO2 and H2/CH4 gas separation performance. Moreover, gas permeation was evaluated at various temperatures (298-373 K) to study the thermodynamic aspect of the process. A characteristic graphite peak was detected at 2θ ∼ 44° in all carbon samples. Scanning electron microscopy (SEM) observations revealed the void-free surface and the asymmetric structure of the carbon membranes. During the permeation test, it was found that gas permeation through the membrane was significantly affected by the temperature of the separation process. The highest permeability of H2, CO2, and CH4 was detected on the composite carbon membrane at a 3 °C/min heating rate with a permeation temperature of 373 K. The thermodynamic study shows that CO2 and H2 have lower activation energies compared to CH4. The transport mechanism of the membrane involved adsorption and activated surface diffusion. The permeation temperature has a large impact on the transport of small penetrants in the carbon matrix.

Microstructural Investigation and On-Site Repair of Thin Pd-Ag Alloy Membranes

Pd membranes act in an important role in H2 purification and H2 production in membrane reactors. Pd-Ag alloy membranes fabricated by consecutive electroless- and electroplating process on alumina tubes exhibited good stability under stringent heating/cooling cycles at a ramp rate of 10 K/min, imitating practical fast initiation or emergency shutdown conditions. Bilayer Pd-Ag membranes can form dense and uniform alloy after thermal treatment for 24 h at 823 K under H2 atmosphere, despite a porous structure due to the development of liquid-like properties above Tamman temperature to enforce the migrativity. On the contrary, alloying under N2 atmosphere resulted in a Pd-enriched layer. This led to a lower H2 flux but superior thermal stability compared to that alloying under H2 atmosphere. The trilayer approach of electroless-plated Pd, electro-polated Ag and electroless-plated Pd is not suitable to achieve homogeneous Pd-Ag alloys, which, on the other hand, presented the occurrence of a small gap between top Pd layer and middle Ag layer, probably due to insufficient wetting during plating process. An on-site repair treatment in analogous to MOCVD (Metal-organic Chemical Vapor Deposition) process was first proposed to extend the lifetime of Pd-Ag membrane, i.e., by vaporizing, and subsequent decomposition of Ag(OOCC2F5) powders to "preferentially" block the pinholes under vacuum and at working temperature of ca. 473-673 K, which effectively reduced the N2 flux by 57.4% compared to the initial value. The H2 flux, however, declined by 16.7% due to carbon deposition on the membrane surface, which requires further investigation. This approach shows some potential for on-site repair without disassembly or cooling to room temperature.

Review of Supported Pd-Based Membranes Preparation by Electroless Plating for Ultra-Pure Hydrogen Production

In the last years, hydrogen has been considered as a promising energy vector for the oncoming modification of the current energy sector, mainly based on fossil fuels. Hydrogen can be produced from water with no significant pollutant emissions but in the nearest future its production from different hydrocarbon raw materials by thermochemical processes seems to be more feasible. In any case, a mixture of gaseous compounds containing hydrogen is produced, so a further purification step is needed to purify the hydrogen up to required levels accordingly to the final application, i.e., PEM fuel cells. In this mean, membrane technology is one of the available separation options, providing an efficient solution at reasonable cost. Particularly, dense palladium-based membranes have been proposed as an ideal chance in hydrogen purification due to the nearly complete hydrogen selectivity (ideally 100%), high thermal stability and mechanical resistance. Moreover, these membranes can be used in a membrane reactor, offering the possibility to combine both the chemical reaction for hydrogen production and the purification step in a unique device. There are many papers in the literature regarding the preparation of Pd-based membranes, trying to improve the properties of these materials in terms of permeability, thermal and mechanical resistance, poisoning and cost-efficiency. In this review, the most relevant advances in the preparation of supported Pd-based membranes for hydrogen production in recent years are presented. The work is mainly focused in the incorporation of the hydrogen selective layer (palladium or palladium-based alloy) by the electroless plating, since it is one of the most promising alternatives for a real industrial application of these membranes. The information is organized in different sections including: (i) a general introduction; (ii) raw commercial and modified membrane supports; (iii) metal deposition insights by electroless-plating; (iv) trends in preparation of Pd-based alloys, and, finally; (v) some essential concluding remarks in addition to futures perspectives.