Ammonia as an effective hydrogen carrier and a clean fuel for solid oxide fuel cells

Hydrogen has attracted much attention as a globally accepted clean energy carrier. Currently, the search for safe and efficient hydrogen storage materials is one of the most difficult challenges for the upcoming hydrogen economy. Boron- and nitrogen-based hydrides, such as metal borohydrides (e.g. NaBH4), ammonia borane (NH3BH3), ammonia (NH3), hydrous hydrazine (N2H4·H2O), and hydrazine borane (N2H4BH3), have received much attention as potential chemical hydrogen storage materials because of their high hydrogen contents and the advantage of CO-free H2 produced. In recent years, substantial efforts have been devoted to research highly efficient catalysts to significantly improve the kinetic properties for hydrogen evolution from the hydrolysis of sodium borohydride and ammonia borane, and selective decomposition of ammonia, hydrous hydrazine and hydrazine borane. Among them, non-noble metal catalysts have been widely considered as potential candidates due to their low cost, abundant reserves, and relatively high catalytic activities. In this review, we focus on the recent advances in non-noble metal catalyst design, synthesis and applications in hydrogen generation from boron- and nitrogen-based hydrides.

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Noble-metal-free nanocatalysts for hydrogen generation from boron- and nitrogen-based hydrides

Fuel cells offer the possibility of zero-emissions electricity generation and increased energy security. We review here the current status of solid oxide (SOFC) and polymer electrolyte membrane (PEMFC) fuel cells. Such solid electrolyte systems obviate the need to contain corrosive liquids and are thus preferred by many developers over alkali, phosphoric acid or molten carbonate fuel cells. Dramatic improvements in power densities have been achieved in both SOFC and PEMFC systems through reduction of the electrolyte thickness and architectural control of the composite electrodes. Current efforts are aimed at reducing SOFC costs by lowering operating temperatures to 500–800 °C, and reducing PEMFC system complexity be developing ‘water-free’ membranes which can also be operated at temperatures slightly above 100 °C.

Fuel cell materials and components

Porous solids in the form of adsorbents and catalysts play a crucial role in various industrially important chemical, energy, and environmental processes. Formulating them into structured configurations is a key step toward their scale up and successful implementation at the industrial level. Additive manufacturing, also known as 3D printing, has emerged as an invaluable platform for shape engineering porous solids and fabricating scalable configurations for use in a wide variety of separation and reaction applications. However, formulating porous materials into self-standing configurations can dramatically affect their performance and consequently the efficiency of the process wherein they operate. Toward this end, various research groups around the world have investigated the formulation of porous adsorbents and catalysts into structured scaffolds with complex geometries that not only exhibit comparable or improved performance to that of their powder parents but also address the pressure drop and attrition issues of traditional configurations. In this comprehensive review, we summarize the recent advances and current challenges in the field of adsorption and catalysis to better guide the future directions in shape engineering solid materials with a better control on composition, structure, and properties of 3D-printed adsorbents and catalysts.

Recent Advances in 3D Printing of Structured Materials for Adsorption and Catalysis Applications.

Review of the Decomposition of Ammonia to Generate Hydrogen

Catalytic ammonia decomposition for hydrogen production on Ni, Ru and Ni-Ru supported on CeO2

Ceria-supported Ni-Ru bimetallic catalysts with different metal loadings have been prepared by co-impregnation, characterized and tested in the production of hydrogen from the catalytic decomposition of ammonia. The bimetallic catalysts showed an excellent catalytic performance in long-term stability tests with respect to monometallic Ru/CeO2 and Ni/CeO2 and in multicycle tests under pure ammonia. The best catalytic performance has been obtained over catalysts with 2.4−5 wt.% Ni, 0.4−0.6 wt.% Ru, and a Ni/Ru wt.% ratio of ca. 7. TOFH2 values exceeding 2 s−1 have been obtained, which are among the highest reported for ammonia decomposition at 400 °C. Raman spectroscopy, XRD, HRTEM, XPS, TPR and H2 chemisorption have revealed the existence of an intimate contact between Ni and Ru and CeO2, which is considered the reason of the excellent catalytic activity and stability observed. A kinetic model has been developed using the Langmuir-Hinshelwood-Hougen-Watson approach for the decomposition of ammonia in a fixed bed reactor. The reaction rate expression of the ammonia decomposition on Ni-Ru bimetallics supported on ceria suggests that the dehydrogenation of the ammonia adsorbed on the surface of the catalyst is the limiting step of the reaction and that ammonia decomposition is inhibited by the presence of H2.

Catalytic ammonia decomposition over Ni-Ru supported on CeO2 for hydrogen production: Effect of metal loading and kinetic analysis

The development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under specific reaction conditions and may not be stable either in air or under high vacuum conditions. The identification of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox properties.

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

Binder-free ceria pastes have been formulated and used to prepare ceria structures with a woodpile arrangement of microchannels through 3D printing. After loading them with nickel, these catalytic structures have been tested for ammonia decomposition to obtain hydrogen, and their performance has been compared with those of conventional Ni/CeO2 powder catalysts and with that of a conventional cordierite honeycomb washcoated with Ni/CeO2. Samples have been characterized by N2 physisorption, inductively coupled plasma optical emission spectrometry (ICP-OES), X-Ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy. At the same reaction temperature and flow rate to catalyst weight ratio (F/W), the 3D-printed ceria structures show a catalytic activity much higher than that of the cordierite honeycomb on a reactor volume basis. Additive manufacturing represents a valuable tool to prepare customized ceria-based catalytic structures for practical application with a variety of geometries not attainable with conventional methods.

Ilaria Lucentini

Papers

Review of the Decomposition of Ammonia to Generate Hydrogen

Catalytic ammonia decomposition for hydrogen production on Ni, Ru and Ni-Ru supported on CeO2

Catalytic ammonia decomposition over Ni-Ru supported on CeO2 for hydrogen production: Effect of metal loading and kinetic analysis

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

Ammonia decomposition over 3D-printed CeO2 structures loaded with Ni