ABSTRACT
Solar-driven thermochemical conversion of H2O and CO2 into sustainable fuels, based on redox cycle, provides a promising path for alternative energy, as it employs the solar energy as high-temperature heat supply and adopts H2O and CO2 as initial feedstock. This review describes the sustainable fuels production system, including a series of physical and chemical processes for converting solar energy into chemical energy in the form of sustainable fuels. Detailed working principles, redox materials, and key devices are reviewed and discussed to provide systematic and in-depth understanding of thermochemical fuels production with the aid of concentrated solar power technology. In addition, limiting factors affecting the solar-to-fuel efficiency are analyzed; meanwhile, the improvement technologies (heat recovery concepts and designs) are summarized. This study therefore sets a pathway for future research works based on the current status and demand for further development of such technologies on a commercial scale.
ABSTRACT
"Electrocatalytic Methods for Ammonia Production" is a new open Special Issue in Materials, which aims to publish original research papers, perspectives and review articles on theoretical and applied research, and shed an inspiring light on electrocatalytic methods of ammonia production [...].
ABSTRACT
In the present work, a flexible carbon sponge is experimentally characterized and proposed as an alternative electrode for advanced vanadium redox flow batteries. Such an electrode is prepared via directly carbonizing the commercially-available and inexpensive melamine formaldehyde resin sponge in argon, to inherit the well-defined and three-dimensional bi-continuous architecture of the melamine sponge with 99.6% porosity and 40â µm average pore size. By applying the carbon sponges as the electrodes, it is demonstrated that the vanadium flow battery at 200â mA cm-2 can yield an energy efficiency of 77.9%, significantly higher than that with commonly-used graphite felt electrodes (72.9%). After a thermal treatment in air, the energy efficiency of carbon sponge can further be improved to 81.2% at mA cm-2 due to introduction of favorable oxygen containing functionalities. The operating stability with the carbon sponge is proven by a 200â cycling test with minor efficiency decay.
ABSTRACT
The green production of ammonia, in an electrochemical flow cell under ambient conditions, is a promising way to replace the energy-intensive Haber-Bosch process. In the operation of this flow cell with an alkaline electrolyte, water is produced at the anode but also required as an essential reactant at the cathode for nitrogen reduction. Hence, water from the anode is expected to diffuse through the membrane to the cathode to compensate for the water needed for nitrogen reduction. Excessive water permeation, however, tends to increase the possibility of water flooding, which would not only create a large barrier for nitrogen delivery and availability, but also lead to severe hydrogen evolution as side reaction, and thus significantly lower the ammonia production rate and Faradaic efficiency. In this work, the water flooding phenomenon in flow cells for ammonia production via electrocatalytic nitrogen reduction is verified via the visualization approach and the electrochemical cell performance. In addition, the effects of the nitrogen flow rate, applied current density, and membrane thickness on the water crossover flux and ammonia production rate are comprehensively studied. The underlying mechanism of water transport through the membrane, including diffusion and electro-osmotic drag, is precisely examined and specified to provide more insight on water flooding behavior in the flow cell.
ABSTRACT
Ion exchange membranes are widely used in fuel cells to physically separate two electrodes and functionally conduct charge-carrier ions, such as anion exchange membranes and cation exchange membranes. The physiochemical characteristics of ion exchange membranes can affect the ion transport processes through the membrane and thus the fuel cell performance. This work aims to understand the ion transport characteristics through different types of ion exchange membrane in direct formate fuel cells. A one-dimensional model is developed and applied to predict the polarization curves, concentration distributions of reactants/products, distributions of three potentials (electric potential, electrolyte potential, and electrode potential) and the local current density in direct formate fuel cells. The effects of the membrane type and membrane thickness on the ion transport process and thus fuel cell performance are numerically investigated. In addition, particular attention is paid to the effect of the anion-cation conducting ratio of the membrane, i.e., the ratio of the anionic current to the cationic current through the membrane, on the fuel cell performance. The modeling results show that, when using an anion exchange membrane, both formate and hydroxide concentrations in the anode catalyst layer are higher than those achieved by using a cation exchange membrane. Although a thicker membrane better alleviates the fuel crossover phenomenon, increasing the membrane thickness will increase the ohmic loss, due to the enlarged ion-transport distance through the membrane. It is further found that increasing the anion-cation conducting ratio will upgrade the fuel cell performance via three mechanisms: (i) providing a higher ionic conductivity and thus reducing the ohmic loss; (ii) enabling more OH- ions to transport from the cathode to the anode and thus increasing the OH- concentration in the anode catalyst layer; and (iii) accumulating more cations in the anode and thus enhancing the formate-ion migration to the anode catalyst layer for the anodic reaction.
