The hybrid sulfur cycle has been investigated as a means to produce CO 2 -free hydrogen efficiently on a large scale through the decomposition of H 2 SO 4 to SO 2 , O 2 , and H 2 O, and then electrochemically oxidizing SO 2 back to H 2 SO 4 with the cogeneration of H 2 . The net effect is the production of hydrogen and oxygen from water. Recently, sulfonated polybenzimidazoles (s-PBI) have been investigated as a replacement for Nafion due to the ability to offer increased process efficiency through the generation of higher acid concentrations at lower potentials. Here, we measure the acid concentrations and individual potential contributions toward the overall operating voltage seen in the SO 2 -depolarized-electrolyzer. We then determine model parameters necessary to predict voltage losses in a cell over a wide range of operating temperatures, pressures, currents and reactant flow rates. The hydrogen production program at the U. S. Department of Energy is examining an array of distributed and centralized hydrogen facilities that could contribute to the hydrogen generation infrastructure.1 Thermochemical cycles are being considered for large scale, centralized facilities due to their potential for high efficiencies at low costs. These cycles involve a series of chemical reactions that result in the splitting of water at much lower temperatures (∼500-1000• C) than direct thermal dissociation (>2500 • C) and at much higher efficiencies than direct water electrolysis.2 Chemical species in these reactions are recycled resulting in the consumption of only energy and water to produce hydrogen and oxygen. Although there are hundreds of possible thermochemical cycles, the hybrid-sulfur (HyS) process is the only all-fluid, two step thermochemical cycle. 3-6The high temperature step (850-950• C) involves the decomposition of H 2 SO 4 to produce oxygen and sulfur dioxide via the following reaction:The SO 2 is separated, cooled, and sent to the SO 2 -depolarized electrolyzer (SDE). The resulting reactions at the anode and cathode, respectively, are:Thus, the overall reaction in the electrolyzer is represented as:Considerable progress was made in the last decade in lowering the operating voltage and increasing the current density of the SDE by moving from a microporous rubber diaphragm separator used by Westinghouse 7 to a perfluorinated sulfonic acid membrane (e.g., DuPont's Nafion). [8][9][10][11][12] For example, Westinghouse was only able to get the cell voltage down to 1.0 V at 400 mA/cm 2 , where we achieved 500 mA/cm 2 at 0.71 V and 1.2 A/cm 2 at 1.0 V using Nafion 212 (N212). However, to achieve overall process efficiency, concentrated sulfuric acid as well as low cell voltage at high current densities are * Electrochemical Society Member.* * Electrochemical Society Student Member. * * * Electrochemical Society Fellow.z E-mail: taylor.garrick@gm.com; weidner@cec.sc.edu necessary. The key issue when using membranes like Nafion that rely on water for their proton conductivity is that high acid concentrations dehydrate ...
Cystatin C (CysC) is a versatile and ubiquitously-expressed member of the cysteine protease inhibitor family that is present at notably high concentrations in cerebrospinal fluid. Under mildly denaturing conditions, CysC forms inactive domain-swapped dimers. A destabilizing mutation, L68Q, increases the rate of domain-swapping and causes a fatal amyloid disease, hereditary cystatin C amyloid angiopathy. Wild-type (wt) CysC will also aggregate into amyloid fibrils under some conditions. Propagated domain-swapping has been proposed as the mechanism by which CysC fibrils grow. We present evidence that a CysC mutant, V57N, stabilized against domain-swapping, readily forms fibrils, contradicting the propagated domain-swapping hypothesis. Furthermore, in physiological buffer, wt CysC can form oligomers without undergoing domain-swapping. These non-swapped oligomers are identical in secondary structure to CysC monomers and completely retain protease inhibitory activity. However, unlike monomers or dimers, the oligomers bind fluorescent dyes that indicate they have characteristics of pre-amyloid aggregates. Although these oligomers appear to be a pre-amyloid assembly, they are slower than CysC monomers to form fibrils. Fibrillation of CysC therefore likely initiates from the monomer and does not require domain-swapping. The non-swapped oligomers likely represent a dead-end offshoot of the amyloid pathway and must dissociate to monomers prior to rearranging to amyloid fibrils. These prefibrillar CysC oligomers were potent inhibitors of aggregation of the Alzheimer's-related peptide, β-amyloid. This result illustrates an example where heterotypic interactions between pre-amyloid oligomers prevent the homotypic interactions that would lead to mature amyloid fibrils.
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