Quantifying Individual Potential Contributions of the Hybrid Sulfur Electrolyzer

The hybrid sulfur cycle has been investigated as a means to produce clean hydrogen efficiently on a large scale by first decomposing H2SO4 to SO2, O2, and H2O and then electrochemically oxidizing SO2 back to H2SO4 with the cogeneration of H2. Thus far, it has been determined that the total cell potential for the hybrid sulfur electrolyzer is controlled mainly by water transport in the cell. Water is required at the anode to participate in the oxidation of SO2 to H2SO4 and to hydrate the membrane. In addition, water transport to the anode influences the concentration of the sulfuric acid produced. The resulting sulfuric acid concentration at the anode influences the equilibrium potential of and the reaction kinetics for SO2 oxidation and the average conductivity of the membrane. A final contribution to the potential loss is the diffusion of SO2 through the sulfuric acid to the catalyst site. Here, we extend our understanding of water transport to predict the individual contributions to the total cell potential

Sulfur Dioxide Crossover during the Production of Hydrogen and Sulfuric Acid in a PEM Electrolyzer

A proton exchange membrane (PEM) electrolyzer has been investigated as a viable system for the electrolysis step in the thermochemical conversion of sulfur dioxide to sulfuric acid for the large-scale production of hydrogen. Unfortunately, during operation, sulfur dioxide can diffuse from the anode to the cathode. This has several negative effects, including reduction to sulfur that could potentially damage the electrode, consumption of current that would otherwise be used for the production of hydrogen, introduction of oxygen and SO2 to the hydrogen stream, and loss of sulfur to the cycle. However, proper water management can reduce or eliminate the transport of SO2 to the cathode. Here we present model simulations and experimental data for the flux of SO2 to the cathode as a function of current density and pressure differential across the membrane and show how water transport influences SO2 crossover. Understanding SO2 crossover is important in evaluating both the lifetime of the electrolyzer and membranes developed to limit SO2 crossover

Effect of Water Transport on the Production of Hydrogen and Sulfuric Acid in a PEM Electrolyzer

The thermochemical cycle involving the interconversion between sulfur dioxide and sulfuric acid is a promising method for efficient, large-scale production of hydrogen. A key step in the process is the oxidation of sulfur dioxide to sulfuric acid in an electrolyzer. Gaseous SO2 fed to a proton exchange membrane (PEM) electrolyzer was previously investigated and was shown to be a promising system for the electrolysis step. A critical factor in the performance of this gas-fed electrolyzer is the management of water since it: (i) is needed as a reactant, (ii) determines the product sulfuric acid concentration, (iii) affects SO2 crossover rate, and (iv) serves to hydrate the membrane. Therefore, we present a coupled mathematical and experimental study on the effect of water on the production of sulfuric acid in a gas-phase PEM electrolyzer. The model is shown to successfully predict the concentration of sulfuric acid as a function of temperature, current density, pressure differential across the membrane, and membrane thickness

Sulfur dioxide crossover during the production of hydrogen and sulfuric acid

A proton exchange membrane ͑PEM͒ electrolyzer has been investigated as a viable system for the electrolysis step in the thermochemical conversion of sulfur dioxide to sulfuric acid for the large-scale production of hydrogen. Unfortunately, during operation, sulfur dioxide can diffuse from the anode to the cathode. This has several negative effects, including reduction to sulfur that could potentially damage the electrode, consumption of current that would otherwise be used for the production of hydrogen, introduction of oxygen and SO 2 to the hydrogen stream, and loss of sulfur to the cycle. However, proper water management can reduce or eliminate the transport of SO 2 to the cathode. Here we present model simulations and experimental data for the flux of SO 2 to the cathode as a function of current density and pressure differential across the membrane and show how water transport influences SO 2 crossover. Understanding SO 2 crossover is important in evaluating both the lifetime of the electrolyzer and membranes developed to limit SO 2 crossover. The hybrid sulfur process is being investigated as an efficient way to produce clean hydrogen on a large scale at efficiencies higher than water electrolysis. where SHE is the standard hydrogen electrode. The water required for Reaction 1 is supplied via the membrane from the cathode. The H + produced in Reaction 1 migrates through the membrane and reduces to hydrogen at the cathode We have successfully carried out Reactions 1 and 2 over a range of operating conditions ͑e.g., temperature, flow rate, and pressure differential͒ and design variations ͑i.e., catalyst loading and membrane type and thickness͒. Unfortunately, a detrimental side reaction occurs when SO 2 crosses the membrane to the cathode and is reduced to sulfur via the reaction The reduction of SO 2 to sulfur at the cathode consumes current that would otherwise be used for the production of hydrogen, introduces oxygen and SO 2 to the hydrogen stream that must be separated, and may increase cell resistance due to sulfur deposits in the electrode. SO 2 crossing the membrane, even if it is not reduced at the cathode via Reaction 3, is lost to the cycle and must be resupplied. For example, researchers at the Savannah River National Laboratory ͑SRNL͒ have observed a sulfur layer between the membrane and the cathode in the liquid-fed anode system, which leads to a significant delamination of the cathode from the membrane. Experimental The experimental setup was similar to that reported in previous papers. Liquid water was fed to the cathode by a metering pump, and gaseous SO 2 was fed to the anode. The cell was maintained at 80°C, and the water was heated to 88°C before being fed to the cathode. The membrane electrode assembly ͑MEA͒ contained Pt black with a loading of 1.5 mg/cm 2 on each side of the membrane. The membranes were either N212 or N115 ͑2 and 5 mil thicknesses, respectively͒. The SO 2 flow rate was maintained so that the conversion rate at the anode was 20%. We have shown previously, however, that conversion and catalyst loading have little effect on the electrolyzer performance. 15 A pressure differential was maintained across the membrane by the use of a globe valve on the exit stream of the cathode. The gaseous feed stream to the anode was maintained at 101 kPa. The electrolyzer was run at a constant current and different pressure differentials, and energy dispersive X-ray ͑EDX͒ elemental analysis was performed on the anode and cathode to determine the buildup of sulfur in the cathode. The electrochemical monitoring technique 18 was used to determine the diffusion coefficient and the solubility of SO 2 in Nafion. The membrane pressure differential was initially maintained at ⌬P = 0 kPa, N 2 was fed to the gas side, and a voltage of 0.31 V was applied. The gas was then switched to SO 2 , with the cell voltage maintained at 0.31 V. The SO 2 crossing the membrane was oxidized to sulfuric acid on the liquid water side and hydrogen evolved on the gaseous SO 2 side. The slight increase in the water flux toward the gaseous SO 2 side due to the electro-osmotic drag was considered negligible for the analysis. The measured limiting current was a result of the mass-transfer-limited flux of SO 2 across the membrane, with the transient data useful for determining the diffusion coefficient and solubility

Solar thermochemical hydrogen (STCH) Processes

There is a significant opportunity to store solar energy using hydrogen if a suitable thermochemical process can be developed. Although there are literally hundreds of cycles to choose from, there are only two real ones. One, the direct thermochemical cycle based on zinc oxide, has garnered significant attention in recent years, and development has proceeded far enough that a pilot-scale reactor has been developed. However, extremely high temperatures required present significant materials challenges that may not be solvable in the near-term, and the rapid quenching step limits process efficiency.The second, the hybrid thermochemical cycle based on sulfur dioxide, combines a low-temperature electrolysis step with a higher temperature decomposition step. Recent techno-economic studies demonstrate solar to hydrogen efficiencies on the order of 15-20% and costs on the order $4.80/kg with potential to reach values of$2.00/kg for large scale solar hydrogen production using the HyS cycle. In the future, additional advances in materials and operating parameters for all feasible thermochemical cycles will need to be demonstrated for commercial adoption of these processes

Solar Thermochemical Hydrogen (STCH) Processes

There is a significant opportunity to store solar energy using hydrogen if a suitable thermochemical process can be developed. Although there are literally hundreds of cycles to choose from, there are only two real ones. One, the direct thermochemical cycle based on zinc oxide, has garnered significant attention in recent years, and development has proceeded far enough that a pilot-scale reactor has been developed. However, extremely high temperatures required present significant materials challenges that may not be solvable in the near-term, and the rapid quenching step limits process efficiency.The second, the hybrid thermochemical cycle based on sulfur dioxide, combines a low-temperature electrolysis step with a higher temperature decomposition step. Recent techno-economic studies demonstrate solar to hydrogen efficiencies on the order of 15-20% and costs on the order $4.80/kg with potential to reach values of$2.00/kg for large scale solar hydrogen production using the HyS cycle. In the future, additional advances in materials and operating parameters for all feasible thermochemical cycles will need to be demonstrated for commercial adoption of these processes

Low-cost nanostructured electrocatalysts for hydrogen evolution in an anion exchange membrane lignin electrolysis cell

© The Author(s) 2019. The aim of this study is to quantify the hydrogen production rate in an anion exchange membrane (AEM) lignin electrolysis cell. Two non-precious and nanostructured metal and metal oxide electrocatalysts were developed and used as the anodic catalysts in a lignin electrolysis process. H2 production rates, energy consumption rates and faradaic efficiency were measured using β-PbO2/MWNTs and Ni-Co/TiO2 electrocatalysts as the anode, where electrochemical depolymerization of lignin occurs. Our results were then compared with recent efforts for lignin electrolysis in the literature. This work demonstrates that the β-PbO2/MWNTs nanocomposite is the more stable and active electrocatalyst in this process. At the end, our results showed that using β-PbO2/MWNTs as the anodic electrocatalyst can enhance lignin oxidation rates, with a corresponding increase in the rate of H2 production at the cathode. As a result, this can lead to high hydrogen evolution rates (∼45.6 mL/h), and increase energy efficiency by 20%, compared to a commercial alkaline water electrolyzer