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The chemical stability, sulfur dioxide transport, ionic conductivity, and electrolyzer performance have been measured for several commercially available and experimental proton exchange membranes (PEMs) for use in a sulfur dioxide depolarized electrolyzer (SDE). The SDE's function is to produce hydrogen by using the Hybrid Sulfur (HyS) Process, a sulfur based electrochemical/thermochemical hybrid cycle. Membrane stability was evaluated using a screening process where each candidate PEM was heated at 80 °C in 60 wt. % H 2 SO 4 for 24 hours. Following acid exposure, chemical stability for each membrane was evaluated by FTIR using the ATR sampling technique. Membrane SO 2 transport was evaluated using a two-chamber permeation cell. SO 2 was introduced into one chamber whereupon SO 2 transported across the membrane into the other chamber and oxidized to H 2 SO 4 at an anode positioned immediately adjacent to the membrane. The resulting current was used to determine the SO 2 flux and SO 2 transport.Additionally, membrane electrode assemblies (MEAs) were prepared from candidate membranes to evaluate ionic conductivity and selectivity (ionic conductivity vs. SO 2 transport) which can serve as a tool for selecting membranes. MEAs were also performance tested in a HyS electrolyzer measuring current density versus a constant cell voltage (1V, 80 °C in SO 2 saturated 30 wt% H 2 SO 4 ). Finally, candidate membranes were evaluated considering all measured parameters including SO 2 flux, SO 2 transport, ionic conductivity, HyS electrolyzer performance, and membrane stability. Candidate membranes included both PFSA and non-PFSA polymers and polymer blends of which the non-PFSA polymers, BPVE-6F and PBI, showed the best selectivity.

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Reduction of greenhouse gases is vital for the long-term environmental health of the planet. While there has been progress in reducing CO2 emissions at large point sources, a significant portion of the CO2 emitted each year in the United States is released from distributed sources, like cars, smaller factories, and farms. Direct capture of CO2 from ambient air is therefore necessary for the ultimate reduction of greenhouse gas emissions in the atmosphere. However, capturing the CO2 in ambient air presents a much greater challenge due to the dilute nature of the CO2, requiring different strategies than carbon capture from concentrated CO2 waste streams. We are developing a novel process for the capture and containment of CO2 from air into a purified, concentrated CO2 stream that can be redirected for use as a feedstock for a wide variety of applications, including chemical manufacturing and syngas formation. This process involves the capture of CO2 in a concentrated KOH solution using a high-surface area air contactor to form potassium carbonate. The potassium carbonate is then efficiently electrolyzed in a hydrogen-assisted process to regenerate the CO2 at a low operating potential, increasing the electrical efficiency of the process while producing concentrated and purified CO2. In addition, water, hydrogen, and KOH are also regenerated as byproducts that can be recycled back into the CO2 capture and electrolysis processes, reducing both overall energy and chemical consumption. Using a custom designed test stand and traditional electrolysis cell design, we have demonstrated voltage of <1.8 V at 100 mA/cm2 for potassium carbonate conversion to carbon dioxide at the anode, with concomitant H2 and KOH production at the cathode, based on potassium-selective ion exchange across the membrane. Furthermore, using a custom 3-compartment electrolysis flow cell and ion-selective membranes, we successfully demonstrated hydrogen-assisted carbonate electrolysis, with hydrogen consumption at the anode, hydrogen and KOH production at the cathode, and CO2 formation from potassium carbonate in an internal flow-through compartment, at 1.4 V at 50 mA/cm2. Further improvements in operating conditions and cell components and design should decrease the operating voltage significantly, to <1 V at 100 mA/cm2. Acknowledgement: The project is financially supported by the Department of Energy’s ARPA-E Office under the Grant DE-AR0001495

Hydrogen production from water electrolysis for mobile and energy storage applications is attractive due to its high efficiency, fast ramp rates, and potential for the clean energy economy. However, current hydrogen production from electrolysis comprises only a small fraction of the global hydrogen market due to the high cost associated with expensive stack materials (membrane, catalyst, and bipolar plates) and electricity consumption of the commercial electrolysis systems. We have developed a high temperature alkaline electrolyte-based water electrolyzer (HTAWE) that operates at 350-550 °C, enabling high reaction rates while limiting thermal degradation compared to solid oxide cells operating above 700 °C. This water electrolyzer is based on our innovative molten hydroxide electrolyte impregnated in a porous ceramic matrix, which has conductivity as high as 0.4 S/cm at 350 °C, enabling rapid hydroxide transport. This HTAWE can simultaneously reduce the electrolyzer cost (by adopting cheap material) and improve energy efficiency (by enabling high-temperature operation). Using an innovative new SrZrO3-based matrix, we have demonstrated exceptional water electrolysis performance using both single and binary hydroxide mixtures. We successfully achieved sustained cell performance of 1.35 V at a current density of 1,000 mA/cm2 with area specific resistance (ASR) 0.1 Ohm-cm2 across the cell at furnace temperature 500 °C. We were also able to demonstrate cell performance of 1.45 V at a current density of 1,000 mA/cm2 with ASR of <0.2 Ohm-cm2 across the cell at a furnace temperature 400 °C. Furthermore, we have demonstrated stable cell performance for >1000 h of continuous operation at 1 A/cm2 with ASR of 0.2 Ohm∙cm2 across the cell.

Ammonia is a very attractive carbon-neutral fuel due to its low cost, high energy density, and existing transportation infrastructure. However, the viable commercial application of direct ammonia fuel cells (DAFC) has not been realized, largely due to materials challenges that lead to poor cell performance. Generally, DAFCs either use low temperature polymer electrolytes, which are limited by low reaction rates and severe ammonia crossover, or use high temperature solid oxide electrolytes, which suffer from severe material thermal degradation. We have developed an intermediate temperature alkaline electrolyte-based DAFC that operates at 400-600 °C, enabling high reaction rates while limiting thermal degradation. This DAFC is based on our innovative molten hydroxide electrolyte impregnated in a porous ceramic matrix, which has conductivity as high as 0.4 S/cm at 350 °C, enabling rapid hydroxide transport. The DAFCs uses state-of-the-art ammonia oxidation catalysts and ammonia-tolerant oxygen reduction reaction catalysts. Our DAFC has demonstrated an OCV of 1.2 V and can achieve 300 mA/cm2 at 0.86 V at 550 ⁰C, and 0.98 V at 600 ⁰C, or >450 mW/cm2 power from pure humidified ammonia. It has low ammonia crossover, thus enabling the high OCV and high power density without catalyst poisoning. Further work on catalyst development and cell design is underway to achieve higher power densities and reduce PGM catalyst loading and operation temperature, making this a more commercially viable technology.

Ammonia is a very attractive carbon-neutral fuel due to its low cost, high energy density, and existing transportation infrastructure. However, the viable commercial application of direct ammonia fuel cells (DAFC) has not been realized, largely due to materials challenges. Generally, DAFCs either use low temperature polymer electrolytes, which are limited by low reaction rates, or use high temperature solid oxide electrolytes, which suffer from severe material thermal degradation. We have developed an intermediate temperature alkaline electrolyte-based DAFC that operates at 400-600 °C, enabling high reaction rates while limiting thermal degradation. This DAFC is based on our innovative molten hydroxide electrolyte impregnated in a porous ceramic matrix, which has conductivity as high as 0.4 S/cm at 350 °C, enabling rapid hydroxide transport. The DAFCs use state-of-the-art ammonia oxidation catalysts and ammonia-tolerant ORR catalysts developed in-house and by our collaborators at SUNY Buffalo and University of Delaware. Our DAFC has demonstrated an OCV of 1.2 V and can achieve 300 mA/cm2 at 0.86 V, or >250 mW/cm2 power from pure humidified ammonia. It is stable for days without loss of activity and has very low ammonia crossover, enabling the high OCV and high power density without catalyst poisoning. Further work on catalyst development and cell design is underway to achieve higher power densities and reduce PGM catalyst loading and operation temperature, making this a more commercially viable technology. Acknowledgements: This work is financially supported by the Department of Energy’s ARPA-E under the award# DE_AR000814