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Title: Test Plan for Characterization Testing of SO2-depolarized Electrolyzer Cell Designs

Abstract

SRNL received funding in FY 2005 to test the Hybrid Sulfur (HyS) Process for generating hydrogen. This technology employs an electrolyzer that uses a sulfur dioxide depolarized anode to greatly reduce the electrical energy requirement. The required current is the same as for conventional electrolysis of water, but the required cell voltage is reduced. The electrolyzer is a key part of HyS technology. Completing the material loop for HyS requires a high temperature decomposition of sulfuric acid to regenerate the sulfur dioxide gas needed for the anode reaction. Oxygen is also produced and could be sold. The decomposition of sulfuric acid is being studied by others in a separately funded task. It is not included in this SRNL task.

Authors:
Publication Date:
Research Org.:
Savannah River Site (SRS), Aiken, SC
Sponsoring Org.:
USDOE
OSTI Identifier:
881306
Report Number(s):
WSRC-TR-2006-00069
TRN: US200613%%418
DOE Contract Number:
DE-AC09-96SR18500
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; ANODES; ELECTROLYSIS; HYDROGEN; OXYGEN; SULFUR; SULFUR DIOXIDE; SULFURIC ACID; TESTING; WATER

Citation Formats

Steimke, J. L. Test Plan for Characterization Testing of SO2-depolarized Electrolyzer Cell Designs. United States: N. p., 2006. Web. doi:10.2172/881306.
Steimke, J. L. Test Plan for Characterization Testing of SO2-depolarized Electrolyzer Cell Designs. United States. doi:10.2172/881306.
Steimke, J. L. Wed . "Test Plan for Characterization Testing of SO2-depolarized Electrolyzer Cell Designs". United States. doi:10.2172/881306. https://www.osti.gov/servlets/purl/881306.
@article{osti_881306,
title = {Test Plan for Characterization Testing of SO2-depolarized Electrolyzer Cell Designs},
author = {Steimke, J. L.},
abstractNote = {SRNL received funding in FY 2005 to test the Hybrid Sulfur (HyS) Process for generating hydrogen. This technology employs an electrolyzer that uses a sulfur dioxide depolarized anode to greatly reduce the electrical energy requirement. The required current is the same as for conventional electrolysis of water, but the required cell voltage is reduced. The electrolyzer is a key part of HyS technology. Completing the material loop for HyS requires a high temperature decomposition of sulfuric acid to regenerate the sulfur dioxide gas needed for the anode reaction. Oxygen is also produced and could be sold. The decomposition of sulfuric acid is being studied by others in a separately funded task. It is not included in this SRNL task.},
doi = {10.2172/881306},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Feb 15 00:00:00 EST 2006},
month = {Wed Feb 15 00:00:00 EST 2006}
}

Technical Report:

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  • This document reports work performed at the Savannah River National Laboratory (SRNL) that further develops the use of a proton exchange membrane or PEM-type electrochemical cell to produce hydrogen via SO{sub 2}-depolarized water electrolysis. This work was begun at SRNL in 2005. This research is valuable in achieving the ultimate goal of an economical hydrogen production process based on the Hybrid Sulfur (HyS) Cycle. The HyS Process is a hybrid thermochemical cycle that may be used in conjunction with advanced nuclear reactors or centralized solar receivers to produce hydrogen by water-splitting. Like all other sulfur-based cycles, HyS utilizes the highmore » temperature thermal decomposition of sulfuric acid to produce oxygen. The unique aspect of HyS is the generation of hydrogen in a water electrolyzer that is operated under conditions where dissolved sulfur dioxide depolarizes the anodic reaction, resulting in substantial voltage reduction. Sulfur dioxide is oxidized at the anode, producing sulfuric acid that is sent to the acid decomposition portion of the cycle. The focus of this work was to conduct single cell electrolyzer tests in order to prove the concept of SO{sub 2}-depolarization and to determine how the results can be used to evaluate the performance of key components of the HyS Process. A test facility for conducting SO{sub 2}-depolarized electrolyzer (SDE) testing was designed, constructed and commissioned. The maximum cell current is 50 amperes, which is equivalent to a hydrogen production rate of approximately 20 liters per hour. Feed to the anode of the electrolyzer is sulfuric acid solutions containing dissolved sulfur dioxide. The partial pressure of sulfur dioxide may be varied in the range of 1 to 6 atm (15 to 90 psia). Temperatures may be controlled in the range from ambient to 80 C. Hydrogen generated at the cathode of the cell is collected for the purpose of flow measurement and composition analysis. The test facility proved to be easy to operate, versatile, and reliable.« less
  • The single cell test system development for the SRNL sulfur dioxide-depolarized electrolyzer has been completed. Operating experience and improved operating procedures were developed during test operations in FY06 and the first quarter of FY07. Eight different cell configurations, using various MEA designs, have been tested. The single cell test electrolyzer has been modified to overcome difficulties experienced during testing, including modifications to the inlet connection to eliminate minute acid leaks that caused short circuits. The test facility was modified by adding a water bath for cell heating, thus permitting operation over a wider range of flowrates and cell temperatures. Modificationsmore » were also identified to permit continuous water flushing of the cathode to remove sulfur, thus extending operating time between required shutdowns. This is also expected to permit a means of independently measuring the rate of sulfur formation, and the corresponding SO{sub 2} flux through the membrane. This report contains a discussion of the design issues being addressed by the single cell test program, a test matrix being conducted to address these issues, and a summary of the performance objectives for the single cell test system. The current primary objective of single cell test system is to characterize and qualify electrolyzer configurations for the following 100-hour longevity tests. Although the single cell test system development is considered complete, SRNL will continue to utilize the test facility and the single cell electrolyzer to measure the operability and performance of various cell design configurations, including new MEA's produced by the component development tasks.« less
  • This document reports work performed at the Savannah River National Laboratory (SRNL) that resulted in a major accomplishment by demonstrating the proof-of-concept of the use of a proton exchange membrane or PEM-type electrochemical cell to produce hydrogen via SO{sub 2}-depolarized water electrolysis. For the first time sulfur dioxide dissolved in liquid sulfuric acid was used to depolarize water electrolysis in a modern PEM cell. The use of such a cell represents a major step in achieving the ultimate goal of an economical hydrogen production process based on the Hybrid Sulfur (HyS) Cycle. The HyS Process is a hybrid thermochemical cyclemore » that may be used in conjunction with advanced nuclear reactors or centralized solar receivers to produce hydrogen by water-splitting. Like all other sulfur-based cycles, HyS utilizes the high temperature thermal decomposition of sulfuric acid to produce oxygen. The unique aspect of HyS is the generation of hydrogen in a water electrolyzer that is operated under conditions where dissolved sulfur dioxide depolarizes the anodic reaction, resulting in substantial voltage reduction. Sulfur dioxide is oxidized at the anode, producing sulfuric acid, that is sent to the acid decomposition portion of the cycle. The focus of this work was to conduct single cell electrolyzer tests in order to prove the concept of SO{sub 2}-depolarization and to determine how the results can be used to evaluate the performance of key components of the HyS Process. A test facility for conducting SO{sub 2}-depolarized electrolyzer (SDE) testing was designed, constructed and commissioned. The maximum cell current is 50 amperes, which is equivalent to a hydrogen production rate of approximately 20 liters per hour. The test facility was designed for operation at room temperature with pressures up to 2 bar. Feed to the anode of the electrolyzer can be water, sulfuric acid of various concentrations, or sulfuric acid containing dissolved sulfur dioxide. Provisions are included to allow variation of the operating pressure in the range of 1 to 2 bar. Hydrogen generated at the cathode of the cell can be collected for the purpose of flow measurement and composition analysis. The test facility proved to be easy to operate, versatile, and reliable. Two slightly different SDE's were designed, procured and tested. The first electrolyzer was based on a commercially available PEM water electrolyzer manufactured by Proton Energy Systems, Inc. (PES). The PES electrolyzer was built with Hastelloy B and Teflon wetted parts, a PEM electrolyte, and porous titanium electrodes. The second electrolyzer was assembled for SRNL by the University of South Carolina (USC). It was constructed with platinized carbon cloth electrodes, a Nafion 115 PEM electrolyte, carbon paper flow fields, and solid graphite back plates. Proof-of-concept testing was performed on each electrolyzer at near-ambient pressure and room temperature under various feed conditions. SDE operation was evidenced by hydrogen production at the cathode and sulfuric acid production at the anode (witnessed by the absence of oxygen generation) and with cell voltages substantially less than the theoretical reversible voltage for simple water electrolysis (1.23 V). Cell performance at low currents equaled or exceeded that achieved in the two-compartment cells built by Westinghouse Electric Corporation during the original development of the HyS Process. Performance at higher currents was less efficient due to mass transfer and hydraulic issues associated with the use of cells not optimized for liquid feed. Test results were analyzed to determine performance trends, improvement needs, and long-term SDE potential. The PES cell failed after several days of operation due to internal corrosion of the titanium electrodes in the presence of sulfuric acid. Although it was anticipated that the titanium would react in the presence of acid, the rapid deterioration of the electrodes was unexpected. The USC cell was constructed of carbon-based components and had excellent corrosion resistance. However, it was a modified design originally based on gaseous reactants, and it had poor mass transfer characteristics when using liquid sulfuric acid feed with dissolved sulfur dioxide. This resulted in substantially increased polarization losses at higher current densities. Future work will focus on operation at higher temperature and pressure, as well as improved cell designs specifically considering the unique flow conditions for SDE operation.« less
  • Over the past several years, Savannah River National Laboratory (SRNL) has led a team of collaborators under the Department of Energy’s (DOE) nuclear hydrogen production program to develop the Hybrid Sulfur (HyS) Process. HyS is a 2-step water-splitting process consisting of high temperature decomposition of sulfuric acid to generate SO 2, followed by the electrolysis of aqueous SO 2 to generate hydrogen and sulfuric acid. The latter is fed back into the high temperature reactor. SRNL designed and built an SO 2-depolarized electrolyzer (SDE) and a test facility. Over 40 SDE’s were tested using different catalysts, membranes and other components.more » SRNL demonstrated that an SDE could be operated continuously for approximately 200 hours under certain conditions without buildup of sulfur at the SDE’s cathode, thus solving a key technical problem with SDE technology. Air Products and Chemicals, Inc. (APCI) is a major supplier of hydrogen production systems, and they have proprietary technology that could benefit from the SDE developed by SRNS, or some improved version thereof. However, to demonstrate that SRNL’s SDE is a truly viable approach to the electrolyzer design, continuous operation for far greater periods of time than 200 hours must be demonstrated, and the electrolyzer must be scaled up to greater hydrogen production capacities. SRNL and Air Products entered into a Cooperative Research and Development Agreement with the objective of demonstrating the effectiveness of the SDE for hydrogen and sulfuric acid production and to demonstrate long-term continuous operation so as to dramatically increase the confidence in the SDE design for commercial operation. SRNL prepared a detailed technical report documenting previous SDE development, including the current SDE design and operating conditions that led to the 200-hour sulfurfree testing. SRNL refurbished its single cell SDE test facility and qualified the equipment for continuous operation. A new membrane electrode assembly (MEA) was fabricated and installed in the single cell electrolyzer (60 cm 2 active cell area). Shakedown testing was conducted, and several modifications were made to the test facility equipment. Seven different MEAs were used during testing. Beginning on May 20, 2013, SRNL was able to test the SDE continuously for 1200 hours, including 1000 hours under power to generate hydrogen at an average rate of 10.8 liters per hour. The SDE was not removed or repaired during the 50-day test and was successfully restarted after each shutdown. The test was intentionally stopped after 1200 hours (1000 hours of hydrogen production) due to funding constraints. Post-test examination of the MEA using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Microanalysis (EDAX) showed no elemental sulfur deposits or sulfur layer inside the cell, thus successfully achieving the test goals. The results demonstrated that the SDE could be operated for extended periods without major performance degradation or the buildup of sulfur inside the MEA. Air Products conducted an assessment of the economic viability of the SDE based on the “as tested” design. The results indicated that the SDE faces significant economic obstacles in its current state. Further development and scale-up are necessary before the SDE is ready for commercialization.« less
  • The Hybrid Sulfur (HyS) process is one of the leading thermochemical cycles being studied as part of the DOE Nuclear Hydrogen Initiative (NHI). SRNL is conducting analyses and research and development for the Department of Energy on the HyS process. A conceptual design report and development plan for the HyS process was issued on April 1, 2005 [Buckner, et. al., 2005] , and a report on atmospheric testing of a sulfur dioxide depolarized electrolyzer (SDE), a major component of the HyS process, was issued on August 1, 2005 [Steimke, 2005]. The purpose of this report is to document work relatedmore » to the design and experimental test plan for a pressurized SDE. Pressurized operation of the SDE is a key requirement for development of an efficient and cost-effective HyS process. The HyS process, a hybrid thermochemical cycle proposed and investigated in the 1970s and early 1980s by Westinghouse Electric Corporation, is a high priority candidate for NHI due to the potential for high efficiency and its relatively high level of technical maturity. It was demonstrated in laboratory experiments by Westinghouse in 1978. Process improvements and component advancements that build on that work are being pursued. One of the objectives of the current work is to develop the SDE in order to permit the demonstration of a closed-loop laboratory model of the HyS process. The heart of the HyS process for generating hydrogen is a bank of electrolyzers incorporating sulfur dioxide depolarized anodes. SRNL planned, designed, built and operated a facility for testing single cell electrolyzers at ambient temperature and near atmospheric pressure during the spring and summer of 2005. The major contribution of the SRNL work was the establishment of the proof-of-concept for utilizing the proton-exchange-membrane (PEM) cell design for the SDE operation. Since PEM cells are being extensively developed for automotive fuel cell use, they offer significant potential for cost-effective application for the HyS Process. This report discusses the modifications necessary to the existing SRNL sulfur dioxide depolarized electrolyzer test facility to allow testing at up to 80 C and 90 psig. Because of the need for significant additional equipment and the ability to infer performance results to higher pressures, it recommends delaying further modifications to support testing at up to 300 psig (the commercial goal) until other, higher priority technical issues are addressed. These issues include membrane material selection, component designs, catalyst type and loading, etc. The factors and rationale that should be considered in developing and executing a detailed test matrix for pressurized operation are also discussed. In addition, an electrolyzer assembly design has been developed to allow the testing of different Membrane Electrode Assemblies (MEA's) as part of the planned FY06 HyS Development Program to complete selection of component design specifications for the HyS electrolyzer. MEA's are used in PEM cells to allow intimate contact and minimal resistance between the electrodes and the electrolyte layer. The pressurized electrolyzer assembly presented in this report will facilitate rapid change-out and testing of various MEA designs as part of the electrolyzer development effort.« less