SO2-depolarized Electrolysis Research Articles

Experimental study and modelling of continuous SO2-depolarized electrolysis in hybrid sulfur cycle for hydrogen production

SO2-depolarized electrolysis (SDE) is the key process in the hybrid sulfur cycle for green hydrogen production, but the detailed SDE characteristics including products generation, current efficiency variation, and side reactions are unclear. Herein, a continuous SDE process was experimentally conducted and theoretically modelled. The H2 and H2SO4 production was accelerated and the current efficiency for both electrodes was improved with a proper increase in current density (50−100 mA/cm2) or temperature (293−318 K), but further increasing current density to 150 mA/cm2 or temperature to 333 K led to a negative trend because of the anode corrosion or declined SO2 solubility. The increasing initial H2SO4 concentration (10−40 wt%) facilitated the cathodic H2 production and current efficiency, while negatively influenced the anodic SO2 conversion. The parasitic reactions occurred at cathode consumed H2 and resulted to the cathodic current efficiency well below 100%. A rigorous mathematical regression model correlating the production rates of H2 and H2SO4 with current density, temperature, and initial H2SO4 concentration was developed, and combined with the analysis of variance, current density was suggested the biggest factor affecting products generation in SDE process. These findings demonstrate a practical avenue for efficient SO2 conversion and H2 production in SDE process of hybrid sulfur cycle.

Performance improvement of the liquid-fed SO2-depolarized electrolyzers for anolyte with high H2SO4 concentration

As the hydrogen producing step of the hybrid sulfur process, SO2-depolarized electrolysis (SDE) needs to be operated with anolyte of high H2SO4 concentration, aiming at the high hydrogen production efficiency. However, the SDE cell is facing the challenge of rising impedance when anolyte H2SO4 concentration is increased. In this study, the performance of liquid-fed SDE cells is improved by using a thinner PEM and three-dimensional anode, especially for the anolyte with high H2SO4 concentration. The underlying mechanism of the SDE performance improvement is also discussed. The results show that effectively reducing the concentration polarization impedance is the key to improving the SDE performance. Significantly different from what is expected, PEM thickness reduction could not sharply decrease ohmic impedance. Instead, the application of thin PEMs can significantly reduce the concentration polarization impedance of the cathode reaction. As for the application of three-dimensional anode, it decreases anode concentration polarization dramatically. Finally, the combination of PEM thickness reduction and three-dimensional anode greatly improves the SDE performance of the electrolyzer. The influence of operating conditions on performance is studied for the optimized cell GF-212. When the anolyte H2SO4 concentration set at 50 wt%, the current density could reach 560 mA/cm2 at 1.0 V cell voltage, 360 mL/min anolyte flow rate and 80 °C.

Comparison of various structure designs of SO2-depolarized electrolysis cell

SO2-depolarized electrolysis (SDE) is the key step of the hybrid sulfur (HyS) process, which is one of the simplest thermochemical cycles for producing hydrogen by water splitting. Exploration and optimization of flow field/structure design is essential for improving the efficiency of SDE. In this work, graphite plates with different flow channels, together with porous graphite felts or carbon papers as diffusion layers are adopted to fabricate SDE cells with different structures. Evaluation of the cell structures is carried out, by comparing the SDE performance and taking into account the fluid resistance (pressure drop) of anode side. The effects of graphite felt compression ratio, hydrophilicity or hydrophobicity of carbon papers, anodic fluid flow rate, and operating temperature on the SDE performance are investigated. Square porous flow fields provided by graphite felts show excellent performance. Serpentine channel covered by hydrophobic carbon paper reveals advantages when adopted on the cathode side. Combination of above two flow fields and using them on anode and cathode sides respectively, could achieve excellent SDE performance. Under the condition of 40 °C and 360 mL/min anolyte flow rate, the current density could reach 760 mA/cm2 at the cell voltage of 1.19 V.

Application of Pt loaded graphite felt in SO2-depolarized electrolyzer

The three-dimensional anodes for SO2 depolarized electrolysis (SDE) cells are prepared by loading Pt/C on high void content graphite felts, with the method of ultrasonic spray and vacuum suction. SEM results confirm the three-dimensional space distribution of Pt in graphite felts, which ensures sufficient contact between Pt/C and SO2 in anolyte. Comparing with the two-dimensional anodic catalyst layer loaded on the proton exchange membrane in a conventional SDE cell, the application of the three-dimensional anode decreases cell impedance greatly and improves the SDE performance significantly. In this study, 0.63 mg/cm2 Pt loading amount shows the best performance when Pt/C is double-side-sprayed on graphite felt, and the current density reaches 1.24 A/cm2 at cell voltage of 1.2 V, as operating at 60 °C and anolyte flow rate of 360 mL/min. The effects of the Pt loading amount, operating temperature and anolyte flow rate on the SDE performance are investigated.

Study on electrochemical impedance spectroscopy and cell voltage composition of a PEM SO2-depolarized electrolyzer using graphite felt as diffusion layer

SO2-depolarized electrolysis (SDE) is the hydrogen-producing step of the hybrid sulfur (HyS) process. In this study, electrochemical impedance spectroscopy (EIS) is used to study the composition of cell voltage of SDE electrolyzers, which apply porous graphite felts as diffusing layers. By measuring the open circuit potential, the possible side reactions and the methods to avoid the side reactions are discussed from the perspectives of thermodynamics and kinetics. According to EIS results obtained at different voltages, the impedance and its components are determined by equivalent circuit fitting calculation. The obtained impedance values of different cells agree very well with their SDE performance results. The voltage loss in the SDE process is split into ohmic impedance loss, kinetics impedance loss and concentration polarization impedance loss. The influences of the graphite felt compression ratio, operating temperature and anolyte flow rate on the impedance and performance of SDE cell are investigated.

Process modeling of a hybrid-sulfur thermochemical cycle combined with solid oxide fuel cell/gas turbine system

This work proposes a novel design of the hybrid sulfur (HyS) thermochemical water-splitting cycle for hydrogen production. The hydrogen production plant is combined with a solid oxide fuel cell/gas turbine (SOFC/GT) system to satisfy the demand for the high-temperature heat from the HyS cycle by exploiting the heat loss of the SOFC stack and the demand for electric power for SO2-depolarized electrolysis. A detailed thermodynamic analysis of the HyS cycle and the SOFC/GT system was conducted on a flowsheet basis to determine the energy requirement and to estimate the effects of the operating conditions on the overall efficiency. As a result, it was found that the LHV efficiency of the combined HyS-SOFC/GT system can reach > 29% (34.32% HHV efficiency), with a net electric efficiency of the SOFC/GT system of 38.83% and consumption of 340.3 kJ molH2-1 high-temperature heat, 148.77 kJ molH2-1 low-temperature heat, and 134.35 kJ molH2-1 electric power.

Electrode optimization for efficient hydrogen production using an SO2-depolarized electrolysis cell

The hybrid sulfur (HyS) cycle offers an alternative route to hydrogen and sulfuric acid production using the SO2-depolarized electrolysis (SDE) cell. This work reports the most efficient SDE operation to date at high sulfuric acid concentrations (∼60 wt%) achieved through the optimization of operating conditions and cell components. We observed that open porosity in the porous transport media (PTM) plays a significant role in SDE performance as it enables efficient acid removal from the catalyst layer. The combination of membrane electrode assembly (MEA) components, such as Sulfonated Diels Alder Poly (phenylene) (SDAPP) membranes and electrodes prepared using SGL 29BC PTM, and operating conditions (103.4 kPagauge at 125 °C) yielded electrolysis potentials <700 mV at 500 mA/cm2 and acid concentrations >60 wt%.

Electrospray Deposition of Catalyst Layers with Ultralow Pt Loading for Cost-Effective H2 Production by SO2 Electrolysis.

The hybrid sulfur (HyS) thermochemical cycle has been considered as a promising approach for the massive production of clean hydrogen without CO2 emissions. The key to advance this technology and to enhance the cycle efficiency is to improve the electrocatalytic oxidation of SO2, which is the pivotal reaction within this process. Hence, this paper investigates, for the first time, the effect of electrospray and air gun deposition techniques and the influence of very low Pt loadings (<0.3 mg Pt/cm2) on catalyst durability and activity. The variation of electrochemical active surface area (ECSA) with the number of cycles demonstrates the significant impact of the electrode fabrication method and catalyst loading on the catalyst durability with considerable ECSA values for electrosprayed electrodes. Electrodes prepared with low platinum loadings (0.05 mg Pt/cm2) exhibit elevated catalyst activity and stability under sulfuric acid conditions and maintain a crucial current density after 5 h of electrolysis. This work extends the understanding of the SO2-depolarized electrolysis (SDE) process and gives suggestions for further improvements in the catalyst layer fabrication, which provides potential support for the large-scale research and application of the HyS cycle.

Characterization of PBI/Graphene Oxide Composite Membranes for the SO2 Depolarized Electrolysis at High Temperature

In this work, polybenzimidazole (PBI) membranes with different graphene oxide (GO) contents (0.5, 1.0, 2.0, and 3.0 wt %) as organic filler have been prepared. The X-ray diffraction confirms the incorporation of the filler into the polymeric membrane. The composite GO-based PBI membranes show better proton conductivity at high temperature (110–170 °C) than the pristine one. Moreover, the hydrophobicity of the PBI membranes is also improved, enhancing water management. The chemical stability demonstrates the benefit of the incorporation of GO in the PBI matrix. What is more, the composite PBI-based membranes show better phosphoric acid retention capability. For the first time, the results of the SO2-depolarized electrolysis for hydrogen production at high temperature (130 °C) using phosphoric acid-doped polybenzimidazole (PBI) membranes with the different GO contents are shown. The benefit of the organic filler is demonstrated, as H2SO4 production is 1.5 times higher when the membrane with a content of 1 wt % of GO is used. Moreover, three times more hydrogen is produced with the membrane containing 2 wt % of GO compared with the non-modified membrane. The obtained results are very promising and provide open research for this kind of composite membranes for green hydrogen production by the Westinghouse cycle.

Modeling and numerical study of SO2-depolarized electrolysis for hydrogen production in the hybrid sulfur cycle

The SO2-depolarized electrolysis (SDE) is one of the main processes in the hybrid sulfur cycle for hydrogen production, the microscopic characteristics of which are still not clear. In this work, a numerical analysis of SDE was conducted. The steady-state, laminar and incompressible 2D model of the electrolytic cell (EC) for carrying out SDE was first developed and verified. Physical field characterization indicated that the velocity was almost uniformly distributed in the flow channel, but higher pressure drops and pumping power consumption existed in the anode channel than in the cathode channel. Meanwhile, the continuous conversion of SO2 to H2SO4 at anode and generation of H2 at cathode along the channel length were determined. Further exploring the impacts of three main operating parameters implied that an appropriate higher electrolytic potential, lower inlet flow rate (not less than 0.005 m/s) and operating temperature were favorable for improving the SDE. After comprehensive consideration of SDE performance and energy consumption, a feasible operating condition of 0.8 V, 303 K, and 0.005 m/s was proposed for SDE process. The developed model and microscopic characterization of SDE will favor the design and optimization of EC for SDE process.

SO2-depolarized electrolysis using porous graphite felt as diffusion layer in proton exchange membrane electrolyzer

The hybrid sulfur (HyS) process, which is composed of SO2-depolarized electrolysis (SDE) reaction and sulfuric decomposition reaction, is one of the simplest thermochemical cycles for producing hydrogen by water splitting. SDE is currently conducted in a proton exchange membrane (PEM) electrolyzer. In this work, a novel PEM electrolyzer structure is proposed. Graphite felt with large void content is used as the diffusion layer. In the electrolyzer, porous graphite felt plays the role of evenly distributing fluid and conducting electricity. The gap between the polar plate and the catalytic layer is occupied by the electrolyte solution and graphite felt, which effectively reduces the ohmic impedance of the electrolyzer. The effects of the main parameters including graphite felt compression ratio, anodic fluid flow rate, and sulfuric acid concentration, as well as temperature are investigated. Under optimized operating conditions, the current density reaches 800 mA/cm2 at cell voltage of 1.094 V, which is remarkably better than reported SDE performance using conventional PEM electrolyzers.

High-performance SO2-depolarized electrolysis cell using advanced polymer electrolyte membranes

Three different proton conducting polymeric membrane materials (Nafion® 115, Nafion® 212, and sulfonated Diels-Alder polyphenylene [SDAPP]) were evaluated for use in SO2-depolarized electrolyzers for the production of hydrogen via the hybrid sulfur cycle. Their performance was measured using different water feed strategies to minimize overpotential losses while maintaining high product acid concentration. Both thin membranes (Nafion® 212 and SDAPP) showed performance superior to that of the thicker Nafion® 115. The SDAPP membrane electrode assembly (MEA) performed well at higher acid concentrations, maintaining low ohmic and kinetic overpotentials. Finally, short-term (100-h) stability tests under constant current conditions showed minimal degradation for the SDAPP and Nafion® 212 MEAs. SDAPP MEA performance approached the targets needed to make the hybrid sulfur cycle a competitive process for hydrogen production (product acid concentration ≥65 wt% H2SO4 at ≤ 0.6-V cell potential and ≥0.5 A-cm−2 current density).

Boosting SO2-depolarized electrolysis with anodic HI for efficient and energy-saving hydrogen production

The SO2-depolarized electrolysis (SDE) is a promising method for SO2 capture and hydrogen production. But the SDE is limited by the high overpotential and slow kinetics of anodic SO2 oxidation reaction (SOR). Herein, to boost SDE process, we add hydroiodic acid (HI) to the anolyte and form HI-assisted SO2-depolarized electrolysis (HASDE). The mechanism of HI assisted SOR is explored, the existence of HI reduces the onset potential and impedance, but increases current density and reaction rate of SOR. HI is supposed to change the single-step SOR into three-step reactions, without changing the overall anodic reaction. A HI concentration of less than or equal to 0.02 mol/L and an operating temperature of 50 °C or less are recommendable. The comparison of SDE and HASDE processes indicates that HASDE delivers a lower cell voltage than SDE during continuous constant-current electrolysis, and has negligible effect on cathodic H2 evolution, while it achieves 27.03–48.65% reduction in electric energy consumption compared with SDE. The present work provides an innovative strategy for improving SDE and energy-saving production of value-added chemicals like H2SO4 and H2.

Review of Anodic Catalysts for SO2 Depolarized Electrolysis for “Green Hydrogen” Production

In the near future, primary energy from fossil fuels should be gradually replaced with renewable and clean energy sources. To succeed in this goal, hydrogen has proven to be a very suitable energy carrier, because it can be easily produced by water electrolysis using renewable energy sources. After storage, it can be fed to a fuel cell, again producing electricity. There are many ways to improve the efficiency of this process, some of them based on the combination of the electrolytic process with other non-electrochemical processes. One of the most promising is the thermochemical hybrid sulphur cycle (also known as Westinghouse cycle). This cycle combines a thermochemical step (H2SO4 decomposition) with an electrochemical one, where the hydrogen is produced from the oxidation of SO2 and H2O (SO2 depolarization electrolysis, carried out at a considerably lower cell voltage compared to conventional electrolysis). This review summarizes the different catalysts that have been tested for the oxidation of SO2 in the anode of the electrolysis cell. Their advantages and disadvantages, the effect of platinum (Pt) loading, and new tendencies in their use are presented. This is expected to shed light on future development of new catalysts for this interesting process.

Solar hydrogen production using direct coupling of SO2 depolarized electrolyser to a solar photovoltaic system

To obtain clean hydrogen production from a renewable energy source, direct coupling of photovoltaic and SO2 depolarized electrolysis (SDE) is one of the most promising alternative options. Both incident solar radiation variability and operating SDE parameters can affect the performance of such system. However, a big challenge is to have an efficient relative sizing. In fact, relative sizing is always a key issue when coupling renewable electric sources to SDE electrolysers. The aim is to determine the operation conditions and the optimal size of the direct coupling of a photovoltaic array to a SDE that ensure maximum hydrogen production while minimizing the energy consumption. In this study, a detailed two part-modeling approach has been used to estimate a system relative sizing for hydrogen production in a desired location. First, a simple model based on the extraction of the maximum power delivered by the photovoltaic array as function of temperature conditions and solar irradiation is developed. The model results are validated with experimental data of commercial PV module Second and in order to predict the maximum hydrogen production of SO2 depolarized electrolysis as a function of the consumed power, the SDE polarization curve is determined using a model and validated by experimental data. For the site of Bouzareah (Algiers), the results indicate that, for the case of global solar radiation on horizontal surface, the optimal combination of the integrated system is of one stack of 14 cells mounted in series and 17 stacks of 14 cells connected in parallel. In this case the global annual loss is 14.11%. For the same site but in the case of the global solar radiation on tilted surface, though the best sizing is still the same combination of series/parallel cells, the global annual loss is 20.80%.

SO2 carry-over and sulphur formation in a SO2-depolarized electrolyser

SO2-depolarized electrolysis (SDE) is considered as one of the most efficient hydrogen production methods. To maximize H2 production in SDE, the phenomena occurring in the cell need to be understood and controlled. In this work, electrochemically driven SO2 carry-over and elemental sulphur formation in cathode space are analysed by various methods: electrochemical, titration and photon correlation spectroscopy (PCS) under different conditions during SDE operation. The results indicate that SO2 carry-over is a fast process in an operating SDE and is difficult to control with only working parameter optimization. The PCS method provides SO2 carry-over information at early stages compared to other methods. Moreover, PCS might be further implemented as an in situ method for SDE system control.

Quantitative analysis of the cell voltage of SO2-depolarized electrolysis in hybrid sulfur process

SO2-depolarized electrolysis (SDE) is the pivotal reaction in hybrid sulfur process. To date, the total cell potential for an SO2-depolarized electrolyzer has been identified to be controlled dominantly by the sulfuric acid concentration of the anolyte and electrolysis temperature. Potential loss in SDE can be separated into four components, i.e., equilibrium potential, anodic polarization overpotential, cathodic polarization overpotential, and ohmic loss. In this work, the potential individual components of SDE were measured and calculated. Results showed that the anodic polarization overpotential exhibited the highest ratio in the cell voltage of SDE reaction, and the kinetics of the anodic reaction was controlled by a distinct reaction process under different cell potentials. This study increases understanding on SDE and provides assistance to improve its performance.

Development and testing of a PEM SO2-depolarized electrolyzer and an operating method that prevents sulfur accumulation

The hybrid sulfur (HyS) cycle is being developed as a technology to generate hydrogen by splitting water, using heat and electrical power from a nuclear or solar power plant. A key component is the SO2-depolarized electrolysis (SDE) cell, which reacts SO2 and water to form hydrogen and sulfuric acid. SDE could also be used in once-through operation to consume SO2 and generate hydrogen and sulfuric acid for sale. A proton exchange membrane (PEM) SDE cell based on a PEM fuel cell design was fabricated and tested. Measured cell potential as a function of anolyte pressure and flow rate, sulfuric acid concentration, and cell temperature are presented for this cell. Sulfur accumulation was observed inside the cell, which could have been a serious impediment to further development. A method to prevent sulfur formation was subsequently developed. This was made possible by a testing facility that allowed unattended operation for extended periods.

In-situ electrochemical impedance spectroscopy measurement of anodic reaction in SO2 depolarized electrolysis process

SO2 depolarized electrolysis (SDE) is an important reaction in the hybrid sulfur process, which is one of the most promising approaches for mass hydrogen production without CO2 emission. The potential loss of SDE process could be separated into four components, i.e., reversible cell potential, anode over potential, cathode over potential, and ohmic loss. Thus far, the total cell potential for the SO2 depolarized electrolyzer has been identified to be dominantly controlled by the sulfuric acid concentration of the anolyte and the temperature of the electrolysis process. In this work, an in-situ electrochemical impedance spectroscopy measurement of the anodic SDE reaction was conducted. The results show that anodic over potential is mainly caused by the SO2 oxidation reaction rather than ohmic resistance or mass transfer limitation. This study extends the understanding of the SDE process and gives suggestions for further improvements in SDE performance.

Pt-based bimetallic catalysts for SO2-depolarized electrolysis reaction in the hybrid sulfur process

SO2-depolarized electrolysis (SDE) is pivotal in the hybrid sulfur process, which is a promising approach for mass hydrogen production without CO2 emission. The anode overpotential of SDE is the key component of electrolysis potential. This factor can be reduced by improving anode reaction kinetics. Such improvement is commonly achieved by employing Pt/AC as anode electrode and catalyst, thus also improving economic and electrocatalytic performances. In this work, anode catalysts for SO2 oxidation reaction are experimentally studied. Platinum-based bimetallic catalysts, including Pt–Pd/C, Pt–Rh/C, Pt–Ru/C, Pt–Ir/C, and Pt–Cr/C, are prepared and characterized. Their electrochemical characteristics for SDE in a once-through mode are investigated in SO2-saturated 30 wt% sulfuric acid at room temperature by various approaches such as cyclic voltammetry, linear sweep voltammetry, and polarization curves. Results show that 60 wt% Pt–Cr/C exhibits the highest electrocatalytic activity for SDE. Further studies on the metal proportion in Pt–Cr/C show that at a Pt:Cr atomic ratio of 1:2, this bimetallic catalyst demonstrates equal or even better electrolysis performance than 60 wt% Pt/C at a significantly lower economic cost.