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 kPa_gauge at 125 °C) yielded electrolysis potentials <700 mV at 500 mA/cm² and acid concentrations >60 wt%.     

Proton conductivity and performance in fuel cells of grafted membranes based on polymethylpentene with radiation-grafted crosslinked sulfonated polystyrene

In this work, a comparative study was carried out of the transport properties and performance in a hydrogen-air fuel cell of the membranes based on polymethylpentene (PMP) with grafted sulfonated polystyrene and the standard Nafion® 212 membrane. Grafted cation-exchange membranes (GCM) were obtained by radiation graft post-polymerization of styrene onto UV-exposed PMP film followed by sulfonation with chlorosulfonic acid. The proton-conductivity of the GCM membrane with an ion-exchange capacity of 2.9 ± 0.1 meq/g reaches 21 ± 1 mS cm^?1 at room temperature and 95% relative humidity, which is twice higher the conductivity of the Nafion® under the same conditions. The GCM-1 H₂-permeability of 2.06?10^?7 cm² s^?1 even slightly lower than that of the Nafion® 212 (2.14?10^?7 cm² s^?1). A comparison of these membranes in the membrane electrode assemblies (MEA) of hydrogen-air fuel cells (FC) shows that the use of the grafted membranes with the high ion-exchange capacity is highly promising. The maximum performance of FC with grafted and Nafion® 212 membrane are both close to 180 mW/cm² at the current density of 400 mA/cm². At the same time, the high degree of crosslinking of sulfonated polystyrene leads to a decrease in conductivity and does not give an advantage in gas permeability.     

First approaches for hydrogen production by the depolarized electrolysis of SO2 using phosphoric acid doped polybenzimidazole membranes

Renewable energy storage and conversion is nowadays a major target for the scientific community. Their conversion into hydrogen is a clear and clean alternative for their storage. This work shows, for the first time, the results of the SO₂ depolarized electrolysis for hydrogen production at high temperature (120–170 °C) using phosphoric acid doped polybenzimidazole (PBI) membranes. A standard and a thermally cure PBI membrane doped with phosphoric acid were used for manufacturing the MEA of two electrolyzers. The benefit of the temperature was demonstrated but an unexpected behavior occurs at voltages higher than 0.8 V when temperature increases. Moreover, the thermally cured membrane shows a superior performance as compared with the standard one. Production of sulfur by reduction of SO₂ becomes an important drawback and advices not operating above 130 °C. Results show that PBI membranes doped with phosphoric acid are suitable for high temperature operation for the sulfur dioxide depolarized electrolysis. Increasing temperature is beneficial up to a certain value of potential, showing a considerable influence in the charge transfer resistance of the system.     

Evaluation of hydrogen and methanol fuel cell performance of sulfonated diels alder poly(phenylene) membranes

Hydrogen fuel cell performance of sulfonated diels alder poly(phenylene) (SDAPP) with IECs ≥ 1.8 meq g⁻¹ is comparable to Nafion 212 under fully humidified conditions at 80 °C. However, as relative humidity is reduced, performance loss is substantial for SDAPP when compared to Nafion 212. This loss can be attributed to the large drop in proton conductivity in SDAPP as relative humidity is reduced; the proton conductivity of SDAPP with an IEC of 2.3 meq g⁻¹ dropped from 0.117 S cm⁻¹ to 0.001 S cm⁻¹ as the relative humidity was reduced from 100% to 25% at 80 °C. Methanol fuel cell experiments using 3 M methanol result in a 60 mV performance improvement at 25 mA cm⁻² when using SDAPP with an IEC of 1.2 meq g⁻¹ instead of Nafion 212. This improvement is due to lower methanol permeability of SDAPP (1.4 meq g⁻¹) over Nafion 212, with SDAPP films having methanol permeabilities less than 25% of Nafion 212.     

Sulphonated (PVDF-co-HFP)-graphene oxide composite polymer electrolyte membrane for HI decomposition by electrolysis in thermochemical iodine-sulphur cycle for hydrogen production

In overall iodine-sulphur (I-S) cycle (Bunsen reaction), HI decomposition is a serious challenge for improvement in H₂ production efficiency. Herein, we are reporting an electrochemical process for HI decomposition and simultaneous H₂ and I₂ production. Commercial Nafion 117 membrane has been generally utilized as a separator, which also showed huge water transport (electro-osmosis), and deterioration in conductivity due to dehydration. We report sulphonated poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) (SCP) and sulphonated graphene oxide (SGO) composite stable and efficient polymer electrolyte membrane (PEM) for HI electrolysis and H₂ production. Different SCP/SGO composite PEMs were prepared and extensively characterized for water content, ion-exchange capacity (IEC), conductivity, and stabilities (mechanical, chemical, and thermal) in comparison with commercial Nafion117 membrane. Most suitable optimized SCP/SGO-30 composite PEM exhibited 6.78 × 10^?2 S cm^?1 conductivity in comparison with 9.60 × 10^?2 S cm^?1 for Nafion® 117. The electro-osmotic flux ofSCP/SGO-30 composite PEM (2.53 × 10^?4 cm s^?1) was also comparatively lower than Nafion® 117 membrane (2.75 × 10^?4 cm s^?1). For HI electrolysis experiments, SCP/SGO-30 composite PEM showed good performance such as 93.4% current efficiency (η), and 0.043 kWh/mol-H₂ power consumption (Ψ). Further, intelligent architecture of SCP/SGO composite PEM, in which hydrophilic SGO was introduced between fluorinated polymer by strong hydrogen bonding, high efficiency and performance make them suitable candidate for electrochemical HI decomposition, and other diversified electrochemical processes.     

Experimental study on water transport in membrane humidifiers for polymer electrolyte membrane fuel cells

This paper presents an experimental setup for the measurement of water transfer in membrane humidifiers for automotive polymer electrolyte membrane (PEM) fuel cells at different process conditions. This setup was used to determine steady-state water permeation through perfluorinated sulfonic acid (PFSA)-based polymer membranes. The process conditions were varied within a relative humidity in the feed stream of RH = 30–90 %, absolute pressures of p = 1.25–2.5 bar, and temperatures of T = 320–360 K. The examined membranes are Nafion® membranes of different thicknesses (Nafion® 211, 212 and 115) and an experimental composite membrane manufactured by W. L. Gore & Associates. It was found that the overall water permeance is affected by both the mass transfer resistance of the membrane and the resistances in the boundary layers of the adjacent gas streams. The overall permeance is a strong function of water activity, with high levels of relative humidity showing the highest overall permeance. The absolute pressure only affects the overall permeance by affecting the diffusion in the boundary layers. Lower pressures are preferable for high overall water permeances. Increasing temperatures favor diffusion in the membrane and the boundary layers but lead to lower sorption into the membrane. The thicker Nafion® membranes show lower overall permeance at higher temperatures, while the overall permeance of the composite membrane shows no dependency on the temperature. Investigation of membrane humidifiers in counter-, co-, and cross-flow shows that the flow configuration in our setup has very little impact on the water flux in the humidifier.     

In-situ sulfonation of targeted silica-filled Nafion for high-temperature PEM fuel cell application

Silica targeted filled in Nafion® membrane is in-situ sulfonated (s-WR-Nafion membrane) to improve the proton conductivity at high-temperature and low relative humidity (RH). The silica introduction as water sorbent in the membrane can both stabilize water balance for polymer electrolyte membrane fuel cells (PEMFCs) and prevent the formation of the water crystalline phase in membrane-electrode assemblies (MEAs) in course of sub-zero temperature operation (so-called cold start) owing to chemical water adsorption. On the other hand, water presence in MEAs provides required operating humidity when temperature increases. Due to the synergistic effect of the targeted filled silica and the efficient –SO₃H based proton conductive channel, high temperature proton conductivity of the Nafion® membrane is therefore significantly improved at low RH. Particularly, proton conductivity of the s-WR-Nafion membrane is raised to 0.07 S/cm at 110 °C and 60% RH. At the same time, mechanical, oxidative and thermal stabilities of the s-WR-Nafion membrane are not decayed due to the integrity of the original structure of Nafion®. As a result, the maximum output power of the s-WR-Nafion membrane-based fuel cell is ca. 65% higher than that of the pristine Nafion® membrane at 110 °C and 20% RH. Above all, the s-WR-Nafion membrane exhibit promising application potential for high-temperature PEMFC.     

Robust proton exchange membrane for vanadium redox flow batteries reinforced by silica-encapsulated nanocellulose

High ion selectivity and mechanical strength are critical properties for proton exchange membranes in vanadium redox flow batteries. In this work, a novel sulfonated poly(ether sulfone) hybrid membrane reinforced by core-shell structured nanocellulose (CNC-SPES) is prepared to obtain a robust and high-performance proton exchange membrane for vanadium redox flow batteries. Membrane morphology, proton conductivity, vanadium permeability and tensile strength are investigated. Single cell tests at a range of 40–140 mA cm⁻² are carried out. The performance of the sulfonated poly(ether sulfone) membrane reinforced by pristine nanocellulose (NC-SPES) and Nafion® 212 membranes are also studied for comparison. The results show that, with the incorporation of silica-encapsulated nanocellulose, the membrane exhibits outstanding mechanical strength of 54.5 MPa and high energy efficiency above 82% at 100 mA cm⁻², which is stable during 200 charge-discharge cycles.     

Exergy analysis of hydrogen production from different sulfur-containing compounds based on H2S splitting cycle

There is a tremendous demand for hydrogen production worldwide but the current H₂ production routes from natural gas and other carbon fuels lead to large greenhouse gas emissions. Intentionally coupled with nuclear power, the sulfur–iodine (S–I) thermochemical water splitting cycle is one of the most widely studied cycles for the large-scale hydrogen production that has environmental benignity. Based on the inspiration of the S–I cycle, a novel chemical cycle called hydrogen sulfide splitting cycle has been proposed for hydrogen production. In addition to the SO₂ production from the reaction of H₂S and sulfuric acid, SO₂ can be produced from the burning (direct oxidation) of hydrogen sulfide or elemental sulfur. And it can also be provided by SO₂ capture from flue gas or other SO₂-containing waste gases. This paper performs exergy analysis on the various SO₂ provisions to the Bunsen reaction that make different routes for hydrogen production from waste sulfur-containing compounds as feedstock. It has been found that the route including SO₂ from direct H₂S oxidation potentially makes the best energy-efficient process of H₂ production. The heat that is generated from H₂S oxidation can be recovered and used to support the energy requirements for other steps of the cycle, making the entire hydrogen production cycle more energy-efficient.     

Proton exchange membrane water electrolysis at high current densities: Investigation of thermal limitations

In this work the thermal limitations of high current density proton exchange membrane water electrolysis are investigated by the use of a one dimensional model. The model encompasses in-cell heat transport from the membrane electrode assembly to the flow field channels. It is validated by in-situ temperature measurements using thin bare wire thermocouples integrated into the membrane electrode assemblies based on Nafion® 117 membranes in a 5 cm² cell setup. Heat conductivities of the porous transport layers, titanium sinter metal and carbon paper, between membrane electrode assembly and flow fields are measured in the relevant operating temperature range of 40 °C – 90 °C for application in the model. Additionally, high current density experiments up to 25 A/cm² are conducted with Nafion® 117, Nafion® 212 and Nafion® XL based membrane electrode assemblies. Experimental results are in agreement with the heat transport model. It is shown that for anode-only water circulation, water flows around 25 ml/(min cm²) are necessary for an effective heat removal in steady state operation at 10 A/cm², 80 °C water inlet temperature and 90 °C maximum membrane electrode assembly temperature. The measured cell voltage at this current density is 2,05 V which corresponds to a cell efficiency of 61 % based on lower heating value. Operation at these high current densities results in three to ten-fold higher power density compared to current state of the art proton exchange membrane water electrolysers. This would drastically lower the material usage and the capital expenditures for the electrolysis cell stack.     

A predictive model and mechanisms associated with hydrogen production via hydrothermal reactions of sulfide

Hydrogen generation and the concurrent formation of sulfur products from hydrothermal reactions of aqueous sulfide solutions at pH values between 9 and 13 and temperatures between 280 and 330 °C were studied. A hydrogen production model was developed by kinetic and statistical analysis of sulfide consumption rates and the ratio of hydrogen produced to sulfide consumed. Results showed that the amount of hydrogen generated in a given reaction may be predicted by a series of equations incorporating starting conditions such as the initial sulfide concentration, pH and temperature. The data from this study suggested that the overall hydrogen generation reaction mechanism consists of one or more elementary reactions which result in the formation of various sulfur products, such as polysulfides and sulfur oxyanions, depending on the reaction conditions. The possible specific sulfur compounds included pentasulfide (S₅²⁻), thiosulfate (S₂O₃²⁻), trithionate (S₃O₆²⁻) and sulfate (SO₄²⁻). The production rate constants of these products increased with temperature, but were independent of pH. Additionally, it was indicated that increasing the reaction temperature and/or pH resulted in the formation of sulfur products with higher oxidation numbers. This work suggests that the optimal mechanism for hydrogen generation via the sulfur redox cycle, taking into account the requirement for sulfide regeneration, is that which forms trithionate as the sole sulfur product.     

Quantitative analysis of proton exchange membrane prepared by radiation-induced grafting on ultra-thin FEP film

Radiation-induced graft polymerization is introduced to effectively fabricate proton exchange membrane based on 12.5 μm fluorinated ethylene propylene (FEP) film. The graft side chains penetrate FEP film and distribute inside the bulk matrix evenly. The membranes exhibit hydrophilic/hydrophobic microphase-separated morphology as well as good thermal stability. The influences of irradiation parameters on the membrane property are investigated and the resulting membranes (named FEP-g-PSSA) exhibit excellent physicochemical properties. Membrane with 27.48% degree of graft and 130.1 mS cm^?1 proton conductivity is employed for fuel cell performance measurement. Under optimized operate conditions (80 °C, 75% relative humidity), the power density could reach up to 0.896 W cm^?2, inspiring for fuel cell application. The mass-transport-controlled polarization of membrane electrode assembly (MEA) based on FEP-g-PSSA membrane is higher than Nafion® 211 within the whole current density range and the gap is widening with increasing current density. At 2.0 A cm^?2, the mass transfer polarization of FEP-g-PSSA reaches up to 0.204 V, far higher than Nafion® 211 (0.084 V). By promoting the compatibility between the ionomer in the catalyst layer and FEP-g-PSSA membrane and optimizing the membrane/catalyst layer/gas diffusion layer interfaces, the fuel cell performance could be significantly enhanced, making the FEP-g-PSSA membranes promising in fuel cell application.     

Evaluation of proton-conducting membranes for use in a sulfur dioxide depolarized electrolyzer

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 SDEs 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% H₂SO₄ for 24 h. Following acid exposure, chemical stability for each membrane was evaluated by FTIR using the ATR sampling technique. Membrane SO₂ transport was evaluated using a two-chamber permeation cell. SO₂ was introduced into one chamber whereupon SO₂ transported across the membrane into the other chamber and oxidized to H₂SO₄ at an anode positioned immediately adjacent to the membrane. The resulting current was used to determine the SO₂ flux and SO₂ transport. Additionally, membrane electrode assemblies (MEAs) were prepared from candidate membranes to evaluate ionic conductivity and selectivity (ionic conductivity vs. SO₂ transport) which can serve as a tool for selecting membranes. MEAs were also performance tested in a HyS electrolyzer measuring current density vs. a constant cell voltage (1 V, 80 °C in SO₂ saturated 30 wt% H₂SO₄). Finally, candidate membranes were evaluated considering all measured parameters including SO₂ flux, SO₂ 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.     

Polybenzimidazole containing ether units as electrolyte for high temperature proton exchange membrane fuel cells

A rapid method to synthesize poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) through a solution polycondensation under microwave irradiation is explored. Synthesis parameters affecting the molecular weight (M_w) of OPBI, including the mass ratio of solvent to P₂O₅, the monomer concentration, and reaction time, are optimized. The main characteristics of OPBI are studied, and the corresponding membrane is prepared through a solvent casting process. A series of sulfuric acid doped OPBI (H₂SO₄/OPBI) hybrid membranes with different acid doping levels (ADLs) are developed. The effects of H₂SO₄ on microstructure, ADL and electrochemical properties of these membranes are explored. Herein, the hybrid membrane shows high proton conductivity (190 mS cm⁻¹) at elevated temperature (160 °C) and anhydrous conditions, high ADL (18.73 mol of H₂SO₄ for OPBI per repeat unit, i.e., ADL = 18.73 mol PRU⁻¹) and excellent dimensional stability (40.3%). All these properties demonstrated that H₂SO₄/OPBI hybrid membrane can be used as an alternative membrane for high temperature proton exchange membrane fuel cells (HT-PEMFCs).     

Comparison of ionically and ionical-covalently cross-linked polyaromatic membranes for SO2 electrolysis

It has been shown that hydrogen, which can be used for energy storage, can be produced efficiently by the membrane based Hybrid Sulfur (HyS) process. During the HyS electrolysis step, SO₂ and H₂O are converted to H₂ and H₂SO₄, which implies that membranes to be used for this process should have both a high proton conductivity and acid stability. In this study ionic and ionic-covalently cross-linked polybenzimidazole (PBI) blended membranes were investigated and compared with Nafion^®212 in terms of their acid stability. Characterization of the membranes, which included monitoring the change in weight, swelling, SEM/EDX, TEM, TGA-MS, FTIR and IEC before and after H₂SO₄ treatment showed that all tested membranes were stable in 80 wt% H₂SO₄ at 80 °C for 120 h. Subsequent HyS electrolysis showed that the blend membranes performed better than Nafion^®115 at current densities below 0.3 A/cm², while performing similar above 0.3 A/cm².     

Numerical Modeling of Bayonet-Type Heat Exchanger and Decomposer for the Decomposition of Sulfuric Acid to Sulfur Dioxide

The growth of global energy demand during the 21st century, combined with the necessity to master greenhouse gas emissions, has led to the introduction of a new and universal energy carrier: hydrogen. The Department of Energy (DOE) proposed using a bayonet-type heat exchanger as a silicon carbide integrated decomposer (SID) to produce the sulfuric acid decomposition product sulfur dioxide, which can be used for hydrogen production within a sulfur–iodine thermochemical cycle. A two-dimensional computational model of SID having a boiler, superheater and decomposer was developed using GAMBIT and fluid. The thermal and chemical reaction analyses were carried out in FLUENT. The main purpose of this study is to obtain the decomposition percentage of sulfur trioxide for the integrated unit. Sulfuric acid (H₂SO₄), sulfur trioxide (SO₃), sulfur dioxide (SO₂), oxygen (O₂), and water vapor (H₂O) are the working fluids used in the model. Concentrated sulfuric acid liquid of 40 mol% was pumped into the inlet of the boiler and the mass fraction of concentrated sulfuric acid vapor obtained was then fed into the superheater to obtain sulfur trioxide. The decomposer region, which houses the pellets, placed on the top of the bayonet heat exchanger acts as the porous medium. As the decomposition takes place, the mass fraction of SO₃ is reduced and mass fractions of SO₂ and O₂ are increased. The percentage of SO₃ obtained from the integrated decomposer was compared with the experimental results obtained from Sandia National Laboratories (SNL). Further, effects of various pressures, flow rates, and acid concentrations on the decomposition percentage of sulfur trioxide were studied.     

Parametric study on electrochemical reforming of glycerol for hydrogen production

Hydrogen production by electrochemical reforming of glycerol was investigated in this study. Within this scope, the performance of the system under different operating conditions was evaluated by parametric studies and optimum operating conditions were determined. The effects of membrane type, membrane pre-treatment procedure and temperature were investigated. System performance was examined also with long-term tests. The formation of hydrogen at the cathode was determined by analyzing the product gases by gas chromatography. Optimum condition for maximum hydrogen production was obtained with the Zn/Zn electrode pair in the presence of 0.4 M glycerol and 0.04 M H₂SO₄ at the anode side, 0.04 M H₂SO₄ at the cathode side and with pre-treated Nafion XL membrane. As the result of performance tests, room temperature and 2 V potential were found to be the most suitable operating conditions.