In-situ and ex-situ comparison of the electrochemical oxidation of SO2 on carbon supported Pt and Au catalysts

Electrochemical characterizations are performed using thin films and commercial carbon supported platinum and gold catalysts for sulfur dioxide oxidation, the primary electrochemical oxidation reaction in the Hybrid-sulfur (HyS) thermochemical process. Electrochemical evaluation of metal thin films qualitatively confirms the higher activity of Au over Pt, AuPt, Pd, and Ir for the electrochemical oxidation of SO₂. Ex-situ testing, using rotating disk electrode (RDE), shows an earlier onset potential for Au/C at low sulfuric acid concentrations (C ≤ 3.5 M) and a higher turnover frequency than Pt/C at sulfuric acid concentrations ranging from 3.5 M to 9 M. In-situ electrolysis experiments using low catalyst loadings (0.1 mg_Au cm⁻², a factor of ≥5 lower than typical loadings) confirm that Au nanoparticles exhibit higher current densities and greater stability than Pt nanoparticles. This is consistent with the thin film screening studies, which showed higher activity with increasing gold content in AuPt thin films. This work reveals an alternative material to state-of-the-art Pt to lower the energy needs and aid the HyS cycle in reaching the target of $2/kg H₂ set forth by the Department of Energy to achieve economic feasibility of large-scale hydrogen generation.     

Performance degradation of solid oxide fuel cells due to sulfur poisoning of the electrochemical reaction and internal reforming reaction

Hydrogen sulfide is known to degrade the solid oxide fuel cell (SOFC) performance by adsorbing on the nickel anode catalyst. In this research, the mechanism underlying such SOFC degradation was evaluated based on a theoretical mathematical modeling approach and the sulfur coverage was calculated from a Temkin-like isotherm which is related to both temperature and hydrogen sulfide (H₂S) concentration. The influences of the cell temperature, H₂S concentration and electrochemical performance on both the sulfur coverage and cell polarization are studied in detail. Two specific models were considered to identify whether sulfur poisoning has a larger impact on cell performance through its effect on the electrochemical reaction or on the internal reforming reaction. It was found that sulfur poisoning has different effects on the hydrogen oxidation reaction and internal reforming reaction, leading to competing changes in cell performance with temperature and H₂S concentration.     

Voltage pulsing for performance recovery of yttria-stabilised zirconia membranes in oxygen/sulfur dioxide separation

The regeneration of yttria-stabilized zirconia (YSZ) membranes exposed to high concentration sulfur dioxide in oxygen at 850 °C using DC voltage pulses was investigated by in-situ impedance spectroscopy. The membranes consisted of a dense YSZ layer as the solid electrolyte coated with two platinum layers as electrodes. On operation in the presence of SO₂, the serial resistance and polarization resistance of the Pt/YSZ cell increased. This is most likely due to the formation of sulfide at the interface area of the electrode and electrolyte combined with sulfur adsorption on electrode surface. DC Voltage pulses were found to have an effect on the charge transfer and mass transfer properties of the Pt/YSZ cell, assisting the removal of sulfur on the cathode surface and leading to performance recovery of the cell. In these experiments, the greatest rate of membrane performance recovery is achieved with a cathodic DC bias of 10 V, applied for 0.08 s. Higher or longer voltage pulses may cause the rate controlling step for the oxygen reduction reaction to shift to oxygen supplied in feed from oxygen surface exchange and diffusion processes. A relatively steady membrane performance was achieved during 20 h SO₂ exposure tests. It is concluded that DC voltage pulses show promise as a method for reducing the performance degradation effects of poisoning due to sulfur containing gases in the fields of fuel cells and in the sulfur family of thermochemical cycles.     

A nano-structured and highly ordered polypyrrole-sulfur cathode for lithium-sulfur batteries

A tubular polypyrrole (T-PPy) fiber is synthesized as a conductive matrix for the cathode of lithium-sulfur secondary battery. The sublimed sulfur is incorporated with the T-PPy by a co-heating process. The location and the content of sulfur show a significant effect on the electrochemical behavior of the composite. A reversible capacity of ca. 650 mAh g⁻¹ is maintained for over 80 cycles for the S/T-PPy composite with 30 wt.% sulfur. The enhanced conductivity, the favorable distribution of the nano-sized sulfur in the T-PPy and the stable retention of polysulfides lead to the improvement of the cycling stability of the sulfur based electrode.     

Effect of H2S in methane/air flames on sulfur chemistry and products speciation

Reaction behavior of H₂S/O₂ under different equivalence ratios in methane/air flames is examined. Three equivalence ratios extending from fuel-lean (Φ = 0.5), stoichiometric (Φ = 1.0), to fuel-rich (Claus condition, Φ = 3.0) are examined. The results revealed that the presence of H₂S prevents hydrogen oxidation in the primary reaction zone, while in the secondary reaction zone oxidation competition occurs between H₂ and H₂S. In presence of oxygen, oxidation of hydrogen sulfide forms sulfur dioxide. However, under Claus conditions, the depletion of oxidant causes the direction of hydrogen sulfide reaction to shifts towards the formation of elemental sulfur. Higher hydrocarbons are formed in trace amounts under Claus conditions wherein sulfur dioxide acts as a coupling catalyst which enhances the dimerization of CH₃ radical to form higher series of hydrocarbons. Under Claus conditions, sulfur deposits are formed in low temperature regions of the reactor including the sampling line. The deposits are analyzed using X-ray powder diffractometer and were found to be cyclo-S₈ (α-sulfur) with orthorhombic crystal structure. The formation of α-sulfur is mainly due to the agglomeration of elemental sulfur (S₂) during its condensation at low temperatures.     

Sulfur trioxide electrolysis studies: Implications for the sulfur–iodine thermochemical cycle for hydrogen production

In this paper we describe our efforts to develop a sulfur trioxide (SO₃) electrolyzer that could lower the temperature of the SO₃ decomposition step in the sulfur–iodine and hybrid sulfur thermochemical cycles. The objective is to develop an alternative to the standard process of converting SO₃ to SO₂, which is thermal decomposition at 830 °C and above. Thermodynamic calculations show that high SO₃ conversions can be obtained at 590 °C if oxygen is removed during the SO₃ decomposition stage. One way of achieving oxygen removal during SO₃ decomposition is electrolysis, if suitable electrode and electrolyte materials can be found. Active oxygen electrode materials are already developed and we have demonstrated suitability of a thin doped-zirconia electrolyte in this study. The main difficulty came in the development of an active and stable SO₃ electrode. Using Ga–V–O/NbB₂/Au electrodes we demonstrated high catalytic activity, but could not achieve acceptable electrochemical performance.     

Numerical analysis of the electrochemical Bunsen reaction for hydrogen production from sulfur–iodine cycle

The sulfur–iodine (S-I) water-splitting cycle is one of the most promising hydrogen production methods. The Bunsen reaction in the cycle affects the flowsheet complexity and thermal efficiency, but an electrochemical technique has recently been applied to make the S-I cycle more simplified and energy efficient. However, the performance of the electrochemical Bunsen reaction, especially the electrode reactions inside the electrolytic cell (EC) are not clear at present. In this work, a two-dimensional numerical model of EC was developed. The detailed reaction process was numerically calculated with considering the coupling of mass transfer and electrochemical reactions, and was verified using experimental data. The effects of various operating parameters on the reactions were investigated. The results showed that the increase of current density significantly improves the conversion rates of reactants. A higher temperature is unfavorable for concentrating H₂SO₄ and HI. Increase in the inlet flow rate reduces the conversion rates of reactants, but the impact declines with further rising flow rate. An optimal operating condition is also proposed. The theoretical simulation study will provide guidance for the improvement of experimental work.     

Electrochemical corrosion of platinum electrode in concentrated sulfuric acid

We report on the electrochemical corrosion of a Pt electrode in strong sulfuric acid. The electrochemical measurements were conducted using a Pt-flag working electrode, Ag/Ag₂SO₄ reference electrode and Pt counter electrode at 25 °C. The measured cyclic voltammograms significantly changed in the H₂SO₄ concentration range of 0.5–18 mol dm⁻³, especially from 14 to 18 mol dm⁻³. After successive potential sweeps for 15 h in 16 mol dm⁻³ H₂SO₄, a weight loss of the Pt-flag electrode was realized. In contrast, a controlled potential electrolysis by cathodic polarization caused a weight gain, which was attributed to sulfur deposition by the H₂SO₄ reduction. The subsequent anodic polarization produced corrosion of the deposited sulfur. Consequently, the alternating polarization generated platinum corrosion, resulted in the production of platinum and sulfur composite particulates in the solution.     

Performance of direct alkaline sulfide fuel cell without sulfur deposition on anode

The paper reports the use of alkaline sulfide as a fuel with advantages of low cost, easy storage and transportation, and high electrochemical activity. The fuel cell fueled with alkaline sulfide, named direct alkaline sulfide fuel cell (DASFC), oxidizes sulfide to sulfur oxyanion without electrochemical catalyst, allowing not only to remove H₂S but also to fully recover energy stored in H₂S as electricity. In this study, particular attention was paid to systematic investigation of the effects of various operating parameters on cell performance, such as NaOH concentration, sulfide concentration, temperature, and electro-catalyst. Higher alkalinity and sulfide concentration of the anolyte were found to lead to higher power output, recording maximum power density of 10.69 mW cm⁻² at 1.0 M sulfide, 3.0 M NaOH, and 80 °C, which greatly increased up to 25.86 mW cm⁻² with the aid of Pt/C. During discharge, DASFC with higher alkaline anolyte exhibited higher current density profile, resulting in more oxidized sulfur oxyanions to be dominant. In the SEM analysis, the anode surface after discharge did not show any distinctive change from the original state, indicating that the anode of DASFC is free from sulfur deposition even at ambient temperature. Considering electricity generation, recovery of sulfur oxyanion, and long-term stability, we tend to believe that DASFC is one sustainable, promising way of tackling H₂S problem.     

Development of the once-through hybrid sulfur process for nuclear hydrogen production

The Once-through Hybrid Sulfur (Ot-HyS) process, proposed in this work, produces hydrogen using the same Sulfur dioxide Depolarized water Electrolysis (SDE) process found in the original Hybrid Sulfur cycle (HyS). In the process proposed here, the Sulfuric Acid Decomposition (SAD) process in the HyS procedure is replaced with the well-established sulfur combustion process. First, a flow sheet for the Ot-HyS process was developed by referring to existing facilities and to the work done by the Savannah River National Laboratory (SRNL) under their reasonable assumptions. The process was then simulated using Aspen Plus with appropriate thermodynamic models. It was demonstrated that the Ot-HyS process has higher net thermal efficiency, as well as other advantages, over competing benchmark processes. The net thermal efficiency of the Ot-HyS process is 47.1% (based on LHV) and 55.7% (based on HHV) assuming 33.3% thermal-to-electric conversion efficiency of a nuclear power plant with no consideration given to the work for the air separation. Hydrogen produced through the Ot-HyS process would be used as off-peak electricity storage, to relieve the burden of load-following and could help to expand applications of nuclear energy, which is regarded as a ’sustainable development’ technology.     

Highly dispersed sulfur in ordered mesoporous carbon sphere as a composite cathode for rechargeable polymer Li/S battery

A mesoporous carbon sphere with the uniform channels (OMC) is employed as the conductive matrix in the sulfur cathode for the lithium sulfur battery based on all-solid-state PEO₁₈Li(CF₃SO₂)₂N-10 wt%SiO₂ electrolyte. Cyclic voltammograms (CV) and electrochemical impedance spectrum (EIS) suggest that the electrochemical stability of the S-OMCs is obviously superior to the pristine sulfur cathode. The S-OMCs composite shows excellent cycling performance with a reversible discharge capacity of about 800 mAh g⁻¹ after 25 cycles. This would be attributed to an appropriate conductive structure in which the active sulfur is highly dispersed in and contacted with the OMCs matrix.     

Process integration of sulfur combustion with claus SRU for enhanced hydrogen production from acid gas

The commercial Claus sulfur recovery process is intended for treating H₂S present in acid gas by recovering sulfur. During this process, hydrogen present in H₂S is inadvertently converted to low grade steam. In the current study, an improved technique for recovering hydrogen and sulfur from acid gas containing H₂S was developed using Aspen HYSYS®. Hydrogen production by thermal decomposition of H₂S was achieved in the tubes of a waste heat exchanger connected in-series with a reaction furnace and followed by Claus sulfur recovery unit (SRU). The energy requirement for the decomposition reaction was supplied through elemental sulfur combustion in the reaction furnace. While H₂S decomposition was defined by a kinetic model in a plug flow reactor, sulfur combustion and H₂S-SO₂ combustion processes were described using Sulsim? Sulfur Recovery model in Aspen HYSYS®. A commercial Claus sulfur recovery unit (SRU) located in Abu Dhabi was considered for process development. Two different process integration schemes differing in hydrogen recovery layout design were analyzed. Based on various performance indicators, including hydrogen and sulfur yields, H₂S conversion rate, and sulfur combustion rate, the most feasible process configuration for maximizing overall process efficiency was identified. The proposed integrated process has the capability for generating hydrogen yield as high as 33% and a simultaneous sulfur recovery of nearly 99%. In addition, the developed processes can significantly curtail the handling load on catalytic section by 11.3% and 16%, respectively, in terms of catalyst bed volume.     

Electrocatalytic hydrogen evolution reaction on sulfur-deficient MoS2 nanostructures

Molybdenum disulfide (MoS₂) is a 2D layered structured material with a Mo:S of 1:2 and is a great attention seeker for hydrogen production through water-splitting. In the present work, we prepared nanostructured MoS_x with different sulfur molar concentrations (x = 2, 1, 0.5) through a one-step hydrothermal method. The decrease in sulfur concentration resulted in a new phase that is MoO₃ with a Mo:S of 1:0.5. The structural, morphological, and optical properties of all the samples were studied through X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR), and Ultraviolet–Visible (UV–Vis) spectroscopy, respectively. Moreover, the electrochemical behavior was studied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and Tafel slope. Optimum properties were observed for Mo:S (1:1) with an onset potential of 96 mV, an overpotential of 130 mV for hydrogen evolution reaction (HER) coupled with a specific capacitance of 889 F/g and low charge transfer resistance of 43 Ω. Further, it was noted that the electrocatalytic activity of MoS₁ was better than that of the composite and bare MoO₃. It is proposed that the excellent electrochemical activity arises from sulfur vacancies which provide active sites for HER and a free path for ions to flow through the material.     

Self-grown one-dimensional nickel sulfo-selenide nanostructured electrocatalysts for water splitting reactions

Herein, one-dimensional (1-D) self-grown nickel sulfide (NiS), and nickel sulfo-selenide (NiSSe) nanostructures on Ni-foam are successfully prepared via a simple and low-cost hydrothermal synthesis route. The S²⁻ and Se²⁻ ions are obtained after decomposition of sulfur and selenium precursors, which on reacting with oxidized Ni²⁺ ions, from the surface of pristine Ni-foam, produces NiS, and NiSSe superstructures. The crystal phase, surface morphology and chemical states of as-grown electrocatalysts are monitored through various measurment tools. Due to synergistic effects, the NiSSe nanostructured electrode provides a superior over potentials as low as 154 and 75 mV for OERs and HERs activity in electrochemical water splitting measurement. Moreover, the NiSSe electrode exhibits an excellent chemical stability compared to the NiS and NiSe electrodes during the electrolysis process. Such an outstanding catalytic performance makes the NiSSe electrode as a potential candidate in water splitting applications.     

Effects of carbon coating on the electrochemical properties of sulfur cathode for lithium/sulfur cell

Carbon-coated sulfur cathodes were prepared by sputtering method and electrochemical properties of lithium/sulfur cells were investigated. As a result of charge/discharge test, sulfur cathode having carbon layer of 180 Å showed the highest capacity of 1178 mA hg⁻¹ at first discharge. Moreover, discharge capacity showed about 500 mA hg⁻¹ until 50th cycle, which is two times larger than that of no-coated sulfur cathode. This capacity increase could be considered due to the decrease of polysulfides dissolution and the enhancement of electrical contact by surface carbon layer. The changes of sulfur cathode during discharge process were investigated by SEM observation, XRD and DSC measurements. From these results, a discharge reaction mechanism of lithium/carbon-coated sulfur cell was suggested.     

Systematically reduced chemical mechanisms for sulfur oxidation and pyrolysis

Recent research on sulfur chemistry has predominantly focused on the high-temperature chemistry typical of flames. The present work initially assesses the ability of a sulfur submechanism featuring 12 sulfur-containing species and 70 reversible reactions to reproduce measured data. The functionality includes the pyrolysis and oxidation of hydrogen sulfide as well as the chemistry of sulfur dioxide. The sensitivity of reaction paths to alternative rate determinations has been analyzed. In particular, the consumption paths of sulfanyl and the rates of the reactions involved in the SO₂-catalyzed radical recombination highlight existing uncertainties. Despite these difficulties, the detailed mechanism generally produces good agreement with experimental data. Most industrial combustion processes are turbulent and turbulence-chemistry interactions cannot be included through high-Damköhler-number limit approximations. Accordingly, chemical kinetic effects need to be accounted for and the implementation of systematically reduced mechanisms has the potential to increase the computational efficiency. The detailed reaction mechanism is thus subsequently reduced to six independent scalars with HSO, HOSO, HOSO₂, HSSH, and S in steady state. The reduced mechanism provides good agreement over the range of conditions studied. Further simplifications are made in the context of oxides of sulfur and a two-step mechanism involving SO, SO₂, and SO₃ is derived and shown to retain good agreement with the experimental data for a more limited set of conditions.     

Morphology and electrochemical behaviour of ruthenium oxide thin film deposited on carbon paper

In order to improve the efficiency of ruthenium dioxide, RuO₂, as an electrochemical capacitor electrode, a RuO₂ thin film is deposited on carbon paper and its structure and properties are evaluated. This new composite material is prepared via solution dip-coating of a Ru-ethoxide precursor and heat conversion. The coating thickness is easily controlled by varying the number of repetitions of the preparation process. The resulting structure consists of a by homogeneously coated RuO₂ film on carbon paper which has a porous graphite matrix. Extensive electrochemical studies have been performed in 1 M H₂SO₄ electrolyte in order to evaluate the properties of the composite as an electrode in an electrochemical capacitor. The composite material shows not only high specific capacitance (620 F g⁻¹) but also good power characteristics.     

Molecular dynamic investigation on sulfur migration during hydrogen production by benzothiophene gasification in supercritical water

Benzothiophene (BT) is a key sulfur-containing intermediate product in the thermal conversion process of coal and heavy oil. The migration process of the sulfur element may affect the thermal utilization design of BT. In this paper, BT was used as a model compound to simulate the supercritical water gasification (SCWG) process by molecular dynamics with a reactive force field (ReaxFF) method, and the laws of hydrogen production and sulfur migration mechanisms were obtained. Increasing the molecule number of supercritical water (SCW) and increasing the reaction temperature can enhance the generation of hydrogen and promote the conversion of organic sulfur to inorganic sulfur. Water was the main source of H₂, and H₂S was the main gaseous sulfur-containing product. SCW had a certain degree of oxidation due to a large number of hydroxyl radicals, which could increase the valence of sulfur. The conversion process of BT in SCW was mainly divided into four stages, including thiophene ring-opening; sulfur separation or carbon chain broke with sulfur retention; carbon chain cleaved, and gas generation. The lumped kinetic parameters of the conversion of sulfur in BT to inorganic sulfur were calculated, and the activation energy was 369.98 kJ/mol, which was much lower than those under pyrolysis conditions. This article aims to clarify the synergistic characteristics of hydrogen production and sulfur migration in the SCWG process of BT from the molecular perspective, which is expected to provide a theoretical basis for pollutant directional removal during hydrogen production by sulfur-containing organic matters in SCW.     

Pulse-reverse electrodeposition of Ni–Mo–S nanosheets for energy saving electrochemical hydrogen production assisted by urea oxidation

Electrochemical hydrogen production from water splitting is one of the effective methods for hydrogen production that has recently attracted particular attention. One of the limitations of the electrochemical water splitting method is the slow oxygen evolution reaction (OER), which leads to an increase in overpotential and a decrease in hydrogen production efficiency. Here, Ni–Mo–S ultra-thin nanosheets were synthesized using the pulse reverse electrochemical deposition technique, and then this electrode was used as an electrode material for accelerating hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Remarkably, the optimized electrode needs only 74 mV to attain the 10 mA cm⁻² current density in HER and require only 1.3 V vs RHE potential in the UOR process. Also, results showed that the replacement of the UOR with the OER process resulted in a significant improvement in the electrochemical production of hydrogen in which for delivering the current density of 10 mA cm⁻² in overall urea electrolysis, only 1.384 V is needed. In addition, outstanding catalytic stability was obtained, after 50 h electrolysis, the voltage variation was negligible. Such outstanding catalytic activity and stability was due to 3-D ultrathin nanosheets, the synergistic effect between elements, and the superhydrophilic/superaerophobic nature of fabricated electrode.     

Fabrication and electrochemical characterization of cobalt-based layered double hydroxide nanosheet thin-film electrodes

A continuous cobalt-based layered double hydroxide (LDH) nanosheet thin-film electrode has been fabricated by drying a nearly transparent colloidal solution of cobalt-based LDH nanosheets on an indium tin oxide (ITO)-coated glass plate substrate. The effects of varying the Al content, the film thickness, and the heating temperature on the electrochemical properties of the as-deposited thin-film electrode have been investigated. A thin-film electrode with a Co/Al molar ratio of 3:1, which has a large specific capacitance of 2500 F cm⁻³ (833 F g⁻¹) and a good high-rate capability, shows the best performance when used as an electrode in thin-film supercapacitors (TFSCs). As the thickness of the thin film was increased from 100 to 500 nm, the specific capacitance of the thin-film electrode remained essentially unchanged, which is due to the porous microstructure generated in the original electrochemical process and the low internal resistance of the thin-film electrode. The specific capacitance of the thin-film electrode showed no observable change after heating at 160 °C, but decreased on further heating to 200 °C, indicating that the electrochemically active Co sites inside the thin-film nanosheet electrode are already essentially fully exposed in the as-prepared material and hence cannot be further exposed through heating. Such a thin-film electrode made up of nanosheets may be a potential economical alternative electrode for use in TFSCs.