Hydrogen sulfide poisoning and recovery of PEMFC Pt-anodes

A simple and effective method for reactivation of H₂S poisoned Pt-anodes is described and the feasibility of the method was examined by single cell tests and 1 kW stack tests. The performance of the H₂S poisoned Pt-anode can be basically recovered by applying a high voltage pulse (1.5 V for 20 s) followed by a low voltage pulse (0.2 V for 20 s) in a single cell. During the 10 poisoning–recovery cycles, the ohmic resistance and electrochemical surface area did not change significantly. The 1 kW stack tests show that the stack performance decayed severely and the maximum power decreased to 0.366 kW (32% of the original value) after exposure to 18 ppm H₂S/H₂ for 2 h at 600 mA cm⁻². The stack performance can be significantly recovered by applying a high voltage pulse (1.5 V for 2 min) followed by a low voltage pulse (0.2 V for 2 min) to each cell. The maximum power recovered to 1.095 kW (97.5% of the original value).     

Plasma assisted dissociation of hydrogen sulfide

Dissociation of hydrogen sulfide has been studied in four different discharges: AC corona, dielectric barrier, streamer, and contracted glow discharge. All experiments were done in a single geometry, close to a plug flow reactor, with the goal of fair comparison. The performance of corona discharge and DBD was studied in the initial gas temperature range of 300–1200 K. A specific energy requirement (SER) was calculated as function of energy input for each type of discharge and compared with earlier experimental results and modeling. The results showed that discharges with high E/n and low specific energy input (corona, DBD, and streamer) perform much worse than those with low E/n (contracted glow discharge) where specific energy input was high and gas temperature was elevated. The SER for non-thermal dissociation was 12–14 eV/molec. However, in the case of the contracted glow discharge, SER decreased to 2.4 eV/molec. This SER is close to the value predicted by thermodynamic equilibrium modeling. Further reduction of SER in a plug flow reactor does not seem possible.     

Mechanism for SOFC anode degradation from hydrogen sulfide exposure

Recent results on solid oxide fuel cells with Ni/YSZ and Ni/GDC anodes reveal a mechanism for permanent performance degradation due to hydrogen sulfide exposure. Our results confirm the temporary performance decline observed by others but also reveal a mechanism for the long term permanent degradation. We find that hydrogen sulfide leads to nickel migration and depletion in the anode, thereby compromising electrical conductivity and cell performance.     

Chemical reactor network modeling of a microwave plasma thermal decomposition of H2S into hydrogen and sulfur

The conventional treatment method for H₂S is the Claus process, which produces sulfur and water. This results in a loss of the valuable potential product hydrogen. H₂S treatment would be more economically valuable if both hydrogen and sulfur products could be recovered. Based on standard heats of formation analysis, the theoretical energy required to produce hydrogen from H₂S dissociation is only 20.6 kJ/mol of H₂ as compared to 63.2 kJ/mol of H₂ from steam methane reforming and 285.8 kJ/mol of H₂ from water electrolysis. Among the many thermal decomposition methods that have been explored in the literature, Micro-wave plasma dissociation of H₂S prevails as the method of choice to attain the best conversion and energy efficiency. Chemical kinetics simulations using an ideal flow reactor network have been carried out on the CHEMKIN-PRO software package and they support these findings. The reactor network simulates the thermal plasma behavior in the plasma torch, the plasma reactor, and the sulfur condenser. Two chemical kinetics mechanisms have been used and the results show an almost complete conversion of H₂S into hydrogen and sulfur in the plasma reactor at an optimum temperature of about 2400 K at atmospheric pressure. While the most challenging task of the process is found to be the plasma cooling rate at the sulfur condenser where very fast quenching is required to conserve the hydrogen product from converting back to H₂S.     

Hydrogen sulfide tolerance of palladium–copper catalysts for PEM fuel cell anode applications

The H₂S-tolerance of Palladium–Copper nanoparticle catalysts supported on carbon black employed on PEMFC anodes operating on H₂S-contaminated anode feeding streams is described. The anode feeding consists of 80% H₂, ∼20% N₂ (in vol.), and 8 ppm H₂S. Catalyst performances were evaluated by estimating the cell potential reduction due to H₂S contamination through lifetime tests. It is found that the H₂S-tolerance of PdCu catalysts depends on the concentration of copper, as confirmed by a catalyst pre-leaching procedure. This methodology is employed in order to remove copper atoms from the catalyst surface, which is confirmed by energy dispersive X-ray spectroscopy and X-ray diffraction techniques. This method generates catalysts with lower copper content and higher susceptibility to H₂S. Moreover, it is proposed a new and effective catalytic activity recovering technique to restore the performance of PdCu catalysts after H₂S exposure.     

Highly efficient TiO2 nanotube photocatalyst for simultaneous hydrogen production and copper removal from water

1-D mesoporous TiO₂ nanotube (TNT) with large BET surface area was successfully synthesized by a hydrothermal-calcination process, and employed for simultaneous photocatalytic H₂ production and Cu²⁺ removal from water. Cu²⁺, across a wide concentration range of 8-800 ppm, was removed rapidly from water under irradiation. The removed Cu²⁺ then combined with TNT to produce efficient Cu incorporated TNT (Cu-TNT) photocatalyst for H₂ production. Average H₂ generation rate recorded across a 4 h reaction was between 15.7 and 40.2 mmol h⁻¹ g⁻¹_catalyst, depending on initial Cu²⁺/Ti ratio in solution, which was optimized at 10 atom%. In addition, reduction process of Cu²⁺ was also a critical factor in governing H₂ evolution. In comparison with P25, its large surface area and 1-D tubular structure endowed TNT with higher photocatalytic activity in both Cu²⁺ removal and H₂ production.     

The effects of hydrogen sulfide on the polymer electrolyte membrane fuel cell anode catalyst: H2S-Pt/C interaction products

The performance of a polymer electrolyte membrane fuel cell (PEMFC) operating on a simulated hydrocarbon reformate is described. The anode feed stream consisted of 80% H₂, ∼20% N₂, and 8 ppm hydrogen sulfide (H₂S). Cell performance losses are calculated by evaluating cell potential reduction due to H₂S contamination through lifetime tests. It is found that potential, or power, loss under this condition is a result of platinum surface contamination with elemental sulfur. Electrochemical mass spectroscopy (EMS) and electrochemical techniques are employed, in order to show that elemental sulfur is adsorbed onto platinum, and that sulfur dioxide is one of the oxidation products. Moreover, it is demonstrated that a possible approach for mitigating H₂S poisoning on the PEMFC anode catalyst is to inject low levels of air into the H₂S-contaminated anode feeding stream.     

Thermal efficiency evaluation of the thermochemical H2S splitting cycle for the hydrogen and sulfur production

A flowsheet of the thermochemical H₂S splitting cycle was designed and simulated for hydrogen and sulfur production. The heat and mass balance, as well as the thermal efficiency of the process, were calculated. A thermal efficiency of 40.865% for hydrogen production was obtained by optimizing the heat exchangers and the EED cell considering waste heat recovery. The effects of five calculation parameters, namely, the sulfuric acid concentration, hydrogen iodide (HI) conversion ratio, molar flow rate of HI_x phase, pressure, and reflux ratio at the distillation column, on thermal efficiency were evaluated. The results indicated that further research on the membrane reactor is needed. The optimized conditions for the over-azeotropic HI solution yield should be prioritized. Furthermore, an H₂SO₄ concentration system should be reasonably designed to reduce the complexity of the process and equipment settings, as well as to improve thermal efficiency.     

Robust optimization of hydrogen network

Process integration is an effective way to reduce hydrogen utility consumption in refineries. A number of graphical and mathematical programming approaches have been proposed to synthesis the optimal network. However, as the operation of refineries encounters uncertainty with the rapidly changing market and deteriorating crude oil, existing approaches are inadequate to achieve robust hydrogen network distribution due to the uncertain factors. In this paper, robust optimization is introduced as a framework to optimize hydrogen network of refineries under uncertainty. In this framework, a number of scenarios representing possible future environments are considered. Both model robust and solution robust are explicitly incorporated into the objective function. A possible optimal network distribution which is less sensitive to the change of scenarios and has the minimum total annual cost is achieved by the tradeoff between the total annual cost and the expected error. Case studies indicate that this method is effective in dealing with hydrogen network design and planning under uncertainty in comparison to the deterministic approach and the stochastic programming method.     

Hydrogen amplification from coke oven gas using a CO2 adsorption enhanced hydrogen amplification reactor

To improve coke oven gas (COG) energy conversion, alternative configurations for amplifying hydrogen from COG are proposed in this paper. In these new configurations, a CO₂ adsorption enhanced hydrogen amplification reactor is combined with a pressure swing adsorption separation unit (PSA) to produce pure hydrogen. Hydrogen production was integrated with desorption gas utilization, in situ CO₂ capture and waste heat recovery to improve COG energy conversion efficiency and decrease CO₂ emissions. To analyze the advantages of the flowsheet modifications, technical and economic performance indicators were used to evaluate and compare the performances of the various system configurations. Simulation results show that the alternative configurations proposed in this paper have higher energy conversion efficiencies, higher hydrogen yields and shorter dynamic payback periods. The variation of technical performance with reaction temperature, pressure, sorbent to carbon ratio and steam to carbon ratio were also analyzed using a sensitivity study. Optimal operating conditions for the CO₂ adsorption enhanced hydrogen amplification reactor were obtained based on the simulation results.     

Effects of general zero-valent metals power of Co/W/Ni/Fe on hydrogen production with H2S as a reductant under hydrothermal conditions

Hydrogen production from water with S²⁻ as a reductant under hydrothermal conditions is an effective new method, in which S²⁻ is oxidized into S₂O₃²⁻, SO₃²⁻ and SO₄²⁻. However, the reactor wall had great effect on hydrogen production as large amount hydrogen is produced with Hastelloy C-22 reactor while almost no hydrogen generated in SUS 316 reactor. Therefore, the influence of main components of Hastelloy C-22 reactor (Co/W/Ni) and SUS 316 reactor (Fe) on hydrogen production with H₂S as the reductant was investigated. The results showed that Fe had negative effect, whereas W, Co and Ni had significant positive effect on improving hydrogen production. These results provided a possible explanation for no hydrogen generated with SUS 316 reactor,and some suggestions for improving hydrogen production. The highest hydrogen production of 199 mL (2 times than the control) was obtained with 4.00 mmol Co, 4.00 mmol W, and 1.00 mmol Ni.     

H2S removal from biogas for fuelling MCFCs: New adsorbing materials

Cu and Zn modified 13X zeolites prepared by ion exchange or impregnation and activated carbons (ACs) treated with KOH, NaOH or Na₂CO₃ solutions were studied as H₂S sorbents for biogas purification for fuelling molten carbonate fuel cells. H₂S sorption was studied in a new experimental set-up equipped with a high sensitivity potentiometric system for the analysis of H₂S. Breakthrough curves were obtained at 40 °C with a fixed bed of 20 mg of the samples under a stream (6 L h⁻¹) of 8 ppm H₂S/He mixture. The adsorption properties of 13X zeolite improved with addition of Cu or Zn:Cu exchanged zeolite showed the best performances with a breakthrough time of 580 min at 0.5 ppm H₂S, that is 12 times longer than the parent zeolite. In general, unmodified and modified ACs were more effective H₂S sorbents than zeolites. Treating ACs with NaOH, KOH, or Na₂CO₃ solutions improved the H₂S adsorption properties: AC treated with Na₂CO₃ was the most effective sorbent, showing a breakthrough time of 1222 min at 0.5 ppm, that is twice the time of the parent AC.     

Effects of H2S addition on hydrogen ignition behind reflected shock waves: Experiments and modeling

Hydrogen sulfide is a common impurity that can greatly change the combustion properties of fuels, even when present in small concentrations. However, the combustion chemistry of H₂S is still poorly understood, and this lack of understanding subsequently leads to difficulties in the design of emission-control and energy-production processes. During this study, ignition delay times were measured behind reflected shock waves for mixtures of 1% H₂/1% O₂ diluted in Ar and doped with various concentration of H₂S (100, 400, and 1600 ppm) over large pressure (around 1.6, 13, and 33 atm) and temperature (1045–1860 K) ranges. Results typically showed a significant increase in the ignition delay time due to the addition of H₂S, in some cases by a factor of 4 or more over the baseline mixtures with no H₂S. The magnitude of the increase is highly dependent on the temperature and pressure. A detailed chemical kinetics model was developed using recent, up-to-date detailed-kinetics mechanisms from the literature and by changing a few reaction rates within their reported error factor. This updated model predicts well the experimental data obtained during this study and from the shock-tube literature. However, flow reactor data from the literature were poorly predicted when H₂S was a reactant. This study highlights the need for a better estimation of several reaction rates to better predict H₂S oxidation chemistry and its effect on fuel combustion. Using the kinetics model for sensitivity analyses, it was determined that the decrease in reactivity in the presence of H₂S is because H₂S initially reacts before the H₂ fuel does, mainly through the reaction H₂S + H ? SH + H₂, thus taking H atoms away from the main branching reaction H + O₂ ? OH + O and inhibiting the ignition process.     

Time-dependent optimization of a large size hydrogen generation plant using “spilled” water at Itaipu 14 GW hydraulic plant

In this paper hydrogen generation and storage systems optimization, related to a very large size hydraulic plant (Itaipu, 14 GW) in South America, is investigated using an original multilevel thermo-economic optimization approach developed by the Authors. Hydrogen is produced by water electrolysis employing time-dependent hydraulic energy related to the water which is not normally used by the plant, named “spilled water”.     

Thermodynamic analysis of combined unit of biomass gasifier and tar steam reformer for hydrogen production and tar removal

A combined unit of biomass gasifier and tar steam reformer (CGR) was proposed in this study to achieve simultaneous tar removal and increased hydrogen production. Tar steam reforming calculations based on thermodynamic equilibrium were carried out by using Aspen Plus software. Thermodynamic analysis reveals that when selecting appropriate operating conditions, exothermic heat available from the gasifier could sufficiently supply to the heat-demanding units including feed preheaters, steam generator and reformer. The effects of gasification temperature (T_gs), reforming temperature (T_ref) and steam-to-biomass ratio (S:BM) on percentages of tar removal and improvement of H₂ production were investigated. It was reported that the CGR system can completely remove tar and increase H₂ production (1.6 times) under thermally self-sufficient condition. The increase of H₂ production is mainly via the water–gas shift reaction.     

Effect of hydrogen sulfide inclusion in syngas feed on the electrocatalytic activity of LST-YDC composite anodes for high temperature SOFC applications

The performance of La_0.4Sr_0.6TiO_3±δ-Y_0.2Ce_0.8O_2−δ (LST-YDC) composite anodes in solid oxide fuel cells significantly improved when 0.5% H₂S was present in syngas (40% H₂, 60% CO) or hydrogen. However, electrochemical impedance spectroscopy measurements at OCV showed that polarization resistance of the cell increased when the concentration of H₂S exceeded 0.5%. Gas chromatographic and mass spectrometric analyses revealed that the rate of electrochemical oxidation of all fuel components improved when H₂S was present in the fuel. Electrochemical stability tests performed under potentiostatic conditions showed that there was no power degradation caused by the presence of H₂S in different feeds, and that there was power enhancement when 0.5% H₂S was present.     

Neural network based optimization for cascade filling process of on-board hydrogen tank

Compressed hydrogen storage is widely used in hydrogen fuel cell vehicles (HFCVs). Cascade filling systems can provide different pressure levels associated with various source tanks allowing for a variable mass flow rate. To meet refueling performance objectives, safe and fast filling processes must be available to HFCVs. The main objective of this paper is to establish an optimization methodology to determine the initial thermodynamic conditions of the filling system that leads to the lowest final temperature of hydrogen in the on-board storage tank with minimal energy consumption. First, a zero-dimensional lumped parameter model is established. This simplified model, implemented in Matlab/Simulink, is then used to simulate the flow of hydrogen from cascade pressure tanks to an on-board hydrogen storage tank. A neural network is then trained with model calculation results and experimental data for multi-objective optimization. It is found to have good prediction, allowing the determination of optimal filling parameters. The study shows that a cascade filling system can well refuel the on-board storage tank with constant average pressure ramp rate (APRR). Furthermore, a strong pre-cooling system can effectively lower the final temperature at a cost of larger energy consumption. By using the proposed neural network, for charging times less than 183s, the optimization procedure predicts that the inlet temperature is 259.99–266.58 K, which can effectively reduce energy consumption by about 2.5%.     

High-capacity hydrogen release through hydrolysis of NaB3H8

NaB₃H₈ has advantages over NaBH₄ and NH₃BH₃, two most widely studied chemical hydrides for hydrogen storage via hydrolysis. NaB₃H₈ has an extraordinary high solubility in water and thus possesses a high theoretical capacity of 10.5 wt% H via hydrolysis, in contrast to 7.5 wt% for NaBH₄ and 5.1 wt% for NH₃BH₃. NaB₃H₈ is reasonably stable in water which makes it unnecessary to add corrosive NaOH as a stabilizer as the case for NaBH₄. Furthermore, hydrolysis of NaB₃H₈ can be catalyzed by a Co-based catalyst with fast kinetics that is comparable to Ru-based catalysts. Therefore, cost-effective hydrolysis of NaB₃H₈ is possible for practical applications. A high capacity of 7.4 wt% H was achieved when water was included in the materials weight.     

Decomposition of hydrogen sulfide (H2S) on Ni(100) and Ni3Al(100) surfaces from first-principles

Spin-polarized density functional theory studies of hydrogen sulfide (H₂S) adsorption and decomposition on Ni(100) and Ni₃Al(100) surfaces were conducted to understand the aluminum (Al) alloying effect on H₂S dissociation. For such purpose, we first determined the near surface structure of fully ordered Ni₃Al alloy along the [100] direction by calculating the Al segregation energy to the surface and then examined the adsorption energies of the adsorbates (H₂S, HS, S, and H) and the activation barriers for the H₂S and HS decomposition by using Climbing Image-Nudged Elastic Band method. We found that regardless of the way to terminate the surface, Al atom in bimetallic Ni₃Al(100) tends to exist in the first surface layer, rather than in the second or third layer, and the Ni₃Al(100) surface can substantially retard the H₂S decomposition by reducing the adsorption energy of sulfur compounds compared to the pure Ni(100) case. Finally, we presented how the Al in Ni₃Al modifies the activity of surface Ni atoms toward the sulfur compounds by calculating the local density of states and charge distribution in alloying components. This work hints the importance of knowing how to properly tailor the reactivity of Ni based materials to enhance the resistance for sulfur poisoning.     

PEFC-type impurity sensors for hydrogen fuels

Hydrogen purity sensor cells were newly developed with the principle of PEFC. By using the phenomena of PEFC's voltage drop seen in the presence of impurities and further minimizing the amount of Pt to make the cells more sensitive to impurities, the sensor cells were prepared. This unique sensing principle was applied to typical impurities in practical hydrogen gases, including CO, H₂S, and NH₃. Sensor responses were derived by analyzing various kinds of dependency on Pt loading, current density, impurity concentration, and operational temperature. Possibility of recovery from impurity poisoning was also studied by varying impurities' supply and potential charge. Consequently, our simple PEFC-type hydrogen purity sensors were verified to have ability to sense ppm-level impurities within 10 min.