Energy consumption for hydrogen production in a single-cell hydrogen sulfide electrolysis system

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476257 B0506 Page 1/7 B0506 Energy consumption for hydrogen production in a single-cell hydrogen sulfide electrolysis system Alexi Sellaman* (1,2), Frédéric Ferrer (1), Paule Salvin (2), Mickaël Rimboud (2), Florent Robert (2) (1) Société Anonyme de la Raffinerie des Antilles, Le Lamentin – Martinique/France; (2) Laboratoire des Matériaux et Molécules en Milieu Agressif, UR4_1, UFR STE Université des Antilles, Schoelcher – Martinique/France; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract Many industrial processes produce hydrogen sulfide as a by-product. If not burnt, it is mainly used to produce only sulfur [1][2]. H2S electrolysis is a promising alternative, producing gaseous hydrogen with sulfur as a by-product. It has been stated that the theorical onset potential of H2S to drive the electrolysis (0.142V) is lower than the one for H2O (1.23V) [3]. However, there is not enough accurate data on energy efficiency in these systems in the literature. This work aims to specify the energy required to produce one kg of hydrogen and to find the best conditions for minimizing this need. The results presented here are obtained by chronoamperometry. Different voltages are applied between electrodes immersed in a single-cell reactor equipped with a system of gas collection. The experiments are carried out in a basic medium with an equimolar solution of NaOH:NaHS. A nickel chrome alloy is used for both electrodes. A comparison of performance at 4 applied voltages (0.6V, 0.8V, 1.0V, 1.2V) shows that in our conditions, the system is more efficient at 1V, with a faradic efficiency of ~85% and 27 kWh/kg of energy consumption. At the same time, 1.2V reached ~85% but with 32 kWh/kg. Although not obtained under optimal conditions, these values are already lower than the 55 kWh/kg generally expected to produce 1 kg of hydrogen by water electrolysis. To assess the profitability and feasibility of this technology on an industrial scale, experiments under more representative dynamic conditions are required, and should enable several parameters to be tested. These findings, however, are the first steps in the development of a pilot plant to produce hydrogen by H2S electrolysis. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476257 B0506 Page 2/7 Introduction Hydrogen sulfide (H2S) is a molecule present in all crude oils [4]. It is present at the SARA (Société Anonyme de la Raffinerie des Antilles) refinery in Martinique. H2S is mainly formed at the diesel formation stage, more specifically during desulfurization. Today, SARA burns this gas at the end of its production line using a flare, in accordance with the procedures described for this gas. This molecule is subject to strict safety protocols due to its toxicity and explosiveness [5]. Hydrogen sulfide is known to cause corrosion problems and immobilization of certain installations. SARA's research and development department is studying various solutions, including electrolysis, to recover this waste. With this method of separating hydrogen sulfide, two products are obtained that can be recycled: hydrogen and sulfur. For the Martinique refinery, this offers two opportunities on the local market: 1. Contribute to the development of the French hydrogen plan by addressing the hydrogen market in Martinique. Today, H2 could be one of the pillars that will support the energies of the future. It has a wide range of applications, including industry, mobility and, above all, energy storage (to overcome the intermittent nature of renewable energies, for example). 2. The sulfur market is already present in certain fields, including acid production in the chemical industry or as a fertilizer blend in agriculture. Furthermore, the H2S produced by SARA is not the only source that can be found in Martinique. This gas is present in the emissions of some fermenting biomasses, but also in wastewater treatment plants. This project therefore has the potential to go beyond the petroleum sector, offering a recovery solution in all sectors where H2S poses health and corrosive problems. Thus, the objective is to create a pilot-scale electrolysis system that can recover H2S as a low-carbon energy source on a tropical island. To do this, we need to take a first step, which will give us an overall view of a potential economic system and the problems we will face. For this, our first approach was to study the energy consumption for gas production as a function of the applied voltage in nonoptimal conditions. 1. Scientific Approach A cyclic voltammetry has been carried out with and without NaHS in a basic medium containing NaOH (figure 1). The NaHS solution's catalyzing effect can be seen, indicating the usefulness of studying it to facilitate electrolysis and expand its field of application. The current differences obtained are significant due to the absence of NaHS, which prevents the oxidation of water or OH-, even at a relatively high potential (1.8 V). EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476257 B0506 Page 3/7 Figure 1 : Cyclic voltammetry with (red) and without (black) NaHS in a basic medium containing NaOH (DOP = difference of potential between A and C) The following reactions take place at the electrodes: • Anodic : NaHS + OH- → S + H2O + Na+ + 2e • Cathodic : (Na+) + H2O + e- → ½ H2 + NaOH To ensure that the only gas produce is hydrogen via H2S electrolysis, the voltages applied are chosen to be between 0.6 V (start of observation of gas production) and 1.2 V (before the theoretical minimum threshold for electrolysis of water in a neutral medium). The aim of this article is to determine trends in current density curves as a function of voltage. It is generally accepted that the higher the voltage, the higher the hydrogen production is. This influence of applied voltage is verified under the conditions set out in Table 1. The electrochemical method used is chronoamperometry to measure the current response to applied voltage over time. Voltage is applied using a 3-electrode system in which the reference electrode is also the counter-electrode. The potentiostat was able to generate a potential difference between the working electrode and the counter-electrode due to this. 2. Experiments/Calculations/Simulations 1.2 Set up A single-compartment cell is used with a capacity of 250 mL, the solution is a mixture of NaOH and NaHS, 125 mL of NaOH and 125 mL of NaHS previously formed by bubbling H2S into a NaOH solution. The quantities are sufficiently large to allow the solution to be reused. The concentrations are not strictly defined for practical reasons. This was considered negligible, as mentioned previously, the main objective of this experiment is to obtain economic trends in relation to the voltage imposed. The electrodes are flat rods made of NiCr22 alloy, with anodic and cathodic surfaces of 10.2 cm2 and 14.4 cm2 respectively with an interelectrode distance of 5 cm. Temperature is 30°C at atmospheric pressure and pH is 14. The hydrogen is recovered in a container that is connected to the cell by a stainlesssteel tube to prevent leaks. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476257 B0506 Page 4/7 Data are collected directly by the potentiostat during chronoamperometry. Then recovered and processed using data processing software. 2.2 Experiment Each voltage variation is applied over a period of 90 minutes (the experiment lasts 4 times 1.5 hours), which is considered sufficient time to observe trends. At the end of the 90-minute process, the potentiostat is turned off to clean the electrodes and return them to the initial conditions prior to each voltage variation. Previous observations have shown that it is not necessary to have a high concentration, as the solution will not be completely exhausted in this time. The experiment is carried out without agitation. It has been proven that under our conditions, the higher the agitation, the more it disturbs the reaction on the surface of the nickelchromium electrodes. 3. Results Table 1: Global data Experimental Conditions Applied Voltage (V) 0.6 0.8 1 1.2 Experimental Measurements Current (mA) 4.63 35.61 132.56 151.47 Standard Deviation of Current (mA) 0.64 6.34 15.86 16.05 Captured Gas Volume (mL) 1 20 77 90 Standard Deviation of Captured Gas Volume (mL) 0 3 5 4 Assumed Hydrogen Proportion (%) 100% 100% 100% 100% Anodic Surface Area (cm²) 10.2 10.2 10.2 10.2 Cathodic Surface Area (cm²) 14.4 14.4 14.4 14.4 Anodic Current Density (mA/cm²) 0.45 3.49 13 14.85 Cathodic Current Density (mA/cm²) 0.32 2.47 9.21 10.52 Calculated from Experimental Results Electrical Energy Consumed (Wh) 0.004 0.043 0.199 0.273 Energy Consumption Based on Collected Gas (kWh/kg) 51 26.1 31.6 37.1 Energy Consumption Based on Exchanged Charge (kWh/kg) 16.1 21.4 26.8 32.2 Faradic Efficiency (%) 31.5% 82.0% 84.8% 86.8% The Faradic Efficiency was calculated from dividing the energy consumption based on exchanged charge by the energy consumption based on collected gas. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476257 B0506 Page 5/7 The gas produced during this experiment was analyzed in SARA's laboratories using gas chromatography GC SCION 456-GC. 5mL samples were taken from 20, 77 and 90mL production using syringes. The curves obtained show that most of the gas is hydrogen content for the 3 samples (from the three voltages applied: 800mV, 1000mV and 1200mV). The other peaks indicate the presence of air in the sample, caused by the gas already in the cell. This experiment confirms that as the voltage applied to the cell increases, so does hydrogen production. The time evolution of current as a function of applied voltage is not linear (figure 2). The current curve at 1.2 V shows a much more pronounced decrease than the others, until it stabilizes and returns to approximately the same values as the 1 V curve. The higher the applied voltage, the larger the initial peak and the more pronounced the decrease in current density. The ratio between the generated current and the applied voltage seems more advantageous at 1V than at 1.2V. At the end of these four experiments, no solid sulfur deposits are found at the bottom of the cells. It is likely that the reaction time or solution concentration should be increased. : Figure 2: Current density vs time for each applied voltage The data also show that hydrogen production is possibly impacted by sulfur passivation. If so, the trend of decreasing current density indicates that passivation is more important and has a greater impact at higher potentials. Passivation does not appear to occur at 0.6 and 0.8V and has very little impact at 1V. It is therefore preferable to stay around 1V to optimize the energy ratio. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476257 B0506 Page 6/7 Figure 3 : Comparison between energy consumption and recovery hydrogen quantities for different applied voltage As can be seen in figure 3, the energy consumption to produce 1 kg of H₂, deduced from the electrical energy exchanged at cell level and the volume of collected hydrogen, is more advantageous at applied voltages above 1 V. It can be seen for each voltage, 0.6 V, 0.8 V, 1.0 V and 1.2 V, that the energy consumption amounts to 51 kWh/kg, 26.1 kWh/kg, 31.6 kWh/kg and 37.1 kWh/kg respectively. 4. Discussion Despite low faradic efficiencies, energy consumption per H2 produced appears attractive even compared with water SOEC technology prevision for 2050 (<40 kWh/kg for SOEC [6] and around 30 kWh/kg here for the 1V experiment). The main problem with H2S electrolysis is the passivation of sulfur [7][8][9] at the anode in a double compartment and at both electrodes in a single-cell compartment. The fact that little or no passivation is observed (particularly between 0.6 and 1V) during all these processes makes NiCr22 of great interest. To confirm the absence of passivation at low voltages when using this material, the next step is to conduct an experiment until the solution is depleted. Several areas of improvement will be investigated in future studies, including concentration control to optimize initial conditions (tension, temperature, concentration). A study on the influence of electrode material and membrane in a double compartment system is under way. Another step is to switch to a dynamic circuit. As explained above, the system in place and the selected parameters are being optimized. These results show that there is an advantage over water electrolysis. By performing these tests on systems close to marketable technologies, H2S electrolysis could prove an effective alternative to the recovery of refinery waste gases. References [1] John S. Eow, Recovery of Sulfur from Sour Acid Gas: A Review of the Technology, 2002 [2] M. Sassi and Ashwani K. Gupta, Sulfur Recovery from Acid Gas Using the Claus Process and High Temperature Air Combustion (HiTAC) Technology, 2008 [3] M. Kumar and Tharamani C. Nagaiah, efficient production of hydrogen from H2S via electrolysis using CoFeS2 catalyst, 2022 EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476257 B0506 Page 7/7 [4] H. A. Elmawgoud, T. M. Elshiekh, M. Abdelkreem, S. A. Khalil, and A. M. Alsabagh, “Optimization of petroleum crude oil treatment using hydrogen sulfide scavenger,” Egyptian Journal of Petroleum, vol. 28, pp. 161–164, 6 2019. [5] https://www.airgas.com/msds/001029.pdf. [6]https://www.irena.org//media/Files/IRENA/Agency/Publication/2020/Dec/IRENA_Green_ hydrogen_cost_2020.pdf [7] J. Narendranath, J. Manokaran, S. S. Sundar, R. Muruganantham, A. A. Shoaibi, C. Srinivasakannan, and N. Balasubramanian, “Electrochemical recovery of hydrogen and elemental sulfur from hydrogen sulfide gas by two-cell system,” Energy Sources, Part A: Recovery, Utilization and Environmental Effects, 2019 [8] M. Velazquez-Rizo and A. C. C. Sepulveda, “Low-temperature direct electrochemical splitting of h2s,” 2022. [9] Kwiyong Kim, Changsoo Lee, “Recent progress in electrochemical hydrogen sulfide splitting: Strategies for enabling Sulfur-tolerant anodic reactions”, Chemical Engineering Journal, 2023. Keywords: EFCF2025, H2, LowTemp. Fuel Cells & Electrolysers, H2S, Industry Remark: This work is licensed under Creative Commons Attribution 4.0 International