Carbon Deposition on Solid Oxide Electrolyser Cells: From CO₂ Electrolysis to Co-Electrolysis

Abstract

Solid Oxide Electrolyser Cells (SOECs) have emerged as a leading technology in the Power-to-X (PtX) transition for converting surplus renewable electricity into e-fuels. Among SOEC processes, co-electrolysis of H₂O and CO₂ in SOEC has attracted significant interest due to its ability to produce syngas, which is a key feedstock for methanol and Fischer-Tropsch synthesis. Unlike separate electrolysis processes, co-electrolysis improves efficiency for methanol production by reducing the number of reactors required and minimizing the energy-intensive gas-shifting steps traditionally needed to achieve the desired H₂/CO ratio in syngas [1]. However, despite its potential, co-electrolysis faces operational challenges, particularly regarding carbon deposition on the fuel electrode at high feedstock utilisations, and long-term degradation that limits system performance [2]. To address these challenges, CO₂ electrolysis has been extensively investigated in previous work [3] as a foundation for understanding carbon-related degradation mechanisms in SOECs before advancing the research of this work to co-electrolysis. During CO 2 electrolysis carbon deposition occurs due to the Boudouard reaction (2CO ⇔ CO₂ + C), which is catalysed by nickel, with a theoretical temperature- and gas-composition-dependent threshold that determines a safe operation range to avoid carbon deposition. Nonetheless, carbon formation can take place below the expected equilibrium due to additional, often interlinked factors such as gas diffusion, electrode overpotentials, and impurities present in the CO₂ feed. These conditions intensify localized carbon deposition, especially at elevated current densities where concentration gradients between the gas channel and the fuel electrode-electrolyte interface become significant. While maintaining a stable voltage is often used as an indicator of operational stability in the presence of carbon-containing gases, recent findings [3] indicate that voltage stability alone is insufficient to rule out carbon deposition. Instead, early detection requires more advanced electrochemical diagnostics. In these recent findings, Electrochemical Impedance Spectroscopy (EIS) was identified as a powerful tool for monitoring carbon deposition during CO₂ electrolysis on SOEC, with characteristic shifts in impedance parameters serving as early warning signs. The ability to detect these changes before irreversible electrode damage occurs, presents an opportunity to develop diagnostic methods that can enable real-time operational adjustments. To further analyse the relationship between electrochemical behaviour and carbon deposition, two Python models were developed: an electrochemical model for estimating Boudouard equilibrium concentrations and cell voltage, and a Laplace-transform-based model to assess the effects of carbon deposition on impedance using an R s C deg (RC) equivalent circuit during step increases in current. The electrochemical model provided valuable insights, though deviations of up to 30% between model and experimental results were observed due to mounting inconsistencies, resistance variations, and cell degradation, underscoring the need for model refinement. Meanwhile, the Laplace transform model’s accuracy was limited by data acquisition challenges. This research also explored whether AC:DC operation, an innovative method of operation patented by Dynelectro that alternates between electrolysis and fuel cell mode [4], can reduce the risk of carbon deposition in SOEC cells. Experimental work at Dynelectro on CO₂ electrolysis on Ni-YSZ ∣ YSZ ∣ LSCF-GDC Solydera cells, involved current step increase tests and long-term operation under both DC and AC:DC conditions. The step increase tests, conducted with 0.8 A increments every 2–5 minutes, tracked impedance changes associated with carbon deposition. Long-term tests (20–50 min) were performed under average currents of 4–4.8 A and AC:DC frequencies of 0.01, 0.1, 1, and 30 Hz. CO concentrations were monitored using Quadrupole Mass Spectrometry (QMS), while EIS provided insights into cell behaviour through equivalent circuit fitting. Results confirmed that carbon deposition can be detected before causing irreversible cell damage by monitoring shifts in EIS parameters, particularly the decrease in summit frequency of the RC element below 0.05 Hz. This work demonstrated that carbon formation during CO₂ electrolysis can occur at outlet compositions with 10-20% less CO than the expected equilibrium concentration of 77.8% CO at 750 °C on Ni-containing SOEC, therefore emphasizing the need to define safe operational boundaries for CO₂ electrolysis in SOECs. Although the impact of AC:DC conditions on carbon deposition remained inconclusive at the single-cell level, findings suggested that it may have a significant effect in stack-level applications, where temperature gradients are more pronounced. Despite these findings in CO₂ electrolysis, their applicability to co-electrolysis remains uncertain due to the added complexity introduced by simultaneous H₂O reduction, reverse water-gas shift (RWGS) and methanation reactions. A critical gap in current research lies in the predictive modelling of carbon deposition during co-electrolysis under realistic operating conditions. While theoretical carbon deposition thresholds can be calculated based on inlet gas partial pressures and temperature, additional local factors such as gas concentration gradients along the cell, impurity-driven overpotentials, and variations in reaction kinetics must be considered to refine predictions. This research aims to address this gap by investigating how these phenomena influence carbon deposition and by extending previous findings into the context of co-electrolysis with two key objectives: Can carbon formation during co-electrolysis in SOECs be predicted and, if so, prevented to ensure a safe operation window? A detailed mathematical model is being developed to quantify carbon deposition under varying CO₂/H₂O ratios and operating conditions. By incorporating reaction kinetics, mass transport effects, and electrochemical behaviour, this model seeks to provide a predictive framework for identifying conditions that minimize carbon risk while maximizing syngas yield. The model is being designed to integrate with Dynelectro’s existing SOEC simulation framework. Experimental validation is a key component of this study, involving SOEC tests under controlled co-electrolysis conditions. Carbon deposition will be assessed using a combination of gas analysis techniques, post-mortem electrode characterization, and real-time impedance spectroscopy. Special attention will be given to correlating EIS-derived parameters with observed carbon buildup, with the goal of refining diagnostic capabilities. The objective is to develop a tool that can detect early-stage carbon deposition based on impedance shifts, enabling operational adjustments to prevent long-term degradation. Can AC:DC operation reduce the risk of carbon deposition during co-electrolysis, enabling higher CO concentrations and more efficient syngas production? AC:DC operation has previously been shown to enhance SOEC durability by reducing localized thermal gradients and minimizing degradation mechanisms associated with steady-state DC operation [4], [5]. In co-electrolysis, where carbon formation is highly sensitive to temperature fluctuations and gas-phase composition, AC:DC operation may provide additional benefits by stabilizing reaction environments. One potential advantage of AC:DC operation is its ability to expand the safe operating range for co-electrolysis. By maintaining higher outlet temperatures, particularly in the driest conditions where steam concentration is lowest, the Boudouard equilibrium can be shifted toward higher CO concentrations, thereby increasing the safety margin for carbon deposition, as shown in Figure 1. Additionally, higher temperatures reduce the Area Specific Resistance (ASR), lowering overall cell overvoltages and possibly further mitigating carbon risk. Beyond thermal stabilization, AC:DC operation may also influence the behaviour of carbon-forming impurities. Certain gas-phase contaminants, such as sulphur species, have been shown to accelerate carbon deposition by increasing electrode overpotential [6], [7]. AC:DC cycling has the potential to either remove or transform these impurities into less reactive forms, delaying the onset of carbon buildup. However, this hypothesis has not been thoroughly tested in co-electrolysis conditions, making it a key focus of ongoing research. If validated, AC:DC operation could provide a practical method for avoiding carbon deposition, improving syngas quality and reducing cell degradation, thereby ensuring long-term stability and effici

Identifiers

DOI

10.1149/ma2025-031176mtgabs

Journal

ECS Meeting Abstracts

Year

2025