Hydrogen via Co-Electrolysis of Water and CO₂: Challenge or Solution for Industrial Decarbonization?

Abstract

The paper investigates the potential of co-electrolysis as a viable pathway for hydrogen production and industrial decarbonization, expanding on previous studies on water electrolysis. The analysis adopts a general and critical perspective, aiming to assess the realistic scope of this technology with regard to current energy and environmental needs. Although co-electrolysis theoretically offers improved efficiency by simultaneously converting H₂O and CO₂ into syngas, the practical advantages are difficult to consolidate. The study highlights that the energetic margins of the process remain relatively narrow and that several key aspects, including system irreversibility and the limited availability of CO₂ in many contexts, significantly constrain its applicability. Despite the growing interest and promising technological developments, co-electrolysis still faces substantial challenges before it can be implemented on a larger scale. The findings suggest that its success will depend on targeted integration strategies, advanced thermal management, and favorable boundary conditions rather than on the intrinsic efficiency of the process alone. However, there are specific sectors where assessing the implementation potential of co-electrolysis could be of interest, a perspective this paper aims to explore.

1. Introduction

1.1. Background on Hydrogen and Co-Electrolysis

The urgent drive for decarbonization has positioned it at the core of global energy policies. Although closely linked to the broader issue of climate change, it also serves as a catalyst for innovation in the energy sector, encouraging the adoption of low-carbon technologies and the transition toward sustainable energy systems [1]. In this context, reducing carbon dioxide emissions from industrial sectors, especially those classified as “hard-to-abate” such as steel, cement, and chemicals, is a relevant issue [2]. Despite renewable energy sources such as solar and wind power have made significant strides in decarbonizing the electricity sector, industrial processes remain heavily reliant on fossil fuels.

To reduce emissions in these energy-intensive industries, technologies such as carbon capture, utilization, and storage (CCUS), direct electrification, and hydrogen-based solutions have emerged as key strategies to reduce emissions in these energy-intensive industries. Hydrogen has gained global recognition as a crucial enabler of decarbonization, providing a pathway to replace fossil fuels in sectors where direct electrification is impractical [3]. Its versatility allows it to be used as a fuel, chemical feedstock, and energy storage medium. Low-carbon hydrogen can be produced either through Steam Methane Reforming (SMR) coupled with CCUS, known as blue hydrogen, or via water electrolysis powered by renewable energy, referred to as green hydrogen. CCUS encompasses CO₂ capture from sources such as power plants or directly from air, transportation mainly via pipelines, underground storage in geological formations, or reuse in industrial processes, which helps close the carbon loop and supports emission reduction goals. Hydrogen and CCUS are often promoted separately across industries, with hydrogen for decarbonizing fuels and CCUS for reducing industrial emissions. As green hydrogen production scales up, attention is turning to technologies that enhance overall efficiency and possibly integrate carbon reuse directly into the production process. Although blue hydrogen integrates hydrogen production with CCUS, its adoption is limited by technical and infrastructural challenges as well as environmental concerns from a life cycle perspective [4,5]. By contrast, co-electrolysis offers a unified solution that uses renewable electricity to simultaneously convert water and CO₂ into synthetic fuels or feedstocks, providing a dual benefit of emission reduction and resource valorization [6]. While still emerging, it can potentially overcome the limits of blue hydrogen and fragmented decarbonization strategies.

Compared to conventional electrolysis, co-electrolysis offers several advantages. The first is the possible integration of CO₂ capture and conversion: instead of merely capturing CO₂ for storage, co-electrolysis transforms it into usable energy carriers [7]. The second is a potentially higher system efficiency: the process utilizes high-temperature solid oxide electrolysis cells (SOECs), which enable better conversion efficiencies than low-temperature electrolysis [8,9]. Challenges such as scaling up technology, ensuring economic viability, and managing system durability remain, but overall co-electrolysis presents a comprehensive pathway for industries aiming to reduce their carbon footprint.

1.2. Scientific Motivation and Objectives of the Work

This article explores the potential of co-electrolysis in decarbonizing hard-to-abate sectors by integrating CO₂ utilization with hydrogen production. Despite existing technological and economic hurdles, co-electrolysis could offer a sustainable alternative to conventional fossil-based processes. The co-electrolysis research reflects a complexity of approaches, primarily divided between two major lines. On one side, there are studies with a strong electrochemical focus, aiming to optimize and understand the underlying technology in detail. On the other hand, much of the recent attention has concentrated on high-temperature co-electrolysis, driven by its appealing characteristics, such as near-100% electricity-to-syngas efficiency and fast reaction kinetics. While justified by the integration potential with catalytic fuel synthesis and syngas production, this focus may have inadvertently overshadowed the possibilities offered by low-temperature systems. As emphasized in recent reviews [8,9], the dominance of high-temperature approaches risks underestimating the advantages of low-temperature electrolysis, such as system modularity, operational flexibility, and potentially lower material constraints. A more balanced and integrated perspective could foster innovation across a broader technological spectrum, unlocking new opportunities for co-electrolysis development.

Building on previous work by one of the authors on water electrolysis [10], this paper examines the progress of co-electrolysis processes for hydrogen production and industrial decarbonization. The aim is to assess the viability of this technology to face current energy and environmental needs. Unlike most review papers on co-electrolysis, such as [8,9], which concentrate primarily on the technological advancements and operational parameters of the process, the present work seeks not only to summarize recent developments but also to situate them within a broader decarbonization framework. The aim is to assess the conditions and application scenarios in which co-electrolysis can serve as a truly viable and impactful pathway for CO₂ mitigation. In addition to revisiting the fundamental aspects and principles of the process, this study also preliminarily examines potential real-world deployments, with particular attention to sectors where concentrated CO₂ streams are already readily available. By exploring these contexts, the work offers a practical perspective on how and where co-electrolysis could be most effectively implemented. Particular attention is given to the enabling conditions for the practical implementation of co-electrolysis, highlighting the major technical limitations to its broader uptake.

This work is organized into three main sections. Section 2 reviews process fundamentals and presents a comprehensive energy analysis. Section 3 outlines recent and potential technological developments, focusing on key advances in materials and system configurations. Section 4 adopts an application-oriented perspective, preliminarily assessing industrial contexts where co-electrolysis could be advantageous from both an energy and strategic standpoint, particularly in sectors where CO₂ is already separated, facilitating integration despite current technical and energy balance challenges.

2. Fundamentals and Energy Analysis of Co-Electrolysis of H₂O and CO₂

Co-electrolysis of water (H₂O) and carbon dioxide (CO₂) is an advanced electrochemical process that converts them into syngas, a mixture of hydrogen (H₂) and carbon monoxide (CO). It is particularly attractive, as illustrated in Figure 1, enabling CO₂ reduction and production of energy carriers and oxygen. Powered by renewable electricity, such as wind, solar, or hydro, the process has strong potential for integration into a fully decarbonized energy system.

For this technology to be considered a viable solution, feasibility must be assessed from multiple perspectives. From an energy standpoint, process efficiency depends on operating conditions, particularly temperature, and integration with renewable sources [11]. In addition, to these factors, technical aspects, such as the durability and performance of electrolyzers, must be carefully evaluated alongside economic factors, including capital and operational costs, to determine the practical viability and potential of co-electrolysis. A comprehensive analysis must consider the entire transformation chain. Since CO₂ is often mixed with other gases, and syngas is a mixture of CO and H₂, separation and purification steps are required before downstream fuel synthesis.

2.1. Operating Principles of Co-Electrolysis

Considering the case illustrated in Figure 1, the electro-reduction reactions at the cathode side are:

H₂O + 2e⁻ → H₂ + ½ O²⁻

(1)

CO₂ + 2e⁻ → CO + O²⁻

(2)

while the corresponding electro-oxidation reaction at the anode side is:

O²⁻ → ½ O₂ + 2e⁻

(3)

Dissociation of H₂O, CO₂, or mixtures requires a well-defined energy input, at least equal to the standard reaction enthalpy at ambient temperature:

H₂O → H₂ + ½ O₂    ∆H = 286 kJ/mol    (at T = 25 °C)

(4)

CO₂ → CO + ½ O₂    ∆H = 283 kJ/mol    (at T = 25 °C)

(5)

Considering the simultaneous availability of the two substances, the basic reaction is the following one:

CO₂ + H₂O + electricity + heat → CO + H₂ + O₂

(6)

Both electrical energy and heat are required to drive these dissociation reactions, with their relative contributions depending on operating temperature, as illustrated in Figure 2. The temperature dependence of ΔG and ΔH was calculated from standard thermodynamic functions—enthalpy, entropy, and heat capacity—using NIST data for the gaseous species involved [12].

The minimum total energy requirement corresponds to the reaction enthalpy (ΔH), with the electrical energy input related to the Gibbs free energy (ΔG) and the remaining part provided as thermal energy (TΔS). The total energy required for co-electrolysis (of H₂O and CO₂) remains constant with temperature; however, as temperature increases, there is a significant shift in the distribution between the different forms of energy supplied. The portion of energy required in the form of electrical input decreases, while the thermal energy contribution increases. This behavior is linked to the temperature dependence of the Gibbs free energy, which leads to a reduction in the theoretical cell voltage (Nernst potential) at higher temperatures.

As can be seen, water electrolysis presents a jump in the total and thermal energy requirements due to the supply of water in the gaseous form for temperatures above the boiling point (100 °C, 1 bar). At around 825 °C the Gibbs free energy for CO₂ electrolysis becomes lower than that for H₂O electrolysis, making CO₂ reduction thermodynamically more favorable at higher temperatures.

The co-electrolysis process typically utilizes solid oxide electrolysis cells (SOECs, which operate at temperatures between 700 and 1000 °C, enabling higher efficiency and lower electricity consumption compared to low temperature electrolysis. SOECs have demonstrated efficiency advantages over proton exchange membrane (PEM) and alkaline electrolysis technologies due to their ability to directly utilize thermal energy [13]. The high temperature environment facilitates the simultaneous reduction of H₂O and CO₂, producing H₂ and CO at the cathode while oxygen is released at the anode. High-temperature co-electrolysis reduces electrical energy demand by lowering the required electrochemical potential, as Gibbs free energy decreases with temperature [14]. Co-electrolysis shifts a portion of the energy input from electricity to heat, which is easier and cheaper to supply. The significant thermal input required adds complexity, cost, and scalability challenges. Material stability and thermal integration are also critical issues in industrial applications [15]. Electrodes and electrolytes must withstand high thermal and chemical stress, requiring advanced materials resistant to degradation over long operational periods. Moreover, the high-temperature operation requires robust insulation and precise thermal control to minimize heat loss and maximize efficiency [16]. Large-scale deployment remains a challenge due to the high initial capital costs and the need for robust engineering solutions to maintain efficiency over extended use.

The electrochemical reduction of CO₂ (CO₂RR) at low temperatures offers a promising alternative to high-temperature CO₂/H₂O co-electrolysis [17]. While co-electrolysis requires temperatures above 600 °C and yields syngas (CO + H₂), CO₂RR enables the direct production of valuable chemicals such as formic acid and methanol under milder conditions using aqueous or ionic electrolytes [18,19]. However, its low-temperature operation limits performance, leading to reduced selectivity, lower energy efficiency, and strong competition from the hydrogen evolution reaction.

2.2. Energy Analysis of Co-Electrolysis

Co-electrolysis simultaneously produces H₂ and CO, with a total energy demand (electricity + heat) similar to that of water electrolysis. Obviously, co-electrolysis becomes less advantageous because part of the energy is used to produce CO. The transformation of low-energy compounds such as CO₂ and H₂O into higher-energy species such as CO and H₂ inevitably requires a considerable energy input, due to the increase in the system’s chemical potential. Power can be estimated from the minimum electrical potential and the number of electrons involved in the reaction. The overall electrochemical conversion of one mole of CO₂ and one mole of H₂O into CO and H₂ requires the transfer of 4 moles of electrons. In standard conditions, the theoretical minimum potential for the co-electrolysis of CO₂ and H₂O is approximately 1.46 V, which is slightly higher than that of water electrolysis alone (1.23 V). Given a unit CO₂ feed of 1 kg/s and the corresponding molar and mass flow rates listed in

Table 1. Molar mass, molar flow rate, mass flow rate, and lower heating value (LHV) of the chemical species involved in the co-electrolysis process.

Chemical Species	Molar Mass (g/mol)	Moles [mol/s]	Mass Flow Rate [kg/s]	LHV [kJ/mol]	LHV [MJ/kg]
H2O	18.02	22.73	0.41
CO2	44.01	22.73	1
CO	28.01	22.73	0.637	283.0	10.1
H2	2.02	22.73	0.046	241.8	120

, the theoretical power required for the process, based on this unit mass flow rate, is estimated in

Table 2. Summary of the theoretical power requirements for the co-electrolysis process in ideal conditions, considering a unitary mass flow rate of CO2 (1 kg/s).

Number of Electrons [mol/s]	Total Charge [C/s]	Minimum Cell Potential, E [V]	P = Q × E [kW]
90.92	8.76 × 106	1.46	12,680.9

.

Under real operating conditions, both the required voltage and current will be higher, leading to increased energy consumption and, consequently, higher operational costs. The required potential applied to the cell can be expressed as

V = E + µ_act,a + µ_act,c + µ_ohmic + µ_conc,a + µ_conc,c

(7)

where µ_act,a and µ_act,c are the activation overpotential at the anode and cathode, µ_ohmic is the ohmic overpotential, and µ_conc,a and µ_conc,c are the concentration overpotential at the anode and cathode that contribute to increasing the required power and energy used in the process. As suggested for water electrolysis by Min et al. in [16], the proper characterization of the energy performance of co-electrolysis requires clear and standardized definitions, including metrics at different boundaries of the system, such as efficiency and specific energy consumption.

The power values reported in Table 2 indicate that, due to the typically large CO₂ flow rates, the required electrolyzer power is significantly high. Some expressions are provided below for evaluating system efficiency for hydrogen production η_H2,syst, system efficiency for syngas production η_syn,syst, specific energy consumption for hydrogen production SEC_H2,syst, and specific energy consumption for syngas production, SEC_syn,syst:

$${\mathsf{\eta }}_{\mathrm{H}2,\mathrm{syst}}={\stackrel{.}{\mathrm{m}}}_{\mathrm{H}2}{\mathrm{LHV}}_{\mathrm{H}2}/({\mathrm{W}}_{\mathrm{stack}}/{\mathsf{\eta }}_{\mathrm{AC},\mathrm{DC}}+\Sigma {\stackrel{.}{\mathrm{W}}}_{\mathrm{BoP}}+\Sigma {\stackrel{.}{\mathrm{Q}}}_{\mathrm{ext}})$$

(8)

$${\mathsf{\eta }}_{\mathrm{syn},\mathrm{syst}}=({\stackrel{.}{\mathrm{m}}}_{\mathrm{H}2}{\mathrm{LHV}}_{\mathrm{H}2}+{\stackrel{.}{\mathrm{m}}}_{\mathrm{CO}}{\mathrm{LHV}}_{\mathrm{CO}})/({\stackrel{.}{\mathrm{W}}}_{\mathrm{stack}}/{\mathsf{\eta }}_{\mathrm{AC},\mathrm{DC}}+\Sigma {\stackrel{.}{\mathrm{W}}}_{\mathrm{BoP}}+\Sigma {\stackrel{.}{\mathrm{Q}}}_{\mathrm{ext}})$$

(9)

$${\mathrm{SCE}}_{\mathrm{H}2,\mathrm{syst}}=({\stackrel{.}{\mathrm{W}}}_{\mathrm{stack}}/{\mathsf{\eta }}_{\mathrm{AC},\mathrm{DC}}+\Sigma {\stackrel{.}{\mathrm{W}}}_{\mathrm{BoP}}+\Sigma {\stackrel{.}{\mathrm{Q}}}_{\mathrm{ext}})/{\stackrel{.}{\mathrm{m}}}_{\mathrm{H}2}$$

(10)

$${\mathrm{SCE}}_{\mathrm{syn},\mathrm{syst}}=({\stackrel{.}{\mathrm{W}}}_{\mathrm{stack}}/{\mathsf{\eta }}_{\mathrm{AC},\mathrm{DC}}+\Sigma {\stackrel{.}{\mathrm{W}}}_{\mathrm{BoP}}+\Sigma {\stackrel{.}{\mathrm{Q}}}_{\mathrm{ext}})/{\stackrel{.}{\mathrm{m}}}_{\mathrm{syn}}$$

(11)

where ṁ represents the flow rates of products, $\stackrel{.}{\mathrm{W}}$ is the electrical power for the cell and the auxiliary system, and $\stackrel{.}{\mathrm{Q}}$ thermal power from external sources. The symbols in the four equations take into account the fact that in an electrolysis system, we usually distinguish three main parts: the stack, the balance of plant (BoP), and the external components. The stack is the core of the electrolyzer, where the electrochemical reaction takes place. It consists of multiple cells with electrodes and membranes, and it is the part that actually splits water into hydrogen and oxygen. The balance of plant includes all the auxiliary subsystems that keep the stack operating under optimal conditions. This covers pumps, valves, heat exchangers, sensors, safety devices, and power electronics inside the electrolyzer module. Finally, the external components are everything outside the electrolyzer module that enables integration with the wider energy system. These may include transformers, compressors, hydrogen storage, purification units, and grid connections.

A comparative overview is provided between conventional water electrolysis and the co-electrolysis of CO₂ and H₂O in

Table 3. Comparison between electrolysis and co-electrolysis.

Parameter	Water Electrolysis (H2O → H2 + ½O2)	Co-Electrolysis (CO2 + H2O → CO + H2)
Reactants required	~9 kg H2O	~9 kg H2O + ~22 kg CO2
Overall reaction	H2O → H2 + ½O2	CO2 + H2O → CO + H2+ O2
Technology used	PEM/Alkaline/SOEC	SOEC
Operating temperature	60–80 °C (ALK, PEM); 700–850 °C (SOEC)	700–850 °C
ΔG°	~237 kJ/mol H2	~257 kJ/mol H2
Minimum energy required	39.6 kWh/kg H2	42.7 kWh/kg H2
Electrical energy input	55–60 kWh/kg H2	79–84 kWh/kg H2
Thermal energy input (net)	~0–1 kWh/kg H2	~5–10 kWh/kg H2
Total energy required	~55–61 kWh/kg H2	~84–94 kWh/kg H2
Main product gas	H2	Syngas (H2 + CO)
CO2 processed per kg of H2	None	~22 kg
Efficiency (on LHV basis)	~60–70%	~45–50%

. The comparison is based on producing 1 kg of hydrogen and includes key technical and energetic parameters such as feedstock requirements and technology used, as well as energy consumption, operating conditions, and overall process efficiency. The values reported in Table 3 refer to highly general conditions, meant to provide a simplified and idealized benchmark. In practice, these figures can vary significantly depending on the specific configuration and integration of the system.

Table 4. Process balances for co-electrolysis at 30–70% efficiency, referenced to a CO₂ mass flow rate of 1 kg/s.

Efficiency of Co-Electrolysis	Electricity Required [kWh]	SEC for Hydrogen Production [kWh/kg]
70%	9100	77.8
50%	12,800	100.2
30%	21,330	151.7

reports some energy balances calculated assuming an ideally stoichiometric co-electrolysis reaction, based on a reference CO₂ feed flow rate of 1 kg/s and some hypothetical efficiencies of the whole process. Considering a higher level of 70% or a more cautious level of 30%, we can obtain the balances reported in Table 4, which usually ranges between 30% and 70% for co-electrolysis, as also discussed in [20].

The apparent energy advantage in co-electrolysis comes from the energy potential of the products, primarily hydrogen (H₂), which has a high calorific value (120 MJ/kg), and carbon monoxide (CO), which, while having a lower calorific value, still has useful industrial applications. The previously shown balances demonstrate some potential advantages of co-electrolysis from a theoretical perspective. However, the advantages of this process can only be significant if system efficiency exceeds at least 50% and if both hydrogen and carbon monoxide can be effectively valorized. By leveraging high-temperature heat, co-electrolysis can consume an energy amount well over that for simple electrolysis per kilogram of hydrogen under ideal conditions. Evaluating the efficiency of an electrolysis process, and by extension that of co-electrolysis, is challenging due to the difficulty in accounting for the various thermochemical irreversibilities involved in real systems.

Co-electrolysis of CO₂ and H₂O is traditionally a high-temperature process, in which both molecules are simultaneously reduced within solid oxide electrolysis cells (SOECs, producing syngas (CO + H₂) efficiently. This is made possible by oxygen ion conduction through ceramic electrolytes at temperatures above 700 °C, enabling a thermodynamically balanced electrochemical reaction. At lower temperatures, the combined electrochemical conversion of CO₂ and H₂O is theoretically feasible, but in practice involves distinct pathways with slow kinetics and significant competition from the hydrogen evolution reaction. This does not constitute true co-electrolysis, as the two half-reactions proceed independently and typically require different catalysts and conditions.

Table 5. Estimated efficiency of co-electrolysis with varying operating temperature.

Operating Temperature [°C]	Estimated Perspective Efficiency [%]
50–80	20–30
200	30–40
400	40–50
600	50–60
800	60–70

shows some estimated projections of process efficiency with respect to temperature. If co-electrolysis is to be considered for applications of meaningful scale, it would necessarily involve significant power input and substantial energy demands. Efficiency values are approximated, as the influence of temperature on performance remains challenging to quantify accurately across different systems.

The analysis in this section shows that co-electrolysis is not energetically favorable for producing hydrogen alone. Its main advantage emerges in integrated systems, where CO₂ utilization and process coupling deliver additional value beyond standalone H₂ generation.

3. Outlook and Recent Developments of Co-Electrolysis of H₂O and CO₂

As previously discussed, solid oxide electrolysis cells (SOECs have emerged as a key technology for converting renewable electricity into syngas (H₂ + CO) through the high-temperature co-electrolysis of CO₂ and H₂O. Obviously, the application of co-electrolysis requires technological development that addresses a range of practical considerations.

Early research, such as that conducted by Ebbesen et al. in [21], investigated the feasibility and reaction mechanisms of syngas production and assessed the impact of co-electrolysis on stack degradation. Indeed, although cathode reactions are widely accepted, the detailed dissociation processes for co-electrolysis remain partly understood [22]. While steam is the primary reactant in electrolysis, CO₂ reduction is governed by thermochemical reactions [23].

More recent efforts have focused on improving catalyst composition to enhance electrochemical activity, efficiency, and stability. Experimental efforts have scaled SOEC technology to pilot and industrial levels (at the moment the level of 10 kW high-temperature co-electrolysis system), achieving syngas production rate of 3.08 Nm³/h and adjustable H₂/CO ratio between 1.3 and 4.8, along with some possible applications in steel blast furnaces, using a steam + CO₂ mixture to produce renewable syngas, demonstrating the potential to replace natural gas in steel production.

Studies on co-electrolysis mainly focus on high-temperature systems, as low-temperature processes are less efficient and not truly integrated.

Table 6. Comparative overview of technical, energy, and performance aspects between high and low temperature electrolysis.

Factor	High-Temperature Co-Electrolysis	Low-Temperature Co-Electrolysis
Energy Requirement	~3.0–4.5 MWh/tCO2 (electrical) + thermal input (~2 MWh/tCO2).	~5.0–6.5 MWh/tCO2 (entirely electrical)
Reaction Kinetics	Fast kinetics at 700–850 °C (current densities up to 1.5–2.5 A/cm2).	Slow kinetics at <150 °C (current densities typically <0.5 A/cm2)
Electrolyte Conductivity	Solid oxide (e.g., YSZ) with oxygen ion conductivity ~0.1–0.2 S/cm at 800 °C.	Low proton or ion conductivity (~10−3–10−2 S/cm), leading to high ohmic losses
Water Phase	Requires steam (≥600 °C); utilizes heat from industrial integration	Requires additional energy to convert liquid water into steam (parasitic losses).
Overall Efficiency	50–60% (electrical-to-syngas); 70–80% with heat recovery	<40% typical; high overpotentials
Cost Reduction	Stack cost: >1500 €/kW; high due to ceramics and balance of plant	Stack cost potentially <800 €/kW; lower materials cost but lower performance

provides a concise comparison of the technical, energy, and performance differences, emphasizing the advantages of high-temperature operation.

Expanding on the considerations of Table 6, high-temperature co-electrolysis offers several advantages. Achieving significant efficiency improvements requires advancements in both membrane development and in electrode material optimization. In this context, there is promising research activity, particularly focused on materials, which shows encouraging potential for further enhancement.

Deka et al. in [24] studied the site-deficient perovskite oxide La_0.7Sr_0.2FeO₃ as a cathode material, showing that iron site doping with Ni or a combination of Ni and Co significantly improved performance. The doped cathode achieved a Faradaic efficiency of up to 100%, demonstrating excellent electrocatalytic properties. Recent advances in high temperature co-electrolysis focus on novel electrode materials. Notably, Soltani et al. [25] highlight perovskites for their bifunctional electrocatalytic properties in oxygen evolution and reduction. Additional results highlight how material design and operating conditions jointly influence the effectiveness and flexibility of solid oxide co-electrolysis systems. Bian et al. in [26] developed a cathode material based on prereduced La_0.7Sr_0.3Fe_0.9Ni_0.1O₃-δ (LSFNi), triggering the formation of uniformly distributed Ni-Fe alloy nanoparticles (~45 nm). Their system achieved current density of ~1.0 A/cm² at 1.5 V and 750 °C, current density of ~2.4 A/cm² at 850 °C, and Faradaic efficiency close to 100%.

Bimpiri et al. in [27] observed that varying the H₂O/CO₂ ratio can increase the H₂/CO ratio although the overall electrochemical performance of the cell remains unaffected. These insights confirm that both catalyst development and feedstock optimization play key roles in tailoring co-electrolysis outputs to specific application needs.

Moreover, SOEC co-electrolysis provides significant flexibility in syngas production, allowing for the precise control of the H₂/CO ratio, which is essential for fuel synthesis. By modifying the operating parameters, the H₂/CO ratio can be adjusted from ~0.1 to ~7 [28,29]. Stable operation for over 100 h without carbon deposits has been demonstrated for operation with flue gases [30]. The harsh operating conditions in co-electrolysis demand specialized materials. Current materials used in solid oxide electrolysis cells (SOEC) are prone to degradation. Researchers are exploring new materials that are both durable and efficient under these extreme conditions, but material limitations remain a significant barrier.

Table 7. Research areas in co-electrolysis, associated key challenges, and potential solutions.

Research Area	Key Challenges	Potential Solutions and Directions
Electrode Materials and Catalysts	Stability and durability: operation >1000 h under relevant conditions; resistance to anode/electrolyte interface degradation	Advanced perovskite oxides, transition metal doping, nanostructured catalysts
Electrochemical Performance	Efficiency and performance: high overall and Faradaic efficiency; current densities >1 A/cm2 with favorable reaction kinetics	Optimized doping, interface engineering, enhanced electrode architecture
Temperature Optimization	Operating conditions: elevated temperatures (700–900 °C) leading to high thermal energy demand, material stress, and complex thermal management	Exploring lower temperature SOECs, hybrid approaches integrating PEM technology
H2/CO Ratio Control	Process integration: precise tuning for downstream applications (e.g., Fischer–Tropsch synthesis)	Adjusting feed gas composition, operating voltage, and catalyst properties
System Scalability	Scalability: transition from laboratory (~10 kW) to industrial-scale systems (>100 MW)	Modular SOEC stacks, integration with renewable energy sources
Integration with Industrial Processes	System compatibility: integration with CO2 separation processes requiring high purity (>95%), with corresponding sizing and energy needs	Direct syngas utilization, coupling with carbon capture and utilization (CCUS) technologies

outlines the key research areas in co-electrolysis, identifying current challenges and summarizing potential technological solutions and research directions. It considers critical aspects such as energy efficiency, material durability, system integration, and scalability, and highlights ongoing efforts to enhance the viability of co-electrolysis technology.

A promising example of such an approach is the CO2Chem project, conducted by Sebastien et al. in [31], which focuses on the development of a polymeric electrolyte electrochemical reactor operating at low temperatures (30–95 °C). The project aims to achieve a one-step conversion of CO₂ and water into methanol, utilizing surplus renewable energy. This approach could overcome some of the limitations of high-temperature SOECs by offering a lower temperature range (30–95 °C), reducing material degradation and simplifying system design. While SOECs have been at the center of research due to their high efficiency and ability to operate at large scales, low-temperature co-electrolysis might offer a complementary solution.

Recent research in high temperature SOECs for CO₂ and H₂O co-electrolysis has made significant advancements even if commercial deployment remains unlikely in the near term. Scaling SOEC technology from laboratory-scale to full-scale industrial systems requires improvements in electrode materials, reactor design, and heat management to ensure minimal degradation and durability. While SOECs offer high efficiency and potential integration with industrial processes, challenges such as low-grade thermal heat, technological complexity, cost, durability, and scalability continue to limit their widespread adoption. Alternative approaches, such as low temperature co-electrolysis and hybrid electrolysis, may enhance the feasibility of electrolytic pathways in the decarbonization landscape. Hybrid configurations that combine water electrolysis (e.g., via PEM technologies) with low temperature CO₂ electroreduction could support energy load stabilization in renewable-based systems while enabling flexible syngas or synthetic fuel production.

4. Potential Application of Co-Electrolysis in Specific Industrial Contexts

Decarbonizing hard-to-abate industrial sectors is a cornerstone of modern energy policy. A major challenge for the real-world implementation of co-electrolysis is that the CO₂ produced in many industrial processes is mixed with other gases and not readily separated, posing significant barriers to its direct use. The separation of CO₂ from combustion gas streams is known to be a highly energy-intensive process, a topic that one of the authors of this work investigated extensively in earlier study [32].

Co-electrolysis enables efficient high temperature conversion of CO₂ and H₂O into syngas, relying on available concentrated CO₂, thermal energy integration, and industrial compatibility. This step fits naturally within an integrated pathway from CO₂ capture to downstream synthesis processes, as illustrated in the flow diagram showing mass and energy interconnections (Figure 3).

Co-electrolysis becomes attractive primarily when CO₂ is already available in a separated or easily recoverable form, and when the system design allows for the integration of thermal energy to reduce the overall electrochemical load. The level of CO₂ purity plays a central role, as energy and cost requirements for separation can vary significantly depending on the industrial sector.

Chemical and petrochemical industries offer promising scenarios, as processes such as ammonia or methanol production generate relatively pure CO₂ streams that can be internally reused for further conversion. A proper assessment of industrial applications should include a sector-specific analysis of CO₂ separation. In the following section, some illustrative cases will be explored where co-electrolysis may conceptually make sense, offering preliminary insights into contexts in which this technology could potentially be integrated with minimal additional energy burden.

4.1. Cement Production

The cement industry stands out as a prime example of a hard-to-abate sector—those industries where emissions are particularly difficult to reduce through electrification or energy efficiency alone. Cement production involves not only significant energy use but also process-related CO₂ emissions from the calcination of limestone. This makes the sector a critical testing ground for emerging decarbonization technologies, including the use of hydrogen as an alternative fuel and the development of carbon capture, utilization, and storage (CCUS) solutions. Hydrogen is being explored as a viable substitute for fossil fuels in cement kilns, helping to reduce emissions while maintaining production performance. A detailed and up-to-date overview of recent developments in the sector can be found in [33,34].

The cement sector represents a more promising case for co-electrolysis too. The cement manufacturing process begins with raw materials such as limestone (CaCO₃), clay, shale, and iron ore. These materials are crushed, blended, and heated in a rotary kiln at temperatures of around 1400–1450 °C. During this process, limestone decomposes into calcium oxide (CaO), which then reacts with silica (SiO₂), alumina (Al₂O₃), and iron oxides (Fe₂O₃) to form the main clinker phases. The resulting clinker is then mixed with other elements such as gypsum (CaSO₄) to produce Portland cement, the key ingredient in concrete. Figure 4 provides a simplified schematic view of the process.

A relatively concentrated stream of CO₂ is generated in the clinker production process; limestone (CaCO₃) is heated in a kiln to produce clinker, which is the primary ingredient for cement. This process involves two key steps:

-

Calcination of Limestone: at high temperatures (approximately 900–1000 °C), limestone (CaCO₃) undergoes a chemical reaction known as calcination. During this reaction, calcium carbonate decomposes into calcium oxide (CaO), also known as lime, and carbon dioxide (CO₂):

CaCO₃ (s) → CaO (s) + CO₂ (g)

(12)

The CaO produced is solid and remains in the kiln, while the CO₂ is released into the atmosphere as a gas.

-

Clinker Formation: the CaO produced in the calcination step then reacts with other materials in the kiln, such as silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃), at higher temperatures (around 1400–1450 °C). These reactions result in the formation of clinker, a solid material that is ground into cement.

About 68% of the total CO₂ emissions from the cement production come from this calcination step. Significant amounts of CO₂ are generated during the process, making these facilities potential candidates for integrating co-electrolysis.

To understand the amount of CO₂ available, consider a medium-to-large-scale plant producing 1 million tons per year.

Table 8. Estimated CO₂ emissions from a cement plant producing 1 million tons of clinker.

Parameter	Value
Annual Cement Production	1,000,000 tons/year
Daily Production	1,000,000 tons ÷ 365 days = 2740 tons/day
Hourly Production	2740 tons ÷ 24 h = 114.17 tons/h
CO2 Emissions from Calcination	0.68 × 114.17 tons = 77.76 tons CO2/hour
Annual CO2 Emissions	77.76 tons/hour × 24 h × 365 days = 681,000 tons CO2/year

provides some relevant data on CO₂ emissions from such a plant. Co-electrolysis, which simultaneously converts CO₂ and H₂O into syngas, offers strong potential in this sector. Analysis of scaling data underscores the importance of size: effective deployment would require systems with substantial capacity and power demands.

4.2. Natural Gas and Biogas Processing Facilities

A field in which co-electrolysis could have interesting developments is natural gas refineries. In these facilities, the separation of CO₂ from the gas is often mandatory and can be carried out using different methods, including sorbents, membranes, or cryogenic techniques. Natural gas refineries represent another promising area for co-electrolysis. In these plants, CO₂ separation is often required to meet pipeline standards and is achieved using various technologies, including amine scrubbing, membranes, or cryogenic processes. Gas separation. Membranes (Figure 5) are attractive for their simplicity but typically do not yield high-purity CO₂ in a single step. This limitation has prompted the development of solvent-assisted hybrid systems that combine membrane separation with chemical absorption.

Often ranging from less than 2% in treated gas to over 15% in sour gas [35], the CO₂ separated in natural gas processing can serve as an ideal feedstock for co-electrolysis.

Table 9. Estimated CO₂ separation potential in natural gas processing facilities handling sour gas.

Parameter	Value
Natural gas treated (per day)	2,000,000 m3
Initial CO2 content (6% of total)	120,000 m3
CO2 separation efficiency	85%
CO2 separated (per day)	102,000 m3
CO2 residual (in treated gas)	18,000 m3
CO2 separated (per year)	37,230,000 m3 (approx. 37.23 million m3)
CO2 separated (mass per day)	102,000 m3 × 1.977 kg/m3 = 201,654 kg (201.65 tons)
CO2 residual (mass per day)	18,000 m3 × 1.977 kg/m3 = 35,346 kg (35.35 tons)

provides an order of magnitude of the available CO₂ for a natural gas processing facility producing 2 million cubic meters of natural gas, with a representative initial content of CO2 equal to 6%.

Another relevant case is that of biogas produced from anaerobic digestion processes, which are widely implemented and could be effectively integrated into co-electrolysis-schemes to produce synthetic fuels. This pathway offers a promising opportunity to combine renewable electricity with carbon-containing feedstocks of biological origin, as in Figure 6.

4.3. Carbon Dioxide Generation and Capture in Corn Fermentation for Bioethanol Production

Another particularly interesting field for the potential application of co-electrolysis of CO₂ and H₂O is bioethanol production. This topic has been widely explored in literature and encompasses various perspectives, as outlined in [36]. One of the key byproducts of bioethanol production is CO₂, which is generated in significant amounts during ethanol fermentation. This makes corn-based bioethanol production an interesting case for CO₂ utilization. Corn fermentation involves several stages:

-

Milling and Starch Preparation in which the corn is ground to release starch, which is then broken down into fermentable sugars through enzymatic hydrolysis.

-

Liquefaction and Saccharification where enzymes (such as amylase) convert starch into simple sugars such as glucose and maltose.

-

Fermentation during which yeast and enzymes convert glucose into ethanol and CO₂:

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

(13)

Figure 7 provides a simplified schematic diagram of the process. This process typically takes 48–72 h and occurs at temperatures between 30–35 °C. Ethanol is separated from the fermentation broth and purified to 95–99% concentration. For every ton of ethanol produced, approximately 0.8–1 ton of CO₂ is released. The CO₂ produced during fermentation is relatively pure, making it suitable for various industrial applications, including the food and beverage industry (carbonation of soft drinks), agriculture (enhancing plant growth in greenhouses), and fuel synthesis (conversion of CO₂ into synthetic fuels via processes such as co-electrolysis).

4.4. Co-Electrolysis of CO₂ and H₂O in the Power Sector: The Case of Geothermal Power Plants

A particularly relevant context for exploring the potential of co-electrolysis in the power sector are vapor-dominated geothermal plants. In these systems, CO₂ is naturally present in the geothermal fluid and must be separated before being released into the atmosphere. This need for CO₂ management, combined with the availability of heat and steam, creates favorable conditions for integrating co-electrolysis, offering a promising opportunity to decarbonize geothermal energy production while generating hydrogen.

Examining for example geothermal energy production in the Monte Amiata and Larderello areas, CO₂ emissions vary between approximately 250 and 520 kg/MWh of electricity produced. While the lower end of this range may be considered acceptable, the higher value is problematic, as it even exceeds the emission levels of natural gas-fired combined cycle power plants (around 350 kg/MWh).

Table 10. CO₂ emissions from selected geothermal fields worldwide.

Geothermal Field	CO2 Concentration (Weight %)	CO2 Emissions (kg/MWh)	References
Monte Amiata (Italy)	~5–8%	250–520	[37]
Larderello (Italy)	~1–5%	Lower than Monte Amiata	[38]
The Geysers (USA)	~0.5–2%	~40–100	[39]
Krafla (Iceland)	~0.5–1.5%	~10–50	[40]
Taupo Volcanic Zone (NZ)	~2–6%	~100–300	[41]
Philippines Fields	~1–4%	~50–200	[42]

presents CO₂ emissions from various geothermal power plants worldwide, highlighting the significant variability influenced by geological and operational factors.

This section presents a quantitative assessment of co-electrolysis integration in geothermal plants, based on case studies selected for their representativeness in plant scale, resource characteristics, and data availability. For illustrative purposes, some significant data from one of the plants in the Larderello area, described in Figure 8 are used.

The Larderello geothermal field contains superheated steam at a temperature of approximately 200 °C with a CO₂ content of around 5%. This means that for every 1 kg/s of geothermal fluid, about 0.05 kg/s consists of CO₂. Therefore, to obtain a CO₂ flow rate of 1 kg/s, a geothermal fluid flow of approximately 20 kg/s would be required. The geothermal plant operates under standard inlet conditions defined by ENEL for design purposes: a pressure of 5 bar absolute, a temperature of 195 °C, and a CO₂ content of approximately 5%.

For a 60 MW turbine, the nominal inlet mass flow rate is set at 111.1 kg/s, serving as a reference value for plant sizing. However, when accounting for auxiliary consumption —most notably the compressors required for CO₂ extraction—the net useful power output does not exceed 53 MW; this means that the generation of 14.72 kWh for each second. The data of the plant are reported in Table 10. Although geothermal energy is a limited and, in some respects, controversial resource, the development of co-electrolysis could enable the exploitation of certain resources currently unused due to high CO₂ concentrations. This is particularly true for saturated vapor two-phase systems, where co-electrolysis could convert the abundant CO₂ and H₂O into syngas or other valuable products, thereby improving the overall system efficiency. By analyzing the data in

Table 11. Mass flow rate from geothermal plant of Valle Secolo, area of Larderello (data from [43]).

Point	ṁ [kg/s]	P [bar]	T [°C]	x CO2
1	111.11	5.00	195	5.0
2	111.11	0.08	41	5.0
3	7.65	0.07	26	72.6
4	7.65	0.272	177	72.6
5	6.10	0.260	33	91.1
6	6.10	1.013	176	91.1

and considering the CO₂ mass flows, it is possible to estimate the power levels involved, as well as the significant amount of hydrogen that could potentially be produced in geothermal plants with dry steam source, according to the mass balances analyzed in Table 1.

In a geothermal plant such as Valle Secolo, where approximately 6.10 kg/s of CO₂ is extracted, the application of co-electrolysis could enable the production of hydrogen at a rate of roughly 0.28 kg/s (≈1 ton per hour). However, such integration would also require a substantial amount of electrical power, which must be accounted for, as it would be subtracted from the net output of the geothermal plant. Assuming a specific electricity consumption of 100 kWh per kilogram of hydrogen in co-electrolysis, as in Table 4, the resulting hydrogen production rate is approximately 0.15 kg/s. From a stoichiometric perspective, this hydrogen flow allows the co-electrolysis of around 3.2 kg/s of CO₂, producing roughly 2.0 kg/s of CO and 2.3 kg/s of O₂, while consuming about 1.3 kg/s of steam. With a CO₂ feed rate of 6 kg/s, the conversion level is therefore about 54%, leaving an unreacted fraction of approximately 2.8 kg/s.

For reference, complete conversion of the 6 kg/s CO₂ feed would require nearly 0.28 kg/s of hydrogen, corresponding to about 100 MW of net electrical power, well above the available capacity of the plant. To evaluate the potential of hydrogen production via co-electrolysis under different operational regimes, we consider a net plant power of 53 MW.

Table 12. Hydrogen production and CO₂ conversion via co-electrolysis at different levels of plant power utilization. The table compares full (100%), half (50%), and partial (20%) power scenarios.

Scenario	Net Power	Energy Req. per Second	H2 Prod.	H2 Daily	CO2 Converted	CO2 Converted	CO Prod.
Full Power	53 MW	14.7 kWh	0.147 kg/s	12.7 t/day	3.2 kg/s	54%	2.0 kg/s
50% Power	26.5 MW	7.35 kWh	0.073 kg/s	6.3 t/day	1.6 kg/s	27%	1.0 kg/s
20% Power	10.6 MW	2.94 kWh	0.029 kg/s	2.5 t/day	0.64 kg/s	10.8%	0.40 kg/s

summarizes the hydrogen production, CO₂ conversion, and related outputs for three levels of power utilization: full power (100%) used as reference scenario, half power (50%), and partial power (20%). Daily hydrogen production is also reported to provide a tangible sense of scale. This comparison highlights how the available electrical energy directly limits the achievable hydrogen yield and the extent of CO₂ conversion.

This highlights the need to carefully balance hydrogen production potential with overall energy efficiency and, potentially, economic feasibility.

4.5. Multi-Criteria Assessment of Industrial Co-Electrolysis Applications

As discussed in the previous sections, while the use of CO₂ streams from industrial contexts (e.g., cement, biogas) has been proposed as a potential feed for co-electrolysis, it is clear that the question of CO₂ purity remains crucial. A high degree of purity is indeed necessary to ensure efficient operation, and the presence of impurities may significantly affect both performance and system durability. Moreover, to evaluate the industrial applicability of solid oxide co-electrolysis (SOEC) beyond purely technical performance, a structured decision-making framework could be useful. Such a framework allows the systematic comparison of industrial sectors according to process-specific boundary conditions, resource availability, and techno-economic drivers.

Following a multi-criteria decision analysis approach, we propose a decision-making matrix that integrates technical (CO₂ supply and heat integration), economic (electricity price and syngas demand), and systemic (operational flexibility, utilities, and regulatory environment) dimensions.

Table 13. Decision-making matrix for selecting co-electrolysis in industrial applications.

Criterion	Quantitative Indicator	Unit	Normalization (0–3)	Weight [%]
CO2 availability	Capture potential × stream purity	tCO2/h, %	0: <0.2 t/h and purity < 80% 1: 0.2–2 t/h 2: >2 t/h 3: >2 t/h and purity > 95%	25
Heat integration	Recoverable thermal power at T > 650 °C	MWth	0: none 1: <5 MWth 2: 5–20 MWth 3: >20 MWth	20
Electricity cost/availability	Average cost or renewable LCOE	€/MWh	0: >90 €/MWh 1: 70–90 €/MWh 2: 50–70 €/MWh 3: <50 €/MWh	15
Syngas demand	Substitutable demand vs. fossil feedstocks	MWh/year	0: none; 1: <10%; 2: 10–30%; 3: >30% of internal demand	15
Operational flexibility	Load modulation range or storage capacity	% load range	0: <20% 1: 20–40% 2: 40–60% 3: >60% plus storage capability	10
Utilities and space	Availability of water, oxygen handling, footprint	Qualitative	0: critical 1: limited 2: adequate 3: abundant	5
Policy/incentives	Value of existing schemes	€/tCO2	0: <30 €/t 1: 30–50 €/t 2: 50–80 €/t 3: >80 €/t	10
Total score	—	—	Weighted sum	100

details a possible selection of criteria and scoring methodology used to rank different industrial applications. Each criterion is defined by a measurable indicator, normalized into discrete scores (0–3), and weighted according to relative importance. The weighted sum provide a sector-specific readiness score for co-electrolysis deployment. It is worth noting that current TRL assessments place co-electrolysis at intermediate stages: while low-temperature systems remain at TRL 4–5, solid oxide co-electrolysis has already reached TRL 6–7, with some pilot-scale demonstrations approaching TRL 8. These findings suggest targeted R&D, policy support, and strategic investments are required to fully exploit the potential of co-electrolysis for industrial decarbonization.

The application of the decision-making matrix highlights that industrial sectors with abundant high temperature waste heat and accessible CO₂ streams, such as steel and cement production, are the most suitable candidates for co-electrolysis deployment.

A quantitative SWOT analysis confirms that while co-electrolysis provides high electrical-to-syngas efficiency and strong heat integration opportunities, it is currently limited by stack durability, CAPEX, and material availability. Compared to alternative decarbonization pathways, co-electrolysis presents unique synergies with existing industrial processes and potential for sector-specific optimization.

5. Conclusions

In this paper, we have analyzed from a general point of view the co-electrolysis of CO₂ and H₂O, a process that enables the simultaneous production of hydrogen and carbon monoxide, resulting in syngas, a key feedstock for synthetic fuels and chemicals. From a thermodynamic point of view, he process appears interesting in the perspective of decarbonization. It benefits from high-temperature operation (700–900 °C), where heat replaces part of the electrical input, thus improving overall energy efficiency. However, the efficiency advantage is only meaningful if both H₂ and CO are valued outputs. If hydrogen alone is the target, the process becomes less efficient due to the energy diverted to CO₂ reduction. For syngas production, especially when using renewable electricity and waste heat, co-electrolysis offers a potentially integrated and sustainable route.

From a thermochemical perspective, co-electrolysis is limited in application and current results; in practice, achieving efficiencies above 30–40% remains challenging. The process becomes more favorable when operated at high temperatures (700–800 °C), particularly in contexts where substantial amounts of waste heat are available. Under these conditions, the energy input requirements can be partially offset, improving the overall viability of the system.

Practical implementation remains challenging. Hydrogen production via co-electrolysis yields 46 kg of H₂ per ton of CO₂, requiring from 10 to 15 MWh/tonnCO₂. Co-electrolysis can be applied in sectors with concentrated CO₂ streams, such as cement production, natural gas processing, bioethanol production, and geothermal energy. While technological complexity, size constraints, costs, and low commercial readiness make large-scale adoption challenging, it is worth noting that several of these sectors already involve CO₂ separation as part of their process requirements. In such cases, integration may be advantageous from an energetic point of view. Downstream syngas use offers further value. Moreover, co-electrolysis contributes to climate neutrality by enabling flexible, low-carbon hydrogen and syngas production. This supports environmental resilience, a key aspect of the broader Industry 5.0 vision. A qualitative-quantitative decision-making matrix was also proposed to assess sector-specific opportunities for co-electrolysis.

Practical implementation of co-electrolysis is constrained by narrow energy margins. The high energy demand relative to the hydrogen yield limits its attractiveness, particularly when compared to alternative hydrogen production technologies. Economic indicators such as capital costs and the Levelized Cost of Hydrogen (LCOH) are critical for assessing the scalability of co-electrolysis. However, a meaningful comparison with benchmark technologies is premature, as the technology is still at a low TRL, and reliable cost baselines or projections cannot yet be established.

In conclusion, while co-electrolysis of CO₂ and H₂O holds conceptual promise, its practical viability in industrial decarbonization hinges on overcoming technical constraints related to energy efficiency, system design, and scalability. Moreover, the significant energy demand associated with large-scale implementation raises additional concerns about the availability of sufficient renewable electricity and waste thermal heat, a key requirement if the process is to contribute meaningfully to low-carbon strategies. Its future role as either a practical solution or a persistent challenge will largely depend on progress in addressing these technical and energy-related limitations. Similarly to other hydrogen-based technologies, co-electrolysis could become especially attractive when integrated with periods of renewable energy surplus, effectively linking energy storage with CO₂ conversion and providing a dual benefit in both decarbonization and flexibility of the power system.