Experimental Clarification of the RWGS Reaction Effect in H₂O/CO₂ SOEC Co-Electrolysis Conditions

Abstract

In the present investigation, modified X-Ni/GDC electrodes (where X = Au, Mo, and Fe) are studied, in the form of half-electrolyte supported cells, for their performance in the RWGS through catalytic-kinetic measurements. The samples were tested at open circuit potential conditions in order to elucidate their catalytic activity towards the production of CO (r_co), which is one of the products of the H₂O/CO₂ co-electrolysis reaction. Physicochemical characterization is also presented, in which the samples were examined in the form of powders and as half cells with BET, H₂-TPR, Air-TPO and TGA re-oxidation measurements in the presence of H₂O. In brief, it was found that the rate of the produced CO (r_co) increases by increasing the operating temperature and the partial pressure of H₂ in the reaction mixture. In addition, the first results revealed that Fe and Mo modification enhances the catalytic production of CO, since the 2wt% Fe-Ni/GDC and 3wt% Mo-Ni/GDC electrodes were proven to perform better compared to the other samples, in the whole studied temperature range (800–900 °C), reaching thermodynamic equilibrium. Furthermore, carbon formation was not detected.

1. Introduction

Solid oxide electrolysis is a contemporary process for CO₂ capture/recycling, which is proven as an attractive method to provide CO₂ neutral synthetic hydrocarbon fuels. In particular, co-electrolysis of H₂O and CO₂ in a solid oxide electrolysis cell (SOEC) yields synthesis gas (CO + H₂), which in turn can be used towards the formation of various types of synthetic fuels [1,2,3] by applying the Fischer-Tropsch process. According to thermodynamics, SOECs offer benefits for endothermic reactions, such as H₂O and/or CO₂ electrolysis at temperatures >700 °C, because a larger part of the required electrical energy can be substituted by thermal energy [4,5]. In addition, high temperature can lead to a significant decrease in the internal resistance of the cell and acceleration of the electrode reaction processes due to fast reaction kinetics [4,5,6,7].

H₂O/CO₂ Co-electrolysis is a much more complex process compared to pure steam or CO₂ electrolysis. This is because three reactions take place simultaneously, namely H₂O electrolysis, CO₂ electrolysis, and the catalytic Reverse Water Gas Shift reaction (RWGS). More specifically, at the cathode of SOECs, two electrochemical reactions take place in parallel at the triple phase boundaries, i.e., H₂O and CO₂ electrochemical reductions. The oxygen ions (O²⁻), produced by these reactions, are moved to the anode, through an oxygen ion-conducting electrolyte (Yttria-Stabilized Zirconia − YSZ), where oxygen (O₂) gas is formed [8,9]:

Cathode:  H₂O + 2e⁻ → H₂ + O²⁻ ΔH_{800 °C} = 151.8 kJ/mol

(1)

CO₂ + 2e⁻ → CO + O²⁻ ΔH_{800 °C} = 185.9 kJ/mol

(2)

Anode:  O²⁻ → ½ O₂ + 2e⁻

(3)

Besides the electrolysis reactions mentioned above, the reversible water gas shift (RWGS) catalytic reaction also occurs at the cathode with the co-existence of CO₂ and H₂O:

Cathode:  H₂ + CO₂ → H₂O + CO ΔH_{800 °C} = 36.8 kJ/mol

(4)

It thus appears that CO can be produced either electrocatalytically by the reduction of CO₂ or catalytically via the RWGS reaction. Questionable conclusions have been proposed in the literature about the role of the RWGS reaction for CO production. Some studies mention that CO is mainly produced electrocatalytically and RWGS has a small participation [10,11,12,13,14,15], while other studies mention that CO is absolutely produced catalytically via the RWGS reaction [16,17]. However, it has not been shown conclusively to which degree the RWGS is responsible for CO production in an SOEC.

Specifically, Mogensen and co-researchers found that the performance of co-electrolysis on an Ni/YSZ cathode varied between the highest pure H₂O electrolysis and the lowest CO₂ electrolysis, being much closer to the prior process [13,14]. This implies the significant contribution of the RWGS reaction in the H₂O/CO₂ SOEC co-electrolysis process and also confirms the co-existence of CO₂ electrolysis. The above conclusion was supported by recently published results on computational modelling of the direct and indirect (with electro-generated H₂) reduction of CO₂ [18,19]. The direction of the Water Gas Shift (WGS) reaction could be forward or backward, depending on the operating conditions, indicating that at high temperatures (>838 °C), CO was produced electrochemically and the WGS was shifted in reverse towards H₂ and CO₂.

In contrast, Yue and Irvine [20] suggested that the RWGS reaction did not contribute critically in catalysing H₂O/CO₂ co-electrolysis on s (La,Sr)(Cr,Mn)O₃ (LSCM) perovskite based cathode. The LSCM cathode displayed higher catalytic activity for pure CO₂ electrolysis than pure H₂O electrolysis, and the performance for H₂O/CO₂ co-electrolysis was much closer to pure CO₂ electrolysis.

On the other hand, Bae et al., Hartvigsen et al., Kee et al., and Zhao et al. considered that CO production on Ni-based cathodes was mainly from the RWGS reaction, whereas the electrochemical processes are dominated by H₂O electrolysis with identical performances for H₂O electrolysis and H₂O/CO₂ co-electrolysis [16,17,21].

According to the above mentioned reports, until now, no agreement has been reached regarding the role of the RWGS reaction in the production of CO. As a result, it is crucial to quantify the degree of CO formation for each reaction. The discrepancies regarding the CO production route can be affected both by the structural characteristics of the electrode/catalyst (specific surface area, reducibility and re-oxidation behaviour, porosity, particle size, ionic and electronic conductivity), but also by the operating conditions (gas composition, temperature, etc.) [9,22]. As reported by Li’s study [10], the heterogeneous thermochemical reactions occur at the external surface of the cathode and they are 20–100 times faster than the electrochemical reactions, which occur close to the electrolyte at the three phase boundaries. Furthermore, structural modifications of the cathode could enhance mass transport and promote CO production through the catalytic RWGS reaction, resulting in H₂O/CO₂ co-electrolysis performance close to that of H₂O electrolysis.

The majority of studies on high temperature H₂O/CO₂ co-electrolysis utilize Ni-containing ceramic cathodes with YSZ and Gadolinia-Doped Ceria (GDC), similarly to the case of H₂O electrolysis [10,11,12,13,14]. Ni-based materials indeed are cheap and exhibit porous structure, high electronic conductivity, appropriate catalytic activity, and a similar Thermal Expansion Coefficient (TEC) with the electrolyte. Consequently, they could act as excellent SOECs cathodes for H₂O/CO₂ co-electrolysis [7,22,23,24,25,26].

However, SOECs comprising Ni-based cathodes face some critical degradation issues, which are more pronounced with an increasing current density [3,12,14,27,28,29,30,31,32,33]. The main reasons for degradation have been reported to be microstructural changes that take place after prolonged co-electrolysis, resulting in passivation and blocking of the Three-Phase Boundaries (TPB) area [12,13,28,30,31,33,34]. Post-mortem microscopy investigations in the Ni/YSZ electrode have shown irreversible damages of the electrode’s microstructure, such as loss of Ni-YSZ contact, decomposition of YSZ, Ni grain growth, loss of Ni percolation (loss of Ni–Ni contact), and even migration of Ni from the fuel electrode [28,30,31,33]. Taking into account the above, it is critical to develop alternative cathode materials for H₂O/CO₂ co-electrolysis with improved structural properties.

Previous studies reported that the electrocatalytic efficiency of SOECs can be improved by alloying transition metals with the state of the art (SoA) Ni catalyst [35,36,37,38,39]. Modification by means of alloying may be a promising strategy to promote the catalytic activity due to ligand and strain effects that change the electronic structure [40] of the active element. Recent studies focused on the Ni–Co alloy with Sm-doped ceria (Ni–Co/SDC) as an alternative material, with enhanced performance for H₂O electrolysis in SOECs [36]. Specifically, the addition of Co increased the intrinsic catalytic activity of pure Ni and simultaneously expanded the active reaction region [36,41]. Other reports have shown that an Ni–Fe bimetallic cathode, mixed with Ba_0.6La_0.4CoO₃ on La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) electrolyte, improved the electrochemical performance of the electrode for H₂O electrolysis [38] and CO₂ electrolysis [39], due to prevention of Ni aggregation. Furthermore, the enhanced performance of the Ni–Fe formulations could be attributed to the increased stability and improved catalytic activity, due to the addition of Fe, as this was concluded by combining experimental results with theoretical Density Functional Theory (DFT) calculations [41].

Our research group have studied the effect of Au and/or Mo doping on the physicochemical and catalytic properties of Ni/GDC for the reactions of catalytic CH₄ dissociation and methane steam reforming [23,42,43,44,45] in the presence of H₂ and H₂S impurities. These modifications resulted in SOFC fuel electrodes with high sulfur and carbon tolerance as well as with improved electrocatalytic activity. Regarding the doping level of Au and Mo, the amount of 3wt% was indicated as an appropriate loading for achieving the most promising results under the examined SOFC conditions. Recently, the 3wt% Au-Ni/GDC electrode was tested under SOEC conditions for the H₂O electrolysis process and exhibited improved electrocatalytic performance compared to Ni/GDC [46]. The enhanced performance of the Au-doped cathode was attributed to the creation of a surface Ni–Au solid solution, which causes weaker interplay of absorbed H₂O_ads and O_ads species with the modified cathode. The result is a durable and resistant electrode to surface oxidation with an improved “three phase boundaries” area, especially at temperatures higher than 800 °C [46].

The presented investigation deals with modified X-Ni/GDC electrodes (where X = Au, Mo, Fe), in the form of half-Electrolyte Supported Cells (ESCs), for their performance in the RWGS through catalytic-kinetic measurements. Ni/GDC, 3wt% Au-Ni/GDC, 3wt% Mo-Ni/GDC, 3wt% Au–3wt% Mo-Ni/GDC, and 2wt% Fe-Ni/GDC modified cathodes were tested at Open Circuit Potential (OCP) conditions to elucidate their catalytic activity towards the production of CO (r_co), which is one of the products from the H₂O/CO₂ co-electrolysis reaction. The latter approach is considered as an attempt to create a reference profile for the catalytic performance of the candidate electrodes, by applying the same H₂O/CO₂/H₂ feed conditions as those under the co-electrolysis operational mode. The samples, both in the form of powders and as half cells, were physicochemically characterized, including specific redox stability measurements in the presence of H₂O.

2. Results and Discussion

2.1. Physicochemical Characterization

2.1.1. Specific Surface Area Values of Au–Mo–Fe-Modified Powders

The specific surface area values (SSA) for the modified-NiO/GDC samples, calcined in air at 600 and 1100 °C, as well as after reduction with H₂ at 850 °C, are presented in

Table 1. BET Specific Surface Area (SSA_BET) of commercial NiO/GDC, and X-modified NiO/GDC samples (where X = Au, Mo, and Fe). The measurements were performed on powders, calcined at 600 °C, 1100 °C, and after H₂-reduction at 850 °C. Error/accuracy of SSA_BET = ±0.2 m² g⁻¹.

Sample Powder	SSABET (m2 g−1)
	T = 600 °C, (oxidized)	T = 1100 °C, (oxidized)	After H2-reduction at T = 850 °C
NiO/GDC	5.3	2.9	2.0
3Au-NiO/GDC	5.4	2.7	2.1
3Mo-NiO/GDC	5.0	2.2	1.7
3Au–3Mo-NiO/GDC	5.1	2.4	1.8
2Fe-NiO/GDC	8.8	4.0	3.4

.

The first remark is that all samples exhibit quite similar and low SSA values, which decrease further by increasing the calcination temperature from 600 °C to 1100 °C and after H₂ reduction at 850 °C. Furthermore, Mo and Au–Mo modification slightly decrease the SSA of Ni/GDC, while the addition of Fe causes a significant increase in the SSA values up to approximately 70%. Taking into account the generally low values and the ±0.2 m² g⁻¹ accuracy limits of the measurements, it can be concluded that the reduced Mo and Au–Mo-Ni/GDC powders have similar SSAs with Ni/GDC. On the other hand, the 70% increase in the SSA of the Fe modified sample can be primarily ascribed to the formation of FeO_x species, in the oxidized form of the sample, which inhibit the decrease of SSA during reduction.

2.1.2. H₂ Reducibility and air Re-Oxidation Behaviour of Au–Mo–Fe-Modified Powders

Temperature Programmed Reduction followed by Oxidation (H₂-TPR, Air-TPO) measurements were performed, in order to investigate the reducibility and re-oxidation behavior of the prepared composite powders, as well as the existence of possible bulk effects due to the modification with Au, Mo, and Fe. The corresponding H₂-TPR and Air-TPO TGA profiles of modified-NiO/GDC samples, calcined at 1100 °C, are presented in Figure 1.

H₂-TPR and air-TPO measurements showed different reduction and oxidation behavior in each of the examined modified samples. In particular, two main reduction peaks (Figure 1A) and one oxidation peak (Figure 1B) are mainly observed, which can be attributed to the reduction of NiO species to Ni and the reverse [42,47]. Moreover, the presence of Au does not seem to affect significantly the reduction of NiO. On the other hand, MoO₃ and, to a certain extent, Fe inhibit the NiO reduction, implying a stronger Ni–O bond. The inhibition effect of MoO₃ is also observed in the case of the ternary 3Au–3Mo-NiO/GDC sample and indicates a bulk interaction between Ni–Au–Mo, which has been thoroughly investigated in previous studies [23,42,43]. In particular, the second broad peak (“shoulder”) at 580 °C, which is observed for the 3Mo-NiO/GDC and 3Au–3Mo-NiO/GDC samples, may be associated with the reduction of MoO₃ species, which are reduced at higher temperatures in the range of 600–770 °C [42,47]. The presence of Fe seems to affect the main reduction peak of NiO, in a similar way as MoO₃, suggesting the possible formation of a solid solution between Ni and Fe. The latter observation is currently under further clarification.

Upon re-oxidation of the samples, the unmodified Ni/GDC as well as the Au modified sample are oxidized practically at the same temperature with a similar TPO profile. The reduced state of the Fe, Mo-, and Au–Mo- modified samples proved to be more resistant to re-oxidation in 33 vol.% air/Ar (6.7 vol.% O₂/Ar), since they had to reach higher temperature for complete re-oxidation.

2.1.3. H₂O Re-Oxidation Profiles of Au-Mo-Fe–Modified Powders

Isothermal-TGA measurements, under 15.5 vol.% H₂O/Ar conditions, were carried out at 650 °C, 700 °C, 750 °C, and 800 °C and the results are depicted in Figure 2 and Figure 3. These measurements investigate the activity of the powders for the H₂O dissociation reaction and their concomitant re-oxidation rate. H₂O acts as an oxidative agent and interacts with the Ni atoms on the surface of each sample towards H₂ and NiO [reaction (5)] [46].

$${\mathrm{H}}_2\mathrm{O}+\mathrm{Ni}\to {\mathrm{H}}_2+\mathrm{NiO}$$

(5)

Thereafter, there is a progressive diffusion of the absorbed oxygen species (O_ads) from the surface in the bulk phase of the sample, which is oxidized further [46,48,49,50]. Figure 2 depicts the H₂O re-oxidation profiles, as an increase of the weight (Δwt%), of the pre-reduced Ni/GDC, 3wt% Au-Ni/GDC, 3wt% Mo-Ni/GDC, 3wt% Au–3wt% Mo-Ni/GDC, and 2wt% Fe-Ni/GDC samples in the temperature range between 650–800 °C.

The re-oxidation profiles of the samples in Figure 2 can be separated in three sections [46]. The first section corresponds to the initial sharp increase in weight and is ascribed [46] to the complete, bulk, re-oxidation of the partially reduced CeO₂ in GDC and specifically of Ce³⁺ to Ce⁴⁺. The second section occurs during the following 12–25 min of the reaction, where the samples, except for 2Fe-Ni/GDC, keep their reduced state without any changes in their weight. This step is associated [46] to the dissociation of bulk NiH species, which are formed during the prior reduction period, and maintains the samples reduction. NiH dissociation is considered as an activated process, because by decreasing the operating temperature from 800 °C to 650 °C, the specific “delay” period increases from 12 min to 25 min. Τhe first two sections are quite similar for the pre-reduced powders, with identical low SSA values (Table 1). On the other hand, the initial re-oxidation behavior, in the first section, of the pre-reduced 2Fe-Ni/GDC is more intense, indicating the strong effect of H₂O.

In the third section of the TG profiles, further re-oxidation takes place [reaction (5)] and the main discrepancies are detected among the samples. In the range of 650–800 °C, the ternary 3Au–3Mo-Ni/GDC sample is the most tolerant in bulk re-oxidation by H₂O, while the binary 2Fe-Ni/GDC sample is the least tolerant. The decrease in temperature (800 → 650 °C) leads to inhibition of the bulk re-oxidation, as observed by the reduced slopes in the TG profiles, whereas the trend among the samples does not change. The high tolerance of the 3Au–3Mo-Ni/GDC sample against bulk re-oxidation by H₂O is attributed to the synergistic interaction of nickel with gold and molybdenum [23,42].

Τhe slope on the linear part of the third section in the TG curves represents the intrinsic dissociation rate of H₂O and correspondingly the re-oxidation rate of Ni atoms on each sample [reaction (5)] [46]. Figure 3 depicts the Arrhenius plots of the oxidation rates of the samples, which were calculated from the slopes on the linear parts of the TG profiles (Figure 2). The Arrhenius plots (Figure 3) show that the binary 2Fe-Ni/GDC sample has the highest H₂O re-oxidation rates, while the ternary 3Au–3Mo-Ni/GDC has the lowest. This is further confirmed by the calculated apparent activation energies (E_a,ap), which are defined from the Arrhenius plots and are reported in

Table 2. Apparent activation energies (kJ mol⁻¹) for the dissociation of H₂O on commercial Ni/GDC [46], and modified Ni/GDC samples. The values correspond to the slopes of the Arrhenius plots in Figure 3.

Sample Powder	Ea, app (kJ mol−1)
Ni/GDC [46]	41
3Au-Ni/GDC [46]	56
3Mo-Ni/GDC	45
3Au–3Mo-Ni/GDC	63
2Fe-Ni/GDC	20

.

The calculated values verify that the binary 2Fe-Ni/GDC sample exhibits the lowest E_a,ap (20 kJ/mol) for the catalytic dissociation of H₂O, whereas the ternary 3Au–3Mo-Ni/GDC sample exhibits the highest E_a,ap (63 kJ/mol). The above behavior can be interpreted by the fact that Ni-Fe interaction reinforces the bond of H₂O_ads, resulting in a higher re-oxidation rate (lower E_a,ap). On the other hand, following the same interpretation, Ni–Au–Mo interaction weakens the bond of H₂O_ads, resulting in a lower re-oxidation rate (higher E_a,ap) [46].

The calculated E_a,ap values are in good agreement with our previous study [46] and other studies [51,52,53] in the literature, which focus on theoretical investigations. The binary 2Fe-Ni/GDC sample is the most active for the thermochemical dissociation of H₂O and is less resistant against bulk re-oxidation. On the contrary, the ternary 3Au–3Mo-Ni/GDC sample exhibits the lowest activity for the thermochemical dissociation of H₂O and the highest tolerance against bulk re-oxidation.

Taking into account the above, H₂O, apart from being the main reactant of the co-electrolysis process, is also considered as a potential poisoning agent of the electrode. This means that the bonding strength of the adsorbed oxygen species, which result from H₂O decomposition, may induce the re-oxidation of the electrode and finally the deactivation of the sample [46,53]. The H₂O poisoning effect in electrolysis and co-electrolysis processes can also be correlated to the CH₄ poisoning effect in SOFC applications, where degradation is enhanced by the strong adsorption bond of CH₄ on the Ni surface [23,42,43,44,45,54,55]. Thus, the stronger binding of the adsorbed oxygen species, which result from the catalytic dissociation of H₂O_ads, can similarly cause a poisoning effect on the Ni surface, leading to faster re-oxidation and finally deactivation of the sample [46,53]. According to Besenbacher et al. [56], the surface solid solution between Au and Ni induces significant modifications in the electronic properties of Ni (Fermi level, work function, and d-band center) and affects the bonding strength of the adsorbed species on the surface of the sample.

In this respect, the Au- and Au–Mo- doping of Ni should shift the d-band center to lower energies, with respect to the Fermi level of nickel, thus inhibiting the interaction and the dissociation of H₂O. Therefore, the 3Au-Ni/GDC and 3Au–3Mo-Ni/GDC samples are suggested to be the least active samples for the dissociation of H₂O, having the highest E_a,app and consequently the lowest binding energies for both H₂O and O_ads. The binary 2Fe-Ni/GDC is suggested to be the most active sample for the dissociation of H₂O, with the lowest E_a,app, resulting in O_ads species with the highest binding energy and the potential to cause faster re-oxidation/poisoning of the electrode [46,53] during SOEC operation.

2.2. Catalytic-Kinetic Measurements of the RWGS Reaction

All the prepared samples, in the form of half-electrolyte supported cells, were tested at Open Circuit Potential (OCP) conditions to elucidate their catalytic activity towards the production of CO, which is one of the products from the H₂O/CO₂ co-electrolysis reaction. Furthermore, the effect of the modification (type of dopant: Au, Mo, Fe) on the catalytic activity for the production of CO, through the RWGS reaction, was also investigated. The latter approach is considered as an attempt to create a reference profile for the catalytic performance of the candidate electrodes, under the same experimental conditions as in co-electrolysis (including the presence of current collector). This is an important step, because it will provide detailed experimental feedback on the possible contribution of the RWGS reaction to the production of CO and the extent of this contribution to the electrochemical reduction of CO₂, during the co-electrolysis mode.

In regards to the “homogenous” catalytic production of CO, the rate was found to be negligible, in the range of r_{co, homogenous} = 0.05 μmol/s, for a fuel feed comprising 30 vol.% He − 24.5 vol.% H₂O − 24.5 vol.% CO₂ − 21 vol.% H₂. Concerning the current collector, Ni-mesh shows some catalytic activity for the production of CO (r_CO), which increases by increasing the operating temperature and the partial pressure of H₂. However, comparative r_CO measurements (see Figure S1 in Supplementary Material) of an Ni/GDC electrode with and without the presence of Ni mesh as the current collector, as well as of the bare Ni mesh, suggest that there is no direct catalytic correlation/contribution of the Ni mesh to the activity of the electrocatalysts. This is mainly explained by the fact that Ni/GDC is a porous electrocatalyst with SSA_BET and thus with more active sites, compared to the metallic nickel mesh, which does not possess similar properties. Consequently, from the point where they co-exist as the electrode and current collector, the catalytic activity is mainly attributed to the electrocatalyst.

Figure 4 presents the comparison of the produced r_CO for the Ni/GDC and for modified-Ni/GDC electrodes, under three different H₂O-CO₂-H₂ mixtures. The corresponding %CO₂ conversions are depicted in Figure 5. The values are low enough to be considered in the differential region, apart from the case of 2Fe-Ni/GDC in the reaction mixture (A), which is relatively high. The % conversion of CO₂ was calculated by the following formula:

$${\mathrm{CO}}_2\mathrm{conversion}(\%)=\frac{{\mathrm{F}}_{\mathrm{CO}}^{\mathrm{out}}}{{\mathrm{F}}_{{\mathrm{CO}}_2}^{\mathrm{in}}}·100$$

(6)

where: ${\mathrm{F}}_{\mathrm{CO}}^{\mathrm{out}}$ and ${\mathrm{F}}_{{\mathrm{CO}}_2}^{\mathrm{in}}$ correspond to the rate (μmol/s) of the produced CO and of the introduced CO₂ in the reactants feed, respectively.

It is shown (Figure 4 and Figure 5) that 2Fe-Ni/GDC and 3Mo-Ni/GDC have the highest catalytic activity of the examined electrodes for the RWGS reaction. In fact, 2Fe-Ni/GDC is the most active in terms of the produced CO. The above performance is observed for all applied fuel feeds (P_H2/P_CO2 = 0.86, 0.50 and 0.22), whereas it is enhanced by increasing (i) the operating temperature and (ii) the partial pressure of H₂ in the fuel feed. The enhanced catalytic performance of the Fe-modified sample, can be primarily ascribed to the possible stronger adsorption and consequent catalytic dissociation of CO₂ on the active sites of the catalyst. This first conclusion is going to be further examined with specific CO₂ Temperature Programmed Desorption (TPD) measurements.

Another noteworthy remark is that the produced CO rates were compared to the thermodynamic equilibrium rates (dash line in Figure 4) for the three different H₂O-CO₂-H₂ reaction mixtures. The equilibrium rates were calculated by using the equilibrium constant (K_eq) formula that is reported in [57,58]. It was found (Figure 4) that Ni/GDC, the binary Au-, and the ternary Au–Mo- modified samples exhibit CO production rates, which are lower than the thermodynamic equilibrium for all the examined reaction conditions. The performances of 2Fe-Ni/GDC and 3Mo-Ni/GDC are closer to the equilibrium, but cannot be considered as thermodynamically limited.

The results from the measurements in Figure 4 can also be presented as Arrhenius plots, depicted in Figure 6, where the derived apparent activation energies (E_{a, app}) for the production of CO and consequently for the RWGS reaction are listed in

Table 3. Apparent activation energies (E_a,app, kJ/mol) for the RWGS reaction on ESCs for three different mixtures: (A) 24.5% H₂O − 24.5% CO₂ − 21% H₂ (P_H2/P_CO2 = 0.86), (B) 28% H₂O − 28% CO₂ − 14% H₂ (P_H2/P_CO2 = 0.50), and (C) 31.5% H₂O − 31.5% CO₂ − 7% H₂ (P_H2/P_CO2 = 0.22).

Sample	Ea, apparent (kJ mol−1) and A* (μmol s−1 g−1), per reaction mixture
	PH2/PCO2 = 0.86		PH2/PCO2 = 0.50		PH2/PCO2 = 0.22
Ni/GDC	20	2.3 × 103	22	3.6 × 103	20	1.0 × 103
3Au-Ni/GDC	85	1.9 × 106	74	5.0 × 105	46	1.5 × 104
3Mo-Ni/GDC	27	6.5 × 103	14	1.1 × 103	21	1.4 × 103
3Au–3Mo-Ni/GDC	43	1.7 × 104	45	1.5 × 104	50	1.6 × 104
2Fe-Ni/GDC	21	3.3 × 103	19	1.6 × 103	21	1.3 × 103

^*A: The pre-exponential factor in the Arrhenius equation, $\mathrm{r}=\mathrm{A}·\exp (-\frac{E_{a,app}}{R·T})$.

.

The Arrhenius plots and the calculated E_{a, app} show that 2Fe-Ni/GDC, 3Μο-Ni/GDC, and Ni/GDC have practically the same and the lowest E_{a, app}. However, the Fe- and Mo- modified samples exhibit higher pre-exponential factors, compared to Ni/GDC, which explains the higher production rates of CO. 3Au-Ni/GDC shows overall the highest E_{a, app}, which is an additional indication for its worst catalytic activity. Finally, the ternary 3Au–3Mo-Ni/GDC sample shows an apparent activation energy, which lies between that for 2Fe-Ni/GDC and 3Au-Ni/GDC. However, the catalytic activity of the ternary sample is the lowest, due to the lower pre-exponential factor compared to the binary Au-modified sample. According to the knowledge of the authors, there are no literature data available for experimentally measured E_{a, app}, for similar samples and reaction conditions. The so far available data come from theoretical investigations and there is a recent study from Cho et al. [41], who performed DFT calculations to evaluate the ability of various transition metals to increase the activity of Ni for the H₂O/CO₂ co-electrolysis. In particular, they computed the activation energies of specific elementary reaction steps and in the case of the RWGS on Ni(111), the E_a was found to be approximately 46 kJ/mole, which is very close to the values that were experimentally calculated in the present study.

The effect of H₂ partial pressure on the catalytic rate of CO production is further verified in Figure 7 for all samples at 900 °C and 800 °C. The 2Fe-Ni/GDC sample is the most active and 3Au–3Mo-Ni/GDC the least one. The 3Au-Ni/GDC catalyst at 900 °C shows similar performance with that of Ni/GDC. In addition, by decreasing the temperature at 800 °C, the catalytic activity of 3Au-Ni/GDC exhibits the highest reduction. This is apparent from the significant decrease in the slope of the corresponding curve (Figure 7) from 800 °C to 900 °C and can be further explained from the calculated E_{a, app} (Table 3), which is the highest from all samples. Finally, the kinetic behavior of all samples suggests that the production rate of CO exhibits a positive order dependence on the partial pressure of H₂.

The fact that in the majority of the samples there is no trend in the E_{a, app}, by varying the pH₂/pCO₂ ratios, primarily shows that all samples, apart from the binary Au-modified, have similar intrinsic catalytic activity for the RWGS reaction. This is also concluded from the remark that the effect of the H₂ partial pressure on the CO production rate is almost linear. On the other hand, in the case of 3Au-Ni/GDC, there is an increase in E_{a, app,} by increasing the pH₂/pCO₂ ratio. This is corroborated from the results in Figure 7, where r_CO seems to reach a plateau at high pH₂. The latter remarks indicate that the majority of the samples have actives sites that follow a similar reaction mechanism, in regards to the dissociative adsorption of H₂ and CO₂ towards the RWGS. On the other hand, the reaction mechanism seems to be different in the case of 3Au-Ni/GDC. In particular, it is implied that the high coverage of adsorbed H₂ may inhibit the CO₂ dissociative adsorption and thus decreases the intrinsic catalytic activity of the specific sample. This suggestion is going to be further clarified through the currently performed H₂O/CO₂ co-electrolysis measurements.

Carbon formation, through the Boudouard reaction (6), was also investigated and the results are presented in Figure 8. The slight scattering of the experimentally measured rates in combination with the theoretical values, where r[CO₂]_inlet is equal to r[CO + CO₂]_outlet, suggest that there is no carbon deposition under the examined reaction conditions. This result is also in accordance with the fact that the Boudouard reaction is not thermodynamically favored above 750 °C [59].

Previous studies [1,59,60] reported that at realistic CO₂/CO concentrations, during CO₂ electrolysis in the range of 650–750 °C, the equilibrium of (6) is shifted towards CO production (reaction (7)) and therefore carbon will not be formed, catalytically, during CO₂ electrolysis. In the case where the cell is operated at OCP conditions, as in our case, any coke deposited within the porous, modified or not, Ni/GDC electrode would be oxidized to CO according to the reverse Boudouard reaction (7) and thereby removed. Furthermore, the addition of steam in the feed is reported to remove carbon depositions according to reaction (8) [1,8]:

Boudouard reaction:  2CO → CO₂ + C

(7)

Reverse Boudouard reaction:  CO₂ + C → 2CO

(8)

Reaction of coke with H₂O:  C + H₂O → CO + H₂

(9)

3. Materials and Methods

3.1. Preparation of Electrocatalysts

The modified cathode powders were prepared via the Deposition-Precipitation (D.P.) and Deposition-Co Precipitation (D.CP.) methods by using the commercial NiO/GDC cermet (65wt% NiO-35wt% GDC, Marion Technologies, Verniolle, France) as the support. The precursors for the 3wt% Au-NiO/GDC, 3wt% Mo-NiO/GDC, 3wt% Au−3wt% Mo-NiO/GDC, and 2wt% Fe-NiO/GDC samples were the HAuCl₄ (Sigma-Aldrich) or/and (NH₄)₆Mo₇O₂₄ (Sigma-Aldrich, St. Louis, MO, USA) and Fe(NO₃)₃x9H₂O (Sigma-Aldrich, St. Louis, MO, USA) solutions, respectively. Full details about the synthesis can be found elsewhere [42,43]. After filtering, the precipitate was dried at 110 °C for 24 h. All dried powders were calcined at 600 °C/90 min and a part of them at 1100 °C/75 min. The first batch was used for the paste preparation, which is described in the following paragraph. The batch at 1100 °C was used for the physicochemical characterization. In this way, the prepared catalysts were studied at similar thermal stress as the calcined electrode-electrolyte assemblies.

3.2. Preparation of Electrolyte-Supported Half Cells

The electrolyte-supported half cells consisted of circular shaped planar 8YSZ electrolyte (by Kerafol) with a 25 mm diameter and 300 μm thickness. The fuel electrode was deposited by using the screen printing technique as reported in previous studies [23,46]. In particular, a paste was prepared by using an appropriate amount of the electrocatalyst (modified NiO/GDC powder), terpineol (Sigma-Aldrich, St. Louis, MO, USA) as the dispersant, and PVB (polyvinylbutyral, Sigma-Aldrich, St. Louis, MO, USA) as binder. After the deposition of the paste, the cell was sintered at 1150 °C with a heating/cooling ramp rate of 2 °C/min. The last temperature is the lowest possible in order to obtain proper adherence of the electrode on the electrolyte, whereas it is equivalent with the calcination temperature of the characterized powders (1100 °C). The examined electrodes were approximately 20 μm thick and their loading varied in the region of 10–12 mg/cm² with a 1.8 cm² geometric surface area (Figure 9). The prepared half cells were adjusted on a ceramic YSZ tube and sealed airtight with a glass sealing material manufactured by Kerafol.

3.3. Physicochemical Characterization

The samples, in the form of powders, were characterized with BET, H₂-TPR, Air-TPO, and TGA re-oxidation measurements in the presence of H₂O.

The BET specific surface area values of the samples were determined from the adsorption isotherms of nitrogen at −196 °C, recorded with a Micromeritics TriStar 3000 apparatus (Micromeritics, Norcross, GA, USA).

The re-oxidation measurements in the presence of H₂O, as well as H₂-TPR and Air-TPO measurements were carried out with a TA Q50 instrument. The H₂O re-oxidation properties of the powders were studied with TGA measurements, at a constant temperature in the range of 650–800 °C. Before the measurement, the samples were pre-reduced in-situ with 80 vol.% H₂/Ar at 800 °C for 100 min and then the feed was changed to 15.5 vol.% H₂O/Ar. The total flow rate (at STP conditions: 0 °C, 1 atm) was 100 cm³/min and the loading of the measured samples was approximately 50 mg. Steam was introduced in the reactor by passing Ar through a saturator, which was maintained at a fixed temperature (65 °C).

3.4. Catalytic-Kinetic Measurements

The prepared half cells were catalytically investigated at Open Circuit Potential (OCP) conditions for the RWGS reaction, in the presence of Ni mesh. The catalytic experiments were accomplished at temperatures between 800–900 °C under various H₂O/CO₂/H₂ mixtures by keeping in all cases the P_H2O/P_CO2 ratio constant. In regards to the experimental part of these measurements, H₂O was added and handled in the feed in the form of steam, as in the SOEC measurements. Before its evaporation, liquid H₂O was pressurized in a container and circulated in the system by means of a liquid water mass flow controller. Then, liquid H₂O was evaporated through lines and valves, heated at 160 °C, to prevent water condensation. The flow rate was fixed at 140 cm³/min (at STP conditions: 0 °C, 1 atm), avoiding any mass transfer limitation effects in the reactor. Reactants and products were determined by using an on-line gas chromatograph (Varian CP-3800) with a thermal conductivity detector. Further details regarding the experimental parameters are indicated in the corresponding Figures.

4. Conclusions

The presented study deals with the kinetic investigation of Ni-based (modified or not) electrodes towards their performance for the RWGS reaction. The samples were examined in the form of electrolyte-supported (half) cells and the measured kinetic parameter was the production rate of CO. The main objective was to clarify the effect of the modification on the catalytic activity for the RWGS, which is considered as a key reaction for the CO production under H₂O/CO₂ co-electrolysis operation. The reaction conditions were similar to those that are applied under co-electrolysis mode.

Redox stability measurements in the presence of H₂O showed that the ternary 3Au–3Mo-Ni/GDC electrode is the least active sample for the dissociation of H₂O, having the highest E_a,app and consequently the lowest binding energy for the H₂O_ads. On the other hand, the binary 2Fe-Ni/GDC is the most active sample for the dissociation of H₂O, thus having the potential to experience faster re-oxidation. Complementary characterization suggests that the interaction of Ni and Fe (FeO_x species in the oxidized form of the sample) during the H₂-reduction process increases the SSA_BET and affects the bulk properties of the binary Ni-Fe/GDC. The interaction can be realized through the possible formation of an Ni-Fe solid solution, which is currently under further clarification and may be responsible for enhancing the catalytic dissociation of H₂O_ads.

The kinetic study of the candidate electrocatalysts showed that Au modification inhibits the catalytic production of CO, through the RWGS reaction, while modification with Fe or Mo induces an enhancement of r_CO. In fact, the 2wt% Fe-Ni/GDC sample is the most active both in terms of %CO₂ conversion and of the produced CO. In addition, a negative synergy was observed for the ternary Au–Mo-Ni modified sample. Specifically, the 2wt% Fe-Ni/GDC and 3wt% Mo-Ni/GDC samples showed similar apparent activation energy for the RWGS reaction as that of Ni/GDC, while the 3wt% Au-Ni/GDC and 3wt% Au–3wt% Mo-Ni/GDC samples showed lower E_a,app. The above performance is observed for all applied fuel feeds (P_H2/P_CO2 = 0.86, 0.50, and 0.22), whereas it is enhanced by increasing (i) the operating temperature and (ii) the partial pressure of H₂.

Consequently, the kinetic behavior of all samples suggests that the production rate of CO exhibits a positive order dependence on the partial pressure of H₂, whereas carbon formation was not detected. In the case of the most active 2Fe-Ni/GDC sample, it is proposed that Fe-modification may enhance the catalytic dissociative adsorption of CO₂ towards the production of CO and this is further investigated. Finally, it is worth mentioning that the RWGS catalytic performance of both Fe- and Mo- modified samples is close to the equilibrium, but cannot be considered as thermodynamically limited.

Overall, the presented results correspond to a reference catalytic profile of the examined modified Ni/GDC samples for the RWGS reaction. These candidate electrocatalysts are currently being examined, as full electrolyte supported cells, in SOEC measurements for the H₂O/CO₂ co-electrolysis reaction to elucidate any additional effects by the applied current. Moreover, further investigation of the Fe-modification on NiO/GDC for the H₂O/CO₂ electrolysis processes will occur in the future.