Performance-enhanced direct ammonia protonic ceramic fuel cells using CeO₂-supported Ni and Ru catalyst layer

Abstract

Ammonia is an exceptional fuel for solid oxide fuel cells (SOFCs), because of the high content of hydrogen and the advantages of carbon neutrality. However, the challenge lies in its unsatisfactory performance at intermediate temperatures (500‒600 °C), impeding its advancement. An electrolyte-supported proton-ceramic fuel cell (PCFC) was fabricated employing BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) as the electrolyte and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ (BSCF) as the cathode. In this study, the performance of PCFC using NH₃ as fuel within an operating temperature range of 500‒700 °C was improved by adding an M(Ni,Ru)/CeO₂ catalyst layer to reconstruct the anode surface. The electrochemical performance of direct ammonia PCFC (DA-PCFC) were improved to different extents. Compared to H₂ as fuel, the degradation ratio of peak power densities (PPDs) of Ni/CeO₂-loaded PCFC fueled with NH₃ decreased at 700‒500 °C, with a decrease to 13.3% at 700 °C and 30.7% at 500 °C. The findings indicate that Ru-based catalysts have a greater promise for direct ammonia SOFCs (DA-SOFCs) at operating temperatures below 600 °C. However, the enhancement effect becomes less significant above 600 °C when compared to Ni-based catalysts.

Graphical abstract

Keywords

ammonia / proton-ceramic fuel cell (PCFC) / anode / M/CeO₂ catalyst layer

1 Introduction

Solid oxide fuel cells (SOFCs) represent one of the cleanest, most efficient, and versatile energy conversion system. One of the key benefits associated with SOFCs is their exceptional fuel flexibility and versatility. Hydrogen is the ideal fuel for achieving sustainable development in green energy. However, the key technical challenge limiting the extensive use of hydrogen is its relatively lower volumetric energy density compared to other fuels [1]. This requires a larger storage volume, leading to increased transportation and storage costs. In addition to hydrogen, carbonaceous fuels such as methane, methanol, and ethanol are also attractive options for SOFCs due to their narrow explosion limits, affordability, and ease of storage [2–4]. The primary challenge posed by carbonaceous fuels in SOFCs lies in the significant performance deterioration resulting from carbon deposition and sulfur poisoning during the carbonaceous fuels conversion process. This problem still needs to be further investigated and solved [5–7]. Ammonia can be a suitable choice as a clean energy carrier and storage medium.

As a hydrogen carrier fuel, ammonia, with features of zero-carbon and a high hydrogen content, offers the advantages of low source cost [8,9], easy storage [10] and transportation [11]. Ammonia is easily liquefied at room temperature [12]. Liquefied ammonia has significant advantages in terms of volumetric energy density in comparison to compressed hydrogen, liquefied hydrogen, and lithium-ion batteries [1]. In addition, it is possible to achieve internal cracking mode of NH₃ in the anode chamber of SOFCs to convert directly to nitrogen and hydrogen. This feature simplifies the design and enhances thermodynamic efficiency, making it a highly promising fuel candidate for the commercialization of SOFCs [13].

Ammonia has been reported to have a great potential as a fuel in fuel cell, through an indirect system [14]. In this process, a reformer is utilized to convert ammonia into hydrogen gas and nitrogen gas before entering the cell. It is also important to note that the decomposition of NH₃ occurs at the working temperature range of SOFCs (773‒1273 K) [15], whose process can be represented by

{\text{NH}}_3=\frac{3}{2}{\text{H}}_2+\frac{1}{2}{\text{N}}_2,

\mathrm{\Delta }H= 46.22\text{kJ} \cdot {\text{mol}}^{-1}.

The concept of the internal direct cracking mode collectively referred to as direct ammonia SOFCs (DA-SOFCs) involves the integration of the endothermic ammonia decomposition reaction and the exothermic electrochemical reaction within the SOFCs. This integration improved thermal efficiency due to the efficient utilization of the heat generated during the electrochemical and ammonia decomposition processes [15]. In recent years, there has been a rapid development in both material and mechanism research on SOFC cathodes [16–21]. To achieve direct internal decomposition, a versatile anode with an excellent catalytic activity for both ammonia decomposition and electrochemical reactions is essential. The choice of appropriate materials and design features plays a crucial role in enabling this process.

Apparently, the attention of researchers has been drawn to the use of ammonia as a sustainable fuel in SOFCs [22–29]. Several experimental studies and pioneering theoretical work on ammonia for SOFCs have been reported. In particular, the proton-ceramic fuel cell (PCFC), which is an ammonia-based fuel suitable for relatively low-temperature operation, has gained high attention [30]. Ammonia-fed cells with NiO-BaCe_0.8Gd_0.2O_2.9 (BCGO) anodes achieved a peak power density of 384 mW/cm² at 750 °C, demonstrating a comparable performance to hydrogen feeding [31]. However, according to Xie et al. [32], the performance of NiO-BaCe_0.9Nd_0.1O_3‒δ (BCNO) as anode fuelled with NH₃ at 700 °C is not as good as the performance achieved with H₂ as fuel. Lin et al. [27] compared the cell performance of hydrogen and ammonia as fuels for PCFC at different temperatures and found that at a high temperature (750 °C), comparable similar power densities were observed for cells operating on H₂ and NH₃, while the performance gap between NH₃ and H₂ increased progressively with the falling of temperature. This indicates that the rate of conversion of NH₃ at a low temperature is low at the anode. The fuel inlet area is the catalytic active area for ammonia conversion [33]. In the absence of a suitable catalyst, temperature-driven ammonia conversion is not sufficient, even at high temperatures.

Over the years, NH₃ decomposition catalysts have been extensively studied for metal-based catalysts, including Ru, Rh, Pd, Pt, Ir, Ni, Co, Fe, etc. [34,35]. Impregnation of Pd at Ag-LSCF ((La_0.60Sr_0.40)_0.95Co_0.20Fe_0.80O_3‒x) composite anode has been reported to enhance NH₃ dissociation by accelerating hydrogen dissolution [36]. Impregnation is common in the field of catalysis, but the process is cumbersome and complex when applied to SOFC anodes. Although Fe-based catalysts are widely used in the catalytic decomposition of NH₃, their activity is much lower than that of Ru [37–39]. Ni not only has the advantages of high activity in H₂ electro-catalytic oxidation, high electronic conductivity and good thermal/chemical compatibility in comparison of other SOFC materials, but also as a non-precious metal has a higher NH₃ decomposition activity than Fe. Therefore, it has broad applicatory prospects [33,40,41].

It is well known that nitrogen desorption is the reaction-limiting step in ammonia decomposition and that metal-nitrogen interactions directly affect the rate of ammonia decomposition reaction [42,43]. Taking this into account, researchers have attempted to increase the catalytic rate of NH₃ decomposition by using a variety of approaches to promote the desorption of the catalyst with nitrogen. It has been shown that increasing the alkalinity of the catalyst support enhances the desorption of nitrogen [44–46]. Bimetallic catalysts are also promising choices for promoting the NH₃ decomposition activity. Transition metals have high catalytic activity for ammonia decomposition. For example, Ni improves activity by promoting desorption of nitrogen molecules from the catalyst surface [47–49].

The rate of ammonia decomposition is affected not only by the active metal of the catalyst, but also by the way the active metal interacts with the support of the catalyst. In addition to having an excellent stability and providing a high specific surface area, the catalyst support material also needs to have a good electrical conductivity. This is because providing electron transfer facilitates the desorption of nitrogen. It was demonstrated that when Ni was used as the active metal, CeO₂ showed an excellent performance as a catalyst support for NH₃ decomposition compared to MgO, Al₂O₃, Y₂O₃, and ZrO₂ .

In this study, the cell performance of hydrogen and ammonia as fuels on electrolyte-supported-PCFC with proton-conducting electrolyte of BaZr_0.1Ce_0.7Y_0.2O_3‒δ (BZCY) and cathode of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‒δ (BSCF) at different temperatures was compared. The high alkalinity of the BCZY support contributes to the catalytic activity of the anode to ammonia decomposition and resistance to hydrogen poisoning [50,51]. In addition, BSCF has excellent electronic and ionic transport properties at low and medium temperatures [52,53]. To improve the peak power density of NH₃ fuel, the anode surface was reconstructed by integrating the M/CeO₂ catalyst with PCFC. The power density of electrolyte-supported direct ammonia PCFC (DA-PCFC) output containing a catalytic layer is higher than that of a conventional single-cell without catalytic layer coverage.

2 Experimental

2.1 Preparation of catalyst and cell fabrication

The PCFC investigated in this study was based on electrolyte-supported cells. The BZCY and BSCF materials were prepared using the sol-gel method. Taking BSCF as an example, Ba(NO₃)₂, Sr(NO₃)₂, Co(NO₃)₂·6H₂O, and Fe(NO₃)₃·9H₂O were sequentially added into deionized water at a specific stoichiometric ratio. The solution was then heated and stirred at 60 °C until clarified. Subsequently, citric acid monohydrate and EDTA were added in a molar ratio of 1.5:1.5:1. After clarification of the solution, the pH was adjusted to approximately 9. The solution was further evaporated with stirring until it formed a gel. It is then placed in an electrically heated blower at 150 °C until carbonized into a solid precursor. Finally, the solid precursor was annealed in a muffle furnace at 900 °C for 6 h. The electrolyte powder underwent annealing at 1000 °C for 5 h.

The electrolyte powder was prepared by ball milling a mixture of BZCY as the starting material, 1 wt.% NiO, and polyvinyl butyral (PVB) as sintering and binder agents. The electrolyte support was prepared by pressing 0.25 g of electrolyte powder into 15 mm disc-shaped sheets using a grinding tool, and then pre-fired at 1000 °C for 2 h. Pine oil alcohol with ethyl cellulose was added to the mixture of Ni and BZCY, and the resulting anode slurry was obtained after sufficient grinding. The electrolyte sheet was coated with anode slurry on one side, dried, and co-fired at 1500 °C for 10 h. Subsequently, the cathode paste was applied on the other side of the electrolyte and subjected to calcination.

The catalysts used in this study were prepared by the co-precipitation method [54]. Taking Ni/CeO₂ as an example, a certain stoichiometric ratio of Ce(NO₃)₂∙6H₂O, Ni(NO₃)₃∙6H₂O, and polyethylene glycol were added to 200 mL of deionized water and stirred until fully dissolved. The prepared precursor solution and 1 mol/L Na₂CO₃ solution were gradually added dropwise to a distillation flask kept at 60 °C with a bottom solution. The pH in the flask was maintained at 8 by continuous stirring. The suspensions were aged at a constant temperature of 60 °C for 16 h. The aged solution was filtered and washed multiple times with deionized water until the pH of the solution was between 6 and 7. Lastly, the samples were dried at 90 °C for 12 h and then calcined at 600 °C for 4 h. The CeO₂-based catalysts referred to in this work use M/CeO₂ collectively. 1Ru/CeO₂, 10Ni/CeO₂, and 1Ru-10Ni/CeO₂ represent CeO-based catalysts with 1 wt.% Ru, 10 wt.% Ni, and 1 wt.% Ru-10 wt.% Ni, respectively.

The catalyst was formulated into a slurry by grinding it with ethyl cellulose-containing pine oil alcohol as the solvent. Subsequently, the catalyst slurry was coated onto the surface of the cell anode using the screen-printing technique. The catalyst-coated cell was then subjected to calcination at 600 °C for 2 h to construct the catalytic layer.

2.2 Characterizations

The crystal structure of the synthesized catalysts was examined using an X-ray diffractometer (XRD, Panalytical, X'Pert3 Powder) with Cu-Kα radiation (the operating current = 40 mA and voltage = 40 kV) in the 2θ range of 10°−70°. Scanning electron microscope (SEM) was used to observe the morphology of the cell after high-temperature sintering. The specific surface area and pore structures of the catalyst samples were analyzed using a gas adsorber (Micrometrics, ASAP 2020) with liquid nitrogen as the adsorptive medium. The H₂ temperature-programmed reduction (H₂-TPR) experiments were conducted using a fully automated chemisorption instrument equipped with a thermal conductivity detector (TCD) (Micrometrics, AutoChem 2920).

The catalytic activity of the M/CeO₂ for fuel gases was evaluated in a fixed-bed quartz tubular micro-reactor. The mass of the powder catalyst is 100 mg, and the bottom was lined with quartz cotton. The reactor is placed in a vertical furnace connected to an external thermal control system. The catalytic activity of the samples for ammonia decomposition was evaluated at 50 °C intervals ranging from 500 to 700 °C, and the analysis of tail gas was performed using gas chromatography (Agilent).

The electrochemical properties tested in this study include open-circuit voltage, dynamic potential polarization test (polarization curve) measured using an electrochemical workstation (Gamry) with a four-probe method. Ag wires with a small amount of Ag paste were used as current collectors on both the anode and cathode sides to ensure proper electrical contact. To prevent air leakage, ceramic glue was used to seal the single cell at one end of the corundum tube. Before the electrochemical test, the anodes were pre-treated in dry H₂ (50 mL/min) for 1 h to ensure the reduction of NiO to Ni. During the electrochemical tests, both H₂ and NH₃ were supplied at a gas flow rate of 100 mL/min. The characterization of porous structure of prepared catalysts was performed by nitrogen adsorption/desorption.

3 Result and discussion

As depicted in Fig.1, the ammonia conversion at different temperatures for the catalysts prepared in this study is demonstrated. As expected, ammonia decomposition increased with increasing temperature for all catalysts and the only product obtained was a 3:1 ratio of H₂ to N₂. The catalytic activity for ammonia decomposition of M/CeO₂ catalysts was higher than that of Ni/BZCY at all tested temperatures. The catalyst 1Ru/CeO₂ exhibits a higher catalytic activity for ammonia decomposition between 500 and 550 °C compared to 10Ni/CeO₂ and bimetallic 1Ru-10Ni/CeO₂. The ammonia conversion of 1 wt.% Ru-based catalyst at 500 °C was 49.3%, whereas it was only 23.4% for the 10Ni/CeO₂ catalyst. However, above 550 °C, the ammonia conversion of the bimetallic catalyst was higher, reaching 96.6% at 650 °C. Although the addition of Ni to the 1 Ru/CeO₂ did not significantly enhance the catalytic activity of ammonia decomposition between 500 and 550 °C, it had a positive effect from 550 to 650 °C.

As presented in Fig.2(a), the XRD patterns of the three samples primarily have diffraction peaks corresponding to the presence of the catalyst support, i.e., CeO₂ phase. The XRD diffractions of 1Ru/CeO₂ and 1Ru-10Ni/CeO₂ reveal the presence of RuO peaks, with a low intensity indicating a high dispersion of the RuO phase . The XRD patterns of 10Ni/CeO₂ and 1Ru-10Ni/CeO₂ both have diffraction peaks corresponding to the NiO phase, indicating a relatively uniform distribution of NiO in both samples.

The results of the H₂-TPR for the synthesized catalysts are presented in Fig.2(b). A H₂ consumption peak above 700 °C was observed for all three catalysts, corresponding to the reduction of the common carrier CeO₂. In addition, the 1Ru/CeO₂ exhibited a H₂ consumption peak at approximately 188 °C, corresponding to the reduction of Ru²⁺ and Ru⁴⁺ to Ru⁰. As for the 10 Ni/CeO₂, two clear H₂ consumption peaks appeared at approximately 300 °C, which was attributed to the reduction of Ni²⁺ to Ni⁰. Regarding the bimetallic 1Ru-10Ni/CeO₂ catalyst, the reduction temperatures of both Ru²⁺ and Ni²⁺ shifted to lower values. This shift could be attributed to the interaction between Ni and Ru.

To assess the weaving properties of the prepared catalysts, the samples were characterized by physical adsorption and desorption. The results are depicted in Fig.2(c) and Fig.2(d). Each sample exhibited a type IV isotherm hysteresis loop at a relative pressure of (P/P₀) range of 0.6–1.0, indicating that the mesoporosity was primarily obtained. Fig.2(d) illustrates that the pore size distribution of Ru is the narrowest, while the incorporation of Ni results in a broader pore size distribution. The surface area of the samples was calculated using the Brunauer–Emmett–Teller (BET) method, which resulted in 1Ru/CeO₂ (19 m²/g) < 10 Ni/CeO₂ (63 m²/g) < 1Ru-10Ni/CeO₂ (68 m²/g). The mesopore distribution is expected to enhance the transport of reactants within the catalytic layer.

The electron microscope scans of the cell sections coated with the three catalysts are presented in Fig.3, showing that the anodes are well-bonded with the electrolyte, ensuring a low contact resistance. Clear observations reveal that the particles in the catalytic layer are smaller than those in the anode layer, attributed to the fact that the catalytic layer has not undergone high-temperature sintering.

Despite the high electrolyte Ohmic drop, the electrolyte-supported single cell was still useful in comparing the catalytic ability of anode materials for different fuels. The open circuit voltage (OCV) of the PCFC fed with H₂ is 1.098, 1.108, 1.119, 1.134, and 1.122 V between 700 and 500 °C, respectively. When using ammonia as the fuel, the OCV of the PCFC is 1.04 V at 700 °C, 0.998 V at both 650 and 550 °C, and 0.975 V at 500 °C. The OCV on H₂ is slightly higher than that on NH₃, and the OCV of the PCFC is increasing with decreasing temperature when using H₂, whereas with NH₃, the OCV of the PCFC gradually decreases.

In addition, as shown in Fig.4(a), the peak power densities (PPDs) of PCFC with hydrogen as fuel are 61.7, 42.0, 28.7, 17.7, and 8.9 mW/cm² for temperatures ranging from 700 to 500 °C. Upon switching from H₂ to NH₃ fuel, the PCFC exhibited PPDs of 49.5, 27.1, 16.4, 8.8, and 4.2 mW/cm² as temperature varied from 700 to 500 °C (Fig.4(b)). The PPDs of the ammonia-fueled PCFC was less than that of the hydrogen-fueled PCFC at all temperatures tested, and the gap between the PPDs of hydrogen and ammonia increased as the temperature decreased. When compared to the PPDs of PCFC in H₂ fuel, the degradation rate of PPDs of NH₃-driven PCFC was 19.8% and 52.8% when the temperature was lowered from 700 to 500 °C.

The poor performance of NH₃ at low temperatures can be attributed to the low partial pressure of H₂ and the simultaneous production of N₂ during NH₃ decomposition. As a result, undecomposed ammonia and the formed N₂ act as dilution gases for H₂, which partially explains the relatively low OCV at low temperatures. From a catalytic perspective, the catalytic activity of the anode for NH₃ degradation decreases with the lowering of temperature, leading to an insufficient H₂ partial pressure for the PCFC. This indicates that the performance of the direct ammonia-fed PCFC is strongly dependent on the catalytic activity of the anode for ammonia decomposition. The decrease in the catalytic activity of the anode for ammonia decomposition with decreasing temperature leads to a growing gap between the PPDs of PCFC fueled with NH₃ and H₂. Ammonia decomposition is a known endothermic reaction, resulting in a temperature difference when NH₃ is used as fuel. It is important to consider this possible temperature gradient when interpreting the experimental data.

The PPDs of PCFC with the Ni/CeO₂ catalytic layer increases to 69.8 mW/cm² when using H₂ as fuel and to 60.5 mW/cm² when using NH₃ as fuel at 700 °C. The PPDs of PCFC increases to 22.8 mW/cm² when using H₂ as fuel at 500 °C and to 15.8 mW/cm² when using NH₃ as fuel. Compared to H₂ as fuel, the degradation ratio of the PPDs of Ni/CeO₂-loaded PCFC fueled with NH₃ decreases at 700−500 °C, with a decrease to 13.3% at 700 °C and 30.7% at 500 °C. Similar to the PCFC loaded with the Ni/CeO₂ catalytic layer, the degradation ratio of the PPDs of NH₃ decreases for both Ru/CeO₂ and Ru-Ni/CeO₂ loaded PCFC. The detailed information can be obtained from Tab.1. The performance of both Ni/CeO₂ and Ru-Ni/CeO₂ loaded PCFC is slightly improved compared to that of PCFC without catalytic layer loading, when fueled with H₂. However, the performance of the PCFC loaded with Ru/CeO₂ is significantly poorer than that of the PCFC without catalyst loading, especially at 650 and 700 °C. This can be attributed to the poor electrical conductivity of the Ru/CeO₂ catalytic layer [55,56]. This drawback is mitigated by the cells with Ru-Ni/CeO₂-loaded anodes, where both H₂ and NH₃ fuels exhibit an improved performance compared to the cells without the catalytic layer. The PPD degradation rate of NH₃ decreases significantly at all temperatures tested. At 600−650 °C, the cell with Ni/CeO₂ catalytic layer-decorated anodes exhibited a lower PPDs degradation of NH₃. However, the PPDs degradation rate of NH₃ in cells with Ru-containing catalysts performed better at 500−550 °C. This result may be attributed to applying the same thickness other than controlling the quality. According to this result, the addition of Ru-based catalysts plays an important role in the enhancement of DA-PCFC performance at 500−550 °C. However, when the temperature is higher than 600 °C, the addition of 1 wt.% Ru did not lead to a significant improvement. The main factors that dominate the PCFC performance are the temperature, catalyst activity, H₂ concentration, etc.

In addition, the 1Ru-10Ni/CeO₂ catalyst was performed for 45 h of stability testing and the results are presented in Fig. S2 in Electronic Supplementary Material. There is a slight increase in ammonia conversion during the first 17 h. The conversion rate then stabilized at approximately 81% until the end of the test. This is a good indication of the durability that catalysts exhibit in ammonia decomposition.

4 Conclusions

To summarize, enhancing the activity of ammonia utilization is achieved by reconstructing the anode surface through the addition of a catalytic layer with CeO₂ as a carrier on the Ni-BZCY anode. The electrochemical performance of the DA-PCFC is improved to varying degrees by the incorporation of three different catalytic layers. The introduction of Ni/CeO₂, Ru/CeO₂, and Ru-Ni/CeO₂ improves the PPDs of NH₃ at 500 °C from 4.2 mW/cm² for the cell without catalytic layer to 15.8, 13.7, and 15.6 mW/cm², respectively. The combination of catalyst with the cell increases the PPDs when using NH₃ as fuel, while maintaining the overall electrochemical performance of the cell and reducing the power density gap between NH₃ and H₂ fed cells. At relatively low temperatures, the introduction of Ru in the catalyst is more positive for the performance enhancement of DA-PCFC. However, when the temperature is higher than 600 °C, the introduction of 1wt.% Ru in the anode of the cell is of little significance. CeO₂ serves as an effective support for ammonia decomposition, and the bimetallic system Ni-Ru/CeO₂, prepared using the co-precipitation method, offers improved metal dispersion. This compensates for the drawback of the poor catalytic activity in ammonia decomposition resulting from Ni agglomeration at the anode caused by high-temperature sintering. The direct integration of the suitable catalyst with the anode significantly simplifies the entire system, reduces costs, and improves energy efficiency.

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