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Article

Cathode Catalyst PdAgCo/C for Optimal Performance of the Alkaline Anion Exchange Membrane Direct Ammonia Fuel Cells

by
Prithiv Vengatasalapathy
1,2,
Fa-Cheng Su
1,
Zi-Jie Su
1,
Kean Long Lim
3,* and
Hsiharng Yang
1,4,5,*
1
Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung City 402, Taiwan
2
Department of Chemical Engineering, ICT Mumbai IOC Bhubaneswar, Bhubaneswar 751013, India
3
Fuel Cell Institute, University Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia
4
Innovation and Development Center of Sustainable Agriculture (IDCSA), National Chung Hsing University, Taichung City 402, Taiwan
5
Smart Sustainable New Agriculture Research Center (SMARTer), Taichung 402, Taiwan
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(9), 825; https://doi.org/10.3390/catal15090825
Submission received: 29 July 2025 / Revised: 27 August 2025 / Accepted: 29 August 2025 / Published: 31 August 2025
(This article belongs to the Section Electrocatalysis)
Browse Figures
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Abstract

This investigation addresses the enhancement of ammonia fuel cell performance using Pd (palladium)- and Co (cobalt)-doped cathode catalysts. Initially, the performance of the Ag/C cathode catalyst in ammonia fuel cells yielded a baseline power density of only 38 mW/cm2. To improve efficiency, Pd and Co were introduced, resulting in the synthesis of a new 15 wt% PdAgCo/C (15 wt% PdAgCo and 85 wt% C) cathode catalyst, which increased the power density to 74 mW/cm2. Further performance enhancement was achieved by using a highly efficient 40 wt% PtIr/C anode catalyst, as reported in the literature, and applying a cathode catalyst loading of 0.5 mg/cm2, raising the power density to 172 mW/cm2. This investigation addresses the successful synthesis of a 15 wt% PdAgCo/C cathode catalyst, which has proven to be a better choice over conventional catalysts, along with the significance of doping Pd and Co with Ag/C in the augmentation of catalytic activity and fuel cell performance. Thus, a series of physicochemical and electrochemical characterizations, the approach for optimization of the working parameters, and the impact analysis of catalyst loading have all resulted in the achievement of an impeccable power density of 332 mW/cm2.

1. Introduction

Developing sustainable and eco-friendly energy solutions has become crucial as the world faces serious challenges such as the energy crisis, global warming, climate change, population growth, and rising living standards; thus, it is necessary to reduce fossil fuel dependence to achieve the goals of carbon reduction and sustainable development [1]. Most of the renewable energy sources that are eco-friendly show an intermittent and unstable nature, which poses a serious threat to the supply of energy to balance the demand [2]. Hydrogen fuel cell technology has attracted widespread attention as an efficient and clean source of energy [3]. However, the limitations associated with the storage and transportation of hydrogen limit its potential for large-scale applications. In contrast, ammonia, with a lower cost of production, easier storage and transportation, and higher energy density, has attracted widespread attention from the scientific and industrial communities as a potential alternative for hydrogen as a source of sustainable and clean energy, supporting the achievement of Net Zero Carbon Emissions (NZCE) by 2050 [4].
The alkaline anion exchange membrane (AEM) direct ammonia fuel cell (DAFC) operates in a low-temperature range of about 50~120 °C and uses ammonia gas or ammonia water (NH4OH) along with oxygen to produce electricity, due to the transfer of electrons via a redox reaction. Since the sole byproducts of the reaction are water and nitrogen, it is thus regarded as an eco-friendly energy source. Given below is the corresponding electrochemical reaction shown by DAFC.
Anode Reaction: 2NH3 + 6OH− → N2 + 6H2O + 6e−, E°298K = −0.77 V
Cathode Reaction: 1.5O2 +3H2O + 6e− → 6OH−, E°298K = 0.4 V
Total Reaction: 2NH3 + 1.5O2 → N2 + 3 H2O, E°298K = 1.17 V
AEM-DAFC offers several advantages: enhanced operating efficiency, reduced cost and energy usage due to the direct utilization of ammonia, reduced emission of NOx in line with environmental friendly approach, reduced probability of catalyst poisoning or deterioration of fuel cell efficiency, and relatively lower temperature of operation compared to traditional fuel cells like traditional high-temperature solid oxide fuel cells (SOFC), thus reducing energy consumption and the risk of corrosion or failure of materials [5]. Despite the aforementioned advantages, it poses various limitations. This challenge arises from the sluggish electrochemical reaction kinetics at lower operating temperatures, which underscores the strong need for an effective electrocatalyst to promote the ammonia oxidation reaction (AOR) [6] and thereby enhance the overall performance of the fuel cell.
Precious transition metals (Pt, Pd, Rh, and Ir) have proven to be the most active catalysts, while coinage metals (gold, silver, and copper) only showed limited activity in catalyzing the oxidation of ammonia and intermediates formed when the surface is oxidized [7]. Platinum, as an electrocatalyst for DAFCs, was first proposed by EL SIMONS et al., using the 54 wt% KOH aqueous solution as the electrolyte [8]. Reza Abbasi et al. illustrated that metals such as Pt, Pd, Au, and Ag exhibit remarkable electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline media [9]. The activity of these electrocatalysts could be enhanced by altering their surface atomic structure, which includes shape, morphology, and strain controls due to their versatile nature. In order to establish cost-effective electrocatalysts for commercial use, Ni/Ni(OH)2 was promoted as an electrocatalyst with good AOR activity in 2010 and showed broad application prospects [10]. Alloys like NiCu have shown good performance in the electrocatalysis of AOR in low-temperature DAFCs [11,12]. Muroyama et al. studied the synergistic effect between metal oxides and platinum and demonstrated that the SnO2-modified Pt/C anode has better performance than commercial Pt/C for the operation of low-temperature DAFCs [13]. Additionally, Habibzadeh et al. reported that a ruthenium (Ru)–polypyridine complex exhibited desirable electrocatalytic activity for ammonia oxidation under ambient conditions, which was the first AOR molecular electrocatalyst in 2019 [14,15]. Moreover, the most efficient anode electrocatalyst for ammonia fuel cells currently is the PtIr/C electrocatalyst [16]. This is because both Pt and Ir have high dehydrogenation capabilities and low affinity for the reactive intermediate species of nitrogen (*N). Pd is another promising choice of metal for the electrocatalyst because the electrocatalytic activity for ammonia’s electrooxidation varies: Ru < Rh < Pd < Ir < Pt [17].
Though there are many promising electrocatalysts, the PdAgCo/C was chosen due to its versatility and potential properties. The significant catalytic activity of Pd, tolerance of Ag towards ammonia, and electronic structure modification for enhanced catalytic activity upon doping a conductive transition metal, like cobalt, lead to improved fuel cell performance [18,19,20]. Nanoparticles with a Pd-rich shell and a Ag/C-rich core showed a mass activity almost 1.5 times that of Pd/C in an alkaline medium [21], and Pd/O binding energy plays a significant role in ORR activity. The surface modification of Pd due to Ag and Co makes it less favorable for the adsorption and oxidation of ammonia [22]. An electropositive transition metal like cobalt participates in the downshift of the d band center of Pd atoms due to compressive strain and the ligand effect, leading to enhanced ORR catalytic activity that may even match the activity of precious metals if optimized [23].
This research was conducted with a focus on the innovation and novelty of the PdAgCo/C cathode electrocatalyst, with a brief investigation of the electrocatalytic performance of Ag/C, PdAg/C, and PdAgCo/C cathode catalysts loaded in combination with the Pt/C and PtIr/C anode catalysts. The materials were synthesized in optimized conditions for the best possible catalytic activity and fuel cell performance. Subsequently, a series of physicochemical and electrochemical characterizations were performed to evaluate the performance of these materials for their characteristics, composition, and effectiveness in the reduction reaction, which includes their catalytic activity, durability, and tolerance to ammonia. In addition, the study also addresses the influence of working parameters such as operating temperature, humidification temperature, and ammonia concentration in the fuel, thereby providing a strong theoretical and experimental basis for optimizing the performance of these parameters in practical applications for better performance. It was found that the PdAgCo/C electrocatalyst exhibits enhanced conductivity, catalytic activity, and electrochemical stability due to silver, palladium, and cobalt, respectively, demonstrating enhanced performance of the fuel cell.

2. Results

2.1. Physicochemical Characterizations

2.1.1. FTIR Analysis of Carbon Black

The initial carbon black carrier’s FTIR characteristic spectrum shows a C=C peak at 1632 cm−1 and an O-H peak at 3462 cm−1. Upon surface modification via acid wash, the carbon black carrier’s FTIR characteristic spectrum exhibits an O-H peak at 3417 cm−1, C-H peak at 2925 cm−1, C=C peak at 1632 cm−1, C-C peak at 1385 cm−1, and C=O peak at 1750–1705 cm−1, thus clearly indicating the functionalization of -COOH and -OH on the surface of the carbon black carrier after surface modification, as shown in Figure 1.

2.1.2. SEM and EDS Analysis of Ag/C

SEM-EDS analysis of the Ag/C cathode catalyst resulted in successful visualization of the granular microstructure through SEM, as shown in Figure 2, and verification of the attachment of silver particles onto the surface of carbon black carrier, with a weight percentage of silver being 40% (Table 1), which is consistent with the reference value provided in the literature [24]. Similarly, upon performing the SEM-EDS analysis for the 15 wt% PdAg/C and 15 wt% PdAgCo/C catalysts, the attachment of catalyst particles and appropriate surface morphologies were successfully verified with the following SEM images from Figure 3. The elemental compositions that show 15% weight of metal are shown in Table 1 and Table 2. In Table 3, through the EDS analysis, the weight ratios of Pd, Ag, and Co are 6.01%, 6.30%, and 6.01%, respectively. After calculating, the ratio of Pd:Ag:Co is 1:1:2.

2.1.3. XRD Analysis of Ag/C, PdAg/C, and PdAgCo/C

In Figure 4a, the XRD study of the Ag/C shows a broad 2θ peak at 26° corresponding to the direction of the (002) crystal plane. Moreover, the Ag peaks of 2θ observed at 38.1°, 44.3°, 66.4°, and 77.4° correspond to the (111), (200), (220), and (311) planes of the Ag crystal [20], respectively, which could be observed. XRD analysis of PdAg/C revealed a broad 2θ peak at 26° corresponding to the direction of the (002) crystal plane. Moreover, Figure 4b shows the PdAg peaks of 2θ observed at 38.6°, 45.0°, 65.1°, and 78.3°; these peaks correspond to the (111), (200), (220), and (311) planes of the PdAg crystal, respectively, which could be visualized. The XRD analysis of PdAgCo/C revealed a broad 2θ peak at 26° corresponding to the direction of the (002) crystal plane. Moreover, Figure 5 shows the PdAgCo peaks of 2θ observed at 38.7°, 44.9°, 65.4°, and 78.5°; these peaks correspond to the (111), (200), (220), and (311) planes of the PdAgCo crystal. Upon interpretation of the XRD plots with the Jade program, it was verified that all the above catalyst particles (Ag, PdAg, and PdAgCo) that were synthesized have successfully been attached to the surface of carbon black.

2.2. Electrochemical Characterization

Numerous transition metal spinels with notable ORR catalytic performance have been reported in the literature [23]. The ORR activity of these spinel catalysts follows a volcano trend in relation to the occupancy of eg orbitals within the octahedral coordination structure. When choosing a catalyst for a DAFC cathode, it is important to consider the ORR activity at the catalyst/aqueous base interface and the extent of ammonia interference. As shown in Figure 6, cathode catalyst loading is set to 0.5 mg/cm2, electrolyte to 3 M KOH and 3 M KOH + 3M NH4OH, rotation rate to 1600 rpm, and scan rate = 10 mV/s on a rotating disk electrode (RDE). Adding NH4OH enhances the ORR activity relative to KOH alone. For PdAgCo/C, E1/2 is 0.48 V in 3 M KOH and 0.52 V in 3 M KOH + 3 M NH4OH, suggesting that PdAgCo/C can act as a Pt-free ORR catalyst, offering an alternative to Pt/C, Ag/C, and Pd/C.

2.3. Impact of Operating Parameters on the Power Density

2.3.1. Anode Fuel Concentration and Composition

As shown in Table 4, the hydroxide (OH−) ions play a key role, as expected, in accelerating the electrochemical kinetics of the anode oxidation reaction [25], thus signifying the impact of the concentration profile of fuel on the performance. The highest efficiency is achieved when the anode fuel composition is 3 M NH4OH+ 3 M KOH, with a power density of 40 mW/cm2, which is significant, almost 4-fold higher than that of ammonia gas. This gain could be attributed to the improved diffusion effect of ammonia in ammonia water, leading to a higher power density, ionization phenomena, and a mixed conduction effect at the anode due to both OH− and electrons reducing anode polarization, thus improving the open circuit voltage (OCV) of the fuel cell [26]. Moreover, KOH enhances the transfer of ions between anode and cathode, thereby improving the overall reaction kinetics of the fuel cell. However, when the concentration of KOH is increased to 5 M, more ammonia passes through the cathode of the fuel cell, aggravating the breakthrough phenomenon and generating more mixed potential, which in turn causes the voltage of the fuel cell to drop, leading to the conclusion of optimized performance at 3 M concentration.

2.3.2. Operating Temperature

As shown in Table 5, there is a significant improvement observed in the performance of the DAFC, both in open circuit voltage (OCV) and current density, upon increasing the operating temperature [17], which is in line with our expectations. It was found that power density saw an almost 4 times increase from 10 to 40 mW/cm2 with the increase in operating temperature from 80 °C to 110 °C, concluding the optimal performance at 110 °C among temperatures from 80 °C to 110 °C.

2.3.3. Cathode Humidification Temperature

Table 6 shows that increasing the cathode humidification temperature helps in enhancing the electrochemical reaction efficiency of ammonia fuel cells, thereby improving the overall performance of the fuel cell. When the cathode humidification temperature reached 90 °C, the ammonia fuel cell had better power density performance, reaching 40 mW/cm2.

2.3.4. Performance Analysis of the Catalysts

The optimized values of the working parameters were set for the performance analysis of all the catalysts that were synthesized. 3 M KOH + 3 M NH4OH was used as the anode fuel, O2 was used as the cathode oxidant, the working temperature was set to 110 °C, and the cathode humidification temperature was controlled at 90 °C. Fuel flow rates for the anode and cathode were set at 32 mL/min and 1 sl/min, respectively. The corresponding anode and cathode catalyst loadings were adjusted to 1 and 0.5 mg/cm2. Figure 7 and Table 7 show that PdAgCo/C has a better power density of 74 mW/cm2, which is significantly higher than 38 mW/cm2 shown by the conventional Ag/C catalyst. The power density analysis for the anode catalysts Pt/C and PtIr/C, in combination with the better-performing PdAgCo/C cathode catalyst, was tested with the same testing conditions. Table 8 shows the better performance of PtIr/C with a power density of 172 mW/cm2 over 74 mW/cm2 of the Pt/C catalyst, clearly indicating that PdAgCo/C-PtIr/C is the required catalyst combination for the optimal performance of the fuel cell.
With the above progress in identifying the appropriate anode-cathode catalyst, we need to further revamp the performance of the fuel cell. In Figure 8 and Table 9, in order to identify the open circuit voltage (OCV) of the fuel [27], the cathode catalyst loading profile was varied: 0.5, 1, 2, and 3 mg/cm2, maintaining a constant anode catalyst loading of 1 mg/cm2 as earlier, and all other test conditions remaining the same. It resulted in optimal fuel cell performance of 291 mW/cm2 at the cathode catalyst loading of 2 mg/cm2. It could be observed that the increased catalyst loading at 3 mg/cm2 is accompanied by a decrease in power density. This could be attributed to the excessive cathode catalyst PdAgCo/C loading on the GDL 340, which has caused material drop off, leading to depreciation in the fuel cell performance. Having achieved the optimal cathode catalyst loading of 2 mg/cm2 for enhanced performance, to further increase the power density, the anode catalyst loading was increased to 2 mg/cm2 along with it [28]. In Figure 9 and Table 10, with all the test conditions remaining the same, an impeccable power density of 332 mW/cm2 is achieved.
In Figure 10, the durability is tested using the DAFC test station (FCED-PD50) for 100 cycles. The test condition is as follows: PtIr/C catalyst as anode, PdAgCo/C catalyst as cathode, same loading of 2 mg/cm2, PiperION-A20-HCO3 TP-85 as alkaline anion exchange membrane, 3 M NH4OH + 3 M KOH as anode fuel, and pure oxygen as cathode fuel. The results show that after 100 cycles, the power density decreased by 40%, from 332 to 200 mW/cm2.
Table 11 compares the cathode catalysts on the AEMFCs and DAFCs performance, showing the differences between non-Pt group metals and Pt group metals for state-of-the-art ORR catalysts. Pt group metals can indeed achieve higher performance than Non-Pt group metals by enhancing the loading or operating at higher temperatures. In the research field of DAFCs, non-Pt catalysts such as Fe-N-C have a power density of 107 mW/cm2, while Pt/C catalysts have a power density of 158.6 mW/cm2. Compared to the Pt/C catalyst, the Fe-N-C catalyst can reach a lower ammonia water concentrations at the same loading of 2 mg/cm2.

3. Materials and Methods

3.1. Materials

The gas diffusion layer (GDL 340) was bought from CeTech, Taichung City, Taiwan. Carbon black (Vulcan XC-72R) was purchased from Cabot Corporation, Boston, MA, USA. The 40 wt% Pt/C and 40 wt% PtIr/C were bought from Johnson Matthey, Taipei, Taiwan. Anion exchange membranes (20 microns thickness of PiperION® Anion Exchange Membrane) and ionomer (PiperION Anion Exchange Dispersion 5% in ethanol) were purchased from Dioxide Materials, Bryan, TX, USA. Isopropyl alcohol (IPA) was bought from Macron Fine Chemicals, Radnor, PA, USA. Chemicals, including silver nitrate, potassium tetrachloropalladate, sodium citrate, sodium borohydride, nitric acid, cobalt chloride, potassium hydroxide, ethanol, and ammonium hydroxide, were bought from Shen Chiu Enterprise Corporation, Tainan City, Taiwan. Argon (Ar) and hydrogen (H2) were supplied by Air Products San Fu Co., Ltd., Taipei City, Taiwan.

3.2. Preparation of the Catalysts

With the area of the gas diffusion layer being 5.2 × 5.2 cm2, different catalyst loadings were tested upon their synthesis to compare their abilities, enhance the anode oxidation reaction, and improve the performance of the fuel cell.

3.2.1. Anode Catalysts

Conventional Pt/C and PtIr/C were synthesized as the anode catalysts. Beginning with the preparation of the Pt/C catalyst, 67.6 mg of the commercial 40 wt% Pt/C was dispersed in 676 mg of DI water. This mixture was then added to 676 mg of IPA. After ensuring the complete homogenization of the above mixture, 338 mg of the commercial 5 wt% PiperION-A5 ionomer was added, ensuring the catalyst-to-ion polymer ratio was 80:20. The mixture was then ultrasonicated with the ultrasonic device (Delta D150H) at a frequency of 40 kHz for a duration of 1 h at room temperature, which led to the successful synthesis of the Pt/C anode catalyst. The same procedure was repeated with 40 wt% commercial PtIr/C instead of Pt/C, using the same materials and quantities, thus fabricating the anode catalyst layer using a commercial catalyst PtIr/C.

3.2.2. Cathode Catalysts

The primary step for the preparation of the cathode catalysts, namely Ag/C, Pt/C (experimental control group), PdAg/C, and PdAgCo/C, is to ensure the availability of carbon black with appropriate surface morphology. Vulcan XC-72R carbon black was subjected to surface modification, where the carbon black particles were mixed with 20% nitric acid inside the boiling flask and circulated under a reflux condensing tube for 12 h at 120 °C to remove surface contaminants and promote functionalization at 15 kgf/cm2. The graphite bipolar plate used a serpentine-type flow channel. The DAFC test station and DAFC component are shown in Figure 11. The modified carbon black particles were then filtered using a suction filter and washed with DI water until all residual nitric acid was removed and the particles were completely neutralized. Neutrality was confirmed by pH measurement (DOGGER, D9UL-PH5011, New Taipei, Taiwan). Finally, the modified carbon black particles were recovered after being dried in the oven at 80 °C for 12 h.
The chemical reduction method was adopted to synthesize Ag/C, using a silver nitrate solution (50 mM, 123.3 mL), sodium borohydride (7.4 mM, 167 mL) as the reducing agent, and sodium citrate (50 mM, 123.3 mL) as the protecting agent, which plays a significant role by preventing the silver nanoparticles from aggregating into a larger particle, thereby ensure being uniform dispersion and desirable surface area, which facilitate further synthesis. Then, 200 mg of the modified carbon black was added to the reduced silver nanoparticles and stirred for 12 h. After this, the mixture was filtered using a suction filter, and the nanoparticles were washed using DI water until they were neutralized. Finally, the 40 wt% Ag/C catalyst is synthesized by drying it in the oven at 80 °C for 12 h.
The PdAg/C cathode catalyst was synthesized via chemical reduction, using tetrachloropalmatine and silver nitrate as precursors and sodium borohydride and sodium citrate as reducing agents, at a 1:1 Pd and Ag weight ratio. The reaction was performed using 200 mL of DI water, containing 0.01 M tetrachloropalmatine, 0.01 M silver nitrate, and 90 mg of modified carbon black, stirring for a duration of 2 h. Next, 0.2 M sodium borohydride was added to the solution and was continuously stirred for 1 h. Finally, a 15 wt% PdAg/C cathode catalyst (15 wt% PdAg, 85 wt% C) was successfully prepared after washing it with DI water and ensuring complete neutralization. The synthesis procedure for PdAgCo/C is the same as that of PdAg/C, except for the addition of 0.01 M cobalt chloride hexahydrate along with 0.01 M tetrachloropalmatine and 0.01 M silver nitrate at the start of the reaction, resulting in the PdAgCo/C catalyst upon successful synthesis. Additionally, the mixed precursor (K2PdCl4, AgNO3, and CoCl2) solution was added dropwise into the basic carbon slurry while stirring at pH of 11.

3.3. Calibration of Working Parameters

3.3.1. Anode Fuel Concentration and Composition

To identify the ideal concentrations of ammonia water and potassium hydroxide (KOH) in the anode fuel and enhance the fuel cell performance, the Pt/C catalyst slurry was coated on the GDL by hand-brushing on an 80 °C hot plate and allowed to activate on the surface of the GDL for 48 h to form the GDE (gas diffusion electrode). After this, the power density of the fuel cell was tested under various fuel concentration profiles, leading to different compositions of the anode fuel, with oxygen being used as the cathode oxidant. Current studies have shown that 3 M NH4OH demonstrates the best performance [40], which will also be examined and verified using different concentration profiles of KOH.

3.3.2. Operating Temperature

To calibrate the operating temperature for optimal fuel cell performance, Pt/C was loaded onto both the anode and cathode at 1 mg/cm2 and subjected to membrane activation treatment for a duration of 48 h [41]. The corresponding power densities shown by the 40 wt% Pt/C catalysts were recorded and compared under operating temperatures of 80, 95, and 110 °C, using oxygen as the cathode oxidant and the optimized concentrations of NH4OH and KOH in the anode fuel. A temperature controller, GX-36LS (TOHAMA Co., Ltd., Hsinchu, Taiwan), was used, and the maximum temperature was restricted to 110 °C, as the anion exchange membrane cannot withstand temperatures higher than 120 °C.

3.3.3. Cathode Humidification Temperature

To calibrate the cathode humidification temperature for optimal fuel cell performance, Pt/C was loaded onto both the anode and cathode at 1 mg/cm2 and subjected to activation treatment for a duration of 48 h. The corresponding power densities shown by the double-sided 40 wt% Pt/C catalysts were recorded and compared under different cathode humidification temperatures, 70, 80, and 90 °C, using oxygen as the cathode oxidant. The previously calibrated operating temperature and concentrations of NH4OH and KOH were used.

3.4. Optimal Load Test

After setting up the optimized working parameters through calibration of the fuel cell, the current density of 15 wt% PdAg/Co was tested for under different loading profiles: 0.5, 1.0, 1.5, 2, and 3 mg/cm2. The catalyst was evenly coated onto a 5.2 × 5.2 cm2 area of GDL 340 on a hotplate at 80 °C. The gas diffusion electrode must be immersed in 1 M KOH solution for 48 h to be activated, and the solution must be replaced every 24 h. Before beginning the test, a 5.5 × 5.5 cm2 area of the PiperION-A20-HCO3 TP-85 alkaline anion exchange membrane is soaked in 0.5 M KOH for a duration of 2 h. This converts the anion membrane from its bicarbonate form to the hydroxide form, as it is mainly composed of piperidine, terphenyl, and 2,2,2-trifluoroacetophenone condensation residue [42]; thus, it has high anionic conductivity and excellent chemical stability. Subsequently, load tests were carried out under different catalyst loadings and the optimal working conditions that were calibrated earlier. The DAFC was tested after the membrane electrode assembly (MEA), which consists of the activated membrane, anode GDE, and cathode GDE. The cell was assembled and secured using a screw and washer, tightened to a torque of 15 kgf/cm2. The graphite bipolar plate used a serpentine-type flow channel. The DAFC test station and DAFC components are shown separated in Figure 12a,b.

4. Conclusions

The study successfully demonstrates that the selection of catalyst, optimization of operating conditions, and catalyst loading are the primary factors in deciding fuel cell performance. Specifically, the activity of the cathode directly impacts the performance of the entire fuel cell. Doping the synthesized Ag/C cathode catalyst with cobalt and palladium facilitated the successful synthesis of a 15 wt% PdAgCo/C catalyst. XRD, SEM, and EDS analyses helped verify the successful attachment of PdAgCo to the carbon carrier. Cyclic voltammetry analysis, reflecting the electrochemical activities and power densities of Pt/C, Ag/C, PdAg/C, and PdAgCo/C, indicated that PdAgCo/C was the superior cathode catalyst. PdAgCo/C’s improved catalytic activity is attributed by cobalt and palladium doping, as these elements modified the electronic structure of the material [43], thereby improving the catalytic activity of the AOR. This improvement in catalytic activity prompted the acceleration of reaction kinetics, thereby improving the power density and energy efficiency of the fuel cell. Similarly, the choice of the anode catalyst also has a significant impact on the performance of fuel cells [44]. Cyclic voltammetry was used to compare the performance of Pt/C and PtIr/C, indicating that PtIr/C shows better performance compared to traditional Pt/C, which may be due to the fact that the PtIr alloy has a stronger catalytic activity and better anti-poisoning performance than pure platinum; this makes the electrode more stable and efficient during the catalytic reaction, thus resulting in the improved performance of the PdAgCo/C-PtIr/C catalyst combination. Subsequently, the optimized cathode and anode loadings of 2 mg/cm2, determined via a series of fuel cell tests under previously optimized conditions with this above catalyst combinations, not only increased the number of active sites but also promoted charge transfer and reduced the internal resistance, thus improving the overall performance of the fuel cell and achieving a significant power density of 332 mW/cm2. Therefore, this study successfully demonstrated that doping the cathode catalyst with Co and Pd can enhance the performance of fuel cells. Future work should focus on identifying potential low-cost and more environmentally friendly alternatives to PtIr/C for the functionalization of carbon black [45].

Author Contributions

Conceptualization, Z.-J.S., P.V. and H.Y.; methodology, H.Y. and P.V.; validation, F.-C.S. and H.Y.; formal analysis, K.L.L.; investigation, P.V. and Z.-J.S.; resources, K.L.L. and H.Y.; data curation, Z.-J.S.; writing—original draft preparation, P.V.; writing—review and editing, P.V. and F.-C.S.; visualization, F.-C.S.; supervision, H.Y.; project administration, Z.-J.S. and H.Y.; funding acquisition, K.L.L. and H.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science and Technology Council of Taiwan, grant number “NTSC 113-2221-E-005-077-MY3” and “NSTC 113-2634-F-005-002”—project Smart Sustainable New Agriculture Research Center (SMARTer), and supported in part by the Ministry of Education, Taiwan, under the Higher Education Sprout Project.

Data Availability Statement

Data is unavailable due to privacy.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Catalysts 15 00825 g001
Figure 1. FTIR spectra depicting the impact of surface modification of the carbon black carrier.
Figure 1. FTIR spectra depicting the impact of surface modification of the carbon black carrier.
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Catalysts 15 00825 g002
Figure 2. SEM images of Ag/C under the magnification of (a) 10K×. (b) 50K×.
Figure 2. SEM images of Ag/C under the magnification of (a) 10K×. (b) 50K×.
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Catalysts 15 00825 g003
Figure 3. SEM images under the magnification 10× and 50K× of (a,b) Ag/C, (c,d) PdAgCo/C.
Figure 3. SEM images under the magnification 10× and 50K× of (a,b) Ag/C, (c,d) PdAgCo/C.
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Catalysts 15 00825 g004
Figure 4. (a) XRD analysis of Ag/C; (b) XRD analysis of PdAg/C.
Figure 4. (a) XRD analysis of Ag/C; (b) XRD analysis of PdAg/C.
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Catalysts 15 00825 g005
Figure 5. XRD analysis of PdAgCo/C.
Figure 5. XRD analysis of PdAgCo/C.
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Catalysts 15 00825 g006
Figure 6. Electrochemical oxygen reduction reaction (ORR) activity comparison between (a) Pt/C, (b) Ag/C, (c) Pd/C, (d) PdAg/C, and (e) PdAgCo/C in 3 M KOH and 3 M KOH + 3M NH4OH solutions at room temperature.
Figure 6. Electrochemical oxygen reduction reaction (ORR) activity comparison between (a) Pt/C, (b) Ag/C, (c) Pd/C, (d) PdAg/C, and (e) PdAgCo/C in 3 M KOH and 3 M KOH + 3M NH4OH solutions at room temperature.
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Catalysts 15 00825 g007
Figure 7. Power density of Ag/C, PdAg/C, and PdAgCo/C.
Figure 7. Power density of Ag/C, PdAg/C, and PdAgCo/C.
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Catalysts 15 00825 g008
Figure 8. Power density with different cathode catalyst loadings of 0.5, 1, 2, and 3 mg/cm2.
Figure 8. Power density with different cathode catalyst loadings of 0.5, 1, 2, and 3 mg/cm2.
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Catalysts 15 00825 g009
Figure 9. Variation in power density with anode catalyst loadings.
Figure 9. Variation in power density with anode catalyst loadings.
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Catalysts 15 00825 g010
Figure 10. Durability test of DAFC.
Figure 10. Durability test of DAFC.
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Catalysts 15 00825 g011
Figure 11. Recirculate system for carbon black modification.
Figure 11. Recirculate system for carbon black modification.
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Catalysts 15 00825 g012
Figure 12. (a) DAFC test station (FCED-PD50) and (b) DAFC components.
Figure 12. (a) DAFC test station (FCED-PD50) and (b) DAFC components.
Catalysts 15 00825 g012
Table 1. Elemental analysis of Ag/C.
Table 1. Elemental analysis of Ag/C.
ElementalWeight %
C55.98
O3.9
Ag40.12
Total100.0
Table 2. Elemental analysis of PdAg/C.
Table 2. Elemental analysis of PdAg/C.
ElementalWeight %
C82.18
O2.24
Pd7.21
Ag7.79
Total100
Table 3. Elemental analysis of PdAgCo/C.
Table 3. Elemental analysis of PdAgCo/C.
ElementalWeight %
C82.18
O3.02
Pd6.01
Ag6.3
Co3.25
Total100
Table 4. Power density under different concentration profiles of NH4OH and KOH.
Table 4. Power density under different concentration profiles of NH4OH and KOH.
ParametersConcentration Profile of Ammonia WaterConcentration Profile of KOH
Sample12345678
Anode fuel composition1 M NH4OH + 3 M KOH3 M NH4OH + 3 M KOH5 M NH4OH + 3 M KOH7 M NH4OH + 3 M KOH3 M NH4OH + 1 M KOH3 M NH4OH + 3 M KOH3 M NH4OH + 5 M KOH3 M NH4OH + 7 M KOH
Cathode oxidantO2
Flow rate of anode fuel (mL/min)32
Flow rate of cathode oxidant (sLmin)1
Operating temperature (°C)110
Anode and cathode catalystPt/C
Power density (mW/cm2)3340393832403531
Table 5. Optimization of anode fuel profiles.
Table 5. Optimization of anode fuel profiles.
Anode fuel composition3 M NH4OH + 3 M KOH
Cathode FuelO2
Flow rate of anode fuel (mL/min)32
Flow rate of cathode oxidant (sL/min)1
Cathode humidification temperature (°C)8095110
Anode and cathode catalystPt/C
Power density (mW/cm2)102640
Table 6. Optimization of cathode humidification temperature.
Table 6. Optimization of cathode humidification temperature.
Anode fuel composition3 M NH4OH + 3 M KOH
Cathode oxidantO2
Flow rate of anode fuel (mL/min)32
Flow rate of cathode oxidant (sL/min)1
Cathode humidification temperature (°C)708090
Anode and cathode catalystPt/C
Power density (mW/cm2)303640
Table 7. Power density of Ag/C, PdAg/C, and PdAgCo/C as cathode with same anode catalyst.
Table 7. Power density of Ag/C, PdAg/C, and PdAgCo/C as cathode with same anode catalyst.
Anode FuelCathode OxidantAnode CatalystCathode CatalystAnode Catalyst Loading (mg/cm2)Cathode Catalyst Loading (mg/cm2)Power Density (mW/cm2)
3 M NH4OH +
3 M KOH		O2Pt/CAg/C10.538
Pt/C40
PdAgCo/C74
PdAg/C57
Table 8. Power density of different anode catalyst loadings.
Table 8. Power density of different anode catalyst loadings.
Anode FuelCathode OxidantAnode CatalystCathode CatalystAnode Catalyst Loading (mg/cm2)Cathode Catalyst Loading (mg/cm2)Power Density (mW/cm2)
3 M NH4OH +
3 M KOH		O2Pt/CPdAgCo/C10.574
PtIr/C172
Table 9. Different cathode catalyst loading on fuel cell performance.
Table 9. Different cathode catalyst loading on fuel cell performance.
Anode FuelCathode OxidantAnode CatalystCathode CatalystAnode Catalyst Loading (mg/cm2)Cathode Catalyst Loading (mg/cm2)Power Density (mW/cm2)
3 M NH4OH +
3 M KOH		O2PtIr/CPdAgCo/C10.5172
1202
2291
3256
Table 10. Different anode catalyst loadings on fuel cell performance.
Table 10. Different anode catalyst loadings on fuel cell performance.
Anode FuelCathode OxidantAnode CatalystCathode CatalystAnode Catalyst Loading (mg/cm2)Cathode Catalyst Loading (mg/cm2)Power Density (mW/cm2)
3 M NH4OH +
3 M KOH		O2PtIr/CPdAgCo/C12291
2332
Table 11. Comparison of different cathode catalysts on performance of AEMFCs and DAFCs.
Table 11. Comparison of different cathode catalysts on performance of AEMFCs and DAFCs.
Cathode CatalystCathode Loading (mgmetal + carbon/cm2)Fuel (Anode/Cathode)Fuel Cell TypeOperating Conditions Temperature (°C)Power Density (mW/cm2)Reference
Pt/C0.8H2/OAEMFC70912[25]
Fe-N-C4H2/OAEMFC801300[29]
Mn-N-C2H2/OAEMFC60480[30]
Co@Co3O4/@N-C2.5H2/OAEMFC70550[31]
Ag/C2H2/OAEMFC50490[32]
PdAg/C (3:1)1H2/OAEMFC60680[33]
Ag/C1H2/OAEMFC70623[34]
Ag-Co/CNT1H2/OAEMFC70431.1[35]
PdNi/N-rGO1 (Pd only)H2/ODirect formate AEMFC60180[36]
LaCr0.25Fe0.25Co0.5O3-δ (LCFCO)1.857 M NH3·H2O +
1 M KOH/CO2-free air		DAFC10030.1[37]
Pt/C23 M NH3·H2O +
3 M KOH/Pure-O2		DAFC80158.6[38]
Fe-N-C21 M NH3·H2O +
3 M KOH/Pure-O2		DAFC80107[39]
PdAgCo/C23 M NH3·H2O +
3 M KOH/Pure-O2		DAFC90332This work
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Vengatasalapathy, P.; Su, F.-C.; Su, Z.-J.; Lim, K.L.; Yang, H. Cathode Catalyst PdAgCo/C for Optimal Performance of the Alkaline Anion Exchange Membrane Direct Ammonia Fuel Cells. Catalysts 2025, 15, 825. https://doi.org/10.3390/catal15090825

AMA Style

Vengatasalapathy P, Su F-C, Su Z-J, Lim KL, Yang H. Cathode Catalyst PdAgCo/C for Optimal Performance of the Alkaline Anion Exchange Membrane Direct Ammonia Fuel Cells. Catalysts. 2025; 15(9):825. https://doi.org/10.3390/catal15090825

Chicago/Turabian Style

Vengatasalapathy, Prithiv, Fa-Cheng Su, Zi-Jie Su, Kean Long Lim, and Hsiharng Yang. 2025. "Cathode Catalyst PdAgCo/C for Optimal Performance of the Alkaline Anion Exchange Membrane Direct Ammonia Fuel Cells" Catalysts 15, no. 9: 825. https://doi.org/10.3390/catal15090825

APA Style

Vengatasalapathy, P., Su, F.-C., Su, Z.-J., Lim, K. L., & Yang, H. (2025). Cathode Catalyst PdAgCo/C for Optimal Performance of the Alkaline Anion Exchange Membrane Direct Ammonia Fuel Cells. Catalysts, 15(9), 825. https://doi.org/10.3390/catal15090825

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