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Article

Pt-Based Electrocatalyst Modified by CsH2PO4/SiP2O7 for Electrochemical Oxidation of NH3 to H2 in Solid Acid Electrolysis Cell

by
Jihoon Kim
1,†,
Daehee Jang
1,†,
Junil Choi
1,
Junbeom Maeng
1,
Hyun Ho Shin
1,
Taiho Park
1 and
Won Bae Kim
1,2,*
1
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang 37673, Gyeongbuk, Republic of Korea
2
Graduate Institute of Ferrous & Energy Materials Technology, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang 37673, Gyeongbuk, Republic of Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2023, 13(4), 707; https://doi.org/10.3390/catal13040707
Submission received: 28 February 2023 / Revised: 4 April 2023 / Accepted: 4 April 2023 / Published: 6 April 2023
(This article belongs to the Special Issue Theme Issue in Honor of Prof. Dr. Jae Sung Lee)
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Abstract

:
Ammonia (NH3) has received much attention as a hydrogen carrier because it can be easily liquefied with a high hydrogen storage density and emits no greenhouse gas during the dihydrogen evolution process. The ammonia oxidation reaction (AOR) in an electrochemical system has an important merit in which a very high-purity dihydrogen gas can be obtained without an additional separation process that is typically needed for thermochemical decomposition processes. Herein, the electrochemical AOR was carried out in a solid acid electrolysis cell (SAEC) at an intermediate temperature around 250 °C, in which a solid composite of CsH2PO4 mixed with SiP2O7 was used as an electrolyte and Pt/C-based electrocatalysts were employed as the electrode materials of both anode and cathode. The Pt/C electrode material was modified with the CsH2PO4/SiP2O7 electrolyte in order to enhance the electrocatalytic activity for the AOR with an improved H2 production rate. Over the SAEC system reported here, a high AOR performance was obtained with a current density of 67.1 mA/cm2 and Faradaic efficiency (FE) of 98.2%. This study can suggest the significant potential of SAEC for the carbon-free H2 production from the selective electrochemical oxidation of NH3.

1. Introduction

The increasing environmental hazards from global warming have triggered extensive research for alternative energy sources to the conventional fossil fuels [1,2]. Dihydrogen (H2) gas has been suggested as the favorable, alternative energy carrier since H2 has a high gravimetric energy density and the only product from its use is clean water. However, the storage and transport of H2 suffers from its low volumetric energy density, necessitating complicated equipment for extreme conditions of temperature and pressure for liquid-state H2 [3]. Therefore, various materials, such as liquid organic hydrogen carriers (LOHC), metal hydrides, and ammonia, have been studied for use as an H2 carrier. Among them, ammonia (NH3) has received great attention as an H2 gas carrier material recently due to its high volumetric density of hydrogen, easy liquefaction and storage conditions (−33 °C or 8 bar at ambient temperature), no greenhouse gas emission during decomposition, and the presence of existing infrastructure in the current society [4,5,6]. H2 evolution from ammonia can be simply performed via a thermal decomposition process described by the following reaction:
NH3 (g) → 1/2 N2 (g) + 3/2 H2 (g)
As ammonia decomposition is favored at high temperatures, this thermal decomposition of ammonia generally requires more than 400 °C to achieve high conversion. Moreover, an additional separation process to obtain a pure dihydrogen gas is necessary because unconverted NH3 from the thermal decomposition process can damage hydrogen application systems [7]. Therefore, thermal decomposition of NH3 to H2 requires high energy, and thus an alternative H2 evolution process from NH3 needs to be developed. Recently, alternative methods of electrolysis processes have been studied to produce high-purity dihydrogen from the electrochemical oxidation of liquid phase ammonia. The electrochemical ammonia oxidation reaction (AOR) for dihydrogen evolution involves following half-cell reactions of oxidation and reduction processes at the anode and cathode, respectively:
Anode: 2 NH3 → N2 + 6 H+ + 6 e−
Cathode: 6 H+ + 6 e− → 3 H2
The AOR is performed over the electrolysis cells that are fabricated by membrane electrolytes sandwiched with anode and cathode electrodes. Thereby, a high-purity dihydrogen gas can be produced at the cathode side under ambient conditions [8,9,10]. Despite this advantage, however, the AOR under ambient condition is still to be improved significantly in terms of the H2 production rate as compared to the thermal decomposition processes. The solid acid electrolysis cell (SAEC), which can operate at intermediate temperature ranges from 200 °C to 300 °C, has attracted its uses for electrochemical oxidation/reduction processes by circumventing some limitations of low temperature electrolysis cells. Solid acid compounds, which are charge-balanced with SO42− and PO43− oxyanions, can be used as an excellent proton-conducting electrolyte for SAECs due to their phase transition properties to superprotonic phases even at the relatively low and intermediate temperatures with high proton conductivities [11]. Among them, cesium dihydrogen phosphate (CsH2PO4) is a representative solid acid, which reveals a high proton conductivity at 250 °C and has a relatively high tolerance against NH3 [12,13]. This CsH2PO4 is transformed to a superprotonic phase at about 228 °C and exhibits a high proton conductivity of 10−2 S cm−1 or greater [14]. In addition, a humidified environment is typically required in order to prevent dehydration and dissociation of CsH2PO4. The CsH2PO4 electrolyte has another issue to be resolved: it suffers from a relatively low mechanical stability, hence several studies about binder additives have been performed for SAEC systems [15,16,17].
Recently, to overcome the limitations of both thermal and electrochemical ammonia decomposition, a hybrid type of thermal-electrochemical ammonia conversion using an SAEC has been proposed. Lim et al. [18] demonstrated a hybrid system of thermal decomposition of ammonia followed by electrochemical dihydrogen production at 250 °C using CsH2PO4 as a proton-conducting electrolyte. This result showed a similar level of high H2 production rates to the studies in the temperature range from 350 to 500 °C, but the high loading weight of Ru catalyst (>10 mgRu cm−2) was required for the ammonia thermal decomposition part. Moreover, an additional Pt electrocatalyst layer for hydrogen oxidation is required for the system. In this sense, developing an efficient electrolyte and lowering the noble metal use are effective and necessary approaches for AOR research using SAEC systems.
In this study, we report the electrochemical AOR using the SAEC system at intermediate temperature to produce high-purity dihydrogen gas with a high production rate. For the enhanced mechanical properties and high proton conductivity in the relatively broad temperature range, the CsH2PO4 electrolyte was composited with a silica-based material of SiP2O7. As CsH2PO4 and SiP2O7 can react to form CsH5(PO4)2 under a hydrothermal condition, the electrolyte provides high proton conductivity in a temperature range from 150 °C to 280 °C [13]. Moreover, Pt/C-based electrocatalyst was blended with the electrolyte composite for a high-performance SAEC to produce H2 from the electrochemical oxidation of NH3. A high rate of 1.33 mmol cm−2 h−1 and faradaic efficiency (FE) of 98.2% for the H2 production were achieved in the SAEC system reported here.

2. Results and Discussion

Scheme 1 illustrates the SAEC cell configuration used in this study. The cell was composed of a stainless steel mesh current collector, electrodes of anode and cathode with electrocatalysts on carbon paper, and a solid-state electrolyte. The electrolyte was prepared by mixing the synthesized CsH2PO4 (CDP) and SiP2O7 (SPO) in a 1:2 molar ratio, which is described in detail in the experimental section. The electrolyte composite and Pt/C catalyst were blended in a series of weight ratios to use as electrode materials and deposited on the carbon paper (Pt/C_CDP/SPO-x, x = 3, 6, 9). The morphology and crystal structure of the electrode materials were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively, to check the fabrication procedure of the electrode paper. The formation of CsH2PO4 and SiP2O7 was examined by XRD, as shown in Figure 1a. Compared to the XRD patterns of each material, the composite materials on the electrode paper indicated weak peaks relating to a phase of SPO, because the CDP and SPO likely reacted to form CsH5(PO4)2 under the hydrothermal condition by the following reaction [13]:
2 CsH2PO4 + SiP2O7 + 3 H2O → 2 CsH5(PO4)2 + SiO2
However, the XRD peak at around 40° of Pt (111) was not detected clearly in the electrode paper because the intensities of XRD peaks referring to the CDP and SPO were much stronger due to the high crystalline properties of CDP and SPO. To investigate the mixing of the Pt/C and electrolyte composite, elemental mappings were carried out in the SEM image, indicating that Pt, Cs, and Si were uniformly dispersed over the electrode surface (Figure 1b). The XRD and SEM results showed that the Pt/C and electrolyte composite were nicely mixed and uniformly deposited on the carbon paper.
To interpret the proton-conducting properties of the CDP/SPO electrolyte, the current density and faradaic efficiency (FE) of dihydrogen evolution reaction were measured. CDP is known in that it can reveal a high proton conductivity in the range of 220 °C to 270 °C by accompanying the phase transition from a monoclinic crystal structure to superprotonic cubic structure over a temperature of 220 °C. Proton conductivity values of CDP and CDP/SPO were estimated by fitting the electrochemical impedance spectroscopy (EIS) data, especially those obtained at high frequency ranges. The proton conductivity values were obtained every 10 °C from 160 °C to 250 °C (Figure 2a). The CDP alone exhibited a high proton conductivity at the temperature of 220 °C, which is in a good agreement with its phase transition feature, but the CDP/SPO showed no sharp decline of proton conductivity below 220 °C [19]. Moreover, the proton conductivity value of CDP/SPO was higher than that of CDP at 250 °C. This indicated that the addition of SPO into CDP could enhance thermal stability as well as proton conductivity of the electrolyte. Using the CDP/SPO as a composited electrolyte, the linear sweep voltammetry (LSV) of hydrogen oxidation reaction (HOR) at 250 °C indicated that the current density increased linearly as the applied voltage increased (Figure 2b), and the FE of H2 evolution was reached around 100%. These results revealed that the CDP/SPO electrolyte can perform with a great proton conductivity at 250 °C.
A series of Pt/C_CDP/SPO electrodes was applied for the AOR over the SAEC system to optimize the weight ratios of Pt/C and electrolyte composite. The linear sweep voltammetry (LSV) curves of the catalysts at 10 mV s−1 scan rate were investigated from 0 V to 0.8 V (Figure S1). The LSV curves showed the increased slopes of current density versus voltage after 0.4 V, indicating that the AOR should occur faster from 0.4 V. Figure 3a indicates the measured current density for the AOR at 0.5–0.8 V using the Pt/C and Pt/C_CDP/SPO-x catalysts. Pt/C_CDP/SPO-6, which is the mixture of Pt/C and electrolyte composite in a 1:6 weight ratio, showed the best current density compared to the Pt/C, without compositing CDP/SPO or Pt/C_CDP/SPO with other ratio mixtures. EIS data were analyzed to find further insight into the charge transfer kinetics of Pt/C_CDP/SPO-x catalysts for the AOR in the SAEC. The EIS measurement was carried out at an open circuit voltage under 60 mL min−1 ammonia flow rate condition. Figure 3b shows the Nyquist plots of Pt/C_CDP/SPO-x catalysts; the electrochemical impedance parameters were evaluated using the equivalent circuit model shown in the inset of Figure 3b. The equivalent circuit consists of one ohmic resistance (Rs), inductance (L1), and two parallel circuits of constant phase element (CPE1, CPE2) and non-ohmic polarization resistance (R1, R2) [15]. The fitted data were summarized in Table S1. L1 was invariant for the catalysts because it is attributed to the inductance of the cables used in the impedance measurement device. As shown in Figure 3b, the Nyquist plots show that the high-frequency x-axis intercepts of Pt/C_CDP/SPO-x catalysts, which are mainly attributed to the ohmic resistance associated with the interfacial resistance between electrolyte and electrode, are nearly constant. R1 and R2 are the polarization resistance at high frequency range and medium–low frequency range, respectively. R1 is contributed by the charge transfer resistance at the electrolyte and electrode interface, and R2 is associated with the chemisorption of reactant and gas transport limitation at the electrode [20]. According to the fitted data, R1 decreased as the mixed CDP/SPO ratio increased, indicating that mixing the electrolyte composite to Pt/C could improve the charge transfer kinetics in the AOR. However, the R2 value of Pt/C_CDP/SPO-9 (12.47 Ω) highly increased compared to the Pt/C_CDP/SPO-3 (9.59 Ω) and Pt/C_CDP/SPO-6 (9.36 Ω). This might be attributed to the excessive electrolyte composite which caused an inhibition of reactant gas transport and chemisorption over the Pt active sites. Both results with current densities and EIS analysis suggest that there is an optimal ratio between the Pt/C and CDP/SPO, and the Pt/C_CDP/SPO-6 could be better for the electron transfer kinetics in the AOR than the Pt/C_CDP/SPO-3 and Pt/C_CDP/SPO-9.
Using the Pt/C_CDP/SPO-6 catalyst, the AOR in the SAEC was carried out along with different ammonia flow rates of 20, 40, 60 mL min−1. Figure 4 shows that the current densities increased as the voltage and ammonia flow rate were increased. The gas chromatography with thermal conductivity detector (GC-TCD) was operated at every 9 min of reaction time to quantitatively measure the dihydrogen production, and the result is summarized in Table 1. The GC-TCD analysis data of cathode outlet gas at 0.8 V at a flow rate of NH3 of 60 mL min−1 are illustrated in Figure S2. The GC-TCD chromatogram showed that H2, N2, and H2O were detected at 1.05, 1.15, and 10.5 min, respectively. As humidified N2 gas was supplied to the cathode, the GC-TCD analysis revealed that only H2 gas was produced at the cathode. Consequently, FE was 98.2% and H2 production rate was 1.33 mmol h−1 cm−2 at an applied potential of 0.8 V at a flow rate of NH3 of 60 mL min−1, and FE appeared to be decreased as the applied potential was decreased from 0.8 V to 0.5 V. Non-faradaic processes appeared to occur at lower potentials because charge transfer reactions over the electrode would be kinetically unfavorable when the applied potential is not sufficiently high to overcome the overpotentials of AOR [21,22].
The electrochemical AOR in the SAEC system was carried out for 20 h at an inlet ammonia flow rate of 60 mL min−1 at 0.8 V for the long-term durability test of the cell. The corresponding current density profile according to the cell operating time is shown in Figure 5. During the initial 1 h, the ammonia oxidation rate decreased almost linearly, and after 1 h, the current density of 30 mA cm−2 was shown. After that, the activity deterioration rate decreased, and the activity of 7.5 mA cm−2 was maintained after 8 h. Furthermore, the deactivation rate of the Pt/C_CDP/SPO-6 catalyst during the long-term test was calculated by differentiating chronoamperometric data and illustrated in Figure S3 to investigate the stability of the catalyst [23]. The deactivation rate became lower than 0.015 mA cm−2 min−1 after 6 h, which signifies that the catalytic activity was maintained with a decline rate of less than 0.1% after 6 h. After the stability test, XRD and SEM analyses of the electrode materials were performed to examine the morphology and crystalline structure of the catalysts. As shown in Figure S4, the XRD pattern of the used electrode material did not change compared to the pristine electrode. If dehydration of CDP took place, the CsH2P2O7 and CsPO3 would be formed, but no peaks referring to the CsH2P2O7 and CsPO3 were detected in the XRD pattern of Pt/C_CDP/SPO-6 measured after the stability test. This XRD result indicated that the Pt/C_CDP/SPO-6 catalyst demonstrates great stability without dehydration of the electrolyte composite. Furthermore, the SEM elemental mapping of Pt, Cs, and Si also showed that the Pt/C and electrolyte composite still dispersed uniformly on the carbon paper. These results indicated that the Pt/C_CDP/SPO-6 catalyst showed highly efficient electrochemical performance with a great stability for the AOR in SAEC.
The amount of ammonia crossover that can possibly take place across the electrolyte membrane during the electrochemical AOR was calculated by detecting and quantifying the ammonia dissolved in the water trap via UV-vis spectrophotometry [24,25] using the indophenol blue method (Figure S5). Preventing ammonia crossover is a highly important strategy in this SAEC system since the produced H2 cannot be used directly in the PEMFC as H2 fuel if any significant amount of ammonia concentration in the gas is involved [26]. The measured data are summarized in Table S2. The amount of crossed over ammonia collected for 60 min at a cell voltage of 0.8 V was 0.083 micromole, and the partial pressure of crossed ammonia for the outlet gas flow rate of 30 mL min−1 would be less than 0.96 ppm. This is under 1 ppm, which is the partial pressure of ammonia that could be accepted for the H2 feed stream directly into the PEMFC [27]. The residual concentrations of ammonia during the typical thermal decomposition processes are known to be about 1000 ppm or greater, which is definitely subject to the additional separation or removal processes of ammonia to use a pure H2, hence the electrochemical AOR process in the SAEC system proposed here should have a highly significant merit to produce a high-purity H2 gas from a carbon-free NH3.
In this work, the Pt/C_CDP/SPO-6 showed a high performance and stability for the electrochemical AOR at intermediate temperatures in the SAEC. Addition of SPO to CDP induced an increase in proton conductivity and an enhancement of thermal stability of the electrolyte, and the charge transfer resistance over the electrode material was further decreased by compositing the Pt/C catalyst with the electrolyte materials. However, excessive addition of CDP/SPO to the Pt/C catalyst appeared to hinder mass transfer and chemisorption of reactant on the active sites; thus, tuning the ratio of electrolyte composite and catalyst materials is significant and will be studied further. As summarized in Figure 6 and Figure S6, the AOR performance in our work showed a high current density as compared to the AOR results performed in low temperatures [28,29,30,31,32,33,34,35]. Furthermore, the H2 production rate was about 1.8 times higher than the hybrid thermal–electrochemical NH3 conversion to H2 in the SAEC system without a thermal cracking layer composed of Ru catalysts [14]. These indicated that the SAEC system has a great potential for electrochemical oxidation of NH3 to H2 and the performance can be significantly enhanced by moderately modifying electrolytes and catalyst materials with appropriate and effective additives.

3. Materials and Methods

3.1. Synthesis of CsH2PO4 and SiP2O7

For the synthesis of cesium dihydrogen phosphate (CsH2PO4) powder, CsCO3 (99% metal basis, Alfa Aesar, Haverhill, MA, USA), and H3PO4 (85 wt% aqueous solution, Alfa Aesar, Haverhill, MA, USA) were mixed in a 1:1 molar ratio in an aqueous solution. Afterwards, methanol was added to the solution, followed by precipitation of CsH2PO4. The precipitate was filtered and dried at 80 °C for 12 h. SiP2O7 was synthesized by mixing SiO2 (99.9% 0.5 micron, Alfa Aesar, Haverhill, MA, USA) and H3PO4 solution in a 1:2 molar ratio. The mixture was heated at 200 °C for 3 h and at 100 °C for 24 h subsequently. After the heat treatment, the mixture was ground and subsequently heated at 122 °C for 24 h and at 700 °C for 3 h. All heating steps were carried out under air condition, and as-prepared CsH2PO4 and SiP2O7 were mixed in a 1:2 molar ratio.

3.2. SAEC Cell Preparation

Anode and cathode were prepared by mixing Pt/C (20 wt% of Pt on carbon black, Alfa Aesar, Haverhill, MA, USA), electrolyte composite in 1:3, 1:6, 1:9 weight ratio in toluene (Pt/C_CDP/SPO-3, 6, 9). The resultant solution was loaded on a carbon paper (TGP-H-90, Toray, Tokyo, Japan) to 0.5 mgPt cm−2 by drop casting. Then, CsH2PO4/SiP2O7 suspended toluene solution was deposited on anode carbon paper and dried at 100 °C to compose the electrolyte layer. In addition, porous stainless steel mesh (Sus 316 150 mesh) was used as both a current collector and mechanical support of the cell. The porous stainless steel mesh, anode paper with electrolyte, cathode paper, and stainless steel mesh were stacked in a steel chamber in a row and pressed at 80 MPa for 5 min to fabricate a solid acid electrolysis cell, and the cell was sealed using PTFE tape.
The gas flow system was illustrated in Figure S7. Ag wires connected to the stainless steel mesh were used as terminals for both electrodes to a potentiostat (VSP-300). The cell was heated from 25 °C to 250 °C for 1 h with a humidified N2 gas flow of 30 mL min−1 on the cathode side and a humidified Ar gas flow of 30 mL min−1 on the anode side. At 250 °C, the flow of Ar gas was reduced to 15 mL min−1 and humidified NH3 was supplied together to the anode side. Humidified conditions (30%) of gas flow at each side were achieved by passing through the bubbler at 70 °C.

3.3. Electrochemical Measurements

To evaluate the ammonia oxidation activities of the solid acid cell, the reaction was performed at 250 °C from 0.5 V to 0.8 V at different NH3 flow rates (20, 40, 60 mL min−1). Electrochemical impedance spectroscopy (EIS) was performed at open circuit voltage (OCV) in the frequency range from 104 Hz to 0.1 Hz. The outlet gas from the cathode was penetrated through a water trap to check the NH3 crossover from the anode side, and then sampled to gas chromatography (Agilent 7890A, Santa Clara, CA, USA) with a thermal conductivity detector (TCD) to estimate the H2 gas production rate. The trapped NH3 was detected by UV-visible spectroscopy (Agilent Technologies Cary 8454, Santa Clara, CA, USA) using the indophenol blue method. The reagent was made by preparing 2 mL of 1 M NaOH solution containing 5 wt% salicylic acid and 5 wt% sodium citrate, followed by addition of 1 mL of 0.05 M NaClO and 0.01 mL of 1 wt% sodium nitroferricyanide aqueous solution.

3.4. Structural Characterization

The morphology of the electrode material was analyzed through scanning electron microscopy (SEM) data using a JSM 7800F instrument operated at 10 kV. Furthermore, X-ray diffraction (XRD) analysis data were obtained by Ultima Ⅳ (Rigaku) using Cu Kα (λ = 1.5418 Å) radiation operated at 40 kV and 30 mA to obtain the crystal structure information of synthesized materials.

4. Conclusions

In summary, we have successfully prepared the electrolyte composite and electrode materials for the SAEC system and applied this to the electrochemical NH3 oxidation to produce a high-purity H2 in the intermediate temperatures around 250 °C. The electrolyte composite was prepared by mixing CDP and SPO to enhance the thermal stability and proton conductivity for the broader temperature range, and it was further used to modify the Pt/C catalysts with different ratios to boost the electrocatalytic activity further. The AOR performance using the Pt/C_CDP/SPO-6 as the electrode material achieved a maximum Faradaic efficiency of 98.2% and dihydrogen production rate of 1.33 mmol h−1 cm−2. Furthermore, in a long-term durability test, it was confirmed that a current density of 7.5 mA cm−2 was maintained for longer than 20 h with no significant morphological changes. We discovered that a proper mixing ratio of the solid acid electrolyte composites and the Pt/C-based catalyst materials is required to optimize the electrolysis performance of NH3 over the SAEC. The modified electrolyte and catalyst indicated a higher performance for dihydrogen evolution from electrochemical ammonia decomposition as compared to the previously reported results in low temperatures and the hybrid thermal–electrochemical NH3 decomposition in the SAEC system. Consequently, this study can give a new approach toward an SAEC that is operable at intermediate temperatures with developments of efficient electrolytes and active electrode catalysts in the research field of ammonia electrooxidation for dihydrogen production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13040707/s1, Figure S1: Linear sweep voltammetry (LSV) curves of Pt/C_CDP/SPO-x catalysts for AOR from 0 to 0.8 V at 10 mV s−1 scan rate; Figure S2: GC-TCD analysis of cathode outlet gas for the AOR using the Pt/C_CDP/SPO-6 catalyst at 0.8 V at a flow rate of NH3 of 60 mL min−1 (a) from 0.8 to 1.2 min and (b) from 1.2 to 15 min; Figure S3: Deactivation rate of the Pt/C_CDP/SPO-6 catalyst at ammonia flow rate of 60 mL min−1 at 0.8 V for 20 h; Figure S4: (a) XRD patterns of the pristine Pt/C_CDP/SPO-6 and after the long-term stability test. (b) SEM images and EDS elemental mapping of Pt, Cs and Si of Pt/C_CDP/SPO-6 after stability test; Figure S5: (a) UV-vis absorption spectra and (b) calibration curve for the indophenol blue method; Figure S6: Comparison of mass activity for AOR over Pt-based catalysts with reported AOR literatures at low temperatures; Figure S7: Schematic illustration of the overall AOR process with the SAEC system; Table S1: Electrochemical impedance parameters of Pt/C_CDP/SPO-x catalysts at open circuit voltage under 60 mL min−1 ammonia flow rate condition; Table S2: Analyses of ammonia crossover during the electrochemical AOR in the SAEC for 60 min. The references [35,36,37,38,39,40,41,42] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, J.K. and H.H.S.; Methodology, J.M.; Formal analysis, J.K.; Investigation, D.J., J.C. and T.P.; Writing—original draft, J.K. and D.J.; Writing—review & editing, W.B.K.; Supervision, W.B.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2019R1A2C2088174) and by the “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20204010600100), and also by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (No. 2021R1A5A1084921).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00707 sch001 550
Scheme 1. Schematic illustration of electrocatalytic hydrogen oxidation reaction (HOR) and ammonia oxidation reaction (AOR) in solid acid electrolysis cell (SAEC).
Scheme 1. Schematic illustration of electrocatalytic hydrogen oxidation reaction (HOR) and ammonia oxidation reaction (AOR) in solid acid electrolysis cell (SAEC).
Catalysts 13 00707 sch001
Catalysts 13 00707 g001 550
Figure 1. (a) XRD patterns of Pt/C, CDP, SPO, and electrode paper. (b) SEM image of electrode material and EDS elemental mapping of Pt, Cs, and Si.
Figure 1. (a) XRD patterns of Pt/C, CDP, SPO, and electrode paper. (b) SEM image of electrode material and EDS elemental mapping of Pt, Cs, and Si.
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Catalysts 13 00707 g002 550
Figure 2. (a) Conductivity values of CDP and CDP/SPO from 160 °C to 250 °C. (b) LSV curves of HOR using CDP/SPO electrolyte at different hydrogen flow rate.
Figure 2. (a) Conductivity values of CDP and CDP/SPO from 160 °C to 250 °C. (b) LSV curves of HOR using CDP/SPO electrolyte at different hydrogen flow rate.
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Figure 3. (a) Current density of Pt/C and Pt/C_CDP/SPO-x catalysts at each potential from 0.5 V to 0.8 V. (b) Nyquist plots of Pt/C_CDP/SPO-x catalysts at open circuit voltage; inset shows the equivalent circuit model. Every analysis was performed at 250 °C under NH3 flow rate of 60 mL min−1.
Figure 3. (a) Current density of Pt/C and Pt/C_CDP/SPO-x catalysts at each potential from 0.5 V to 0.8 V. (b) Nyquist plots of Pt/C_CDP/SPO-x catalysts at open circuit voltage; inset shows the equivalent circuit model. Every analysis was performed at 250 °C under NH3 flow rate of 60 mL min−1.
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Catalysts 13 00707 g004 550
Figure 4. Chronoamperometry tests at potential from 0.5 V to 0.8 V under NH3 flow rate of (a) 20 mL min−1, (b) 40 mL min−1, and (c) 60 mL min−1. (d) Current density of Pt/C_CDP/SPO-6 catalyst at each potential from 0.5 V to 0.8 V. Every analysis was performed at 250 °C using Pt/C_CDP/SPO-6 catalyst.
Figure 4. Chronoamperometry tests at potential from 0.5 V to 0.8 V under NH3 flow rate of (a) 20 mL min−1, (b) 40 mL min−1, and (c) 60 mL min−1. (d) Current density of Pt/C_CDP/SPO-6 catalyst at each potential from 0.5 V to 0.8 V. Every analysis was performed at 250 °C using Pt/C_CDP/SPO-6 catalyst.
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Figure 5. Long-term stability test of Pt/C_CDP/SPO-6 at 0.8 V at NH3 flow rate of 60 mL min−1 for 20 h.
Figure 5. Long-term stability test of Pt/C_CDP/SPO-6 at 0.8 V at NH3 flow rate of 60 mL min−1 for 20 h.
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Figure 6. Comparison of AOR current density against Pt loading mass with reported AOR studies at low temperature [28,29,30,31,32,33,34,35].
Figure 6. Comparison of AOR current density against Pt loading mass with reported AOR studies at low temperature [28,29,30,31,32,33,34,35].
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Table
Table 1. Summary of AOR results of Pt/C_CDP/SPO-6 at 250 °C.
Table 1. Summary of AOR results of Pt/C_CDP/SPO-6 at 250 °C.
NH3 Flow
(mL min−1)		WHSV
(h−1 Pt)		Voltage
(V)		Current Density
(mA cm−2)		H2 Production Rate
(mmol h−1 cm−2)		Faradic Efficiency
(%)
20183.60.517.40.16847.9
0.629.60.31352.4
0.742.10.48056.5
0.849.90.56556.1
40367.20.524.60.31262.9
0.640.90.65279.0
0.754.00.81975.1
0.861.80.98275.1
60550.80.524.30.37876.1
0.643.40.74585.0
0.759.21.1697.0
0.867.11.3398.2
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Kim, J.; Jang, D.; Choi, J.; Maeng, J.; Shin, H.H.; Park, T.; Kim, W.B. Pt-Based Electrocatalyst Modified by CsH2PO4/SiP2O7 for Electrochemical Oxidation of NH3 to H2 in Solid Acid Electrolysis Cell. Catalysts 2023, 13, 707. https://doi.org/10.3390/catal13040707

AMA Style

Kim J, Jang D, Choi J, Maeng J, Shin HH, Park T, Kim WB. Pt-Based Electrocatalyst Modified by CsH2PO4/SiP2O7 for Electrochemical Oxidation of NH3 to H2 in Solid Acid Electrolysis Cell. Catalysts. 2023; 13(4):707. https://doi.org/10.3390/catal13040707

Chicago/Turabian Style

Kim, Jihoon, Daehee Jang, Junil Choi, Junbeom Maeng, Hyun Ho Shin, Taiho Park, and Won Bae Kim. 2023. "Pt-Based Electrocatalyst Modified by CsH2PO4/SiP2O7 for Electrochemical Oxidation of NH3 to H2 in Solid Acid Electrolysis Cell" Catalysts 13, no. 4: 707. https://doi.org/10.3390/catal13040707

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