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Review

Recent Advances in Electrocatalysts for Ammonia Oxidation Reaction

1
Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
2
Department of Chemical Engineering, Kangwon National University, Chuncheon 24341, Republic of Korea
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(5), 803; https://doi.org/10.3390/catal13050803
Submission received: 8 March 2023 / Revised: 14 April 2023 / Accepted: 24 April 2023 / Published: 26 April 2023
(This article belongs to the Special Issue Theme Issue in Honor of Prof. Dr. Jae Sung Lee)

Abstract

:
Ammonia (NH3) is a clean energy source that can either be directly used as fuel or a hydrogen carrier due to its high energy density and high hydrogen content. The NH3 electro-oxidation reaction (AOR) is the main reaction in both direct NH3 fuel cells and NH3 electrolysis. The AOR is thermodynamically favorable; however, the sluggish kinetics of the reaction can result in issues such as high overpotential, slow reaction rate, deactivation, etc. To overcome this, multiple strategies have been discussed to develop electrocatalysts that maintain a robust reaction rate in low overpotential regions. In this review, the fundamentals of AOR, including thermodynamics, kinetics, and experimental techniques, are studied. This review also focused on recent progress for catalyst modifications and their effects, with a particular focus on Pt- or Ni-based electrocatalysts. Additionally, vacant rooms needed to be developed was pointed, and a way to overcome the limitations was suggested. The fundamentals and efforts to prepare catalysts reviewed in this work will be effective in proposing and designing new robust electrocatalysts leading to advance AOR in practice.

1. Introduction

Ammonia (NH3) is the most potential carbon-free energy carrier due to its remarkable hydrogen storage capacity (17.7 wt%) and energy density (3000 Wh kg−1). Additionally, NH3 can be effectively liquefied at ambient temperature and mild pressure under 8 bar, which makes it superior in terms of storage and transportation compared to other hydrogen carriers [1,2,3]. Furthermore, contrary to the traditional Haber-Bosch process, the synthesis of NH3 using a clean electrochemical approach resulted in a decrease in CO2 emission, which makes NH3 a sustainable hydrogen transport media and carbon-free fuel itself [4,5,6,7,8,9].
With increasing attention to NH3 as a green energy source, various reactions, such as NH3 oxidation and decomposition, have been actively studied [10,11,12,13,14,15]. The use of NH3 can be mainly divided into two ways; one is to produce electricity by directly feeding it as a fuel like methanol and hydrogen in the fuel cell, and the other is to produce hydrogen via its electrolysis [16,17]. Considering the standard reduction potential for the NH3 oxidation and reduction ( N 2 + 6 H 2 O + 6 e 2 N H 3 + 6 O H , E ° = 0.77   V   v s .   S H E ) relative to that for the hydrogen ( 2 H + + 2 e H 2 , E ° = 0.00   V   v s .   S H E ) required for operating in both energy devices, it has a great benefit not only as a fuel but also as a hydrogen carrier. For example, the theoretical minimal cell potential for hydrogen production in NH3 electrolysis, requiring 0.06 V, is much smaller than the value in water electrolysis, requiring 1.23 V [18,19].
With these considerations, it is no doubt that robust electrochemical direct NH3 oxidation (AOR, Ammonia oxidation reaction) is critical to use NH3 in diverse energy conversion devices. Despite the thermodynamic advantage of AOR, its fundamental challenges associated with AOR include sluggish kinetics involving multiple rate-determining steps, unintentional side reactions, and irreversible reactions, such as poisoning [20,21,22]. Thus, this AOR mainly suffers from large overpotentials, imperfect faradaic efficiency, and low reaction rates and stability. To overcome those challenges, active catalyst candidates have been proposed based on noble metals (Pt, Ir, Pt-based alloy, etc.), transition metals (Ni, Cu, Co, etc.), and so on [18,23,24,25].
In this mini-review, at first, we mainly discuss essential AOR fundamentals, including thermodynamics, kinetics corresponding to reaction mechanisms, and experimental techniques. Then, we review recent advances in catalyst design using Pt- and Ni-based electrocatalysts, showing strong progress for AOR, in more detail. Finally, we end this review by providing prospects and perspectives in terms of catalyst design and additional engineering as shown in Scheme 1. We believe that the insight and discussion for the recent achievements discussed in this mini-review will serve as useful resources for the further development of more practical and efficient AOR.

2. Fundamentals of Ammonia Oxidation

NH3 can be used as a fuel directly in NH3 fuel cells, and furthermore, it can be utilized as a hydrogen carrier to produce hydrogen fuel in NH3 electrolytic cells, as shown below [16,28]:
NH 3   Fuel   cell
A n o d e   r e a c t i o n :   2 N H 3 + 6 O H N 2 + 6 H 2 O + 6 e
C a t h o d e   r e a c t i o n :   3 2 O 2 + 3 H 2 O + 6 e 6 O H
O v e r a l l   r e a c t i o n :   2 N H 3 + 3 2 O 2 N 2 + 3 H 2 O
NH 3   Electrolytic   cell
A n o d e   r e a c t i o n :   2 N H 3 + 6 O H N 2 + 6 H 2 O + 6 e
C a t h o d e   r e a c t i o n :   6 H 2 O + 6 e 3 H 2 + 6 O H
O v e r a l l   r e a c t i o n :   2 N H 3 N 2 + 3 H 2
Notably, NH3 oxidation reaction (Ammonia oxidation reaction, AOR) seems to be the determining factor in regards to the overall performance of both electrochemical energy conversion devices. Thus, the robust reaction under low overpotential is the most important to develop energy applications using NH3.
In the Pourbaix diagram for N2-H2O in Figure 1a, thermodynamic behavior for AOR depending on pH and potential is discussed [9]. The red line represents the theoretical standard reduction potential for the reduction of N2, which corresponds to the required potential for the NH3 oxidation to N2 and H2. In the universal pH range, AOR is more thermodynamically favorable than the oxygen evolution reaction (OER), and thus, the challenges coming from the competitive reaction of OER seem not to be dominant. However, experimental observations have shown significant gaps from this theoretical expectation due to extremely high overpotential requirements. Thus, in fact, experimental AOR can be competitive with OER; furthermore, the produced oxygen can additionally oxygenate undesired nitrogen species of N2O, NO, and so on [29,30]. These challenges can be expected based on kinetic limitations for AOR.
The reaction steps include 6 electrons releasing accompanied with 2 NH3 molecules and OH anions converting to N2 and 3 H2 molecules in alkaline media, which have been expected in two different ways, as shown in Figure 1b. In the Oswin-Salomon mechanism, an NH3 molecule (NH3,ads) is adsorbed onto the surface of catalysts and is further oxidized to be an adsorbed N atom (Nads) with releasing electrons [31]. Finally, the two Nads atoms are recombined together to form an N2 molecule, which subsequently desorbs from the catalyst surface. However, this reaction pathway is limited by two main factors; at low current densities, the rate-determining step is the dehydrogenation reaction from NH2,ads to NHads; at high current densities, the recombination of two Nads becomes the rate-determining step. Additionally, the strongly adsorbed Nads on the catalyst surface can cause catalyst poisoning.
On the other hand, Gerischer and Mauerer proposed another mechanism (Gerischer–Mauerer mechanism), where the first step is the dehydrogenation of NH3,ads to NHx,ads (x = 1 or 2) intermediates using OH with releasing electrons [32]. The obvious difference between this mechanism and the Oswin–Salomon mechanism is that the two dehydrogenated NHx,ads molecules are dimerized and then further dehydrogenated into N2,ads. Generally, dimerization is the most representative sluggish step for AOR [20,21]. Experimental reports especially using Pt catalysts, mainly support AOR mechanisms using the Gerischer–Mauerer suggestions [1,20,33].
Meanwhile, in order to directly interpret experimental observations, including intermediates and by-product formation and catalyst deactivation, recently, extended AOR mechanisms have been proposed based on theoretical calculation and experimental observations, as shown in Figure 1c [34]. The intermediate of N2Hx+y proposed in the Gerischer–Mauerer mechanism for N2 production was experimentally revealed by in-situ/operando measurements using FTIR, ATR-SEIRAS, and online electrochemical MS [2,35,36]. Although the pathway of NOx formation, causing catalyst poisoning, has not been clearly identified yet, their formation during AOR under high overpotential was also experimentally revealed using in-situ techniques [37].
Besides the theoretical fundamentals of AORs, it is also important to use basic experimental techniques to evaluate the performance of the reaction. The electrochemical activity, e.g., the current density, is investigated using cyclic voltammetry (CV) with a conventional three-electrode configuration (using a working electrode, a reference electrode, and a counter electrode). Figure 1d represents a CV plot obtained using a Pt working electrode measured in 1 M KOH electrolyte with NH4+ (red line) and without NH4+ (black line). In the case of electrolytes without NH4+, peaks are observed attributing to hydrogen desorption in 0.05–0.45 VRHE and Pt surface oxidation from 0.45 VRHE while anodic scanning. Notably, peaks representing hydrogen adsorption and Pt oxide reduction were observed during the cathodic scanning [38,39]. Interestingly, in the case of electrolytes with NH4+, a distinct oxidation peak is observed at 0.45 VRHE and the maximum current density is shown at 0.75 VRHE, which suggests NH3 oxidation has occurred [1]. The observed peak at 0.2 VRHE during cathodic scanning represents the desorption of NH3 and H2.
The actual amounts of NH3 can be quantified using diverse detection tools, such as UV-vis spectrometer, ion chromatography, and so on. Typically, a colorimetric method using the indophenol blue indicator has been widely suggested [40,41]. The indicator comprising hypochlorite, phenolate, and ferricyanide catalyst turns the solution from yellow to green due to the Berthelot reaction with NH4+ in the solution. The collected adsorption data at 655 nm are shown in Figure 1e,f. Based on the standard calibration curve drawn using the known concentrations of NH4+, the amount of NH4+ presented in the solution can be determined. In addition, N2 produced by AOR is also quantitatively investigated using the gas chromatography [42]. The by-products can be quantified using the Watt and Chrisp method for N2H4 and Griess method for the NOx species [43,44,45].
To determine the performance of AOR, the major three efficiencies must be determined using the quantified results as discussed above [40,42]. The first one is NH3 removal efficiency (R%), which corresponds to the conversion of NH3 to nitrogen-containing compounds as described in Equation (3), where [NH3]0 and [NH3]t are the initial concentration of NH3 added and remaining NH3 in the electrolyte after electrolysis, respectively. The NH3 removal efficiency indicates the conversion of NH3 to all products containing N elements, for example, N2, NO2, NO3, and so on.
NH 3   r e m o v a l   e f f i c i e n c y ,   R   ( % ) = [ N H 3 ] 0 [ N H 3 ] t [ N H 3 ] 0 × 100
Another important factor is the selectivity for the NH3 to N2 conversion as described in Equation (4), where [N2], [NO3], and [NO2] represents the concentration of N2, NO3, and NO2 able to be produced from AOR, respectively.
S e l e c t i v i t y   o f   NH 3 N 2 c o n v e r s i o n ,   S N   ( % ) = [ N 2 ]   [ N O 2 ] + [ N O 3 ] + [ N 2 ] × 100
The last one is the faradaic efficiency, which represents the performance to release electrons from NH3-N2 conversion against the total amount of released electrons, is given in Equation (5). Q theoretically released e, indicates the overall amounts of generated electrons during potential constant electrolysis, as observed in the J-t plot, and the other Q, actually released e, represents the number of electrons generated from the only NH3-N2 conversion. In addition, the constant of 6 indicates that 6 electrons can be generated from two NH3 molecules to produce one N2 molecule.
F a r a d a i c   e f f i c i e n c y ,   F E   ( % ) = 6   Q   a c t u a l l y   r e l e a s e d   e Q   t h e o r e t i c a l l y   r e l e a s e d   e × 100
So far, fundamentals, challenges, experimental investigation methods, and efficiencies to determine AOR were overviewed. Considering what is discussed above, to improve the performance, strategies to design effective catalysts are highly required. In the next section, promising strategies to prepare effective electrocatalysts and how each strategy affects AOR’s performances will be reviewed.
Figure 1. (a) Pourbaix diagram for the N2-H2O system. Reproduced with permission [9]. Copyright 2022, MDPI. (b) Schematic diagram of the NH3 oxidation pathways. (c) The extended AOR mechanism includes NOx species. The O-S mechanism indicates the Oswin-Salomon mechanism, and the G-M mechanism indicates the Gerischer-Mauerer mechanism. * indicates adsorption on the catalyst surface. Reproduced with permission [34]. Copyright 2022, Elsevier B. V. (d) Cyclic voltammograms of Pt plate in the presence (red line) and absence (black line) of 0.1 M NH3 in 1 M KOH with a scanning rate of 20 mV s–1. (e) UV-Vis absorption spectra of indophenol assays with various NH3 concentrations. (f) Calibration curve used for the NH3 quantification. Reproduced with permission [41]. Copyright 2023, Elsevier B. V.
Figure 1. (a) Pourbaix diagram for the N2-H2O system. Reproduced with permission [9]. Copyright 2022, MDPI. (b) Schematic diagram of the NH3 oxidation pathways. (c) The extended AOR mechanism includes NOx species. The O-S mechanism indicates the Oswin-Salomon mechanism, and the G-M mechanism indicates the Gerischer-Mauerer mechanism. * indicates adsorption on the catalyst surface. Reproduced with permission [34]. Copyright 2022, Elsevier B. V. (d) Cyclic voltammograms of Pt plate in the presence (red line) and absence (black line) of 0.1 M NH3 in 1 M KOH with a scanning rate of 20 mV s–1. (e) UV-Vis absorption spectra of indophenol assays with various NH3 concentrations. (f) Calibration curve used for the NH3 quantification. Reproduced with permission [41]. Copyright 2023, Elsevier B. V.
Catalysts 13 00803 g001

3. Electrocatalysts for Ammonia Oxidation

This section mainly investigates two groups of NH3 oxidation catalysts and their performances: (1) Pt-based electrocatalysts and (2) Ni-based electrocatalysts.

3.1. Pt-Based Electrocatalysts

3.1.1. Pure Pt Electrocatalysts

Noble metals (Ru, Rh, Pd, Pt, and Ir) and transition metals (Au, Ag, and Cu) have been considered to be used as catalysts in AOR. Ru, Rh, and Pd typically have too strong of an affinity for Nads to produce N2, whereas transition metals have relatively too weak NH3 adsorption to dehydrogenate NH3 [46]. However, Pt is satisfactory in terms of NH3 adsorption and dehydrogenation capacity, NHx,ads dimerization and product desorption; for this reason, Pt has been considered the most active catalyst and widely studied to be used in AOR [20]. To develop the catalytic performance of Pt with reducing the disadvantage of high cost, multiple strategies have been proposed, such as morphology and shape modification, exposure facet engineering, and so on [18,47,48]. In this part, recent efforts are mainly reviewed about the way to increase surface area and to provide catalytically active facets of Pt (100). Furthermore, it is discussed how each suggestion affects AOR performance.
Guntae Kim et al. synthesized two different types of Pt electrodes, which have uneven dense morphology or flower-shape depending on electrocatalytically synthetic condition [42]. The unevenly dense shaped Pt (CP-Pt) electrode was prepared using chronopotentiometry (CP) at a constant current of 2 mA, whereas the flower-shaped Pt (X CV-Pt, where X is the number of CV cycles; 100, 200, 300, 400, 500 and 600) electrode was deposited using cyclic voltammetry (CV) in the potential range of −0.4 to 0.2 VAg/AgCl, as shown in Figure 2a. Unlike during the CP process, H2 bubbles are generated via water reduction while scanning potential negatively than hydrogen reduction potential during the CV process. The evolution and detachment of the H2 bubble at a particular Pt site induce a flower-shaped Pt formation.
To investigate the effect of morphology on AOR activity, the CV process was conducted with 1 M NH3 in 5 M KOH. From Figure 2b, the current density began to increase at −0.65 VAg/AgCl representing AOR and exhibited a maximum current density around −0.3 VAg/AgCl; the current density then decreased due to strong adsorption of Nads. The CV-Pt electrodes exhibited a higher peak current density than the CP-Pt electrodes; notably, the 600 CV-Pt electrode recorded a maximum current density of 560 mA cm−2, which is three times higher than that of the CP-Pt electrode, as shown in Figure 2b,c. Since all electrodes have similar polycrystalline crystal structures and crystallinity of Pt as investigated using X-ray diffraction (XRD), it can be excluded the possibility of increasing the activity caused by a preferable crystal structure or facet for AOR. Instead, the major contribution to the increased current density is the enhanced active surface area for flower-shaped CV-Pt compared to unevenly dense structured CP-Pt. This work clearly reveals that the morphology, as well as the surface area of the electrocatalyst, is one of the critical factors affecting AOR performances.
W.B. Hu et al. prepared Pt particles with different morphology on indium tin oxide film (ITO) substrate by electrodeposition with controlling the current density from 0.2 mA cm−2 to 1 mA cm−2 [49]. With an increased current density, the Pt growth rate at the edge of the nucleus for Pt is faster than the rate at the other sites of the nucleus, resulting in a morphological change from spherical to flower-like.
To compare AOR activity between different electrodes, CVs were measured using Pt electrodes with spherical and flower-like morphologies with 0.1 M NH3 in 1 M KOH. To prevent false evaluation due to irregular loading amounts of Pt on the electrode, the obtained CV data were normalized with the loaded catalyst amounts. The flower-like Pt particles exhibited a peak current density of 0.063 mA μg−1, which is three times higher than that of the spherical Pt particle. It is mainly due to the difference in the surface area accompanied by each morphology. In fact, the flower-shaped Pt electrode exhibited two times higher electrochemical active surface area (ECSA) than the Pt spherical Pt electrode. Interestingly, when comparing AOR performance divided into ECSA of each Pt electrode, still, the flower-shaped Pt electrode showed advanced activity compared to spherical Pt. Besides the large surface area, the tips and sharp-edged flakes comprising flower morphology develop additional surface electric fields, affecting the enhancement of AOR kinetics.
Facet engineering is another important area to explore in terms of improving AOR kinetics. Yu Chen et al. synthesized Pt nanocubes (Pt-NCs) with dominant (100) facets using a facile hydrothermal method involving the agents of polyvinyl pyrrolidone (PVP), 2-methylimidazole, and ascorbic acid [50]. Each agent is critical in the preparation of the desired catalyst; for example, PVP served as the stabilizing agent, 2-methylimidazole was used as (100) facet-selective agent, and ascorbic acid was used to form nanocubes by controlling the reducing rate of Pt. The preferential (100) facet in the Pt-NCs was revealed by TEM images, as shown in Figure 3a,b.
The AOR activity of the Pt-NCs was evaluated using CV with 0.1 M NH4OH in 1 M KOH and was compared with that of commercial Pt nanoparticles (Pt-NCs-JM) purchased from Johnson Matthey Corporation. In order to remove any possibility of affecting AOR coming from the different surface areas of Pt samples, the AOR activities were normalized using the collected ECSA value, as shown in Figure 3c. For both Pt samples, a remarkable peak indicating AOR starts at 0.5 VRHE and the highest current density was observed at 0.7 VRHE. Pt-NCs exhibited a peak current density of 5.1 mA cmECSA−2, whereas Pt-NCs-JM exhibited a peak current density of 2.6 mA cmECSA−2. Given that Pt-NCs-JM comprises polycrystalline Pt, which has diverse Pt facets, it can be inferred that the advanced AOR performance in Pt-NCs is mainly due to the dominant Pt (100) facets in the sample.
Catalytic stability during electrolysis is another important factor in determining AOR performance. Generally, the strong adsorption of nitrogen atoms (Nads) on Pt catalysts leads to active site deactivation, resulting in severe instability. The catalytic stability of the Pt electrodes was evaluated using constant potential electrolysis at 0.6 VRHE with the same electrolyte conditions. From Figure 3d, the time-profiled current density over the mass of the loaded Pt catalysts (J-t) showed that Pt-NCs exhibited a higher initial current density and lower current density decay rate relative to Pt-NCs-JM. The Pt (100) facets are able to improve initial catalytic activity and stability for AOR by providing an appropriate binding strength for the intermediate.
Juan M. Feliu et al. also prepared (100) facet-dominant cubic platinum nanoparticles (Pt NPs) using the water-in-oil microemulsion method [51]. The Pt (100) facet fraction was adjusted by controlling the concentration of HCl added during synthesis. As the concentration of HCl is increased from 0% to 25%, the (100) fraction also increases from 10% to 44%; when the concentration of HCl is increased beyond 25%, the (100) fraction begins to decrease. To investigate how the (100) facet fraction affects AOR performance, CV was measured with 0.1 M NH3 in 0.2 M NaOH. The Pt sample with a (100) facet fraction of 44% exhibited a current density six times higher than that of the sample with a (100) fraction of 10%. This work obviously suggested a way to increase desired Pt (100) fraction via adjusting the concentration of HCl agents and, furthermore, demonstrated the effect of the Pt (100) fraction on the AOR performance.
The promising effect of Pt (100) facet engineering was theoretically supported by density functional theory (DFT) studies [52,53]. Mavrikakis et al. compared the onset potential and activation barrier of Pt (100) to Pt (111) for each Gerischer-Mauerer and Oswin-Salomon mechanism, as shown in Figure 3e [54]. For the Gerischer–Mauerer mechanism, on Pt (100), the PDS is the deprotonation of HNNHads to NNHads with the onset potential of 0.29 V, which is similar to that of 0.28 V on Pt (111). However, NH2,ads dimerization to N2H4, has a higher barrier of 2.32 eV on Pt (100) than the activation barrier of 1.07 eV on Pt (111). It represents the ability of HNNHads deprotonation on Pt (100) is close to on Pt (111). However, N2H4, formation by NH2,ads dimerization on Pt (111) is more favorable than the dimerization on Pt (100). On the other hand, based on the Oswin-Salomon mechanism on Pt (100), the potential determining step (PDS) is the deprotonation of NH2, ads to NHads with the onset potential of 0.87 V, and the Nads dimerization to N2 has a low activation barrier of 0.53 eV. On the other hand, on Pt (111), the PDS (NHads to Nads) has an onset potential of 0.52 V, which is lower than that of Pt (100), and the Nads dimerization has a high activation barrier of 2.24 eV, which is much higher than that of Pt (100). It indicates that Nads formation on Pt (100) is more difficult than the intermediate formation on Pt (111) referred by onset potentials. However, once Nads formed, its dimerization and desorption on Pt (100) is more preferred than that on Pt (111) when comparing the activation barrier. Considering the theoretically calculated data, Pt (100) facet can provide the lowest activation barrier for N2 formation following the Oswin-Salomon mechanism and also can explain experimental advances in Pt (100) engineering.
Figure 3. (a) The low and (b) the high magnified TEM images of Pt-NCs. (c) the CV curves normalized using the value of ESCA with a potential scan rate of 5 mV s−1 and (d) the time-profiled current density over the mass loaded on the electrode (J-t curves) at 0.6 VRHE for Pt-NCs and Pt-NPs-JM. All AOR performances were measured with 0.1 M NH4OH in 1 M KOH. Reproduced with permission [50]. Copyright 2020, Elsevier B. V. (e) Free-energy diagram for AOR on Pt (100) at 0.0 V. In purple are NHx,ads (x = 0 or 1 or 2) dimerization elementary steps with the respective transition-state (TS) energies. N + N mechanism indicates the Oswin-Salomon mechanism, and * indicates adsorption on the catalyst surface. Reproduced with permission [54]. Copyright 2021, Elsevier B. V.
Figure 3. (a) The low and (b) the high magnified TEM images of Pt-NCs. (c) the CV curves normalized using the value of ESCA with a potential scan rate of 5 mV s−1 and (d) the time-profiled current density over the mass loaded on the electrode (J-t curves) at 0.6 VRHE for Pt-NCs and Pt-NPs-JM. All AOR performances were measured with 0.1 M NH4OH in 1 M KOH. Reproduced with permission [50]. Copyright 2020, Elsevier B. V. (e) Free-energy diagram for AOR on Pt (100) at 0.0 V. In purple are NHx,ads (x = 0 or 1 or 2) dimerization elementary steps with the respective transition-state (TS) energies. N + N mechanism indicates the Oswin-Salomon mechanism, and * indicates adsorption on the catalyst surface. Reproduced with permission [54]. Copyright 2021, Elsevier B. V.
Catalysts 13 00803 g003

3.1.2. Pt-Ir Bimetal Electrocatalysts

Pt is considered the most active AOR catalyst among noble metals. However, it should overcome the catalytic deactivation caused by the strong adsorption of Nads; this behavior is called surface poisoning. Pt-based binary and ternary catalysts with Ir, Ru, Rh, and Pd have been suggested to reduce surface poisoning and improve AOR performance [23,55,56,57,58]. Among these, Ir is the most representative candidate for Pt-based binary and ternary catalysts because it has a sufficient NH3 dehydrogenation capacity and appropriately low Nads affinity. For example, Pt-Ir bimetal catalysts can decrease potential onset relative to a single Pt catalyst and reduce surface poisoning by altering the electron state of Pt and weakening the adsorption strength for Nads [59,60]. In this part, recent studies on Pt-Ir alloy synthesized in various ways and their origins for advancing AOR activity are discussed.
Minhua Shao et al. synthesized a catalyst by depositing Ir onto Pt (100) facets on carbon substrate (Ir-decorated Pt NCs/C) using the modified solvothermal method [59]. The uniform distribution of Ir on Pt was revealed by Energy-dispersive X-ray spectroscopy (EDS) mapping images in Figure 4a. The effect of Ir on Pt was evaluated by comparing LSVs between Ir-decorated Pt NCs/C and pure Pt NCs/C, in which current density was normalized using ECSA in the electrolyte of 0.1 M NH3 in 0.1 M KOH. In Figure 4b, although both Ir-decorated Pt NCs/C and Pt/NCs exhibited pre-peaks at 0.30 VRHE, indicating NH3 adsorption, the current density of Ir-decorated Pt NCs/C arose from a much lower onset potential of 0.43 VRHE than that of Pt NCs/C started at 0.55 VRHE. Furthermore, the Ir-decorated Pt NCs/C exhibited a current density of 1.26 mA cm−2, which was two times higher than that of Pt NCs/C; this confirms that Ir decorated on Pt results in not only reduced overpotential but also increased AOR kinetics.
Besides advancing in kinetic, catalytic stability was also improved by Ir decoration on Pt. The current density decay rates of the Ir-decorated Pt NCs/C and Pt NCs/C were compared using constant potential electrolysis at 0.6 VRHE. The current density of the Ir-decorated Pt NCs/C decayed in 100 s, whereas that of the Pt NCs/C decayed immediately. Considering the major origin of instability observed for Pt electrocatalysts, it reveals that Ir decoration on Pt is effective in preventing Pt surface poisoning by storing Nads.
To further investigate the role of Ir in this regard, the differences in Gibbs free energies of the Pt (100) and Ir-decorated Pt (100) were monitored using DFT calculations for each step of the AOR pathways, as shown in Figure 4c. The Ir-decorated Pt (100) exhibited a relatively low activation energy barrier of 0.74 eV for the dominant rate-determining step compared to pure Pt (100), which exhibited an energy barrier of 0.96 eV. From the experimental and theoretical investigations, it is found that the critical role of Ir on Pt is to improve reaction kinetics by lowering the energy barriers and to increase stability by preventing Pt poisoning.
Elena A. Baravona et al. synthesized Pt-Ir alloys using the polyol method under a pH of 7 (named PtIr (1)) and 8.5 (named PtIr (2)) [60]. As shown in Figure 4d, the AOR activity for both Pt-Ir alloys and the single Pt was evaluated using CV with 0.5 M NH4OH in 1 M KOH, where the measured current densities were normalized using the value of ECSA of each sample. In terms of peak current density, PtIr (1) was less effective than pure Pt; however, PtIr (1) had a clear advantage in onset potential compared to pure Pt. PtIr (1) showed an onset potential for AOR at −1.07 VMSE, which was 0.13 V less than that of the single Pt. The results indicate that the PtIr (1) alloy is better at facilitating high catalytic AOR activity in the low potential region than the single Pt.
Furthermore, the catalytic stabilities of Pt and PtIr (1) were evaluated using a time-profiled potential constant electrolysis at −0.8 VMSE, as shown in Figure 4e. The PtIr (1) showed a higher initial current density and slower current density decay than that of the single Pt. These advances in terms of onset potential, the current density at low potential, and stability can mainly be attributed to the electronic effect between Pt and Ir atoms in the PtIr (1) alloy. Meanwhile, PtIr (2) exhibited an even worse AOR performance than pure Pt in terms of kinetics and stability; this is due to the formation of Pt oxide on the surface of the electrode during synthesis, unlike PtIr (1) and Pt which contain metallic Pt state. In the XPS spectra of Pt 4f, as shown in Figure 4f, PtIr (2) showed additional peaks at 72.9 eV and 74.4 eV, which corresponds to the oxidized Pt, while that of the metallic Pt is at 71.6 eV. The Pt oxide on PtIr (2) inhibited the adsorption of NH3, resulting in a high onset potential and low stability even as alloying Ir with Pt.
Pt-based ternary metal alloying is another promising strategy for advancing AOR performance. Particularly, the addition of a third transition metal onto Pt-Ir alloy has been mainly studied. Gan Wu et al. prepared a ternary PtIrNi alloy on supports comprising SiO2 and CNT-COOH using the sonochemical-assisted method, as shown in Figure 5a [61]. The supports can provide a large surface area for the deposition of the three metals.
The AOR activities of various electrodes, including Pt/SiO2-CNT-COOH, PtIr/SiO2-CNT-COOH, and PtIrNi-SiO2-CNT-COOH, were compared using CVs with 0.1 M NH3 in 1.0 M KOH; their current densities collected during CV were normalized by precious-group-metal (PGM) mass, as shown in Figure 5b. PtIrNi/SiO2-CNT-COOH exhibited the lowest onset potential, followed by PtIr/SiO2-CNT-COOH and Pt/SiO2-CNT-COOH; this confirms that the presence of Ni in the alloy reduces the onset potential. For example, PtIrNi/SiO2-CNT-COOH exhibited the lowest onset potential at 0.4 VRHE and the highest current density of 124.0 A g−1. PtIr/SiO2-CNT-COOH and Pt/SiO2-CNT-COOH even recorded more positive values of onset potential, as much as 0.05 VRHE and 0.15 VRHE, respectively, compared to that from PtIrNi/SiO2-CNT-COOH. Moreover, the Ni ternary alloy exhibited the highest stability, which was measured at a constant 0.65 VRHE, among all Pt-based electrodes, as shown in Figure 5c.
The enhancement of AOR performance by alloying with Pt, Ir and Ni was elucidated by DFT calculations. According to Gibbs’s free energy changes for each elemental step of AOR, as shown in Figure 5d, the Pt-Ir alloy had a largely reduced thermodynamic barrier for dehydrogenation compared to the single Pt. Moreover, as alloying Ni with Pt-Ir, the barrier was further reduced significantly. By adding Ni to Pt-Ir alloy, d orbital states can be largely increased, resulting in facilitating NHads formation and adsorption, which is the major rate-determining step in the single Pt. This work demonstrated that a ternary metal alloy is a promising approach to preparing electrocatalysts for AOR in terms of onset potential and stability. However, even if AOR performance has been largely developed by multi-modification using Pt, for example, morphology control, facet engineering, alloy, and so on, the high cost of Pt and Pt-based electrodes remain a significant challenge for advancing AOR in practice. Therefore, the rational design of electrocatalysts comprising non-precious elements is highly required. We summarized the AOR performance on Pt-based catalysts in Table 1.

3.2. Ni-Based Metal Electrocatalysts

Pt and Pt-based noble metals have been considered active catalysts in AOR because of their low overpotential and high N2 selectivity; however, their high cost and susceptibility to deactivation have proven to be significant challenges. Recently, non-noble metal-based electrocatalysts have been proposed as an alternative to Pt-based electrocatalysts [76,77,78]. Ni has been studied as a potential catalyst to replace noble metals because it is intrinsically satisfactory for adsorbed NH3 dehydrogenation and NHx,ads dimerization and shows experimental activity in AOR with its abundancy [20,40,76]. Notably, Ni exhibited significantly promising long-term stability for a few hours, while Pt-based catalysts were deactivated within a few seconds. However, formation of by-products, N2H4, NO2, NO3, and so on, leads to decrease selectivity for N2 production [79]. To overcome the limitation and further improve AOR activity, Ni-based binary and ternary catalysts have been studied [80,81,82]. This section focuses on recent research regarding combinations of Ni with hetero-atoms and their effects on AOR.
Ni alloying with Cu elements has been widely proposed to promote AOR performance by overcoming the challenges of imperfect reaction rate and N2 selectivity [19].
Shanwen Tao et al. synthesized Ni-Cu layered hydroxides (Ni0.8Cu0.2 LHs) on carbon fiber cloth using the hydrothermal method, followed by electrochemical activation over 200 CV cycles in the potential range of −0.2 VAg/AgCl to 1.0 VAg/AgCl, as shown in Figure 6a [24]. The AOR performance of Ni0.8Cu0.2 LHs was evaluated using CV with 55 mM NH4Cl in 0.5 M NaOH and compared with that of bare Ni(OH)2. In Figure 6b, both electrodes exhibited an increase in the current density at 0.43 VAg/AgCl, indicating the oxidation from hydroxide to oxyhydroxide, such as the transformation of Ni0.8Cu0.2OOH from Ni0.8Cu0.2 LHs (Ni0.8Cu0.2(OH)2) and NiOOH from Ni(OH)2, respectively. In the potential over 0.43 VAg/AgCl representing the AOR region, Ni0.8Cu0.2 LHs showed a rapid increase, and it represents a higher current density than that from Ni(OH)2. Additionally, the Tafel slope of Ni0.8Cu0.2 LHs was much smaller than that of Ni(OH)2, indicating the electron transfer within Ni0.8Cu0.2 LHs is relatively fast, as shown in Figure 6c. From this, it can be posited that the addition of Cu to the Ni oxyhydroxide improved both electron transfer and AOR kinetics. The first origin of AOR enhancement for Ni0.8Cu0.2 LHs is a large surface area compared to Ni(OH)2. Moreover, the adjusted strength of NH3 adsorption promotes the formation of the first intermediate of the Ni1-xCuxOOH(NH3)ads.
Furthermore, the catalytic stability of Ni0.8Cu0.2 LHs was evaluated using constant potential electrolysis of 0.55 VAg/AgCl under the same electrolytic conditions discussed above. As shown in Figure 6d, the addition of NH4Cl resulted in an increase in current density for both Ni0.8Cu0.2 LHs and bare Ni(OH)2 electrodes. Interestingly, Ni0.8Cu0.2 LHs showed a stable current density of 35 mA cm−2, which was approximately seven times higher than that of bare Ni(OH)2 for at least 3 h. The NH3 removal efficiency, determined at 0.55 VAg/AgCl, reached 84%, whereas bare Ni(OH)2 recorded an NH3 removal efficiency of 58%. Based on all the results discussed above, the Cu-doped Ni LDH was able to enhance AOR performance in terms of activity, stability, and ultimate NH3 removal efficiency.
Masaharu Nakayama et al. prepared Ni2+ and Cu2+ co-intercalated electrocatalysts on layered MnO2 (NiCu/MnO2) using a two-step synthetic process: the electrodeposition of tetrabutylammonium cations (TBA+)/MnO2 at a constant potential of 1.0 VAg/AgCl and then the ion exchange reaction, carried out by immersing the catalyst in a solution containing Ni2+ and Cu2+ [26]. As shown in Figure 7a, TBA/MnO2 showed a peak at 7.14°, which represents a (001) plane of layered MnO2 and a d-spacing of 1.23 nm. On the other hand, in NiCu/MnO2, the peak of the (001) plane split and appeared at 10° and 12°, and the d-spacing decreased to 0.88 nm and 0.75 nm. The reduced d-spacing indicates that the bulky TBA+ ions were exchanged into Ni2+ and Cu2+ ions. Additionally, uniformly electrodeposited surface was identified through the SEM images as shown in Figure 7b,c.
The electrocatalytic AOR performance for NiCu/MnO2 was evaluated using CV with 55 mM NH4Cl in 0.5 M NaOH, as shown in Figure 7d. NiCu/MnO2 exhibited a peak at 0.53 VHg/HgO, confirming the oxidation from Ni2+ to Ni3+, which was followed by a rapid increase in current density, an indication of facile AOR. Although Ni/MnO2 also exhibited two distinct increases in current density, attributing to Ni2+ oxidation at 0.65 VHg/HgO and AOR, the subsequent increase in current density was much lower than that of NiCu/MnO2. These results obviously represented that the addition of Cu2+ into Ni2+ intercalated MnO2 reduced the onset potential for Ni2+ oxidation and increased the current density for AOR.
Moreover, the NiCu/MnO2 showed promising catalytic stability during constant potential electrolysis at 0.6 VHg/HgO for 24 h, as shown in Figure 7e. Even if the current density steadily decreased as time passed, the current density was recovered in the 2nd-run, in a re-prepared fresh electrolyte after 24 h. This indicates that the current density decay was not originated from deactivation or electrode corrosion but from the rapid consumption of NH3 at the NiCu/MnO2 electrode. In addition, using online gas chromatography, the desired N2 product selectivity was estimated to be 97.4%. This work revealed that the addition of Cu2+ in Ni2+ intercalated in MnO2 could enhance AOR performance by mediating the electron transfer between Ni2+ and NH3 and facilitating the oxidation of adsorbed NH3.
Lately, to further develop AOR performance, researchers have suggested adding the third hetero-metal to Ni-Cu-based electrocatalysts. Junmin Xue et al. synthesized triple Ni-Cu-Fe oxyhydroxides onto carbon paper (NiCuFe electrode) using electrodeposition at a constant potential of −1.15 VHg/Hg2Cl2, followed by electrochemical activation [27]. The AOR activity of the NiCuFe electrode was evaluated using CV with 55 mM NH4Cl in 0.5 M NaOH and compared with that of the NiCu electrode. The NiCuFe electrode exhibited a rapid increase in current density over 0.36 V Hg/Hg2Cl2, and its current density reached 60 mA cm−2 at 0.7 V Hg/Hg2Cl2, whereas the current density of the NiCu electrode also increased over 0.45 V Hg/Hg2Cl2 and its current density only reached 30 mA cm−2 at 0.7 V Hg/Hg2Cl2. This shows that the addition of Fe to NiCu significantly reduced the onset potential for AOR and increased the current density generated by AOR. Moreover, in the stability test during the constant potential electrolysis at 0.55 V Hg/Hg2Cl2, the NiCuFe electrode exhibited an initial current density of 40 mA cm−2, which was two times higher than that of the NiCu electrode. The current density decreased as NH3 was consumed. The NiCuFe electrode recorded a high NH3 removal efficiency of 95 % quantified using the indophenol method, and a selective NH3 conversion to N2 also occurred, with a minor product of NO3.
The effect of doping Fe into NiCu electrode on AOR activity was additionally revealed using DFT calculation. According to DFT calculation, the first step of the reaction, the adsorption of NH3 on an oxygen atom bonded to Ni and the NiCu electrode, showed the highest energy barrier of 4.91 eV among the tailored elementary steps. The addition of Fe, which has relatively low electronegativity, to the NiCu electrode, resulted in a significant increase in electron density around oxygen atoms and lowered the energy barrier to 1.08 eV. This suggests that Ni-Cu alloys doped with low electronegative elements, such as Fe, are able to accelerate the adsorption of NH3 and ultimately improve AOR kinetics.
Besides Ni and Cu combination, diverse mixture of hetero-atoms and each element’s effects have been experimentally proposed for AOR. Jianhui Wang et al. prepared physically mixed Ag and Ni electrocatalysts (Ag/Ni) by facile hand-mixing method [83]. The Ag/Ni was measured AOR performances by CV with 0.5 M NH3 in 1.5 M NaOH and compared its activity with pristine Ag and Ni, respectively. The pristine Ag represented no catalytic activity in the potential region of CVs, whereas the pristine Ni exhibited a current density increase from 0.6 VHg/HgO. However, its dominant product from electrolysis was not N2 but O2, indicating OER is dominant rather than AOR on the electrode having Ni alone. On the other hand, the Ag/Ni showed a significant current density increase from lower onset potential at 0.46 VHg/HgO, which is a much higher current density than pristine Ni. Furthermore, Ag/Ni produced N2 as a major product, representing its superior activity for AOR. The faradaic efficiency for N2 production was recorded at 59.8%, while constant potential electrolysis at 0.7 VHg/HgO for 2 h. Furthermore, from the control experiments of CVs with and without NH3, as shown in Figure 8a, the synergistic effect of Ag and Ni for AOR was discussed. As anodic scanning with NH3, the peak for Ag+ oxidation to Ag2+, typically observed during the scan without NH3, disappeared and which indicates the oxidized Ag+ can form a strong Ag+–NH3 complex, suppressing further NH3 oxidation. On the other hand, pristine Ni exhibited similar CV results regardless of NH3 presence in the solution because the competitive reaction of OER was more favorable than AOR on the Ni-only surface. Interestingly, Ag/Ni facilitated each Ag+–NH3 complex formation and OH adsorption on Ni, and finally, they synergistically promoted to produce N2 by applying potentials over 0.4 VHg/HgO, as shown in Figure 8b.
The studies for developing AOR using Pt and Ni-based electrocatalysts have been mainly reported, whereas the research using other elemental compositions has been rarely studied. Furthermore, although new groups of catalysts, for example, CuO, CuSn(OH)6, TiO and so on, were suggested, their AOR performances still need to be advanced [77,84,85]. Their recorded AOR activities are summarized in Table 2. It is highly required not only to search for new latent electrocatalyst candidates but also to advance their AOR performance using effective strategies.

4. Summary and Perspective

NH3 has been gaining attention as a sustainable fuel and a carbon-free hydrogen carrier with a high energy density and high hydrogen content. Given this, the electrochemical NH3 oxidation (AOR) is a key reaction for generating electricity in NH3 fuel cells and producing H2 fuel in NH3 electrolyzers. However, the sluggish kinetics of AOR give rise to severe drawbacks of overpotential and reaction rate, even though it is thermodynamically favorable. These experimental challenges have been overcome by developing modified electrocatalysts, as reviewed in this work.
Pt electrocatalysts have been considered for use in AOR due to their superior NH3 affinity and rapid AOR kinetics; because of this, Pt electrocatalysts have been further studied by exploring the diverse strategies modifying morphology and exposure facet on the surface, alloying, and so on. Particularly, a Pt-Ir was able to significantly improve the reaction kinetics of AOR by effectively reducing the energy barrier. Meanwhile, recently, cost-effective non-noble metal catalysts, especially Ni-based catalysts, have also been intensively suggested as alternatives to Pt. In particular, Ni-Cu bimetal catalysts exhibited the most promising AOR performance in terms of reaction rate and stability by adjusting the affinity of NH3 and intermediates on Ni and Cu.
However, still searching for robust new catalysts and their development is necessary in order to perform AOR in practice. Especially to develop cost-effective non-noble metal-based electrocatalysts for AOR, several challenges need to be overcome. First, selective AOR in low overpotential regions is important. However, typical non-noble metal-based electrocatalysts exhibited AOR performance in high overpotential, and thus, the AOR competes with competitive reactions, such as water oxidation or NH3 oxidation to NO3 or NO2, and so on. However, it was found that forming alloys with hetero-atoms can alter the electronic structure of catalysts resulting in reducing the NH3 adsorption energy barrier and effectively reducing the overpotential [27,61,89].
Another important factor is AOR rate, which can be determined by current density. Doping or alloying can also be effective in increasing the current density by stabilizing reaction intermediates and enhancing reaction kinetics. In addition, the engineering morphology of electrocatalysts can be a key suggestion to facilitate reactions. The nano-sized electrocatalyst provides lots of active catalytic sites in large surface areas, leading to advanced current densities for AOR. Moreover, combining electrocatalysts with conductive supports or substrates comprising electrodes can promote significantly improved AOR performance. Substrates, such as carbon paper, carbon nanotube (CNT), Ni foam, and so on, have large surface areas and high intrinsic conductivities, reducing electron transfer resistance within the electrocatalysts coupled with the substrates and improving the AOR activity [61,88,91].
Unlike catalyst modifications, it has been rarely studied evaluating the total performance, including AOR and the counter reduction reaction in single-cell and engineering environmental conditions. For example, in order to effectively produce energy from NH3 fuel or to extract H2 from NH3 electrolysis in practice, it is necessary to construct a single-cell typed real device composed of the developed catalysts and evaluate performance using the device. To our knowledge, the best faradaic efficiency of N2 and H2 production was 90 % and 90 %, respectively, from NH3 during current density constant electrolysis at 200 mA cm−2, where the potential difference of 0.85 V was measured in the single-cell composed with two Pt electrodes [42]. In addition, the total performance can be improved by optimizing reaction environmental conditions. For instance, the reaction operating temperature can be a critical factor directly affecting to the reaction rate [16,92,93,94,95]. As referring to the previous work, the device’s current density was largely increased to 250 mA cm−2 at 60 °C compared to the current density of 80 mA cm−2 measured at room temperature. Therefore, to use NH3 as a promising energy source efficiently, it is highly necessary to engineer and optimize device set-up and operating conditions.
Besides developing catalysts, a fundamental understanding of the reaction mechanism is critical to further improve AOR. Currently, the AOR mechanism has been mainly investigated as suitable for Pt catalysts. However, actual reaction mechanisms or rate-determining steps can be different depending on the types of electrocatalysts. The DFT calculations have been widely applied to propose reaction mechanisms and elucidate the key effect for the catalytic modification [20,96,97]. With advances in theoretical investigation, observations of reaction mechanisms experimentally are important by monitoring adsorbed intermediates. Recently, efforts to investigate actual reaction mechanisms on electrocatalysts have been reported using in-situ or operando characterizations [1,21,98]. Considering the progress, the AOR has significant potential to develop as practically by combining catalytic modification and device engineering and fundamental studies.

Author Contributions

Conceptualization and writing—original draft preparation by J.H.J. and S.Y.P., writing review and editing, and supervision by D.H.Y. and Y.J.J. Funding acquisition by Y.J.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (No. 20224000000440, Sector coupling energy industry advancement manpower training program), and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Trade, Industry & Energy (MOTIE) (No. 20223030030090).

Data Availability Statement

The datasets used and analyzed during the current study are available from the corresponding references listed.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Schematic diagram of NH3 oxidation reaction performance improvement strategies covered in this paper. Schematic of Structural Manipulation: Reproduced with permission [26]. Copyright 2021, American Chemical Society. Schematic of Doping: Reproduced with permission [27]. Copyright 2021, Wiley–VCH.
Scheme 1. Schematic diagram of NH3 oxidation reaction performance improvement strategies covered in this paper. Schematic of Structural Manipulation: Reproduced with permission [26]. Copyright 2021, American Chemical Society. Schematic of Doping: Reproduced with permission [27]. Copyright 2021, Wiley–VCH.
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Figure 2. (a) Schematic illustration of the CP-Pt and CV-Pt electrodes. (b) CVs results and (c) Comparison of peak current density and current density at −0.4 VAg/AgCl for CV-Pts and CP-Pts. All electrocatalytic measurements were conducted with 1 M NH3 in 5 M KOH. Reproduced with permission [42]. Copyright 2021, Royal Society of Chemistry.
Figure 2. (a) Schematic illustration of the CP-Pt and CV-Pt electrodes. (b) CVs results and (c) Comparison of peak current density and current density at −0.4 VAg/AgCl for CV-Pts and CP-Pts. All electrocatalytic measurements were conducted with 1 M NH3 in 5 M KOH. Reproduced with permission [42]. Copyright 2021, Royal Society of Chemistry.
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Figure 4. (a) HAADF-STEM images and corresponding EDS mapping images of Ir-decorated Pt NC. (b) the LSV curves were measured with 0.1 M NH3 in 0.1 M KOH, where the current densities were normalized using the value of ECSA. (c) Gibbs free energy changes for each step of AOR pathways on Pt(100) and Ir-decorated Pt(100). Reproduced with permission [59]. Copyright 2020, Elsevier B. V. (d) The CV curves measured with 0.5 M NH4OH in 1 M KOH, where current densities were normalized using the value of ECSA, (e) J-t curves at −0.8 VMSE of Pt, PtIr (1) and PtIr (2) and (f) Pt 4f XPS spectra for PtIr (1) and PtIr (2). Reproduced with permission [60]. Copyright 2013, Elsevier B. V.
Figure 4. (a) HAADF-STEM images and corresponding EDS mapping images of Ir-decorated Pt NC. (b) the LSV curves were measured with 0.1 M NH3 in 0.1 M KOH, where the current densities were normalized using the value of ECSA. (c) Gibbs free energy changes for each step of AOR pathways on Pt(100) and Ir-decorated Pt(100). Reproduced with permission [59]. Copyright 2020, Elsevier B. V. (d) The CV curves measured with 0.5 M NH4OH in 1 M KOH, where current densities were normalized using the value of ECSA, (e) J-t curves at −0.8 VMSE of Pt, PtIr (1) and PtIr (2) and (f) Pt 4f XPS spectra for PtIr (1) and PtIr (2). Reproduced with permission [60]. Copyright 2013, Elsevier B. V.
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Figure 5. (a) Schematic illustration for the synthesis of PtIrNi/SiO2-CNT-COOH. (b) the CV curves where current densities were normalized using precious-group-metal (PGM) mass and (c) the J-t curves at 0.65 VRHE of Pt, PtIr, PtNi, PtIrNi1, PtIrNi2, PtIrNi3 deposited on SiO2-CNT-COOH, and commercial PtIr/C. The AOR performance was measured in Ar-saturated with 0.1 M NH3 in 1.0 M KOH. (d) Free-energy diagram for elementary steps of NH3 dehydrogenation on Pt(100) Pt3Ir(100), Pt3IrNi(100). Reproduced with permission [61]. Copyright 2020, American Chemical Society.
Figure 5. (a) Schematic illustration for the synthesis of PtIrNi/SiO2-CNT-COOH. (b) the CV curves where current densities were normalized using precious-group-metal (PGM) mass and (c) the J-t curves at 0.65 VRHE of Pt, PtIr, PtNi, PtIrNi1, PtIrNi2, PtIrNi3 deposited on SiO2-CNT-COOH, and commercial PtIr/C. The AOR performance was measured in Ar-saturated with 0.1 M NH3 in 1.0 M KOH. (d) Free-energy diagram for elementary steps of NH3 dehydrogenation on Pt(100) Pt3Ir(100), Pt3IrNi(100). Reproduced with permission [61]. Copyright 2020, American Chemical Society.
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Figure 6. (a) Schematic illustration for the preparation of Ni0.8Cu0.2 LHs on carbon fiber cloth substrate, (b) the CVs and (c) the Tafel plots for Ni0.8Cu0.2 LHs and bare Ni(OH)2. (d) the J-t curves at 0.55 VAg/AgCl. The AOR performance was measured with 55 mM NH4Cl in 0.5 M NaOH. Reproduced with permission [24]. Copyright 2017, Elsevier B. V.
Figure 6. (a) Schematic illustration for the preparation of Ni0.8Cu0.2 LHs on carbon fiber cloth substrate, (b) the CVs and (c) the Tafel plots for Ni0.8Cu0.2 LHs and bare Ni(OH)2. (d) the J-t curves at 0.55 VAg/AgCl. The AOR performance was measured with 55 mM NH4Cl in 0.5 M NaOH. Reproduced with permission [24]. Copyright 2017, Elsevier B. V.
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Figure 7. (a) The XRD patterns of TBA/MnO2 (black line), Ni/MnO2 (blue line), Cu/MnO2 (green line), NiCu/MnO2 before AOR (upper red line) and NiCu/MnO2 after AOR (lower red line). (b) The low and (c) the high-magnified SEM images of NiCu/MnO2 deposited on carbon paper. (d) The CV curves and (e) the J-t curves at 0.6 VHg/HgO for NiCu/MnO2, Ni/MnO2, and Cu/MnO2. The AOR performance was measured with 55 mM NH4Cl in 0.5 M NaOH. Reproduced with permission [26]. Copyright 2021, American Chemical Society.
Figure 7. (a) The XRD patterns of TBA/MnO2 (black line), Ni/MnO2 (blue line), Cu/MnO2 (green line), NiCu/MnO2 before AOR (upper red line) and NiCu/MnO2 after AOR (lower red line). (b) The low and (c) the high-magnified SEM images of NiCu/MnO2 deposited on carbon paper. (d) The CV curves and (e) the J-t curves at 0.6 VHg/HgO for NiCu/MnO2, Ni/MnO2, and Cu/MnO2. The AOR performance was measured with 55 mM NH4Cl in 0.5 M NaOH. Reproduced with permission [26]. Copyright 2021, American Chemical Society.
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Figure 8. (a) The CV curves of Ni, Ag and Ag/Ni on the glassy carbon electrode. The AOR performance was measured with and without 0.5 M NH3 in 1.5 M NaOH. (b) Schematic mechanism of Ag and Ni in the AOR. Reproduced with permission [83]. Copyright 2022, Royal Society of Chemistry.
Figure 8. (a) The CV curves of Ni, Ag and Ag/Ni on the glassy carbon electrode. The AOR performance was measured with and without 0.5 M NH3 in 1.5 M NaOH. (b) Schematic mechanism of Ag and Ni in the AOR. Reproduced with permission [83]. Copyright 2022, Royal Society of Chemistry.
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Table 1. A brief summary of experimental studies on AOR using various Pt-based electrocatalysts.
Table 1. A brief summary of experimental studies on AOR using various Pt-based electrocatalysts.
CatalystsElectrolyteOnset Potential
(VRHE)
Peak Current DensityRef
Pt-based catalysts500 CV-Pt1 M NH3
+ 5 M KOH
0.420.27 mA cmECSA−2[42]
Flower-like Pt0.1 M NH3
+ 1 M KOH
0.50.48 mA cmECSA−2[49]
Pt nanocubes (Pt-NCs)0.1 M NH4OH
+ 1 M KOH
0.55.1 mA cmECSA−2[50]
Pt nanoparticles (Pt NPs)0.1 M NH3
+ 0.2 M NaOH
0.551.96 mA cmECSA−2[51]
Ir-decorated Pt0.1 M NH3
+ 0.1 M KOH
0.431.26 mA cmECSA−2[59]
PtIr nanoparticle0.5 M NH4OH
+ 1 M KOH
0.40.11 mA cmECSA−2[60]
PtIrNi/SiO2-CNT-COOH0.1 M NH3
+ 1 M KOH
0.42.48 mA cm−2[61]
PtNC/C0.1 M NH3
+ 1 M KOH
0.483.89 mA cmECSA−2[48]
Pt nanosheet0.1 M NH3
+ 1 M KOH
0.570.32 mA cmECSA−2[62]
Pt thin film0.1 M NH3
+ 0.2 M NaOH
0.50.212 mA cmECSA−2[63]
Pt nanofilm0.1 M NH3
+ 1 M KOH
0.452.15 mA cm−2[1]
C-Pt/SnO20.1 M NH3
+ 1 M KOH
0.451.6 mA cm−2[64]
Pt/PBI/MWNT-CeO20.1 M NH3
+ 1 M KOH
0.450.26 mA cmECSA−2[65]
Pt/C-ITO1 M NH4OH
+ 1 M KOH
0.541.35 mA cmECSA−2[66]
Y2O3-modified Pt/Si0.1 M NH3
+ 1 M KOH
0.450.19 mA cmECSA−2[21]
Pt/NGA0.1 M NH3
+ 1 M KOH
0.50.64 mA cmECSA−2[67]
Pt90Ir100.1 M NH3
+ 1 M KOH
0.580.37 mA cm−2[68]
Pt-Ir nanocubes0.1 M NH3
+ 1 M KOH
0.451.32 mA cm−2 ECSA[69]
Pt/Ir/MWCNT0.1 M NH3
+ 0.1 M KOH
0.380.23 mA cm−2 ECSA[70]
PtIrZn0.1 M NH3
+ 0.5 M NaOH
0.30.56 mA cm−2[71]
PtIrCu HCOND0.1 M NH3
+ 1 M KOH
0.35122.9 A g−1 Pt[72]
PtIrZn2/CeO2-ZIF-80.1 M NH3
+ 1 M KOH
0.350.64 mA cm−2[73]
Pt90Ru10/C1 M NH4OH
+ 1 M KOH
0.50.91 mA cmECSA−2[74]
Pt6Ru-NCs0.1 M NH3
+ 1 M KOH
0.51.02 mA cmECSA−2[75]
Pt85Pd15/rGO0.1 M NH3
+ 1 M KOH
0.471.46 mA cm−2[57]
Table 2. A brief summary of experimental studies on AOR using various non-noble metal electrocatalysts.
Table 2. A brief summary of experimental studies on AOR using various non-noble metal electrocatalysts.
CatalystsElectrolyteOnset
Potential (VRHE)
StabilityN2
Selectivity (%)
Removal
Efficiency (%)
Ref
Ni-based catalystsNi0.8Cu0.2 LHs0.55 M NH4Cl
+ 0.5 M NaOH
1.3935 mA cm−2 for 3 h-84[24]
NiCu/MnO20.055 M NH4Cl
+0.5 M NaOH
1.63 mA cm−2 for 4 h97.4-[26]
NiCuFe0.055 M NH4Cl
+ 0.5 M NaOH
1.4320 mA cm−2 for 12 h-89.4[27]
Ag/Ni0.5 M NH3
+ 1.5 M NaOH
1.3710 mA cm−2 for 350 h59.8-[83]
NiCu/CP0.055 M NH4Cl
+ 0.5 M NaOH
1.478 mA cm−2 for 2 h-78[19]
Wire-in-plate nanostructured Ni(OH)2-Cu2O@CuO1 M NH3
+ 1 M KOH
1.3760 mA cm−2 for 32 h--[86]
NiCu DHTs0.05 M NH4OH
+ 0.1 M NaOH
1.310.5 mA cm−2 for 1 h--[87]
NiCu/BDD0.5 M NH3
+ 0.5 M NaOH
1.355 mA cm−2 for 7 h88.644.8[88]
NiCuCo-S-T/CP0.2 M NH4Cl
+ 1 M NaOH
1.24105 mA cm−2 for 12 h81.298[89]
NiO-TiO20.2 M NH4NO3
+ 1 M NaOH
1.53-9096.4[90]
Cu-based catalystsCuO1 M NH3
+ 1 M KOH
1.19200 mA cm−2 for 400 h62.8-[84]
CuSn(OH)610 mM NH3
+0.5 M K2SO4
1.56-84.5-[85]
Ti-based catalystTiO0.1 M NH3
+ 0.5 M NaOH
0.40.01 mA cm−2 for 2 h--[77]
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MDPI and ACS Style

Jang, J.H.; Park, S.Y.; Youn, D.H.; Jang, Y.J. Recent Advances in Electrocatalysts for Ammonia Oxidation Reaction. Catalysts 2023, 13, 803. https://doi.org/10.3390/catal13050803

AMA Style

Jang JH, Park SY, Youn DH, Jang YJ. Recent Advances in Electrocatalysts for Ammonia Oxidation Reaction. Catalysts. 2023; 13(5):803. https://doi.org/10.3390/catal13050803

Chicago/Turabian Style

Jang, Ji Hee, So Young Park, Duck Hyun Youn, and Youn Jeong Jang. 2023. "Recent Advances in Electrocatalysts for Ammonia Oxidation Reaction" Catalysts 13, no. 5: 803. https://doi.org/10.3390/catal13050803

APA Style

Jang, J. H., Park, S. Y., Youn, D. H., & Jang, Y. J. (2023). Recent Advances in Electrocatalysts for Ammonia Oxidation Reaction. Catalysts, 13(5), 803. https://doi.org/10.3390/catal13050803

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