Ammonia Electro-oxidation Reaction Research Articles

Sm2+ ions boost ammonia electrooxidation reaction on samarium oxide anode for hydrogen generation in non-aqueous electrolyte

Sm2+ ions boost ammonia electrooxidation reaction on samarium oxide anode for hydrogen generation in non-aqueous electrolyte

Tuning Cu2O morphologies of Cu2O/Ni foam electrodes for the control of reactivity and nitrogen selectivity in direct ammonia electrooxidation reaction

Direct electrochemical oxidation of ammonia (AEO) using non-precious metal catalysts is an energy-efficient and low-carbon-footprint method for ammonia removal from wastewater. In this work, Cu2O with three types of morphologies, including flower, particle, and sheet, were electrodeposited on Ni foam substrates (Cu2O/NF) for direct AEO. The Cu2O flower-deposited NF electrode showed the largest electrochemical surface area (19.6 cm2) and the smallest Cu2O size (200–600 nm) among all Cu2O/NF electrodes. In the ammonia solution, the peak current densities of direct AEO from cyclic voltammograms (CVs) at 10 mVs−1 on the Cu2O flower, Cu2O particle, and Cu2O sheet-deposited NF electrodes reached 4.6, 3.1, and 2.2 mAcm−2, respectively. The result showed that, at an initial pH of 11 and an applied potential of 0.95 V vs. Hg/HgO, the order of three types of Cu2O morphologies with respect to ammonia removal efficiency and rate is: Cu2O flower (52%, 4.1×10−3 min−1) > Cu2O particle (38%, 2.6×10−3 min−1) > Cu2O sheet (30%, 1.8×10−3 min−1). Interestingly, ammonia oxidation to N2 on the Cu2O sheet-deposited NF electrode exhibited the highest selectivity (SN2: 60%), which was much higher than the Cu2O flower-deposited NF electrode (SN2: 31%). The high SN2 of Cu2O sheets was ascribed to the presence of the stable Cu(I) sheet structure during direct AEO, confirmed by X-ray photoelectron spectroscopy analysis. On the other hand, the Cu(II)-rich flower structure facilitated the formation of NO3-. Our findings demonstrate that the surface morphologies of Cu2O deposited on the NF electrode show significant stability difference of Cu(I), which determines reactivity and N selectivity during direct AEO.

FeOOH上掺入Ni和Cu引起的羟基和含氮中间物的共 吸附行为实现加速氨氧化反应中的脱氢过程

Owing to its high hydrogen content and carbon-free nature, ammonia is a promising energy carrier used in powering fuel cells and an alternative oxidation substrate to water in electrolytic hydrogen production devices. However, the insufficient mechanistic understanding and the lack of inexpensive and efficient catalysts for the ammonia electro-oxidation reaction (AOR) have hampered the development of the ammonia-based energy system. In this work, novel Ni and Cu co-doped porous FeOOH nanorods (NiCu-FeOOH) synthesized by a light-induced chemical precipitation method can serve as the AOR catalyst with efficient catalytic activity (1.41 V at an anodic current density of 10 mA cm−2) and enhanced stability in an aqueous ammonia solution. According to the experimental data and theoretical calculation results, the synergistic effect of heterogeneous Ni and Cu atoms makes Ni and Fe sites on the surface of NiCu-FeOOH exhibit a suitable electronic structure to coadsorb nitrogenous intermediates and hydroxyl groups at the top of the volcano plot and thereby accelerate dehydrogenation in the AOR. The backward shift of the rate-determining step (RDS) (⋆NH2 + ⋆OH formation step shifts to ⋆N2H3 + ⋆OH formation step) and the lower energy barrier of the RDS (0.86 eV) reveal that the Ni and Cu co-doping makes FeOOH crystals active for the AOR. The coadsorption reaction pathway involved in nitrogenous intermediates and hydroxyl groups has been in-novatively proposed to effectively describe and simulate the AOR process, which opens a new horizon to design low-cost and stable AOR catalysts.

Self-Terminating Surface Reconstruction Induced by High-Index Facets of Delafossite for Accelerating Ammonia Oxidation Reaction Involving Lattice Oxygen.

Ammonia (NH3 ) is a promising hydrogen (H2 ) carrier for future carbon-free energy systems, due to its high hydrogen content and easiness to be liquefied. Inexpensive and efficient catalysts for ammonia electro-oxidation reaction (AOR) are desired in whole ammonia-based energy systems. In this work, ultrasmall delafossite (CuFeO2 ) polyhedrons with exposed high-index facets are prepared by a one-step NH3 -assisted hydrothermal method, serving as AOR pre-catalysts. The high-index CuFeO2 facet is revealed to facilitate surface reconstruction into active Cu-doped FeOOH nanolayers during AOR processes in ammonia alkaline solutions, which is driven by the favorable Cu leaching and terminates as the 2p levels of internal lattice oxygen change. The reconstructed heterostructures of CuFeO2 and Cu-doped FeOOH effectively activate the dehydrogenation steps of NH3 and exhibit a potential improvement of 260mV for electrocatalytic AOR at 10mA cm-2 compared to the pre-restructured phase. Further, density functional theory (DFT) calculations confirm that a lower energy barrier of the rate-determining step (*NH3 to *NH2 ) is presented on high-index CuFeO2 facets covered with Cu-doped FeOOH nanolayers. Innovatively, lattice oxygen atoms in Fe-based oxides and oxyhydroxide are involved in the dehydrogenation steps of AOR as a proton acceptor, broadening the horizons for rational designs of AOR catalysts.

Platinum on nitrogen doped graphene and tungsten carbide supports for ammonia electro-oxidation reaction

Platinum on nitrogen doped graphene and tungsten carbide supports for ammonia electro-oxidation reaction

Facile Fabrication of a Foamed Ag3CuS2 Film as an Efficient Self-Supporting Electrocatalyst for Ammonia Electrolysis Producing Hydrogen.

Ammonia (NH3) is one of the hydrogen carriers that has received extensive attention due to its high hydrogen content and carbon-free nature. The ammonia electro-oxidation reaction (AOR) and the liquid AOR (LAOR) are integral parts of an ammonia-based energy system. The exploration of low-cost and efficient electrocatalysts for the AOR and LAOR is very important but very difficult. In this work, a novel self-supporting AOR and LAOR bifunctional electrocatalyst of a Ag3CuS2 film is synthesized by a simple hydrothermal method. The Ag3CuS2 film without a substrate shows efficient catalytic activity and enhanced stability for NH3 electrolysis in both aqueous ammonia solution and liquid ammonia, including an onset potential of 0.7 V for the AOR and an onset potential of 0.4 V for the LAOR. The density functional theory calculations prove that compared to Cu atoms, Ag atoms with appropriate charge density on the surface of Ag3CuS2 are more electrocatalytically active for NH3 splitting, including the low energy barrier in the rate-determining *NH3 dehydrogenation step and the spontaneous tendency in the N2 desorption process. Overall, the foamed Ag3CuS2 film is one of prospective low-cost and stable electrocatalysts for the AOR and LAOR, and the self-supporting strategy without a substrate provides more perspectives to tailor more meaningful and powerful electrocatalysts.

Electrochemical ammonia oxidation reaction on defect-rich TiO nanofibers: Experimental and theoretical studies

Ammonia is a promising platform molecule for the future renewable energy infrastructure owing to its high hydrogen content and carbon-free nature. The development of inexpensive and efficient catalysts for ammonia electro-oxidation reaction (AOR) is essential to ammonia-based economy. In this paper, a self-supporting AOR catalyst of ultra-elongated defective TiO nanofibers grown on Ti foil (TiO/Ti foil) is designed. By the comparisons of electrochemical measurement results, the efficient AOR activity of TiO/Ti foil has been presented in alkaline ammonia solution when the voltage is higher than 0.4 V. Density functional theory (DFT) calculations illustrate that the surface oxygen vacancies in TiO play a vital role in facilitating AOR activity by reducing the energy barrier in the rate-determining ∗HNNH2 formation step and also promoting N2 desorption. These results demonstrate that defective TiO is a promising low-cost and stable catalyst for AOR, which is a potential electrode for efficient H2 production by ammonia electrolysis and direct ammonia fuel cells.

Ammonia Oxidation Activity Enhancement of Platinum with N-Doped Graphene and Tungsten Carbide Support

Ammonia electrooxidation reaction (AOR) involving multistep electron-proton transfer is a significant reaction for fuel cells, hydrogen production and understanding nitrogen-cycle. Platinum has been established as the best electrocatalyst for AOR in aqueous alkaline media. 1 In previous studies, it has been observed that nitrogen-doped graphene 2 (NDG) and tungsten carbide 3 (WC) as support could help in increasing activity for some other electrochemical reactions.In this study, Pt/NDG and Pt/WC/NDG were synthesized by a wet chemistry method 4 and their activities were compared to commercial Pt/C. Structure and morphology of the obtained electrocatalysts were examined by TEM, XPS and XRD. Moreover, Ir atoms were deposited on the surface of electrocatalyst Pt/WC/NDG via a modified self-deposition process. 5 LSV and chronoamperometry methods were applied to illustrate the activity and stability of electrocatalysts, respectively. LSVs of electrocatalysts in 0.1M KOH + 0.1M NH3 aqueous solution at a scan rate of 50 mV s-1 are elucidated in Figure 1.It was observed that Pt/NDG exhibited a specific activity of 0.472 mA cm-2 which was 44% higher than commercial Pt/C, thus establishing NDG as a more efficient support for AOR than carbon. Furthermore, WC as support also has an impact on AOR activity (30 % increase in activity in comparison to Pt/NDG) with Pt/WC/NDG having a specific activity of 0.61 mA cm-2. Further surface modification with Ir results in the best electrocatalytic activity for AOR with Pt-Ir/WC/NDG (specific activity 0.94 mA cm-2) having almost thrice the current density of commercial Pt/C. Pt-Ir/WC/NDG also demonstrated the lowest onset potential of 0.48 V vs RHE. It was also found that Pt/WC/NDG delineated the best stability among the catalysts, thus highlighting the role of WC in improving stability for AOR.This work adds insights regarding the role of NDG and WC as efficient supports along with significant impact of Ir surface modification on AOR activity of Pt. Figure 1. LSVs of electrocatalysts in 0.1M KOH + 0.1M NH3 aqueous solution at 50 mV s-1

Towards continuous ammonia electro-oxidation reaction on Pt catalysts with weakened adsorption of atomic nitrogen

One of the key challenges for ammonia-fed anion exchange membrane fuel cells is to the ammonia electro-oxidation reaction (AEOR) at anode, which has sluggish kinetics and generates atomic nitrogen (Nads) poisoning the Pt catalyst. In this study, a comparative study on Pt/Ta3N5, Pt/Ta2O5, Pt/carbon black, and Pt plate are conducted in order to clarify the promoting effect of the support materials for Pt catalysts. X-ray photoelectron spectroscopy analysis and density functional theory calculations reveal that the support materials significantly affect the electron condition of the Pt, resulting in the tuned adsorption energy of Nads on Pt surface. The electrochemical analysis demonstrates that the weakened adsorption of Nads lowers the coverage of Nads on Pt surface, resulting in the enhanced performance and stability of Pt catalysts for AEOR. In particular, Pt/Ta3N5 catalyst exhibits a current density of 5.92 mA cm−2 of AEOR at −0.34 V vs. SCE, which is higher than that of Pt/Ta2O5 (2.56 mA cm−2, −0.35 V vs. SCE) and Pt/C (4.45 mA cm−2, −0.26 V vs. SCE). The achievements in this study demonstrate the importance of controlling the type of supports for the development of an active electrocatalyst for continuous AEOR.

Detection of Intermediate in Ammonia Oxidation Reaction Using Aggregation-Induced Emission Phenomenon

Ammonia electro-oxidation reaction (AOR) is a vital reaction for solving energy and environment problems as it is the anodic reaction in direct ammonia fuel cells (DAFCs), ammonia electrolyzer and in the remediation of wastewater containing ammonia.1-2 Designing high-performance electrocatalysts for AOR requires thorough understanding of mechanism and intermediates produced. Techniques like in-situ FTIR have provided information about intermediates produced during AOR in aqueous alkaline media.3-4 However, the mechanism is still not well understood and more proof of exact intermediates are needed. Furthermore, these in-situ methods are complex with multiple procedures requiring extreme precision.N2Hx is an important intermediate in the dimerization step according to Gerischer and Mauerer mechanism. In this work, aggregation-induced emission (AIE) phenomenon was used for the first time to target and detect hydrazine (N2H4) as an intermediate during AOR using commercial Pt/C as electrocatalyst. Tetraphenylethene-CHO (TPE-CHO) molecule as AIEgen was used for this purpose. Combination of photoluminescence (PL) spectra showing turn-off fluorescence along with the electrochemistry results illustrated the presence of hydrazine as an intermediate. In Figure 1, fluorescence spectra in THF/water mixture with 90% of water fractions for different electrocatalyst loading keeping the AIE concentration same is depicted. It was observed that with increase in catalyst concentration, PL spectra intensity decreased considerably indicating towards the desired reaction of intermediate produced during reaction with the already present AIE on GC electrode. Control experiments using graphene and TPE were performed to elucidate the results further. Mass spectra of the product formed after reaction between hydrazine hydrate and TPE-CHO was obtained and compared with the one obtained during electrochemical reaction. A very simple and novel method for detection of an intermediate, hydrazine in AOR using AIE phenomenon is demonstrated opening new strategy for mechanism understanding of an electrochemical reaction. Figure 1. Fluorescence spectra in THF/water mixture with 90% of water fractions for different electrocatalyst loading keeping the AIE concentration same.

Investigation of cubic Pt alloys for ammonia oxidation reaction

As a promising fuel candidate, ammonia has been successfully used as anode feed in alkaline fuel cells. However, current technology in catalysts for ammonia electro-oxidation reaction (AOR) with respect to both cost and performance is inadequate to ensure large scale commercial application of direct ammonia fuel cells. Recent studies found that alloying Pt with different transition metals and controlling the morphology of catalysts can improve the AOR activity, and thus potentially can solve the cost issue. Herein, (100)-terminated Pt-M nanocubes (M = 3d-transition metals Fe, Co, Ni, Zn) are synthesized via wet-chemistry method and their catalytic activities toward AOR are evaluated. The addition of Fe, Co, Ni and Zn elements can enhance the AOR activity due to decrease in oxophilicity of platinum and bifunctional mechanism. Pt-Zn exhibits the maximum mass activity and specific activity with values of 0.41 A/mgPt and 1.69 mA/cm2 that are 1.6 and 1.8 times higher than Pt nanocubes, respectively. Pt-Fe, Pt-Co and Pt-Ni nanocubes also illustrate higher mass and specific activities compared to Pt nanocubes.

Surface engineering in improving activity of Pt nanocubes for ammonia electrooxidation reaction

Ammonia (NH3) electro-oxidation reaction (AOR) is an important reaction in direct NH3 fuel cells, NH3 electrolyzer, and NH3-based electrochemical sensors. However, its slow kinetics and structure-sensitive properties require specific electrocatalyst designs. In this study, Ir-decorated Pt nanocubes are developed as unique surface engineered model catalysts. For the first time, we find that a trace amount of Ir (less than 2%) could increase the AOR activity of Pt nanocubes by more than twice. Theoretical simulation results also illustrate that the surface-decorated Ir could lower the energy barrier in the rate determining *NH formation step on Pt surfaces, thus increasing activity in accordance with experimental findings. Additionally, Ir and Ni(OH)2-decorated Pt nanocubes can significantly improve the durability. Our findings demonstrate the importance of surface engineering in catalyst synthesis and pave the way of advanced electrocatalyst designs for ammonia oxidation and other catalytic reactions.

Hydrogen generation from ammonia electrolysis on bifunctional platinum nanocubes electrocatalysts

Abstract The ammonia electrolysis is a highly efficient and energy-saving method for ultra-pure hydrogen generation, which highly relies on electrocatalytic performance of electrocatalysts. In this work, high-quality platinum (Pt) nanocubes (Pt-NCs) with 4.5 nm size are achieved by facile hydrothermal synthesis. The physical morphology and structure of Pt-NCs are exhaustively characterized, revealing that Pt-NCs with special {100} facets have excellent uniformity, good dispersity and high crystallinity. Meanwhile, the electrocatalytic performance of Pt-NCs for ammonia electrolysis are carefully investigated in alkaline solutions, which display outstanding electroactivity and stability for both ammonia electrooxidation reaction (AEOR) and hydrogen evolution reaction (HER) in KOH solution. Furthermore, a symmetric Pt-NCs||Pt-NCs ammonia electrolyzer based on bifunctional Pt-NCs electrocatalyst is constructed, which only requires 0.68 V electrolysis voltage for hydrogen generation. Additionally, the symmetric Pt-NCs||Pt-NCs ammonia electrolyzer has excellent reversible switch capability for AEOR at anode and HER at cathode, showing outstanding alternating operation ability for ammonia electrolysis.

PtSnO2/C and Pt/C with preferential (100) orientation: High active electrocatalysts for ammonia electro-oxidation reaction

Pt/C and PtSnO2/C (20 wt% of metal loadings on carbon) containing cubic Pt nanoparticles with preferential Pt(100) orientation synthesized by an alcohol-reduction process using KBr as a shape directing agent and also PtSnO2/C and Pt/C without morphological control were used as electrocatalysts for ammonia electro-oxidation reaction (AmER) in alkaline media. The catalytic activity of the materials were investigated by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). X-ray diffraction (XRD) showed Pt peaks attributed to the face-centered cubic (FCC) structure as well SnO2 cassiterite. Transmission electron microscopy (TEM) and Scanning transmission electron microscopy (STEM) images shows cubic Pt nanoparticles (PtNPs) as well SnO2 highly dispersed onto carbon support and surrounding the PtNPs. CV experiments show that the peak current density for AmER on Pt/C (100) is 2.83 times higher than on Pt/C. The presence of tin onto the catalysts (PtSnO2/C and PtSnO2/C (100)) promoted a decrease of about 70 mV on the onset potential for AmER. The current density at 60 min of CA measurements on PtSnO2/C (100) was 1.51 times higher than on Pt/C (100), and on PtSnO2/C was 1.76 times of Pt/C. The EIS experiments show that the charge transfer resistance decreases as the catalytic activity of the materials for AmER increases. The high catalytic activity of the materials with platinum (100) preferentially orientation might be due to the higher tolerance for the Nads poisoning intermediates. The presence of SnO2 might prevent platinum surface from deactivation caused by intermediates formed during the AmER process.

Binary and Ternary Nanocubes As Electrocatalysts for Ammonia Electrooxidation Reaction

Ammonia electrooxidation reaction (AOR) plays a pivotal role in H2 production, direct ammonia fuel cells, electrosynthesis, electrochemical sensors as well as understanding of nitrogen cycle.1-2 AOR kinetics is slow requiring electrocatalysts among which Pt and Ir are the two best choices.2 AOR is also very sensitive to structure of electrocatalyst as (100) facets exhibit maximum activity.3-4 Among alloys, PtIr has shown the best performance but there are inconsistent reports on the amount of Ir required for enhancing the activity.5 Designing more durable electrocatalysts for AOR has also been a bottleneck. In this context, Ni could be a possible solution according to a report based on theoretical calculations.6 In this study, PtIr (several ratios), PtNi and PtIrNi cubic electrocatalysts with (100) facets were prepared by wet chemistry methods with CO as selective capping agent. Structure, morphology and composition of the obtained electrocatalysts were examined by TEM, EDX, XRD and ICP-AES. Quantitative estimation of (100) surface sites were carried out by GeO2 irreversible adsorption and analysis of H desorption peaks in KOH solution. Currents shown in Figure 1 were normalized by electrochemical surface areas obtained via the Cu UPD method. LSV and chronoamperometry methods were used to elucidate the activity and stability of electrocatalysts, respectively. It was found that only a trace amount of Ir could increase the activity of Pt cubes by more than twice. As illustrated in Figure 1, PtIr (85:1) cubes exhibited specific activity of 1.25 mA cm-2 compared to Pt cubes with 0.58 mA cm-2. Increasing the amount of Ir further decreased the activity. PtNi cubes performed better than Pt cubes with respect to stability although its specific activity of 0.51 mA cm-2 was slightly less than Pt cubes. Addition of trace amount of both Ir and Ni to Pt cubes, i.e. ternary PtIrNi cubes demonstrated the best performance in terms of stability. These PtIrNi cubes had specific activity of 0.72 mA cm-2 which was 41% more than Pt cubes. Additionally, PtIr cubes showed the least value of onset potential which was 0.429 V vs RHE as can be seen in Figure 1. This work adds new insights showing that a minute amount of Ir could improve the AOR of Pt significantly. Furthermore, Ni plays a crucial role in improving stability for AOR. Figure 1. LSVs of electrocatalysts in 0.1M KOH + 0.1M NH3 aqueous solution at 50 mV s-1 References Zhong, C., Hu ,W. B., and Cheng Y. F., Journal of Materials Chemistry A 2013, 10, 3216-3238.Siddharth, Kumar, Yattung Chan, Lu Wang, and Minhua Shao, Current Opinion in Electrochemistry 2018, Vol 9, 151-157.Liu, J., Fan, X., Liu, X., Song, Z., Deng, Y., Han, X., Hu, W. and Zhong, C., ACS applied materials & interfaces 2017, 9(22), pp.18856-18864.Zhang, C., Hwang, S.Y. and Peng, Z., Journal of Materials Chemistry A 2013, 1(45), pp.14402-14408.Li, Z.F., Wang, Y. and Botte, G.G., Electrochimica Acta 2017, 228, pp.351-360.Yang, Y., Yu, H., Cai, Y., Ferrando, R. and Cheng, D., Journal of Physics: Condensed Matter 2018, Vol 30(28). Figure 1

Pt-Zn Nanocube for Enhanced Mass and Specific Activities in Ammonia Electro-Oxidation Reaction (AOR)

Many studies have revealed that only platinum and iridium can selectively oxidize ammonia to nitrogen, as the two combines a good dehydrogenation capacity with a sufficiently low affinity for the poison Nads [1]. While platinum is expensive, one effective approach to reduce platinum loading and increase catalytic activity is alloying platinum with other metals and forming binary or ternary alloys. In one research, Jiang [2] introduced zinc into platinum and iridium electrode by electrodeposition and demonstrated an increase of ammonia electro-oxidation reaction (AOR) catalytic activity. In this study, Pt-Zn cubic nanoparticles were synthesized and loaded onto carbon black. The shape of Pt-Zn nanoparticles was mostly cubic illustrated by TEM images and the atomic ratio of Pt to Zn was around 4 to 1 according to ICP-OES. These Pt-Zn nanoparticles exhibited an improvement in AOR activity compared with cubic Pt nanoparticles and commercial Pt nanoparticles normalized to ECSA, termed specific activity, as shown in figure 1. The AOR peak current of Pt-Zn nanoparticles specific activity was 2.15*10-3 A/cm2 which is 1.1 and 17.1 times higher than cubic Pt and commercial Pt nanoparticles respectively. The possible reasons for increase of AOR activity will be discussed in detail. Reference [1] A. C. A. de Vooys, M. T. M. Koper, R. A. Van Santen and J. A. R. Van Veen, J. Electroanal. Chem., 2001, 506, 127–137. [2] J. Jiang, Electrochemistry Communications, 75 (2017) 52-55. Figure 1

Electrochemical Oxidation of Liquid Ammonia on Pt(100) Electrode: First-Principles Study

Ammonia is a good source of hydrogen and has an advantage in comparison with molecular hydrogen for fuel cell application with respect to the storage in liquid phase.1,2,3 Moreover, the raw energy density of liquid ammonia is higher than for liquid hydrogen. Ammonia fuel cells were intensively developed using aqueous solution of ammonia while the developing direct ammonia fuel cells based on liquid ammonia has less attention. Fuel cells based on liquid ammonia NH3(l) have the significant advantages in comparison with aqueous solution of NH3(aq): liquid ammonia NH3(l) is three times energy denser than the saturated aqueous solution of ammonia;alkaline solution of NH3(aq) is corrosive to metallic containers, whereas the infrastructure for liquid NH3(l) production, storage and distribution is already well developed;there is no formation of hindering poisoning NOx species. The mechanism of electrochemical oxidation of aqueous NH3(aq) is well studied,4,5,6 whereas the detailed mechanism of oxidation of liquid NH3(l) is not known. Hanada et al.7 in experimental study of NH3(l) electrolysis in support of LiNH2, NaNH2, and KNH2 electrolytes show that the products of the reaction are hydrogen gas (on the cathode) and nitrogen gas (on the anode) and the highest activity of the reaction is taking place with KNH2 electrolyte. The authors attributed the products H2 and N2 to the reduction of NH3 and oxidation of NH2 -, respectively, however, the mechanism was not substantiated. It is known that the reaction of NH3 electrooxidation is very sensitive to the surface of Pt electrode and occurs at surface sites with (100) geometry,8 whereas the surface sites with (111) and (110) geometries will be blocked by hollow nitrogens.4 In this study we present the detailed steps of the electrochemical oxidation of liquid ammonia on the Pt(100) anode surface in support of KNH2 electrolyte. We model the electrochemical reaction by solid/liquid interface using density functional theory method developed in our previous work.4 One of the possible pathways of the electrochemical oxidation of NH3(l) is going through spontaneous adsorption of NH3(l) to the surface forming NH3(top) (1), the following oxidation of NH3(top) forming NH2(top) (2), then breaking the hydrogen bond with the just formed NH3(l) molecule and transformation to NH2(bridge) (3), and the following oxidation of NH2(bridge) forming the bridging nitrogen (reactions (4)-(5)). The final product of the NH3(l) oxidation is the atomic nitrogen in bridging position (see Fig.). (1) NH3(l) → NH3(top) (2) NH3(top) + NH2 - → NH2(top) + NH3(l) + e- (3) NH2(top) → NH2(bridge) (4) NH2(bridge) + NH2 - → NH(bridge) + NH3(l) + e- (5) NH(bridge) + NH2 - → N(bridge) + NH3(l) + e- The process of the surface accumulation of the bridging nitrogens is going until the moment when the spontaneous dimerization of N(bridge) will happen. The result of the dimerization is N2(hollow) (6). The following spontaneous desorption of N2(hollow) from the surface results the molecular nitrogen dissolved in the solution (7). (6) N(bridge) + N(bridge) → N2(hollow) (7) N2(hollow) → N2(l) References R. Lan, S. Tao "Ammonia as a suitable fuel for fuel cells", Frontiers in Energy Res., 2, 35, 2014.D.J. Little, M.R. Smith, III, T.W. Hamann "Electrolysis of liquid ammonia for hydrogen generation", Energy Environ. Sci., 8, 2775, 2015.D. Cheddie "Ammonia as a hydrogen source for fuel cells: A review", InTech DOI:10.5772/47759, 2012.D. Skachkov, V.R. Chitturi, Y. Ishikawa "Combined first-principles molecular dynamics/density functional theory study of ammonia electrooxidation on Pt(100) electrode", J. Phys. Chem. C, 117, 25451, 2013.I. Katsounaros, M.C. Figueiredo, F. Calle-Vallejo, H. Li, A.A. Gewirth, N.M. Markovic, M.T.M. Koper "On the mechanism of the electrochemical conversion of ammonia to dinitrogen on Pt(100) in alkaline environment", J. Catalysis, 359, 82, 2018.K. Siddharth, Y. Chan, L. Wang, M. Shao "Ammonia electro-oxidation reaction: Recent development in mechanistic understanding and electrocatalyst design", Curr. Opinion in Electrochem., 9, 151, 2018.N. Hanada, S. Hino, T. Ichikawa, H. Suzuki, K. Takai, Y. Kojima "Hydrogen generation by electrolysis of liquid ammonia", Chem. Comm., 46, 7775, 2010.F.J. Vidal-Iglesias, J. Solla-Gullon, V. Montiel, J.M. Feliu, A. Aldaz "Ammonia selective oxidation on Pt(100) sites in an alkaline medium", J. Phys. Chem. B, 109, 12914, 2005. Figure 1

Construction of Ir-Co/C nanocomposites and their application in ammonia oxidation reaction

Ammonia electro-oxidation reaction has been paid much attention in recent years because it is widely used in various fields. However, the electro-oxidation of ammonia is a complex process, especially the reaction intermediate has a slow toxic effect on the electrode. To reduce the catalyst poisoning by the absorption of intermediate, the constituent and structure of the catalysts must be deeply researched. In this work, carbon-supported Ir-Co alloy nanocomposites were prepared by pre-deposited method. X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and high resolution transmission electron microscopy (HRTEM) were used to measure the character of the nanocomposites. As expected, the Ir-Co/C nanocomposites revealed superior electro-catalytic activity towards the oxidation of ammonia in 0.4M NaClO4 solution. The Ir-Co/C-modified GCE demonstrated high sensitivity for ammonia from a range of 0–2.5M when using amperometry technique to simulate the ammonia-detected environment. At last, real sensor analysis was served to investigate the ammonia-detected performance of the Ir-Co/C, demonstrating high sensitivity and quick response compare to Ir/C and commercial Pt-Ru/C.

Ammonia Oxidation Reaction Mechanism on Pt-Ir Alloys: A Surface Enhanced Infrared Absorption Spectroscopy Study

Energy and environment are two of the most primary concerns of this century, thus leading to holistic research across numerous disciplines.1 Ammonia electro-oxidation reaction (AOR) is extremely crucial with regards to energy and environment as it holds spotlight in several fields such as wastewater remediation, analytical chemistry and as a source of renewable energy in direct ammonia fuel cells. In addition, AOR is also useful by being the producer of hydrogen via electrolysis.2 Ammonia has a greater promise compared to other compounds like ethanol and methanol due to its carbon-free nature, well-defined production, storage and transportation network and no net increase in carbon dioxide content of the environment.3 However, inadequate performance due to slow kinetics and expensive nature of electrocatalysts have impeded the utilization of ammonia oxidation technology on a large scale. Consequently, large amount of efforts have been put over the years to understand the mechanism properly which can assist in developing highly active and stable electrocatalysts. In this study, a powerful in-situ surface enhanced infrared absorption spectroscopy (SEIRAS) technique with the attenuated total reflection (ATR) was used to examine the mechanism and intermediates over Pt-Ir nanofilm deposited on a silicon hemispherical prism. Pt-Ir was chosen as it is the most promising alloy as electrocatalyst for AOR till date. Intermediates such as NO at 1478-1488 cm-1 previously found over Pt electrode6 and NO2 at 1331 cm-1 earlier found with CeO2-modified platinum catalyst7 were also observed in this study with Pt-Ir, thus establishing their presence at higher potential range. Most importantly, a weak band at 2140 cm-1 representing azide intermediate was also observed for the 1st time using FTIR, previously only seen using in-situ Raman spectroscopy.8 This work, confirming azide intermediate presence at higher potential and affirming the NO and NO2 for Pt-Ir too, provided much clearer understanding of the mechanism and intermediates within the most renowned Gerischer and Mauerer framework, henceforth opening pathway for further efficient AOR electrocatalyst design in future. Fig 1. Time-resolved IR spectra of the Pt-Ir nanofilm acquired simultaneously with the linear sweep voltammogram in 0.2 M NH3-10mM PBS buffer solution (pH 6.96). The reference spectrum was taken at 0.1V. References Liu, J., Liu, B., Ni, Z., Deng, Y., Zhong, C., & Hu, W., Electrochimica Acta 2014, 150, 146-150.Diaz, L. A., Valenzuela-Muñiz, A., Muthuvel, M., & Botte, G. G., Electrochimica Acta 2013, 89, 413-421.Zhong, C., Hu ,W. B., and Cheng Y. F., Journal of Materials Chemistry A 2013, 10, 3216-3238.Vitse, Frederic, Cooper, M., and Botte, G. G., Journal of Power Sources 2005, 1, 18-26.Satyapal, Sunita, et al. Catalysis Today 2007, 3, 246-256.Matsui, T., Suzuki, S., Katayama, Y., Yamauchi, K., Okanishi, T., Muroyama, H., and Eguchi, K.. Langmuir 2015, 31, 11717-11723.Katayama, Y., Okanishi, T., Muroyama, H., Matsui, T., and Eguchi, K., ACS Catalysis 2016, 3, 2026-2034.Vidal-Iglesias, F. J., Solla-Gullón, J., Pérez, J. M., and Aldaz, A.. Electrochemistry Communications 2006, 8, 102-106. Figure 1

High Mass and Specific Activity for Ammonia Electro‐oxidation through Optimization of Dispersion Degree and Particle Size of Pt‐Ir Nanoparticles over N‐Doped Reductive Graphene Oxide

AbstractPlatinum and iridium co‐supported on N‐doped reductive graphene oxide (N‐rGO) electrocatalysts (Pt−Ir/N‐rGO) were synthesized and investigated for ammonia electro‐oxidation reaction (AOR) in alkaline media. Pt and Ir particles were deposited over the N‐rGO substrate through a potentiodynamic electrodeposition method. The deposition parameters, lower potential limit (EL) and scan rate, play a vital role on the dispersion degree and particle size of Pt−Ir bimetallic nanoparticles over N‐rGO. Additionally, the comparable current densities and a better poisoning inhibition are obtained compared to the commercial Pt (20 wt%)/C indicating that a synergistic effect among Pt, Ir and N‐rGO substrate is existing in favour on the electrocatalytic activity. The N‐rGO substrate with more active sites, higher excellent electrochemical activity and well dispersion for Pt and Ir particles anchoring is a suitable substrate for AOR and would be extended to other catalytic reactions.