Catalysts For Oxidation Of Ammonia Research Articles

A promising catalyst for catalytic oxidation of chlorobenzene and slipped ammonia in SCR exhaust gas: Investigating the simultaneous removal mechanism

As the emission limits for NOx become further stringent, development of a synergistic control technology to prevent emissions of slipped ammonia (NH3) and chlorinated organics from steel production and waste incineration is both a prospect and a challenge for the environmental issues. In this work, the designed ruthenium-based catalyst (RuNb/ST) showed the excellent chlorobenzene (CB)/NH3 synergistic removal performance and N2/CO2 selectivity above 300 ℃. The activity of NH3 and CB were suppressed at relatively low temperature due to the competitive adsorption of CB and NH3. Whereas, there was a significant improvement in the N2/CO2 selectivity at relatively high temperature. The removal mechanism was studied by experiments, in which the accumulation of chlorine species, the role of NO2 and the oxidation of CB were considerable. In contrast to the carrier, the vast majority of the chloride ions adsorbed onto the active sites after dissociating from CB could be efficiently removed in form of Cl2 via the Deacon Reaction catalyzed by RuO2. CB was predominantly converted into CO2 and H2O with a fraction persisting as organics. Partial of organics were attacked by Cl ions to generate organic chlorine-containing by-products. The presence of NO2 molecule was found to promote the cleavage of the benzene ring within CB, and expedited the carbon migration towards the ultimate product of CO2, resulting in a notable reduction of byproducts.

Tuning surficial atomic configuration of Pt–Ir catalysts for efficient ammonia oxidation and low-temperature direct ammonia fuel cells

Low-temperature direct ammonia fuel cell via ammonia oxidation reaction (AOR) is one of the most attractive ways for ammonia utilization. The Pt–Ir alloyed catalysts have been proven greatly efficient in AOR to overcome the sluggish kinetics and complex multi-electron processes; however, it still remains challenging for further improvement because of the complexity of alloyed types and unclear fundamental understanding. Herein, we systematically fabricated a series of PtxIry/XC-72 catalysts via tuning the Pt/Ir compositions and these catalysts demonstrate significant composition-dependent behaviours in AOR, whereas the Pt favours increasing current densities and the Ir facilitates reducing onset potentials. Importantly, the actual surface Pt/Ir atomic configuration is totally different with the classic view for homogeneous alloys by combining catalytic and characteristic analyses. The Pt prefers to be segregated on the topmost surface. Then the quantified correlations between current density/onset potential and Pt composition in Pt − Ir nanoparticles (NPs) were further established and discussed. The onset potential of AOR is significantly reduced by Ir incorporation while it is insensitive to Ir contents. Accordingly, the well-studied Pt5Ir5/XC-72 catalyst was further employed in an alkaline exchange membrane fuel cell, exhibiting a peak power density (230.0 mW cm−2). Our work provides insights into the effect of Pt/Ir compositions on AOR, with the goal of catalysts engineering for AOR and ammonia fuel cells.

Designing a Bifunctional Pt/Cu-SSZ-13 Catalyst for Ammonia-Selective Catalytic Oxidation with Superior Selectivity

Ammonia-selective catalytic oxidation (NH3-SCO) is an effective technology to solve the problem of ammonia leakage from diesel vehicles, while ammonia oxidation catalysts (AOC) still suffer from insufficient low-temperature activity and poor nitrogen selectivity. Herein, we prepared a bifunctional Pt/Cu-SSZ-13 catalyst containing an SCO component (Pt) and a selective catalytic reduction (SCR) component (Cu). This bifunctional catalyst exhibited excellent catalytic performance during NH3 oxidation under practical conditions containing water vapor, achieving >85% N2 selectivity over the whole temperature range. Catalysts were systematically characterized by Brunauer–Emmett–Teller, X-ray diffraction, inductively coupled plasma optical emission spectrometry, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, NH3 temperature-programmed desorption, high-angle annular dark-field scanning transmission electron microscopy, and X-ray absorption fine structure analysis. The results showed that metallic Pt nanoparticles were mainly present on the exterior surface of the Pt/Cu-SSZ-13 zeolite catalyst, while dispersed Cu2+ cations were present in the interior of the zeolite. Based on operando diffuse reflectance infrared Fourier transform spectroscopy with mass spectrometry experiments, it was confirmed that on Pt/SSZ-13, over-oxidation of NH3 produced large amounts of N2O and NOx at low and high temperatures, respectively, resulting in a poor N2 selectivity. With Cu present, in contrast, the byproduct (NO) would further react with adsorbed NH3 on Cu Lewis acid sites to produce N2 by a tandem SCR reaction, thus contributing to the superior N2 selectivity of the Pt/Cu-SSZ-13 bifunctional catalyst.

High throughput screening of noble Metal-Free High-Entropy alloys catalysts for selective catalytic oxidation of NH3

The high price and scarcity of Pt require the development of alternative low-cost catalysts for selective catalytic oxidation of ammonia (NH3-SCO). This work uses first-principles-based high-throughput calculations to evaluate noble metal-free high-entropy alloys (NMF-HEAs) as potential NH3-SCO candidate catalysts to replace Pt. Firstly, by employing empirical rules to determine the formation probability of single-phase solid solution, 350 NMF-HEAs are found to be single-phase solid solution among the vast configuration space (about 104) consisting of 20 non-noble metals. Among these, NMF-HEAs containing Cu and Zn elements are identified as promising catalysts based on the reactivity and selectivity rules of the NH3-SCO process. Further analysis of elemental composition and distribution on configurations with N-atom adsorption energies reveals that the co-existence of Cu and Zn elements and the symmetric distribution of surface atoms is conducive to tuning N-binding ability in the optimal region. Finally, full reaction paths of NH3-SCO processes on the optimal CuZnNiMoCo HEA are calculated and confirm their activity comparable to that of Pt catalysts and better selectivity. This study gains insight into the effects of the composition and surface configuration of HEAs on the NH3-SCO process and provides a theoretical framework for designing and optimizing efficient catalysts of the NH3-SCO process.

Towards FIB-SEM Based Simulation of Pore-Scale Diffusion in SCR Catalyst Layers

The diffusivity in the upper Cu-Chabazite layer of a dual layer ammonia oxidation catalyst with a lower Pt layer was investigated. In a first step, the pore structure of the upper Cu-Chabazite catalyst layer was determined by Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) slice&view tomography. From the FIB-SEM data the 3D pore structure of the catalyst was reconstructed and diffusion simulations were performed on the reconstructed pore geometry, resulting in an estimated effective diffusivity of Deff/Dgas = 0.31. To validate the FIB-SEM derived estimates of the diffusivity, measurements of CO oxidation on the dual layer catalyst were performed, where the CO was oxidized in the lower Pt-layer while the upper SCR layer served as an inactive diffusion barrier. In this way, the effective diffusivity can be determined from the measured CO conversion. An effective diffusion coefficient of Deff/Dgas = 0.11 was obtained from the CO oxidation measurements, three times lower than the value obtained from the FIB-SEM data, but in line with previous literature data for the effective diffusivity in monolith washcoat layers. Additional NH3 oxidation experiments were performed on the dual layer catalyst. The results were well reproduced by a reactor model applying the effective diffusion coefficient obtained by the CO oxidation experiments. The origin of this apparent inconsistency is currently not understood and requires further investigation.

Tailored Bimetallic Ni-Sn Catalyst for Electrochemical Ammonia Oxidation to Dinitrogen with High Selectivity.

Direct electrochemical ammonia oxidation reaction (eAOR) is an efficient and sustainable strategy to process wastewater containing ammonia, and it endures overoxidation and severely competitive oxygen evolution reaction (OER). Herein, we synthesized a Ni(OH)2/SnO2 composite catalyst by a multistep strategy and applied it to the eAOR process. Ni(OH)2/SnO2 exhibited a N2-N Faradaic efficiency (FEN2-N) of 84.2%, with a N2 partial current density (jN2-N) of 2.7 mA cm-2 at 1.55 V vs reversible hydrogen electrode (RHE) in 0.5 M K2SO4 with 10 mM NH3-N (pH 11). The oxophilic Sn promoted NH3 absorption on Ni sites while suppressing the OER. As the active species, NiOOH accelerated the dimerization of intermediates (*NH2 or *NH) to form N2.

Electrocatalytic Ammonia Oxidation by a Low-Coordinate Copper Complex.

Molecular catalysts for ammonia oxidation to dinitrogen represent enabling components to utilize ammonia as a fuel and/or source of hydrogen. Ammonia oxidation requires not only the breaking of multiple strong N-H bonds but also controlled N-N bond formation. We report a novel β-diketiminato copper complex [iPr2NNF6]CuI-NH3 ([CuI]-NH3 (2)) as a robust electrocatalyst for NH3 oxidation in acetonitrile under homogeneous conditions. Complex 2 operates at a moderate overpotential (η = 700 mV) with a TOFmax = 940 h-1 as determined from CV data in 1.3 M NH3-MeCN solvent. Prolonged (>5 h) controlled potential electrolysis (CPE) reveals the stability and robustness of the catalyst under electrocatalytic conditions. Detailed mechanistic investigations indicate that electrochemical oxidation of [CuI]-NH3 forms {[CuII]-NH3}+ (4), which undergoes deprotonation by excess NH3 to form reactive copper(II)-amide ([CuII]-NH2, 6) unstable toward N-N bond formation to give the dinuclear hydrazine complex [CuI]2(μ-N2H4). Electrochemical studies reveal that the diammine complex [CuI](NH3)2 (7) forms at high ammonia concentration as part of the {[CuII](NH3)2}+/[CuI](NH3)2 redox couple that is electrocatalytically inactive. DFT analysis reveals a much higher thermodynamic barrier for deprotonation of the four-coordinate {[CuII](NH3)2}+ (8) by NH3 to give the copper(II) amide [CuII](NH2)(NH3) (9) (ΔG = 31.7 kcal/mol) as compared to deprotonation of the three-coordinate {[CuII]-NH3}+ by NH3 to provide the reactive three-coordinate parent amide [CuII]-NH2 (ΔG = 18.1 kcal/mol) susceptible to N-N coupling to form [CuI]2(μ-N2H4) (ΔG = -11.8 kcal/mol).

Experimental approach for reducing nitrogen oxides emissions from ammonia–natural gas dual-fuel spark-ignition engine

In this study, characteristics of nitrogen oxides (NOx) emissions from an ammonia–natural gas dual-fuel operation were investigated by conducting spark-ignition engine experiments under the maximum load and a low engine speed (∼1,100 rpm). The feasibility of using a conventional selective catalytic reduction (SCR) system to reduce the NOx emissions was also investigated. For the experiments, an 11-L six-cylinder spark-ignition engine was used with appropriate modifications to use natural gas and ammonia together as the fuel. Several species comprising NOx emissions (NO, NO2, and N2O) and ammonia were measured together using a Fourier transform infrared analyzer (FT-IR). An aftertreatment system, including an SCR system and an ammonia oxidation catalyst (AOC), for an original natural gas-fueled engine was used to reduce engine-out NOx using the unburned ammonia as a reducing agent.From the experimental results, it was found that the NO concentration first increased on the addition of ammonia, as the fuel NOx emissions were produced. However, the total NO concentration became saturated with increasing ammonia fraction, owing to the increased amount of unburned ammonia. Specifically, considering the trend of the burned gas temperature and its value at exhaust valve opening (EVO) timing, it is inferred that nitrogen oxide was reduced owing to unburned ammonia after the exhaust valve opening timing, where the temperature range of combustion product was suitable to trigger selective non-catalytic reduction (SNCR). The remaining ammonia, after the SNCR, was then virtually oxidized to reduce the remaining NO while passing SCR system, showing an almost zero level (<10 ppm) at the catalyst-out level. N2O and NO2 were produced in the order of tens and hundreds of ppm, respectively. Thus, despite the high level of global warming potential (GWP) of N2O, its contribution to global warming effect was not significant. On the other hand, the unburned methane was particularly increased at some point with poor combustion efficiency to offset the effect of decrement in carbon dioxide due to its high level of GWP.

Catalysts for electrochemical ammonia oxidation: Trend, challenge, and promise

Developing sustainable and clean energy technology is critial to address the current energy and environmental crisis. As an energy carrier, hydrogen has been getting attention due to its abundance, high energy density, and zero carbon emission. Due to the difficulty of hydrogen storage and transport, the electro-oxidation of ammonia shows great potential for renewable energy since it satisfies both energy supply and environmental protection as a carbon-free fuel with high hydrogen content. However, ammonia electro-oxidation is a sluggish reaction and shows low durability. Understanding the complicated mechanism and intermediates is conducive to developing efficient electrocatalysts with high catalytic activity and durability. This review begins with the ammonia oxidation technologies and their principles. The two typical mechanisms for ammonia electro-oxidation, Oswin-Salomon mechanism and Gerischer-Mauerer mechanism, are discussed, and attention to the poisoning species is included. Then, the electrocatalysts for ammonia reaction are classified into Pt-based catalysts and non-noble metal-based catalysts, and their performances are reviewed with various strategies. Based on the progress on the electrochemical ammonia oxidation catalysts, the challenges and future insight on ammonia oxidation reaction are discussed and proposed.

Weakening the N-H Bonds of NH3 Ligands: Triple Hydrogen-Atom Abstraction to Form a Chromium(V) Nitride.

Weakening and cleaving N-H bonds is crucial for improving molecular ammonia (NH3) oxidation catalysts. We report the synthesis and H-atom-abstraction reaction of bis(ammonia)chromium porphyrin complexes Cr(TPP)(NH3)2 and Cr(TMP)(NH3)2 (TPP = 5,10,15,20-tetraphenyl-meso-porphyrin and TMP = 5,10,15,20-tetramesityl-meso-porphyrin) using bulky aryloxyl radicals. The triple H-atom-abstraction reaction results in the formation of CrV(por)(≡N), with the nitride derived from NH3, as indicated by UV-vis and IR and single-crystal structural determination of Cr(TPP)(≡N). Subsequent oxidation of this chromium(V) nitrido complex results in the formation of CrIII(por), with scission of the Cr≡N bond. Computational analysis illustrates the progression from CrII to CrV and evaluates the energetics of abstracting H atoms from CrII-NH3 to generate CrV≡N. The formation and isolation of CrV(por)(≡N) illustrates the stability of these species and the need to chemically activate the nitride ligand for atom transfer or N-N coupling reactivity.

Ptirm (M=Zn, Ni) Ammonia Oxidation Catalysts for Anion-Exchange Membrane Direct Ammonia Fuel Cells

Low-temperature direct ammonia fuel cells (DAFCs) use carbon-neutral ammonia as a fuel, which has attracted increasing attention recently due to ammonia's low source-to-tank energy cost, easy transport and storage, and wide availability. However, current DAFC technologies are greatly limited by the kinetically sluggish ammonia oxidation reaction (AOR) at the anode. Herein, we report an AOR catalyst, in which ternary PtIrZn nanoparticles with an average size of 2.3 ± 0.2 nm were highly dispersed on a binary composite support comprising cerium oxide (CeO2) and zeolitic imidazolate framework-8 (ZIF-8)-derived carbon (PtIrZn/CeO2-ZIF-8) through a sonochemical-assisted synthesis method. The PtIrZn alloy, with the aid of abundant OHad provided by CeO2 and uniform particle dispersibility contributed by porous ZIF-8 carbon (surface area: ∼600 m2 g−1), has shown highly efficient catalytic activity for the AOR in alkaline media, superior to that of commercial PtIr/C. The rotating disk electrode (RDE) results indicate a lower onset potential (0.35 vs. 0.43 V), relative to the reversible hydrogen electrode at room temperature, and a decreased activation energy (∼36.7 vs. 50.8 kJ mol−1) relative to the PtIr/C catalyst. Notably, the PtIrZn/CeO2-ZIF-8 catalyst was assembled with a high-performance hydroxide anion-exchange membrane to fabricate an alkaline DAFC, reaching a peak power density of 91 mW cm−2. Unlike in aqueous electrolytes, supports play a critical role in improving uniform ionomer distribution and mass transport in the anode. PtIrZn nanoparticles on silicon dioxide (SiO2) integrated with carboxyl-functionalized carbon nanotubes (CNT–COOH) were further studied as the anode in a DAFC. A significantly enhanced peak power density of 314 mW cm−2 was achieved.

Design composite oxide supported Pt catalyst for catalytic wet air oxidation of ammonia with a high concentration in chloride system

Massive discharge of ammonia nitrogen wastewater not only causes eutrophication of the water body but also has a toxic effect on humans and living things. How to deal with ammonia nitrogen wastewater is a crucial topic for researchers. Here, a novel catalyst of Pt@Ti–Si where platinum was supported on a composite oxide of titanium oxide (TiO2) and silicon oxide (SiO2) via a one-pot method was successfully synthesized for catalytic wet air oxidation (CWAO) of ammonia with a high concentration (more than 2000 ppm). Due to the improved specific surface area of SiO2 and the excellent acid-base resistance of TiO2, the prepared composite oxide-supported platinum catalyst has excellent catalytic performance and good stability for CWAO with a high concentration of ammonia. At 200 °C and the O2 pressure of 2 MPa for 2 h, the 1%Pt@Ti10–Si1 catalyst has a 96.32% conversion of 2470 ppm ammonia and 97.15% selectivity to N2 and has good catalytic performance even after five cycles. Under the same reaction conditions, when the chloride concentration in the system is 3000–10000 ppm, the CWAO reaction can be inhibited at an early stage and promote conversion and selectivity at a later stage. The results show that the catalyst has good tolerance to chloride ions, and the treated ammonia nitrogen wastewater can be used for subsequent biochemical processes. Therefore, the developed novel catalyst in this study is effective for the CWAO of highly concentrated ammonia and has potential industrial application value.

Effects of IrO2 nanoparticle sizes on Ir/Al2O3 catalysts for the selective catalytic oxidation of ammonia

Gaseous ammonia (NH3), especially that emitted from selective catalytic reduction with NH3 (NH3–SCR) units in the atmosphere is potentially harmful to both human health and the environment. The noble metal catalytic oxidation technology placed behind the SCR unit has been considered to be a promising method for decreasing NH3 slip. In this study, we prepared Ir/Al2O3 catalysts using two different crystal structures of Al2O3 (α–Al2O3 or γ–Al2O3) as supported by an impregnation method and evaluated the performance of the selective catalytic oxidation of NH3 (NH3–SCO). We found that the Ir/γ–Al2O3 catalyst performed better than the Ir/α–Al2O3 catalysts, with 99% of NH3 converted at 260 °C. The X–ray absorption fine structure revealed that the active species on the Ir/Al2O3 catalysts are mainly oxidized IrO2. High–angle annular dark–field images of aberration–corrected scanning transmission electron microscopy revealed that IrO2 species over 1 nm in diameter are beneficial for activity. In situ diffuse reflectance for infrared Fourier transform spectroscopy studies suggested that the differences in NH3–SCO activity were caused by the activity of reactive oxygen species, and the formation of –NH2 may be the decisive step of the NH3 oxidation reaction.

Promotional catalytic activity and reaction mechanism of Ag-modified Ce0.6Zr0.4O2 catalyst for catalytic oxidation of ammonia

Ce1-xZrxO2 composite oxides (molar, x = 0-1.0, interval of 0.2) were prepared by a cetyltrimethylammonium bromide-assisted precipitation method. The enhancement of silver-species modification and catalytic mechanism of adsorption-transformation-desorption process were investigated over the Ag-impregnated catalysts for low-temperature selective catalytic oxidation of ammonia (NH3-SCO). The optimal 5 wt.% Ag/Ce0.6Zr0.4O2 catalyst presented good NH3-SCO performance with >90% NH3 conversion at temperature (T) ≥ 250°C and 89% N2 selectivity. Despite the irregular block shape and underdeveloped specific surface area (∼60 m2/g), the naked and Ag-modified Ce0.6Zr0.4O2 solid solution still obtained highly dispersed distribution of surface elements analyzed by scanning electron microscope-energy dispersive spectrometer (SEM-EDS) (mapping), N2 adsorption-desorption test and X-ray diffraction (XRD). H2 temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) results indicated that Ag-modification enhanced the mobility and activation of oxygen-species leading to a promotion on CeO2 reducibility and synergistic Ag0/Ag+ and Ce4+/Ce3+ redox cycles. Besides, Ag+/Ag2O clusters could facilitate the formation of surface oxygen vacancies that was beneficial to the adsorption and activation of ammonia. NH3-temperature programmed desorption (NH3-TPD) showed more adsorption-desorption capacity to ammonia were provided by physical, weak- and medium-strong acid sites. Diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments revealed the activation of ammonia might be the control step of NH3-SCO procedure, during which NH3 dehydrogenation derived from NHx-species and also internal selective catalytic reduction (i-SCR) reactions were proposed.

Selective catalytic oxidation of ammonia to nitric oxide via chemical looping

Selective oxidation of ammonia to nitric oxide over platinum-group metal alloy gauzes is the crucial step for nitric acid production, a century-old yet greenhouse gas and capital intensive process. Therefore, developing alternative ammonia oxidation technologies with low environmental impacts and reduced catalyst cost are of significant importance. Herein, we propose and demonstrate a chemical looping ammonia oxidation catalyst and process to replace the costly noble metal catalysts and to reduce greenhouse gas emission. The proposed process exhibit near complete NH3 conversion and exceptional NO selectivity with negligible N2O production, using nonprecious V2O5 redox catalyst at 650 oC. Operando spectroscopy techniques and density functional theory calculations point towards a modified, temporally separated Mars-van Krevelen mechanism featuring a reversible V5+/V4+ redox cycle. The V = O sites are suggested to be the catalytically active center leading to the formation of the oxidation products. Meanwhile, both V = O and doubly coordinated oxygen participate in the hydrogen transfer process. The outstanding performance originates from the low activation energies for the successive hydrogen abstraction, facile NO formation as well as the easy regeneration of V = O species. Our results highlight a transformational process in extending the chemical looping strategy to producing base chemicals in a sustainable and cost-effective manner.

Rational ensemble design of alloy catalysts for selective ammonia oxidation based on machine learning

High-throughput computation and machine learning studies are conducted for the rational design of ensembles of alloy catalysts for selective catalytic oxidation of NH3 (NH3-SCO).

Theoretical design principles of metal catalysts for selective ammonia oxidation from high throughput computation

High throughput calculation is performed to reveal the ruling laws of catalytic performance and further screen potential alloy catalysts with high activity and selectivity for selective catalytic oxidation of ammonia (NH3-SCO).

Role of the Cu content and Ce activating effect on catalytic performance of Cu-Mg-Al and Ce/Cu-Mg-Al oxides in ammonia selective catalytic oxidation

Copper-containing mixed metal oxides were studied as catalysts for selective catalytic oxidation of ammonia. The performed research shows a high correlation between Cu amount and catalyst efficiency, especially dinitrogen selectivity. Copper species, mainly in the form of highly dispersed CuO oxides, were found to be responsible for high N2 selectivity, which decreased with increasing Cu loading and the formation of CuO bulk-like species. The addition of cerium seemed to have an effect on material’s catalytic activity and N2 selectivity. The increase in selectivity towards N2 of Ce-impregnated materials was attributed to the formation of copper-cerium redox couples on the material’s surface. The catalytic tests performed under different NH3/O2 molar ratios revealed that the developed materials exhibit high catalytic activity in a wide range of ammonia concentrations and oxygen contents.

(Invited) Advanced Catalysts for Efficient Electrochemical Ammonia Oxidation

Ammonia (NH3) has proved to be an effective alternative to hydrogen in low-temperature fuel cells via its direct ammonia oxidation reaction (AOR)(1). However, the kinetically sluggish AOR has prohibitively hindered the attractive direct ammonia fuel cell (DAFC) applications. Here, we report an efficient AOR catalyst, in which ternary PtIrM (M: Ni or Zn) alloy nanoparticles well dispersed on a binary composite support consisting of porous silicon dioxide (SiO2) and carboxyl-functionalized carbon nanotube (PtIrNi/SiO2-CNT-COOH) through a sonochemical-assisted synthesis method (2). The PtIrNi alloy nanoparticles, with the aid of abundant OHad provided by porous SiO2 and the improved electrical conductivity by CNTs, exhibit remarkable catalytic activity for the AOR in alkaline media. It is evidenced by a lower onset potential (∼0.40 V vs reversible hydrogen electrode (RHE)) at room temperature than that of commercial PtIr/C (ca. 0.43 V vs RHE). Increasing NH3 concentrations and operation temperatures can significantly enhance AOR activity of this PtIrNi nanoparticle catalyst. Specifically, the catalyst at the temperature of 80 °C exhibits a much lower onset potential (∼0.32 V vs RHE) and a higher peak current density, indicating that DAFCs operated at a higher temperature are favorable for increased performance.References(1) Adli, N. M.; Zhang, H.; Mukherjee, S.; Wu, G., Review—Ammonia Oxidation Electrocatalysis for Hydrogen Generation and Fuel Cells. Journal of The Electrochemical Society 2018, 165 (15), J3130-J3147.(2) Li, Y.; Li, X.; Pillai, H. S.; Lattimer, J.; Mohd Adli, N.; Karakalos, S.; Chen, M.; Guo, L.; Xu, H.; Yang, J.; Su, D.; Xin, H.; Wu, G., Ternary PtIrNi Catalysts for Efficient Electrochemical Ammonia Oxidation. ACS Catalysis 2020, 10 (7), 3945-3957.

(Invited) Intermediate Temperature Direct Ammonia Fuel Cell Using Alkaline Electrolyte System

Ammonia is a very attractive carbon-neutral fuel due to its low cost, high energy density, and existing transportation infrastructure. However, the viable commercial application of direct ammonia fuel cells (DAFC) has not been realized, largely due to materials challenges that lead to poor cell performance. Generally, DAFCs either use low temperature polymer electrolytes, which are limited by low reaction rates and severe ammonia crossover, or use high temperature solid oxide electrolytes, which suffer from severe material thermal degradation. We have developed an intermediate temperature alkaline electrolyte-based DAFC that operates at 400-600 °C, enabling high reaction rates while limiting thermal degradation. This DAFC is based on our innovative molten hydroxide electrolyte impregnated in a porous ceramic matrix, which has conductivity as high as 0.4 S/cm at 350 °C, enabling rapid hydroxide transport. The DAFCs uses state-of-the-art ammonia oxidation catalysts and ammonia-tolerant oxygen reduction reaction catalysts. Our DAFC has demonstrated an OCV of 1.2 V and can achieve 300 mA/cm2 at 0.86 V at 550 ⁰C, and 0.98 V at 600 ⁰C, or >450 mW/cm2 power from pure humidified ammonia. It has low ammonia crossover, thus enabling the high OCV and high power density without catalyst poisoning. Further work on catalyst development and cell design is underway to achieve higher power densities and reduce PGM catalyst loading and operation temperature, making this a more commercially viable technology.