Electrochemical Ammonia Synthesis Research Articles

Roles of Interface and Surface of Electrode Catalysts in Ammonia Electrochemical Synthesis with Proton-Conducting Ceramic Fuel Cells

Investigation of energy carriers is of critical importance for chemical energy storage and transport technologies in combination with the large-scale utilization of renewable energy. Ammonia is a promising energy carrier in terms of carbon-free fuel, high energy density, and easy liquefaction (1).Electrochemical approaches can provide a more efficient reaction process and allow more flexible operations for ammonia production, compared with the industrial Haber–Bosch process. In this study, we investigated direct electrochemical synthesis of ammonia using proton-conducting ceramic fuel cells (PCFCs) operating at high temperatures around 773 K. A ceramic proton conductor, BaCe1-xYxO3 (BCY), was used for an electrolyte membrane.In our previous studies, metal catalysts of Ru and Fe were investigated with a variety of electrode structures such as metal nanoparticle (2, 3) and cermet electrode (4). In terms of metal nanoparticle catalyst, Ru-doped BCY (BCYR) was examined (2,3). The ammonia formation rate was not so high with BCYR (ca. 10-11 mol/s cm2). To improve the ammonia formation rate, Ru-BCY and Fe-BCY (or K-Al-Fe-BCY) cermet electrodes were investigated. When pure N2 was supplied to the cathode, low ammonia formation rate was observed (ca. 10-11 mol/s cm2). When a N2/H2 gaseous mixture was supplied to the cathode, higher ammonia formation rate was observed; ammonia formation rate with Fe-BCY electrode was increased up to ca. 10-10 mol/s cm2 with an increase in cathodic polarization. In addition, our recent study showed a significant improvement of ammonia formation rate with pure Fe electrode (ca. 10-8 mol/s cm2) (5). Considering the above, the roles of interface and surface of electrode catalysts on ammonia electrochemical synthesis should be investigated for clarifying the reaction mechanism and developing effective cell design. In this study, we conducted the deuterium isotope analysis for ammonia electrochemical synthesis to investigate the contributions of interface between electrode and electrolyte, i.e., triple-phase boundary, and electrode surface for ammonia formation processes as well as relevant kinetic measurements of ammonia formation on Fe catalyst (Fig. 1a).The cell configuration was Pt|BCY|Fe-BCY for a cermet electrode. Also, Pt|BCY|Fe cell with pure iron electrode was fabricated to investigate the ammonia formation mechanism. Ammonia formation rate was measured with the configuration of Pt, H2 H2O-Ar |BCY|Fe, H2-N2. Ammonia formed in the cathode was trapped in an H2SO4 solution. Then, the obtained solution was analyzed by ion chromatography. The ammonia formation was also observed using an FTIR spectrometer with a multiple refraction cell to perform the deuterium isotope analysis.Significantly high ammonia formation rate was observed using the Pt|BCY|Fe cell: 1~6x10-8 mol/s cm2 at -1.5 V and 623 K with changing a flow rate in the cathode, which is the best performance for ammonia electrochemical formation under the H2 flow condition. This suggests that Fe surface may play a dominant role in ammonia formation rather than the triple-phase boundary between Fe and BCY. Deuterium isotope analysis was carried out with Pt, D2-H2O-Ar|BCY|Fe, H2-N2. Observed FTIR spectra showed the formation of NH3 and NH2D under the condition (Fig. 1b). The absorbance of NH2D was very weak, which suggested that most of hydrogen atoms in the ammonia molecule originated from H2 in the cathode. In the case of Pt, H2-H2O-Ar|BCY|Fe, D2-N2, ND3 was observed as a main product under the D2 flow condition in the cathode. Considering the above, the electrochemical promotion of catalyst (EPOC) for ammonia formation will be induced via promotion of N2 dissociation on Fe. N2 dissociation is the rate determining step in the ammonia formation reaction. Possible mechanism for the promotion of N2 dissociation is electron back donation with an increase in cathodic polarization. This mechanism gives a hint for designing an electrochemical reactor to promote efficient ammonia formation. Acknowledgements This work was supported by CREST, Japan Science and Technology Agency (JPMJCR1441). References D. Miura, T. Tezuka, Energy, 68(15), 428-436 (2014).J. Otomo, N. Noda, F. Kosaka ECS Trans., 68, 2663-2670 (2015).F. Kosaka, T. Nakamura, J. Otomo, J. Electrochem. Soc., 164, F1323-F1330 (2017).F. Kosaka, T. Nakamura, A. Oikawa, J. Otomo. ACS Sustainable Chem. Eng., 5(11), 10439–10446 (2017).J. Otomo, et al., 435c, 2019 AIChE Annual Meeting, Orlando, USA. Figure 1

Electrochemical Devices with Ru Catalysts, Hydrogen-Permeable Membranes, and Phosphate Electrolytes for Ammonia Synthesis from Nitrogen and Water

Ammonia is an indispensable bulk chemical for artificial fertilizers and various industries, and it is now expected to be utilized as a chemical energy carrier. When ammonia is synthesized from nitrogen and water which are abundant in the atmosphere with electricity from renewable energies, ammonia is the most ideal chemical fuel utilizable to fuel cells, combustion engines, hydrogen converters, and so on. Conventional industrial ammonia synthesis, so called Haber–Bosch process, currently consumes 1 or 2% of human consumption energy which is mostly from fossil resources. The point is that the most part of consumed energy is for hydrogen production for Haber–Bosch process, since Haber–Bosch synthesis of ammonia is exothermic process releasing heat from high energetic hydrogen to form relatively lower energetic ammonia. Therefore, it is hard to reduce the energy consumption when ammonia is produced from hydrogen. On the other hand, because ammonia is essentially lower energetic chemicals than hydrogen, the energy consumption for ammonia synthesis can be reduced if ammonia is synthesized from nitrogen and water with minimum electricity. Furthermore, a small single electrochemical device has significant advantages over a combination of water electrolysis and Haber–Bosch process; the absence of operation and storage of hydrogen gas, easy start and stop of single devise, simple heating and cooling equipment, distributed small scale devises like fuel cells.The present work indicates that a combination of Ru ammonia catalysts, Pd hydrogen-permeable membranes, phosphate electrolytes enables ammonia synthesized from nitrogen and water at ~250ºC and ~1.0 MPa. The combination of endothermic water electrolysis and exothermic ammonia synthesis in an electrochemical cell leads thermally neutral process without heat loss. In the temperature range of 200~250ºC, the equilibrium of ammonia synthesis reaction shifts to ammonia formation with moderate pressurizing to ~1 MPa. Ammonia is intrinsically decomposed to nitrogen and hydrogen at higher temperature according to the equilibrium. The temperature of 200~250ºC is challenging for Ru ammonia catalysts, because they are typically used at 350~500ºC for typical ammonia synthesis. The presence of Pd hydrogen-permeable membranes blocks absorption of produced ammonia into proton-conductive electrolytes and penetration of water from the anode side into produced ammonia. The maximum ammonia formation rate was 11.1 nmol s-1 cm-2 at 20 mA cm-2, 250ºC, 1.0 MPa, 10 cm3 min-1 of N2 flow and vapor of 1 micro-L liquid H2O with 10 cm3 min-1 of Ar flow. At that time, current efficiency for ammonia formation was 16% and hydrogen was produced with the remaining 79% of current. According to the thermodynamic equilibrium of ammonia synthesis, 36% conversion of hydrogen to ammonia is the equilibrium limit at 250ºC and 1.0 MPa, so that attainment of equilibrium was 44%. As Ru ammonia catalysts, Ru/Cs+/MgO, Ru/Cs+/CeO2, and Ru/CeO2 were examined. Ru/Cs+/MgO showed the highest ammonia synthesis rate under elevated pressure conditions. The point of this cell for higher ammonia formation rates is the operation of H2/N2 ratio at 0.07 which is much lower than 3 of stoichiometry. Ammonia synthesis with Ru catalysts at the low temperature regime was strongly suppressed by the presence of hydrogen in spite of one of the reactants, so that the excess nitrogen need to be fed to the reactor. Suppression of nitrogen activation by the presence of hydrogen is found to be the key factor for further developments.For understanding the properties of adsorption of nitrogen and hydrogen, infrared spectroscopy and temperature programmed desorption measurements have been performed. The presence of Cs+ promoter has been known that it makes electron rich at the nitrogen adsorption sites of Ru. However, the present work found that the presence of Cs+ promoter affected the hydrogen adsorption to be weakened.The further strategy for the development of present electrochemical ammonia synthesis cell will be discussed in the presentation. Figure 1

Opportunities for Intermediate Temperature Renewable Ammonia Electrosynthesis

Production of ammonia using only renewable energy is achievable through various routes; however, direct electrochemical conversion technologies have achieved significant attention. Despite this attention, the promise of electrochemical routes for ammonia synthesis largely rests on the technologies' ability to achieve performance limits which approach that of the Haber-Bosch process (state of the art). The thermodynamics and kinetics of electrochemical ammonia synthesis is largely dependent on operating conditions (temperature, pressure and voltage) and material (catalyst and electrolyte) properties. There is a growing interest in defining realistic performance targets which would make renewable ammonia derived from electrochemical systems a reality. However, most efforts thus far have only explored optimizing single technology specific performance metrics such as faradaic efficiency. Unfortunately, optimization of this single performance metric often occurs at the expense of the rate of production. Here, we aim to outline the performance targets achievable for renewable ammonia produced through intermediate temperature electrosynthesis. Through exploring the thermodynamic and kinetic challenges, we will highlight the optimum expected rate of production and energy efficiency for intermediate routes of electrosynthesis. We also review current experimental reports focused on intermediate routes of ammonia electrosynthesis and detail materials related opportunities in catalyst and solid-electrolyte design. Finally, we discuss some of the challenges related to reporting these desired metrics due to measurement error, and offer solutions to mitigate these challenges.

Metal‐Organic‐Framework‐Derived Cobalt‐Doped Carbon Material for Electrochemical Ammonia Synthesis under Ambient Conditions

AbstractElectrocatalytic nitrogen reduction reaction (NRR), as a potential alternative to industrial ammonia synthesis (Haber‐Bosch method), can achieve “green” and sustainable ammonia production under environmental conditions. However, the NRR process is still limited by the slow cleavage of the N≡N triple bond and the existence of a competitive hydrogen evolution reaction. For this reason, many researchers have studied the construction and synthesis of high‐efficiency nitrogen reduction electrocatalysts. Herein, a Co‐based metal‐organic framework (ZIF‐67) was used to prepare Co/C‐900 composites through high‐temperature pyrolysis (900 °C) for ammonia synthesis under environmental conditions. The Co/C‐900 catalysts exhibited excellent NRR performance, with a high NH3 yield of 4.66 μmol cm−2 h−1 at −0.3 V (vs. RHE) and a faradaic efficiency of 11.53 % in 0.1 M potassium hydroxide. This work provides a simple strategy for the design of porous, non‐noble‐metal electrocatalysts for electrochemical NRR.

Development of a sustainable energy system utilizing a new molten-salt based hybrid thermal energy storage and electrochemical energy conversion technique

In the present study, an integrated solar and wind based cogeneration system is developed for power and ammonia production. A new molten-salt based thermal energy storage and electrochemical energy conversion technique is utilized, which includes electrochemical ammonia synthesis, direct ammonia fuel cell, and thermal energy storage system. A transient energy and exergy analyses is performed on the developed system considering the hourly variations in wind velocities as well as solar intensities. The energy efficiency of the overall system on monthly average days varies between a minimum value of 29.4% and a maximum value of 38.7% across the year. In addition, the exergy efficiency of the developed system is found to vary between 30.2% and 39.8%. Furthermore, the daily ammonia production is found to reach peak value of 4292.3 kmol/day during the year. Moreover, the maximum discharge capacity of the molten salt-based direct ammonia fuel cell system attains a value of 31.8 MWh across the year. Several parametric studies are also performed to assess the effects of varying operating conditions on the performance of the developed system as well as subsystem units.

Electrochemical synthesis of ammonia: Progress and challenges

Ammonia, as an essential chemical and a promising fuel, has been predominantly synthesized by the Haber-Bosch process since the beginning of the 20th century. However, the harsh conditions, high energy consumption, and large carbon footprint of this century-old process enforce human being to develop clean and sustainable ammonia synthesis technologies. The electrochemical synthesis of ammonia has advantages of mild operating conditions, zero emission of carbon dioxide, capability to store renewable electricity in chemical bonds, and possibilities for distributed ammonia production. Benefiting from these merits, the electrochemical synthesis of ammonia is expected as a promising alternative to the Haber-Bosch process and has been extensively explored. Research on the electrochemical synthesis of ammonia by nitrogen reduction reaction (NRR) can be principally divided into several aspects including the development of catalysts, innovations on the configurations of electrolytic cells, strategies to improve the selectivity, and verification of ammonia synthesis results. Moreover, the electrochemical reduction of nitrogenous pollutants has also been tried to produce ammonia. In this review, we summarize the recent progress on these points and conclude with perspectives on challenges and future directions of electrochemical ammonia synthesis.

Editors’ Choice—Economic Considerations for Low-Temperature Electrochemical Ammonia Production: Achieving Haber-Bosch Parity

Production of ammonia through coupling renewable energy with electrolysis cells will undoubtedly aid in reducing carbon dioxide emissions from the ammonia production industry. However, if the cost for electrochemical routes does not reach a Haber-Bosch parity point, then it is unlikely that electrochemical ammonia synthesis will become industrially viable. This promotes a strong need for analyses that explore the economics of various system designs and production scales, to assess what systems and scales can attain Haber-Bosch price parity. Here, we aim to define the Haber-Bosch parity targets for various production scales. We then explore the economic considerations for two electrochemical systems for ammonia synthesis. The first system contains a single electrolysis cell where nitrogen and water are the sole reactants. The second system explores a two-staged electrolysis system. The first stage consists of a water electrolysis cell where water serves as the reactant and hydrogen and oxygen are the products. The second stage consists of a nitrogen electrolysis cell where the reactants are nitrogen and hydrogen and ammonia is the product. We emphasize the important role production scale plays in meeting Haber-Bosch price parity, and highlight the key challenges for electrochemical ammonia production.

Robust Active Site Design of Single-Atom Catalysts for Electrochemical Ammonia Synthesis

Electrochemical ammonia synthesis forms a key part of sustainable chemical synthesis. Single-atom catalysts have emerged as a promising class of electrocatalysts that could be capable of electroche...

Unsaturated p-Metal-Based Metal–Organic Frameworks for Selective Nitrogen Reduction under Ambient Conditions

Electrochemical ammonia synthesis that utilizes renewable electricity in the nitrogen reduction reaction (NRR) has recently been remarkably considered. Of particular importance is to develop efficient electrocatalysts at low costs. Herein, highly selective nitrogen capture using porous aluminum-based metal-organic frameworks (MOFs) materials, MIL-100 (Al), is first designed for the electrochemical nitrogen fixation in alkaline media under ambient conditions. Owing to the unique structure, MIL-100 (Al) exhibits remarkable NRR properties (NH3 yield: 10.6 μg h-1 cm-2 mgcat.-1 and Faradaic efficiency: 22.6%) at a low overpotential (177 mV). Investigation indicates that the catalyst shows excellent N2-selective captures due to the unsaturated metal sites binding with N2. More specifically, as the Al 3p band can strongly interact with N 2p orbitals, Al as a main group metal presents a high and selective affinity to N2. The utilization of multifunctional MOF catalysts delivers both high N2 selectivity and abundant catalytic sites, resulting in remarkable efficiency for NH3 production.

Palladium Nanothorn Assembly Array for Efficient Electroreduction of Nitrogen to Ammonia

Controllable preparation of palladium-based nanomaterials is of great significance for electrochemical ammonia synthesis. Herein, a micelle-assisted replacement strategy was reported for the in sit...

Encapsulating vanadium nitride nanodots into N,S-codoped graphitized carbon for synergistic electrocatalytic nitrogen reduction and aqueous Zn-N2 battery

Electrochemical nitrogen reduction reaction (ENRR) is a promising strategy for mild electrosynthesis of ammonia but needs highly efficient electrocatalytic centers. Herein, vanadium nitride nanodots embedded in ultrathin N,S-codoped carbon matrix (denoted as VN@NSC) are developed to serve as a promising catalyst for efficient ENRR, showing robust stability, high NH3 yield and pH independence for the electrochemical synthesis of ammonia. Besides, VN@NSC was employed as N2 cathode for assembled Zn-N2 aqueous battery, which shows a significant potential for practical applications. Such remarkable activity of VN@NSC is ascribed to structural advantages and synergistic interaction among the components. Specifically, N,S-codoped carbon (NSC), as a stable support with sufficient carbon defects, not only plays vital role in size-limited growth of VN nanoparticles but also acts as a brilliant “capturer” for adsorption and activation of N2 molecules. Meanwhile, VN nanodots encapsulated in the NSC “capturer” function as a highly efficient “converter” for N2 to NH3, and this inlaid structure could effectively prevent VN nanodots shedding and deactivation resulted from hydrogen spillover. Furthermore, 15N isotopic labeling experiments give sufficient evidence for accurate detection of NH3 yields of VN@NSC, and electrochemical-based in situ Fourier transform infrared spectroscopy provides reasonable insight into the underlying ENRR mechanism.

Nanostructured bimetallic Ni–Fe phosphide nanoplates as an electrocatalyst for efficient N2 fixation under ambient conditions

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions has been considered as a promising approach for green ammonia synthesis; however, non-precious metal-based catalysts with excellent NRR electrocatalytic performance have been scarce. Herein, self-supported bimetallic electrocatalysts of Ni12P5/FeP4 nanoplates grown on carbon cloth were synthesized by a combined process of hydrothermal technology and low-temperature phosphidation. The catalyst exhibits excellent electrocatalytic N2 fixation performance with a high NH3 yield rate of 3.08 × 10−10 mol s−1 cm−2 (16.40 μg h−1 mgcat−1 ), Faradaic efficiency (39.9%) as well good durability at − 0.1 V in 0.1 M Na2SO4 at ambient conditions, which are attributed to the distinctive nanostructure and rational composition. Our study paves the way for expanding the scope of NRR research and designing more efficient electrochemical ammonia synthesis in the future.

A Pd/MnO2 Electrocatalyst for Nitrogen Reduction to Ammonia under Ambient Conditions

Electrochemical ammonia synthesis, which is an alternative approach to the Haber–Bosch process, has attracted the attention of researchers because of its advantages including mild working conditions, environmental protection, and simple process. However, the biggest problem in this field is the lack of high-performance catalysts. Here, we report high-efficiency electroreduction of N2 to NH3 on γ-MnO2-supported Pd nanoparticles (Pd/γ-MnO2) under ambient conditions, which exhibits excellent catalytic activity with an NH3 yield rate of 19.72 μg·mg−1Pd h−1 and a Faradaic efficiency of 8.4% at −0.05 V vs. the reversible hydrogen electrode (RHE). X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization shows that Pd nanoparticles are homogeneously dispersed on the γ-MnO2. Pd/γ-MnO2 outperforms other catalysts including Pd/C and γ-MnO2 because of its synergistic catalytic effect between Pd and Mn.

Pore-Size-Tuned Pd Films Grown on Ni Foam as an Advanced Catalyst for Electrosynthesis of Ammonia

Electrochemical ammonia synthesis is considered as an alternative sustainable method to the traditional Haber–Bosch process because of the reduced energy consumption and mild operation conditions. The key is to design efficient electrocatalysts for facile adsorption and activation of nitrogen. Herein, a nanoporous Pd film with a tunable pore size has been in situ grown on Ni foam (nPd/NF) via a micelle-assisted replacement method using NF as a reductant and substrate, where polystyrene-b-poly­(ethylene oxide) and 1,3,5-triisopropylbenzene serve as a pore-forming agent and a swelling agent, respectively. Combining the continuous large nanoporous structure and self-supported properties, the obtained nPd/NF exhibits excellent catalytic performance for ammonia synthesis with a high NH3 yield of 18.27 μg h–1 mgcat–1 and a faradaic efficiency of 10.36% in 0.1 M Na2SO4. This work provides a fascinating route for the rational design of self-supported porous noble metals with a tunable pore size for effective electrosynthesis of ammonia.

Nitrogen reduction to ammonia at ambient conditions using hydrochar prepared from cigarette filters as catalyst

Ammonia synthesis via the electrochemical method is an environmentally-friendly alternative to the Haber-Bosch process. In this work, carbon materials made from fresh cigarette filters were used as the working electrode for the electrochemical synthesis of ammonia. TiO2 was found to exist in the hydrochar of cigarette filters with a mass loading of 0.8 wt%, and showed positive impact on the catalytic performance of the hydrochar. A decent NH3 yield of 6.02 μg mgcat.−1 h−1 and a FE of 8.96% were achieved at −0.1 V versus reversible hydrogen electrode (RHE) in N2-saturated 0.05 M H2SO4. The design of experiments proved that TiO2, which was contained in the hydrochar, indeed played a great role in the catalytic reaction of ammonia synthesis.

High‐Performance Nitrogen Fixation over Mo Atom Modified Defective α‐MnO2 (001)

AbstractThe production of ammonia under ambient conditions through electrocatalytic nitrogen reduction reaction is significant but challenging. The lack of an available active NRR electrocatalyst with a high selectivity impedes the development of the electrochemical ammonia synthesis. In this work, via first‐principles density functional theory calculations, we investigated various transition metal atoms (Fe, Cr, V, and Mo) substitution modified defective (with an oxygen vacancy) α‐MnO2 (001) as NRR electrocatalyst. It was found that the incorporation of a single Mo atom substituent at Mn site of α‐MnO2 (001) shows the best performance of nitrogen fixation through enzymatic mechanism with a favorable limiting potential of −0.14 V. The superior selectivity for NH3 over H2 is observed with high Faradaic efficiency of 99 %. The ‐like characteristics of occupied states around the Fermi level of Mo active center maximizes the p‐d orbital hybridization and thus promotes the stabilities of *N2 and *N2H, which leads to the superior NRR performance. These findings provide theoretical guidance to the development of highly efficient NRR electrocatalysts via defect engineering.

Ru-doped phosphorene for electrochemical ammonia synthesis

The electrochemical ammonia synthesis has attracted increasing attention due to its energy saving characteristics. However, developing novel electrocatalysts and their mechanism remain great challenges. Here, several transition metal (TM) atoms doped on phosphorene were studied as N2 fixation electrocatalysts by using density functional theory (DFT) calculations. The results demonstrate that single Ru atom doped phosphorene shows an excellent catalytic activity for ammonia synthesis via the enzymatic pattern. A small overpotential of 0.696 V is achieved for this process. The effect of oxidation in the catalyst was also discussed in our work. Oxidation deactivates the catalyst, which should be avoided in the experiment. Our outcomes offer a novel perspective for single-atom catalytic ammonia synthesis with phosphorene as a substrate.

Two-Dimensional Layered SnO2 Nanosheets for Ambient Ammonia Synthesis

Industrial production of ammonia remains dependent on the energy-intensive Haber–Bosch process, causing huge environmental burden. Electrochemical ammonia synthesis is a promising alternative approach capable of producing ammonia under ambient conditions via nitrogen reduction reaction on electrocatalysts. Although promising, this approach is still challenged by poor selectivity and low yield rate, requiring advanced electrocatalysts with rational designs for efficient nitrogen reduction reaction. Here, we design and synthesize SnO2 nanosheets with a unique two-dimensional layered architecture, potentially as an efficient electrocatalyst for ambient nitrogen reduction to ammonia. First, SnO2 nanosheets with a thickness of few nanometers expose oriented crystalline (101) facets as a majority, preferably for nitrogen adsorption and reduction. Second, the presence of oxygen vacancies in SnO2 nanosheets creates defective Sn nanostructures with dangling bonds, which can significantly enhance the nitrogen adsorption via strengthening the local binding effect between Sn and N atoms. Last, the two-dimensional architecture enables the presence of abundant uncoordinated surface atoms that are thermodynamically unstable, in which nitrogen adsorption and dissociation mostly take place, thus greatly enhancing the catalytic reactivity. Density functional theory calculations found that the adsorption energy of N2 molecules on the (101) facet with a two-coordinate oxygen vacancy is 10.45 kJ mol–1, larger than that on the (110) facet (4.60 kJ mol–1). The strong binding between N2 and abundantly exposed (101) facets facilitates the nitrogen adsorption and dissociation. Hence, two-dimensional layered SnO2 is a promising electrocatalyst candidate for electrochemical ammonia synthesis. The ammonia production performance has been experimentally evaluated via construction of a three-electrode electrochemical cell, in which the as-prepared SnO2 nanosheets grown on a carbon cloth serves as the working electrode, displaying an ammonia yield rate of 38.18 μg h–1 mg–1 and a Faradaic efficiency of 11.33%, both of which are higher than those achieved by most recently reported metal-based electrocatalysts. The ammonia production capability is also found to be stable, with a 93% retention of ammonia yield rate after six cycles. Such a substantial improvement opens a window of opportunity for breakthroughs in the development of ambient ammonia synthesis technologies.

Chlorine-doped carbon for electrocatalytic nitrogen reduction

The electrochemical reduction of stable dinitrogen (N2) molecules into value-added chemicals such as ammonia has been attracting substantial research interests recently, while the bottleneck of catalyst design exists in the rational synthesis of catalytic centers with efficient N2 chemisorption and subsequent activation. In this work, we demonstrated a facile approach of doping carbon black with a trace amount of chlorine, which allowed for the enhancement of its nitrogen reduction capability. Structural characterizations revealed the successful doping of the chlorine element into the carbon framework, accompanied by the polarization of the surface charge density. The positively charged carbon sites behaved as good NRR catalytic centers, with a maximum ammonia production rate of 4.3 μg h−1 cm–2 and a corresponding Faradaic efficiency of 7.2%, thus suggesting a potential approach for enhancing the nonmetal catalysts for electrochemical ammonia synthesis.

Protonic ceramic electrolysis cells for fuel production: a brief review

Proton-conducting oxides exhibit significant hydrogen ion (proton) conductivity at intermediate temperatures around 300–600 °C. Owing to their distinguished features compared to high-temperature oxygen ion-conducting oxide electrolytes and low-temperature proton-conducting polymer electrolytes, diverse electrochemical applications based on the proton-conducting oxides have attracted great attention for efficient energy conversions. This review particularly aims to introduce protonic ceramic electrolysis cells (PCECs) and their extended applications. The constituent materials, recent developments, remaining issues in PCECs as well as the application integrated with PCEC for electrochemical ammonia synthesis will be presented. In addition, for each section, the relevant prospects and recommendations for future research directions will be discussed.