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TOWARD EFFICIENT ELECTROCHEMICAL GREEN AMMONIA CYCLE

Periodic Reporting for period 1 - TELEGRAM (TOWARD EFFICIENT ELECTROCHEMICAL GREEN AMMONIA CYCLE)

Reporting period: 2020-11-01 to 2022-04-30

Ammonia is one of the most important chemicals used as fertilizer and in many other applications. Its production is achieved through a process responsible for about 1-2% of total CO2 emissions worldwide. Ammonia is also a potentially formidable energy vector, with high volumetric energy density, large hydrogen content and, unlike H2, it can be liquefied at room temperature and low pressure for storage and transportation.
The main objective of the TELEGRAM project is to demonstrate, at a laboratory scale, a complete green ammonia carbon energy cycle. The main goal will be reached through the following objectives:
• Objective 1: Development of novel catalysts for ammonia synthesis with a faraday efficiency >50% and/or a production rate ≥ 10-7 mol cm-2 s-1.
• Objective 2: Understanding of the reaction mechanisms by detailed structural characterization, atomistic simulations and in-operando spectroscopy analyses.
• Objective 3: Multi stage ammonia reactor with optimised design and catalysts, with production rate of at least 10-7 mol cm-2 s-1.
• Objective 4: Direct ammonia fuel cell (DAFC) with optimised design using catalysts with < 0.05 mg cm-² of Pt group metals, achieving a power density of at least 100 mW cm-2 with a chemical to electricity efficiency > 25%.
• Objective 5: Full electricity-ammonia-electricity cycle powered by renewable energy sources with 95% of the combined efficiencies of ammonia generation and DAFC.
The development of catalysts has been focused on three approaches: novel high entropy alloys (HEAs), nanostructured catalysts, and bi-metallic catalysts.
The compositions with higher estimated activity have been selected through atomistic simulations among 3000 HEAs. The more promising have been produced and characterized. The highest ammonia production rate (1.6 x 10-10 cm-2 s-1) has been experimentally achieved with a CoCrFeMnNi alloy in 5M KOH. The best faraday efficiency was 20% in 1M KOH. The activity of CoCrFeMnNi for ammonia oxidation has been also evaluated.
Gold based (Au loading <0.02 mg cm-2) and iron oxide based nanostructured catalysts have been prepared and characterized for ammonia synthesis. The highest production rate, of 4 x 10-11 mol cm-2 s-1 was obtained for Au on Ni foam, while a remarkable faraday efficiency of 40% has been achieved with Mo-Au.
The catalysts have been tested in the membrane electrode assembly (MEA), designed and manufactured with Nafion, in a test-rig with H2 or water feed at the anode and N2 gas at the cathode. Ammonia can be detected via gas phase IR spectroscopy and by the Berthelot method. The first catalysts have shown ammonia generation. However, the influence of the operating conditions is relevant, so the stability window has to be determined.
Multiphysics models for both the direct ammonia fuel cell (DAFC) and the MEA have been developed. The DAFC model has been verified with literature data. The first DAFC prototype has been designed and manufactured employing materials stable in high-concentration of ammonia at 70°C. The open circuit voltage (OCV) of the DAFC with Pt at the anode and cathode reaches levels comparable to literature data (400 mV). Bimetallic catalysts (NiCu) have been prepared by electrodeposition on stainless steel felt and tested. The first results seem promising, with OCV of about 300 mV, corresponding to about 25-35% voltage efficiency. However, work to improve stability is still required.
The devices will be used for the proof of concept of the complete ammonia cycle powered by renewable energy sources. To this aim, a test bench has been setup, capable of emulating fluctuating electrical inputs and loads.
The use of HEAs for ammonia synthesis is a quite innovative and promising approach: the first article was published in 2021 [1] and showed 11.4 µg h–1 cm–2 NH3 production rate with CoCuFeNiRu. The production rate achieved in TELEGRAM adopting a CoCrFeMnNi alloy is 9.8 µg h–1 cm–2, therefore comparable with the literature, with the advantage of avoiding the use of Ru as precious material.
Nanostructured catalysts have been largely employed in literature for ammonia synthesis. Fe3O4/Fe catalysts [2] exhibit production rate of 0.07 µg cm-2 h-1 and 8% faraday efficiency. The values obtained in TELEGRAM (1.22 µg cm-2 h-1 and 20%) are both higher. A recent paper using Au nanoparticles on hydrophobic carbon fiber [3] claims for an excellent performance with rate of 40.6 µg h−1 mg−1 and great efficiency of 31.3%. In TELEGRAM, with Mo-Au catalysts on hydrophobic carbon fiber, values beyond the state of the art have been obtained, with higher faraday efficiency (40%) and rate (150 µg h−1 mg−1).
For the DAFC with Pt based electrodes, adopted as benchmark, the obtained OCV value (400 mV) is comparable to that expected from literature [4]. The current level, however, still needs improvement. This is due, in part, to the fact that measurements in the DAFC have been performed at 25°C, while typically at least 60°C is adopted. Nevertheless, a comparison performed using the 3-electrode measurements at room temperature and at 60°C, indicates that we can expect a doubled DAFC current at 60°C.
The possibility to produce green NH3, with zero CO2 emission, using earth abundant materials is expected to have political impact, contributing to establish a solid European innovation base and to build a sustainable energy system, through the development of two enabling technologies in the field of electrochemistry: the synthesis of ammonia and its use as a fuel in the DAFC. We may expect also an environmental impact since the onsite distributed production of green ammonia, would contribute to the decarbonisation of the global economy of the future, supplanting the energy intensive process and reducing the transportations. The expected increased access to ammonia as fertilizer is associated with social and economic benefits especially for developing countries or isolated regions, with lack of infrastructures. Moreover, the use of ammonia as a fuel would reduce vulnerability of consumers to fluctuating energy prices.
The technologies TELEGRAM is addressing, however, are still at a nascent stage. Therefore, there is a strong impact related to technical and innovation aspects. The basic knowledge the TELEGRAM consortium is acquiring is fundamental to collect scientific proofs of the technological feasibility of its basic concept and to build a know how crucial for designing better performing devices. The knowledge is being used to create a European team with basic skills on catalysts and electrochemical systems that can develop further research, assessing innovative strategies to produce green fuels. In the last five years, among the 1500 papers published on nitrogen reduction, less than 5% have been produced in EU countries. This clearly indicates that there is a lack of knowledge in the field in EU, and the TELEGRAM consortium is contributing to fill this gap. The project is also attracting young researchers, with 4 PhD students and 2 Post Doc researchers already working on the project, thus TELEGRAM is contributing to create the next generation of European scientists who will boost the transition of Europe to a sustainable energy system.

[1]Zhang, D. et al. Adv. Funct. Mater. 2021, 31, 2006939.
[2]Hu L. et al, ACS Catalysis 2018, 8, 9312.
[3]Zhang, J. et al, Adv. Science 2020, 7, 2002630.
[4]S. Suzuki, et al, J. Power Sources 2012, 208, 257.
Direct Ammonia Fuel Cell under operation with the zoom on internal parts
Catalysts approach for ammonia synthesis and test cell
Test bench for devices evaluation under fluctuating power load