Not so long time ago, the ENRR was considered hardly possible to conduct, because there arose a doubt the literature is plagued by false-positive measurements. In ENRR measured ammonia concentrations were often within the range of ambient ammonia contamination. Some ENRR works produced ammonia by (unintentionally) reducing other, more reactive forms of nitrogen (NOx, nitrates, nitrites) or getting ammonia from a decomposition of the catalyst, while reporting reduction of inert dinitrogen to ammonia. Several rigorous experimental protocols were published during the last couple of years (from 2018), 1,2,3,4,5 and these made it easier to distinguish reliable results from the false-positive ones. In general, the ENRR reports that follow the rigorous synthesis protocols are considered valid. If they do not follow the protocols, but still produce ammonia concentrations which are far above contamination levels, the works are also considered valid.
Taking a starting point in the reasoning above, we evaluated 120 articles and found out about 20% of ENRR works in the last three years are reliable. The result of this evaluation emphasizes that researchers should consider rigorous experimental protocols more seriously if they wish to stimulate progress in the field. We took the reliable works and evaluated them for power-to-fuel efficiency, by relating the low heating value of ammonia and total energy input to produce ammonia using ENRR.
Our analysis shows that some of the reliable ENRR reports have power-to-fuel efficiency of a maximum 40% which is comparable to the current Haber-Bosch ammonia plants which are often in the range of 35-60%.6 Although energy efficiency of ENRR (40%) is lower than that of the modern Haber-Bosch plant (60%), such process would be probably relevant for the market where lower capital costs are more important than higher operating costs. About a third of the mostly reliable ENRR reports have energy efficiency between 20-35%. The others have less than 20% energy efficiency and need more improvements to be comparable with the Haber-Bosch process in terms of power-to-fuel efficiency. The results are surprising and show electrochemical synthesis of ammonia is maturing and can compete with the Haber-Bosch process, however, improvements are needed to bring the energy efficiency higher up.
References
(1) Suryanto, B. H. R.; Du, H.-L.; Wang, D.; Chen, J.; Simonov, A. N.; MacFarlane, D. R. Challenges and Prospects in the Catalysis of Electroreduction of Nitrogen to Ammonia. Nat. Catal. 2019, 2 (4), 290–296. https://doi.org/10.1038/s41929-019-0252-4.
(2) Andersen, S. Z.; Čolić, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A.; Statt, M. J.; Blair, S. J.; Mezzavilla, S.; Kibsgaard, J.; Vesborg, P. C. K.; Cargnello, M.; Bent, S. F.; Jaramillo, T. F.; Stephens, I. E. L.; Nørskov, J. K.; Chorkendorff, I. A Rigorous Electrochemical Ammonia Synthesis Protocol with Quantitative Isotope Measurements. Nature 2019, 570 (7762), 504–508. https://doi.org/10.1038/s41586-019-1260-x.
(3) Tang, C.; Qiao, S.-Z. How to Explore Ambient Electrocatalytic Nitrogen Reduction Reliably and Insightfully. Chem. Soc. Rev. 2019, 48 (12), 3166–3180. https://doi.org/10.1039/C9CS00280D.
(4) Du, H.-L.; Gengenbach, T. R.; Hodgetts, R.; MacFarlane, D. R.; Simonov, A. N. Critical Assessment of the Electrocatalytic Activity of Vanadium and Niobium Nitrides toward Dinitrogen Reduction to Ammonia. ACS Sustain. Chem. Eng. 2019, 7 (7), 6839–6850. https://doi.org/10.1021/acssuschemeng.8b06163.
(5) Greenlee, L. F.; Renner, J. N.; Foster, S. L. The Use of Controls for Consistent and Accurate Measurements of Electrocatalytic Ammonia Synthesis from Dinitrogen. ACS Catal. 2018, 8 (9), 7820–7827. https://doi.org/10.1021/acscatal.8b02120.
(6) Smith, C.; K. Hill, A.; Torrente-Murciano, L. Current and Future Role of Haber–Bosch Ammonia in a Carbon-Free Energy Landscape. Energy Environ. Sci. 2020, 13 (2), 331–344. https://doi.org/10.1039/C9EE02873K.