Role for Standardization in Electrocatalytic Ammonia Synthesis: A

May 31, 2019 - Role for Standardization in Electrocatalytic Ammonia Synthesis: A Conversation with Leo Liu, Lauren Greenlee, and Douglas MacFarlane ...
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Role for Standardization in Electrocatalytic Ammonia Synthesis: A Conversation with Leo Liu, Lauren Greenlee, and Douglas MacFarlane Downloaded via 185.14.195.58 on August 13, 2019 at 11:29:31 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Lance Seefeldt, and T. Leo Liu from Utah State University report the surprising observation that decomposition of their tetragonal Mo2N catalyst contributed to measured ammonia concentrations during what is presumably electrocatalysis. They raise the point that without sufficient control experiments (that are inconsistently applied through the electrocatalytic nitrogen reduction reaction (ENRR) research community), it would be straightforward to conclude that the tetragonal Mo2N catalyst is producing ammonia with higher efficiency than it actually is. To obtain further insight on the matter, we spoke with Leo Liu, the corresponding author of the study and Assistant Professor in the Department of Chemistry & Biochemistry at Utah State University. We also consulted with two independent experts in the field, Lauren F. Greenlee, Assistant Professor and Ralph E. Martin Leadership Chair in the Department of Chemical Engineering at the University of Arkansas, as well as Doug MacFarlane, Australian Laureate Fellow and Professor at the Monash University School of Chemistry. Insightful related discussions by these authors and others have been previously published in ACS Energy Letters and elsewhere.1−3 Conversation with Leo Liu. (1) What is the significance of and motivation for your ACS Energy Letters article? How are you hoping that your findings will impact the field? Electrocatalytic nitrogen reduction is becoming a hot topic in electrocatalysis regarding energy conversion, witnessing an increasing number of publications on using a variety of heterogeneous catalysts. Different catalysts have been theoretically and experimentally claimed to be effective for dinitrogen activation. In this ACS Energy Letters paper,4 we investigated tetragonal phase Mo2N as a potential catalyst for electrochemical dinitrogen (N2) reduction reaction (ENRR). However, instead of electrochemically catalyzing reduction of N2, our results revealed that this metal nitride material undergoes chemical decomposition to produce ammonium (NH4+). Although demonstrating a failure of using MoN2 for electrocatalytic N2 reduction, our study highlighted several critical issues of which awareness needs to be increased in order to establish reliable ENRR catalysis. First, if nitrogen atoms are contained in potential electrocatalysts like Mo2N, then these nitrogen atoms can be the source of the formation of NH4+ as a result of decomposition of potential electrocatalysts. Thus, despite claimed catalytic ENNR using metal nitride and other N-containing materials in prior studies, our results call serious attention to carefully evaluate the catalysis

arge-scale ammonia synthesis is critical for the mass production of fertilizers and therefore support of industrialized farming at scale to produce sufficient food for an ever-growing global population. Additionally, efficient, low-cost methods for ammonia production bring promise for the future landscape of low carbon footprint energy technologies as ammonia can be used as a renewable energy carrier. Traditionally, the predominant method for industrial-scale ammonia synthesis has been the Haber−Bosch process, which carries a number of disadvantages. The H2 required to react with N2 is largely sourced from steam methane reforming, which is mainly fossil fuel-dependent (specifically natural gas). Therefore, this process for ammonia production contributes to emission of greenhouse gases. Furthermore, generation of H2 feedstock as well as the high temperatures and pressures required to drive the Haber−Bosch process is highly energy intensive, consuming approximately 2% of the energy supply globally on an annual basis. Lastly, the Haber−Bosch process is inefficient, therefore consuming far more energy and materials than are actually converted to ammonia. As such, it becomes clear that more efficient and environmentally friendly ammonia production methods (ones that require lower energy input, decreased fossil fuel use) are required in order to ensure future food security while mitigating negative environmental impact. In order to address this need, there exist significant research efforts worldwide to develop efficient, selective, and lowtemperature and -pressure dinitrogen reduction electrocatalysts that can be integrated into electrochemical systems driven by renewable energy sources. A wide variety of catalyst materials and structures are being pursued by the research community, and mechanistic insight into catalytic reactions is being sought. However, discussion has recently arisen among researchers in this field regarding the impact of extraneous sources of nitrogen and ammonia in laboratory environments. Ambient ammonia is difficult to eliminate or account for experimentally because it exhibits regional and seasonal variability due to farming cycles. Ammonia possesses high aqueous solubility, and this polar molecule readily adsorbs to the surface of a variety of materials. This creates a challenging environment for laboratories to measure ammonia production (and therefore determine Faradaic efficiency) during small-scale experiments and to control for fluctuating ambient ammonia concentrations between experiments. This research community maintains that there has been some variability in published articles when it comes to providing detailed explanations of important experimental parameters including choice of control experiments and how measurements were collected. In their new ACS Energy Letters Energy Express article (DOI: 10.1021/acsenergylett.9b00648), Bo Hu, Maowei Hu, © 2019 American Chemical Society

Received: May 23, 2019 Accepted: May 23, 2019 Published: May 31, 2019 1432

DOI: 10.1021/acsenergylett.9b01123 ACS Energy Lett. 2019, 4, 1432−1436

Energy Focus

Cite This: ACS Energy Lett. 2019, 4, 1432−1436

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ACS Energy Letters

ENRR catalysts such as metal nitrides and nitrogen-doped carbon materials, the chemical decomposition of these materials could produce ammonia and highly affect the final catalysis evaluation. In addition, as stated above (question 1), even a tiny amount of contamination such as NH3 and NOx can significantly influence measurements of ammonia formation and thus lead to different conclusions in terms of yield, rate, and selectivity. We were motivated to study Mo2N for ENRR as it is a known catalyst for the chemical hydrogenation of N2. In addition, Mo2N has been considered a stable and efficient catalyst for nitrogen activation by both DFT calculation and experimental studies reported by others. We are very cautious of applying nitrogen-containing materials as electrocatalysts for ENRR and would like to carefully study their catalytic performance for nitrogen reduction. Therefore, Mo2N was selected as the catalyst in this study. (3) What are the most important steps/standardizations for the field to integrate in order for technologies for the electrocatalytic production of ammonia to progress? As we addressed in the article, the most important thing is to confirm the true catalysis of ENRR. The first task is to avoid any source of contamination. We recommend making sure there is no nitrate, nitrite, and other forms of contamination in the feeding N2 gas. As discussed above, the direct method to prove real catalysis is the 15N2 labeling experiment. Although many studies reported the 15N2 labeling experiment, the result is unreliable because researchers practicing ENRR are seldom aware of 15N2 gas contamination. Therefore, 15N2 gas pretreatment and background control are barely conducted to make sure no 15N-ammonia and 15NOx exist in the feeding gas. In addition, in most of the ENRR papers, 15N2 controlled experiments are just conducted as a qualitative analysis. The 15 N2 labeling experiment needs to also be demonstrated catalytically. Moreover, the other strong evidence for catalysis is turnover (>1), which has seldom been clearly addressed in the electrocatalytic nitrogen reduction studies. Actually, most of the studies did not show a turnover for the nitrogen electrocatalytic reduction. Often the amount of produced ammonia was much less than that of the loaded catalysts, and therefore, such studies need to exercise caution in claiming catalytic ENRR. (4) Would you care to share any follow-up studies or future directions your lab may be working on to advance the field? In spite of the increasing popularity of electrochemical and photolytically NRR studies, the field is still in its infancy stage. Based on our work published in ACS Energy Letters4 and reported results by other groups, we will continue to identify reliable ENRR catalysts. An ongoing research effort in my group is to examine pure metals as potential ENRR catalysts, particularly ones (e.g. Zn and Bi) with low activity for hydrogen evaluation, a major side reaction competing with the ENRR Faraday efficiency. With a reliable catalyst, we can systematically evaluate effects of reaction conditions on the ENRR performance, such as solvents (protic versus aprotic), pKa, temperature, and N2 pressure. In addition, reliable ENRR catalysts will allow us to further develop in-depth mechanistic understandings through both experimental and computational approaches, and lead to improved catalyst designs. Conversation with Lauren Greenlee. (1) Could you provide some background on the influence of extraneous ammonia sources on literature reports of the ENRR for ammonia production? How is this impacting the status of the field?

Figure 1. Leo Liu is an Assistant Professor in the Department of Chemistry & Biochemistry at Utah State University (https://www. tianbiaoliu.org/). Photo courtesy of Utah State University.

of these materials, which echoes a recent study examining the ENRR activity of vanadium and niobium nitrides reported by MacFarlane et al.5 In addition, elaborate control experiments are crucial to confirm true ENRR catalysis. Especially 15N2 labeling experiments are necessary to provide direct evidence of the cleavage of the N2 triple bond. In this regard, it is important to make sure there are no contaminating compounds including 15NH3 and 15NOX (X = 1 and 2) in the 15N2 gas as they can be converted to 15NH4+ under typical ENRR reaction conditions. However, many reported studies are not careful enough in conducting control experiments, resulting in overestimated or even unreliable results for ENRR. Moreover, it is important to carefully report the turnover frequency (TOF) and turnover number (TON) in an ENRR study, which is dependent on correct evaluation of the loading of a catalyst and the yield of NH4+. Therefore, it is very urgent to uncover these existing problems to promote healthy development of the NRR field. I hope our findings will raise substantial cautiousness for taking every detail into consideration when evaluating the ENRR and also photolytic NRR performance of a catalyst. (2) How extensively could catalyst degradation be influencing nitrogen reduction reaction (NRR) electrocatalysis efficiency/selectivity reports in the literature? Why did you choose tetragonal Mo2N as the catalyst material for your study reported in ACS Energy Letters? The chemical decomposition originates from the stability of the potential catalyst materials. As explained below, it will extensively affect catalysis performance. According to reported ENRR studies, the amount of product, ammonia, is usually as low as nanomoles because of low Faraday efficiency and low catalytic rates. For nitrogen-containing materials as potential 1433

DOI: 10.1021/acsenergylett.9b01123 ACS Energy Lett. 2019, 4, 1432−1436

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ACS Energy Letters

gloss over these details in pursuit of claiming high production rates and Faradaic efficiencies. The result of this scenario is that, as a field, we may be pursuing pathways to electrocatalyst development that are actually not productive (e.g., the recent results of Hu et al.,4 which report that Mo2N is not in fact an electrocatalyst for nitrogen reduction to ammonia but is actually decomposing in the electrochemical environment). The status of the field will ultimately remain stalled at a stage where it is unclear which catalyst design(s) can be identified as routes to efficient and active surfaces for nitrogen reduction if a careful and rigorous approach to measuring ambient ammonia is not implemented across the field. (2) Briefly, what are the most important steps that can be taken to improve the quality of measurements/evaluation of electrocatalyst selectivity and efficiency? What are some promising trends and opportunities in the field currently? Most studies lack quantitative isotope studies with 15N2 gas (which also should be purified to remove any ammonia or nitrate impurities). Any study claiming a new or improved catalyst design or electrochemical engineering design (e.g., different electrolyte) needs to verify spectrophotometric results with a quantitative 15N study, where the 15NH3 produced (and the Faradaic efficiency calculated) is the same amount as that produced with 14N, within experimental error. Perhaps even a double verification is necessary at this point in the field, i.e., NMR and ion chromatography results that quantitatively support spectrophotometric results. It would be helpful to quantitatively measure H2 and N2 and confirm that no hydrazine is produced to work toward an overall mass balance, but the quantitative isotope study is really the key. The other important step is for the field to be publishing socalled “negative” resultswhere researchers test a catalyst composition and do not produce ammonia or find that the catalyst degrades or find some other explanation for their ammonia measurement. The field is at a stage where it is just as important to understand all of the negative results as it is to identify positive results and thus potential catalyst design/ composition directions for further development. In conjunction with this aspect, publications should contain the full details of controls/background measurements, repeats, and variability of ambient ammonia measurements, as well as full reporting of how ammonia production rates and Faradaic efficiencies are calculated. (3) What are the performance benchmarks for alternate methods for ammonia production via NRR? Does electrocatalysis need to catch up, and if so, in what way? Currently, the primary benchmarks are the catalyst-massnormalized ammonia production rate (e.g., μg mg−1 h−1) and Faradaic efficiency (portion of current that goes to nitrogen reduction rather than hydrogen evolution). These metrics will continue to be the primary metrics used until the field moves toward highly efficient and highly active catalysts. At that point, we might see other metrics emerge, such as energy and cost efficiencies, but for the current state of the electrocatalyst field, the production rate and Faradaic efficiency serve the field well in creating comparable baseline metrics. It would also be helpful, as mentioned in Hu et al., to also see researchers reporting the TOF or TON, which takes into account quantitative catalyst loading and can show differences in catalysts with different multimetallic compositions. (4) Would you care to share any current or future directions your lab may be working on to advance the field?

Figure 2. Lauren Greenlee is currently Assistant Professor and Ralph E. Martin Leadership Chair at the Ralph E. Martin Department of Chemical Engineering at the University of Arkansas (https://sites.uark.edu/greenlee/lauren-f-greenlee-ph-d/). Photo courtesy of the University of Arkansas.

Extraneous ammonia sources exist in every lab, and the level of ambient ammonia adsorbed to surfaces and in the air can vary from day to day, lab to lab, and even seasonally. We spent several years taking daily to weekly background measurements of ambient ammonia in our laboratory as we were also doing electrocatalysis experiments, and we found that our ambient ammonia measurements in blank control samples actually vary across seasons. This result (unpublished) makes sense to us because in Arkansas we are surrounded by agricultural activities, and the result shows that you have to measure ammonia background for every single experiment. This variable but consistent presence of ammonia will directly influence researchers’ calculations of production flow rate and Faradaic efficiency when performing low-temperature ENRR studies. One of the major concerns of current and recent literature reports is the lack of detailed reporting and accounting of this ambient ammonia contribution. In many papers, it is unclear whether the authors have performed background and control measurements for every single electrocatalyst test, whether the results (including background/control measurements) are repeatable, and whether the authors subtracted this ammonia contribution from their measurements of ammonia production rate and Faradaic efficiency. Often argon and nonelectrochemical nitrogen control experiments are hidden in the Supporting Information of a paper; therefore, it is not necessarily obvious that ambient ammonia contributes significantly to the measured electrocatalytic ammonia produced. However, upon close inspection of some papers, I have seen ambient/background measurements contribute anywhere from 10 to greater than 25% of the spectrophotometric absorbances measured. Authors tend to 1434

DOI: 10.1021/acsenergylett.9b01123 ACS Energy Lett. 2019, 4, 1432−1436

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ACS Energy Letters Our current and near-future efforts focus on understanding how modifying and controlling the electrocatalyst surface chemistry and the electrocatalyst/electrolyte interface impact the performance of the catalyst for the NRR. However, we have focused thus far not necessarily on measuring ammonia production but understanding the interactions of relevant molecules with the electrocatalyst surface, and we are starting to look at how the electrocatalyst surface chemistry changes under operando conditions. Our focus comes from having tested a suite of catalysts over several years, realizing the challenges with ammonia measurements in general, and realizing how little we understand in terms of how to control and favor nitrogen reduction over hydrogen evolution, even for catalysts that are theoretically predicted to support nitrogen reduction. Our view is that if we can develop a better fundamental understanding of our materials platform through coordinated experimental and theoretical research, we will be able to contribute to advancements across the field through new knowledge of how electrocatalyst/electrolyte interfaces can enable nitrogen reduction to ammonia. Conversation with Doug MacFarlane.

their origins in the century-old Haber−Bosch process, which uses hydrogen derived from coal or natural gas as the source of the H-content of the ammonia and also the energy input to the process. The net result is that more than 1% of global greenhouse gas emissions are associated with this process alone. However, without chemical fertilizers, global food production could only be a fraction of its current levels, and demand will certainly increase in line with a growing global population. Transitioning ammonia production toward sustainable sources of energy therefore becomes an important challenge for the next decade. The second context emerges from the recognition that the capacity for wind and solar energy generation in some remote areas of the world is enormously high and is certainly sufficient to satisfy global energy needs for many decades to come. Northwestern Australia is a good example, where photovoltaic cells could be as much as 3 times more productive on an annual basis than those in northern Europe. However, transporting the energy from such remote regions becomes the challenge. A readily dispatchable form of the energy is required that can be readily piped and shipped. Hydrogen, from electrolysis or a solar−thermal cycle, is a possibility, but the concept of moving around large quantities of hydrogen is challenging from a safety point of view and there is a substantial energy cost to most of the available approaches; for example, the energy cost of hydrogen liquefaction is around 1/ 3 of its energy content. Ammonia is therefore being considered as a viable energy carrier if it can be produced efficiently because its transportation technology is already very well developed in the fertilizer industry. Of course, “green” hydrogen can also be used in a traditional Haber−Bosch ammonia synthesis to produce “green” ammonia. However, the combined process would be only about 40% energy efficient; therefore, there is plenty of scope for a more efficient technology such as direct NRR to improve upon this. An NRR process is also very likely to be considerably less capital intensive than a combined water electrolysis + Haber−Bosch process. (2) How does the work reported in ACS Energy Letters fit into the field, and to what extent could it shape future directions? The key to efficient NRR is a highly active and selective electrocatalyst, and there are many groups investigating a wide range of materials for this purpose. An interesting prospect in this regard are metal nitrides, many of which have the potential to support the fascinating, but speculative, Mars−van Krevelen mechanism. In this mechanism, NH3 is released from the nitride layers to be replaced by insertion of new N atoms into the layers from N2 reduction. However, there are many ways that the yield of such a process can be influenced by external contamination or catalyst breakdown. The work by Hu et al.4 investigates the NRR on Mo2N in detail and shows that, contrary to earlier reports, the ammonia produced is substantially from catalyst breakdown and that there is little true N2 reduction involved. Critically, they used 15N2-based experiments to track, unequivocally, the source of the N2. This key experiment has often been absent from previous studies that have claimed to observe NRR. Our own group has recently5 raised similar doubts about the Mars−van Krevelen mechanism operating in vanadium and niobium nitrides, also contrary to earlier reports. Together, these studies begin to suggest that nitrides, as a broad family of catalyst materials, may not be stable enough for use in the

Figure 3. Dr. Doug MacFarlane is currently Australian Laureate Fellow and Professor at the School of Chemistry at Monash University (https://www.monash.edu/science/schools/chemistryold/our-people/staff/macfarlane). Photo courtesy of Monash University.

(1) Could you comment on the importance of sustainable ammonia production and the promise and shortcomings of electrocatalytic nitrogen reduction to address this need? Ammonia production from sustainable energy sources has two main contexts, and in both cases the need is quite urgent in the bigger picture of transitioning away from fossil fuels. The first context involves the development of a production process from renewables for ammonia as a fertilizer source. Currently, ammonia- and nitrate-based chemical fertilizers make up a substantial fraction of fertilizer supply worldwide. These have 1435

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NRR process. This is not to say that the Li+-mediated NRR processes that have been described by a number of groups are not workable; these mechanisms probably involve in situ production of a surface coating of Li3N, followed by its reactive removal. In other words, nitrides as a transient or intermediate species may be viable but not as stable long-lived electrocatalysts. More broadly, these studies underline the need for careful confirmation of the source of the nitrogen that is detected as ammonia in NRR research. Not only the nitrogen-containing catalysts, such as the nitrides studied here, need to be treated with caution. Many catalysts contain nitrogen in their substrate structures; others are prepared from nitrate or ammonium precursors. Beyond these obvious sources, ammonia and NOx compounds including nitrates are common contaminants in the laboratory environment and gas supplies. Nitrates are readily reducible under NRR-type conditions. Even laboratory gloves can be a significant source of nitrate contamination. Without question, the 15N2 experiments described by Hu et al.4 here should be considered to be a vital component of any reliable NRR study. It is also vital to remove 15NH3 and 15NOx species from the 15N2 supply, and to confirm that these are at an acceptably low level, in order that such “proof of NRR” tests are valid. (3) How much of a challenge is catalyst selectivity for NRR vs HER? What can be done to improve selectivity and how much progress has been made? Selectivity versus HER is undoubtedly a key requirement of a successful NRR electrocatalyst. Currently, there are no stable catalyst materials that have been clearly proven to produce better than about 20% Faradaic efficiency, the remainder being HER. Even if some value can be extracted from the hydrogen produced, after separation, it is relatively expensive hydrogen in energy terms. This adds an additional challenge to making the NRR process viable from a technoeconomic point of view. One approach to combatting H2 generation is to move to aprotic media such as aprotic solvents and/or ionic liquids, as demonstrated in recent publications from our group. The challenge in this area is to optimize the proton source concentration to be just sufficient to satisfy the needs of the NRR process; too high a concentration and H2 begins to dominate, too low and proton supply becomes limiting to the rate of NRR. (4) Would you care to share any current or future directions your lab may be working on to advance this field? Another approach to minimizing HER versus NRR is to carefully engineer the nanolevel design of the conductive support layer to have a high overpotential for the HER process and then to decorate the surface with an optimized surface concentration of catalyst nanoparticles. This was part of the approach behind our ACS Energy Letters paper earlier this year,6 the polymorphic engineering of MoS2 in that case providing a low HER activity surface on which Ru nanoparticles were decorated to provide the NRR active sites. This type of surface structure design and optimization almost certainly offers the promise of successfully tipping the balance away from HER in favor of NRR, and its many possibilities provide numerous avenues for researchers to fruitfully pursue in the future.

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AUTHOR INFORMATION

Notes

Views expressed in this Energy Focus are those of the author and not necessarily the views of the ACS. The author declares no competing financial interest.

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REFERENCES

(1) 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. (2) 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. Nature Catalysis 2019, 2, 290. (3) Minteer, S. D.; Christopher, P.; Linic, S. Recent Developments in Nitrogen Reduction Catalysts: A Virtual Issue. ACS Energy Letters 2019, 4 (1), 163. (4) Hu, B.; Hu, M.; Seefeldt, L.; Liu, T. L. Electrochemical Dinitrogen Reduction to Ammonia by Mo 2N: Catalysis or Decomposition. ACS Energy Letters 2019, 4 (5), 1053. (5) 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 Sustainable Chem. Eng. 2019, 7 (7), 6839. (6) Suryanto, B. H. R.; Wang, D.; Azofra, L. M.; Harb, M.; Cavallo, L.; Jalili, R.; Mitchell, D. R. G.; Chatti, M.; MacFarlane, D. R. Mo2S Polymorphic Engineering Enhances Selectivity in the Electrochemical Reduction of Nitrogen to Ammonia. ACS Energy Lett. 2019, 4 (2), 430.

Christina MacLaughlin, Development Editor, ACS Energy Letters

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