Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Review

Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf Antonio Jose´ Martı´n,1 Tatsuya Shinagawa,1 and Javier Pe´rez-Ramı´rez1,*

The electrocatalytic nitrogen reduction reaction (eN2RR) is an emerging route complementing one of the pillars of the chemical industry, the Haber-Bosch (HB) process. Its flexibility expands suitable operating conditions from highly pure nitrogen and hydrogen streams and high temperature to very mild ones, such as air and water at ambient conditions, which can expand the ammonia synthesis toward the on-site production of carbon-neutral fertilizers when powered, for instance, by sunlight (ammonia artificial leaf). This review uses performance maps to (1) provide a bird’s-eye view of the gap separating it from practical implementation and (2) identify sources of inefficiency, mostly associated with the reduced ability of available catalysts to operate with high energy efficiency. In addition, we discuss basic aspects influencing the design of an ammonia artificial leaf and comment on future directions.

INTRODUCTION In a quaint twist of history, the first artificial synthesis of ammonia might have occurred powered by electricity during electrocatalytic experiments carried out by Humphrey Davy1 as early as in 1807 (although Lord Rayleigh was unable to reproduce them2), at a time when characterization techniques were virtually non-existent and variations of the electrolyte sourness upon reaction were given as experimental evidence. The first related patent was filled in 1898,3 whereas Fichter and Suter produced and reliably quantified it for the first time in 1922.4 However, the electrocatalytic nitrogen reduction reaction (eN2RR) was largely abandoned in favor of the thermal route in the early twentieth century. The Nobel prizes awarded to Fritz Haber (1918), Carl Bosch (1931), and Gerhard Ertl (2007) recognized the impact that the Haber-Bosch (HB) process (for producing ammonia from nitrogen and hydrogen) has in sustaining a growing human population, after making possible the large-scale production of artificial fertilizers used in ca. 50% of crop yields.5 Such an instrumental industry necessitates ca. 2% of the global supply of energy6 to provide the reactors with high temperature (400 C–500 C), high pressure (100–200 bar), purified nitrogen, and, critically, hydrogen. We emphasize the need of hydrogen as the HB process consumes ca. 50% of its global production, which is nowadays largely based (>95%) on transformations of fossil resources.7 As a result, approximately 2 tons of carbon dioxide accompanies the production of each ton of NH3, which represents around 1% of total emissions of the greenhouse gas.8 In addition, the recently discovered non-conventional wells of natural gas tend to concentrate in remote zones, suggesting that transportation costs would counteract the current low price of natural gas.9 On a different note, HB plants are economically feasible only on the scale of thousands of metric tons per day10 and can transform around 60% of the fossil-based

The Bigger Picture The manufacture of nitrogen-rich fertilizers is one of the main pillars supporting an increasingly populated planet, which, in turn, relies on the ability to produce ammonia on a massive scale. The largely optimized Haber-Bosch process dominates this centenarian industry despite requiring high pressure and temperature, purified N2, and critically, H2 produced currently almost exclusively from fossil sources. In this review, we aim to provide a systematic view of the highly appealing—yet still in early development—electrocatalytic routes. Their less constrained operating conditions and natural coupling with renewable energy sources open the door to a more sustainable alternative. Its realization could facilitate the onsite production of fertilizers with a neutral carbon footprint, hence directly addressing two of the UN Sustainable Global Goals, namely Zero Hunger and Climate Action.

Chem 5, 1–21, February 14, 2019 ª 2018 Elsevier Inc.

1

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

energy input into ammonia.7 These facts depict the boundaries of the successful HB process, namely the dependence on fossil resources, harsh operating conditions, and rigidity in the scale. Consequently, we can visualize a complementary process entirely driven by renewable energy at ambient conditions with water and air acting as reagents and suitable for small-scale production. On account of the parallelism between the photosynthetic process and the herein considered on a nitrogen basis, we propose a term, ‘‘ammonia artificial leaf’’ (NH3-leaf), to describe a device able to produce ammonia from air, water, and sun. Developing the palette of processes between the HB process and the NH3-leaf is one of the most fascinating scientific challenges with major societal and environmental implications currently ongoing. This task is being tackled from many different perspectives that can be classified according to the form in which the energy is supplied, but all share the need of a catalyst to activate the nitrogen-nitrogen bond. Efforts on thermo-, photo-, and electrocatalytic processes and combinations thereof are quickly catching up with the vast optimization literature on the HB process, although their applicability still seems distant.7,11,12 Strikingly, the route whereby ammonia is synthesized in diazotrophic microorganisms develops at ambient conditions in nitrogenase enzymes, albeit at least 25% of the energy is directed toward production of hydrogen.13,14 The nitrogenase enzymes are composed of two proteins, one of them acting as an oxidizing unit (Fe protein) where ATP is oxidized,14 and the reducing unit where electrons coming from the Fe protein and available protons from the medium are transferred to nitrogen in a metal-centered catalytic cofactor. The similarity of the electrocatalytic route—where electrons flow from the anode to the cathode—with this scheme and some recent progress in its understanding are largely inspiring the efforts currently devoted to mimicking Nature in electrochemical cells over a wide range of temperatures, as recently summarized in a number of reviews describing developments in cell configurations15,16 and/or catalysts.11,14,17,18 They all conclude that the potential of the flexible electrocatalytic route to emerge as an alternative to the HB process is intact, albeit poor catalytic and electrical performance of available materials veto its upscaling. Beyond this general picture, however, quantitative descriptions of (1) the gains in terms of efficiency required, (2) limiting phenomena, and (3) most promising families of materials and/or strategies are lacking.19 With this in mind, we analyze the development of the variety of electrocatalytic alternatives operating under conditions, from those close to HB (N2 and H2 as feedstocks and high temperature) to those that can be found in an NH3-leaf (room temperature, air, and water) upon extensive literature survey. In a sustainable framework, water appears as the most advisable source of hydrogen atoms in the ideal scenario, albeit other more complex alternatives may be envisaged, such as molecular hydrogen obtained from biosources or water splitting powered by renewable energy. Our approach here allows a fair energetic comparison among different technological solutions and the identification of limiting phenomena in a more global context. These analyses are expressed as performance maps, which we also applied to shape the state of the art of available catalytic materials and associated knowledge. We conclude with some remarks on the design of an NH3-leaf. All in all, we aspire to convey a clear picture of the distance separating the progressive electrification in the real world of the ammonia synthesis, which might curiously parallelize—with some delay—the shift in the gravity center between electric and thermal engines currently under way in terrestrial vehicles after ca. 120 years of development.

2

Chem 5, 1–21, February 14, 2019

1Institute

for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.10.010

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

SETTING A BASIS FOR COMPARISON FOR eN2RR PATHWAYS Our first goal was to provide the reader with a bird’s-eye comparison among reported eN2RR routes and a clear perspective of the improvement required to comply with practical requirements. We chose the energy efficiency and the current density (correlated with the reaction rate of ammonia production) as two representative parameters to determine the global status of this technology. Notably, implementations for the eN2RR vary drastically depending on the operating temperature, use of H2O or H2 as proton sources, or solid/liquid electrolyte, among other factors. This fact largely hinders direct comparison among routes and is the origin of the variety of classifications used to cluster reports (for example by temperature,16 proton source,15 or catalyst nature11,14). With this idea in mind, we surveyed the open literature and defined a common set of conditions under which the energy efficiency of all reports would be evaluated. More specifically, we considered the amount of energy required to carry out the eN2RR as the sum of the following (detailed explanations can be found in the Supplemental Information):  Energy consumed in the electrocatalytic cell, consisting of the sum of the thermodynamic value, the energy dissipated because of non-perfect catalysis (related to kinetic overpotential), and the Joule effect caused by ohmic resistance: this value is directly calculated from the reported cell voltage or estimated in every report.  Energy required to produce the consumed H2. We added, when necessary, this energy by assuming its production by a state-of-the-art water electrolyzer, which shows a high energy efficiency of circa. 70%.20 This term allows direct comparison among routes using H2O and H2, since the energetic implications are considerable. The efficiency X-to-power (X = heat, wind, light.) associated with the source of electrical power is not considered.  Energy required to raise temperature of the reagents above 25 C. This approach allows a first estimation of the influence of using high temperature on the overall energy expenditure. The energy efficiency was thus calculated by comparing the sum of these three components with the thermodynamic value of DH = 22.5 MJ kgNH3 1 associated with the reaction 1/2N2 + 3/2H2O = NH3 + 3/4O2. This approach, based on the environmental implications of the eN2RR, thus sets as base conditions that protons are provided, directly or indirectly, by water and that all energy inputs have an electrical nature, given the easy coupling of renewable energies with electrocatalytic processes. In Figure 1 each contribution is quantified for a group of representative reports with high energy efficiency operated at high temperature using H2 as proton source. A first inspection clarifies that the energy used to produce ammonia from hydrogen and nitrogen in the electrocatalytic cell (gray contribution) is similar to the one required to obtain the consumed molecular hydrogen (magenta), and, in turn, of the same order of 20 MJ kgNH3 1. The thermal contribution (red) is noticeably less important. From this, the relevance of considering the proton source in the general problem of the eN2RR framework is ostensible as well as the relative importance of each component. One must consider that the aforementioned energy efficiency of the HB process of ca. 60% is calculated starting from natural gas (it requires ca. 30.4 MJ kgNH3 1) and thus cannot be directly compared with eN2RR routes. When one considers the hybrid system coupling the hydrogen production from electrolytic water splitting with the thermocatalytic step used in the HB process, i.e., we apply the

Chem 5, 1–21, February 14, 2019

3

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Figure 1. Contributions to the Energetics of the eN2RR from H2O and N2 at High Temperature Contributions to the total energy needed to synthesize NH 3 from N 2 and H 2 O for representative reports operating at high temperature starting from H 2 and N 2 . The x values of black dots represent the energy used in the electrochemical cell (gray zone). The magenta contribution accounts for the energy needed to produce the required amount of H 2 in a water electrolyzer, and the red contribution arises from the thermal energy required to elevate the temperature of the reactants to the operating conditions. The void dots thus represent points directly comparable. The minimum value of DH  = 22.5 MJ kg 1 for the total energy is indicated for reference.

same criterion as in the case of eN2RR, the energy efficiency drops to ca. 56% (ca. 40 MJ kgNH3 1).7 We thus conclude that any eN2RR system entirely driven by electricity must surpass this figure to be competitive with this also sustainable hybrid facility.

THE PERFORMANCE LANDSCAPE OF THE eN2RR Within this framework, we mapped performance in the eN2RR keeping the distinction among different temperatures as the clustering criterion to facilitate the interpretation of the results. A general view of the field is provided in Figure 2A, where the current density is compiled as a function of the energy efficiency for each temperature range, as split into (1) the high-temperature route (above 500 C), (2) the intermediate-temperature route (100 C–500 C), and (3) the low-temperature route (below 100 C). The target set by the United States Department of Energy (DOE) is highlighted in the top-right corner of the figure, which is the predicted performance required for the eN2RR to be competitive in a centralized production scheme, i.e., to directly compete with the HB process.21 As easily observable, reports concentrate at the bottom-left corner, far from the set DOE target (note the logarithmic scale in the current density), thus clearly conveying the large room for improvement existing in transforming electrical into chemical energy (energy efficiency), and the modest scale of the reaction rates, mainly a direct reflection of the early stage of development of catalytic materials. An equivalent map showing total production rates of NH3 is shown in Figure S1, where this aspect is even more clearly highlighted. A closer look at the map based on the temperature classification sheds additional light. The red-colored region (high temperature; above 500 C) appears horizontally in the middle of the figure, revealing the narrow range of current densities available irrespective of the efficiency, which, in some cases, approaches the practical threshold. On the other hand, blue-colored dots (low temperature; below 100 C) do not surpass reaction rates observed at high temperature and are mostly clustered at low energy efficiencies (<20%). Notably, the more scattered orange-colored

4

Chem 5, 1–21, February 14, 2019

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Figure 2. Performance Maps of the eN2RR for Each Temperature Regime The current density (i.e., the reaction rate) and its corresponding energy efficiency are plotted for each temperature range in (A), while the energy efficiency is compiled as a function of the temperature in (B). In (A), the target set by the DOE is highlighted in the green-colored region. In (B), the energy efficiency for a mixed water splitting-HB process is indicated by HB H2O (gray zone). The intensity of the background color is positively correlated with the density of data points, as obtained by a kernel density analysis. Note that the robustness of statistical analysis increases with the population size. Bibliographic references and relevant data for each report are provided in Table S1. See also Figures S1 and S2.

group of dots (intermediate temperature; 100 C–500 C) seems to retain the ability to offer high energy efficiency while exhibiting high current densities, though not simultaneously. Looking at these data from a different angle suggests that there exist a variety of underlying factors determining the performance. The impact of the operating temperature is more apparent when it is plotted versus the energy efficiency (Figure 2B). The label HBH2O indicates the performance of the hybrid thermo-electrocatalytic system. A simple look to the figure makes it clear that the vast majority of purely electrically driven eN2RR routes reported to date are at a disadvantage with regard to the hybrid alternative. A closer inspection reveals that the highest efficiencies reported to date are positively correlated with the temperature, thus reinforcing its role as a significant parameter. This fact is compatible with a field where catalysts are not well developed, since higher temperatures reduce the need for highly active catalysts. Nonetheless, we must note that for the vast majority of reports there is no correlation, also compatible with a field dominated by catalyst screening but also suggesting other limiting factors at work. In fact, the type of fed reactants is one of the major sources of performance variations in the N2-fixation system. Ammonia synthesis by the HB process operates from a pure N2 and H2 feed, while the ideal NH3-leaf will produce ammonia from air and H2O upon sunlight illumination. These two extremes put four different combinations of nitrogen and hydrogen sources as suitable, namely H2/N2, H2O/N2, H2/air, and H2O/air, which generally develop according to the configurations and reactions in Table 1. Our performance maps visibly capture the chasm created by these combinations (Figure 3). The choice of the hydrogen source (i.e., H2 versus H2O) creates a gap separating their performances because of the anodic half-reaction and/or the charge-carrier transport, particularly at low and high temperatures. In the low-temperature regime, the use of H2O seems to confine reports to the bottom-left corner of Figure 3 as a result of the sluggish kinetics of the water oxidation reaction

Chem 5, 1–21, February 14, 2019

5

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Table 1. Feed Reactants and Half-Reactions in the Electrocatalytic Synthesis of Ammonia Reactants

Anode

Cathode Reactant

Reactiona

H2 / 2H + 2e

N2 or air

N2 + 6H+ + 6e = 2NH3

H2O

2H2O / O2 + 4H+ + 4e

N2 or air

N2 + 6H+ + 6e = 2NH3

none

2O2 / O2 + 4e

H2O and N2 or air

3H2O + N2 + 6e = 3O2 + 2NH3

Reactant

Reaction

H2/N2 or air

H2

H2O/N2 or air H2O/N2 or air

+

a Potential parasitic cathodic reactions are, where applicable, the hydrogen evolution reaction (HER; 2H+ + 2e = H2) and oxygen reduction reaction (ORR; O2 + 4H+ + 4e = 2H2O or O2 + 2H+ + 2e = H2O2).

(i.e., oxygen evolution reaction with H2O as the hydrogen source) relative to the hydrogen oxidation reaction (H2 as the hydrogen source). Similarly, the use of H2O as the hydrogen source at high temperature brings the same effect, albeit the small number of available data points makes any discussion tentative at this point. In the intermediate temperature range, however, the difference brought by the nature of the hydrogen source is not easily identifiable, most likely because of the different nature of some of the processes proposed in this range, as explained below. With regard to the nitrogen source, systems with air have only been examined at low temperatures, with performances characterized by poor energy efficiency and low current densities. This fact points to the significance of the coexistent O2 in the N2 stream, likely resulting from the kinetically disfavored smaller N2 fugacity as well as its possible side reaction (O2 reduction; see Table 1). Therefore, although the use of H2O/air for the electrochemical production of ammonia is the most desirable option, the state of the art discourages its consideration as a short-term objective and gives it a ‘‘holy grail’’ character. Parallel maps expressed in terms of total production of NH3 (a commonly reported value in the literature) spanning an extended range of energy efficiencies can be found in Figure S2.

LIMITING FACTORS AND CURRENT STATUS OF UNDERSTANDING IN THE eN2RR After analyzing critical parameters from the technological implementation standpoint, such as temperature and reactants, this section deepens the pursuit of the origin of performance inefficiencies. Indeed, by looking at Figure 2 one can notice significant diversity in the performance present for each combination of the reactant feed beyond detected regularities. We thus took one step ahead to assess the performance gap imposed by the state-of-the-art catalysts and modes of operation. We analyzed the selectivity toward ammonia (faradic efficiency) and excess potential with regard to thermodynamics (overpotential) and looked for relations in each temperature regime. Although the limiting phenomena in electrocatalytic devices such as fuel cells typically drift from materials design to kinetics as the operating temperature decreases,22 additional sources of inefficiencies can be found in this particular case, such as the decomposition of ammonia at high temperature. The High-Temperature (>500 C) Route Reported systems operating at high temperature also share with the HB process a preference for using H2 as reactant (Figure 3). In the high-temperature regime the proton-conductive solid electrolyte is the common choice, where the migration of proton connects reactions described in the first two rows of Table 1. Under these conditions, limitations are expected to surge from poor electronic and/or ionic

6

Chem 5, 1–21, February 14, 2019

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Figure 3. Performance Maps of the eN2RR for Each Reactants Feed The current density and its corresponding energy efficiency are plotted for different reactant combinations in the low-temperature regime (below 100  C) in (A), in the intermediate temperature regime (100  C–500  C) in (B), and in the high-temperature regime (above 500  C) in (C). Energy efficiencies above 100% are explained by the NEMCA effect (see Figure 4 and its associated discussion). The intensity of the background color is positively correlated with the density of data points, which were obtained by a kernel density analysis. Bibliographic references and relevant data for each report are provided in Table S1. See also Figures S2 and S3.

conductivity in the catalyst layers and/or the electrolyte, the recombination of protons in the cathode (HER), the ammonia decomposition, and kinetics. The exploration of catalysts has been largely limited so far to combinations of silver and/or palladium in bulk form23–25 or oxides under bulk26 or doped or modified27,28 form. The performance map is provided in Figure 4A. Although differences in the performance among catalyst classes seem apparent, no clear trend or clustering can be observed for each catalyst family, implying that the surface catalysis cannot be the sole performance descriptor. A closer look finds acceptable faradic efficiencies at relatively low overpotentials between 0.5 and 1.2 V, as expected from high energy efficiencies in Figure 2. In this regard, the scarcity of dots in the map reflects a fairly unexplored field, albeit a considerable number of reports could not be included because of incomplete reporting of electrocatalytic performance (see Table S1). In turn, this fact is largely due to the use of solid electrolytes, whose conductivity is postulated to be the other major performance-determining factor. A recent study29 points to the high significance of proton conductivity in known materials, which was positively correlated with the performance. In this context, a number of studies focused on developing optimized perovskite-based30–32 or fluoritebased23,24,33 electrolytes. This partly explains the modest current densities reported at reasonable overpotentials (Figure 2A), given the large voltage drops under these circumstances. In this regard, recent developments in protonic solid electrolytes have partially removed this limitation. Specifically, a highly conductive BaZr1 x yCexYyO3 demonstrated an overall reaction rate as high as on the order of 10 9 mol s 1 cm 2 toward ammonia,34,35 which, however, was accompanied with an appreciable faradic efficiency toward H2 (up to 60%),34 possibly highlighting the significance of the parasitic HER and/or ammonia decomposition. In a different approach, cells where the electrolyte and the cathode share the same nature are promising candidates for high performance, as exemplified in Figure 4B (asterisklabeled dot in Figure 4A), where the perovskite nature of both components led to

Chem 5, 1–21, February 14, 2019

7

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Figure 4. Performance Map of the eN2RR in the High-Temperature Regime The faradic efficiency and its corresponding overpotential are plotted in the high-temperature regime (above 500  C) in (A) for representative reports labeled according to the catalyst used. The intensity of the background color is positively correlated with the density of data points, which were obtained by a kernel density analysis. Bibliographic references and relevant data for each report are provided in Table S1. See also Figure S5. Shown in (B) is the evolution of the faradic efficiency with temperature and cross-section for a cell where the solid electrolyte and the cathodic and anodic catalysts share the perovskite structure. Data are extracted from the report labeled with an asterisk in (A). Panel (B) is adapted from Wang et al. 26

remarkable performance over a wide range of temperatures. Altogether, our discussion here suggests that the development of active and selective surface catalysts and highly conductive protonic solid electrolytes, as well as the construction of their effective interface, are the keys to improving the performance. In parallel, the eN2RR finds an additional limitation in the thermodynamic instability of ammonia at high temperature, which is reflected as a loss of faradic efficiency. In the temperature range considered in this subsection, the Gibbs free energy of ammonia formation is noticeably positive (>70 kJ mol 1; Figure S4), meaning that the decomposition of ammonia into hydrogen and nitrogen is thermodynamically favored (the equilibrium composition of ammonia is well below 1%). Some reports estimate a loss of up to 40% of the reacted hydrogen resulting from the back reaction.26,30 This thermodynamic constraint calls for strategies to minimize the undesired back reaction, such as quenching stages and/or quick removal of NH3. Noticeably, a parallel functioning of the electro- and thermal routes has been reported, which makes the evaluation of the electrocatalytic contribution harder. Note that in some systems, faradic efficiencies higher than 100% appear (Figure 4A and Table S1), where an effect is in action, called non-faradic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion of catalysis (EPOC), originally reported by Vayenas and coworkers.36 According to the proposed mechanism, the spillover of charged species (e.g., H+) on the catalyst surface directly influences the surface potential of the catalysts, which controls the activation energy and in turn changes the reaction rate, being completely distinct from the electrocatalytic process that relies on the reduction-oxidation reaction (faradic process). Furthermore, the supply of the positively charged species to the catalyst surface presumably disfavors the electrophobic ammonia decomposition, leading to the increase of the overall

8

Chem 5, 1–21, February 14, 2019

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Figure 5. Performance Map of the eN2RR in the Intermediate-Temperature Regime The faradic efficiency and its corresponding overpotential are plotted in the intermediate-temperature regime (100  C–500  C) in (A) for representative reports separated in modes of operation. The intensity of the background color is positively correlated with the density of data points, which were obtained by a kernel density analysis. Bibliographic references and relevant data for each report are provided in Table S1. See also Figure S5. In (B), the lithium-mediated cycle for ammonia production is composed of electrochemical and thermal-based reactions to achieve high selectivity. The scheme is extracted from the report labeled with an asterisk in (A). Panel (B) is reproduced from McEnaney et al. 42 with permission from the Royal Society of Chemistry.

reaction rate of the ammonia synthesis.28 Its detailed description falls outside of our scope, and the reader is referred to the extensive available literature.37 In summary, efficient eN2RR at high temperature critically depends on a complex interplay among catalytic activity, selectivity, and materials conductivity, accompanied with a large influence of thermally driven processes such as thermocatalytic side pathways or ammonia decomposition. Penalized by low current densities, more efforts are required in the use of water as proton source to upgrade its potential as a complementary technology to the HB process on small scales. The Intermediate-Temperature (100 C–500 C) Route In this range of temperatures, most of the effects described at high temperature are still at work. In accordance, cell configurations resembling those at higher temperature have been repeatedly tested. Nevertheless, cells based on liquid electrolytes are also under examination, together with other ingenious schemes where the role of the electrocatalytic route is to regenerate a mediating compound. This picture corresponds to a vibrant field where incipient approaches compete. The use of solid electrolytes has also been the most frequently reported option in this temperature route, which, however, has not witnessed its success so far. In addition to the protonic solid oxides detailed in the previous section, cells based on oxide-ion conductive solid electrolytes have been employed,38 which rely on redox reactions described in the third row of Table 1. The performance map in this case (Figure 5A) captures a large clustering of data points in the low-performance region for these solid-electrolyte-based systems, irrespective of the type of charge carrier. Sharing the limitations in conductivity described at high temperature, the expectedly larger influence of the electrokinetics as the temperature decreases

Chem 5, 1–21, February 14, 2019

9

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

demands more efficient catalysts to improve the selectivity in this approach. Precious metals, with special attention to ruthenium-based compositions39,40 and mixed oxide38,41 catalysts, fill the catalog of systems examined. In this context, another class of electrolyte had emerged facilitating more satisfactory results, as easily deduced from Figure 5A: molten electrolytes, which being liquid presumably exhibit higher ion conductivity and thus possess a potential to achieving higher overall performance while allowing for the operation at lower temperatures. One successful example was reported in 2014,43 where a molten NaOHKOH was employed as an electrolyte where nanosized Fe2O3 was dispersed, achieving a faradic efficiency of 35% toward ammonia from N2 and H2O at 200 C. The oxide ion was presumed to function as the charge carrier. In another example of the potential of this approach, producing nitride ions at the cathode as the charge carrier in molten eutectic at 400 C44,45 led to an appreciable faradic efficiency (72%) with an energy efficiency above 50% (Figure 3B),44 being one of the highest reported. The following reactions are proposed to be at work in such systems: oxidation: 2N3 + 3H2 /2NH3 + 6e ;

(Equation 1)

reduction: N2 + 6e /2N3 :

(Equation 2)

A step ahead in the optimization of molten eutectic-based systems, a lithium-mediated cycle was proposed in 2017 (reproduced in Figure 5B),42 which achieved nearly 90% faradic efficiency with impressive current densities surpassing 450 mA cm 2 (Figure 3B), though at the expense of lower energy efficiencies. The cycle comprises LiOH electrolysis to produce metallic Li (Equation 3), direct reaction of metallic Li with N2 to form Li3N (Equation 4), and release of NH3 by reaction with H2O (Equation 5): 4LiOH/4Li + 2H2 O + O2 ;

(Equation 3)

6Li + N2 /2Li3 N;

(Equation 4)

Li3 N + 3H2 O/3LiOH + NH3 ;

(Equation 5)

where the overall reaction is 2N2 + 6H2 O/4NH3 + 3O2 . Note that by carrying out the series of described reactions in separated vessels, the possibility of competitive undesired reactions such as the HER is circumvented in all steps, facilitating the high faradic efficiency reported. The contact of the metallic lithium with air thus becomes the only source of inefficiency in this respect, which demands handling conditions non-ideal from the practical point of view. The other source of inefficiency arises from the overpotential required to obtain metallic lithium—a minimum of ca. 2 V—which thus sets a clear target for future improvements. A preliminary economic analysis carried out by the authors suggests that very cheap prices for electricity ($0.02 kWh 1) could make this technology competitive with HB. From this overview of the intermediate-temperature range, we can conclude that, compared with the high-temperature-operated systems, a decrease of the operating temperature allows a larger flexibility in terms of configurations able to achieve high selectivity toward ammonia, very reasonable current densities, and, in principle, more convenient operating conditions. We note that, as in the case of high temperature, NEMCA-type systems have also been reported46 (see Figure 5A, sitting at the top of the map), which adds versatility, albeit the decomposition of ammonia still prevails thermodynamically at these temperatures (Figure S4). The main limitations in the promising molten eutectic-based strategies are associated with the need of

10

Chem 5, 1–21, February 14, 2019

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Figure 6. Performance Map of the eN2RR in the Low-Temperature Regime The faradic efficiency and its corresponding overpotential are plotted in the low-temperature regime (<100  C) in (A) for representative reports labeled according to the catalyst used. The intensity of the background color is positively correlated with the density of data points, which were obtained by a kernel density analysis. Bibliographic references and relevant data for each report are provided in Table S1. See also Figure S5. Shown in (B) is the ionic-liquid promoted system based on an Fe catalyst proposed by MacFarlane and co-workers. 52 The structure of the ionic liquids used and their interaction with N 2 are represented. The panel is extracted from the report labeled with an asterisk in (A) and reproduced from Zhou et al. 52 with permission from the Royal Society of Chemistry.

large overpotentials, making energy efficiency values drop at high current densities (see Figure 2B), and with the presumable lack of stability in long-term operation. Nevertheless, the number of investigations on the molten-eutectic-based system is still very limited (see Table S1). For example, catalytic materials are mainly limited to Ni43,44,47 and stainless steel,42,48 and no engineering efforts are yet present in current cell designs, which makes it reasonable to think optimistically about the near future. The Low-Temperature (<100 C) Route The synthesis of ammonia at a low temperature is obviously preferred from the point of view of the equipment and operational costs, and is also favored by thermodynamics, since below 100 C ammonia is the predicted stable phase. At temperatures lower than 100 C water can be employed as the liquid electrolyte at the ambient pressure, which functions as a proton carrier connecting the redox half-reactions. It is thus not surprising that low-temperature and aqueous-electrolyte conditions have attracted the largest amount of efforts so far, although the solid protonic conductor Nafion has been the choice for the catholyte on occasions.49,50 However, this is unfortunately in inverse relation to reported performances. At low temperature, the main expected limitation lies in electrokinetics, as is generally the case for fuel cells22 or water electrolyzers.20 This is readily confirmed by the performance map in Figure 6, where most data points appear near the x axis irrespective of the catalyst type, visualizing an almost invariably low faradic efficiency (<10%). Such regularity suggests an underlying general obstacle, which takes the form of the side reaction of hydrogen formation, albeit the formation of hydrazine (N2H4) could

Chem 5, 1–21, February 14, 2019

11

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

potentially become a competitive pathway. The usually kinetically facile proton coupling outperforms the more complex eN2RR for reasons computational chemistry has partially shed light upon (see next section). The main constraint in this case is thus the selectivity in aqueous environments.51 The catalog of catalytic materials that has been investigated is mainly composed of transition metals, most of them noble metals such as Pt,49 Pd,53 Rh,54 or Au,55 although inspired by the catalysts used in the HB process, Fe56–58 and Mo56,59 are also under focus. Only very recently the families of materials have expanded to nitrogen-doped carbon60 and multicomponent ones where Au nanoparticles strongly interact with non-reducible oxidic supports and results in improved performance.61,62 In comparison with other electrocatalytic reduction processes of small molecules such as O263 or CO2,19 the variety of materials screened so far is remarkably small, pointing to the incipient stage this appealing electrocatalytic process finds to date. As mentioned, faradic efficiencies remain low (<10%) in all cases except at impractically large overpotentials (see cluster at around 5 V) and for some outliers with faradic efficiencies up to 60%. These recent reports address the selectivity problem from different perspectives; one approach is to develop an electrocatalyst composed of HER-inactive materials that in turn has a potential to be eN2RR selective. This strategy was recently demonstrated using oxide-Au catalysts, which achieved faradic efficiencies of around 10% in aqueous environment rationalized by the formation of amorphous metallic particles62 and the positive polarization of the Au surface in proximity to the oxide favoring N2 anchoring.61 Another approach is the use of (near) aprotic solvents,51,64 among which ionic liquids are appealing candidates to suppress the HER, as already demonstrated, for example, in the electrocatalytic reduction of CO2.65 Ionic liquids can realize in the vicinity of the active site (1) low water content, thus suppressing the HER; (2) a larger concentration of the reactant given the higher N2 solubility, which both kinetically and thermodynamically favors the eN2RR; and (3) stabilization of intermediates, thus altering the energetic landscape of the reaction. In 2017 MacFarlane and coworkers52 achieved by using this strategy (see Figure 6B) as high as 60% faradic efficiency at ambient temperature and pressure at low overpotentials—at the expense of very modest current densities associated with transport problems—sitting at the top of the performance map. A follow-up study by the same authors partially overcame this limitation by using an aprotic fluorinated solvent-ionic liquid mixture in which the water content and cathodic catalysts were initially optimized to obtain ca. 30% faradic efficiency and current densities one order of magnitude higher.58 Even though the high cost of systems based on ionic liquids puts some burden on its potential for practical applications, understanding them might facilitate its replication in more practically oriented systems. We now briefly describe homogeneous molecular catalysts, whose implementation is favored by the mild conditions associated with the low-temperature route. The operation of these systems present differences with regard to their heterogeneous counterparts, since in general they require a reducing agent and a proton source present in the reaction medium. In parallel, since the potential is not directly applied to the catalyst and, thus, the electric current is not directly measured, their direct comparison with other classes of catalysts in the current review might be misleading, and hence we have not included these in the performance maps (data from selected reports are nonetheless included in Table S1). The discovery in 1965 of a ruthenium complex capable of coordinating N2 triggered large interest including the opening of entire institutes in countries such as Russia,66 although it was not until 2003 that the first molecular catalyst able to reduce nitrogen into ammonia under ambient

12

Chem 5, 1–21, February 14, 2019

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

conditions was reported.67 Molybdenum as the metal center with tetradentate [HIPTN3N]3– triamidoamine ligands produced approximately 8 equiv of NH3 relative to the metal center mediated by a reducing agent. The other Mo complexes reported to date include those from Nishibayashi and coworkers,68,69 achieving over 11 equiv of NH3. In the last decade, among others, the catalog of metal centers has expanded to Fe,70,71 yet at very low temperatures ( 78 C). Among the advantages of molecular catalysts, it can be mentioned that the large degree of control over the geometrical and chemical environment around the metal center by careful selection of the ligands—which finds a clear synergy with research on nitrogenase enzymes—alongside the promising selectivities reported of up to 72%.14 On the other hand, the separation of the ammonia product, the catalyst, and the reaction medium becomes a non-negligible issue for its practical application. The reader can find more detailed descriptions of these catalysts in other studies.72 All in all, the design of an electrocatalytic system (in combination with the surrounding electrochemical environment) able to simultaneously facilitate the eN2RR at low overpotentials while hindering the parasitic HER in aqueous media remains the largest challenge to large-scale application. Often overlooked, other relevant consideration is the separation of anhydrous ammonia from the aqueous medium, adding technological complexity downstream.

EFFORTS TOWARD MECHANISTIC UNDERSTANDING OF THE eN2RR The experimental picture depicted in the previous section shapes a field where a significant gap is present separating it from practical implementation, in large part attributable to a lack of accessible performance descriptors (i.e., catalyst design principles). Even though the revival of the interest in the low-temperature eN2RR was largely triggered by Nørskov and coworkers73 in 2000 by suggesting the lowtemperature electrochemical route to mimic the FeMo cofactor in the nitrogenase enzyme, the level of mechanistic understanding is at a distance from the insights available for the HB process.74 The reduction of a nitrogen molecule into ammonia demands the transfer of six electrons and protons to the nitrogen molecule. The possible network of intermediates is thus considerable; however, the first-principle density functional theory (DFT) calculations have emerged as a powerful tool to help elucidate the reaction thermodynamics for electrocatalysis. Assuming the concerted proton-electron transfer (CPET) in each elementary step, the energy levels of the possible surface intermediates can be computed.75 Not until 2012 did Sku´lason publish the first mechanistic study on associative nitrogen reduction, settling on the molecular model most widely accepted.76 In 2015, the reactivity of transition metals was theoretically explored based on the linear scaling relationships.77 As a result of these efforts, the current consensus establishes that the eN2RR toward ammonia is likely to happen following one of the routes depicted in Figure 7A. Numbers denote the order of the CPET, and arrows show to which nitrogen atoms each CPET is directed. In the dissociative path, the nitrogen molecule dissociatively adsorbs on the surface followed by alternating hydrogenation of both *N (where * denotes a surface site), producing two ammonia molecules. This pathway is favored over surfaces with highly negative N2 adsorption energies, such as those found in early-transition metals (Sc, Y, Ti, or Zr). The analysis of the energetic landscape predicts the formation of *NH2 (i.e., steps 3 and 4 in Figure 7A) to be the potentialdetermining step.76 At slightly milder dissociation energies, the rate-limiting step is presumably the dissociation of N2. In the associative alternating path, the nitrogen

Chem 5, 1–21, February 14, 2019

13

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Figure 7. Mechanistic Insights into the eN2RR Proposed reaction mechanisms in the literature are depicted in (A). The numbers in circles denote the order of the proton-electron transfer to the nitrogen designated by the arrow. For example, in the ‘‘alternating’’ path, an electron-proton pair will be first transferred to the outer nitrogen (1), whereas the second electron-proton pair will be transferred to the nitrogen directly bonding to the surface (2). In (B), the onset potentials for the eN 2 RR and the HER obtained by the DFT calculations are compared. Bibliographic references and relevant data for each report are provided in Table S1. See also Figure S6.

molecule retains its integrity upon end-on adsorption and undergoes alternating hydrogenations similarly to the dissociative case, whereas in the associative distal path the hydrogenation of the anchoring nitrogen atom starts only after release of the distal nitrogen atom. Over transition metals, the distal pathway is considered favored76 because of the large energetic barrier associated with step 2 of the alternating one. Over these materials, the potential determining step is predicted to be the formation of *N2H (step 1) for middle- and late-transition metals, whereas early ones find limitation in the formation of *NH2 (flat surfaces, step 5) or *NH3 (stepped, step 6). Volcano plots based on these limiting elemental steps predict Mo, Rh, and Fe as those metals with lower-onset overpotentials (ca. 0.4–0.5 V). A special case of the associative alternating mechanism, called enzymatic, starts from a side-on configuration of the adsorbed nitrogen molecule where both nitrogen atoms bind to the surface, and has been recently proposed for thermal synthesis of ammonia on Rh1Co3 clusters78 and over single-atom Mo supported on defective boron nitride under electrochemical conditions, with a thermodynamic onset overpotential of only 0.19 V.79 In parallel, Sku´lason proposed a different mechanistic pathway over transition metal nitrides based on a Mars-van Krevelen mechanism (labeled as MvK in Figure 7A) whereby ammonia evolves from surface nitrogen atoms, creating a vacancy, which is subsequently replenished by molecular nitrogen. The reduction of the newly available nitrogen surface atom to the second ammonia molecule completes the catalytic cycle.80,81 It is of utmost interest to compare the rich chemistry of the nitrogen reduction reaction toward ammonia in different systems with that occurring in the electrochemical environment. An attentive consideration of how the necessary redox process occurs and the role of hydrogen in each case expose fundamental differences that need to be reflected in the catalyst design. More precisely, in the case of the HB process, where one nitrogen molecule is reduced by three hydrogen molecules to form

14

Chem 5, 1–21, February 14, 2019

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

ammonia, the reaction proceeds via the dissociative mechanism over the state-ofthe-art promoted iron82 and ruthenium83 catalysts. The overall reaction rate is determined by the step of dissociative adsorption of nitrogen, which experiences subsequent hydrogenation from adsorbed neighbor hydrogen ad-atoms on the surface.82,83 In this regard, the oxidation of hydrogen and reduction of nitrogen happen simultaneously over adjacent atoms and there is no significant competing side reaction. A diametrically opposed process occurs over nitrogenase enzymes, over which an extensive research corpus develops to unveil the functioning of the most efficient one, the FeMo nitrogenase, composed of the Fe and FeMo proteins, which produces ammonia concomitant to molecular hydrogen according to Equation 6: N2 + 8H + + 16MgATP + 8e /2NH3 + H2 + 16MgADP + 16Pi:

(Equation 6)

The oxidation and reduction half-reactions take place in separate sites of the enzyme. In fact, the rate-determining step has been recently claimed to be the hydrolysis of ATP in the Fe protein (i.e., its oxidation to ADP and phosphate), which leaves open the question of the intrinsic catalytic activity of the FeMo unit.14 In a still ongoing debate, some key aspects of the underlying mechanism have nonetheless already been clarified, such as the intimate relationship of molecular hydrogen and ammonia formations. The reaction proceeds through an associative pathway driven by a complex and precise atomic configuration, where polypeptide chains around the FeMo cofactor avoid proximity of water molecules to the active site—presumably to maximize the selectivity toward ammonia—whereas the Fe atoms surrounding the Mo center possess different oxidation states and are likely to be the N2 adsorption centers.13,14,84 Though still looking at an incomplete puzzle, these pieces suggest the need of multifunctional active sites with exquisite geometric and electronic properties as well as an unavoidable link to the hydrogen formation. Homogeneous molecular catalysts are arguably less complex than the biological and heterogeneous systems and have provided very valuable information on general effects exerted over the N2 molecule by the enzyme structure. From theoretical and experimental evidence on nitrogenase mimic complexes, the occurrence of an associative pathway (whether distal or alternating remains unclear, see Figure 7) is confirmed, alongside some light shed on the chemical rationale behind the structure of the FeMo cofactor. This structure seems to be mostly directed to trap and facilitate the weakening and cleavage of the N–N bond. More precisely, the presence of (1) early-transition-metal centers, (2) multiple Fe atoms coordinated to C and/or S, (3) electron-rich ligands, and (4) the formation of hydrides have all been directly related to the first step of the reaction.14,85 On the other hand, much less is known about how this structure drives subsequent ones. Interestingly, experiments on complexes incorporating alkali metals (somehow mimicking the alkali-promoted Fe catalyst for HB) suggest that oxygen bridges between Fe and alkali atoms are centers for nitrogen fixation, whereas computational studies point to their positive charge playing the role of histidine and arginine residues found near Fe atoms in the nitrogenase.86 From the knowledge acquired in thermal, biological, and molecular systems, the importance of populating the surface of electrocatalysts with centers able to increase p-back bonding to weaken the triple N–N bond can be concluded. However, a wider extrapolation needs to be done carefully, since over thermal, biological, and homogeneous molecular systems the oxidation process also occurs over the catalytic unit, and importantly, over the latter two the production of molecular hydrogen seems concomitant to the nitrogen reduction, as already mentioned. Since the formation of hydrogen represents a loss of efficiency, a highly performing eN2RR electrocatalyst must simultaneously inhibit the HER.

Chem 5, 1–21, February 14, 2019

15

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Quantitative analysis of the free energy of the surface intermediates for both reactions allows for the determination of the thermodynamic onset potential, at which all elementary steps become exergonic. The thermodynamic onset potential calculated in the literature for both the eN2RR and the HER is plotted in Figure 7B and classified according to different families of materials (references can be found in Table S1 and Figure S6). The dashed line represents onset potentials of the same value for both reactions. The area below this line delimits favorable conditions for the eN2RR versus HER. A quick inspection clearly reveals the selectivity challenge previously mentioned, as the vast majority of investigated materials locate above the line. The d-block transition metals (blue colored), the most commonly reported family of materials, are visibly expected to favor the HER.76 Notably, introducing a high number of defects by reducing the size down to atomic clusters (green colored) shows a detrimental effect, as apparent from the reduction in the onset potential for the HER.87 These results dictate the limit of the performance achievable using d-metals, which are constrained by the linear-scale relations among the energy levels of the surface intermediates. In this regard, modification of d-block metals with other classes of materials, e.g., p-block elements, has proved to be successful in breaking the linear-scale relations for other aqueous electrocatalytic reactions such as the electroreduction of CO2.88 In this context, oxides89 still favor the HER except on IrO2 (its stability under eN2RR potentials is seriously compromised), but some chalcogenide-based materials (Mo single atoms anchored on Mo2S)90 and mononitrides (CrN, WN, and Mo2N)91,92 form a cluster below the line, indicating their potential as selective eN2RR catalysts with modest onset potentials. We note that, typically, these families of materials are poorly conductive in bulk form, which calls for a strategy such as nanostructuring, while avoiding its full reduction under the eN2RR. Recently, the anchoring of atoms to defective graphitic substrates M@CX (where X denotes the number of anchoring C atoms) is suggested to show comparable HER and eN2RR onset potentials for the case of Mo@C293 and (Ti, V, Fe, Co)@C4.94 Note that this criterion does not consider the comparison between adsorption energies of *N and *H. Adsorption of protons must be thermodynamically less favored compared with that of nitrogen, since a large coverage of protons may eventually hinder the eN2RR over an otherwise efficient catalyst for the eN2RR. In a similar manner, the decomposition of partially reduced intermediates, as proved in biological systems, may play a detrimental role.95 Summarizing, circumventing high overpotentials for the eN2RR and the concomitant efficient suppression of the HER are the main challenges for which understanding tools must complement experimental efforts. This combination may lead to a more complete picture of the process, with exploration of routes including decoupled electron and proton transfer steps, the influence of potential independent steps, or a detailed study of the adsorption modes of the intermediates. Interestingly, a recent application of infrared spectroscopy to the eN2RR over Au revealed intermediates not compatible with the expected associative distal mechanism,96 suggesting a more complex mechanistic picture at work than the one currently available.

LOOKING TO THE FUTURE: THE DISTRIBUTED PRODUCTION OF FERTILIZERS BY AMMONIA ARTIFICIAL LEAVES The coupling of the scalability of electrolyzers with the distributed nature of renewable energy sources may open the door to a new paradigm in agricultural development. Consider that the current manufacture of fertilizers such as NH4NO3 depends on the centralized production of NH3 by the HB, thus adding significant

16

Chem 5, 1–21, February 14, 2019

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

Figure 8. Synthesis of Ammonia and Fertilizers from Water, Air, and Sunlight: An Ammonia Artificial Leaf (A) An exploded view of a proposed scheme for an ammonia artificial leaf, relying on a photoanode and dark cathode, with air fed through a gas diffusion layer electrode. (B) The on-site, on-demand direct synthesis of fertilizers could be realized with an ammonia artificial leaf carrying out both reduction and oxidation reactions of nitrogen species.

transportation costs to the final price in addition to energy and environmental concerns. In stark contrast, a situation whereby ammonia could be produced from air, water, and renewable energy would realize the on-site production of fertilizers at the crop fields. We call a device realizing this an NH3-leaf. A photoelectrocatalytic approach, where the photon absorber and the electrode are coupled at a device scale, can be considered the most feasible for the development of the NH3-leaf. Figure 8A includes a proposed scheme in which the anodic reaction takes place on a photoanode (a photocathode would also be conceivable) and the N2 reduction on a dark cathode. This design addresses the low solubility of N2 leading to performance constraints52 by the use of a gas-diffusion electrode, which allows for the gaseous N2 access to the active sites. This ideal NH3-leaf mainly comprises the gas-diffusion electrode, the proton-conductive liquid electrolyte, the ion-permeable membrane, and the light harvester. Air containing the reactant N2 is fed to the hydrophobic back side of the gas-diffusion electrode (label 1), whose front is in contact with the liquid electrolyte (water, supplied to the cathodic chamber as shown by label 3). Protons consumed in the cathodic half-reaction are provided through the membrane by the anodic chamber, which is filled with the same liquid electrolyte that also serves as the reactant for the anodic reaction (water, supplied to the anodic chamber as shown by label 5). The anodic catalyst layer is deposited on the surface of a light harvester. We must note that the presence of oxygen in the cathodic compartment in this ideal case adds the very significant catalytic challenge of silencing its reduction reaction, which, at potentials required to trigger the eN2RR, would run at a large overpotential (>1.1 V). This is confirmed by the very marked decrease in performance observed in the scarce works reporting air fed to the cathodic compartment.49,50 On a different level, the design of such a device must coordinate different disciplines, and thus demands an integral effort to achieve practical levels of efficiency.19 Current activities on the development of artificial leaves for water

Chem 5, 1–21, February 14, 2019

17

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

splitting and CO2 conversion97,98 may pave the way toward the NH3-leaf concept given their large technological resemblance. A step ahead, the aforementioned scheme still demands the synthesis of NH4NO3 from the ammonia raw material. It is based on the high-temperature catalytic oxidation of ammonia (Ostwald process) via formation of NOx (x = 1,2) according to Equations 7, 8, and 9. 4NH3 + 5O2 /4NO + 6H2 O;

(Equation 7)

2NO + O2 /2NO2 ;

(Equation 8)

3NO2 + H2 O/2HNO3 + NO:

(Equation 9)

Even though the complexity of this stage is far away from that found in the HB process, the presence of only nitrogen, oxygen, and hydrogen in NH4NO3 opens the door to its integrated production from air and water in an NH3-leaf. With this idea in mind, the anodic compartment must carry out oxidation of a nitrogen species toward nitrate. The oxidation of N2 is a possibility99 represented by the global Equation 10. Note the side production of H2, required to keep stoichiometry and pH constant. Alternatively, the oxidation of ammonia generated in the anode to nitrate would lead to the general Equation 11, yielding both NH4NO3 and NH3 as products.100 The scheme shown in Figure 8A would require minimal modifications to perform desired reactions. However, we note that the electro-oxidation processes proposed have been almost entirely unexplored, given their lack of utility for the nitrogen transformation industry.99 The development of anodic electrocatalysts thus adds to this optimized approach. The appeal of this challenging concept is apparent when one considers that, in this situation, the stream containing the diluted fertilizer could directly be used at the fields with no intermediate step (scheme in Figure 8B), thus compacting the complex and contaminating chain currently needed (fossil fuel extraction and processing + HB + Ostwald + transportation) into an autonomous and simple single device requiring the same inputs demanded by any crop field, namely sun and water. N2 + 3H2 O/NH4 NO3 + H2 ;

(Equation 10)

4N2 + 9H2 O/3NH4 NO3 + 2NH3 :

(Equation 11)

CONCLUSION AND OUTLOOK After the analysis of the state of the art of the eN2RR, the flexibility of the electrochemical approach is manifested in the varieties of operating conditions and architectures reported so far. The establishment of a common set of operating conditions allowed a general perspective whereby the need to improve both current density and energy efficiency became apparent. Mediocre catalytic performance is a common denominator at all temperatures, with special impact at low ones (<100 C), where high overpotential and poor selectivity have been travel partners so far. The low electronic and ionic conductivity of materials and the very early stage of schemes need to be additionally considered at high (>500 C) and intermediate temperatures, respectively. To date, devices or cycles operating at intermediate temperatures are those closer to practicality. Understanding efforts have been provided so far by theoretical means and show the difficulty of inhibiting the parasitic formation of hydrogen in protic environments while achieving favorable energetic landscapes for the eN2RR. We put forward a deeper mechanistic understanding to be brought about by the

18

Chem 5, 1–21, February 14, 2019

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

combination of experimental and theoretical efforts, together with rational, highthroughput screening of materials, which will help make this fascinating field evolve rapidly. This picture corresponds to an incipient field. We understand the eN2RR in the medium term as a complementary technology to the incumbent HB process and only coupled to sustainable resources. We thus consider it a priority to orient efforts toward developing small-scale electrocatalytic systems for distributed ammonia synthesis, with special emphasis on ammonia artificial leaves, as a way to complete the revolution started by Fritz Haber 120 years ago.

SUPPLEMENTAL INFORMATION Supplemental Information includes details on calculations, six figures, and one table and can be found with this article online at https://doi.org/10.1016/j.chempr.2018. 10.010.

ACKNOWLEDGMENTS This work was funded by ETH Zu¨rich.

AUTHOR CONTRIBUTIONS A.J.M., T.S., and J.P.-R. conducted the literature survey and prepared the manuscript. J.P.-R. conceived the idea and coordinated efforts.

REFERENCES AND NOTES 1. Davy, H. (1807). On some chemical agencies of electricity. Phil. Trans. 97, 1–56. 2. Strutt, J.W. (1807). Observations on the oxidation of nitrogen gas. J. Chem. Soc. Trans. 71, 181–186. 3. Nithack, F. (1898). Electrolyse water, containing nitrogen, dissolved under a pressure of 50-100 atmospheres. Deutsche Reichs patent 95,532. 4. Fichter, F., and Suter, R. (1922). Zur Frage der kathodischen Reduktion des elementaren Stickstoffs. Helv. Chim. Acta 5, 246–255. 5. Stewart, W.M., Dibb, D.W., Johnston, A.E., and Smyth, T.J. (2005). The contribution of commercial fertilizer nutrients to food production. Agron. J. 97, 1–6. 6. Jackson, R.B., Canadell, J.G., Le Que´re´, C., Andrew, R.M., Korsbakken, J.I., Peters, G.P., and Nakicenovic, N. (2015). Reaching peak emissions. Nat. Clim. Change 6, 7–10. 7. Wang, L., Xia, M., Wang, H., Huang, K., Qian, C., Maravelias, C.T., and Ozin, G.A. (2018). Greening ammonia toward the solar ammonia refinery. Joule 2, 1–20. 8. Appl, M. (2011). Ammonia, 2. production processes. In Ullmann’s Encyclopedia of Industrial Chemistry, B. Elvers, ed. (WileyVCH Verlag GmbH), pp. 139–210. 9. World Energy Council. (2016). World Energy Resources (World Energy Council). 10. Okuzumi, N., Jones, B.D., and Nielsen, S.E. (2001). Start-up of the world largest ammonia plant. Proceedings of NITROGEN 2001.

11. Guo, C., Ran, J., Vasileff, A., and Qiao, S.-Z. (2018). Rational design of electrocatalysts and photo(electro)catalysts for nitrogen reduction to ammonia (NH3) under ambient conditions. Energy Environ. Sci. 11, 45–56. 12. Zheng, Y., Jiao, Y., Jaroniec, M., and Qiao, S.Z. (2015). Advancing the electrochemistry of the hydrogen-evolution reaction through combining experiment and theory. Angew. Chem. Int. Ed. 54, 52–65. 13. Hoffman, B.M., Lukoyanov, D., Yang, Z.-Y., Dean, D.R., and Seefeldt, L.C. (2014). Mechanism of nitrogen fixation by nitrogenase: the next stage. Chem. Rev. 114, 4041–4062. 14. Foster, S.L., Bakovic, S.I.P., Duda, R.D., Maheshwari, S., Milton, R.D., Minteer, S.D., Janik, M.J., Renner, J.N., and Greenlee, L.F. (2018). Catalysts for nitrogen reduction to ammonia. Nat. Catal. 1, 490–500. 15. Shipman, M.A., and Symes, M.D. (2017). Recent progress towards the electrosynthesis of ammonia from sustainable resources. Catal. Today 286, 57–68. 16. Kyriakou, V., Garagounis, I., Vasileiou, E., Vourros, A., and Stoukides, M. (2017). Progress in the electrochemical synthesis of ammonia. Catal. Today 286, 2–13. 17. Deng, J., In˜iguez, J.A., and Liu, C. (2018). Electrocatalytic nitrogen reduction at low temperature. Joule 2, 846–856. 18. Cui, X., Tang, C., and Zhang, Q. (2018). A review of electrocatalytic reduction of dinitrogen to ammonia under ambient conditions. Adv. Energy Mater. 8, 1800369.

19. Larraza´bal, G.O., Martı´n, A.J., and Pe´rezRamı´rez, J. (2017). Building blocks for high performance in electrocatalytic CO2 reduction: materials, modification strategies, and device engineering. J. Phys. Chem. Lett. 8, 3933–3944. 20. Carmo, M., Fritz, D.L., Mergel, J., and Stolten, D. (2013). A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 38, 4901–4934. 21. Advanced Research Projects Agency— Energy—US Department of Energy. (2016). Refuel. https://arpa-e.energy.gov/?q=arpa-eprograms/refuel. 22. Vielstich, W., Gasteiger, H.A., and Yokokawa, H. (2009). Handbook of Fuel Cells: Fundamentals Technology and Applications (John Wiley and Sons). 23. Xie, Y.-H., Wang, J.-D., Liu, R.-Q., Su, X.-T., Sun, Z.-P., and Li, Z.-J. (2004). Preparation of La1.9Ca0.1Zr2O6.95 with pyrochlore structure and its application in synthesis of ammonia at atmospheric pressure. Solid State Ionics 168, 117–121. 24. Wang, J.-D., Xie, Y.-H., Zhang, Z.-F., Liu, R.-Q., and Li, Z.-J. (2005). Protonic conduction in Ca2+-doped La2M2O7 (M = Ce, Zr) with its application to ammonia synthesis electrochemically. Mater. Res. Bull. 40, 1294– 1302. 25. Marnellos, G., and Stoukides, M. (1998). Ammonia synthesis at atmospheric pressure. Science 282, 98–100. 26. Wang, W.B., Cao, X.B., Gao, W.J., Zhang, F., Wang, H.T., and Ma, G.L. (2010). Ammonia synthesis at atmospheric pressure using a

Chem 5, 1–21, February 14, 2019

19

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

reactor with thin solid electrolyte BaCe0.85Y0.15O3-a membrane. J. Membr. Sci. 360, 397–403. 27. Skodra, A., and Stoukides, M. (2009). Electrocatalytic synthesis of ammonia from steam and nitrogen at atmospheric pressure. Solid State Ionics 180, 1332–1336. 28. Vasileiou, E., Kyriakou, V., Garagounis, I., Vourros, A., and Stoukides, M. (2015). Ammonia synthesis at atmospheric pressure in a BaCe0.2Zr0.7Y0.1O2.9 solid electrolyte cell. Solid State Ionics 275, 110–116. 29. Yoo, C.-Y., Park, J.H., Kim, K., Han, J.-I., Jeong, E.-Y., Jeong, C.-H., Yoon, H.C., and Kim, J.-N. (2017). Role of protons in electrochemical ammonia synthesis using solid-state electrolytes. ACS Sustain. Chem. Eng. 5, 7972–7978. 30. Wang, W.B., Liu, J.W., Li, Y.D., Wang, H.T., Zhang, F., and Ma, G.L. (2010). Microstructures and proton conduction behaviors of Dy-doped BaCeO3 ceramics at intermediate temperature. Solid State Ionics 181, 667–671. 31. Li, Z., Liu, R., Wang, J., Xu, Z., Xie, Y., and Wang, B. (2007). Preparation of doubledoped BaCeO3 and its application in the synthesis of ammonia at atmospheric pressure. Sci. Technol. Adv. Mater. 8, 566–570. 32. Chen, C., and Ma, G. (2008). Preparation, proton conduction, and application in ammonia synthesis at atmospheric pressure of La0.9Ba0.1Ga1-xMgxO3-a. J. Mater. Sci. 43, 5109–5114. 33. Omata, T., and Otsuka-Yao-Matsuo, S. (2001). Electrical properties of proton-conducting Ca2+-doped La2Zr2O7 with a pyrochlore-type structure. J. Electrochem. Soc. 148, E252– E261. 34. Klinsrisuk, S., and Irvine, J.T.S. (2017). Electrocatalytic ammonia synthesis via a proton conducting oxide cell with BaCe0.5Zr0.3Y0.16Zn0.04O3-d electrolyte membrane. Catal. Today 286, 41–50. 35. Vasileiou, E., Kyriakou, V., Garagounis, I., Vourros, A., Manerbino, A., Coors, W.G., and Stoukides, M. (2016). Electrochemical enhancement of ammonia synthesis in a BaZr0.7Ce0.2Y0.1O2.9 solid electrolyte cell. Solid State Ionics 288, 357–362. 36. Brosda, S., Vayenas, C.G., and Wei, J. (2006). Rules of chemical promotion. Appl. Catal. B 68, 109–124. 37. Ishihara, T. (2014). Non-faradaic electrochemical modification of catalytic activity (NEMCA). In Encyclopedia of Applied Electrochemistry, G. Kreysa, K. Ota, and R.F. Savinell, eds. (Springer), pp. 1375–1380. 38. Amar, I.A., Petit, C.T.G., Mann, G., Lan, R., Skabara, P.J., and Tao, S. (2014). Electrochemical synthesis of ammonia from N2 and H2O based on (Li,Na,K)2CO3Ce0.8Gd0.18Ca0.02O2-d composite electrolyte and CoFe2O4 cathode. Int. J. Hydrogen Energy 39, 4322–4330. 39. Imamura, K., Matsuyama, M., and Kubota, J. (2017). Ammonia synthesis from nitrogen and water by electricity using an electrochemical

20

Chem 5, 1–21, February 14, 2019

cell with Ru catalyst, hydrogen-permeable PdAg membrane, and phosphate-based electrolyte. ChemistrySelect 2, 11100–11103. 40. Kishira, S., Qing, G., Suzu, S., Kikuchi, R., Takagaki, A., and Oyama, S.T. (2017). Ammonia synthesis at intermediate temperatures in solid-state electrochemical cells using cesium hydrogen phosphate based electrolytes and noble metal catalysts. Int. J. Hydrogen Energy 42, 26843–26854. 41. Kosaka, F., Noda, N., Nakamura, T., and Otomo, J. (2017). In situ formation of Ru nanoparticles on La1-xSrxTiO3-based mixed conducting electrodes and their application in electrochemical synthesis of ammonia using a proton-conducting solid electrolyte. J. Mater. Sci. 52, 2825–2835. 42. McEnaney, J.M., Singh, A.R., Schwalbe, J.A., Kibsgaard, J., Lin, J.C., Cargnello, M., Jaramillo, T.F., and Nørskov, J.K. (2017). Ammonia synthesis from N2 and H2O using a lithium cycling electrification strategy at atmospheric pressure. Energy Environ. Sci. 10, 1621–1630. 43. Licht, S., Cui, B., Wang, B., Li, F.-F., Lau, J., and Liu, S. (2014). Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3. Science 345, 637–640. 44. Murakami, T., Nishikiori, T., Nohira, T., and Ito, Y. (2003). Electrolytic synthesis of ammonia in molten salts under atmospheric pressure. J. Am. Chem. Soc. 125, 334–335. 45. Yusuf, B., and Ibrahim, D. (2017). Performance assessment of electrochemical ammonia synthesis using photoelectrochemically produced hydrogen. Int. J. Energy Res. 41, 1987–2000. 46. Ouzounidou, M., Skodra, A., Kokkofitis, C., and Stoukides, M. (2007). Catalytic and electrocatalytic synthesis of NH3 in a H+ conducting cell by using an industrial Fe catalyst. Solid State Ionics 178, 153–159. 47. Kim, K., Lee, S.J., Kim, D.-Y., Yoo, C.-Y., Choi, J.W., Kim, J.-N., Woo, Y., Yoon, H.C., and Han, J.-I. (2017). Electrochemical synthesis of ammonia from water and nitrogen: a lithiummediated approach using lithium-ion conducting glass ceramics. ChemSusChem 11, 120–124.

52. Zhou, F., Azofra, L.M., Ali, M., Kar, M., Simonov, A.N., McDonnell-Worth, C., Sun, C., Zhang, X., and MacFarlane, D.R. (2017). Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids. Energy Environ. Sci. 10, 2516–2520. 53. Wang, J., Yu, L., Hu, L., Chen, G., Xin, H., and Feng, X. (2018). Ambient ammonia synthesis via palladium-catalyzed electrohydrogenation of dinitrogen at low overpotential. Nat. Commun. 9, 1795. 54. Liu, H.-M., Han, S.-H., Zhao, Y., Zhu, Y.-Y., Tian, X.-L., Zeng, J.-H., Jiang, J.-X., Xia, B.Y., and Chen, Y. (2018). Surfactant-free atomically ultrathin rhodium nanosheet nanoassemblies for efficient nitrogen electroreduction. J. Mater. Chem. A 6, 3211–3217. 55. Di, B., Qi, Z., Fan-Lu, M., Hai-Xia, Z., MiaoMiao, S., Yu, Z., Jun-Min, Y., Qing, J., and XinBo, Z. (2017). Electrochemical reduction of N2 under ambient conditions for artificial N2 fixation and renewable energy storage using N2/NH3 cycle. Adv. Mater. 29, 1604799. 56. Tsuneto, A., Kudo, A., and Sakata, T. (1994). Lithium-mediated electrochemical reduction of high pressure N2 to NH3. J. Electroanal. Chem. 367, 183–188. 57. Shi, G., Yu, L., Ba, X., Zhang, X., Zhou, J., and Yu, Y. (2017). Copper nanoparticle interspersed MoS2 nanoflowers with enhanced efficiency for CO2 electrochemical reduction to fuel. Dalton Trans. 46, 10569– 10577. 58. Suryanto, B.H.R., Kang, C.S.M., Wang, D., Xiao, C., Zhou, F., Azofra, L.M., Cavallo, L., Zhang, X., and MacFarlane, D.R. (2018). Rational electrode-electrolyte design for efficient ammonia electrosynthesis under ambient conditions. ACS Energy Lett. 3, 1219–1224. 59. Yang, D., Chen, T., and Wang, Z. (2017). Electrochemical reduction of aqueous nitrogen (N2) at a low overpotential on (110)oriented Mo nanofilm. J. Mater. Chem. A 5, 18967–18971. 60. Liu, Y., Su, Y., Quan, X., Fan, X., Chen, S., Yu, H., Zhao, H., Zhang, Y., and Zhao, J. (2018). Facile ammonia synthesis from electrocatalytic N2 reduction under ambient conditions on N-doped porous carbon. ACS Catal. 8, 1186–1191.

48. Cui, B., Zhang, J., Liu, S., Liu, X., Xiang, W., Liu, L., Xin, H., Lefler, M.J., and Licht, S. (2017). Electrochemical synthesis of ammonia directly from N2 and water over iron-based catalysts supported on activated carbon. Green Chem. 19, 298–304.

61. Shi, M.-M., Bao, D., Wulan, B.-R., Li, Y.-H., Zhang, Y.-F., Yan, J.-M., and Jiang, Q. (2017). Au sub-nanoclusters on TiO2 toward highly efficient and selective electrocatalyst for N2 conversion to NH 3 at ambient conditions. Adv. Mater. 29, 1606550.

49. Lan, R., Irvine, J.T.S., and Tao, S. (2013). Synthesis of ammonia directly from air and water at ambient temperature and pressure. Sci. Rep. 3, 1–7.

62. Li, S.-J., Bao, D., Shi, M.-M., Wulan, B.-R., Yan, J.-M., and Jiang, Q. (2017). Amorphizing of Au nanoparticles by CeOx-RGO hybrid support towards highly efficient electrocatalyst for N2 reduction under ambient conditions. Adv. Mater. 29, 1700001.

50. Lan, R., and Tao, S. (2013). Electrochemical synthesis of ammonia directly from air and water using a Li+/H+/NH4+ mixed conducting electrolyte. RSC Adv. 3, 18016–18021. 51. Singh, A.R., Rohr, B.A., Schwalbe, J.A., Cargnello, M., Chan, K., Jaramillo, T.F., Chorkendorff, I., and Nørskov, J.K. (2017). Electrochemical ammonia synthesis—the selectivity challenge. ACS Catal. 7, 706–709.

63. Shao, M., Chang, Q., Dodelet, J.-P., and Chenitz, R. (2016). Recent advances in electrocatalysts for oxygen reduction reaction. Chem. Rev. 116, 3594–3657. 64. Zhang, L., Mallikarjun Sharada, S., Singh, A.R., Rohr, B.A., Su, Y., Qiao, L., and Nørskov, J.K. (2018). A theoretical study of the effect of a

Please cite this article in press as: Martı´n et al., Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf, Chem (2018), https://doi.org/10.1016/j.chempr.2018.10.010

non-aqueous proton donor on electrochemical ammonia synthesis. Phys. Chem. Chem. Phys. 20, 4982–4989. 65. Rosen, B.A., Salehi-Khojin, A., Thorson, M.R., Zhu, W., Whipple, D.T., Kenis, P.J.A., and Masel, R.I. (2011). Ionic liquid-mediated selective conversion of CO2 to CO at low overpotentials. Science 334, 643–644. 66. Allen, A.D., and Senoff, C.V. (1965). Nitrogenopentammineruthenium(II) complexes. Chem. Commun. (Camb.), 621–622. 67. Yandulov, D.V., and Schrock, R.R. (2003). Catalytic reduction of dinitrogen to ammonia at a single molybdenum center. Science 301, 76–78. 68. Arashiba, K., Miyake, Y., and Nishibayashi, Y. (2010). A molybdenum complex bearing PNPtype pincer ligands leads to the catalytic reduction of dinitrogen into ammonia. Nat. Chem. 3, 120–125. 69. Arashiba, K., Kinoshita, E., Kuriyama, S., Eizawa, A., Nakajima, K., Tanaka, H., Yoshizawa, K., and Nishibayashi, Y. (2015). Catalytic reduction of dinitrogen to ammonia by use of molybdenum-nitride complexes bearing a tridentate triphosphine as catalysts. J. Am. Chem. Soc. 137, 5666–5669. 70. Anderson, J.S., Rittle, J., and Peters, J.C. (2013). Catalytic conversion of nitrogen to ammonia by an iron model complex. Nature 501, 84–87. 71. Chalkley, M.J., Del Castillo, T.J., Matson, B.D., Roddy, J.P., and Peters, J.C. (2017). Catalytic N2-to-NH3 conversion by Fe at lower driving force: a proposed role for metallocenemediated PCET. ACS Cent. Sci. 3, 217–223. 72. Roux, Y., Duboc, C., and Gennari, M. (2017). Molecular catalysts for N2 reduction: state of the art, mechanism, and challenges. ChemPhysChem 18, 2606–2617. 73. Rod, T.H., Logadottir, A., and Nørskov, J.K. (2000). Ammonia synthesis at low temperatures. J. Chem. Phys. 112, 5343–5347. 74. Ertl, G. (2008). Reactions at surfaces: from atoms to complexity (Nobel lecture). Angew. Chem. Int. Ed. 47, 3524–3535. 75. Nørskov, J.K., Bligaard, T., Rossmeisl, J., and Christensen, C.H. (2009). Towards the computational design of solid catalysts. Nat. Chem. 1, 37–46. 76. Sku´lason, E., Bligaard, T., Gudmundsdo´ttir, S., Studt, F., Rossmeisl, J., Abild-Pedersen, F., Vegge, T., Jo´nsson, H., and Nørskov, J.K. (2012). A theoretical evaluation of possible transition metal electro-catalysts for N2 reduction. Phys. Chem. Chem. Phys. 14, 1235– 1245.

77. Montoya, J.H., Tsai, C., Vojvodic, A., and Nørskov, J.K. (2015). The challenge of electrochemical ammonia synthesis: a new perspective on the role of nitrogen scaling relations. ChemSusChem 8, 2180–2186. 78. Ma, X.-L., Liu, J.-C., Xiao, H., and Li, J. (2018). Surface single-cluster catalyst for N2-to-NH3 thermal conversion. J. Am. Chem. Soc. 140, 46–49. 79. Zhao, J., and Chen, Z. (2017). Single Mo atom supported on defective boron nitride monolayer as an efficient electrocatalyst for nitrogen fixation: a computational study. J. Am. Chem. Soc. 139, 12480–12487. 80. Abghoui, Y., Garden, A.L., Hlynsson, V.F., ´ lafsdo´ttir, H., and Bjo¨rgvinsdo´ttir, S., O Sku´lason, E. (2015). Enabling electrochemical reduction of nitrogen to ammonia at ambient conditions through rational catalyst design. Phys. Chem. Chem. Phys. 17, 4909–4918. 81. Abghoui, Y., and Sku´lason, E. (2017). Electrochemical synthesis of ammonia via Mars-van Krevelen mechanism on the (111) facets of group III-VII transition metal mononitrides. Catal. Today 286, 78–84. 82. Ertl, G. (1983). Primary steps in catalytic synthesis of ammonia. J. Vac. Sci. Technol. A. 1, 1247–1253. 83. Honkala, K., Hellman, A., Remediakis, I.N., Logadottir, A., Carlsson, A., Dahl, S., Christensen, C.H., and Nørskov, J.K. (2005). Ammonia synthesis from firstprinciples calculations. Science 307, 555–558. 84. van der Ham, C.J.M., Koper, M.T.M., and Hetterscheid, D.G.H. (2014). Challenges in reduction of dinitrogen by proton and electron transfer. Chem. Soc. Rev. 43, 5183– 5191. 85. MacLeod, K.C., and Holland, P.L. (2013). Recent developments in the homogeneous reduction of dinitrogen by molybdenum and iron. Nat. Chem. 5, 559–565. 86. McWilliams, S.F., and Holland, P.L. (2015). Dinitrogen binding and cleavage by multinuclear iron complexes. Acc. Chem. Res. 48, 2059–2065. 87. Howalt, J.G., Bligaard, T., Rossmeisl, J., and Vegge, T. (2013). DFT based study of transition metal nano-clusters for electrochemical NH3 production. Phys. Chem. Chem. Phys. 15, 7785–7795. 88. Shinagawa, T., Larraza´bal, G.O., Martı´n, A.J., Krumeich, F., and Pe´rez-Ramı´rez, J. (2018). Sulfur-modified copper catalysts for the electrochemical reduction of carbon dioxide to formate. ACS Catal. 8, 837–844. 89. Ho¨skuldsson, A´.B., Abghoui, Y., Gunnarsdo´ttir, A.B., and Sku´lason, E. (2017).

Computational screening of rutile oxides for electrochemical ammonia formation. ACS Sustain. Chem. Eng 5, 10327–10333. 90. Zhao, J., Zhao, J., and Cai, Q. (2018). Single transition metal atom embedded into a MoS2 nanosheet as a promising catalyst for electrochemical ammonia synthesis. Phys. Chem. Chem. Phys. 20, 9248–9255. 91. Abghoui, Y., and Sku´lason, E. (2017). Computational predictions of catalytic activity of zincblende (110) surfaces of metal nitrides for electrochemical ammonia synthesis. J. Phys. Chem. C 121, 6141–6151. 92. Matanovic, I., Garzon, F.H., and Henson, N.J. (2014). Electro-reduction of nitrogen on molybdenum nitride: structure, energetics, and vibrational spectra from DFT. Phys. Chem. Chem. Phys. 16, 3014– 3026. 93. Wang, Z., Yu, Z., and Zhao, J. (2018). Computational screening of a single transition metal atom supported on the C2N monolayer for electrochemical ammonia synthesis. Phys. Chem. Chem. Phys. 20, 12835–12844. 94. Choi, C., Back, S., Kim, N.-Y., Lim, J., Kim, Y.-H., and Jung, Y. (2018). Suppression of the hydrogen evolution reaction in electrochemical N2 reduction using singleatom catalysts: a computational guideline. ACS Catal. 8, 7517–7525. 95. Weare, W.W., Schrock, R.R., Hock, A.S., and Mu¨ller, P. (2006). Synthesis of molybdenum complexes that contain ‘‘hybrid’’ triamidoamine ligands, [(hexaisopropylterphenylNCH2CH22NCH2CH2N-aryl]3-, and studies relevant to catalytic reduction of dinitrogen. Inorg. Chem. 45, 9185–9196. 96. Yao, Y., Zhu, S., Wang, H., Li, H., and Shao, M. (2018). A spectroscopic study on the nitrogen electrochemical reduction reaction on gold and platinum surfaces. J. Am. Chem. Soc. 140, 1496–1501. 97. California Institute of Technology, Joint Center for Artificial Photosynthesis. (2018). Joint Center for Artificial Photosynthesis. http://solarfuelshub.org/. 98. A-LEAF. (2018). A-leaf. http://www.a-leaf.eu/. 99. Chen, J.G., Crooks, R.M., Seefeldt, L.C., Bren, K.L., Bullock, R.M., Darensbourg, M.Y., Holland, P.L., Hoffman, B., Janik, M.J., Jones, A.K., et al. (2018). Beyond fossil fuel-driven nitrogen transformations. Science 360, eaar6611. 100. Jewess, M., and Crabtree, R.H. (2016). Electrocatalytic nitrogen fixation for distributed fertilizer production? ACS Sustain. Chem. Eng. 4, 5855–5858.

Chem 5, 1–21, February 14, 2019

21