Abstract:Lithium-mediated nitrogen reduction is a proven method to electrochemically synthesize ammonia; yet the process has so far been unstable, and the continuous deposition of lithium limits its practical applicability. One...
“…Therefore, active lithium and inert proton source can facilitate N 2 activation and HER suppression, respectively. To date, lithium-mediated NRR is developing quickly ( Andersen et al., 2019 , 2020 ; Lazouski et al., 2019 , 2020 ) and has become an important technological roadmap in the field of electrochemical NRR. However, despite the wide attention raised by researchers and an increasing number of reports ( Andersen et al., 2019 ; Suryanto et al., 2021 ), the mechanistic understanding on nitrogen reduction in the lithium-mediated process is still in its infancy, which hampers performance enhancement.…”
Section: Introductionmentioning
confidence: 99%
“…Thus, nitrogen reduction/ammonia synthesis can proceed as long as there is residual active lithium deposit on the electrode, even if the current is cutoff. Figure 1 Schematic diagram of lithium-mediated nitrogen reduction reaction Four possible mechanisms of lithium-mediated nitrogen reduction reaction in literature, labeled as CC ( Akira et al., 1994 ; Gao et al., 2020 ; Lazouski et al., 2019 ), EE ( Schwalbe et al., 2020 ), CE ( Andersen et al., 2020 ), and EC ( Ma et al., 2017 ; Zhang et al., 2019 ) model. C and E represent chemical and electrochemical steps, respectively.…”
Summary
Green synthesis of ammonia by electrochemical nitrogen reduction reaction (NRR) shows great potential as an alternative to the Haber-Bosch process but is hampered by sluggish production rate and low Faradaic efficiency. Recently, lithium-mediated electrochemical NRR has received renewed attention due to its reproducibility. However, further improvement of the system is restricted by limited recognition of its mechanism. Herein, we demonstrate that lithium-mediated NRR began with electrochemical deposition of lithium, followed by two chemical processes of dinitrogen splitting and protonation to ammonia. Furthermore, we quantified the extent to which the freshly deposited active lithium lost its activity toward NRR due to a parasitic reaction between lithium and electrolyte. A high ammonia yield of 0.410 ± 0.038 μg s
−1
cm
−2
geo and Faradaic efficiency of 39.5 ± 1.7% were achieved at 20 mA cm
−2
geo and 10 mA cm
−2
geo, respectively, which can be attributed to fresher lithium obtained at high current density.
“…Therefore, active lithium and inert proton source can facilitate N 2 activation and HER suppression, respectively. To date, lithium-mediated NRR is developing quickly ( Andersen et al., 2019 , 2020 ; Lazouski et al., 2019 , 2020 ) and has become an important technological roadmap in the field of electrochemical NRR. However, despite the wide attention raised by researchers and an increasing number of reports ( Andersen et al., 2019 ; Suryanto et al., 2021 ), the mechanistic understanding on nitrogen reduction in the lithium-mediated process is still in its infancy, which hampers performance enhancement.…”
Section: Introductionmentioning
confidence: 99%
“…Thus, nitrogen reduction/ammonia synthesis can proceed as long as there is residual active lithium deposit on the electrode, even if the current is cutoff. Figure 1 Schematic diagram of lithium-mediated nitrogen reduction reaction Four possible mechanisms of lithium-mediated nitrogen reduction reaction in literature, labeled as CC ( Akira et al., 1994 ; Gao et al., 2020 ; Lazouski et al., 2019 ), EE ( Schwalbe et al., 2020 ), CE ( Andersen et al., 2020 ), and EC ( Ma et al., 2017 ; Zhang et al., 2019 ) model. C and E represent chemical and electrochemical steps, respectively.…”
Summary
Green synthesis of ammonia by electrochemical nitrogen reduction reaction (NRR) shows great potential as an alternative to the Haber-Bosch process but is hampered by sluggish production rate and low Faradaic efficiency. Recently, lithium-mediated electrochemical NRR has received renewed attention due to its reproducibility. However, further improvement of the system is restricted by limited recognition of its mechanism. Herein, we demonstrate that lithium-mediated NRR began with electrochemical deposition of lithium, followed by two chemical processes of dinitrogen splitting and protonation to ammonia. Furthermore, we quantified the extent to which the freshly deposited active lithium lost its activity toward NRR due to a parasitic reaction between lithium and electrolyte. A high ammonia yield of 0.410 ± 0.038 μg s
−1
cm
−2
geo and Faradaic efficiency of 39.5 ± 1.7% were achieved at 20 mA cm
−2
geo and 10 mA cm
−2
geo, respectively, which can be attributed to fresher lithium obtained at high current density.
“…A similar strategy has already been used successfully to increase the selectivity of electrochemical N 2 reduction (where the parasitic reaction is hydrogen evolution). [41][42][43][44] Limited access to holes can for instance be achieved by limiting conductivity to the surface. For TiO 2 this could be achieved by controlling the thickness of a non-conducting TiO 2 lm on the electrode surface.…”
Nitric acid is manufactured by oxidizing ammonia where the ammonia comes from an energy demanding and non-eco-friendly, Haber-Bosch process. Electrochemical oxidation of N2 to nitric acid using renewable electricity could...
“…Crucially, the dependence on metallic lithium results in a built-in requirement for high potential losses given the negative reduction potential of Li + . The organic electrolyte is also highly resistive, which results in an incredibly low energy efficiency (10,14).The SEI layer itself could be a source of instability. During NH3 synthesis, the organic electrolyte continues to undergo reduction and product accumulation on the electrode surface, which increases resistance (10).…”
mentioning
confidence: 99%
“…Crucially, the dependence on metallic lithium results in a built-in requirement for high potential losses given the negative reduction potential of Li + . The organic electrolyte is also highly resistive, which results in an incredibly low energy efficiency (10,14).…”
The Haber-Bosch process converts nitrogen (N2) and hydrogen (H2) into ammonia (NH3) over iron-based catalysts. Today, 50% of global agriculture uses Haber-Bosch NH3 in fertilizer. Efficient synthesis requires enormous energy to achieve extreme temperatures and pressures, and the H2 is primarily derived from methane steam reforming. Hence, the Haber-Bosch process accounts for at least 1% of global greenhouse gas emissions (1). Electrochemical N2 reduction to make NH3, powered by renewable electricity under ambient conditions, could provide a localized and greener alternative. On page xxx of this issue, Suryanto et al.(2) report highly efficient and stable electrochemical N2 reduction based on a recyclable proton donor. This study builds on earlier work showing that an electrolyte containing a lithium salt in an organic solvent with a sacrificial proton donor was unique in its ability to unequivocally reduce N2 (3,4). In both studies, it is still unclear why lithium is so critical.Neighboring fields of homogeneous and bio-catalysis provide insight. The nitrogenase enzyme selectively reduces N2 to NH3 with a Faradaic efficiency of 65% at ambient N2 pressure (5), far higher than has been achieved with heterogeneous catalysts (see the figure). Studies of nitrogenase and homogeneous mimics have revealed the crucial role of proton donation rate to activate N2. Nitrogenase moderates access of protons to active sites through internal channels through an anhydrous and hydrophobic protein matrix; electrochemical studies showed that the isolated catalytic cofactor in aqueous solution undergoes a catastrophic loss of efficiency (6). The biomimetic compound reported in 2003 by Yandulov and Schrock (7) could reduce N2 efficiently only if the proton source and reducing agent were added slowly. Chalkley et al. (8) later showed that moderate proton donating ability led to optimal efficiency.Singh et al.'s models predict that inhibiting proton access to the electrode, so that N2 adsorption is no longer blocked, enhances selectivity (9). Nonetheless complete inhibition of access to protons will prevent NH3 formation; hence their model implies that moderate access to protons leads to optimum rates, albeit possibly at the cost of selectivity. Aqueous solutions provide unhindered proton access, and so aqueous electrochemical paradigms produce NH3 in quantities indistinguishable from background contamination (4). However, in 1993, Tsuneto et al. reported efficient NH3 synthesis in an organic electrolyte containing a small amount of ethanol as a proton source and a lithium salt, noting that nonlithium salts yielded negligible NH3 (3). Later isotopic labeling experiments proved that only a lithium ion (Li + ) electrolyte could unequivocally reduce N2 (4).Under ambient conditions, lithium metal can dissociate the stable N2 bond (3); however, such strong N2 binding generally results in even stronger binding to hydrogen (12). Moreover, in the homogeneous systems and nitrogenase, nitrogen hydrogenation precedes N≡N bond scission (7, 11). As s...
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