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| DOI | 10.1126/science.abi8329 |
| Is lithium the key for nitrogen electroreduction? | |
| Olivia Westhead; Rhodri Jervis; Ifan E. L. Stephens | |
| 2021-06-11 | |
| 发表期刊 | Science
 |
| 出版年 | 2021 |
| 英文摘要 | 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 ][1]). Electrochemical N2 reduction to make NH3, powered by renewable electricity under ambient conditions, could provide a localized and greener alternative. On page 1187 of this issue, Suryanto et al. ([ 2 ][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 unmatched in its ability to unequivocally reduce N2 ([ 3 ][3], [ 4 ][4]). In both studies, it is still unclear why lithium is so critical.
Neighboring fields of homogeneous and biocatalysis provide insight. The nitrogenase enzyme selectively reduces N2 to NH3 with a faradaic efficiency of 65% at ambient N2 pressure ([ 5 ][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 in the activation of 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 ][6]). The biomimetic compound reported in 2003 by Yandulov and Schrock ([ 7 ][7]) could reduce N2 efficiently only if the proton source and reducing agent were added slowly. Chalkley et al. ([ 8 ][8]) later showed that moderate proton-donating ability led to optimal efficiency.
![Figure][9]
Toward ideal ammonia synthesis
The relative performance of various systems is compared to the “ideal” electrode (blue stars, see text) based on calculations in ([ 15 ][10]) (filled symbols, 1 bar N2, and open symbols, higher pressures). Triangles represent the recyclable proton donor of Suryanto et al. , and circles indicate sacrificial ethanol proton donors. Potentials versus the reversible hydrogen electrode (RHE) were corrected for ohmic losses and can become more negative during operation. Calculated nitrogenase values are also presented in ([ 15 ][10]).
GRAPHIC: KELLIE HOLOSKI/ SCIENCE
Singh et al. 's models predict that inhibiting proton access to the electrode, so that N2 adsorption is no longer blocked, enhances selectivity ([ 9 ][11]). Nonetheless complete inhibition of access to protons will prevent NH3 formation; hence, their model implies that moderate access to protons leads to optimum N2 reduction 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 ][4]). However, in 1994, 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 ][3]). Later isotopic labeling experiments proved that only a lithium ion (Li+) electrolyte could unequivocally reduce N2 ([ 4 ][4]).
Under ambient conditions, lithium metal can dissociate the stable N2 bond ([ 3 ][3]); however, such strong N2 binding generally results in even stronger binding to hydrogen ([ 10 ][12]). Moreover, in the homogeneous systems and nitrogenase, nitrogen hydrogenation precedes N≡N bond scission ([ 7 ][7], [ 11 ][13]). As such, dissociative N2 binding may not be a prerequisite to nitrogen reduction. Rather, the solid electrolyte interphase (SEI) formed in Li+ batteries may be the key. When a Li+ battery is initially charged, electrolyte decomposition products form a layer on the anode surface. This SEI layer is electronically insulating but Li+ conducting and protects the battery from further electrolyte decomposition ([ 12 ][14]). An SEI layer is also formed in Li+-mediated N2 reduction ([ 13 ][15]). This layer could mimic the hydrophobic and anhydrous environment housing the catalytic cofactor in nitrogenase.
The lithium-mediated paradigm is the most efficient and reproducible system to date but still has vast scope for optimization. Factors that can improve the efficiency, activity, and stability include N2 partial pressure ([ 2 ][2], [ 3 ][3], [ 13 ][15]), choice of proton donor ([ 2 ][2], [ 3 ][3]), potential cycling ([ 13 ][15]), electrolyte cation ([ 14 ][16]), and use of a gas diffusion electrode ([ 14 ][16]) (see the figure). These optimization efforts have resulted in substantial improvements since the verification of the continuous LiClO4-based system in 2019 ([ 4 ][4]). In particular, the work by Suryanto et al. represents a crucial step toward longer-term stability. The tetraalkyl phosphonium salt stably shuttles protons from the anode as the cation to donate them to nitrogen reduced at the cathode to form an ylide. Critically, this salt is not consumed like the previously reported sacrificial alcohol donor. The salt also enhances ionic conductivity, which allows this system to achieve high NH3 production rates (60 nmol s−1 cm−2) in 20-hour experiments at 20 bar N2.
Despite these advances, no reported system is ideal. The ideal system would operate at negligible overpotential (that is, toward 100% potential efficiency), with high current densities (>1 A/cm2) because of high turnover frequencies, have a lifetime of at least 5 years, and achieve 100% selectivity to NH3 (see blue stars in the figure). The best turnover numbers are still only ∼105 per site, well below the ideal of ∼1010 per site. 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 ([ 13 ][15], [ 14 ][16]).
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 ([ 13 ][15]). Battery science could provide key insights for improving the stability and effectiveness of the N2 reduction SEI, which is still uncharacterized and unoptimized. An effective SEI may even enable the use of water as a proton donor.
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, Imperial College London Data Repository, (2021); |
| 领域 | 气候变化 ; 资源环境 |
| URL | 查看原文 |
| 引用统计 | |
| 文献类型 | 期刊论文 |
| 条目标识符 | http://119.78.100.173/C666/handle/2XK7JSWQ/329900 |
| 专题 | 气候变化 资源环境科学 |
| 推荐引用方式 GB/T 7714 | Olivia Westhead,Rhodri Jervis,Ifan E. L. Stephens. Is lithium the key for nitrogen electroreduction?[J]. Science,2021. |
| APA | Olivia Westhead,Rhodri Jervis,&Ifan E. L. Stephens.(2021).Is lithium the key for nitrogen electroreduction?.Science. |
| MLA | Olivia Westhead,et al."Is lithium the key for nitrogen electroreduction?".Science (2021). |
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