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Pei et al. J Mater Inf 2023;3:26 https://dx.doi.org/10.20517/jmi.2023.35 Page 9 of 14

Reaction mechanism of electrochemical NRR
As shown in Figure 4A, one reaction mechanism for the electrocatalytic synthesis of ammonia, namely the
enzymatic mechanism, is depicted, and an additional consecutive mechanism is presented in Supplementary
Figure 1A. The distal mechanism and the alternative mechanism are also detailed in Supplementary Figure
1B. A common feature among them is that the N atom at one end of N is adsorbed on the catalyst, while
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the N element at the other end is not adsorbed. However, the N element at the far end preferentially reacts
with the H proton. In the distal mechanism, N atoms leaving the surface of the catalyst preferentially react
with H protons and release as ammonia, leaving *N adsorbed on the catalyst, which will start hydrogenation
and form ammonia. An alternative mechanism involves hydrogenating two N atoms using six proton-
electron pairs to form two NH molecules. Contrarily, in the enzymatic pathway, the N molecule is first
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decomposed into adsorbed *N atoms, and then, *N is gradually hydrogenated into ammonia. The
hydrogenation process follows the same pathway as the alternative mechanism. In the consecutive
mechanism, one of the N atoms undergoes hydrogenation first and subsequently reacts with the remaining
N atoms, resulting in the formation of the second NH molecule. Owing to the distinct structures and
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performances of catalysts, the NRR reaction mechanism often varies. Nevertheless, numerous studies have
shown that the initial or final step can be considered the potential determinant independent of NRR
mechanisms [73-75] . The proton step with the maximal positive free energy change (∆G ) is defined as the
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PDS. It is well known that the ideal catalyst for an electrochemical NRR should meet the following criteria:
(1) The ∆G of the two key steps is below 0.55 eV, that is, ∆G *N 2 →*NNH ≤ 0.55 eV and ∆G *NH 2 →*NH 3 ≤ 0.55 eV,
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so the performance of the catalyst in question is superior to that of the best pure metal or nanometal cluster
- E < 0, which proves that N has good selectivity in catalysts.
catalyst; (2) E ads(*N 2 ) ads(*H) 2

For a prompt evaluation of TM @C N catalysts regarding their NRR performance, we calculated the ∆G for
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the initial and final stages of NRR under open-circuit conditions (U = 0). The resulting values, along with
corresponding diagrams, are presented in Table 2 and Supplementary Figure 2. Four classifications are
considered to evaluate the critical steps, with the critical point set at 0.55 eV. Consequently, six systems met
the above criteria, demonstrating decent catalytic activity in electrochemical NRR. In the case of Pt @C N ,
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the associated free energy diagram is presented in Figure 4B. The Pt @C N exhibits a propensity for NRR
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via enzymatic mechanisms, wherein N adopts a side-on configuration. In the enzymatic mechanism, the
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initial step of hydrogenation involves the formation of a chemical bond between a hydrogen atom and one
of the N atoms, followed by alternating bonding to the two N atoms until a second NH molecule is
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produced. The ∆G change is 0.24 eV, establishing the final hydrogenation step as the PDS of the entire
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electrochemical NRR. Similarly, we investigated the NRR pathway on Ru @C N , as depicted in Figure 4C,
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revealing a ∆G of 0.35 eV.
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The relevant intermediate geometries of Pt @C N for each step are presented clearly in Figure 4D. It is
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evident that throughout the entire NRR process, the initial step (*N → *NNH) is identified as the PDS.
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Briefly speaking, for Pt @C N catalysts, the corresponding limiting potential (U ) value is -0.24 V, while for
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Ru @C N catalysts, the U value is -0.35 V. Consequently, by applying the U to the surfaces of Pt @C N
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and Ru @C N catalysts, it is ensured that all electron transfer steps occur spontaneously without any uphill
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energy barriers, which is beneficial to the production of NH , where the reaction process of Ru @C N also
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follows an enzymatic mechanism.
It is worth emphasizing that the development of efficient NRR catalysts remains challenging due to the
competition with HER [66,76,77] . An ideal NRR catalyst would demonstrate significantly higher NRR activity
and considerably lower HER activity. To assess selectivity, we calculated the N adsorption energy and
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hydrogen adsorption energy on the designed TM @C N catalysts using Equation (2). A more negative
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difference between N adsorption energy and hydrogen adsorption energy indicates higher selectivity for
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