Adsorption-regulated precise synthesis of atomically-dispersed bimetallic Fe-Co sites on carbon for electrocatalytic nitrogen reduction reaction

The intriguing features of single-atom catalysts (SACs) could bring catalysis into a new paradigm, however, controllably synthesising SACs with desired SA loadings and coordination forms are challenging. Here, we report an adsorption-regulated approach to precisely control the synthesis of bimetallic Fe-Co SAs on carbon. Bacterial cellulose (BC) is utilised as an adsorption regulator to controllably impregnate Fe3+/Co2+ on BC and through carbonisation to anchor Fe-Co SAs on BC-derived carbon via bimetallic [(O-C2)3Fe-Co(O-C2)3] coordination with desired Fe/Co contents and atomic ratios. Under electrocatalytic nitrogen reduction reaction (NRR) conditions, [(O-C2)3Fe-Co(O-C2)3] is operando transformed to [(O-C2)3Fe-Co(O-C)C2] that promotes and sustains NRR performance. A superb ammonia yield of 574.8 ± 35.3 μg h-1 mgcat.-1 with an exceptional faradaic eciency of 73.2 ± 4.6% are obtained from an electrocatalyst with the highest bimetallic Fe-Co site density. The exemplied synthetic approach would be of generically applicable to controllably anchor SAs on carbon that enables meaningfully investigate and rationally design SACs.


Background
Owing to their ultimate mass-catalytic activity, simple active site con guration and readily tuneable electronic structures, single-atom catalysts (SACs) have emerged as a class of catalysts with exempli ed superiority over other forms of catalysts [1][2][3][4] . Although still in an early stage, it has been widely assented that such intriguing features of SACs could bring the catalysis into a new paradigm, nevertheless, key challenges must be met 5,6 . One of the key challenges hindering rational design and development of SACs is the lack of insight into the performance-SA loading relationship, due mainly to our inability to precisely control the synthesis of SACs with desired loading densities and active site coordination forms. For electrocatalytic N 2 reduction reaction (NRR), Au, Ag, Ru, Pd, Mo, Mn, Y, Sc, Cu, Fe, Co and Ni based singleatom electrocatalysts (SAECs) have been reported  . The vast majority of such SAECs have been fabricated by anchoring SAs to carbon support almost exclusively via metal-nitrogen (M-N x ) or metal-carbon (M-C x ) coordination (Supplementary Table 1)   . To date, the carbon supported SAECs have been fabricated via two common approaches. One anchors SAs to carbon supports through calcination, while another employs metal ions-impregnated carbonisable precursors to anchor SAs via carbonisation [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][24][25][26][27] . The former normally involves a simple one-pot synthetic procedure, however, through the calcination process to precisely control SAs loading on a pre-carbonised support is di cult 5,6,19 . The late involves multiple synthetic steps (e.g., metal ion adsorption, carbonisation and acid etching), but has potentials to achieve controllable synthesis, although it has yet been realised.
Over the past few years, numerous SAECs with the atomically-dispersed Fe-Co, Ni-Fe and Zn-Co bimetallic sites on N-doped carbon supports have been constructed via M-N x coordination and demonstrated to possess higher electrocatalytic activities than their mono-metallic SA counterparts towards oxygen and carbon dioxide reduction reactions [28][29][30][31] . Recently, Wang et al. reported a NRR electrocatalyst with Ncoordinated Fe-Cu clusters anchored on C 3 N 4 modi ed carbon nanotubes to achieve an NH 3 yield rate of 10 μg h -1 mg cat.
-1 with a FE of 24.8 ± 0.8% at -0.45 V (vs. RHE) 33 . Both studies attributed the enhanced NRR performance to the bimetallic sites induced synergistic modulations on the electronic structures. Despite the demonstrated attractions of bimetallic SAECs, one can easily apprehend the tough challenge involved in the controllable synthesis of such bimetallic SAECs.
Here, we report an adsorption-regulated approach to precisely control the synthesis of oxygencoordinated bimetallic Fe-Co SAs sites on carbon with the desired Fe and Co contents, and Fe/Co atomic ratios as e cient SAECs for NRR. The bacterial cellulose (BC) with rich oxygen groups is utilised as an adsorption regulator to realise controllable adsorption of Fe 3+ /Co 2+     The synchrotron-based X-ray absorption near-edge structure (XANES) and the extended X-ray absorption ne structure (EXAFS) spectra were obtained to further validate the Fe/Co-O-C coordination con gurations. The Fe K edge XANES spectra (  Fig. 17b). It should note that under the experimental conditions, NH 3 is the sole NRR product and N 2 H 4 is undetectable ( Supplementary Fig. 18). Although the observed steady-state cathodic current densities are increased with the applied cathodic potentials ( Supplementary Fig. 17a), the determined R NH3 and FE (Fig. 3a) Fig. 3b and Supplementary Fig. 19, the yielded 14 Fig. 25 and Table 3) was synthesised and its NRR performance was evaluated. Supplementary Fig. 26 Table 2). Notably, the total Fe and Co SAs contents in Fe/Co-O-C-r are also closely approximated each other, therefore, for a given total Fe/Co content, the NRR performance of Fe/Co-O-Cr is likely determined by their Fe/Co atomic ratio. In fact, the NRR performance of Fe/Co-O-C-r increases as the Fe/Co atomic ratios closer to the unity, and Fe/Co-O-C-1.0 with a Fe/Co atomic ratio of 0.99 (Supplementary Table 2) exhibits the best NRR performance. This could be due to that Fe/Co-O-C-r with a Fe/Co atomic ratio approaching the unity possesses higher density of the bimetallic Fe-Co sites that dictate the NRR performance. Nonetheless, to our knowledge, no existing analytical technique is capable of quantitatively determining the density of such atomically-dispersed bimetallic Fe-Co sites.
Fortunately, the controllable synthetic approach used in this work enables us to precisely control both the content and the atomic ratio of Fe and Co. As such, if we assume that for a given Fe/Co-O-C-r, the bimetallic Fe-Co sites are homogeneously distributed and the maximum possible bimetallic Fe-Co site density is determined by the lower Fe or Co content in a sample, then according to Supplementary Table  5 (Fig. 3f), signifying that the trend of the bimetallic Fe-Co site density matches the trend of the experimentally determined NRR performance. This provides us with a reasonable con dence that the density of the bimetallic Fe-Co sites is a decisive factor for NRR performance of Fe/Co-O-C-r, nevertheless, further elucidation on the NRR activity origin of the bimetallic Fe-Co sites is needed.
Mechanistic studies. The experimentally identi ed Fe-Co sites con gurations were used to construct the bimetallic Fe-Co sites structural model for the density functional theory (DFT) calculations. Fig. 4a shows  Fig. 27) or concurrently adsorb on Fe-Co site via side-on adsorption (Fig. 4b). With side-on adsorption, the calculated charge density difference ( Supplementary Fig.  28) discloses that the electrons in d orbitals of Fe and Co are transferred to the empty π* orbitals of N 2 .
As a result, the adsorbed N 2 gains 0.72 efrom the bimetallic site, and the N-N bond is elongated from 1.120 Å (free gaseous N 2 state) to 1.225 Å, suggesting that the adsorbed N 2 is activated. Notably, the calculated reaction free energy of the rst hydrogenation step (*N 2 + H + + e − → *NH-N) for side-on adsorption on Fe-Co site is 0.61 eV, which is approximately half of that for the end-on adsorption on Fe (1.16 eV) and Co (1.30 eV) sites, con rming that the side-on adsorbed N 2 on the bimetallic Fe-Co site is favourable for NRR.

The catalytic activity origin and NRR pathway on the bimetallic [(O-C 2 ) 3 Fe-Co(O-C 2 ) 3 ] sites were
subsequently investigated in details. Supplementary Fig. 29 shows the projected density of states (PDOS) before and after side-on adsorption on the bimetallic Fe-Co site. The broadened Fe-3d and Co-3d states after N 2 adsorption addicting to the overlapped Fe, Co and N states imply the hybridized Fe/Co 3d orbitals with N 2p orbital. The overlapped PDOS between Fe-3d and *N 2 -2p, and Co-3d and *N 2 -2p are distributed below and above the Fermi energy, signifying a back-bonding between the bimetallic Fe-Co site and *N 2 48 . These suggest that the electrocatalytic NRR activity of Fe/Co-O-C-r electrocatalysts is resulted from the effective hybridisation of N 2p orbital with the synergistically con gured bimetallic Fe-Co site.
We consequently calculated the Gibbs free energy diagram of side-on adsorption NRR pathway and intermediates structures on [(O-C 2 ) 3 Fe-Co(O-C 2 ) 3 ] corresponding to each reaction step (Fig. 4c). The  Fig. 30) and concurrently on Fe-Co site via the side-on adsorption (Fig. 4e), respectively. The calculated charge density differenceunveils that the side-on

Discussion
In summary, we exempli ed an adsorption-regulated synthetic strategy to fabricate atomically dispersed Fe-Co SAs supported on carbon in a precisely controllable manner via metal-oxygen coordination.
The quantitative relationships de ning Fe 3+ /Co 2+ distributions between the adsorption solution and bacterial cellulose adsorbent, and the percentage of the adsorbed Fe 3+ /Co 2+ converting to Fe/Co SAs were unveiled and used to guide the controllable synthesis of bimetallic Fe-Co SACs with desired Fe/Co contents and atomic ratios. We demonstrated that the controllably synthesised SACs can be used to meaningfully depict the composition-performance relationship. The catalyst with a Fe/Co atomic ratio of 1:1 possesses the highest bimetallic Fe-Co site density and exhibits the best NRR performance. The exempli ed approach would be of generically applicable to controllably anchor a wide spectrum of other SAs on carbon. We envisage that an ability to controllably synthesise SACs with variety of desired compositions is essential for depicting the factual composition-performance relationships that enable rationally design and development of SACs. 15 N 2 isotope labelling experiments. When quality assurance required, 15 N isotopic labelling experiments were conducted using 15 N 2 as the feeding gas with identical experimental procedure as that of 14