Moving beyond bimetallic-alloy to single-atom dimer atomic-interface for all-pH hydrogen evolution

Single-atom-catalysts (SACs) afford a fascinating activity with respect to other nanomaterials for hydrogen evolution reaction (HER), yet the simplicity of single-atom center limits its

further modification and utilization. Obtaining bimetallic single-atom-dimer (SAD) structures can reform the electronic structure of SACs with added atomic-level synergistic effect, further

improving HER kinetics beyond SACs. However, the synthesis and identification of such SAD structure remains conceptually challenging. Herein, systematic first-principle screening reveals

that the synergistic interaction at the NiCo-SAD atomic interface can upshift the d-band center, thereby, facilitate rapid water-dissociation and optimal proton adsorption, accelerating

alkaline/acidic HER kinetics. Inspired by theoretical predictions, we develop a facile strategy to obtain NiCo-SAD on N-doped carbon (NiCo-SAD-NC) via in-situ trapping of metal ions followed

by pyrolysis with precisely controlled N-moieties. X-ray absorption spectroscopy indicates the emergence of Ni-Co coordination at the atomic-level. The obtained NiCo-SAD-NC exhibits

exceptional pH-universal HER-activity, demanding only 54.7 and 61 mV overpotentials at −10 mA cm−2 in acidic and alkaline media, respectively. This work provides a facile synthetic strategy

for SAD catalysts and sheds light on the fundamentals of structure-activity relationships for future applications.

Owing to the high energy storage density and zero carbon emission, hydrogen (H2) fuel from water electrolysis has been regarded as the most promising alternative to fossil fuels1,2.

Strikingly, the hydrogen evolution reaction (HER) plays an essential role in electrochemical water splitting for energy conversion. Various water electrolyzers demand different pH values of

the electrolyte, such as proton exchange membrane electrolysis in strong acid, seawater electrolysis in neutral medium, and commercial water electrolysis in strong base3. To meet the above

requirements, pH-universal HER catalysts with superior performance in both acidic and alkali media are highly regarded; however, they are barely accessible4. Platinum (Pt) and Pt-based

catalysts are still the best-known pH-universal HER electrocatalysts, but their limited availability and high cost hinder their large-scale applications5. Therefore, exploring non-precious

metal-based electrocatalysts with Pt-like pH-universal HER activity is highly desired, yet challenging.

To date, numerous earth-abundant HER electrocatalysts including oxides, hydroxides, alloys, phosphides, nitrides, sulfides, and their hybrids, have been identified as promising HER

catalysts3,6,7,8,9,10,11. However, satisfactory Pt-like activity has been seldomly achieved and only a few of them can be simultaneously active in both acidic and alkaline media3. Recently,

single-atom catalysts (SACs) with nearly 100% atom economy and unique electronic properties compared to their regular nanoparticle (NPs) counterparts have attracted immense scientific

attention in the field of photo/electro/thermo-catalysis12,13,14. Most SACs contain isolated single metal sites coordinated with the neighboring nitrogen atoms in carbon matrix (M-NC), which

are only capable of catalyzing simple elementary reactions15. Due to the simplicity of the single-atom center, the possibilities for further modification of the active site in SACs are

extremely limited, hindering their wide range of applications16. In response to this, recent exploration suggests that tuning the coordination site to sulfur/phosphorus or by introducing

secondary metal atom to construct metal–metal dual atom sites (single-atom dimer: SAD) can further modulate the electronic structure of SACs and boost their intrinsic activity, attributed to

the unique atomic interface and synergistic effect of dual-metal site17,18,19. Recently, Fe-Co, Zn-Co, and Ni-Fe dual-metal sites have been demonstrated as efficient bifunctional oxygen

electrocatalysts (oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) and for CO2 reduction reaction (CO2RR)20,21,22. Zhang et al.23 synthesized noble metal-based Pt-Ru dimer

using the advanced atomic layer deposition technique and showed comparable HER performance to commercial Pt in acidic media. However, the evidence for the formation of a single metal–metal

bond from X-ray absorption spectroscopy (XAS) was unclear due to the existence of additional atomic clusters in the sample23. Although SADs are explored towards ORR/OER/CO2RR, a generalized

cost-effective and versatile strategy to fabricate pH-universal low-cost HER catalyst with targeted dimeric sites at atomic precision along with appropriate identification of the dimeric

structure and deeper understanding of the dual-metal atom synergism has never been achieved and remain elusive.

Herein, we report a transition metal-based SAD (TM-SAD) atomic interface, which can efficiently catalyze complex HER in a wide pH range (0–14). At first, systematic density functional theory

(DFT) screening reveals that among various TM-SADs, the synergistic interaction between the Ni-Co at the atomic level in the SAD configuration can significantly upshift the d-band center,

thereby accelerating water dissociation and boosting pH-universal HER activity. Motivated by DFT prediction, we develop a facile methodology to synthesize NiCo-SAD on N-doped carbon

(NiCo-SAD-NC) via in situ trapping of targeted metal ions in the polydopamine sphere followed by annealing with precisely controlling the N-moieties. State-of-the-art techniques including

X-ray absorption near edge structure (XANES), extended X-ray absorption fine spectra (EXAFS), aberration-corrected scanning transmission electron microscopy (AC-STEM), and X-ray

photoelectron spectroscopy (XPS) along with theoretical calculation are employed to analyze the detailed structure of the NiCo-SAD-NC, which reveal the emergence of Ni-Co bond with strong

electronic coupling at the atomic level. The as-prepared NiCo-SAD-NC exhibits an exceptional pH-universal HER activity, which requires only 54.7 and 61 mV overpotential at −10 mA cm−2 in

acidic and alkaline media, respectively, outperforms the NiCo-NP and monoatomic Ni/Co-SACs. The activity of NiCo-SAD-NC is comparable/superior to commercial Pt-C/Pt-SAC, as well as superior

to most of the recently reported TM-based single-atom electrocatalysts.

We first employed DFT calculations for screening various bimetallic TM-SAD structures stabilized on the N-doped carbon for HER based on their electronic parameters. To evaluate the stability

of TM-SAD structures (heteronuclear: CoCu-SAD-N6C, NiCo-SAD-N6C, CoFe-SAD-N6C, CoMn-SAD-N6C; homonuclear: CuCu-SAD-N6C, NiNi-SAD-N6C, CoCo-SAD-N6C, FeFe-SAD-N6C, MnMn-SAD-N6C), we

calculated the formation energies (Ef), displayed in Fig. 1a and Supplementary Table 1. All the selected TM-SAD structures were thermodynamically stable, revealed from their respective

negative values of Ef, which also exhibited an increasing trend with the number of outermost 3d orbital valence electrons. Interestingly, we found that there was a consistent trend for the

average Mulliken charges distribution (Δq) of TM-SAD center with the formation energy of TM-SAD-N6G structures, except for the CoCu/CuCu-SAD-N6C (Fig. 1a and Supplementary Fig. 1a). The

formation energy increased with the increase of Δq in the homo/heterostructures of SAD, suggesting that higher Δq confirmed the higher thermodynamically stable SAD structure24. In addition,

the water adsorption energy exhibited a linear correlation with the Mulliken charge transfer from the metal active site, suggesting that higher charge transfer from the active site exhibited

stronger water adsorption strength (Supplementary Fig. 1b). The differential charge density distribution revealed that a significant charge resides between the metal atoms and N

coordination, with charge accumulated at the center and depleted at the metal center, consistent with the Mulliken charge analysis (Fig. 1b). The TM atoms with lower electronegativities

showed a higher tendency to donate more electrons to N atoms and form stronger TM-N bonds in the SAD structure. Meanwhile, the projected partial density of states of various TM-SAD showed

that the d-orbitals of the TM atoms were mainly distributed around the Fermi level (Supplementary Fig. 2). We further investigated whether there was a correlation between the d-band center

and formation energy, and we found a poor linear relationship between them (Fig. 1a); however, 3d band center of SAD displayed a linear correlation with kinetic barrier of H2O dissociation

(Fig. 1d). Especially, the comprehensive d-band centers of Co and Ni atoms in the NiCo-SAD-N6C (−0.87 eV) were the nearest to the Fermi level compared to the other TM-SADs, except for

NiNi-SAD-N6C, demonstrating its superior ability to facilitate water dissociation and enhanced proton adsorption, beneficial for HER25. In addition, a significant overlap between the Co 3d

orbitals of NiCo/CoMn-SAD-N6C and Fe 3d orbitals of FeFe-SAD-N6C with the O 2p orbitals of adsorbed H2O near the Fermi level consolidated that the Co and Fe atoms in their respective TM-SADs

acted as the principal active site for activating the H2O dissociation, boosting the HER (Supplementary Fig. 3).

a Formation energies corresponding to the charge depletion (Mulliken charge) and d-band center of various TM-SAD-N6C. b Different charge density distribution of TM-SAD-N6C. The charge

depletion and accumulation are denoted by green and red colors, respectively. c Minimum energy paths of water splitting reactions on TM-SAD-N6C (blue: Co and gray: Ni). d Linear correlation

between the d-band center and kinetic energy barrier. e Free energy diagram of hydrogen adsorption and f free energy changes of the hydronium (ΔGH*) and hydroxide (ΔGOH*) desorption step for

TM-SAD-N6C.

For the thermodynamic assessments of these TM-SAD structures towards HER, we calculated the kinetic energy barrier for the H2O dissociation (Fig. 1c). After the optimization of all the

possible structures for the initial/final states in the H2O splitting process, we recognized that the NiNi-SAD-N6C was inactive and incapable of H2O adsorption; therefore, further

examination was ignored (Supplementary Discussion 1). Figure 1c revealed that the NiCo-SAD-N6C exhibited the lowest transition state energy barrier for H2O dissociation compared to other

TM-SADs, suggesting that the NiCo-SAD-N6C with the highest H2O dissociation rate can be regarded as a promising candidate for alkaline HER (Supplementary Table 2a). Interestingly, we found

that the H2O dissociation energy barrier for the TM-SADs decreased with the increase in the 3d band center, further corroborating the reactivity trends (Fig. 1d). In addition, we also

extended our calculation for alkaline HER activity with other metal (Cr, Mo, and Zn)-based SAD, summarized in Supplementary Table 2b. As revealed in Supplementary Table 2b, the NiCo-SAD-N6C

still exhibited the best alkaline HER activity compared to other SAD based on the evaluated kinetic barrier of water dissociation and free energy of *H and *OH desorption. In addition, for

the Co and Ni single-atom sites (Co-SA-N4C and Ni-SA-N4C), the calculation revealed that the hydronium (H*) and hydroxide (OH*) species auto-recombined to generate H2O molecule after the

optimization process in the final state of H2O dissociation, suggesting that only Co and Ni single-atom sites were extremely sluggish for H2O dissociation, further validating the beneficial

role of synergistic interaction at the Ni-Co atomic interface in the SAD configurations. To evaluate the potential-determining-step (UL) for HER, the free energies for the desorption of H*

and OH* were calculated, where the |UL| should be equal to the larger value among the |ΔGOH*| and |ΔGH*|. Figure 1e, f revealed that for CoCu-SAD-N6C, NiCo-SAD-N6C, CoFe-SAD-N6C,

CuCu-SAD-N6C, CoCo-SAD-N6C, and MnMn-SAD-N6C, the |ΔGOH*| was more uphill than that of |ΔGH*|, suggesting the |ΔGOH*| as the |UL|. In contrast, the |UL| for CoMn-SAD-N6C and FeFe-SAD-N6C

were determined by the |ΔGH*|. By comparing the |UL| of the TM-SADs, three potential candidates with the lowest theoretical overpotentials were observed in the following order: NiCo-SAD-N6C

(0.460 eV) < CoMn-SAD-N6C (0.465 eV)