Tailoring Water Dissociation Energy by Platinum Single-Atom Catalyst Coupled with Transition Metal/metal Oxide Heterostructure for Accelerating Alkaline Hydrogen Evolution Reaction

High-activity catalysts in alkaline media are compelling for durable hydrogen evolution reaction (HER). Single-atom catalysts (SACs) provide an effective approach to reduce the amount of precious metals meanwhile maintain their catalytic activity. However, the sluggish activity of SACs for water dissociation in alkaline media has extremely hampered advances in highly efficient hydrogen production. Herein, we developed a platinum SAC immobilized NiO/Ni heterostructure (PtSA-NiO/Ni) as an alkaline HER catalyst. It was found that Pt SACs coupled with NiO/Ni heterostructure enable the tunable binding abilities of hydroxyl ions (OH*) and hydrogen (H*), which efficiently tailors the water dissociation energy for accelerating alkaline HER. In particular, the dual active sites consisting of metallic Ni sites and O vacancies modified NiO sites near the interfaces of NiO/Ni in PtSA-NiO/Ni have preferred adsorption affinity for H* and OH* groups, respectively, which efficiently lowers the energy barrier of water dissociation of Volmer step. Moreover, anchoring Pt single atoms at the interfaces of NiO/Ni heterostructure induces more free electrons on Pt sites due to the 2 elevated occupation of the Pt 5d orbital at the Fermi level and reaches a near-zero H binding energy (ΔGH*, 0.07 eV), which further promotes the H* conversion and H2 evolution. Further enhancement of alkaline HER performance was achieved by constructing PtSA-NiO/Ni nanosheets on the Ag nanowires to form a hierarchical threedimensional (3D) morphology that provides abundant active sites and accessible channels for charge transfer and mass transport. Consequently, the fabricated PtSANiO/Ni catalyst displays extremely high alkaline HER performances with a quite high mass activity of 20.6 A mg for Pt at the overpotential of 100 mV, which is 41 times greater than that of the commercial Pt/C catalyst, significantly outperforming the reported catalysts.

elevated occupation of the Pt 5d orbital at the Fermi level and reaches a near-zero H binding energy (ΔGH*, 0.07 eV), which further promotes the H* conversion and H2 evolution. Further enhancement of alkaline HER performance was achieved by constructing PtSA-NiO/Ni nanosheets on the Ag nanowires to form a hierarchical threedimensional (3D) morphology that provides abundant active sites and accessible channels for charge transfer and mass transport. Consequently, the fabricated PtSA-NiO/Ni catalyst displays extremely high alkaline HER performances with a quite high mass activity of 20.6 A mg -1 for Pt at the overpotential of 100 mV, which is 41 times greater than that of the commercial Pt/C catalyst, significantly outperforming the reported catalysts.

Introduction.
Hydrogen (H2) has been regarded as the most promising energy carrier alternative to fossil fuels due to the environmental friendliness nature and high gravimetric energy density. 1,2 Electrocatalytic water splitting powered by wind energy or solar technologies for hydrogen generation is considered a sustainable strategy. 3 For an optimal electrocatalyst, minimizing the energy barrier and increasing the active sites are desirable for boosting the hydrogen evolution reaction (HER). 4-6 Despite the significant progress that has been presented in nonprecious catalysts, the HER performances are still second to platinum (Pt)-based materials due to its optimal binding ability with hydrogen. [7][8][9][10] However, the high cost and scarcity of Pt extremely hamper its large-scale application in electrolyzers for H2 production. Single-atom catalysts (SACs) provide an effective approach to reduce the amount of Pt meanwhile maintain its high intrinsic activity. [11][12][13][14] Recently, electrocatalytic HER in an alkaline condition has attracted more attention because catalyst systems are generally unstable in acidic media, resulting in safety and cost concerns in practice. Unfortunately, the alkaline HER activity of Ptbased catalysts is approximately two orders of magnitude lower than that in the acidic condition caused by the high activation energy of the water dissociation step. [15][16][17][18] Alkaline HER process involves two electrochemical reaction steps: (step (i)) electroncoupled H2O dissociation to generate adsorbed hydrogen hydroxyl (OH*) and hydrogen (H*) (Volmer step), and (step (ii)) the concomitant interaction of dissociated H* into molecular H2 (Heyrovsky or Tafel step). 19,20 In particular, the additional energy in step (i) is required to overcome the barrier for splitting strong OH-H bond, leading to a hamper of Pt SACs for alkaline HER application. Therefore, reducing the water dissociation energy in Volmer step (step (i)) for Pt single-atom catalyst in alkaline media becomes vital for large-scale H2 production of industrialization.
Some strategies have been developed to improve Pt SACs HER activity. For instance, employing the microenvironment engineering to immobilize single Pt atoms in MXene nanosheets (Mo2TiC2Tx) and onion-like carbon nanospheres supports could greatly reduce the H adsorption energy (ΔGH) and, thus, facilitates the release of H2 molecular. 21,22 Besides, Pt single atoms anchored alloy catalysts (Pt/np-Co0.85Se SAC) were constructed as an efficient HER electrocatalyst, 23 in which np-Co0.85Se can largely optimize the adsorption/desorption energy of hydrogen on atomic Pt sites, thus improving the HER kinetics. Furthermore, by utilizing the electronic interaction between the Pt atoms and the supports, single-atom Pt anchored 2D MoS2 (PtSA-MoS2), 24 nitrogen-doped graphene nanosheets (PtSA-NGNs) 25 and porous carbon matrix (Pt@PCM) 26 show enhanced electrocatalytic HER efficiency due to the higher d band occupation near Fermi level, which can provide more free electrons for boosting the H* conversion. Despite significant progress in Pt SACs, these methods are difficult to decrease the energy barrier of water dissociation in the Volmer step (step (i)). Generally, the H2O dissociation and H* conversion happen on different catalytic sites. 27 Especially, the HER activities of Pt-based catalysts in alkaline conditions are governed by the binding ability of hydroxyl species (OH*), [28][29][30] and the alkaline HER kinetics could be optimized by independently regulating the binding energy of reactants (OH and H*) on dual active sites. [31][32][33] Inspired by these findings, the energy barrier of Pt SCAs for H2O dissociation in Volmer step (step (i)) in alkaline media could be decreased by incorporating or creating the dual active sites in the catalyst to independently modulate the binding energy of reactants (OH* and H*).
In this work, we developed a three-dimensional (3D) nanostructured electrocatalyst consisting of two-dimensional (2D) NiO/Ni heterostructure nanosheets supported single-atom Pt attached on one-dimensional (1D) Ag nanowires (Ag NWs) conductive network (PtSA-NiO/Ni). Density functional theory (DFT) calculations reveal that the dual active sites consisting of metallic Ni sites and O vacancies modified NiO sites near the interfaces of NiO/Ni heterostructure in PtSA-NiO/Ni show the preferred adsorption affinity toward OH* and H*, respectively, which efficiently facilitates water adsorption and reaching a barrier-free water dissociation step with a lower energy barrier of 0.11 eV in Volmer step (step (i)) for PtSA-NiO/Ni in the alkaline condition compared with that of PtSA-NiO (0.34 eV) and PtSA-NiO (1.27 eV) catalysts.
Additionally, anchoring Pt single atoms at the interfaces of NiO/Ni heterostructure induces more free electrons on Pt sites due to the elevated occupation of the Pt 5d orbital at Fermi level and the more suitable H binding energy (ΔGH*, 0.07 eV) than that of Pt atoms at the NiO (ΔGH*, 0.93 eV) and Ni (ΔGH*, 0.26 eV), which efficiently promotes the H* conversion and H2 desorption, thus accelerating overall alkaline HER. (step (ii)). Furthermore, the Ag NWs supported 3D morphology provides abundant active sites and accessible channels for charge transfer and mass transport. As a result, the fabricated PtSA-NiO/Ni catalyst exhibits outstanding HER activity with a quite lower overpotential of 26 mV at 10 mA cm -2 in 1 M KOH. The mass activity of PtSA-NiO/Ni is 20.6 A mg -1 Pt at the overpotential of 100 mV, which is 41 times greater than that of the commercial Pt/C catalyst, significantly outperforming the reported catalysts. This work provides a new design principle toward single-atom catalyst systems for efficient alkaline HER.

Results
Synthesis and characterization of PtSA-NiO/Ni catalyst. The fabrication process of PtSA-NiO/Ni on Ag NWs is illustrated in Figure 1. In brief, the synthesized Ag NWs by a typical hydrothermal method 34 were first loaded on the flexible cloth to form a conductive network. Then Ni/NiO composite is attached to the Ag network by the facile electrodeposition process. 35 In detail, the Ag NWs network loaded cloth is immersed in nickel acetate aqueous solution followed by an electrochemical process with -3.0 V versus SCE (saturated calomel electrode) for 200 s ( Figure S1), forming the uniformly distributed nanosheets on the Ag network ( Figure S2). Transmission electron microscopy (TEM, Figure S3a-b) images, high-resolution TEM (HRTEM, Figure S3c) image, fast Fourier transform (FFT, Figure S3d), and elements mapping ( Figure S4) images clearly show that the metallic Ni nanoparticles uniformly embed in amorphous NiO nanosheets. Besides, the X-ray diffraction (XRD, Figure S5) pattern shows that only metallic Ni signal without the peaks of NiO can be detected, and X-ray photoelectron spectroscopy (XPS, Figure S6) spectra suggest both metallic Ni and Ni oxide exists in Ni/NiO sample, further confirming the composition of metallic Ni on amorphous NiO. Interestingly, the deposited composition can be facilely controlled by performing various voltage in the nickel acetate aqueous solution. 35 Specifically, as above discussion, a high voltage of -3 V versus SCE will generate the Ni/NiO composite on Ag NWs (NiO/Ni), whereas a lower voltage of -1 V versus SCE could prepare the amorphous NiO on Ag NWs (NiO, Figure S7  To quantitate the electronic state structural information, the white-line peak evolution of Pt can be clearly described by the differential XANES spectra (ΔXANES, Figure S17) by subtracting the spectra from that of Pt foil. The valence state of Pt can be quantitatively examined by the integration of the white-line peak in ΔXANES spectra. As shown in Figure 3c (Figure 2d-g). To more precisely clarify the atomic dispersion and coordination conditions of Pt, the wavelet transform (WT) analysis was carried out due to its more efficient resolution ability in K spaces and radial distance, 48,49 in which the atoms at similar coordination conditions and distances could be discriminated. 50 Consequently, a locally enhanced electric field with a half-moon shape area around the Pt site was generated ( Figure S19c-d), which is more intensive than that of PtSA-NiO  Additionally, the PtSA-NiO/Ni exhibits a smaller Tafel slope of 27.07 mV dec -1 than PtSA-NiO (37.54 mV dec -1 ), PtSA-Ni (37.32 mV dec -1 ), NiO/Ni (58.67 mV dec -1 ), and Pt/C catalyst (41.69 mV dec -1 ), which suggests a typical Volmer-Tafel mechanism for alkaline HER and implies that the rate-determining step of PtSA-NiO/Ni is the H2 desorption (Tafel step) rather than the H2O dissociation (Volmer step). 56 For real applications, HER catalyzing stability is another essential factor. As present in Figure 5f, the PtSA-NiO/Ni shows high durability in the alkaline electrolyte with negligible loss in HER performance for 5000 cycles or 30 hours. The characterizations of PtSA-NiO/Ni after the stability test, including HAADF-STEM image, elements mapping, and double-layer capacitance ( Figure S28-30

Methods
Synthesis of Ag NWs. An oil bath method was used to synthesize Ag NWs according to our previous report. 58 Specifically, a mix solution consisting of ethylene glycol, FeCl3 Electrochemical measurements. All electrochemical measurements were finished by an electrochemical workstation (CHI 660E) with a three-electrode configuration, in which fabricated catalysts were directly employed as the working electrode, graphite sheet acted as a counter electrode, saturated calomel electrode acted as a reference electrode. All the presented potential in this work was transferred to RHE according to an experimental method. 59 LSV with 95% iR-corrections were tested under the potential range from 0.05 to -0.5 V and the scan rate of 5 mV s -1 . EIS was obtained by a frequency range from 100 k to 0.1 Hz with an overpotential of 230 mV vs RHE. For the preparation of 3D Pt/C@Ni foam, 5 mg 20 wt% Pt/C was dispersed in 0.9 mL alcohol containing 0.1 mL 5 wt% Nafion solution to form a homogeneous ink. Then, the obtained ink was coated on the Ni foam and dried in air to form a porous Pt/C@Ni foam electrode.
DFT theoretical calculations. All the structural optimizations, charge density difference analysis, Bader charge analysis, and energy calculations were carried out based on DFT as implemented in the Vienna Ab-initio Simulation Package (VASP). [60][61][62] The projector-augmented-wave (PAW) method was implemented to calculate the interaction between the ionic cores and valence electrons. 63,64 The Perdew-Burke-Ernzerhof approach of spin-polarized generalized gradient approximation (GGA-PBE) was used to describe the exchange-correlation energy. 65 Calculations were performed with the cut-off plane-wave kinetic energy of 500 eV, and 8×4×1 k-mesh grids were employed for the integration of the Brillouin zone. Electronic relaxation was undertaken to utilize the conjugate-gradient (CG) method 66 with the total energy convergence criterion being 10 -5 eV. Geometry optimization was employed by the quasi-Newton algorithm 67,68 until all the residual forces on unconstrained atoms less than 0.01 eV/Å. Climbing image nudge elastic band (CI-NEB) calculations 69 were employed for finding transition barriers with the initial configuration of H2O absorbed on the catalyst surface and final configuration of OH + H absorbed on the catalyst surface. To ensure the initial configuration correctly, an H2O molecule was deposited on the catalyst surface and relaxed for calculating its local minimum total energy on different sites, and the last one is the initially stable configuration. The final configuration is also found by relaxing OH and H near the H2O absorbed site of the initial configuration. Next, The equation for calculating adsorption enthalpy ∆ H* as the following: ∆EH*=Eslab+H-Eslab- where ΔEH* represent the H adsorption energy and Δ H 2 O * represent the H2O adsorption energy, and ΔEZPE represents the difference related to the zero-point energy between the gas phase and the adsorbed state.