Single-atom Mo-tailored high-entropy-alloy ultrathin nanosheets with intrinsic tensile strain enhance electrocatalysis

Direct methanol fuel cells are promising energy conversion devices but are hindered by high Pt loadings and poor tolerance to COads poisoning in the methanol oxidation reaction (MOR). Conventional strategies such as nanostructuring, alloying Pt with a limited set of transition metals, morphology control, and support optimization tune Pt binding energies but leave abundant contiguous Pt-Pt ensembles that promote COads formation, limiting activity and durability. High-entropy alloys (HEAs) offer a large compositional space and can isolate Pt atoms and redistribute charge to modulate intermediate binding for multi-electron MOR, but precise atomic-level design to simultaneously enhance activity, CO tolerance, and stability remains challenging. This study aims to integrate atomically dispersed Mo promoters into tensile-strained PdPtNiCuZn HEA ultrathin nanosheets to dilute Pt ensembles, tailor the electronic environment, and steer MOR along a CO-free, formate-dominated pathway with high activity and durability.

Prior work has improved MOR catalysts by architectural control, transition-metal alloying with Pt, morphology engineering, and support effects, yet the electron modulation is constrained by limited alloying elements and persistent Pt-Pt ensembles that foster COads (refs. 10–18). HEAs have emerged to broaden compositional freedom, redistribute charge, and provide multiple active sites to modulate intermediate binding in MOR (refs. 19–25). Synergistic strain and ligand effects, as well as single-atom promoters on noble-metal hosts, have enhanced alcohol oxidation and mitigated CO poisoning in related systems (refs. 11, 31–35). Despite advances in HEA synthesis and performance, atomically precise tailoring to both isolate Pt sites and engineer the electronic microenvironment for CO-free MOR with high stability has remained elusive.

Synthesis: A one-pot liquid-phase method in oleylamine (OAm) at 200 °C was developed to prepare single-atom Mo-tailored HEA ultrathin nanosheets (NSs). For Mo1-PdPtNiCuZn SAHEA NSs, Pt(acac)2, Pd(acac)2, Ni(acac)2, and Zn(acac)2 were dissolved in OAm, degassed at 80 °C, purged with N2, followed by addition of Mo(CO)6 and L-ascorbic acid (AA), brief evacuation, then heating to 200 °C. Cu(acac)2 in OAm/toluene was added dropwise, and the mixture reacted for 2 h before quenching and solvent washing. Controlled growth proceeds via: (i) Pd-rich atomic-thick NS formation, (ii) reduction/diffusion of Pt, Ni, Cu, Zn into Pd NSs, (iii) formation of atomically dispersed Mo sites on NS surfaces. Senary (Mo1-PdPtCoNiCuZn) and septenary (Mo1-PdPtFeCoNiCuZn) SAHEA NSs were synthesized analogously by adding Co and Fe precursors. Nanoparticle (NP) analogs were obtained when Cu precursor was co-introduced at the beginning. HEA NPs without Mo were also synthesized for controls. Characterization: Structure and morphology were analyzed by TEM, HAADF-STEM, HRTEM, AFM, XRD, STEM-EDS/EDX mapping, ICP-OES/ICP-MS, XPS, EELS line scanning, and XAFS (XANES/EXAFS/WT). Ultrathin 2D graphene-like NSs displayed fcc Pd-type diffraction without phase segregation, broadened peaks indicating lattice distortion, average thickness ~1.69 nm (AFM), polycrystalline structure with concave defects (HRTEM), clear (111) lattice fringes (HAADF-STEM/FFT), and lattice spacing variation 0.233–0.237 nm evidencing ~−4.4% intrinsic tensile strain. EDS mapping showed homogeneous Pd/Pt/Ni/Cu/Zn distribution with isolated Mo atoms. XPS indicated Pd, Pt, Ni, Cu, Zn predominantly metallic; Mo in oxidized states. EELS detected Mo M4,5 edges at isolated atomic columns. Mo K-edge XANES exhibited higher edge energy and white line than Mo foil with a pre-edge shoulder consistent with distorted MoOx [MoO6]; FT-EXAFS showed a dominant Mo–O peak (~1.33 Å) and absence of Mo–Mo scattering, confirming atomically dispersed surface Mo. Electrochemistry: Catalysts (SAHEA NSs, SAHEA NPs, HEA NPs) were supported on carbon (Ketjen Black) by sonication and ethanol washing. Electrochemical measurements used a three-electrode setup (GCE working, SCE reference, Pt counter), CHI 660E workstation. CV in N2-saturated 0.1 M HClO4 (50 mV s−1) assessed ECSA via Hupd with 0.21 mC cm−2 criterion. MOR was measured in N2-saturated 1.0 M KOH + 1.0 M CH3OH at 50 mV s−1; catalysts were pre-activated by CV in 1.0 M KOH. Chronoamperometry (CA) at 0.77 V vs RHE assessed durability. CO stripping was performed by holding at −0.88 V vs SCE under CO for 15 min, then scanning in fresh electrolyte. In-situ FTIR (ATR-EC cell, MCT detector) recorded spectra from 0.20 to 1.30 V vs RHE every 50 mV in 1.0 M KOH + 1.0 M methanol. Computations: DFT (VASP, PAW, PBE-GGA) with SQS-generated fcc HEA slabs (32 atoms), 500 eV cutoff, Monkhorst-Pack 4×4×1 k-mesh, spin-polarized. Top two layers relaxed to <0.02 eV Å−1 force, energy convergence 1e−5 eV. Models: PdPtNiCuZn, Mo1-PdPtNiCuZn, and strained Mo1-PdPtNiCuZn surfaces. Bader charges, PDOS/TDOS, adsorption energies (CH3OH, CO, CO2), and free-energy diagrams for CO and CO2 (formate) pathways were calculated.

- Successfully synthesized single-atom Mo-tailored PdPtNiCuZn HEA ultrathin nanosheets (SAHEA NSs) with isolated Mo atoms on the surface and intrinsic tensile strain (~−4.4%). Average thickness ~1.69 nm; lattice spacings 0.233–0.237 nm. - Composition (ICP-MS): Pd/Pt/Ni/Cu/Zn/Mo = 24.1/14.6/26.3/22.1/11.1/1.8 (at.%). Uniform Pd, Pt, Ni, Cu, Zn distribution with isolated Mo atoms (EDS/EELS/XAFS); Mo present in oxidized states; absence of Mo–Mo EXAFS peak confirms single-atom Mo. - ECSA (m2 g−1): Mo1-PdPtNiCuZn SAHEA NSs 70.21; SAHEA NPs 59.89; HEA NPs 53.32; Pt/C 58.29. - MOR performance in 1.0 M KOH + 1.0 M CH3OH: Mo1-PdPtNiCuZn SAHEA NSs delivered highest ECSA-normalized specific activity of 16.55 mA cm−2, outperforming SAHEA NPs, HEA NPs, and Pt/C. - Mass activity: 24.55 A mgPt−1 and 11.62 A mg(Pd+Pt)−1 at peak potential, which are 18.13× and 8.58× those of commercial Pt/C, respectively. - Charge transfer: Lowest electrochemical impedance among compared catalysts at 0.75 V vs RHE (EIS), indicating superior conductivity/kinetics. - Stability: Sustained higher current density than Pt/C over 10,000 s CA at 0.77 V vs RHE; extended to 10 h with current density remaining 1.15 A mgPt−1. Activity restored upon electrolyte refresh; morphology and composition preserved (minimal aggregation) with XPS unchanged post-test. - CO tolerance and pathway: CO stripping peak (~0.75 V) markedly suppressed vs Pt/C; in-situ FTIR shows strong formate bands (1585, 1378, 1349, 1313 cm−1), no CO band across 0.20–1.30 V, and emergence of CO2 (2341 cm−1) at higher potentials. Onset potential for formate formation 0.45 V on SAHEA NSs vs 0.55 V on Pt/C. - Mechanistic insights: Diluted Pt-Pt ensembles by HEA mixing and tensile strain expand Pt–Pt distances, reducing COads formation (requires ≥3 contiguous Pt). Oxophilic Mo single atoms modulate adjacent Pt electronic structure, favoring formate pathway and accelerating kinetics. - DFT: Mo incorporation makes Pt more electron-rich and Mo electron-deficient (Bader); PDOS indicates Ni and Mo as electron-depletion centers while Pt/Pd/Cu/Zn coupling facilitates electron transfer; d-band center upshifts with Mo and strain, strengthening intermediate/*OH adsorption. Mo1-PdPtNiCuZn disfavors CO adsorption, lowers energy barriers along CO2 (formate) pathway, with more favorable RDS free energy at U = 0 and achieving ideal RDS equilibrium at U = 0.67 V vs PdPtNiCuZn. Tensile strain further lowers ΔG of RDS.

The work addresses the MOR challenges of CO poisoning and limited activity/durability of Pt-based catalysts by atomically integrating oxophilic Mo single atoms into a PdPtNiCuZn HEA ultrathin nanosheet host that imparts intrinsic tensile strain. HEA mixing and strain dilute and separate Pt sites, suppressing the formation of COads that requires contiguous Pt ensembles, while Mo promoters tune the d-band of adjacent Pt to balance reactant activation and intermediate binding. Electrochemical testing shows large gains in mass and specific activities and robust durability, with CO-stripping suppression and in-situ FTIR confirming a shift to a formate-dominated pathway without detectable CO intermediates. DFT corroborates that Mo and strain upshift the d-band center, enhance adsorption of key intermediates and *OH, weaken CO adsorption, and lower the free-energy barrier at the RDS along the CO2 pathway, enabling thermodynamically favorable deep oxidation to CO2. These results validate the strategy of single-atom tailoring and strain engineering in HEAs to achieve CO-tolerant, high-activity MOR electrocatalysts.

A general synthesis was developed for single-atom Mo-tailored HEA ultrathin nanosheets with intrinsic tensile strain (quinary Mo1-PdPtNiCuZn, senary Mo1-PdPtCoNiCuZn, and septenary Mo1-PdPtFeCoNiCuZn). The representative Mo1-PdPtNiCuZn SAHEA NSs achieved outstanding MOR performance in alkaline media, with mass activities of 24.55 A mgPt−1 and 11.62 A mg(Pd+Pt)−1 and excellent durability, far surpassing Pt/C. In-situ FTIR and DFT show that isolated oxophilic Mo atoms and tensile strain collectively create a favorable electronic microenvironment for isolated Pt, steering MOR to a formate-dominated, CO-free pathway and lowering kinetic/thermodynamic barriers. This strategy offers a practical route to engineer atomically precise, strain-modulated HEA catalysts, with potential extension to broader compositions and electrochemical reactions.