Synergistic effect between Co single atoms and Pt nanoparticles for efficient alkaline hydrogen evolution

In the pursuit of sustainable energy solutions, the efficiency of the hydrogen evolution reaction (HER) in alkaline conditions has been a significant challenge, primarily due to the sluggish dissociation of water molecules on platinum (Pt) catalysts. Addressing this critical issue, our study introduces an innovative Pt-Co@NCS catalyst. This catalyst synergistically combines Pt nanoparticles with Co single atoms on a nitrogen-doped carbon scaffold, overcoming the traditional bottleneck of slow water dissociation. Its unique porous concave structure and nitrogen-enriched surface not only provide abundant anchoring sites for Co atoms but also create a conducive hydrophilic environment around the Pt particles. This design leads to a drastic improvement in the water dissociation process, as demonstrated by CO stripping and deuterium labeling experiments. Achieving an outstanding current density of 162.8 mA cm−2 at −0.1 V versus RHE, a Tafel slope of 26.2 mV dec−1, and a superior nominal mass activity of 15.75 mA μgPt −1, the Pt-Co@NCS catalyst represents a significant step forward in enhancing alkaline HER efficiency, indicating promising advancements in the field.

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Introduction
Hydrogen is a carbon-free and environmentally benign energy carrier that could be efficiently produced through electrochemical hydrogen evolution reaction (HER) by water splitting [1][2][3][4][5].While significant progress has been made in acidic operations, alkaline hydrogen evolution is preferred in industrial settings for improved corrosion resistance and higher hydrogen purity.However, translating the advancements from acidic to alkaline HER remains challenging due to mechanistic differences.Notably, platinum, the benchmark catalyst in acidic HER, exhibits a sluggish kinetic profile in alkaline environments owing to weak adsorption of hydroxide ions and suboptimal water dissociation [6,7].This hinders the Volmer step and reduces the availability of H 3 O + for hydrogen spillover in the Volmer-Heyrovsky and Volmer-Tafel pathways [8,9].Such limitation highlights the pressing need for innovative catalyst designs and material modifications via fine-tuning catalyst-electrolyte interaction to expedite the reaction kinetics.For instance, creating an acid-like local environment could accelerate water dissociation and H 3 O + generation in alkaline media [10][11][12][13].
In recent years, innovative catalysts have been specifically engineered to enhance the local environment at the Ptgroup interface on traditional oxide supports such as CeO 2 , TiO 2 , and SiO 2 [14][15][16][17][18]. Unfortunately, such oxide supports are inferior to carbon support due to limited charge transfer and extra energy losses on internal resistance at large current densities [19][20][21][22].The requirement for improved water dissociation kinetics has led to the utilization of nanostructured, nitrogen-doped carbon supports.This approach has revealed a synergistic interaction with water at the defective nitrogen-doped sites, significantly enhancing HER performance.Recent research has shown that the coordination environment of atomic sites can be readily adjusted through the manipulation of pyrolysis cycles and temperatures, facilitating the coexistence of single atoms and particles in singleatom/particle composites (SA/PCs) [23][24][25].Furthermore, the integration of diverse metal species into SA/PCs, particularly those comprising heterogeneous elements, not only diversifies the selection of metals but also provides more versatile control strategies, thereby deepening our understanding of the structure-activity relationship [26][27][28][29][30]. Notably, the interaction between transition metal single atoms and Pt particles effectively catalyzes the water splitting in acidic environments, attributed to their superior OH − affinity.Furthermore, composite catalysts, created by depositing Pt particles onto nitrogen-doped graphene supports and further modified with Fe, Mn or Ni atoms, are capable of catalyzing HER.This is augmented by enhanced diffusion kinetics within hierarchical supports, rendering the HER process more efficient in alkaline media [31][32][33][34][35][36][37].The success of this approach underscores the significance of meticulously integrating single atoms with metal particles, marking a substantial advancement in boosting catalytic efficiency and stability.
Building on the progress outlined in the research on catalysts for HER, our investigation delves deeper into the realm of optimizing catalyst design for energy conversion processes, particularly focusing on the alkaline HER.Herein, we constructed a hierarchically porous N-doped carbon scaffold (NCS) catalyst containing Co single atoms and Pt nanoparticles for alkaline HER (shown in figure 1).The porous structure was crafted through the polymerization of mphenylenediamine and etching of silicon oxide coating.Pt and Co precursors were then pyrolyzed in the confined space of NCS and transformed into tiny Pt nanoparticles and atomically dispersed Co-N 4 sites.Such Pt-Co@NCS catalyst leveraged the synergistic interaction of Pt nanoparticles and Co single atoms and the nanoscale geometrical effects, thus significantly enhancing water dissociation by promoting OH − adsorption to the Co single atoms in an alkaline environment.The unique porous structure also minimized the potential aggregation of metal species, enhancing both the long-term activity and stability of Pt-Co@NCS.These results highlighted the importance of synergistic interaction between nanoparticles and single atoms, offering new insights into developing more effective HER catalysts.

Synthesis of N-doped carbon scaffold (NCS)
200 ml of deionized water, 80 ml of ethanol, and 0.6 ml of ammonia were mixed at 70 • C. 0.7 g of resorcinol and 2.1 g of m-phenylenediamine were dissolved in the above mixture before adding 3 ml of tetraethyl orthosilicate and 1.48 ml of formaldehyde solution for reaction at 70 • C for 12 h.The obtained powder was filtered, washed, dried, and calcined at 1000 • C for 2 h in an argon-filled tube furnace.

Synthesis of Pt-Co@NCS
200 mg of dry NCS powder was dispersed in 100 ml of water/ethanol mixture (1:1 v/v) under 30 min sonication.266 µl of H 2 PtCl 6 (8 wt% in water) and 22 mg of anhydrous cobalt chloride were added and sonicated for another 30 min.Rotary evaporation was used to obtain a smooth and dry slurry for further annealing in a tube furnace.The annealing was performed in an H 2 /Ar mixture (30/70 sccm) at 150 • C for 2 h and then in pure H 2 (30 sccm) at 700 • C for another 2 h to obtain Pt-Co@NCS with 5 wt% Pt loading.Control samples with various Pt and Co loadings were obtained by tuning the metal feeding.

Electrochemical measurement
8 mg of Pt-Co@NCS was dispersed in 450 µl of ethanol, 450 µl of water, and 100 µl of 5% Nafion solution for 30 min.100 µl of the dispersion was drop-casted onto 1 cm 2 of carbon paper and dried under a baking lamp to achieve a catalyst loading of 0.8 mg cm −2 (0.04 mg Pt cm −2 ) as the working electrode.Alkaline hydrogen evolution was conducted in 1 M KOH in a two-compartment H-type reactor with a Nafion 211 membrane as the separated membrane.A Hg/HgO electrode and a Pt mesh were used as the reference and counter electrodes.N 2 was bubbled for 10 min to remove the dissolved oxygen before measurement.Internal resistance of the system was measured by AC impedance spectra ranging from 10 000 to 1 Hz.The IR-corrected linear scanning voltammetry (LSV) was performed at a scanning speed of 2 mV s −1 after compensating 90% of the internal resistance.Cyclic voltammetry (CV) for stability was performed at 10 mV s −1 .CO-stripping was conducted in a CO-saturated 1 M KOH electrolyte via CVs from 0.06 to 1.0 V vs. RHE at 10 mV s −1 twice, where complete removal of the adsorbed CO was confirmed in the 2nd cycle.The baseline was obtained in N 2 at 0.06 V before and after the test.

Flow electrolyzer operation
Gas diffusional electrodes (GDEs) were prepared by spraycoating the catalyst ink onto oxygen-treated Toray-60 carbon papers at a catalyst loading of 0.8 mg cm −2 .The ink was prepared by dispersing 20 mg of catalyst in 2.45 ml of isopropanol, 2.45 ml of DI water, and 100 µl of 5% Nafion solution with 30 min sonication.The flow cell was assembled with two stainless-steel cover plates, two gold-coated copper plates as the current collector, two monopolar graphite plates with parallel paths for electrolyte distribution, one piece of the GDE (4 × 4 cm 2 ) as cathode, a dimensionally stable anode (Ru/Ir alloy on Ti mesh, 4 × 4 cm 2 ) and a Selemion AEM membrane (4.5 × 4.5 cm 2 , AGC Engineering).The flow rate was controlled by two Watson Marlow 120 S peristaltic pumps, and the actual flow rate was determined by a measuring cylinder.An aqueous electrolyte containing 1 M KOH was supplied to both cathodic and anodic chambers at 4 ml min −1 .The cell voltage was applied to the current collectors by an Autolab PGSTAT30 workstation.Before measurement, the cathode was activated in a CV from 0 to −2 V at 50 mV s −1 for 20 cycles.Chronoamperometry was measured under the full cell voltage of 1-3.5 V for at least 5 repeating cycles.The stability test was conducted in chronopotentiometry (CP) mode for 6 h at a full cell current of 1 A. Catalyst activity after the stability test was verified by LSV at 5 mV s −1 .

Results
The morphology of the Pt-Co@NCS catalyst was elucidated through a combined toolset of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM).The Pt-Co@NCS has a typical carbon sphere morphology with an average diameter of 400 ∼ 500 nm in figure 2(a).The HAADF-STEM image in figure 2(b) illustrates the homogeneous distribution of metallic Pt particles in the confined space across the hierarchically mesoporous carbon substrate, whose size is approximately 2-4 nm.Complementary XRD patterns (figure S1) The interplay between Pt nanoparticles and Co single atoms within the Pt-Co@NCS catalyst was further probed through x-ray photoelectron spectroscopy (XPS).Figure 2(f) presents the Pt 4f XPS spectra, where a discernible shift in binding energy is observed.The Pt-Co@NCS sample exhibits a binding energy of approximately 0.3 eV higher than its monometallic Pt@NCS variant.This shift indicates the altered electronic environment of Pt due to the presence of Co, reflecting the intricate electronic interactions at the nanoscale interface within the catalyst system [38,39].Similar phenomena are observed in the Pt L 3 -edge x-ray absorption near edge spectra (XANES) in figure 2(g).The white line of Pt-Co@NCS positively shifts by approximately 0.5 eV over Pt foil owing to charge transfer between Pt nanoparticles and substrate, which is larger than that of monometallic Pt@NCS.Upon binding with Pt, the XPS peak of Co (figure S5) undergoes a negative shift of 0.4 eV, with the Co 2p 3/2 peak in Pt-Co@NCS decreasing from 780.7 eV to 780.3 eV.Consequently, the XPS of Co exhibits a converse trend to support the interactions between Pt NPs and Co SA.Meanwhile, the decrease in white line intensity and extension to higher energies in Pt-Co@NCS indicate that Co cations only affect the surface Pt atoms rather than doping into the lattice of the Pt crystals [40].This is confirmed by the wavelet transforms in figure 2(h).The first shell of the Pt-Co@NCS is reduced by ∼0.1 angstroms compared to that of the Pt foil, together with a weakened second shell intensity.Detailed analysis of the R space diagram and its fitting in figures 2(i), S6, and table S1 reveal that only one of the 12 coordinated atoms around each Pt center is replaced by a Co atom on average.The corresponding Pt-Co bond is approximately 0.08 angstroms shorter than the Pt-Pt bond (∼2.76 Å) in a standard Pt crystal.In contrast, monometallic Pt@NCS shows an unaltered crystal structure to bulk Pt, whose Pt-Pt bond length of the first shell is ∼2.76 Å in figures S7, S8, and table S2.The SA nature of Co in Pt-Co@NCS was validated by the Co K-edge XANES spectra in figure 3(a).Compared with the Co standards (Co 3 O 4 , CoO, and Co foil), Pt-Co@NCS and monometallic Co@NCS exhibit a valence state below +2.Such observation is aligned with the anticipated electronic signature of atomically dispersed Co sites.Individual Co atoms could also be visualized as bright spots in close vicinity to Pt nanoparticles in the STEM images in figures 2(d) and (e).The infusion of Pt on the Co@NCS matrix ostensibly modulates the electronic landscape of Co.This is evidenced by the diminution of the pre-edge feature at 7712.4 eV due to the depletion in the state density of Co orbital.Likewise, the escalated white line intensity at 7723.3 eV in the Pt-Co@NCS composite substantiates owing to strong inter-metal interaction.The electron density around the Co atoms is tuned by nearby Pt nanoparticles, thus potentially enhancing the catalytic efficacy [41,42].The notable influence of Pt on Co is also discernible in the R space distribution in figure 3(b).The absence of metallic Co-Co bonding in Co@NCS suggests that the majority of Co exists in SA states.This feature is observable in Pt-Co@NCS owing to the bonding between Co and surrounding Pt atoms in short distances, thus manifesting a transition state between SA and bulk metal.The fitting results in figures 3(c) and (d) (as well as figures S9-S10 and tables S3-S4) provide a quantitative insight into the inter-metal interaction.The bond length of Co-Pt in Pt-Co@NCS is significantly larger than the typical Co-N bond length in Co@NCS (2.71 vs. 2.17 Å), which could also be seen in the wavelet transforms in figures 3(e) and (f).Additionally, the N1s XPS spectra in figure S11 display a variety of nitrogen species, such as pyridinic N, pyrrolic N, and pyridinic oxide on the NCS surface, which provides the anchoring sites for Co atoms.Unlike the typical multi-peak profiles in CoO and Co 3 O 4 , Co@NCS displays a single-peak Co-N pattern with shorter bond lengths due to the atomic dispersion.In contrast, Pt-Co@NCS exhibits both the SA characteristics and the multiple peaks associated with the Pt-Co bonding.The synergy between Co single atoms and Pt nanoparticles could promote sluggish water dissociation in alkaline HER.
The alkaline HER performance was first evaluated by standard 3-electrode measurements in H-cells.As depicted in figure 4(a), Co@NCS shows a limited current density of 0.24 mA cm −2 at −0.1 V vs. RHE, owing to the low activity of isolated Co atoms.In contrast, Pt-Co@NCS with a nominal Pt loading of 1.25 wt% exhibits a substantially enhanced current density of 59.3 mA cm −2 at −0.1 V, rivaling the performance of commercial Pt/C catalysts with 20 wt% Pt loading.Subsequent analyses of Pt-Co@NCS at varying Pt loadings reveal a direct correlation between metal content and HER performance.These findings are supported by elemental analysis (ICP-MS) in table S5.A remarkable improvement in current densities to 108.2 and 162.8 mA cm −2 at −0.1 V vs. RHE can be seen at a nominal Pt loading of 2.5 and 5 wt%.Notably, the Pt-Co@NCS sample with 2.5 wt% Pt gives the best Tafel  S12), the production rate of hydrogen reaches a point where it is limited more by mass transport phenomena than by the availability of active catalytic sites.This indicates an optimal Pt threshold, beyond which extra Pt will not proportionally contribute to the catalytic efficiency.
Semi-quantitative insights into the atomic composition of the catalyst system were provided by CV curves in figures S13 and S14.Characteristic features of hydrogen underpotential deposition (H upd ) on the Pt surface appear in the 0.1-0.4V vs. RHE.The presence of Co atoms in the catalyst is evidenced by distinct current peaks at approximately 0.4 V and 0.7 V in both forward and reverse scans owing to the specific electrochemical interactions of Co with the electrolyte that induce changes in the hydrogen adsorption and desorption processes on the catalyst surface.Furthermore, lower charge transfer resistance in Pt-Co@NCS is evidenced by the electrochemical impedance spectroscopy measurements in figures S15, S16, and table S7, which could be another reason for its enhanced HER performance compared to commercial Pt/C.The charge transfer resistance (R ct ) of Pt-Co@NCS is measured as 0.149 Ω at −0.1 V vs. RHE, significantly lower than that of Pt/C (0.416 Ω).We further employed a fingerprint technique in the electrochemical behavior of Pt catalysts using carbon monoxide (CO) as a probe molecule.As shown in figure 4(d), no discernible peak is found in the CV curves in monometallic Co@NCS, reflecting the inability of Co toward CO adsorption, the control sample of Pt@NCS displays a pronounced peak at around 0.66 V, similar to that of Pt/C (in figure S17).The synergistic interaction of Pt nanoparticles and nearby Co single atoms will result in a much earlier CO-stripping at around 0.4 V in Pt-Co@NCS in figure S18.This lower stripping voltage is attributed to the oxidation of adsorbed CO on Pt particles near Co atoms than on a plain Pt surface [43,44].Peak splitting may appear at a lower CO oxidation potential of ∼0.35 V due to stronger interaction between Co single atoms and subnanometric Pt clusters at the atomic scale than with Pt nanoparticles.Meanwhile, the stripping peak associated with CO oxidation on the Pt surface at ∼0.7 V co-exists, confirming the typical characteristics of Pt nanoparticles in Pt-Co@NCS.Such observations are referenced by control samples of Pt electrochemically deposited on Co@NCS in figure S18.
The exceptional HER performance of Pt-Co@NCS in alkaline media is not only attributed to the electronic coupling (synergy) between Pt nanoparticles and Co single atoms.To a more significant extent, this is correlated with the enhanced adsorption capability of single Co atoms for OH − ions, which promotes sluggish water dissociation in alkaline conditions.A comprehensive deuterium labeling study of the electrochemical behavior of Pt@NCS and Pt-Co@NCS in D 2 O was conducted.The detailed findings on the kinetic isotope effect (KIE) are presented in figure 4(e) using linear sweep voltammetry (LSV).Notably, the Pt-Co@NCS exhibits a KIE value of up to 2.10 at overpotentials of −0.4 V in D 2 O, far surpassing the corresponding KIE value of 1.34 for Pt@NCS.This indicates a higher sensitivity of Pt-Co@NCS to OH − ions than Pt@NCS.As the Co content increased from 1.25% to 2.5% and 5% (figure S19), the KIE values for the three samples decreased from 1.93 to 1.56 and 1.51, respectively.This reduction in KIE indicates that higher Co loadings reduce the isotopic effect, the decreased KIE values imply that the reaction mechanism becomes less dependent on isotopic differences, likely due to more efficient catalytic sites provided by Co single atoms.CO stripping tests were conducted in 1 M KOH solution in D 2 O to further corroborate our hypothesis.The significant inhibition and positive shift by 30 mV of the CO stripping peak at ∼0.4 V in D 2 O, as shown in figure 4(f), can be attributed to the KIE.The replacement of hydrogen atoms with the heavier deuterium in the electrolyte modifies the adsorption and desorption processes of CO, affecting the electrochemical reaction's kinetics and energetics.This results in a higher energy barrier for the CO stripping process in D 2 O, causing the observed inhibition and positive potential shift of the CO stripping peak.This is distinct from the minimal influence of the KIE on pure Pt systems.A notable recovery in the synergistic CO stripping peak could be observed when changing the electrolyte from D 2 O to H 2 O for Pt-Co@NCS.The corresponding LSV curves in figure S20 also confirm the linear relationship between the CO stripping peak area and the current density at various overpotentials, thereby establishing a direct correlation between the synergistic sites and HER performance.From the literature, the rate-determining step in Pt-Co@NCS is primarily associated with the adsorption of OH − ions during the water dissociation process [45].The presence of synergistic interaction between Pt nanoparticles and Co single atoms significantly promotes the rapid dissociation of water in alkaline solutions.A strong KIE effect is expected in Pt-Co@NCS due to limited HER performance in D 2 O.
To elucidate the mechanism behind the enhanced water dissociation observed on Co single atoms, we have employed density functional theory (DFT) calculations.These calculations aimed to explore the interaction between water molecules and the catalytic sites within our system, which comprises a unique structure of a Pt 3 Co cluster in conjunction with a Co-N 4 -C configuration, as depicted in figure S21.Specifically, we calculated the adsorption energy of water on this novel structure and compared it with the energies on pure Co-N 4 -C and the Pt (111) surface.As shown in figure S22, our findings reveal that the adsorption energy on the synergistic structure (1.26 eV) is significantly higher than those on pure Co-N 4 -C (0.94 eV) or the Pt (111) facet (0.46 eV), indicating a strong preference for water adsorption on the central Co atom within the Co-N 4 -C motif.Figure S23 depicts the atomic configurations of a single H 2 O molecule dissociation step.The HO * -Pt 3 Co-CoN 4 C model exhibits a notably low energy barrier for the Volmer step, with an energy barrier of 0.41 eV, and the process is exothermic by 0.41 eV.In contrast, the HO * -CoN 4 C model shows a higher energy barrier for the Volmer step, at 0.52 eV, and the process is exothermic by 0.52 eV.The Pt (111) model, on the other hand, has an energy barrier of 0.96 eV, and the process is endothermic by 0.81 eV.This indicates that the dissociation of H 2 O is significantly easier on the HO * -Pt 3 Co-CoN 4 C model, leading to a faster proton supply, which can help overcome the efficiency loss in the Volmer reaction.Furthermore, the charge density difference of HO * -Pt 3 Co-CoN 4 C compared to Pt 3 Co-CoN 4 C (figure S21) clearly reveals that the Co atom donates its electrons mainly to nearby N atoms and the OH − group through orbital hybridization.These results also support the more positive oxidation state of Co atoms under working conditions.Therefore, these findings validate the strong charge transfer occurring on HO * -Pt 3 Co-CoN 4 C, which promotes the sluggish water dissociation step, enhancing catalytic activity.Consequently, combining the adsorption energy and the kinetics of the Volmer step, we have a comprehensive understanding of the underlying mechanism of alkaline HER on the optimized HO * -Pt 3 Co-CoN 4 C electrocatalyst.This preference suggests that the synergistic system effectively promotes the generation of proton and hydroxide ions, key intermediates for the alkaline HER.
In addition to its superior catalytic activity, Pt-Co@NCS exhibits enhanced stability compared to commercial Pt/C.As shown in the CP at 40 mA cm −2 in figure 5(a), Pt-Co@NCS experiences a minimal voltage increase of 2 mV after an 18 h continuous experiment compared to >10 mV increase in Pt/C.LSV curves before and after the stability test were also recorded in figure 5(b).The current density at an overpotential of 100 mV slightly decreases from 57.8 to 54.1 mA cm −2 for Pt-Co@NCS, while Pt/C significantly drops from 45.5 to 25.8 mA cm −2 .Flow electrolyzer operation was also conducted using membrane electrode assembly (MEA) setup in figure 5(c) to further evaluate the alkaline HER performance.At a practical Pt loading of 40 µg cm −2 (one-fourth of that of Pt/C), Pt-Co@NCS still exhibits a superior performance in water electrolysis.The full cell voltage at 1 A current is only 2.62 V for Pt-Co@NCS, much lower than the 2.75 V for Pt/C.It also demonstrates a more stable voltage profile in the stability measurement at 1 A current in figure 5(d).The morphological changes of the catalysts before and after the stability test were provided in figures 5(e)-(h).Pt-Co@NCS retains a structure identical to its initial state, whereas significant agglomeration is observed in commercial Pt/C.The improved stability of Pt-Co@NCS can be attributed to the confinement effects of its porous structure and the anchoring of metal sites.

Conclusion
In conclusion, the rate-limiting water dissociation in alkaline hydrogen evolution can be effectively addressed by a synergistic Pt-Co@NCS catalyst containing Pt nanoparticles and Co single atoms on a hierarchically porous N-doped carbon substrate.The key to its remarkable performance lies in the unique porous concave structure paired with a nitrogen-rich defective surface, which collectively enhances the hydrophilicity and catalytic interaction around the Pt sites.Rigorous CO stripping and deuterium labeling experiments confirm the promoted water dissociation pathway related to synergistic interactions.The Pt-Co@NCS catalyst not only exhibits a substantial increase in activity, as reflected by its high current density and nominal mass activity but also a favorable kinetics for hydrogen evolution from the reduced Tafel slope.Our research highlights the essential role of tailoring the microenvironment around catalytic sites, especially for alkaline applications, to enhance activity and stability.This work offers a framework for designing advanced catalysts to optimize hydrogen production.

Future perspectives
Based on the demonstrated effectiveness of the Pt-Co@NCS catalyst in enhancing the HER efficiency in alkaline media, the future of material development for SA and crystalline synergistic catalysis appears highly promising.This field is expected to further explore the vast potential of high-density atomically dispersed metal catalysts on various materials.Research could focus on diversifying the types of metal atoms and doping elements in the substrate to tailor catalytic properties for specific reaction beyond HER, as well as exploring the long-term stability and scalability of these advanced catalysts.Moreover, the development of in situ and real-time techniques for monitoring synergistic catalytic processes at the atomic level is crucial for understanding the dynamic interactions within the catalyst microenvironment.Through these efforts, the catalysis community is likely to make significant strides in creating more efficient, durable, and economically viable catalysts to support sustainable energy technologies.

Figure 1 .
Figure 1.Schematic illustration of the synergistic alkaline hydrogen evolution in the Pt-Co@NCS.

Figure 2 .
Figure 2. Structural analysis of Pt-Co@NCS.(a) SEM image of the hierarchically porous Pt-Co@NCS.Inset shows the TEM image of concave structure for promoted diffusion kinetics; (b) Corresponding STEM-HAADF image and (c) EDX elemental mapping; (d), (e) Atomic resolution STEM images, highlighting the co-existence of Pt nanoparticles and Co single atoms; (f) Pt 4f XPS spectra; (g) Pt L 3 -edge XANES spectra, (h) Fourier transformed Pt L 3 -edge EXAFS spectra in the R-space, and (i) corresponding wavelet transforms of Pt samples.

Figure 3 .
Figure 3. Elucidation of Co single-atom features in Pt-Co@NCS.(a) The Co K-edge XANES and (b) corresponding EXAFS spectra of Co 3 O 4 , CoO, Co foil, Co@NCS, and Pt-Co@NCS; (c), (d) Experimental data (solid lines) and fitting results (dotted lines) for Pt-Co@NCS and Co@NCS.These spectra are k2-weighted, without phase correction; (e), (f) Wavelet transforms of the EXAFS signals.

Figure 4 .
Figure 4. Alkaline hydrogen evolution performance.(a) IR-corrected HER performance of Co@NCS, Pt-Co@NCS, and commercial Pt/C catalysts in 1 M KOH; (b) Tafel slope analysis of Pt-Co@NCS at various Pt loadings (1.25, 2.5, and 5 wt%); (c) Comparative mass activity of Pt-Co@NCS and commercial Pt/C at an overpotential of 100 mV; (d) CO stripping experiments for Co@NCS, Pt@NCS in 1 M KOH electrolyte; (e), (f) LSV and CO stripping experiments for Pt-Co@NCS in H 2 O and D 2 O.

Figure 5 .
Figure 5. Durability tests for alkaline HER.(a) Chronopotentiometry profiles over 18 h of Pt/C and Pt-Co@NCS at 40 mA cm −2 ; (b) LSV curves of Pt-Co@NCS and Pt/C before (BOL) and after (EOL) prolonged HER testing; (c) Setup of the flow electrolyzer and polarization curves with a Ru/Ir anode in 1 M KOH; (d) 6 h stability test under a constant current of 1 A; (e), (f) TEM images of commercial Pt/C, and (g), (h) Pt-Co@NCS before and after HER stability test.