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[Hiroaki Maeda](https://orcid.org/0000-0001-9552-7478), Eunice Jia Han Phua, [Yuta Sudo](https://orcid.org/0009-0006-8312-4966), Sayoko Nagashima, Wentai Chen, Mayumi Fujino, [Kenji Takada](https://orcid.org/0000-0002-7531-6865), [Naoya Fukui](https://orcid.org/0000-0003-4021-3193), [Hiroyasu Masunaga](https://orcid.org/0000-0002-0939-2114), [Sono Sasaki](https://orcid.org/0000-0001-7374-9854), [Kazuhito Tsukagoshi](https://orcid.org/0000-0001-9710-2692), [Hiroshi Nishihara](https://orcid.org/0000-0002-6568-5640)

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[Synthesis of Bis(diimino)palladium Nanosheets as Highly Active Electrocatalysts for Hydrogen Evolution](https://mdr.nims.go.jp/datasets/b09bd0a0-9507-49f6-99b3-a3be386aca4b)

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Synthesis of Bis(diimino)palladium Nanosheets as Highly Active Electrocatalysts for Hydrogen EvolutionSynthesis of Bis(diimino)palladium Nanosheets as HighlyActive Electrocatalysts for Hydrogen EvolutionHiroaki Maeda,*[a] Eunice Jia Han Phua,[b] Yuta Sudo,[c] Sayoko Nagashima,[a] Wentai Chen,[c]Mayumi Fujino,[c] Kenji Takada,[a] Naoya Fukui,[a] Hiroyasu Masunaga,[d] Sono Sasaki,[e, f]Kazuhito Tsukagoshi,[g] and Hiroshi Nishihara*[a, c]Development of efficient electrocatalysts for hydrogen evolu-tion reactions (HERs) is necessary to achieve environmentallyfriendly and sustainable hydrogen production. To reduce costand to circumvent the scarcity of platinum, the most efficientcatalyst for HER, it is essential to develop catalysts usingubiquitous base metals or minimal amounts of precious metals.Bis(diimino)metal (MDI) coordination nanosheets are potentialHER catalysts because their electric conductivities, two-dimen-sionality, and porous structures provide large surface areas andefficient mass and electron transfer. In addition, with sparsemetal arrangements in their chemical structures, nanosheetscan reduce the amount of metal needed. We synthesizedbis(diimino)palladium coordination nanosheets (PdDI) as acoordination polymer composed of bis(diimino)palladium, withsemiconducting characteristics, using gas-liquid interfacial syn-thesis and electrochemical oxidation. These electrochemicallysynthesized PdDIs exhibit remarkable catalytic performancewith overpotential reaching 10 mAcm� 2 of 34 mV, a Tafel slopeof 47 mVdec� 1, and an exchange current density of 2.1 mAcm� 2after appropriate activation. This performance is closely com-parable to that of metallic platinum. An ex-situ investigation ofthe activation process revealed that reduction of the divalentPd center in bis(diimino)palladium produced a composite ofPd(0) species and PdDI, combining high catalytic activity withsmooth electron transfer.IntroductionHydrogen is a clean, efficient energy source for sustainablesociety because it does not emit CO2 and because it canpotentially be produced from various sources. It is essential tosupply society’s energy needs based on hydrogen. However,there are still challenges to achieving mass production, trans-portation, and storage, and to improving infrastructure.[1–4]Inexpensive, stable hydrogen production is critical to reach thegoal. Hydrogen evolution reaction (HER), a cathodic reactioninvolving electrolysis of water, is considered the most environ-mentally friendly and sustainable hydrogen productionmethod.[5–7] Although platinum’s catalytic performance is highlyefficient, its rarity and cost are problematic. Hence, alternativecatalysts have been developed, with[8–16] or without metals,[17,18]or with nano-amounts of precious metals to reduce theirconsumption.[19,20] π-Conjugated conductive coordination nano-sheets composed of square-planar metal complexes with fourligating hetero atoms are candidates for efficient HER electro-catalysts because the metal complex sites exhibit HER catalyticactivities and their porous, two-dimensional, π-conjugatedstructures provide large surface areas, smooth mass transport,and electron transport between catalytic active sites andelectrodes, enhancing catalytic activity.[21–24] Various studieshave evaluated HER catalytic performances of coordinationnanosheets composed of bis(dithiolato)metals and theiranalogs.[25–30] We previously reported electrocatalytic perform-ances of bis(dithiolato)metal complex nanosheets (MDT, M=Pd,Pt)[28,31] for HER with overpotentials (η10) of ca. 410 mV for PtDTand 560 mV for PdDT at a current density of 10 mAcm� 2.[28]However, further improvement is still required for production ofhighly active and efficient electrocatalysts. Recently, we directlymodified glassy carbon electrodes (GCEs) with bis(diimino)metalcoordination nanosheets (MDI, M=Co, Ni, Cu) composed ofmetal ions and hexaaminobenzene (HAB, Figure 1a) ligands byelectrochemical oxidation in the space of a few minutes.[29,32,33][a] H. Maeda, S. Nagashima, K. Takada, N. Fukui, H. NishiharaResearch Institute for Science and Technology, Tokyo University of Science,2641 Yamazaki, Noda, Chiba 278-8510, JapanE-mail: h-maeda@rs.tus.ac.jpnisihara@rs.tus.ac.jp[b] E. Jia Han PhuaDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1Hongo, Bunkyo-ku, Tokyo 113-0033, Japan[c] Y. Sudo, W. Chen, M. Fujino, H. NishiharaGraduate School of Science and Technology, Tokyo University of Science,2641 Yamazaki, Noda, Chiba 278-8510, Japan[d] H. MasunagaJapan Synchrotron Radiation Research Institute (JASRI), Kouto, Sayo-cho,Sayo-gun, Hyogo 679-5198, Japan[e] S. SasakiFaculty of Fiber Science and Engineering, Kyoto Institute of Technology, 1Matsugasaki Hashikami-cho, Sakyo-ku, Kyoto 606-8585, Japan[f] S. SasakiRIKEN SPring-8 Center, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan[g] K. TsukagoshiResearch Center for Materials Nanoarchitectonics (MANA), National Institutefor Materials Science (NIMS), 1–1 Namiki, Tsukuba 305-0044, JapanSupporting information for this article is available on the WWW underhttps://doi.org/10.1002/chem.202403082© 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution and re-production in any medium, provided the original work is properly cited.Wiley VCH Freitag, 06.12.20242499 / 387785 [S. 1/8] 1Chem. Eur. J. 2024, e202403082 (1 of 7) © 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbHChemistry—A European Journal www.chemeurj.orgResearch Articledoi.org/10.1002/chem.202403082http://orcid.org/0000-0001-9552-7478http://orcid.org/0009-0006-8312-4966http://orcid.org/0000-0002-7531-6865http://orcid.org/0000-0003-4021-3193http://orcid.org/0000-0002-0939-2114http://orcid.org/0000-0001-7374-9854http://orcid.org/0000-0001-9710-2692http://orcid.org/0000-0002-6568-5640https://doi.org/10.1002/chem.202403082http://crossmark.crossref.org/dialog/?doi=10.1002%2Fchem.202403082&domain=pdf&date_stamp=2024-12-06NiDI-modified GCE exhibited the highest HER catalytic perform-ance to date with η10=227 mV and a Tafel slope of131 mVdec� 1.[32] Therefore, we can expect that a PdDI nano-sheet composed of palladium ion and HAB ligand will performas an efficient electrocatalyst. An additional advantage of PdDInanosheets is their porous framework realizing a low metalatom density, suggesting the possibility of realizing a highlyactive catalyst with a small number of precious metals.In this study, we newly synthesized PdDI as one of the MDIfamilies using chemical oxidation and electrochemical oxida-tion, C-PdDI, and E-PdDI, respectively (Figure 1b and c). HERcatalytic performance investigations revealed the remarkablecatalytic activity of activated E-PdDI which is comparable tothat of Pt electrodes. Hence, we succeeded in producing ahighly active HER electrocatalyst with fewer palladium atomsthan platinum atoms in metallic Pt because of the sparsearrangement of Pd in the PdDI structure. Our findings showcasethat PdDI is a useful coordination polymer to produce efficientHER catalysts with small amounts of noble metal.Results and DiscussionC-PdDI was synthesized at a gas-liquid interface by thefollowing procedure. An aqueous ammonia solution ofHAB ·3HCl and potassium tetrachloropalladate(II) (K2PdCl4) wasprepared under Ar. Then, atmospheric oxygen was slowlyintroduced to the reaction vessel as an oxidizing agent to allowoxidation-assisted MDI formation (Figure 1b).[32,34] The initialcolorless solution gradually turned dark yellow, then dark green,and finally PdDI was formed as a black film at the aqueoussurface (Figure S1). The resulting PdDI film was transferred ontosubstrates for characterization (Figure 2a). SEM observationsand energy-dispersed X-ray spectroscopy (EDS) mapping re-vealed that C-PdDI has a sheet-like structure with a uniformdistribution of Pd, N, and C (Figure 2b). AFM topography imagerevealed a rough surface morphology of C-PdDI with a sheet-like structure ca. 150 nm thick. (Figure 2c).The chemical composition and structure of C-PdDI wereinvestigated using X-ray photoelectron spectroscopy (XPS) andRaman spectroscopy. The narrow XP spectra of C-PdDI oncarbon tape showed Pd 3d and N 1s peaks (Figure 2d). Thebinding energy of Pd 3d peaks at 338.0 and 343.1 eVcorresponding to the 3d5/2 and 3d3/2, respectively, suggested adivalent oxidation state of Pd.[35–37] The N 1s peak can bedeconvoluted into two peaks at 398.4 eV and 399.9 eV, whichare assigned to quinoid (C=N) and benzoid (C� N) amines,respectively.[38–40] The calculated elemental ratio of Pd :N was1 :3.8, matching the theoretical ratio (1 :4) of Pd(o-phenyl-enediimine)2. Raman spectra of C-PdDI showed a broad peakFigure 1. (a) Representative chemical structure of MDI coordination nano-sheets composed of metal ions (Mn+) and hexaaminobenzene (HAB) ligand.Schematic illustration of (b) chemical oxidation at the gas-liquid interfaceand (c) electrochemical oxidation methods for PdDI synthesis.Figure 2. (a) Photograph of C-PdDI transferred onto a glass substrate. (b)SEM image and EDS mapping, (c) AFM topography image and height profilesat the corresponding white line, (d) XPS of Pd 3d and N 1s regions with thepeak deconvolution result (dashed gray lines), and (e) Raman spectrum of C-PdDI. The peak marked by an asterisk is derived from a Si substrate.Wiley VCH Freitag, 06.12.20242499 / 387785 [S. 2/8] 1Chem. Eur. J. 2024, e202403082 (2 of 7) © 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbHChemistry—A European Journal Research Articledoi.org/10.1002/chem.202403082 15213765, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202403082 by National Institute For, Wiley Online Library on [08/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensefrom 1300 to 1700 cm� 1 (C=C stretching in an aromatic ring), apeak at 2999 cm� 1 (N� H stretching), and two peaks at 454 and644 cm� 1 corresponding to Pd� N stretching and the breathingmode of the five-membered PdN2C2 ring, respectively (Fig-ure 2e).[41,42] The IR spectrum also exhibited peaks at 1420 and3210 cm� 1 that are assignable to C� N stretching and N� Hstretching, respectively (Figure S2). In addition, broad oxidationand reduction waves were observed at ca. 0.23 and ca. 0.0 V vs.ferrocenium/ferrocene (Fc+/Fc) in the cyclic voltammogram(Figure S3). These results revealed that C-PdDI is a coordinationpolymer consisting of bis(diimino)palladium complexes.An X-ray diffraction pattern converted from a grazing-incidence X-ray scattering (GIXS) image showed a peak at 2θ=17.5°, corresponding to d=3.3 Å. This matches typical values ofthe interlayer distance of MDIs, suggesting that PdDI has alayered structure (Figure S4).[29,32–34,43,44] Although crystallineMDIs typically give diffraction peaks derived from the in-planeperiodicity in the small 2θ range, an observed broad peakaround 2θ=9° implies low periodicity of C-PdDI in the in-planedirection. TEM observations also revealed a film-shapedstructure with an amorphous nature (Figure S5).Electrical conductivity of pelletized C-PdDI was1.3×10� 3 Scm� 1 at 300 K using a four-probe method underhelium (Figure S6a). An increase of conductivity with temper-ature observed in the temperature-dependent conductivitymeasurement indicated a semiconductive nature of PdDI withan activation energy of 0.22 eV (Figure S6b).Synthesis of E-PdDI was carried out in aqueous ammoniasolution in the presence of HAB, K2PdCl4, and NaBF4 by applyingan oxidation potential of 0.20 V vs. Ag/AgCl to a workingelectrode under Ar for 3 min (Figure 1c). Potential for electro-chemical oxidation was decided based on the cyclic voltammo-gram of the precursor solution for the E-PdDI synthesis(Figure S7). The voltammogram showed redox couples at thepotential range between � 1 V and � 0.4 V and at � 0.28 V andan oxidation peak at 0.085 V vs. Ag/AgCl. These peaks can beassigned to the Pd deposition and the first and the secondoxidation reactions of the HAB ligand, respectively, from thecomparison with the cyclic voltammograms of the HAB ligandand K2PdCl4 dissolved in 0.1 M NaBF4/0.1 M NH3 aqueoussolutions. Hence, we selected 0.20 V vs. Ag/AgCl as theoxidation potential for E-PdDI because it is positive enough tooxidize the HAB ligand to promote the PdDI formation. Thispotential is also positively sufficient to prevent the reduction ofPd(II) ions to Pd(0). Thus, we can eliminate the effect of thepalladium deposition on the E-PdDI synthesis. An SEM image ofE-PdDI formed on an Au/glass electrode shows a clear contrastbetween PdDI-modified and bare Au electrode areas. EDSspectra recorded at each area revealed that Pd is present onlyin PdDI-modified areas, based upon the peak appearance for PdLα (2.83 keV) and Pd Lβ1 (2.99 keV) at Point 1, but not at Point 2(Figure 3a). AFM observation of E-PdDI prepared on an Au/glasselectrode showed a rough film formation 10–30 nm in thickness(Figure 3b). Further characterization of E-PdDI by XPS andRaman spectroscopy showed good agreement with C-PdDIdescribed above, indicating a chemical structure of E-PdDIidentical with C-PdDI (Figure 3c and d). Furthermore, XP spectraof E-PdDI on an Au/glass electrode recorded at multiple pointsgave similar peak shapes, positions, and intensities, revealingthe uniform formation of E-PdDI on the electrode surface(Figure S8). Moreover, E-PdDI exhibited oxidation and reductionpeaks at respective potentials of ca. 0.20 and � 0.24 V vs. Fc+/Fc, corresponding to the redox reaction of [PdN4]+/[PdN4]0(Figure S9).Linear sweep voltammetry (LSV) of E-PdDI, C-PdDI, and E-NiDI on GC rotating disk electrodes (GC-RDEs) was performedto evaluate HER catalytic performance in 0.5 M H2SO4 (pH=0.48) solution at a rotational rate of 1600 rpm. Before electro-chemical measurements of the MDIs, cyclic voltammetry (CV)was conducted in the potential range between +0.23 V and+0.93 V vs. RHE for 5 cycles and +0.23 V and � 0.37 V vs. RHEfor 3 cycles in 0.5 M H2SO4 solution to remove the residue onthe surface. The XP spectra of E-PdDI after the CV treatmentsuggested E-PdDI contains about 15% of Pd(0) species becauseapplying the negative potential induced the reduction of Pd(II)to Pd(0) (Figures S10, S11). The density of Pd atoms (NPd) wascalculated as 1.8×1017 atom cm� 2 using the inductively coupledplasma atomic emission spectroscopy (ICP-AES) (Table S2). Asshown in Figure S12, the HER catalytic activity of E-PdDI wasgradually enhanced by repeating potential scans. Ramanspectra of E-PdDI after the 1st, 5th, 10th, and 50th potentialsweep in the negative direction did not show a significantdifference from the initial spectrum, suggesting that E-PdDIremained on the electrode (Figures 4a and S13). In contrast,Figure 3. (a) SEM image and EDS spectra recorded at the PdDI-modified area(point 1: magenta dot) and the bare Au/glass area (point 2: blue dot). (b)AFM topography image and height profiles at the corresponding white line,(c) XPS of Pd 3d and N 1s regions with the peak deconvolution result(dashed gray lines), and (d) Raman spectrum of E-PdDI formed on an Au/glass electrode.Wiley VCH Freitag, 06.12.20242499 / 387785 [S. 3/8] 1Chem. Eur. J. 2024, e202403082 (3 of 7) © 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbHChemistry—A European Journal Research Articledoi.org/10.1002/chem.202403082 15213765, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202403082 by National Institute For, Wiley Online Library on [08/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseXPS showed the appearance of additional peaks at 335.3 and340.6 eV assigned to Pd(0) species.[45] The atomic existing ratioof Pd(0):Pd(II):N gradually changed from 0 :1 :4.58 in the as-prepared sample, to 1.55 :1 :4.45 with potential sweeps (Fig-ure 4b and Table S1). These results imply that Pd(II) ions in[PdN4] complexes were reduced to Pd(0) during the potentialsweep in the reductive potential region, resulting in acomposite electrocatalyst of Pd(0) species and PdDI. After 50sweeps, ca. 60% of Pd(II) in [PdN4] was reduced to Pd(0). Inaddition, the XP spectra recorded at multiple points of E-PdDIafter the 50th potential sweep exhibited that the reductionfrom Pd(II) to Pd(0) uniformly occurred on the electrode withthe ratio of Pd(0) to 69�5% (Figures S15 and S16). The peak at399.7 eV in the N 1s region implied the presence of HAB ligandafter reducing Pd(II) to Pd(0). ICP-AES suggested the NPd wasmaintained at 1.8×1017 atom cm� 2 after 50 sweeps. Further-more, η10 values decreased concomitantly with the increase ofthe Pd(0) atomic ratio (Table S1 and Figure S14). Hence, Pd(0)species potentially act as the most active sites for HER electro-catalysis and the conductive PdDI provides smooth electrontransport between the active sites and electrodes.Then, we compared the catalytic performance of E-PdDIactivated by multiple LSV scans with the other metal electrodesand MDIs. As shown in Figure 5a, the almost overlapping LSVcurves of Pt and newly prepared E-PdDI after the activationsuggest the remarkable HER catalytic activity of E-PdDI. The E-PdDI showed an onset potential difference (Eonset) of 13 mV,giving a current density of 1 mAcm� 2. Furthermore, E-PdDIexhibited η10 of only 34 mV, a Tafel slope of 47 mVdec� 1, andan exchange current density (j0) of 2.1 mA cm� 2 (Figure 5b). Inaddition, the performance of E-PdDI was greater than C-PdDItransferred on GC RDE because the directly formed PdDI on theelectrodes realizes the smooth electron transport from theelectrodes (Table 1, Figure S17). In our previous work, NiDIsprepared using chemical and electrochemical oxidations alsoexhibited a similar trend.[29] As shown in Table 1, these valuesare extremely comparable to those of Pt metal electrodes, themost efficient electrocatalysts known for HER. Notably, thisperformance is significantly superior to that of Pd metalelectrodes and reported E-MDI (M=Ni, Co, Cu) nanosheets,[29]revealing the excellent catalytic activity of E-PdDI (Figure 5 andTable 1). Considering all Pd(0) species in E-PdDI contribute tothe HER catalysis, the turnover frequency (TOF) of E-PdDI atη10=34 mV was estimated as 0.25 s� 1 based on the NPd=1.8×1017 atomcm� 2 from the ICP-AES and the ratio of Pd(0)from the multiple-point XPS measurement (69%) (see details inSupporting Information). However, calculating accurate num-bers of active sites in CONASHs is challenging because theactive sites on the surface area may prior contribute to catalysisdue to the limitation of the accessibility to the active pointsinside multilayered CONASHs due to their stackingstructure.[46–48] Hence, the actual TOF of E-PdDI is possibly largerthan the estimation in this study. In the case of the Pd metalelectrode, only the Pd atoms on the surface can perform asactive sites because the electrolyte solution cannot penetrateinside the electrode. The TOF of the Pd electrode at the samepotential was calculated as 1.1 s� 1 based on the NPd on thePd(111) surface (1.5×1015 atom cm� 2, see details in SupportingInformation). In addition, chronopotentiometry measurementsat j= � 10 mAcm� 2 exhibited that E-PdDIs maintained theoperating potentials for 6 and 12 hours (Figure S18a). Thepotential fluctuation observed during the measurements mayattributed to the oxygen evolution reaction occurring on thecounter electrode, which covered the electrode surface withbubbles and inhibited electron transfer. Raman spectra of E-PdDIs after the 6- and 12-hour measurements showed peaks at450 and 640 cm� 1, which were also observed in the as-preparedE-PdDI (Figures S18b and c). XP spectra of E-PdDI after 12-hourmeasurements showed that almost all palladium was reducedto zero valence due to the long-time application of reductionpotential. However, the N 1s peak was still observed, suggestingFigure 4. (a) Raman spectra and (b) XPS of E-PdDI after the 1st, 5th, 10th,and 50th potential sweep in the negative direction.Figure 5. (a) LSV curves and (b) the corresponding Tafel plots of E-PdDI onGC RDE, E-NiDI on GC RDE, Pt, and Pd electrodes in a 0.5 M H2SO4 solution.Table 1. HER electrocatalytic performance.Eonset/mV η10/mV Tafel slope/mV dec� 1j0/mAcm� 2E-PdDI 13 34 47 2.1C-PdDI 9 108 99 0.80Pt 2 35 42 1.2Pd � 68 216 151 0.40E-NiDI � 103 237 127 0.128E-CoDI28) � 186 340 147 0.048E-CuDI28) � 386 508 119 0.001Wiley VCH Freitag, 06.12.20242499 / 387785 [S. 4/8] 1Chem. Eur. J. 2024, e202403082 (4 of 7) © 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbHChemistry—A European Journal Research Articledoi.org/10.1002/chem.202403082 15213765, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202403082 by National Institute For, Wiley Online Library on [08/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensethe presence of HAB ligand on the surface (Figure S19). The NPdafter the 12-hour measurement was calculated as 1.6×1017 atomcm� 2 from the ICP-AES, corresponding to that before thechronopotentiometry experiment (1.8×1017). This series ofresults indicates that the Pd species did not leach even after thereduction from Pd(II) to Pd(0) during the long-term operationunder a highly acidic condition.Theoretical calculations by Ji et al. will help understand thesuperior catalytic activity of E-PdDI.[49] HER consists of thefollowing three elementary processes.Volmer step : H3Oþ þ e� !* Hþ H2OHeyrovsky step : * Hþ H3Oþ þ e� ! H2 þ H2OTafel step : * Hþ* H! H2Where *H indicates a hydrogen atom adsorbed on a catalyst.When each process is a rate-determining step in HER, the Tafelslope ideally takes 120, 40, and 30 mVdec� 1, respectively.[50] HERcan proceed in two pathways, the Volmer-Heyrovsky and theVolmer-Tafel processes, and both involve the Volmer step whichis the adsorption reaction of a hydrogen atom on a catalystsurface. When the Gibbs free energy of hydrogen adsorptionΔG(*H) is near zero, high catalytic performance is expected dueto the well-balanced adsorption and desorption of hydrogen(e.g. ΔG(*H) for Pt: � 0.09 eV). According to the first-principlescalculations, ΔG(*H) of the Pd center in PdDI is 1.45 eV, which isfar from zero and close to the ΔG(*H) of E-NiDI as 1.34 eV.Hence, the predicted catalytic performance of PdDI is lowerthan Pt, but close to NiDI. However, the experimentallyobserved catalytic performance of activated E-PdDI is superiorto the NiDI. Furthermore, the Tafel slope values of E-PdDI andNiDI are 47 and 127 mVdec� 1, respectively, suggesting thattheir respective rate-determining steps are the Heyrovsky andVolmer steps. Hence, the difference in the rate-determiningstep may contribute to enhancing the catalytic performance ofE-PdDI.ConclusionsWe created PdDI by chemical and electrochemical oxidationmethods. Electrochemically synthesized PdDI on GC electrodesexhibited a catalytic performance with η10=34 mV, a Tafel slopeof 47 mVdec� 1, and j0=2.1 mAcm� 2 after activation. Thisremarkable performance exceeds that of Pd metal electrodesand conventional MDI (M=Ni, Co, Cu) nanosheets and isextremely comparable to that of Pt electrodes. Summarizing theresults, PdDI produces electrocatalysts with remarkable per-formance despite using very small amounts of precious metals,promising to advance the goal of achieving a hydrogen society.Experimental SectionMaterials: Hexaaminobenzene trihydrochloride (HAB ·3HCl) wassynthesized according to the literature.[51,52] Chemical reagents andorganic solvents were purchased from commercial sources (TCI,FUJIFILM Wako Pure Chemicals, and ALDRICH) and used withoutfurther purification. Commercially available Au/glass (ca. 30 nmgold layer deposited on glass substrates purchased from Kenis)were used as gold substrates. Highly oriented pyrolytic graphite(HOPG) was purchased from Alliance Biosystems, Inc. (Grade SPI-1/210×10×2 mm) and the clean surface was obtained by removing thesurface layers with adhesive tape just before use. P-type Si(100)wafers (%0.02 Ωcm, AS ONE Corporation) were cut into ca.1.5×1.5 cm pieces to use as Si substrates.Equipment: Scanning electron microscopy and energy-dispersed X-ray spectroscopy were performed using JCM-7000 NeoScope(JEOL). X-ray photoelectron spectroscopy data were obtained usingPHI 5000 VersaProbe and VersaProbeIII (ULVAC-PHI). Al Kα (15 kV,25 W) was used as the X-ray source, and the beam was focused ona 100 μm2 area. The spectra were analyzed using the MultiPakSoftware and standardized using a C 1s peak at 284.6 eV. Atomicforce microscopy (AFM) was carried out using a Hitachi AFM5000IIwith an SI-DF40P2 cantilever in the DFM mode. Raman spectrawere collected using an NRS-5500 (JASCO) with a 532-nm excitationlaser. IR spectra were recorded using an FT/IR-6100 (JASCO) undervacuum conditions. TEM observations were performed at 200 kV ofthe accelerating voltage using a JEM-2100F (JEOL). Grading-incidence X-ray scattering (GIXS) measurements at λ=1 Å wereconducted at Beamline BL05XU in Super Photon ring-8 GeV(SPring-8). A HORIBA D-55S pH meter was used to test the pHvalues of H2SO4 solutions for catalytic performance evaluation.Inductively coupled plasma atomic emission spectroscopy wasperformed using an ICPE-9820 (Shimazu).SynthesisC-PdDI: 7.5 mL of aqueous potassium tetrachloropalladate(122.4 mg, 375 μmol) solution was prepared with degassed waterunder an argon atmosphere to obtain a pale-yellow solution. Tothis solution, 7.5 mL of concentrated aqueous ammonia was added.15 mL of the resulting colorless solution was added to 10 mL of1 mM of aqueous ligand solution and then exposed to atmosphericair at room temperature for over 8 days. A black film was formed atthe water surface and black precipitation was formed in thereaction solution.E-PdDI: Before an electrochemical synthesis, the glassy carbonelectrodes were polished with 0.03 μm Al2O3 powder dispersion onpolishing pads, then rinsed with water and dried under argon flow.The polished glassy carbon electrodes for the electrochemicalsynthesis were pretreated by scanning cyclic voltammetry in 0.1 MH2SO4 from 0.0 V to 2.2 V (vs. Ag/AgCl) for 25 cycles with a scan rateof 0.1 V/s to activate the surface by anodic polarization. HAB ·3HCl(4.4 mg, 16 μmol), K2PdCl4 (7.8 mg, 24 μmol), and NaBF4 (0.22 g,2.0 mmol) were dissolved in a 0.1 M ammonia solution (20 mL) inan Ar-purged glove box. Electrochemical synthesis was conductedusing the glassy carbon as a working electrode (3 mmϕ or 4 mmϕ),a Pt coil as a counter electrode, and an Ag/AgCl referenceelectrode. A constant potential (0.20 V vs. Ag/AgCl) was applied for3 min to form E-PdDI on the working electrode. The modifiedelectrode was rinsed with water and then dried under vacuum. Auglass electrodes were also used as working electrodes for electro-chemical synthesis.E-NiDI: E-NiDI was synthesized according to the preparationprocedure for E-PdDI. For electrochemical oxidation synthesis of E-Wiley VCH Freitag, 06.12.20242499 / 387785 [S. 5/8] 1Chem. Eur. J. 2024, e202403082 (5 of 7) © 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbHChemistry—A European Journal Research Articledoi.org/10.1002/chem.202403082 15213765, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202403082 by National Institute For, Wiley Online Library on [08/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseNiDI, HAB ·3HCl (2.2 mg, 8 μmol), Ni(OAc)2 · 4H2O (3.0 mg, 12 μmol),and NaBF4 (0.11 g, 1.0 mmol) were dissolved in a 0.1 M ammoniasolution (10 mL) in an Ar-purged glove box, and the oxidationpotential of 0.58 V vs. Ag/AgCl was applied to the workingelectrodes.Conductivity measurements: The C-PdDI obtained from vacuumfiltration were first ground using a mortar and pestle beforepressing into a pellet. The pelletized form was then approximatelysliced into flat rectangular strips for the resistivity measurements.The direct-current resistivity measurements were performed withthe pelletized C-PdDI using the standard four-probe method.Electrical contacts were obtained by gluing four gold wires (15 μmdiameter) to the pellet with carbon paste.Electrochemical MeasurementsCyclic voltammetry of C-PdDI and E-PdDI: Cyclic voltammetry used athree-electrode configuration electrochemical cell with 1 Macetonitrile solution of tetrabutylammonium hexafluorophosphate(nBu4NPF6) as the electrolyte solution, a Pt coil as the counterelectrode and an Ag+/Ag electrode as the reference electrode (anAg wire immersed in a 10 mM AgClO4/ 100 mM nBu4NClO4/acetonitrile solution). A C-PdDI/HOPG substrate or an E-PdDI-modified GC electrode was used as the working electrode. A 650DTelectrochemical analyzer (BAS) performed the potential control anddata collection. The recorded potentials were adjusted from Ag+/Ag to Fc+/Fc using the difference in potentials between the tworedox couples.Linear sweep voltammetry of E-PdDI, E-NiDI, and C-PdDI for electro-catalytic performance evaluation: The electrocatalytic performanceevaluations were carried out by ALS 750E electrochemical analyzer(BAS) and RRDE-3A rotating ring disk electrode apparatus (BAS) in aconventional three-electrode cell. All the electrodes except an Ag/AgCl electrode were purchased from BAS. The Ag/AgCl electrode(an Ag wire in a saturated KCl solution) or reversible hydrogenelectrode (RHE) was used as a reference electrode, and a Pt wirewas used as a counter electrode. The following equation calibratedthe potentials recorded using the Ag/AgCl reference (EAg/AgCl) to thepotential vs. RHE (ERHE).ERHE ¼ EAg=AgCl þ 0:199 þ 0:059pH (1)GC RDEs modified with MDIs were used as working electrodes. TheE-PdDI- and E-NiDI-modified electrodes were prepared using thesynthesis method described above. The C-PdDI-modified electrodeswere prepared by transferring C-PdDI film synthesized by the gas-liquid interfacial method using the Langmuir-Schaefer method.Before the measurements, the solution was fully purged with argonfor 30 minutes. Then, cyclic voltammetry (CV) was performed in thepotential range between +0.23 V and +0.93 V vs. RHE for 5 cyclesand +0.23 V and � 0.37 V vs. RHE for 3 cycles in 0.5 M H2SO4solution to remove the residue on the surface. The rotation ratewas set at 1600 rpm during linear sweep voltammetry measure-ments.Chronopotentiometry measurement of E-PdDI for long-term stabilitytest: Chronopotentiometry was carried out by an HZ-Pro S4 (HokutoDenko) using an Ag/AgCl, an activated E-PdDI on GC RDE, and a Ptwire as a reference, working, and counter electrodes, respectively,in an Ar-purged 0.5 M H2SO4 solution. The rotational rate was set at1600 rpm during the measurements.Inductively coupled plasma atomic emission spectroscopy: Replace-able GC RDEs (4 mmϕ) were used as working electrodes for E-PdDIsynthesis. After the CV treatment, 50-cycle LSV measurement, and12-hour chronopotentiometry measurement, the GC disks weredetached from the electrode attachments and immersed into HCl/H2O2 solution (1 mL: 1 mL) to decompose and dissolve E-PdDIs onthe electrodes. The solutions were diluted to 50 mL for themeasurements. The number of Pd atoms in the unit area (NPd atomcm� 2) was calculated from the following equation:NPd ¼c� 10� 6 � 50�NAMPd � 1000� ð0:2� 0:2� pÞ(2)Where, c, NA, and MPd indicate the concentration of Pd detected byICP measurement (ppb), the Avogadro constant (6.02×1023 atommol� 1), and the molar mass of Pd (106.42 gmol� 1).AcknowledgementsThis work was financially supported by JSPS KAKENHI (GrantNumber: JP19H05460, 22K14569, 22K05055, and 24H00468) andthe White Rock Foundation. GIXS experiments were performedat BL05XU in SPring-8 (Hyogo, Japan). XPS measurements weresupported by the Advanced Research Infrastructure for Materi-als and Nanotechnology in Japan (ARIM) of the Ministry ofEducation, Culture, Sports, Science and Technology (MEXT)(JPMXP12-A-22-UT-0007, JPMXP12-A-23-UT-0025, and JPMXP12-A-24-UT-0037). TEM observations were supported by Prof. Y.Idemoto (Department of Pure and Applied Chemistry, TokyoUniversity of Science) and Dr. T. Ichihashi (Research EquipmentCenter, Tokyo University of Science).Conflict of InterestsThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.Keywords: Bis(diimino)palladium · Coordination nanosheet ·Electrochemistry · Electrocatalyst · Heterogeneous catalysis ·Hydrogen evolution reaction[1] H. Ishaq, I. Dincer, C. Crawford, Int. J. Hydrog. Energy 2022, 47, 26238.[2] C. Kim, S. H. Cho, S. M. 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Min, Langmuir 2015, 31, 1196.Manuscript received: August 15, 2024Accepted manuscript online: November 28, 2024Version of record online: ■■■, ■■■■Wiley VCH Freitag, 06.12.20242499 / 387785 [S. 7/8] 1Chem. Eur. J. 2024, e202403082 (7 of 7) © 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbHChemistry—A European Journal Research Articledoi.org/10.1002/chem.202403082 15213765, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202403082 by National Institute For, Wiley Online Library on [08/12/2024]. 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Thecatalytic activity evaluation forhydrogen evolution reaction revealedthe remarkable catalytic performanceof activated PdDI which is comparableto that of metallic platinum despiteusing a tiny number of preciousmetals.H. Maeda*, E. Jia Han Phua, Y. Sudo, S.Nagashima, W. Chen, M. Fujino, K.Takada, N. Fukui, H. Masunaga, S.Sasaki, K. Tsukagoshi, H. Nishihara*1 – 8Synthesis of Bis(diimino)palladiumNanosheets as Highly Active Electro-catalysts for Hydrogen EvolutionWiley VCH Freitag, 06.12.20242499 / 387785 [S. 8/8] 1 15213765, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202403082 by National Institute For, Wiley Online Library on [08/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Synthesis of Bis(diimino)palladium Nanosheets as Highly Active Electrocatalysts for Hydrogen Evolution Introduction Results and Discussion Conclusions Experimental Section Synthesis Electrochemical Measurements Acknowledgements Conflict of Interests Data Availability Statement