Amorphous conversion in pyrolytic symmetric trinuclear nickel clusters trigger trifunctional electrocatalysts

Abstract

The pursuit of multifunctional electrocatalysts holds significant importance due to their comprehension of material chemistry. Amorphous materials are particularly appealing, yet they pose challenges in terms of rational design due to their structural disorder and thermal instability. Herein, we propose a strategy that entails the tandem (low-temperature/250–350 °C) pyrolysis of molecular clusters, enabling preservation of the local short-range structures of the precursor Schiff base nickel (Ni₃[2(C₂₁H₂₄N₃Ni_1.5O₆)]). The temperature-dependent residuals demonstrate exceptional activity and stability for at least three distinct electrocatalytic processes, including the oxygen evolution reaction (η₁₀ = 197 mV), urea oxidation reaction (η₁₀ = 1.339 V), and methanol oxidation reaction (1358 mA cm⁻² at 0.56 V). Three distinct nickel atom motifs are discovered for three efficient electrocatalytic reactions (Ni1 and Ni1′ are preferred for UOR/MOR, while Ni2 is preferred for OER). Our discoveries pave the way for the potential development of multifunctional electrocatalysts through disordered engineering in molecular clusters under tandem pyrolysis.

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Introduction

Developing clean and renewable energy sources to address the energy crisis has attracted worldwide interest. In the era of next-generation energy, water splitting for hydrogen production and fuel cell applications has garnered a lot of attention.¹ Overall water splitting requires the oxygen evolution reaction (OER),² while fuel cells necessitate the urea oxidation reaction (UOR)³ and methanol oxidation reaction (MOR).⁴ Multifunctional electrocatalysts are materials that can efficiently catalyze multiple electrochemical reactions, such as OER, UOR and MOR.⁵ These electrocatalysts are important for various technologies, such as fuel cells,⁶ electrolyzers,⁷ and metal–air batteries,⁸ as they can improve the overall performance and efficiency of these devices. Importantly, multifunctional electrocatalysts can enable the development of integrated systems that perform multiple electrochemical reactions in a single device, leading to more compact and efficient energy conversion systems.⁹

The design and synthesis of efficient and stable multifunctional electrocatalysts present significant challenges. This is because those different electrochemical reactions take place via different mechanisms. The OER comprises four-electron transfer steps, resulting in the evolution of O₂. In contrast, the UOR proceeds through six-electron transfer steps, converting urea into CO₂ and N₂. The mechanism of the MOR is more complex, involving six-electron transfers. Therefore, designing a multifunctional catalyst for OER, UOR, and MOR that simultaneously meets criteria for uniformity, efficiency, and stability is a challenge.^10–12

Amorphous materials are characterized by long-range disorder and only short-range order.^13,14 Their local atomic environment and electronic states can be modulated to enhance the intrinsic reactivity of each site, meeting the requirements of multifunctional catalysts. The abundant defects and inherent structural disorder greatly facilitate charge transfer and ion diffusion.¹⁵ Therefore, they possess more abundant active sites, wider compositional ranges, and higher structural flexibility.¹⁶ However, the design of amorphous materials poses challenges because of their disordered and unstable structures. Currently, there is no universal approach that can effectively control highly efficient structures. The intrinsic correlation between structure and activity remains unclear.^17,18

To study the detailed or characteristic structure of amorphous materials, scientists often study their transition from crystalline to amorphous. By studying the degree and the path of amorphous change, along with modulating defects or disorders in crystals, structural insights into amorphous materials can be obtained. Based on the above theory, our experience of the effect of pyrolysis on coordination molecular clusters can play a good demonstration role.^19,20 We have studied the process of coordination of molecular clusters from crystalline to amorphous and later to crystalline during pyrolysis and determined the in situ reconstruction process and information about the amorphous structure.²¹ However, previous studies often focused on the stage of high-temperature pyrolysis transformation (above 400 °C), where the structure is easy to characterize and clear in crystalline materials. The transformation mechanism of tandem pyrolysis (low temperature <400 °C) in amorphous materials has not been well studied, and the difficulty of research is greater due to the lower degree of amorphous material.

Therefore, it is crucial to design a tandem pyrolysis system with clear crystalline and amorphous transition processes to clarify the electrochemical performance changes and active site selectivity design of electrocatalysts during crystalline and amorphous transition processes. Herein, we propose a tandem pyrolysis approach that retains partial local coordination environments of the crystalline phase, providing the possibility of highly dispersed active sites. Controlling amorphous structures through tandem pyrolysis triggers a chemical environment of metals in the crystalline phase that can be retained, unlike high-temperature pyrolysis of organic frameworks, which causes structural collapse due to traditional carbon graphitization. Surprisingly, obtained disordered materials from symmetric trinuclear nickel clusters exhibit three excellent electrocatalytic functionalities (OER/UOR/MOR) with distinct selectivity, Ni1, Ni1′ and Ni2, where this paves the way for potentially developing multifunctional electrocatalysts through disordered engineering and provides insights into establishing structure–property relationships in amorphous materials.

Results and discussions

The schematic illustration of tandem pyrolysis in Ni₃ crystal was shown in Scheme 1. The single-crystal X-ray diffraction data found Ni₃ belongs to the triclinic crystal system P1̄ space group. The nickel (Ni) atoms are coordinated by two O bridges in an opposing direction, with Ni1 and Ni1′ being connected to Ni2 via μ2-O1 and μ2-O1′ bridge connections, two O bridges in opposite directions, Ni atoms are chelated by N on the ligand imine and O on the phenolic hydroxyl group. The oxygen on the free hydroxyl group on the ligand and O on the DMF molecule are not involved in the coordination (Fig. S2†). The bond lengths of Ni1–N1 and Ni1–N2 in the compound are 2.012 (4) Å and 2.026 (4) Å, respectively. The bond lengths of Ni1–O1, Ni1–O2, and Ni1–O5 in the compound are 1.997 (3) Å, 2.029 (3) Å, and 2.215 (3) Å, respectively.

Scheme 1 Schematic illustration of the crystal transformation via tandem pyrolysis. Schematic diagram depicts the tandem pyrolysis induced transformation of crystalline Ni₃ into amorphous a285, and its subsequent high-temperature pyrolysis reconstruction into a crystalline structure. The transformation process is monitored by TG-MS, which detects the overflow of organic small molecule fragments during pyrolysis.

The TG curve is shown in Fig. 1a, mass losses and corresponding temperatures during the three stages of pyrolysis are 3.33%, 22.6%, and 21.74%, respectively. The pyrolysis process of the compounds is divided mainly into three stages: (i) the first stage is the loss of guest molecules at 213 °C and the main source of H₂ (m/z = 2, where hydrogen is obtained from the condensation of aromatic structures), CH₃⁺ (m/z = 15), and DMF (m/z = 73). (ii) The second stage at 213–308 °C is the basic pyrolysis stage of the complex, and the main small molecular fragments lost include CH₂CHOH⁺ (m/z = 31), CO₂ (m/z = 44), CH₂CHOHCH₂⁺ (m/z = 46). (iii) The third stage starts at 310 °C and lasts until 900 °C. There is a rapid weight loss stage between 308 °C and 413 °C, accompanied by the loss of C₄H₂⁺ (m/z = 50), C₄H₅⁺ (m/z = 53) and C₆H₅⁺ (m/z = 76). According to TG-MS data (Fig. 1b and c), following chemical reactions occurred in this process:

2C₂₁H₂₄N₃Ni_1.5O₆ → C_xH_x−1N_yNi_y/2O_z + H₂, DMF + chain alkanes (1)

C_xH_x−1N_yNi_y/2O_z → hydrocarbons (C_xH_x−1CHOHCH) + N-containing, C hybrid compounds + amorphous state NiO_x (2)

Hydrocarbons (C_xH_x−1CHOHCH) → Ni/NiO_x@NCs (3)

2C₂₁H₂₄N₃Ni_1.5O₆ → amorphous state NiO_x → Ni/NiO_x@NCs (4)

Fig. 1 Tracking structural evolution in tandem pyrolysis to uncover the mechanism of amorphous and crystalline transformation products. (a) TG and DTG curves of Ni₃; (b and c) TG-MS curves of organic fragments from decomposition products m/z = 2, H₂; m/z = 15, CH₃⁺; m/z = 73, DMF; m/z = 31, CH₂CHOH⁺; m/z = 44, CO₂; m/z = 46, CH₂CHOHCH₂⁺; m/z = 50, C₄H₂⁺; m/z = 53, C₄H₅⁺; m/z = 76, C₆H₅⁺ derived from Ni₃. (d) HRTEM images of a250, a285, a300 and c325; the inset is the Fourier transform, showing lattice stripes.

Scanning and transmission electron microscopy (SEM and TEM) were utilized to examine the morphology, size, and microstructure of pyrolysis products. The sample subjected to tandem-temperature pyrolysis exhibited an amorphous structure, as evidenced by diffuse rings in fast Fourier transform (FFT) patterns and high-resolution TEM (HRTEM) images (Fig. 1). a250 and a285 samples were confirmed to be amorphous by TEM analysis of heat-treated samples (Fig. 1d). The corresponding FFT patterns (insets of Fig. 1d) displayed a diffuse ring for the edge and a halo pattern for the core, further corroborating the amorphization in the edge region.²² After collapse of the structure, c325 particles agglomerated, but no obvious difference was observed in the microscopic morphology before and after agglomeration (Fig. S4†). This indicates that the sample tends towards stability.²³ Further analysis verified that main products after pyrolysis were Ni and NiO, along with a peripheral carbon layer. Due to the uniform distribution of metal, oxide, and carbon layers, the nickel metal is largely exposed, which may facilitate faster electron transfer for improved electrocatalytic performance.²⁴

The representative peaks at 44.5°, 51.8°, and 76.4° are assigned to (111), (200), and (220) planes of metallic nickel (JCPDS 04-0850),²⁵ respectively, indicating the amorphous nature of c325. When the pyrolysis temperature reaches 250 °C, the crystal structure has changed, and wide peaks appear at about 2θ = 8.59° for a285 and a300, which proves that the compound has changed from a crystalline state to an amorphous and crystalline transition state (Fig. 3a). Compared with the Ni/NiO standard card, the crystal plane index of c325 contains mainly NiO (JCPDS 47-1049)²⁶ and Ni (JCPDS 04-0850) after pyrolysis at 325 °C.

The surface chemical states of a250, a285, a300, and c325 were examined by XPS. The surface chemical composition consisted of C, N, O, and Ni (Fig. 2b). The high-resolution Ni 2p spectrum displayed a peak at 855.8 eV for Ni 2p_3/2 of Ni³⁺, along with two satellite peaks at 862.3 eV and 880.6 eV, indicating the oxidation of surface Ni to Ni(II) or Ni(III) in the pyrolysis products. Compared to standard XPS data for nickel compounds, the peak positions of the c325 samples at 856.8 eV (Ni 2p_3/2) and 874.3 eV (Ni 2p_1/2) exhibited a positive shift versus standard NiO positions, while typical NiO satellite peaks were at 860.6 eV and 877.9 eV; the two satellite peaks of pyrolysis samples (862.3 eV and 880.6 eV) shifted forward, suggesting that the surface Ni existed predominantly as Ni(III). The literature standard peak positions for Ni(III) in NiO_x were 855.5 eV (Ni 2p_3/2) and 873.3 eV (Ni 2p_1/2).²⁷ Compared to standard positions, the peak positions of synthesized samples were positively shifted (Fig. 2c), indicating that the nickel valence state on the surface of our prepared samples was higher than the typical Ni(III). Since high-valent nickel serves as active sites for methanol adsorption, this may promote high activity for the methanol oxidation reaction.²⁸

Fig. 2 Physical property and chemical bonding environment analysis. (a) XRD patterns of Ni₃, a285, a300, and c325 at 2θ = 3–80. (b) XPS spectra of a250, a285, a300 and c325. (c) Ni 2p_3/2. (d) Atomic pair distribution function (PDF) for a250, a285, a300 and c325. (e) In situ Ni K-edge XANES spectra of a250, a285, a300 and c325, the inserted picture is a magnification of XANES spectra in the region. (f) Fourier transform (FT) curves and Ni K-edge EXAFS spectra of Ni₃.

Using PDF analyses, more information on the structure and particle size of Ni species can be obtained. The small crystallite sizes cause deviations on the absolute scale for G(r) and intensity ratios. At short range (below γ of 10 Å, Fig. 2d), both PDFs are practically equal, not only in the position of the peaks but also in their intensity. The peaks at 2.48 Å, 3.52 Å, 4.30 Å, 5.56 Å, and 6.56 Å belong to Ni⁰. 3.01 Å, 5.10 Å, and 6.61 Å, which correspond to Ni–O, are consistent with XANES results. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy analyses were conducted. The XANES spectrum at the Ni K-edge revealed that compound Ni₃ exhibited the highest white line intensity, whereas the Ni foil displayed the lowest intensity.²⁹

The experimental findings indicate that at 285 °C, the nickel (Ni) valence state lies between Ni²⁺ and Ni³⁺ (Fig. 2e). After tandem pyrolysis, the Ni valence state in each sample exceeds that of the initial sample. Notably, in the a250 sample, the Ni valence state remains lower than the original, implying retention of its metallic phase during pyrolysis. The reciprocal space (R-space) analysis reveals two distinct coordination peaks at 1.61 and 2.15 Å (uncorrected for phase shift) (Fig. 2f), assignable to Ni–O/N and Ni–Ni interactions, respectively. Specifically, a285 predominantly exhibits the Ni–O/Ni–N form, with the 2.15 Å peak corresponding to the single scattering path of Ni–Ni.

The spectral analysis of a300 closely resembles that of nickel, maintaining stability at approximately 2.15 Å. For a250, a285, a300, and c325, dual peaks in the EXAFS spectrum are evident, appearing at 1.73 Å and 2.14 Å, respectively. Focusing on Fourier-transformed (FT) peaks near 1.73 Å, a detailed analysis reveals an average Ni–Ni spacing marginally shorter than that of Ni³⁺ (2.49 Å), indicating the presence of coordination unsaturation and structural distortion in the local coordination of nickel (Ni–O or Ni–N).³⁰ Concomitantly, close to 2.14 Å, the FT peak resulting from the single scattering of the Ni–Ni path suggests the existence of metallic phases. The observable strength of the Ni–Ni bond peak in a300 and Ni–O coordination in c325 imply that 300 °C represents a temperature node for the structural transition from an amorphous to a crystalline state. This structural insight underscores the complex interplay between tandem pyrolysis and evolution of nickel coordination environments in examined samples.³¹

The catalytic performance of prepared electrocatalysts is evaluated by electrochemical testing, and the OER conversion mechanism is as follows:⁵

Ni + 2OH⁻ → Ni–O + H₂O + 2e⁻

Ni–O + 2OH⁻ → Ni + O₂ + 2H₂O + 2e⁻

UOR: Ni + OH⁻ → Ni–OH

6Ni(OH)₂ + 6OH⁻ → 6NiOOH + 6H₂O + 6e⁻ + CO(NH₂)₂ → 6Ni(OH)₂ + N₂ + CO₂(g).

MOR: Ni + OH⁻ → Ni–OH

Ni–CH₃OH* → OH⁻–CH₃O* → CH₂O* → CHO* → CO* → COOH* → CO₂ + H₂O.

Due to the pronounced electronic correlation effect within the three-dimensional orbitals of nickel atoms, catalysts based on nickel have consistently been a focal point in OER research. The redox peak of Ni(II)/Ni(III) is susceptible to oxidation, showcasing the potential of nickel-based materials for finely tuning electronic structures and serving as effective active sites for methanol oxidation. This insight prompted an investigation into the electrocatalytic performance of OER/UOR/MOR under alkaline conditions. The electrocatalytic OER performance of all catalysts (a250, a285, a300, and c325) was assessed via LSV using a standard three-electrode system in a nitrogen-saturated 1 M KOH electrolyte. In Fig. 3a, a285 exhibited a mere 197 mV overpotential to achieve a reference current density of 10 mA cm⁻². Tafel plots, derived from polarization curves to gain deeper insight into electrochemical reaction kinetics (Fig. S9†), revealed a Tafel slope of 156.21 mV dec⁻¹ for a285 and 161.44 mV dec⁻¹ for a300.

Fig. 3 Oxygen evolution reaction (OER) catalytic performance. (a) Overpotentials at typical current densities and comparison of Tafel slopes. (b) EIS plots. (c) Durability test at 1.2 and 1.8 V vs. RHE for 25 h of a285. LSV curve after time stability test. (d) Normalized LSV curve of current density.

The data demonstrates that a285 and a300 also exhibit good dynamics at high current densities of 50 mA cm⁻². Fig. 3b illustrates the equivalent circuit diagram used for fitting, where R_s represents the contact resistance, which can be respectively represented as charge transfer resistance (R_ct) and Warburg impedance (Z_w), with R being the internal resistance,³² the value of the X-axis intercept of the semicircle, which was 4.30 Ω cm² before the first cyclic voltammetry cycle. The Faraday process can be further divided into two parts: charge transfer and mass transfer.³³ The Nyquist plot shows that a285 has the smallest semicircle radius, indicating the minimum charge transfer resistance (R_ct). This could be attributed to the rapid reaction kinetics of electrocatalytic water oxidation or the large specific surface area.³⁴ We quantified the electrochemically active surface area (ECSA) through electrochemical double-layer capacitors (C_dl). CV curves in the non-Faraday region, recorded at various scanning rates (20, 40, 60, 80, and 120 mV s⁻¹) in 1.0 M KOH, are presented. The C_dl of a300 was determined to be 12.00 mF cm⁻² by calculating the difference in current density at half of the slope using the linear relationship (j_a − j_c)/2. Following closely, the C_dl value of a285 was measured as 8.72 mF cm⁻² (Fig. S10†).

Recognizing the long-term durability as a pivotal indicator in the development of high-performance electrocatalysts, these findings contribute to a comprehensive assessment of electrochemical characteristics and potential applications of studied materials.³⁵ The stability of a285 was evaluated through chronoamperometry in 1.0 M KOH at a constant overpotential of 50 mV, as depicted in Fig. 3c. Following more than 24 hours of long-term stability testing, the potential required to maintain current density exhibited minimal variation. Subsequent to stability testing, LSV was conducted again, the overpotential was measured as 260 mV at 10 mA cm⁻², underscoring the excellent stability of the a285 sample in long-term oxygen evolution reactions under alkaline conditions.

We normalized the current density based on the corresponding roughness factor (R_f) obtained from ECSA results (Fig. 3d). At a 350 mV overpotential, the specific current density of a300 was 0.04 mA cm⁻², while that of a285 was 0.06 mA cm⁻². The higher specific current density of a285 compared to the control sample a300 suggests that a285 is inherently more active. This phenomenon may arise from the synergistic effect of exposing more active sites in unique structural features of a285, along with the accelerated electron transfer and significantly improved OER activity induced by Ni³⁺ species on the oxidation surface.

The UOR performance was evaluated in oxygen-saturated 1 M KOH + 0.33 M urea. For a300, a sharp increase in the anodic current was observed at an onset potential of 1.339 V vs. RHE. The electrode potentials at 10 mA cm⁻² were 1.355 V, 1.347 V, and 1.339 V for a250, a285, and a300 electrodes, respectively. The Tafel slope of a300 was as low as 10.15 mV dec⁻¹, lower than a285 (12.65 mV dec⁻¹) or c325 (20.59 mV dec⁻¹) (Fig. 4a). This indicates that the reaction follows the Volmer-Tafel mechanism where the chemical recombination of adsorbed H is the rate-determining step. The low Tafel slope of a300 signifies high UOR catalytic activity arising from fast and efficient charge and mass transfer during the catalytic process.³⁶

Fig. 4 UOR and MOR catalytic performance. (a) Potential and Tafel slope of the four catalysts at 10 and 50 mA cm⁻² in 1.0 M KOH + 0.33 M urea. (b) EIS curves and equivalent circuit of the electrocatalysts. (c) Long-term stability evaluation of a300 at 10 mA cm⁻² using the chronoamperometry method. (d) CV curves at 50 mV s⁻¹ in 1 M KOH + 0.5 M methanol. (e) Ratio of forward peak (I_f) to reverse peak (I_b) during forward scanning. (f) Tafel slope fitted according to the d-plot curve.

In Fig. 4b, the electrochemical impedance spectrum of the catalyst after the addition of urea solution reveals that the a300 catalyst exhibits the optimal kinetic process. The electrochemical double-layer capacitance fitting curve, derived from the cyclic voltammetry curve within the valid Ladi interval (Fig. S15†), confirms that the a300 sample possesses the highest C_dl value (C_dl = 2.31 mF cm⁻²), indicative of a relatively large electrochemical specific surface area. Utilizing eqn (4), turnover frequency (TOF) values for the catalyst were calculated as 0.22 s⁻¹, 0.26 s⁻¹, 0.11 s⁻¹, and 0.16 s⁻¹, respectively. The a300 catalyst demonstrates robust intrinsic activity. In Fig. 4c, the chronopotentiometric curve of a300 over 24 hours reveals sustained performance, and subsequent LSV curve scanning indicates a rise in potential and a decrease in the number of active sites. This decline in active sites, coupled with the extended sample consumption over time, suggests that the favorable UOR response is likely to be attributable to the presence of abundant active sites in the catalyst.

The MOR performance of the pyrolysis electrocatalyst was assessed through CV in 1.0 M KOH and 0.5 M methanol solution. Prior to MOR testing, a CV activation consisting of 50 cycles was conducted in a solution of 1 M KOH and 0.5 M CH₃OH until a stable curve shape was achieved. The resulting CV curve exhibits two distinct oxidation peaks, with the forward peak current (I_f) during forward scanning and reverse peak current (I_b) during reverse scanning occurring around 0.55 V. I_f is associated with the formation of intermediate product CO during methanol oxidation, while I_b corresponds primarily to the oxidation of adsorbed intermediate species that are not fully oxidized.³⁷ The peak current density of c325 is notably the highest at 1358 mA cm⁻², surpassing that of a250 (388 mA cm⁻²), a285 (490 mA cm⁻²), and a300 (453 mA cm⁻²) (Fig. 4d). This performance is superior to that of most reported Ni-based electrocatalysts, underscoring the efficacy of c325 in methanol oxidation reactions.

The Tafel slopes for methanol electrooxidation of a250, a285, a300, and c325 were 44.01, 30.74, 33.53, and 21.57 mV dec⁻¹, respectively (Fig. 4f). Compared to other groups, c325 exhibited the lowest Tafel slope, indicating significantly enhanced kinetics for methanol electrooxidation. Key indicators of electrocatalytic efficiency and catalyst tolerance to carbonaceous intermediates are I_f and I_b values.³⁸ The I_f/I_b ratios were 1.22, 1.23, 1.11, and 1.26 for a250, a285, a300, and c325, respectively (Fig. 4e). This improved electrooxidation performance is attributed primarily to abundant nickel active sites and unsaturated coordination in the amorphous material continually generated by decomposition of the complex framework.³⁹ The c325 sample exhibited a C_dl of 2.4 mF cm⁻² (Fig. S16†), indicating a large electrochemically active surface area. The fracturing of the peripheral carbon skeleton and formation of N–Ni, Ni–O, and Ni–Ni bonds likely exposed more Ni nanocrystals, increasing the specific surface area and MOR activity. Based on electrochemical results, we demonstrate the tremendous potential of amorphous and crystalline derivatives prepared from Ni₃-derived precursors for OER, UOR and MOR electrocatalytic applications.⁴⁰

DFT calculations were performed to elucidate the underlying mechanism for the enhanced OER performance. Ni sites play an important role in the catalytic process, as the density of states (DOS) of the Ni2 atom in Ni₃ crystals and a285 is concentrated at the Fermi level. Adsorption of electrons on the Ni2 surface can shift the d-band center of the Ni2 atom in a285 closer to the Fermi level (Fig. 5b). The higher OER activity could be attributed to stabilized OH⁻ adsorption on the Ni2 surface, which accelerates O₂ evolution (Fig. 5d). The remarkable OER electrocatalytic activity and stability of a285 can be ascribed to following reasons. As demonstrated by combined TG-MS, XPS, XANES, and DFT analyses, the amorphous structure provides rich 3d diffusion channels for charge carriers and contains highly dispersed Ni2 sites with defined charge states and bonding. This enables selective and efficient catalysis of specific reactions. The outstanding electrocatalytic performance of a285 further validates the structural advantages of the nickel complex.

Fig. 5 Theoretical calculation. (a) Density of states (DOS) (N 2p) of (a) Ni₃-crystal, (b) a285, and (c) c325 crystal. (d) D-band of nickel atoms.

Conclusions

In summary, we have uncovered modulation mechanisms of electrocatalytic oxygen/urea/methanol oxidation reactivity correlated with crystalline-to-amorphous transitional motifs in nickel clusters mediated by prudent tandem-temperature pyrolysis through rigorous experimentation combined with theoretical calculations. In situ TG-MS technology coupled with X-ray distribution function analysis was utilized to investigate the crystalline-to-amorphous transformation of Ni₃ during tandem pyrolysis. A homogeneous amorphous material was generated in the pyrolysis process, primarily attributed to inadequate ligand fracturing, which facilitated electron transfer. Density functional theory calculations further revealed that the three amorphous Ni atoms were located near the Fermi level. The Ni2, Ni1 and Ni1′ components not only offer abundant diffusion pathways for hydroxyl ions but also possess highly dispersed nickel metallic sites, a concentrated density of states, and definitive interfacial bonding. Moreover, Ni2, Ni1 and Ni1′ nanoparticles catalyze oxygen evolution reactions (a285) and urea (a300)/methanol (c325) oxidation reactions with remarkably high selectivity and efficiency (η₁₀ = 197 mV, η₁₀ = 1.339 V and 1358 mA cm⁻² at 0.56 V, respectively). Such attributes render this material among the most exceptional nickel-based electrocatalysts for these reaction systems, unveiling new opportunities to enable enhanced oxygen evolution through electronic structure engineering of nickel metal (or amorphous compounds) for water splitting applications.

Data availability

All experimental data associated with this article have been included in the main text and ESI.†

Author contributions

Xu Peng and Hai-Bin Yu conceived the idea, co-wrote the paper and supervised the whole experimental procedure and data analysis. Li Li, Hui-Feng Zhao, Mei-Xing Gan, Tao Zhang, Jia-Ning Li, Shi Tao and Jing Peng performed the experiments, analyzed the data and wrote the manuscript. All the authors discussed the results, commented on and revised the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Thousand a Young Talents Program of China, Fundamental Research Funds for the Central Universities (2018KFYXKJC009), Wuhan Knowledge Innovation Special Project (2023020201020422), and Natural Science Foundation of the Jiangsu Higher Education Institutions (23KJA430001).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2270095. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc01696c

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