Nanocrystalline CoO_x glass for highly-efficient alkaline hydrogen evolution reaction

Abstract

Hydrogen evolution reaction (HER) is a vital step for green-hydrogen production in commercial alkaline water electrolyzers. Although various electrocatalysts have been developed, the relationship between the structure and HER activity has not been clearly understood. Herein, we report nanocrystalline CoO_x glass composed of mixed amorphous parts and crystalline domains on Ni foam (NF) (denoted as (10CeCrP)CoO_x–NF–HER) for alkaline HER. We find that (10CeCrP)CoO_x–NF–HER exhibits high catalytic activity (for example, −0.354 V at 200 mA cm⁻² without iR correction) and good stability at high current density. Our experimental results reveal that the synergistic effects between the nanocrystalline domains and amorphous matrix improve the HER kinetics dramatically because: (1) the amorphous CoO_x enhances the pseudocapacitive K⁺ adsorption, leading to high surface water affinity, (2) the mixed crystalline and amorphous structure improves the stability of CoO_x in the HER process, leading to long-term catalytic stability, and (3) the high water and hydrogen concentrations on its surface provide abundant feedstocks for HER and promote the hydrogen transportation and conversion. Our findings may provide an insightful understanding for the enhanced catalytic performance of poor-crystalline electrocatalysts in HER, and open a new avenue for the design of high-performance HER electrocatalysts.

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1 Introduction

Hydrogen gas (H₂) from sustainable energy sources has been widely recognized as one of ideal candidates for next-generation fuels.^1–3 For decades, water splitting in alkaline solution through electricity from renewable powers, such as sun and wind, is considered as one of the mature technologies to produce high-purity green hydrogen.⁴ Nevertheless, the cost for the hydrogen produced in this way is still too high to be applied on large scale because of the low HER efficiency.^5,6 Therefore, it is urgent to develop efficient electrocatalysts for HER.^2,7,8 Pt-group-based electrocatalysts are efficient for HER and have been extensively investigated, but the limited reservoir and high cost severely hinder their practical applications.^9–11 Accordingly, noble-metal-free electrocatalysts have been developed. Particularly, poor crystalline (amorphous or glassy) electrocatalysts are mostly promising.^12–14 It is well known now that most electrocatalysts undergo in situ structure reconstruction in the HER process.¹⁵ Although intensive efforts have been undertaken to improve the HER performance, few studies focus on investigating the relationship between the in situ reconstructed structure and the HER performance. The formation of nanocrystals, phase segregation, and rearrangement of bonding states possibly occur during the reaction.^16–19 Therefore, it is necessary to understand the in situ structure reconstruction during HER, and its effect on the HER basic steps, which shall provide an insightful mechanism and guide the development of novel HER electrocatalysts.

In this work, we find that cobalt oxides (CoO_x) with variable crystallinities supported on Ni foam (NF) can be obtained in the HER process. The electrocatalysts after the HER tests are denoted as (P)CoO_x–NF–HER, (CrP)CoO_x–NF–HER, and (10CeCrP)CoO_x–NF–HER, respectively, while those before the tests as (P)CoO_x–NF, (CrP)CoO_x–NF, and (10CeCrP)CoO_x–NF, respectively (Scheme 1). CoO_x in (10CeCrP)CoO_x–NF–HER is amorphous with nanocrystalline CoO_x domains embedded, which is considered as nanocrystalline glass,^20,21 whereas those in (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER are polycrystalline CoO_x with small and large crystal grains, respectively. (10CeCrP)CoO_x–NF–HER achieves high electrochemical activity at high current density and good stability because the synergistic effects between the nanocrystalline and amorphous CoO_x can enhance the reaction kinetics.

Scheme 1 Schematic process to fabricate CoO_x with different crystalline structures.

2 Results and discussion

(10CeCrP)CoO_x–NF, (CrP)CoO_x–NF and (P)CoO_x–NF were prepared by electrodeposition (see the Experimental section in the ESI†). Typically, 0.2 M KHCO₃, Co foam, and NF were used as the basic electrolyte, anode, and cathode, respectively. The salt couples, CeCl₃ + CrCl₃ + NaH₂PO₂, CrCl₃ + NaH₂PO₂, and NaH₂PO₂ were used as an additive to obtain (10CeCrP)CoO_x–NF, (CrP)CoO_x–NF and (P)CoO_x–NF, respectively. For all the samples, we first measured their electrochemical performance in 1.0 M KOH. The linear sweep voltammetry (LSV) curves show that (10CeCrP)CoO_x–NF exhibits a higher current density than (CrP)CoO_x–NF, (P)CoO_x–NF and NF at the same potential (Fig. S1†), indicating its best electrochemical activity. Then, a chronopotentiometry test at 200 mA cm⁻² was performed to study the stability of the electrocatalyst. During the stability test, (10CeCrP)CoO_x–NF shows lower applied potential and smaller potential increment than (CrP)CoO_x–NF and (P)CoO_x–NF (Fig. 1a & S2†), further confirming its high catalytic performance. The LSV curves of (10CeCrP)CoO_x–NF, (CrP)CoO_x–NF and (P)CoO_x–NF after the HER stability test (denoted as (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER, respectively) were measured and compared with those of their fresh counterparts and Pt/C–NF. Notably, (10CeCrP)CoO_x–NF–HER needs smaller potential to achieve high current density (>150 mA cm⁻²) than Pt/C–NF. We can also see that (10CeCrP)CoO_x–NF–HER has negligible reduction in the current density compared with (10CeCrP)CoO_x–NF, while (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER are obviously poorer than (CrP)CoO_x–NF and (P)CoO_x–NF in the HER activity, respectively (Fig. 1b). Meanwhile, (10CeCrP)CoO_x–NF–HER shows a lower Tafel slope than (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER (Fig. 1c). Most importantly, the electrochemical active surface area of (10CeCrP)CoO_x–NF–HER is larger than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER (Fig. S3†), suggesting it has more electrocatalytic sites. The turnover frequency value (TOF) of (10CeCrP)CoO_x–NF–HER is larger than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER (calculation methods are described in the ESI and Fig. S4†), indicating its higher activity (Fig. 1d). Additionally, we further study (yCeCrP)CoO_x–NF–HER (y = the volume of 0.1 M CeCl₃ in the additive) and (CeP)CoO_x–NF–HER without CrCl₃ in the additive. (10CeCrP)CoO_x–NF–HER shows higher current density than (CeP)CoO_x–NF–HER, (5CeCrP)CoO_x–NF–HER, (20CeCrP)CoO_x–NF–HER and (50CeCrP)CoO_x–NF–HER, suggesting that the ratio of Ce, Cr and P in the electrolyte for the fabrication of (10CeCrP)CoO_x–NF is optimal (Fig. S5†). The Faraday efficiency of (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER is 97.5%, 95.6% and 93.9% in average, respectively, demonstrating the high conversion efficiency of (10CeCrP)CoO_x–NF–HER for hydrogen production (Fig. S6†). (10CeCrP)CoO_x–NF–HER also shows better electrocatalytic activity than many of the alkaline HER electrocatalysts with similar chemical compositions (Table S1†).

Fig. 1 Electrochemical performances. (a) Stability measurement at 200 mA cm⁻², (b) LSV curves at a scan rate of 5 mV s⁻¹, (c) Tafel plots, and (d) TOF curves.

To reveal the electrocatalytic sites, the samples were characterized by various ex situ methods, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman scattering, and X-ray photoelectron spectroscopy (XPS). The surfaces of (10CeCrP)CoO_x–NF, (CrP)CoO_x–NF and (P)CoO_x–NF before the HER are flat (Fig. S7†). After the 35 h-HER test, the surfaces of all samples change. The SEM images of (10CeCrP)CoO_x–NF–HER show that the surface of NF is covered by flower-like nanosheets with ∼200 nm in length and ∼30 nm in thickness (Fig. 2a–c). The SEM images of (CrP)CoO_x–NF–HER show that the nanosheets on the surface are large (∼500 nm in length and 50 nm in thickness) (Fig. 2d–f). For (P)CoO_x–NF–HER, the surface is also composed of nanosheets with ∼200 nm in length and ∼30 nm in thickness (Fig. 2h and i). The flower-like aggregated nanosheets on (10CeCrP)CoO_x–NF–HER may provide an open structure for the HER. The energy dispersive spectra (EDS) show the Co, O and Ni signals on the surfaces of the samples clearly, where Co and O are from CoO_x and Ni is from NF. The contents of Ce, Cr and P in (10CeCrP)CoO_x–NF–HER, Cr and P in (CrP)CoO_x–NF–HER and P in (P)CoO_x–NF–HER are quite low, suggesting that Ce, Cr and P do not play an important role in the HER (Fig. S8–S10 and Tables S2–S4†).

Fig. 2 SEM images of (10CeCrP)CoO_x–NF–HER (a–c); (CrP)CoO_x–NF–HER (d–f) and (P)CoO_x–NF–HER (g–i).

The TEM images are in good agreement with their corresponding SEM images, where the (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER catalysts consist of nanosheets. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and the corresponding EDS maps further confirm the uniform distributions of Co and O in (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER (Fig. 3a–c and S11†). The selected area electron diffraction (SAED) pattern of (10CeCrP)CoO_x–NF–HER is blurred, while the SAED patterns of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER show dash-like and diffraction spots, respectively. These results confirm that (10CeCrP)CoO_x–NF–HER exhibits a poor-crystalline structure, while (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER are composed of polycrystalline CoO_x with small and large crystals, respectively (Fig. 3d, g & j), which are further confirmed using the fast Fourier transform (FFT) images (Fig. 3e, h & k). The high-resolution TEM (HRTEM) images show that (10CeCrP)CoO_x–NF–HER is mainly composed of disordered lattice fringes, corresponding to the amorphous structure. A few domains with an interplanar spacing of 0.25 nm can be observed inside the amorphous matrix, corresponding to the CoO_x crystals with the (311) planes (Fig. 3f). For (CrP)CoO_x–NF–HER, the HRTEM image shows ordered lattice fringes. The interplanar spacing is 0.25 nm, corresponding to the Co₃O₄ (311) planes (Fig. 3i). The HRTEM image shows that (P)CoO_x–NF–HER consisted of ordered lattice fringes with interplanar spacings of 0.25 and 0.29 nm, which are related to the (311) and (220) planes of Co₃O₄, respectively. The HRTEM results reveal that (10CeCrP)CoO_x–NF–HER is composed of the amorphous CoO_x matrix with nanocrystals embedded, while (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER are dominated by polycrystalline CoO_x with different crystal sizes.

Fig. 3 (10CeCrP)CoO_x–HER: (a) TEM image (inset: HAADF image), (b and c) EDS elemental maps, (d) SAED pattern, (e) FFT image, and (f) HRTEM image. (CrP)CoO_x–NF–HER: (g) SAED pattern, (h) FFT image, and (i) HRTEM image. (P)CoO_x–NF–HER: (j) SAED pattern, (k) FFT image, and (l) HRTEM image.

The XRD patterns show that (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER have broad peaks within 10–30° and no other obvious peaks except for those of Ni (Fig. 4a) due to the low loading of CoO_x, which is below the XRD detection limit. The ex situ Raman spectrum of (10CeCrP)CoO_x–NF–HER (Fig. 4b) shows three characteristic peaks from low to high Raman shift: one for the stretching mode of the spinel tetrahedral unit (CoO₄) (E_g), one for the breathing mode of CoO₄ (F_2g) and one for the symmetrical vibration mode of the spinel octahedral unit (CoO₆) (A_1g).^22–24 (CrP)CoO_x–NF–HER also gives the three characteristic peaks. (P)CoO_x–NF–HER gives one E_g, two F_2g and one A_1g characteristic peaks. Therefore, all these electrocatalysts are composed of CoO_x with spinel-like structures. Additionally, the Raman spectra of those samples before and after the HER suggest that the crystallinity of CoO_x is improved in the HER process (Fig. S12†).

Fig. 4 (a) XRD patterns and (b) Raman spectra. XPS spectra for: (c) Co 2p and (d) O 1s.

The XPS spectra show that there are eight main peaks for Co 2p, including Co 2p_3/2 and 2p_1/2 peaks for both of Co³⁺ and Co²⁺ in Co₃O₄, and their corresponding satellite peaks (Fig. 4c),^25–27 and three main peaks for O 1s, including the Co–O, Co–OH and O–H bonds of the water molecule (Fig. 4d),^25,28 for all samples. We see that the chemical states of cobalt in all electrocatalysts are close to those in Co₃O₄.^25,29 It is worth noting that the binding energies of Co³⁺ and Co²⁺ in (10CeCrP)CoO_x–NF–HER are lower than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER by 0.4–0.5 eV, indicating that (10CeCrP)CoO_x–NF–HER may have more oxygen defects than (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER,^30–32 which can be beneficial to the HER basic steps.³³ What's more, we can see that the shapes, the intensities, and the locations of Co characteristic peaks in the Co 2p XPS spectra of (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER, (P)CoO_x–NF–HER and those before the HER are different, further suggesting the reconstruction during the HER (Fig. S13†). Meanwhile, the XPS spectra confirm no obvious Ce, Cr and P in all the tested samples, indicating that they play a negligible role in the HER further (Fig. S14–S16†). Based on the discussions above, CoO_x with different crystallinities can be obtained by controlling the salt couple in the electrolyte. The nanocrystalline/amorphous CoO_x composite in (10CeCrP)CoO_x–NF–HER endows CoO_x with a special physical structure and chemical states, which shall promote the HER performance.

To reveal the surface reconstruction, the potential–time dependent curve was measured, and in situ Raman spectroscopy was performed. First, the potential–time dependent curves of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER measured at current density (j) > 5 mA cm⁻² show bends at the bottom of slopes, which are called the “coup de fouet” phenomena (circled in Fig. S17a & b†),³⁴ indicating that the skeleton of CoO_x partially collapses in the HER. For (10CeCrP)CoO_x–NF–HER, no obvious “coup de fouet” phenomena can be observed on its curve (Fig. S17c†), suggesting the skeleton of CoO_x is well retained. Therefore, the amorphous matrix of (10CeCrP)CoO_x–NF–HER is much more tolerant to the reaction, which prevents the structure from collapsing. The in situ Raman spectra were measured to understand the in situ structural reconstruction further (Fig. 5a & S18†). The Raman spectra show that the characteristic A_1g peaks of (P)CoO_x–NF–HER and (CrP)CoO_x–NF–HER disappear at open circuit potential (OCP) before the HER (Fig. S18†). All the CoO_x characteristic peaks of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER disappear at j = 8–10 mA cm⁻². Notably, the CoO_x characteristic peaks of (P)CoO_x–NF–HER and (CrP)CoO_x–NF–HER cannot recover fully after returning to OCP for 10 minutes (Fig. S18†). Therefore, the structural changes of CoO_x in (P)CoO_x–NF–HER and (CrP)CoO_x–NF–HER are irreversible. For (10CeCrP)CoO_x–NF–HER, the Raman spectra show that the A_1g characteristic peak maintains well at OCP before the HER. When at j = 8–10 mA cm⁻², the characteristic A_1g peaks disappear as the defects in CoO₆ increase.³⁵ It is noticed that the characteristic E_g and F_2g peaks maintain well at j = 8–10 mA cm⁻². Most importantly, we can see that at the 10th minute after the HER, the A_1g peak is recovered and close to those measured before the HER test and in air (Fig. 5a). These results indicate that the CoO_x skeleton in (10CeCrP)CoO_x–NF–HER has better stability than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER in the HER process.

Fig. 5 (a) Raman spectra of (10CeCrP)CoO_x–NF–HER. (b) CV curves of the samples from −0.2 to 0.25 V vs. RHE (scan rate = 5 mV s⁻¹). (c) Potential–time dependent curves of the samples measured at 10 mA cm⁻². (d) Bode phase–frequency curves and resistance–frequency curves at −0.01 V vs. RHE.

Our studies show further that the CoO_x skeleton in (10CeCrP)CoO_x–NF–HER could provide more exposed Co sites to promote the HER basic steps.³⁶ The amount of exposed Co sites can be quantified by integrating the area of the reduction peak at a potential interval of 0–0.1 V on the cyclic voltammetry (CV) curve.³⁷ We can see that the Co(II)/Co(0) reduction peak on (10CeCrP)CoO_x–NF–HER can be clearly observed at ∼0.03 V (Fig. 5b).³⁸ Its area is larger than those on (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER (Fig. 5b), suggesting that the amount of exposed Co sites on (10CeCrP)CoO_x–NF–HER is more than those on (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER, which should be responsible for its superior HER performance too.

In the HER process, the pseudocapacitive adsorbed K⁺ could present near the cathode due to the effect of the electric field. Meanwhile, water molecules can combine with the K⁺ ions to form cation–water adducts (K⁺(H₂O)_x).³⁹ Therefore, the enhanced local K⁺ concentration could improve the concentration of the water molecules near the surface of the cathode. The amount of pseudocapacitive adsorbed K⁺ ions can be estimated from the potential–time dependent curves (Fig. 5c, S17 & S19†). We see that the potential decreases first (the slope part) and converges to a value over the time. The pseudocapacitive K⁺ adsorption takes place at the time interval of the slope mainly, and saturates at the end of the slope. Then, the HER takes place after the slope (or the platform) (Fig. 5c & S17†). The density of charge (Q, mC cm⁻²) can be obtained by timing the current density with the retention time of the slope, which is related to the amount of pseudocapacitive adsorbed K⁺ ions near the surface.³⁷ Typically, the potential–time dependent curves measured at j = 10 mA cm⁻² show that the Q value of (10CeCrP)CoO_x–NF–HER (200 mC cm⁻²) is much larger than those of (CrP)CoO_x–NF–HER (70 mC cm⁻²) and (P)CoO_x–NF–HER (40 mC cm⁻²) (Fig. 5c). We can also see that the Q values of (10CeCrP)CoO_x–NF–HER obtained from the potential–time dependent curves at the different current densities are larger than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER (Fig. S19†). Therefore, (10CeCrP)CoO_x–NF–HER can attract more pseudocapacitive K⁺ ions than (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER.

The Bode plots of the resistance–frequency and phase angle–frequency could be used to evaluate the K⁺ adsorption further. The peak height at 0.1–1 Hz in the Bode phase angle–frequency curve represents the properties of K⁺ adsorption.⁴⁰ The frequencies within 1–100 Hz of the Bode phase–frequency curve at the phase angle ∼−45° can be utilized to estimate the K⁺ adsorption strength further.⁴¹ (10CeCrP)CoO_x–NF–HER has smaller frequencies than (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER, indicating that the retention time of pseudocapacitive K⁺ on (10CeCrP)CoO_x–NF–HER is longer than that on (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER. Therefore, the K⁺ adsorption on (10CeCrP)CoO_x–NF–HER is more tight than that on (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER (Fig. 5d). Correspondingly, the amount of K⁺(H₂O)_x adducts on (10CeCrP)CoO_x–NF–HER is more than those on (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER, leading to the improved water concentration.

As we know, the HER undergoes the following radical steps under alkaline conditions as shown in eqn (1)–(3) (Volmer–Heyrovsky mechanism):⁴²

H₂O + e⁻ + catalyst → catalyst-H + OH⁻ (Volmer step) (1)

Catalyst-H + H₂O + e⁻ → catalyst-H₂ + OH⁻ (Heyrovsky step) (2)

Catalyst-H₂ → catalyst + H₂ (dissociation) (3)

The Tafel slope values (75–120 mV dec⁻¹) confirm that the HER on these electrocatalysts undergoes the basic steps described in eqn (1)–(3), and the Volmer step (eqn (1)) is the rate-determining step (Fig. 1c).⁴² From eqn (1)–(3), we can see that the electrocatalyst surface should satisfy the following requirements for high HER activity: (i) fast charge transportation; (ii) high water affinity and water cleavage; (iii) appropriate OH⁻ adsorption strength; (iv) high concentration of hydrogen on the surface; and (v) fast hydrogen conversion. The Nyquist plot of the electrochemical impedance spectrum (EIS) can be utilized to characterize the charge transfer. The EIS spectra show that the charge transfer resistance of (10CeCrP)CoO_x–NF–HER (∼25 Ω) is lower than those of (CrP)CoO_x–NF–HER (∼30 Ω) and (P)CoO_x–NF–HER (∼33 Ω) (Fig. 6a). Therefore, the charge transfer kinetics on (10CeCrP)CoO_x–NF–HER is more beneficial than those on (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER. It could not only promote the charge transfer processes of the HER radical steps, but also hamper the collapse of the CoO_x skeleton by the Co self-reduction.⁴³

Fig. 6 (a) Nyquist plots of (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER at −0.13 V. (b) In situ Raman spectra of (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER at 1450–1800 cm⁻¹. (c) FT-IR ATR spectra of (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER after the treatment in the NH₃/KOH mixed solution. (d) In situ Raman spectra of (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER at OCP, j = 9 and 10 mA cm⁻² at 900–1200 cm⁻¹ Raman shift.

Water affinity and cleavage are also crucial in alkaline HER. We have proved that the enhanced pseudocapacitive K⁺ adsorption on (10CeCrP)CoO_x–NF–HER leads to the high concentration of K⁺(H₂O)_x adducts on its surface, resulting in the improved water affinity. To further confirm the enhanced water affinity on (10CeCrP)CoO_x–NF–HER, in situ Raman spectra were measured at 10 mA cm⁻² within the range of 1450–1800 cm⁻¹. The peak at 1600 cm⁻¹ is attributed to the O–H bending vibration of the water molecule on the surface (δ_O–H).⁴⁴ Clearly, the δ_O–H intensity on (10CeCrP)CoO_x–NF–HER is stronger than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER, suggesting the amount of adsorbed water on (10CeCrP)CoO_x–NF–HER is more than those on (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER. Therefore, the water affinity on (10CeCrP)CoO_x–NF–HER is higher than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER (Fig. 6b).

The water molecule could be cleaved on the skeleton of Co oxide: Co–O + H₂O → Co–O–H⁺ + OH⁻.^45–47 A red shift of ∼7 cm⁻¹ for E_g on (10CeCrP)CoO_x–NF–HER in the HER suggests the water molecule cleavage and the formation of Co–O–H⁺ (Fig. 5a),^46,48 where H⁺ can be seen as a proton. Therefore, the acidity of Co–O–H⁺ shall play a crucial role in the HER. The Fourier transform infrared spectroscopy with the attenuated total reflectance method (FT-IR ATR) can quantify the acidity of H⁺ in Co–O–H⁺ on the surface of the electrocatalyst by using NH₃ as the probe molecule.^49,50 The FT-IR ATR spectrum of (P)CoO_x–NF–HER shows the characteristic peak at 1380 cm⁻¹ due to the N–H bending vibration of free NH₃ molecules on the surface.^49,50 For the FT-IR ATR spectrum of (CrP)CoO_x–NF–HER, the characteristic peaks at 1376 and 1408 cm⁻¹ are attributed to the N–H bending mode of free NH₃ on the surface and the deformation vibration of NH₄⁺, respectively. For the FT-IR ATR spectrum of (10CeCrP)CoO_x–NF–HER, three characteristic peaks can be observed at 1373, 1442 and 1690 cm⁻¹, which are related to the N–H bending vibration of free NH₃, N–H deformation vibration of NH₄⁺ and stretching vibration of NH₄⁺, respectively (Fig. 6c). We see that the intensities of the NH₄⁺ characteristic peaks on (10CeCrP)CoO_x–NF–HER are higher than those on the other two samples, suggesting the protonation of NH₃ is more beneficial on (10CeCrP)CoO_x–NF–HER (Fig. S20†). Correspondingly, Co–O–H⁺ on the surface of (10CeCrP)CoO_x–NF–HER has stronger acidity than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER. Therefore, the HER performance could be improved because of the high acidic micro-environment on (10CeCrP)CoO_x–NF–HER.

The OH⁻ affinity on the electrocatalyst is also crucial for the HER. According to eqn (1)–(3), we know that the OH⁻ affinity should be neither too strong nor too weak. To study the OH⁻ affinity, in situ Raman spectra at OCP and j = 9–10 mA cm⁻² were measured (Fig. 6d). The characteristic peak at ∼1060 cm⁻¹ is attributed to the Co–OH⁻ bonding (Co–OH⁻).^51,52 For (P)CoO_x–NF–HER, no characteristic peak of Co–OH⁻ can be observed in the tests, suggesting few OH⁻ adsorbed on its surface. For (CrP)CoO_x–NF–HER, the Co–OH⁻ characteristic peaks can be observed not only at j = 9 and 10 mA cm⁻², but also at OCP, suggesting the stronger OH⁻ binding on the surface, which may lead to surface poisoning. For (10CeCrP)CoO_x–NF–HER, the characteristic peaks of Co–OH⁻ can be observed when j = 9 and 10 mA cm⁻² only. Therefore, the OH⁻ affinity of (10CeCrP)CoO_x–NF–HER is weaker than that of (CrP)CoO_x–NF–HER, but much stronger than that of (P)CoO_x–NF–HER, indicating that (10CeCrP)CoO_x–NF–HER shows the optimal OH⁻ affinity for the HER.

As we know, the low proton concentration in the alkaline electrolyte (10⁻¹⁴ order of magnitude) leads to low hydrogen concentration on the surface, which is one of the largest obstacles to improve the kinetics of the HER.⁵³ Therefore, improving the concentration of hydrogen on the surface shall lead to improved HER kinetics. To quantify the hydrogen concentration and study the hydrogen transportation kinetics in depth, the amount of hydrogen adsorption capacity and the resistance of hydrogen transportation were characterized by the second parallel components, C_φ and R₂, which represent the hydrogen adsorption pseudo capacitance and resistance of hydrogen adsorption,⁵⁴ respectively, by using a fitted electronic circuit with specific parameters (Fig. S21 & Table S5†). The C_φ values of (10CeCrP)CoO_x–NF–HER are much larger than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER, that is, the hydrogen adsorption values of (10CeCrP)CoO_x–NF–HER are larger than those of (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER at the same potential (Fig. 7a). The high hydrogen adsorption amount on (10CeCrP)CoO_x–NF–HER shall provide enough hydrogen resources for the HER.

Fig. 7 (a) C_φ–potential plots, (b) lg(R₂)–potential plots, and (c) polarization curves in different solutions. (d) Curves of the KIE ratio value vs. potential.

Besides the hydrogen adsorption amount, the hydrogen mobility on the surface of the electrocatalyst is also important, which can be quantified using the slope of the lg R₂vs. potential plot.⁵⁵ (10CeCrP)CoO_x–NF–HER has a smaller slope (13.4 mV dec⁻¹) than (CrP)CoO_x–NF–HER (16.6 mV dec⁻¹) and (P)CoO_x–NF–HER (15.4 mV dec⁻¹) (Fig. 7b), indicating that the hydrogen transportation on (10CeCrP)CoO_x–NF–HER is more beneficial than those on (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER. The surface hydrogen transfer kinetics can be further estimated from the H/D kinetic isotope effects (KIEs).⁵⁵ The KIE ratio value, defined by the current density ratios between those measured in aqueous KOH/H₂O and KOH/D₂O (j_H/j_D), is considered as evidence for the hydrogen transfer that shall affect the reaction rate, especially when the KIE ratio value > 1.⁵⁶ The LSV curves of (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER measured in 1.0 M KOH/D₂O exhibit significantly lower current density than those in 1.0 M KOH/H₂O. The KIE ratio values of (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER over the entire potential range are larger than 1, suggesting that the proton transfer in the HER process is evolved as the rate-determining step (Fig. 7c and d).⁵⁷ Importantly, the KIE ratio value could be used as a descriptor of the proton transfer rate.⁵⁸ Among (10CeCrP)CoO_x–NF–HER, (CrP)CoO_x–NF–HER and (P)CoO_x–NF–HER, (10CeCrP)CoO_x–NF–HER possesses the lowest KIE values at the same potentials (Fig. 7d), suggesting the enhanced hydrogen transfer kinetics for (10CeCrP)CoO_x–NF–HER.^56,58 The C_φ–potential plots, lg(R₂)–potential plots and KIE ratio values suggest that (10CeCrP)CoO_x–NF–HER has high hydrogen amount, small hydrogen transfer resistance and fast hydrogen mobility on the surface.

The systematic investigation verifies that the nanocrystalline/amorphous CoO_x composite of (10CeCrP)CoO_x–NF–HER results in its excellent HER performance by promoting the basic steps of alkaline HER. Firstly, amorphous CoO_x enhances pseudocapacitive hydrate K⁺ adsorption, leading to high water concentration on the surface. Secondly, mixed nanocrystalline and amorphous CoO_x in (10CeCrP)CoO_x–NF–HER improves the stability of the CoO_x skeleton during the HER, enabling exposure of more Co sites expose and leading to high water molecule cleavage ability and hydrogen stabilization. Finally, the high water and hydrogen concentrations on the (10CeCrP)CoO_x–NF–HER surface provide abundant feedstocks for processing the HER basic steps and effectively promote the hydrogen transportation and conversion. The full process of HER promotion is summarized in Scheme 2.

Scheme 2 Proposed mechanism of the nanocrystalline/amorphous CoO_x composite in (10CeCrP)CoO_x–NF–HER.

3 Conclusions

In summary, we present a novel strategy to fabricate nanocrystalline CoO_x glass with a nanocrystalline CoO_x/amorphous CoO_x mixed structure on NF ((10CeCrP)CoO_x–NF–HER) as a high-performance HER electrocatalyst. (10CeCrP)CoO_x–NF–HER has high electrochemical activity (−0.354 V at 200 mA cm⁻² without iR correction) and good stability in alkaline electrolyte. The factors that improve the HER performance of (10CeCrP)CoO_x–NF–HER are: (1) the high water and hydrogen concentrations, (2) the stable interwoven nanocrystalline and amorphous structure, (3) the high water molecule cleavage ability and hydrogen stabilization, and (4) the fast hydrogen transportation and conversion. Our findings may not only present an insightful understanding on the promotion mechanisms in the HER on the electrocatalyst's surface, but provide practical strategies for the design of electrocatalysts towards high-performance HER in alkaline electrolyte.

4 Materials and methods

4.1 Chemicals

All chemicals were utilized as received without any treatment. Deionized (DI) water was supplied by a Barnstead Nanopure water system (resistivity: 18.3 MΩ cm⁻¹) and used for the preparation of all aqueous solutions. Cobalt foam (200 × 300 × 1.7 mm) was purchased from Teng'er hui Electronic Co., Ltd. Nickel foam (NF, 200 × 300 × 1.7 mm, 110 ppi) was purchased from Kunshan Guangjiayuan New Material Co., Ltd. Ethanol (C₂H₅OH, AR, 95%), hydrochloric acid (HCl, AR, 32%), and potassium hydroxide (KOH, AR, 98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium bicarbonate (KHCO₃, AR, ≥99.99%), sodium hypophosphite (NaH₂PO₂, AR, ≥99.99%), chromium(III) chloride hexahydrate (CrCl₃·6H₂O, AR, ≥99.99%) and cerium(III) chloride hexahydrate (CeCl₃·6H₂O, AR, ≥99.99%) and heavy water (D₂O, >99% D atom) were purchased from Shanghai Aladdin Biochemical Technology Co, Ltd.

4.2 Electrocatalyst fabrication

A piece of NF (2 cm × 3 cm) was used as the substrate and first cleaned by ultrasonication for 10 min with ethanol, 6 M HCl aqueous solution, and DI water, respectively. A piece of Co foam (3 cm × 3 cm) was used as the Co source and first cleaned by ultrasonication for 10 min with ethanol, 6 M HCl aqueous solution, and DI water, respectively.

The electrolyte to prepare (P)CoO_x–NF (denoted as electrolyte 1) contains 0.2 M KHCO₃ and 0.1 M NaH₂PO₂. The electrolyte for (CrP)CoO_x–NF (denoted as electrolyte 2) included 200 μL of 75 mM CrCl₃ and electrolyte 1. To obtain (yCeCrP)CoO_xNF, different volumes of 0.1 M CeCl₃ solution (y, y = 5, 10, 20 and 50 μL) were added in the electrolyte 2. To obtain (CeP)CoO_x–NF, 10 μL of 0.1 M CeCl₃ was added in electrolyte 1. A piece of NF (3 cm × 2 cm) after treatment was used as the cathode with an area of 2 cm × 2 cm in electrolyte. A piece of Co foam (3 cm × 3 cm) was used as the anode with an area of 2 cm × 3 cm in electrolyte. The total volume of all the electrolytes is 15 mL. The current density was calculated based on the immersed geometry area of the cathode. In all the fabrication processes, a current density of 2.5 mA cm⁻² was used and the electrodeposition time was 30 minutes. After that, the final products were taken out and washed thoroughly with DI water, and extra water was absorbed by filter paper. For comparison, commercial Pt/C loading on NF (Pt/C–NF) was fabricated via the following steps. 20 mg of 20% Pt/C powder, 50 μL of 5% Nafion solution, 475 μL of ethanol, and 475 μL of DI water were mixed with the assistance of a 30 minute ultrasonication to form a homogeneous ink. Then, 200 μL of the ink was cast onto one piece of NF (1 cm × 1 cm) and dried using an infrared lamp.

4.3 Materials characterization

The morphology was determined using a scanning electron microscope (SEM) (ZEISS-Merlin) and a transmittance electron microscope (TEM) (JEM-F200) coupled with an EDX spectrometer. Powder XRD measurements were conducted on a Rigaku rotating anode diffractometer with a monochromatic Cu K_α X-ray source. Raman spectra were recorded using a confocal laser Raman system with a 532 nm laser and an acquisition time of 20 s. X-ray photoelectron spectroscopy (XPS) spectra were collected on a Thermo Fisher Scientific Theta Probe with Mg K_α (hν = 1253.6 eV) as the excitation source. The loading mass of CoO_x on NF was calculated by using an electronic balance (d = 0.0001 g, Mettler Toledo). Typically, 5 pieces of samples and 5 pieces of NF with the same size were weighed, and the average weight difference between the sample and NF was estimated as the loading mass of CoO_x on NF.

4.4 Electrochemical test

The electrochemical tests towards the HER performance were conducted on an electrochemical workstation (ModuLab XM) with a standard three-electrode cell, where the as-prepared samples, a graphite rod, and a standard Hg/HgO (1 M KOH) electrode with an additional salt bridge were used as the working, the counter, and the reference electrodes. Potentials were converted to the reversible hydrogen electrode (RHE) via the Nernst equation (E vs. RHE = E vs. Hg/HgO + 0.0591 × pH + 0.098 V). There was no iR correction in our test. The electronic impedance spectroscopy (EIS) plots were measured in the frequency range of 0.01–10⁶ Hz with an amplitude of 10 mV. The EIS raw data were fitted using ZView software.

4.5 TOF estimation

The electrochemical surface area (ECSA) of the as-prepared sample was estimated from its double layer capacitance (C_dl), which has been measured using a simple cyclic voltammetry method. Here the applied potential window was the non-Faraday region.⁵⁹ Then the current was only generated for charging of the double layer, which was expected to have a linear relationship with the active surface area. The calculated slope is two times of C_dl. The value of ECSA can be calculated according to the following equation:

ECSA = C_dl/C_s

According to the previous literature, the specific capacitance value C_s is 0.040 mF cm⁻².⁶⁰ The electrochemical active species of the tested samples is Co₃O₄. The number of surface-active sites per real surface area for Co₃O₄ (determined by the main species of electrocatalysts) was calculated using the following equation:⁸

The plot of current density can be converted into a TOF plot according to:

4.6 Faraday efficiency

Faraday efficiency of the HER was measured using a gas chromatograph (GC) (7890B, Agilent Technologies) with a thermal conductivity detector (TCD). The working electrode area was 0.5 cm² and the current was 10 mA. The hydrogen gas generated from an alkaline electrolyte cell was used as the standard gas.

4.7 Quantification of the surface acidity

All the measurements were established using the Fourier transform infrared spectroscopy with attenuated total reflectance (TENSOR II) method. NH₃ was used as the probe molecule. The samples were immersed in a mixture of 1.0 M KOH + 3% NH₃ for 15 minutes, then taken out and placed on a piece of filter paper to remove the excess liquid. The NF with NH₃ was used as the background, which was prepared by the following process: A piece of NF was soaked in 1.0 M KOH + 3% NH₃ for 15 minutes first, then excess liquid was removed using a piece of filter paper. The background was removed before the measurement began.

4.8 In situ Raman spectroscopy

In situ Raman spectra were measured by using a Teflon in situ Raman cell with 1 mm thickness in 1.0 M KOH electrolyte. The purified electrolyte was circulated throughout the cell to remove the bubbles. The Raman excitation wavelength was 532 nm with 100% power ratio and the acquisition time was 20 s. The current and potential were directly controlled using an electrochemical workstation (CHI 760E).

4.9 Estimation of H/D kinetic isotope effect (KIE) values

Experiments to obtain the H/D kinetic isotope effect (KIE) values were carried out by measuring the LSV curves in 1.0 M KOH aqueous solution (KOH/H₂O) and 1.0 M KOH D₂O solution (KOH/D₂O), respectively. The current densities at a certain potential were abbreviated as j_H and j_D. The KIE values were defined as:^61,62

j _H and j_D should be contrasted at the same overpotential. The overpotentials in KOH/H₂O solution were calculated using the following formula:

Overpotential/V = E vs. Hg/HgO + 0.0591 × pH + 0.098 V

The LSV curves in KOH/D₂O were measured by using Ag/AgCl (saturated KCl) as the reference electrode, and the overpotentials of D₂ generation in KOH/D₂O solution were calculated using the following formula:

Overpotential/V = E vs. Ag/AgCl + 0.0591 × pH + 0.198 V + (−0.013 V)

0.198 V represents the equilibrium potential of Ag/AgCl (saturated KCl), and −0.013 V is the equilibrium potential of the D⁺/D₂ couple (−0.013 V vs. RHE).⁶³

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its ESI.† Raw data generated for this study are available from the corresponding author on reasonable request.

Author contributions

Jinxian Feng: conceptualization, methodology, software, data curation, and writing – review & editing. Lulu Qiao: data curation and writing – review & editing. Pengfei Zhou: data curation and writing – review & editing. Haoyun Bai: writing – review & editing. Chunfa Liu: writing – review & editing. Chon Chio Leong: writing – review & editing. Yu-Yun Chen: supervision. Weng Fai Ip: writing – review & editing. Jun Ni: funding acquisition, formal analysis, supervision, and validation. Hui Pan: conceptualization, methodology, supervision, validation, funding acquisition, and project administration.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

H. P. is thankful for the support of Science and Technology Development Fund (FDCT) from Macau SAR (0081/2019/AMJ, 0154/2019/A3, 0033/2019/AMJ, and 0102/2019/A2), Multi-Year Research Grants (MYRG2018-00003-IAPME and MYRG2022-00026-IAPME) from Research & Development Office at University of Macau, and Shenzhen-Hong Kong-Macao Science and Technology Research Programme (Type C) (SGDX20210823103803017) from Shenzhen.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta08073g

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