Ultra-Fast and In-Depth Reconstruction of Transition Metal Fluorides in Electrocatalytic Hydrogen Evolution Processes.

Hitherto, there are almost no reports on the complete reconstruction in hydrogen evolution reaction (HER). Herein, the authors develop a new type of reconfigurable fluoride (such as CoF2 ) pre-catalysts, with ultra-fast and in-depth self-reconstruction, substantially promoting HER activity. By experiments and density functional theory (DFT) calculations, the unique surface structure of fluorides, alkaline electrolyte and bias voltage are identified as key factors for complete reconstruction during HER. The enrichment of F atoms on surface of fluorides provides the feasibility of spontaneous and continuous reconstruction. The alkaline electrolyte triggers rapid F- leaching and supplies an immediate complement of OH- to form amorphous α-Co(OH)2 which rapidly transforms into β-Co(OH)2 . The bias voltage promotes amorphous crystallization and accelerates the reconstruction process. These endow the generation of mono-component and crystalline β-Co(OH)2 with a loose and defective structure, leading to an ultra-low overpotential of 54 mV at 10 mA cm-2 and super long-term stability exceeding that of Pt/C. Moreover, DFT calculations confirm that F- leaching optimizes hydrogen and water adsorption energies, boosting HER kinetics. Impressively, the self-reconstruction is also applicable to other non-noble transition metal fluorides. The work builds the fundamental comprehension of complete self-reconstruction during HER and provides a new perspective to conceive advanced catalysts.

Introduction

For oxygen evolution reaction (OER) pre‐catalysts in alkaline media, it has been well established that their intrinsic activity originates from the reconstructed intermediates, such as oxides,[ ] hydroxides,[ ] and oxy‐hydroxides.[ ] Unfortunately, for hydrogen evolution reaction (HER) catalysts, to date, only a few studies show that the surface reconstruction can take place and contribute to improving HER performance. Meanwhile, these reports seem to present debatable active sites (after reconstruction), such as reduction to metals[ , ] or hydroxylation to hydroxides[ ] and oxy‐hydroxides.[ ] Moreover, most of the original catalysts are preserved due to the limited reconstruction degree. Such a complex structure makes it difficult to identify the real origin of activity in the catalysts. Hence, the rational design of rapidly and completely reconfigurable pre‐catalysts during alkaline HER and the revelation of the underlying reconstruction mechanism are of great significance. The key constraint of in‐depth reconstruction is the generation of dense reconstruction layer due to sluggish mass transfer process, which blocks further mass transfer according to Mai et al.[ ] For deeper or complete reconstruction, the designed pre‐catalysts should possess the advantages of rapid mass transferability and the formation of a loose reconstructible layer. Existing studies found that the introduction of F− makes the surface of catalysts more hydrophilicity,[ ] thus favoring to fully react with electrolyte to realize preliminary reconstruction. The reconstructible layer presents a highly mesoporous hierarchical structure,[ ] which enables more electrolyte penetration for deepening the reconstruction. Thus, the introduction of more fluorine into pre‐catalysts probably makes them easier to implement deeper or even complete reconstruction.

Accordingly, a new class of single‐phase target pre‐catalysts, transition metal fluorides, including CoF2, NiF2, and FeF3 (H2O)0.33, are developed for alkaline HER. As expected, fluorides undergo a successive and rapid self‐reconstruction process owing to the leaching of F− during HER, resulting in great reductions in overpotentials. Especially for CoF2, it not only exhibits extraordinary catalytic activity comparable to the benchmark Pt/C but also outputs high stability exceeding Pt/C. By means of in situ Raman together with ex situ X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and spherical aberration correction electron microscopy (AC‐TEM) analyses, we systematically uncover the successive self‐reconstruction process of CoF2, in which the mono‐component hexagonal crystalline phase, β‐Co(OH)2, is presented as about 5 nm nanoparticles (NPs) interconnected into porous and defective interwoven nanosheets (NSs). Also, by combining the density functional theory (DFT) calculations, we further disclose the reconstructive evolution process.

Results

Synthesis and Structure Characterizations of Pre‐Catalysts

The synthetic process of CoF2 including the growth of precursor and subsequent fluoridation is depicted in Figure . The rosy red precursor with a hexagram star morphology grew on the pre‐cleaned carbon cloth (CC) via a facile hydrothermal strategy (Figure S1A, Supporting Information). From the XRD pattern, it is well‐indexed that the cobalt hydroxide fluoride (CoF1.3(OH)0.7, JCPDS No. 18–0405) was thoroughly transformed into pink single‐phase cobalt fluoride (CoF2, JCPDS No. 71–1969) by reacting with hydrogen fluoride (HF) vapor resulting from the thermal decomposition of NH4F (Figures S1B, S2, and S3, Supporting Information). Both oxygen and fluorine signals were detectable by XPS, indicating the adsorbed oxygen species on catalyst surfaces due to exposure to air (Figure S4A, Supporting Information). The relevant peak of adsorbed oxygen species appears at 532.2 eV in the O 1s spectrum (Figure S4B, Supporting Information).[ ] Meanwhile, the peaks belonging to CoF2 locate at 783.0 and 799.1 eV in Co 2p spectrum and 684.8 eV in F 1s spectrum (Figure S4C, D, Supporting Information).[ , ] The remaining peaks of 787.8 and 804.7 eV derive from the satellites of Co 2p3/2 and Co 2p1/2.[ ]

Figure 1

A) Synthetic scheme of hexagram star CoF2 supported on CC. B) FESEM image of CoF1.3(OH)0.7. C) FESEM, D) TEM, E) HAADF‐STEM (inset, EELS spectra integrated from red box region), F) HRTEM, and G) HAADF‐STEM (inset, SAED pattern) images of CoF2. H–J) Corresponding EDS elemental mappings. Scale bar: (B, C) 2 µm; (D) 500 nm; (E) 20 nm; (F) 5 nm; (G–J) 200 nm, 5 nm−1 (inset).

From field emission scanning electron microscope (FESEM) images, it is clearly seen that CoF1.3(OH)0.7 has the shape of a regular hexagram star, with the lateral diameter of about 6 µm and thickness in the range of about 2 to 3 µm (Figure 1B and Figure S5, Supporting Information). After fluorination, the hexagram star structure as a precursor is well preserved and stacked by 3D irregular ribbons with a rough face, which has the high homogeneity of chemical components from edge to center (Figure 1C–E, Figures S6 and S7, Supporting Information). Their lattice fringes with an interplanar distance of approximately 2.64 Å are well indexed to the (101) plane of CoF2 (Figure 1F). The rings in the selected area electron diffraction (SAED) pattern of the hexagram star are indexed to the lattice planes of

(110), (101), (200), (210), and (301) (Figure 1G), demonstrating the polycrystalline nature of the prepared CoF2. Additionally, the scanning transmission electron microscopy (STEM) element mapping further confirms the homogeneity of CoF2 components (Figure 1H–J).

Electrochemical Activity and Stability of HER

Dynamic Activity Variation During HER of Pre‐Catalysts

To prove that CoF2 is capable of speedy realizing reconstruction for promoting catalytic activity during HER, we first tested successive LSV curves under the potentials ranging from −0.8 to −1.3 V versus Hg/HgO in 1 m KOH without iR correction (Figure and Figure S8, Supporting Information). Note that the HER catalytic activity is dynamic with a drastic reduction in overpotential with successive LSV scans, similar to the previous reports on OER,[ , ] indicating the possibility of reconstruction of CoF2 during HER. When subjected to consecutive LSV scanning for only 50 cycles for the initial CoF2 (Figure 2B), the overpotential strikingly decreases by approximately 73 (@10 mA cm−2) and 118 mV (@400 mA cm−2), resulting in ultra‐low overpotentials of 54 mV at 10 mA cm−2 and 251 mV at 400 mA cm−2. If further increasing LSV scans, the degree of decrement remains unchanged. The consecutive EIS plots were also recorded at HER occurring. Obviously, a continuous decline in charge transfer resistance (R ct) is observed with increasing EIS scans, and the final R ct of 2.0 Ω following 12 cycles is as low as 36.4% of the initial CoF2 value (Figure 2C). As expected, the HER catalytic activity and charge transfer ability of CoF2 can be rapidly and significantly enhanced in the course of HER under alkaline electrolyte.

Figure 2

A) Consecutive LSV curves, B) corresponding overpotentials (@10, 400 mA cm−2), C) consecutive EIS plots, and D) i‐t curve of CoF2. E) LSV curves and F) Tafel slopes of R‐CoF2, R‐NiF2, and R‐FeF3 (H2O)0.33 in comparison with commercial Pt/C. G) LSV curves of R‐CoF2 and Pt/C before and after successive 3000 cycles CV acceleration. H) i‐t curves of R‐CoF2 and Pt/C.

To further observe the dynamic variations, the chronoamperometry (i‐t) curve under a constant potential of −1.3 V versus Hg/HgO was examined. As shown in Figure 2D and Video S1, Supporting Information, its behavior matches well with the LSV changes and involves three key processes driven by the applied potential. Apparently, the current density of the i‐t curve suddenly decreases at the beginning of 2 s. Then, a sharp rise lasts for approximately 5 min. Subsequently, the moderate increase sustains for approximately 10 min and finally keeps nearly at a constant. In contrast, with the initial sample, the obtainable current density is as high as 2.5 times after 15 min of consecutive electrolysis. More importantly, the similarity of dynamic variation during HER is also observed in other non‐noble metal fluorides (NiF2 and FeF3 (H2O)0.33) (Figures S9–S12, Supporting Information), which further proves the versatility of transition metal fluoride reconstruction.

Comparison of HER Activity and Stability for Reconstruction‐Derived Components and Pt/C

Additionally, we compared the HER activities of reconstructed fluorides (named as R‐CoF2, R‐NiF2, and R‐FeF3 (H2O)0.33) supported on CC with commercial Pt/C supported on CC as a benchmark. From Figure 2E, it is obvious that R‐CoF2 only needs an ultra‐low overpotential of 54 mV comparable to Pt/C (34 mV) but lower than that of R‐NiF2 (101 mV) and R‐FeF3 (H2O)0.33 (159 mV). Its Tafel slope (56 mV dec−1) is also closer to that of Pt/C (42 mV dec−1, Figure 2F). While the current density exceeding to about 294 mA cm−2, the overpotentials of R‐CoF2 are lower than those of Pt/C, and its limit current density can be reached up to 1200 mA cm−2, indicating a promising application in industrialization. Moreover, there is no degradation for R‐CoF2 after the successive CV scans of 3000 cycles and 110 h continuous electrolysis (Figure 2G, H, and Figure S13, Supporting Information). By comparison, the overpotential of Pt/C shifts negatively by about 11 mV at 10 mA cm−2, and its current density degrades dramatically with about 68.9% only after 24 h. These demonstrate the reconstruction‐derived component delivers superior catalytic activity and remarkable robustness for HER.

Mechanism Analysis of Reconstruction Processes During HER

In Situ Capture of Structure Evolution Processes

To uncover the dynamic variation of catalytic activity during HER, in situ Raman spectroelectrochemistry was applied for real‐time monitoring of structure information. The signal was captured at the potentials window from −0.9 to –1.3 V versus Hg/HgO in 1 m KOH (Figure and Figure S14, Supporting Information). First, the local structure of fresh CoF2 was recorded without the application of potential and electrolyte. It can be seen that the detectable bands centered at 183, 457, 504, and 664 cm−1 belong to the Co‐F vibrations in fresh CoF2. But all of these bands entirely disappear when soaked in 1 m KOH. Noticeably, three new broad bands of 422, 496, and 685 cm−1 originated from the Co—OH vibration in α‐Co(OH)2 come into being.[ ] To further corroborate the role of the electrolyte, we dropped the fresh powder sample ink on a transparent glass and soaked it in the electrolyte to observe the color change. Strikingly, the pink CoF2 sample turns into blue bluish in a split second, further into brown in a few minutes, and the existence of F− in the electrolyte is supported by ionic chromatography (Video S2 and Figure S15, Supporting Information). This fact demonstrates that the alkaline electrolyte triggers the rapid breakage of Co‐F coordination in CoF2 and supplies an immediate complement of hydroxide ions (OH−) to form Co—OH coordination in α‐Co(OH)2, the α‐Co(OH)2 is unstable and it will fastly transform into β‐Co(OH)2. While the potentials are applied, the new bands slightly shift to a high wave number, indicating the transformation from α‐Co(OH)2 to β‐Co(OH)2. Besides, the bands of β‐Co(OH)2 become stronger, and the increasing trend is kept until the potential of −1.1 V, confirming the increment of β‐Co(OH)2. With the potentials further rising, the character bands attributable to β‐Co(OH)2 are gradually weakened, which could be attributed to the violent production of bubbles at high potentials, affecting the signal capture.[ ]

Figure 3

A) In situ Raman spectra of CoF2 measured during HER. B) XRD patterns of CoF2 at point A–C in the i‐t curve. C) Co 2p, D) O 1s, and E) F 1s spectra of CoF2 at point A–C in i‐t curve and initial CoF2.

Ex Situ Capture of Structure Evolution Processes

Further structure information of species generated during HER was explored by XRD patterns (Figure 3B). We selected three vital points in an i‐t curve as representative of key courses (Figure 2D). At point A, only a broad diffraction peak can be detectable. Combining with the results of in situ Raman, Video S2, and ionic chromatography, Supporting Information, it is deduced that the broad diffraction peak is derived from stable amorphous β‐Co(OH)2. At point B, a minor crystallization peak assignable to the (101) plane of hexagonal β‐Co(OH)2 (JCPDS No. 30–0443) phase emerges at 2θ = 37.9°. While at point C, apart from the intensity of the above peak increase, four new crystallization peaks corresponding to the (001), (100), (102), and (110) planes of β‐Co(OH)2 appear in the XRD pattern as well, suggesting bias potential promotes amorphous crystallization.

The chemical states of samples at three vital points and initial CoF2 were also investigated. In Figure S16, Supporting Information, the relative intensity of O 1s peaks increases under HER conditions while that of F 1s peaks decreases in comparison with initial CoF2. Furthermore, Co 2p and O 1s spectra shift to low binding energies (BEs), yet the shifting degree of BE maintains unchanged after point A (Figure 3C,D). Such peak shifts can be caused by the structural evolution from the Co‐F coordination environment to that of Co—OH.[ , ] Although the F 1s spectra display a similar shift trend, the F 1s peak of the sample (point B) further shifts to lower BE and thoroughly vanishes in the sample (point C) (Figure 3E). Gradual shifts and even disappearance of F 1s signal further illustrates that the continuous leaching of F− triggers the successive reconstruction during HER. Besides, the peaks appear at higher BEs and become stronger in the O 1s spectrum, reflecting the adsorbed potassium salts on the catalyst surface.[ ]

Identification of Morphological Evolution Processes

The aberration‐corrected high angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) was conducted to unveil the morphological evolution in HER. Only after 2 s duration under HER test (point A), the polycrystalline ribbon of CoF2 is fully converted into an amorphous NS interwoven structure of β‐Co(OH)2 with plenty of structural defects (Figure and Figure S17, Supporting Information). The formation of large surface area and structural defects of NSs favor the contact with more alkaline electrolytes for further reconstruction.[ ] After continuous electrolysis for 5 min (point B), the amorphous NSs are transformed into low crystallinity NPs of β‐Co(OH)2. These NPs possess abundant defects and interconnect to form a sponge‐like loose structure (Figure 4C and Figure S18, Supporting Information), allowing more alkaline electrolyte infiltration for catalytic reactions and deeper reconstruction. After complete reconstruction at point C, the catalyst is composed of ultra‐small (≈5 nm) and homogeneous interconnected polycrystalline NPs of β‐Co(OH)2 with abundant defects (Figure 4D–G and Figure S19A–C, Supporting Information). Furthermore, after complete reconstruction, it inherits the loose and interwoven structure of NSs and the hexagram star frame of CoF2, confirmed by the FESEM image (Figure S19D, Supporting Information). Besides, atomic‐resolution HAADF‐STEM image (Figure 4H) elucidates that Co atoms distribute in a layered arrangement, in which the nearest distances between two layers of atoms are 4.64 and 2.73 Å, matching with the (001) and (100) crystal faces of the β‐Co(OH)2, respectively. STEM‐EDS mappings certify the uniform distribution of reconstruction components and no detectable F signal over the entire region (Figure 4I–L and Figure S20, Supporting Information).

Figure 4

A) HAADF‐STEM image of initial CoF2. B–D) HAADF‐STEM images of CoF2 at point A–C (inset, SAED pattern). E) FESEM, F) TEM, G) HRTEM (inset, local FFT‐filtered HRTEM image, FFT pattern), and H) atomic‐resolution HAADF‐STEM (inset, FFT pattern) images of CoF2 at point C. I–L) HAADF‐STEM image and corresponding EDS elemental mappings of CoF2 at point C. Scale bar: (A) 50 nm; (B–D) 10 nm, 2 nm−1 (inset); (E) 200 nm; (F) 10 nm; (G) 5 nm, 2 nm−1 (inset); (H) 1 nm, 5 nm−1 (inset); (I–L) 50 nm.

DFT Calculations of Reconstruction Processes

DFT calculations were further conducted to understand the dynamic reconstruction information of HER, which involves Gibbs free energy of hydrogen adsorption (∆GH*) and H2O adsorption energy (ΔEH2O) of four vital states of CoF2, CoF2–x(OH)x, CoF2–y(OH)y (x

Figure 5

A–D) Optimized structure models, E) Calculated free energy diagram (inset, optimized H* adsorption structure models), and F) H2O adsorption energy diagram (inset, optimized H2O adsorption structure models) of CoF2, CoF2–xOHx, CoF2–y(OH)y, and β‐Co(OH)2.

Discussion

The results prove that the obtained target transition metal fluorides are a kind of ideal pre‐catalysts for the realization of ultra‐fast complete self‐reconstruction during alkaline HER. This process depends on three vital factors, namely the unique surface structure of fluorides, alkaline electrolyte, and bias voltage. The fluorides possessing a surface F‐atom enrichment structure with more hydrophilic characters endows the feasibility of continuous self‐reconstruction. When contacting the alkaline electrolyte, the F− is rapidly leached from the fluorides lattice due to high ionicity,[ ] and the OH− derived from alkaline electrolyte immediately coordinates with Co sites to form amorphous α‐Co(OH)2, then rapidly transformed into β‐Co(OH)2, as confirmed by the previous reports.[ , , ] This preliminary reconstruction brings about striking morphology and structure changes from the dense‐packed irregular ribbons of crystalline CoF2 to interwoven NSs of amorphous β‐Co(OH)2 with abundant structure defects, which enables to contact alkaline electrolyte as much as possible further reconstruction. Moreover, it also contributes to a remarkable reduction in ∆GH*, thus providing favorable kinetics for the HER process. As the reaction proceeds, bias potential drives the amorphous crystallization and boosts the process of reconstruction.[ ] Further F− leaching urges amorphous NSs chalked into interconnected low crystallinity NPs with plenty of defects for deeper reconstruction and causes the preferable HER kinetics. Finally, all the F− ions are leached and the complete reconstruction is realized for pre‐catalysts. The completely reconstructed catalyst is featured by a nanoscale catalytic unit with abundant lattice defects and large specific surface structure, consistent with reported OER pre‐catalysts such as Co2(OH)3Cl[ ] and NiMoO4 xH2O.[ ] This unique structure for the reconstruction‐derived mono‐component β‐Co(OH)2 crystalline phase contributes to the penetration of alkaline electrolyte, exposing more active sites, shortening path charge transfer, thus accelerating the HER process.[ , ]

Conclusion

In summary, we construct a new class of transition metal fluoride pre‐catalysts that is capable of achieving ultra‐fast and complete self‐reconstruction, dramatically boosting the HER activity in alkaline media. It only requires an ultra‐low overpotential of 54 mV to deliver the current density of 10 mA cm−2 close to commercial Pt/C catalysts, and long‐term stability of 110 h outperforming Pt/C. The experimental results and theoretical calculations co‐confirm the occurrence of the dynamic reconstruction process of fluorides under three vital conditions including the unique surface structure, alkaline electrolyte, and bias voltage. All of them not only accelerate the mass transfer for continuous and deeper reconstruction but also boost kinetics processes of alkaline HER. The reconstruction‐derived mono‐component hydroxides crystalline phase with a nanoscale structure is responsible for the greatly enhanced performance for HER. Our work provides a very important insight for the reconstruction of pre‐catalysts during HER and opens up a novel strategy to design highly efficient catalysts.

Experimental Section

Preparation of Pre‐Catalysts

The synthesis process of CoF2 was as follows: 1) Co(NO3)2 6H2O, NH4F, and CH4N2O were successively dissolved in a certain amount of ultrapure water with the molar ratio of 1:4:5 and transferred into reaction kettle after the mixture becomes homogeneous red solution; 2) then added pre‐cleaned CC and mixed into a Teflon‐lined stainless autoclave and the container maintained at 120 °C for 2 h and cooled down to room temperature naturally; 3) the product repeatedly washed with ultrapure water for several times, and finally dried under vacuum at 80 °C for 4 h, the rosy red product loaded on CC were obtained (CoF1.3(OH)0.7); 4) the obtained CoF1.3(OH)0.7 and NH4F were placed in a tube furnace and annealed at 480 °C for 1 h under an inert atmosphere and cooled down to room temperature naturally; 5) the pink product supported on CC was repeatedly washed with ultrapure water for several times and dried under vacuum for 4 h. Finally, the pink CoF2 pre‐catalyst supported on CC was obtained. The synthesis process of NiF2 was similar to CoF2 just replacing Co(NO3)2 6H2O with Ni(NO3)2 6H2O. Because iron fluoride is unstable at high temperature, so FeF3 (H2O)0.33 was obtained at 300 °C for 4 h.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

P.J., H.J., and S.M. conceived and designed the studies. P.J. synthesized the materials, performed their electrochemical properties, analyzed all the data, and wrote the paper. D.Z. contributed to the materials synthesis and electrochemical measurements. R.Y. and J.W. conducted HAADF‐STEM and HRTEM observations. P.W. performed the DFT calculations, computational models, and result analyses. X.P. and P.J. carried out in situ Raman characterization. D.C., J.Z., Z.P., J.W., and S.M. provided helpful suggestions and revised the manuscript. All authors discussed the results and commented on the manuscript.

Supporting Information

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Supporting Information

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Supporting Information

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