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

Abstract 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 presents a highly mesoporous hierarchical structure, [9] 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 CoF 2 , NiF 2 , and FeF 3 (H 2 O) 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 CoF 2 , 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 CoF 2 , in which the mono-component hexagonal crystalline phase, -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.

Synthesis and Structure Characterizations of Pre-Catalysts
The synthetic process of CoF 2 including the growth of precursor and subsequent fluoridation is depicted in Figure 1A. 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 (CoF 1.3 (OH) 0.7 , JCPDS No. 18-0405) was thoroughly transformed into pink single-phase cobalt fluoride (CoF 2 , JCPDS No. 71-1969) by reacting with hydrogen fluoride (HF) vapor resulting from the thermal decomposition of NH 4 F (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). [10] Meanwhile, the peaks belonging to CoF 2 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). [8,11] The remaining peaks of 787.8 and 804.7 eV derive from the satellites of Co 2p3/2 and Co 2p1/2. [12] From field emission scanning electron microscope (FESEM) images, it is clearly seen that CoF 1.3 (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 CoF 2 ( 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 CoF 2 . Additionally, the scanning transmission electron microscopy (STEM) element mapping further confirms the homogeneity of CoF 2 components ( Figure 1H-J).

Dynamic Activity Variation During HER of Pre-Catalysts
To prove that CoF 2 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 2A 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, [5a,13] indicating the possibility of reconstruction of CoF 2 during HER. When subjected to consecutive LSV scanning for only 50 cycles for the initial CoF 2 ( 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 CoF 2 value ( Figure 2C). As expected, the HER catalytic activity and charge transfer ability of CoF 2 can be rapidly and significantly enhanced in the course of HER under alkaline electrolyte.
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 (NiF 2 and FeF 3 (H 2 O) 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-CoF 2 , R-NiF 2 , and R-FeF 3 (H 2 O) 0.33 ) supported on CC with commercial Pt/C supported on CC as a benchmark. From Figure 2E, it is obvious that R-CoF 2 only needs an ultra-low overpotential of 54 mV comparable to Pt/C (34 mV) but lower than that of R-NiF 2 (101 mV) and R-FeF 3 (H 2 O) 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-CoF 2 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-CoF 2 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. www.advancedsciencenews.com www.advancedscience.com

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 3A and Figure S14, Supporting Information). First, the local structure of fresh CoF 2 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 CoF 2 . 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 -Co(OH) 2 come into being. [14] 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 CoF 2 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 CoF 2 and supplies an immediate complement of hydroxide ions (OH − ) to form Co-OH coordination in -Co(OH) 2 , the -Co(OH) 2 is unstable and it will fastly transform into -Co(OH) 2 . While the potentials are applied, the new bands slightly shift to a high wave number, indicating the transformation from -Co(OH) 2 to -Co(OH) 2 . Besides, the bands of -Co(OH) 2 become stronger, and the increasing trend is kept until the potential of −1.1 V, confirming the increment of -Co(OH) 2 . With the potentials further rising, the character bands attributable to -Co(OH) 2 are gradually weakened, which could be attributed to the violent production of bubbles at high potentials, affecting the signal capture. [14c]

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 www.advancedsciencenews.com www.advancedscience.com 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 -Co(OH) 2 . At point B, a minor crystallization peak assignable to the (101) plane of hexagonal -Co(OH) 2 (JCPDS No. 30-0443) phase emerges at 2 = 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 -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 CoF 2 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 CoF 2 . 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. [14b,15] 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. [16]

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 rib-bon of CoF 2 is fully converted into an amorphous NS interwoven structure of -Co(OH) 2 with plenty of structural defects ( Figure  4A,B 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. [17] After continuous electrolysis for 5 min (point B), the amorphous NSs are transformed into low crystallinity NPs of -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 ultrasmall (≈5 nm) and homogeneous interconnected polycrystalline NPs of -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 CoF 2 , 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 -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).

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 (∆G H* ) and H 2 O adsorption energy (ΔE H2O ) of four vital states of CoF 2 , CoF 2-x (OH) x , CoF 2-y (OH) y (x<y), and -Co(OH) 2 . Notably, the F atoms of CoF 2 , CoF 2-x (OH) x , and CoF 2-y (OH) y migrate to the surface after geometry optimization (Figure 5A-C and Figure S21A-C, Supporting Information), which enables them easier to contact electrolytes for achieving spontaneous and continuous reconstruction. [8a] For -Co(OH) 2 , the Co atoms migrate to the surface after geometry optimization ( Figure 5D and Figure S21D, Supporting Information). Theoretically, the ∆G H* for ideal catalysts is close to 0 eV, suggesting suitable adsorption and desorption strength of hydrogen. Herein, the computational ∆G H* value of CoF 2 is 1.51 eV ( Figure 5E). Such value is significantly reduced to 0.36 eV with the F − leaching and OH − coordination for CoF 2-x (OH) x , and further decreases with more OH − substituted for F − sites (0.33 eV for CoF 2-y (OH) y ). This proves that F − leaching can effectively reduce ∆G H* , hence promoting the HER kinetics, which matches well with the results of electrochemical tests. In comparison with CoF 2-y (OH) y , a slight increase in the |∆G H* | value is observed for -Co(OH) 2 (−0.44 eV). Combining with the morphology and structure analyses of CoF 2-y (OH) y and -Co(OH) 2 , we can deduce that the excellent HER catalytic activity of -Co(OH) 2 Figure 5. A-D) Optimized structure models, E) Calculated free energy diagram (inset, optimized H* adsorption structure models), and F) H 2 O adsorption energy diagram (inset, optimized H 2 O adsorption structure models) of CoF 2 , CoF 2-x OH x , CoF 2-y (OH) y , and -Co(OH) 2 . mainly originates from reconstruction-derived ultra-small and defective nanostructure. In addition, the H 2 O adsorption energy also shows a similar variation trend ( Figure 5F).

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, [8a] and the OH − derived from alkaline electrolyte immediately coordinates with Co sites to form amorphous -Co(OH) 2 , then rapidly transformed into -Co(OH) 2 , as confirmed by the previous reports. [5b,14a,18] This preliminary reconstruction brings about striking morphology and structure changes from the dense-packed irregular ribbons of crystalline CoF 2 to interwoven NSs of amorphous -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 ∆G H* , thus providing favorable kinetics for the HER process. As the reaction proceeds, bias potential drives the amorphous crystallization and boosts the process of reconstruction. [19] 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 Co 2 (OH) 3 Cl [13a] and NiMoO 4 xH 2 O. [20] This unique structure for the reconstruction-derived mono-component -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. [7,21]

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 precatalysts during HER and opens up a novel strategy to design highly efficient catalysts.

Experimental Section
Preparation of Pre-Catalysts: The synthesis process of CoF 2 was as follows: 1) Co(NO 3 ) 2 6H 2 O, NH 4 F, and CH 4 N 2 O 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 Teflonlined 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 (CoF 1.3 (OH) 0.7 ); 4) the obtained CoF 1.3 (OH) 0.7 and NH 4 F 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 CoF 2 pre-catalyst supported on CC was obtained. The synthesis process of NiF 2 was similar to CoF 2 just replacing Co(NO 3 ) 2 6H 2 O with Ni(NO 3 ) 2 6H 2 O. Because iron fluoride is unstable at high temperature, so FeF 3 (H 2 O) 0.33 was obtained at 300°C for 4 h.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.