Electro-oxidation reconstitution of aluminium copper MOF-derived metal oxyhydroxides for a robust OER process

Abstract

It is common knowledge that the O₂ evolution reaction (OER) is a crucial half-reaction in the electrolysis of water. However, it is currently difficult to create inexpensive OER electrode materials in a way that is efficient, simple, and environmentally friendly. In this research, metal oxy-hydroxides with numerous oxygen defects (M-OOHv) are created at surface of Cu foam (CF) using a unique, straightforward electro-oxidation reconstitution (ER) process. Different spectroscopic and microscopy methods are used to analyse the electrode characteristics of Al₂Cu-MOF@M-OOHv-ER/CF; electrochemical measurements display a lower overpotential (η) of 366 mV @ 10 mA cm⁻² and a Tafel slope of 95.2 mV dec⁻¹ in 1.0 M KOH. X-Ray diffraction (XRD), scanning electron microscopy (SEM), and Raman studies confirm the phase transition of the metal–organic framework (MOF) to the M-OOH, which acts as the active site to boost the OER activity. Through spectroscopic and microscopic investigations, it is determined that the efficiency of bimetallic electrode materials and oxygen vacancies in the M-OOHv have an impact on the electron power density. The manufactured electrode material additionally showed good durability for 50 hours. As a result, the newly developed Al₂Cu-MOF@M-OOHv-ER/CF nanomaterial has greater potential for both electrolysis of water and other energy storage equipment.

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1 Introduction

The requirement for clean, nondepletable energy reservoirs is a hot topic because of the current energy crisis. Functional materials for distinctly efficient hydrogen evolution reactions (HER) and oxygen evolution reactions (OER) attract considerable attention.^1–6 A highly active electrode material for the OER is urgently required because the OER has a sluggish kinetics as compared to the HER. Due to excellent oxygen evolution electrocatalytic performance, contemporary IrO₂ and RuO₂, etc. are valuable metals for electrocatalysts. However IrO₂ and RuO₂ are not frequently employed in actual commercial applications because of their small reserves, high cost, and unstable nature.^7–11 There is a lot of interest in transition metal-based materials (TMMs) as alternatives to precious metal-based materials because they have enormous reserves and various electronic structures that make them appropriate for binding to intermediates.^12–14 Metal oxides (MOs), metal hydroxides (MOHs), and metal oxy-hydroxides (MOOHs) are a few of the compounds that have a major impact on the success of the fundamental O₂ evolution process. Directly produced TMMs are insufficient because the OER requires a large surface area and a certain shape to expose more active sites.^15–19

Therefore, metal–organic frameworks (MOFs) have regarded as electrocatalysts for OER due to their improved specific surface area as well as periodic structure.²⁰ It is tough to further boost the electrocatalytic performance of MOFs on account of their poor electrical conductivity and catalytic activity.^20–22 Calcination at high temperatures can convert MOFs with high catalytic activity to the metal oxides. In addition, the presence of a carbon layer can improve the electrical conductivity.^23–25 While high-temperature calcination can increase the conductivity and introduce more catalytically active sites, it is also very energy-intensive, waste-producing, and impure.^26–28 This makes it necessary to find out an effective, uncomplicated, and ecologically friendly technique to create electrocatalysts that perform wonderfully, which is still an obstacle. However, it has been shown that M-OOHs exhibit outstanding OER electrocatalytic efficacy and developing the corresponding M-OOHv from MOFs with high specific surface area will considerably enhance OER efficiency.^29,30 Furthermore, scientists studied the bimetallic MOF, such as Ling et al., who synthesized the Fe_0.67Co_0.33OOH on nickel foam for OER activity. The electrocatalytic result of Fe_0.67Co_0.33OOH displays 45.3 mV dec⁻¹ and a Tafel slope of 244 mV @ 10 mA cm^−2.31 Frang et al. developed the defect rich (Co, Fe)₃O₄ for the OER process. Current density at 10 mA cm⁻² alongside Tafel slope around 41 mV dec⁻¹, the electrochemical performance related to (Co, Fe)₃O₄ indicated a smaller overpotential of 286 mV. Due to the enhanced electrochemical result, the enrichment of active centers and defective species occurred.³²

In the present study, an Al₂Cu-MOF@M-OOHv-ER/CF nanomaterial is developed by an electro-oxidation reconstitution procedure from an Al/Cu bimetallic organic framework (Al₂CuMOF) on copper foam, resulting in a metal hydroxide with copious oxygen vacancies (M-OOHv) on the interface of Al₂Cu-MOF/CF. The substrate, which is copper foam, has a high porosity and improves the conductivity in addition to exposing a lot of active centers. Additionally, the efficacy of O₂ evolution is greatly enhanced when oxygen defect engineering is used in conjunction with the M-OOH. Due to the formation of defects in the crystal structure, the way the electrons are organized can change. This improves the conductivity and accelerates the adsorption of water molecules. With a morphology similar to saw-toothed edges, Al₂Cu-MOF@M-OOHv-ER/NF manifests huge electrochemically active surface area (ECSA), a BET surface area of 173 m² g⁻¹, and stable long-term operation without decay for 50 hours at 1.77 V. Its overpotential is just 366 mV measured @ 10 mA cm⁻¹. The MOF to M-OOHv phase change seen by in situ Raman indicates oxygen defects. In addition, many analytical methods were used to quantify the structural modifications that prove oxygen defects in the MOOHv and the bimetal synergistic effect, which regulate the electron density of states, and are responsible for the enhanced OER catalytic activity. Furthermore, the newly developed Al₂Cu-MOF@M-OOHv-ER/CF nanomaterial has vast application capability for complete water splitting and can be produced in an effective, straightforward, and environmentally friendly way and has significant oxygen evolution performance.

2 Experimental

2.1. Chemicals and reagents

Aluminum nitrate (Al(NO₃)₃·9H₂O, ≥98%), copper(II) nitrate (Cu(NO₃)₂·3H₂O, 99%), terephthalic acid (C₈H₆O₄, H₂BDC), N,N-dimethylformamide (C₃H₇NO, DMF), absolute ethanol (C₂H₆O, 95.0%), and kalium hydroxide (KOH, ≥85%), all were obtained from Sigma-Aldrich. There was no need to purify the aforementioned chemical reagents because they were all of analytical grade.

2.2. Synthesis of Al₂Cu-MOF/CF

First, to get rid of any surface impurities, the CF (1 cm × 1 cm) was ultrasonically treated for 10 min in 1.0 M HCl, CH₃CH₂OH, and DI water. The precursors used for the fabrication of Al₂Cu-MOF/CF were Cu(NO₃)₂·3H₂O (160 mg, 0.5 mmol), terephthalic acid (265 mg, 1.65 mmol), and Al(NO₃)₃·9H₂O (280 mg, 1.1 mmol). All these precursors were then disintegrated in mixture of 50 mL DMF, 5 mL of CH₃CH₂OH, along with 5 mL of DI H₂O. Consequently, this uniform mixture was then shifted to a 100 mL hydrothermal reactor along with the already cleaned CF and then placed in the electric furnace for six hours at 160 °C. The autoclave was then allowed to cool at ambient temperature. Then the fabricated Al₂Cu-MOF/CF was repeatedly washed with absolute ethanol and double distilled H₂O to eliminate any remaining organic ligands after the hydrothermal process. At 60 °C for 6 hours, the newly synthesized bimetallic Al₂Cu-MOF/CF was then dried, and stored for further characterization.

2.3. Synthesis of Al₂Cu-MOF@M-OOHv-ER/CF

For the fabrication of Al₂Cu-MOF@M-OOHv-ER/CF, the electrochemical method was used. For this purpose, the synthesized Al₂Cu-MOF/CF was employed as the working electrode, while a platinum wire served as the counter electrode as well as Ag/AgCl served as standard reference electrode. The M-OOHv (oxygen defect metal hydroxide) was electrochemically deposited on the surface of Al₂Cu-MOF using 10 CV cycles in the potential window (0 to 2 V) vs. Ag/AgCl inside 1 M aqueous solution of KOH at a sweeping speed of 5 mVs⁻¹. The deposited electrocatalyst (Al₂Cu-MOF@M-OOHv-ER/CF) was desiccated in electric oven at 60 °C.

2.4. Material characterization

The crystalline phases of different electrodes were investigated using an XRD (Advance D8 Bruker) with Cu Kα radiation and an accelerating potential of 40 kV in the 2θ range of 10–80°. Raman spectroscopy was used to analyze the material's phase change after it was manufactured (Alpha 300 R) under a He–Ne laser (excitation at 632 nm). The electrode surface morphologies were explored using SEM, and the elemental compositions of the electrodes were investigated using SEM-EDX (Quanta 200-FEG). FT-IR was recorded using a JASCO 6800 FTIR spectrometer with scans ranging from 4000 to 400 cm⁻¹. A Quantachrome Nova2200e BET surface area analyzer was utilized at 80 K to evaluate the specific surface areas.

2.5. Electrochemical measurement

The electrode material that was created acted as the functioning electrode, the counter electrode was made of a Pt wire and standard electrode was made of silver/silver chloride. This was all done in a Pyrex electrolytic cell with 1 M KOH as the electrolyte. An alkaline environment is generally utilized to analyze the O₂ evolution process because the O₂ evolution reaction includes the creation of defects after the OH⁻ loses electrons. Using the Nernst equation, the potential values in this reading were translated into the RHE with the following eqn (1):³³

E_RHE = E_Ag/AgCl + 0.059 pH + E⁰_Ag/AgCl (1)

In the non-faradaic region in range of −0.2 V alongside 0.01 V (vs. RHE), electrode ECSAs were measured by cyclic voltammetry (CV) in a sweeping range of 10–50 mV s⁻¹ (vs. RHE). The double-layer capacitance (C_dl) and ECSA were measured using the given expressions (2) and (3):^34,35

Herein, C_dl, C_sp and ECSA are the double layer capacitance (mF), specific capacitance of the flat electrodes (NF, 0.04 mF cm⁻²) and electrochemical active surface area (cm²), respectively. By making a graph of the overpotential (η) vs. the current density, then Tafel slope was found. This Tafel slope gave information about the kinetic mechanism of the OER for the manufactured materials. Linear sweep voltammetry (LSV) with a 95% iR correction was used to get the OER polarization curves. The overpotential of the OER at 25 °C was computed with the given expression (3):³⁶

η = E(RHE) − 1.23 V. (4)

With the use of the EIS approach, the OER mechanism has been further examined at frequencies ranging between 100 Hz and 100 kHz and an alternating current (AC) potential voltage around 5 mV at the open circuit voltage. The durability of the fabricated material in long-term was measured with chronoamperometry at the cell constant of 0.75 V.

3 Results and discussion

3.1. Structural, morphological, and textural characteristics

To investigate the physical features of the catalyst, the XRD patterns of Al₂Cu-MOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF were explored in the 2θ range of 10 to 80°. The distinctive diffraction peaks detected in Al₂Cu-MOF/CF at 25.85°, 43.07°, and 53.16° correspond to the 002, 100, and 105 indexed planes, and other peaks located at 15.56° and 27.56° correspond to the 110 and 444 indexed planes, which were well consistent with the Al and Cu MOF as previously reported.^37,38 The existence of a bimetallic peak in Al₂Cu MOF indicates the successful fabrication of Al₂Cu MOF. Fig. 1 illustrates that the slight shifting in the 2θ value and variation in the peak intensity suggested the change in MOF phase to the metal hydroxy phase during the electro-oxidation reconstitution (ER). The absence of peaks at 60.29° and 62.82° in Al₂Co-MOF@MOOHv-ER/CF also confirms the phase transformation. Furthermore, phase transition and structure of the fabricated material were assessed with Raman spectroscopy as shown in Fig. 1(b). The presence of the characteristics peaks at 714.24, 925.42, 1181.21, 1452.23, and 1630.75 cm⁻¹ is attributed to Al₂Cu-MOF/CF. The material displays the characteristic peak of MOOH at 608 cm⁻¹.⁴⁷ The variation in peak intensity at 608 cm⁻¹ and the production of MOOHv on the interface of Al₂Cu-MOF confirm the phase transition and oxygen defects that are present in the Al₂Cu MOF@M-OOHv-ER/CF.

Fig. 1 (a) XRD diffractogram and (b) Raman spectroscopy of Al₂CuMOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF.

Additionally, the molecular structural changes of Al₂Cu-MOF/CF and Al₂CuMOF@M-OOHv-ER/NF were determined with FT-IR as seen in Fig. 2(a). The stretching bands in Al₂Cu-MOF/NF and Al₂CuMOF@M-OOHv-ER/NF at 1610 cm⁻¹ and 1390 cm⁻¹ are due to vibrations that concerns carboxyl group from the ligand H₂BDC.^39,40 Additionally, the benzene ring's C–H bonding stretching from H₂BDC appeared at 780 cm⁻¹, and the other peaks located below 700 cm⁻¹ are ascribed due to metal oxide (M–O) vibration modes in Al₂CuMOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF.⁴¹ The presence of FT-IR peaks demonstrates that Al₂CuMOF/CF, along with H₂BDC, was successfully developed.

Fig. 2 (a) FTIR, (b and c) SEM micrograph and (d) textural properties of Al₂Cu-MOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF.

Scanning electron microscopy (SEM) micrographs of Al₂CuMOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF are displayed at 500 nm in Fig. 2(b and c). The Al₂CuMOF/CF, which is composed of two-dimensional smooth irregular nanosheets, develops uniformly over the CF. Fig. 2(c) demonstrates that electro-oxidation reconstitution of the material results in a more abrasive and rougher Al₂Cu MOF surface, exposing more active edge sites for improving catalytic activity.^41,42 In addition, a rough surface implies the production of new compounds, which might reduce the effectiveness of catalysis. Fig. 2(d) illustrates the textural properties of the fabricated material measured under a nitrogen atmosphere at a comparative pressure (P/P₀) of 0 to 1.0. The BET isotherm displays the surface areas of Al₂CuMOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF to be 68 and 173 m² g⁻¹, respectively.^44–46 The BET result suggests that the high surface area of the bimetallic MOF increases the electrocatalytic efficiency of the material toward the OER activity.

3.2. Electrocatalytic OER performance

All OER activities regarding Al₂Cu-MOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF were investigated in 1.0 M aqueous solution (KOH) configured with a three electrode system whose potential ranging from 0 to 2.0 V. Fig. 3(a) depicts about CV polarization curve of Al₂Cu-MOF@M-OOHv-ER/CF exhibited a higher current density than those of Al₂Cu-MOF/CF, RuO₂ and Cu foam.

Fig. 3 (a) Cyclic voltammogram at various scanning rates, (b) LS voltammetry, (c) overpotential of Al₂Cu-MOF/CF, Al₂Cu MOF@M-OOHv-ER/CF, RuO₂ and Cu foam @ 10 mA cm⁻² and (d) Tafel slope of Al₂Cu-MOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF@ 10 mA cm⁻².

Electro-oxidation reconstitution (Fig. 3(b)) showed the higher current density than Al₂Cu-MOF/CF, RuO₂ and Cu foam. Additionally, the Al₂Cu-MOF@M-OOHv-ER/CF nanostructured arrangements with a saw-tooth morphology may assist the uniformly dispersed M-OOHv in attaining the adsorption of OH ions at surface of the electrocatalyst that intensifies conductance of the material and exhibited OER's potent electrochemical performance.⁴³ LSV curves were used to calculate the Tafel slopes for each electrode, so that the whole electrode's OER reaction dynamics could be investigated more closely. Furthermore, the Al₂Cu-MOF@M-OOHv-ER/CF (366 mV) delivers a better overpotential than Al₂Cu-MOF/CF (428 mV), RuO₂ (352 mV) and Cu foam (460 mV) as shown in Fig. 3(c). This may be the result of the M-OOHv adsorption at surface of Al₂Cu-MOF/CF.⁴⁴ For the kinetic mechanism the Tafel slope of Al₂Cu-MOF@M-OOHv-ER/CF had the lowest value (22.3 mV dec⁻¹) and delivered the fastest OER reaction kinetics than the other fabricated electrode as demonstrated in Fig. 3(d).

OER competence may also be investigated by calculating the electrochemical active surface area (ECSA) via using value of double layer capacitance (C_dl) with the help of eqn (2) and (3). Al₂Cu-MOF@M-OOHv-ER/CF exhibited the most exposed dynamic areas compared to the other fabricated electrode. The ECSA can be calculated using cyclic voltammograms at various scan rates as represented in Fig. 4(a and c). From these curves the C_dl values were calculated by plotting a graph between the change in current density and scan rate. All obtained C_dl results of Al₂Cu-MOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF were 7.5 mF and 30.2 mF, respectively, as shown in Fig. 4(b and d). The ECSA results for Al₂Cu-MOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF are 187.5 and 755.0 cm², respectively.

Fig. 4 (a and c) Cyclic voltammogram, (b and d) C_dl plot profile, and (e) normalized plot profile of Al₂Cu-MOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF.

It is probable that the produced saw-tooth type of morphology of Al₂Cu-MOF@M-OOHv-ER/CF will exhibit the greatest ECSAs that will boost its OER electrochemical performance.⁴⁵

For intrinsic activity, the ECSA normalized LSV curve is another way to evidence the remarkable OER activity of all prepared samples. The ECSA-normalized LSV curve is shown in Fig. 4(e). The obtained LSV curve for prepared samples has been normalized by ESCA 7.5 m² g⁻¹ and 30.2 m² g⁻¹ for Al₂Cu-MOF/CF and Al₂Cu-MOF@M-OOHv-ER/CF. The given graph exhibits a higher current density normalized by ECSA in the case of Al₂Cu-MOF@M-OOHv-ER/CF as compared to Al₂Cu-MOF/CF; therefore, the prepared OER electrocatalyst can be used for commercial and business applications.

To find out more about the OER kinetics, EIS of the fabricated electrode was conducted. The EIS was evaluated in 1.0 M KOH and its frequency ranges through 100 Hz to 100 KHz. Utilizing the Nyquist curves, we were able to determine the resistance values of all electrocatalysts. Compared to the Al₂Cu-MOF/CF (14 Ω), the Al₂Cu-MOF@M-OOHv-ER/CF electrode depicts extraordinarily low R_ct value of (5.7 Ω), which corresponds to the higher oxygen evolution efficiency and lower Tafel slope results. As a consequence, the Al₂Cu-MOF@M-OOHv-ER/CF exhibits strong catalytic activity, and good electron transfer capacity.^46–50

In addition to electrocatalytic properties, the practical application of Al₂Cu-MOF@M-OOHv-ER/CF was determined through chronoamperometry. The Al₂Cu-MOF@M-OOHv-ER/CF operates for 50 h without experiencing considerable attenuation and has better stability than RuO₂ (Fig. 5(b)).

Fig. 5 (a) EIS of Al₂Cu-MOF/CF and Al₂Cu MOF@M-OOHv-ER/CF, (b) chronoamperometry of Al₂Cu MOF@M-OOHv-ER/CF and RuO₂, (c) XRD stability and (d) CV stability cycles of Al₂Cu-MOF@M-OOHv-ER/CF.

The structural durability of the Al₂Cu-MOF@M-OOHv-ER/CF was also determined with XRD analysis. The XRD structure stability result suggests that the electrocatalyst is stable over 50 hours, as shown in Fig. 5(c). Additionally, there was no discernible change between the CV curves before and after the durability test, demonstrating exceptional endurance of Al₂Cu MOF@M-OOHv-ER/CF, as shown in Fig. 5(c).

4 Conclusions

In conclusion, a novel technique has been created to transform Al₂Cu-MOF/CF into Al₂Cu-MOF@M-OOHv-ER/CF via electro-oxidation reconstitution that has many oxygen defects on the surface. When analysed with a three-electrode system under 1.0 M aqueous solution, and a current density of around 10 mA cm⁻² Al₂Cu-MOF@M-OOHv-ER/CF showed lesser overpotential around 224 mV. The Al₂Cu-MOF@M-OOHv-ER/CF also displays minimal Tafel slope of 95.2 mV dec⁻¹ having larger ECSA with a C_dl of 30.2 mF. In situ Raman spectroscopy discloses active sites for the OER when the MOF phase transforms into the MOOHv phase. Various analytical techniques of the structure determination helped in betterment of catalytic activity were responsible for the bimetal synergistic effect & oxygen defects in MOOHv, which affects electron density of states. The newly developed Al₂Cu-MOF@M-OOHv-ER/CF, which resembles a rough surface, can be produced using a quick, easy, and environmentally friendly process and has remarkable potential for O₂ evolution. The in situ electrooxidation reconstitution process proposed in this project offers a unique way to transform MOF materials into derivatives that are catalytically active.

Author contributions

All authors have equal contribution.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors express their gratitude to Princess Nourah bint Abdulrahman University Researchers Supporting Project (Grant No. PNURSP2023R70), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

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