Cadmium Functionalized Zeolitic Imidazolate Framework (Cd-ZIF-8) Electrocatalysts for Efficient CO₂ Conversion to Syngas

Abstract

The electrochemical conversion of Carbon dioxide (CO₂) into value-added fuels and chemicals represents a promising approach to mitigating greenhouse gas emissions and addressing energy demands. Among the various catalysts studied, metal-doped zeolitic imidazole frameworks (ZIFs) have emerged as effective materials for the selective electrochemical CO₂ reduction reaction (eCO₂RR). In this study, we employ a series of cadmium (Cd) doped ZIF-8 composites synthesized using a straightforward approach to enhance CO₂ electroreduction performance. The results indicate that the electroreduction of CO₂ on Cd-doped ZIF-8 catalysts facilitates the production of syngas (CO and H₂), without the generation of any liquid fuel. This leads to a total faradaic efficiency (FE) that approaches 100%. The ideal Cd–ZIF-8 composite, consisting of 10% Cd and 90% Zn, exhibits favorable selectivity with high faradaic efficiencies of 78% for CO and 22% for H₂. Additionally, the syngas ratio may be readily modified from 4:1 to 1:3 (H₂/CO) by varying the applied voltage during the CO₂ conversion procedure. This tunability highlights the potential of Cd-ZIF-8 as an efficient catalyst for customizable syngas production from CO₂. These findings open new avenues for catalyst design, supporting the development of scalable CO₂ reduction technologies aimed at environmental sustainability and renewable energy solutions.

1. Introduction

Chemically produced gas, commonly termed syngas, consists of carbon monoxide (CO) and hydrogen (H₂). Various methods can be employed to prepare it, including steam reforming of methane, partial oxidation of hydrocarbons, and gasification of biomass. Syngas is a versatile fuel gas mixture with multiple applications. It possesses the capacity to produce several commodities such as chemicals, fuels, and energy. Different volumetric ratios of syngas, specifically 33.33/66.67, 50/50, 66.67/33.33 and 80/20 were employed as initial substrates in the production of synthetic fuels such as synthetic natural gas, methanol, and dimethyl ether using the Fischer-Tropsch (F-T) process. Syngas can be utilized in gas turbines for the purpose of generating power. It is considered a significant alternative to fossil fuels because of its ability to be derived from many feedstocks, including biomass and waste, resulting in a lower greenhouse gas emissions compared to fossil fuels.

Carbon dioxide is a colorless, odorless gas that plays a crucial role in the carbon cycle. The escalating concentrations of CO₂ in the atmosphere are a primary factor contributing to global climate change. The present atmospheric concentration of CO₂ stands at 414 parts per million (ppm). Elevated levels of CO₂ in the atmosphere are impacting natural ecosystems. The monitoring and management of carbon dioxide and CO₂ levels in the environment are of utmost importance in safeguarding human health and safety. The prioritization of the four Rs, namely, reduce, reuse, recycle, and recover, is imperative in this context. The utilization of CO₂ as a raw material for the production of diverse chemicals, including methane, formic acid, alcohol, and hydrocarbons, represents an innovative approach. Employing the eCO₂RR technique to generate syngas can effectively tackle the aforementioned problem while simultaneously mitigating the greenhouse effect.

Electrochemical CO₂ reduction presents a promising approach to reduce greenhouse gas emissions and generate valuable products from CO₂, thanks to its exceptional selectivity, durability, energy efficiency, and adaptability. However, unresolved challenges remain, including the need to improve catalyst efficiency and durability, optimize reaction conditions, and reduce expenses.

Metal-organic frameworks (MOFs) are a class of crystalline porous materials characterized by the presence of metal ions or nodes that are interconnected by organic linkers. MOFs have garnered significant attention due to their remarkable attributes, such as high porosity, considerable surface area, adjustable pore size, great tunability, and outstanding stability. MOFs present a compelling prospect for a range of applications, such as gas storage and separation, catalysis, drug delivery, and sensing, owing to their unique properties. − MOFs have been utilized for the conversion of CO₂, a crucial measure in the endeavor to reduce greenhouse gas emissions. MOFs can serve as catalysts in CO₂ conversion processes to effectively convert CO₂ to usable goods or fuels. MOFs are a highly sought-after option for achieving sustainable energy generation and environmental remediation, owing to their ability to efficiently and effortlessly decrease the level of CO₂ emissions. ZIFs are a significant subset of MOFs, where most series consist of Zn or Co as the metal core and imidazole as linkers. The ZIF-8 exhibits notable thermal and chemical stability, setting it apart from other MOFs in notable manners.

The utilization of Cd²⁺ and its composite materials as catalysts is commonly seen due to its cost-effectiveness, notable catalytic activity, and inherent stability. The utilization of Cd²⁺ anchored ZIFs has demonstrated encouraging synergistic potential. − Previous studies have provided evidence that Cadmium (Cd²⁺) exhibits a notable FE in the conversion of CO₂ to CO, despite its relatively low current density. − Hence, the objective of this study is to employ composite materials consisting of Cd²⁺ and ZIF-8 for the eCO₂RR. Cd²⁺ species were immobilized using a straightforward, low-temperature chemical coating approach to improve their catalytic capabilities and CO₂ conversion activity. The analysis and utilization of the resultant catalysts for the conversion of eCO₂RR into syngas were assessed in aqueous solutions of KHCO₃ under ambient conditions. The experimental apparatus that has been built enables the detection of even the tiniest of eCO₂RR products with exceptional accuracy. Furthermore, the electrocatalytic characteristics of the materials were thoroughly evaluated in H-cells and flow cells to generate pure syngas at different applied potentials.

2. Experimental Section

2.1. Materials

The following chemicals were obtained from Sigma-Aldrich, US: zinc nitrate hexahydrate (Zn­(NO₃)₂.6H₂O) (99.95%), cadmium nitrate hydrate (Cd­(NO₃)₂.6H₂O) (99.0%), 2-methyl imidazole (99.0%), potassium bicarbonate (99.9%), and potassium hydroxide (99.5%). Methanol (CH₃OH) (99.8%) was obtained from Sharlu (Sharjah, United Arab Emirates).

2.2. ZIF-8 Preparation

The preparation of ZIF-8 was conducted using the methodology outlined by Lee et al. A solution was prepared by dissolving 1.31 g of Zn­(NO₃)₂ in 45 mL of methanol within a 100 mL beaker. 2.87 grams of 2-methyl imidazole were dissolved in a separate beaker until a transparent solution was obtained. In a 150 mL round-bottom flask, the two transparent solutions were combined and agitated at ambient temperature for a duration of 1 h. Centrifugation was used to separate the synthetic white ZIF-8 suspension at a speed of 8,00 rpm for a duration of 10 min. The white crystal underwent three rounds of methanol washing. For the Cd incorporation into ZIF-8, similar method was applied with adding Cd⁺² with different weight ratios (10, 20, and 30%).

2.4. Electrocatalyst Preparation

2.4.1. Fabrication of Electrodes for H-type Cell

A dispersion was prepared by dispersing 10 mg of the ZIF-8 or Cd-ZIF-8 catalyst in a 1 mL mixture consisting of 750 μL of 2-propanol, 200 μL of DI water, and 50 μL of Nafion 5%. The solution was subjected to 10 min of sonication. Subsequently, a volume of 100 microliters (μL) of the suspension was carefully poured onto a 1.0 cm² sheet of conductive carbon paper and allowed to dry at ambient temperature.

2.4.2. Fabrication of Electrodes for Flow Cell

The functioning electrodes were prepared by using the spray painting technique. A volume of 100 μL of the ink that was created as previously described was maintained at a consistent air flow and pressure. Subsequently, it was applied onto a gas diffusion electrode (GDE) using the spray gun technique.

2.5. Characterization

Field emission scanning electron microscopy (FESEM, Tescan Lyra-3) was utilized to identify the morphological and detailed microstructural characteristics of the materials. Additional methods utilized for the analysis of the samples were X-ray diffraction (XRD) using the Rigaku MiniFlex instrument, transmission electron microscopy (TEM) using Jeol JEM-2100F, Fourier transform infrared (FT-IR) analysis using the Thermo instrument, BET surface analysis using the Triplex instrument, gas chromatography using a barrier ion discharge detector (GC-BID) from Shimadzu, and potentiostat analysis using the Gammray 620 instrument.

2.6. Electrochemical Investigations

The investigation focuses on the performance of the eCO₂R using a H-cell setup that incorporates a silver chloride electrode (Ag/AgCl) as the reference electrode. The counter electrode employed in the experiment was a platinum mesh. The working electrode utilized in this study was a ZIF-8-based film that was produced on conductive carbon paper. The electrodes in the cell are coupled to a potentiostat, specifically, the Gammray 620. The performance evaluation of eCO₂R was conducted through the utilization of linear sweep voltammetry (LSV) technique. The overpotential was calculated at various current densities with the current being normalized to the geometric surface area of the electrode. The tests of cyclic voltammetry (CV) and LSV were conducted using a solution of 0.1 M potassium bicarbonate (KHCO₃).

All potentials were normalized to the reversible hydrogen electrode (RHE) using the following Nernst equation

$$E_{\mathrm{RHE}}=E_{(\mathrm{Ag}/\mathrm{AgCl})}^{°}+0.059[\mathrm{pH}]+E{\mathrm{calculated}}_{(\mathrm{Ag}/\mathrm{AgCl})}$$ 1

where E° is standard, the value for Ag/AgCl is equal to 0.199 V.

The range of potential was seen to be between 0.0 and −1.4 V versus RHE. The electrochemical impedance spectroscopy (EIS) experiment involved the manipulation of frequency within the range of 105 to 0.1 Hz, while maintaining the same electrolyte and electrodes for the liquid-solid junction.

The flow cell configuration was composed of three primary components. The initial component consisted of electrolyte compartments, with one compartment containing the catholyte, 0.5M KHCO₃, and the second compartment containing the anolyte, 1.0M KOH. The second constituent is the cell, comprising the cathode section, where the CO₂ gas flows on one side of the GDE and the catholyte flows on the other side. The anode section is connected to the anolyte. The two cellular components are segregated by a proton-permeable membrane, facilitating the movement of the generated H⁺ ions from the anode to the cathode. The pump is the third element of the flow cell and is responsible for the movement and circulation of the catholyte and anolyte between the electrolyte compartments within the cell. In a similar manner to that of the H-cell, the reference electrode is linked to the working electrode (GDE) on the cathode side and the counter electrode on the anode side, all of which are connected to the potentiostat workstation (Gammray 620).

The FE efficiency was calculated using the formula

$$\mathrm{Faradaic}\mathrm{efficiency}(\mathrm{FE})=\frac{\mathit{nZF}}{Q}=\frac{\mathit{nZF}}{\mathit{It}}=\frac{\mathit{nZF}}{\mathit{IV}/v}\ast 100\%$$ 2

n is the amount of product detected (number of moles, mol); Q is the total charge passed through the system, recorded during electrolysis (coulombs, C); F is the Faraday constant (96485 C/mol); Z is the number of electrons required to obtain 1 molecule of the product; I is the recorded current (A); t is the time required to fill the sampling loop (s); V is the volume of the sampling loop (cm³); and ν is the recorded flow rate (ml/s).

3. Results and Discussion

The structural and compositional integrity of synthesized Cd-ZIF-8 was confirmed through various analytical techniques. Figure a demonstrates the synthesis of Cd-ZIF-8, by incorporating the Cd²⁺ with the Zn²⁺ along with the imidazole linker. Figure b displays the XRD data of both parent ZIF-8 and Cd-ZIF-8. The XRD patterns acquired for ZIF-8 in this investigation demonstrated a significant level of agreement with the simulated XRD patterns. The XRD patterns of Cd-ZIF-8 exhibited distinct peaks corresponding to ZIF-8, however, no identifiable peaks associated with Cd²⁺ were seen. These XRD findings provide evidence of the successful incorporation of Cd²⁺ into the ZIF-8 framework without disrupting its crystallinity. Further morphological and structural characterization was conducted using SEM and TEM to examine the surface features and spatial distribution of Cd²⁺ within the ZIF-8 framework. Figure c,d depicts the SEM images of ZIF-8, displaying consistent dodecahedron crystals. No significant differences in shape and aggregation of Cd nanoparticles were noticed when ZIF-8 was compared with the Cd-loaded sample. The SEM findings were corroborated following the execution of the TEM (Figure e,f). The TEM images revealed homogeneous crystals of the ZIF-8 framework. The homogeneous dispersion of the elements (C, N, Zn, and Cd) was validated using elemental mapping, as depicted in Figure g–j. The Zn phase observed in ZIF-8 was determined to be Zn²⁺ as a result of its interaction with the nitrogen atoms in the organic imidazole linkers. Additionally, the framework exhibited the presence of a doped Cd²⁺ phase. Moreover, the N₂ Isotherm (Figure S2) was investigated with the aid of the BET surface analyzer. The parent material ZIF-8 showed a high surface area around 1750 m²/g, which is a characteristic value for the Zn-MOF. Upon the loading of Cd⁺² into the framework, the high surface area was maintained in the case of 10% and 20% loading, which confirms the uniform incorporation of Cd²⁺ inside the framework. Very small drop in the surface area (1610 m²/g) was observed in the case of 30% Cd/ZIF-8. Overall, the combined XRD, SEM, and TEM analyses confirm that Cd²⁺ ions are uniformly incorporated into the ZIF-8 framework, preserving both the structural integrity and the high surface area of the material. This successful integration of Cd²⁺ into ZIF-8 highlights the stability and adaptability of the framework, establishing a solid foundation for further electrochemical applications. To evaluate the electrocatalytic performance of Cd²⁺ doped ZIF-8 catalysts, a comprehensive electrochemical analysis was conducted, assessing their activity, reaction kinetics, and surface characteristics. The electrocatalysts (ZIF-8, 5% Cd-ZIF-8, 10% Cd-ZIF-8, 20% Cd-ZIF-8, and 30% Cd-ZIF-8) were examined using LSV in CO₂-saturated 0.1 M KHCO₃ electrolytes to assess their overall electrocatalytic activity. The results of ZIF-8 and 10%-Cd-ZIF-8 are shown in Figure a, while the LSVs of all samples are given in Figure S1. The pure ZIF-8 exhibited the lowest current density of approximately 5 mA cm^–2 at 1.5 VRHE. Upon loading ZIF-8 with 10% Cd²⁺, the current density increased substantially to 14 mA cm^–2. However, further increases in Cd²⁺ content to 20% and 30%, led to a decrease in current density. The Tafel slope was determined from the polarization curves (Figure S3) in order to acquire a deeper understanding of the mechanism and kinetics of the eCO₂RR. The Tafel values for electrodes ZIF-8, 10% Cd-ZIF-8, 20% Cd-ZIF-8, and 30% Cd-ZIF-8 were determined to be 300, 75, 92, and 330 mV dec^–1, respectively. The 10% Cd-ZIF-8 exhibited a lower Tafel slope, reflecting faster reaction kinetics and enhanced adsorption of the CO₂ ^•‑ intermediate.

1.

(a) Schematic diagram of the synthesis of Cd-ZIF-8. (b) XRD of ZIF-8 and Cd-doped ZIF-8. (c, d) SEM of 10% Cd-ZIF-8 (e, f), TEM of 10% Cd-ZIF-8, and (g–j) elemental mapping of 10% Cd-ZIF-8.

2.

(a) The LSV curves of ZIF-8 and 10% Cd-ZIF-8₃ curves in CO₂-saturated 0.1 M KHCO₃. (b) Cdl slopes of ZIF-8, 10% Cd-ZIF-8, 20% Cd-ZIF-8, and 30% Cd-ZIF-8. (c) Nyquist plots of ZIF-8, 10% Cd-ZIF-8, 20% Cd-ZIF-8, and 30% Cd-ZIF-8. (d) FE of ZIF-8, 10% Cd-ZIF-8, 20% Cd-ZIF-8, and 30% Cd-ZIF-8.

The electrochemically active surface area (ECSA) is a significant factor that can be determined by estimating the double-layer capacitance (Cdl). The capacitance of the Cdl was assessed through the acquisition of cyclic voltammograms (Figure S4) at various scan rates (50, 100, 150, 200, and 250 mV s^–1) and the subsequent graphing of the capacitive current against the scan rate. According to the data presented in Figure b, it can be observed that 10% Cd-ZIF-8 had the greatest Cdl value of 1.4 mF cm^–1. This was followed by 20% Cd-ZIF-8 with a Cdl value of 1.20 mF cm^–1. Conversely, pure ZIF-8 and 30% Cd-ZIF-8 exhibited the lowest Cdl values of 0.90 and 0.6 mF cm^–1, respectively. The interaction between the electrode and electrolyte interface was investigated using EIS. The charge transfer resistance (Rct) was determined by using the Nyquist plot (Figure c). The elevated frequency is attributed to the eCO₂RR in relation to CO (mass transfer), whereas the diminished frequency may be associated with the electrolysis process (HER). The Rct values for the electrodes ZIF-8, 10% Cd-ZIF-8, 20% Cd-ZIF-8, and 30% Cd-ZIF-8 were. Thus, Cd doping significantly enhanced charge transfer rates, with the 10% Cd²⁺ loading showing the lowest Rct, indicating an optimal interaction between the electrode and electrolyte for CO₂ reduction. Overall, the linear sweep voltammetry, Tafel slope, electrochemically active surface area, and impedance analyses confirm that 10% Cd²⁺ loading in ZIF-8 provides an optimal balance between enhanced charge transfer, improved surface area, and favorable reaction kinetics for CO₂ reduction. These findings highlight the catalytic potential of Cd²⁺ doped ZIF-8, particularly at the 10% level, in advancing the efficiency of electrochemical CO₂ conversion.

To evaluate the CO₂ reduction performance of the electrocatalysts (ZIF-8, 10% Cd-ZIF-8, 20% Cd-ZIF-8, and 30% Cd-ZIF-8), chronoamperometry measurements were conducted at various applied potentials for 1 h. The resulting products were then quantified using an online-connected GC-BID. Figure d presents a comparison of the CO faradic efficiency (FE) of the electrodes. The four electrocatalysts exhibited comparable patterns in the generation of CO. A low CO production was recorded at a lower applied voltage. The augmentation of the potential resulted in a substantial increase in the CO FE%. The maximum FE% was observed at the applied voltage of -1.1 Vs RHE. An additional augmentation in the potential resulted in a reduction in the FE%. The decrease in the CO FE at higher negative potentials (-1.3 V versus RHE) is caused by the restricted CO₂ and polarization losses resulting from the heightened HER. The current densities of the electrocatalysts increased with applied voltage, with 10% Cd-ZIF-8 showing the highest FE values of 57% for CO and 43% for H₂, at -1.1 Vs RHE.

The optimized electrode from the H-cell, 10% Cd-ZIF-8, was also evaluated in a flow cell system, which offers a more practical configuration. A comparison was made between the LSV and FE acquired from the flow cell and the results obtained from the H-cell. The data presented in Figure a demonstrate a notable disparity in current density between the flow cell and the H-cell. The enhanced diffusion of CO₂ gas into the catalyst surface can be attributed to the GDE. Furthermore, the flow system provides a continuous fresh electrolyte supply to the surface of the electrode, enhancing the overall reaction process. Due to the separation of electrolyte compartments in the flow system, it is possible to utilize two distinct electrolytes, unlike the H-cell. The presence of KOH in the anolyte functions as a proton source as a result of the oxygen evolution reaction occurring at the anode, offering improved efficiency over KHCO₃. As shown in Figure b, c, the FE% trend exhibited a distinct variation in the flow cell scenario. The potential of -1.1 Vs RHE exhibited the greatest FE% values, reaching 78.0%. Furthermore, the enhanced flow cell exhibits a better flow efficiency, which can be observed through the computation of the partial current density, which represents the current employed in the conversion of CO₂. Figure S5 demonstrates that the current was about 13-fold greater than that of the H-cell. Figure d illustrates the long-term stability of the 10% Cd-ZIF-8 catalyst in a flow cell, as measured by the current density over time. The graph demonstrates a consistent current density for the catalyst across a 10-hour period, indicating stable performance during prolonged operation. The pink data points on the secondary y-axis show the FE for CO production, which remains relatively steady throughout the duration, with minor fluctuations. This stability in both current density and CO FE suggests that the 10% Cd-ZIF-8 catalyst maintains robust electrocatalytic activity and durability in a continuous CO₂ reduction environment to syngas without production of any liquid products as shown in Figures S6 and S7.

3.

(a) Comparative LSV for 10% Cd-ZIF-8 using a H-cell and flow cell. FE of (b) H-cell and (c) flow cell. (d) Long-term stability of 10% Cd-ZIF-8 using the flow cell.

The results of this study demonstrate that cadmium doping and cell configuration play crucial roles in enhancing the electrocatalytic performance of ZIF-8 for CO₂ reduction. The electronic interactions between Cd²⁺ and Zn in ZIF-8 support yield negligible electronic disruptions, alongside the tuning of the metal d-bandwidth of Cd and Zn, which effectively modulates the binding strength of the intermediates *COOH and *CO, thereby stabilizing *COOH while diminishing the effect of CO poisoning. The developed Cd-ZIF-8 electrocatalyst facilitates selective, stable, and scalable CO₂ electrolysis to CO, demonstrating an exceptional CO FE. According to similar studies, the formation of CO through the *COOH intermediate is considered the favorable pathway on Cd-ZIF-8 surfaces due to the relatively lower energy barrier associated with this mechanism. Cd-based electrocatalysts exhibit a low energy barrier of approximately 0.3 eV for generating *COOH, while the free energy difference between *COOH and HCOO* during the first proton-electron transfer reaction is higher, around 0.5 eV. This indicates that the selective formation of *COOH is more thermodynamically favorable on Cd-ZIF-8. Furthermore, during the second proton-electron transfer step leading to *CO and *HCOOH formation, the lower Tafel slope of Cd-ZIF-8 suggests enhanced reaction kinetics for CO production. , Cadmium doping significantly improves the current density and reaction kinetics, with the 10% Cd-ZIF-8 catalyst achieving an optimal performance. This balance in Cd content allows for effective electron transfer and strong CO₂ adsorption, as indicated by the lower Tafel slope and higher ECSA observed. These findings suggest that controlled Cd incorporation into the ZIF-8 framework enables enhanced interaction with CO₂ and efficient generation of syngas, thus positioning Cd-ZIF-8 as a promising catalyst for CO₂ electroreduction.

Moreover, the comparison between H-cell and flow cell configurations highlights the importance of reactor design in maximizing the catalyst efficiency. The flow cell configuration demonstrated superior performance, attributed to enhanced CO₂ diffusion and continuous electrolyte replenishment. This setup not only increased the current density but also improved FE for CO production, achieving 78% FE at an applied potential of -1.1 Vs RHE. The flow cell’s ability to maintain stable performance while maintaining the electrocatlysts composition (Figure S8) over extended periods further underscores its practical advantages for scalable CO₂ reduction applications.

Overall, this study shows that the synergy between cadmium doping and an optimized cell configuration can significantly enhance the CO₂ reduction capabilities of ZIF-8. These insights pave the way for the future development of MOF-based catalysts and advanced reactor designs aimed at sustainable and efficient conversion of CO₂ to valuable fuels and chemicals.

4. Conclusions

In this study, we successfully synthesized cadmium-doped ZIF-8 (Cd-ZIF-8) catalysts, demonstrating their effectiveness for electroreduction of CO₂ to syngas (CO and H₂) with tunable ratios at different potentials. Characterization techniques confirmed uniform Cd²⁺ incorporation, preserving the framework’s structural integrity and high surface area. Electrochemical analysis showed that Cd-doped ZIF-8, particularly the 10% Cd variant, achieved a higher current density and favorable reaction kinetics, optimizing the CO₂ conversion efficiency.

Notably, the flow cell configuration provided superior performance compared to the H cell, enhancing CO₂ diffusion and current density. The 10% Cd-ZIF-8 in the flow cell achieved a high FE of 78% for CO at −1.1 V_RHE and demonstrated stable performance over an extended operation. These results indicate that Cd-ZIF-8 catalysts hold significant promise for efficient and scalable CO₂ conversion, paving the way for future applications in sustainable fuel and chemical production.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c10149.

• LSV of ZIF-8 and Cd-ZIF-8 (Figure S1), BET of ZIF-8 and Cd-ZIF-8 (Figure S2), Tafel slopes of ZIF-8 and Cd-ZIF-8 (Figure S3), Cdl of ZIF-8 and Cd-ZIF-8 (Figure S4), partial current densities of 10%-Cd-ZIF-8 (Figure S5), GC chromatogram of Cd-ZIF-8 (Figure S6), the UPLC-RI chromatogram of 0.5M STD and 10%Cd-ZIF-8 at -1.1 V (Figure S7), and the XRD of 10% Cd-ZIF-8 before and after the stability (Figure S8) (PDF)

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.