Electrocatalytic performance of NiNH2BDC MOF based composites with rGO for methanol oxidation reaction

Present work comprehensively investigated the electrochemical response of Nickel-2 Aminoterephthalic acid Metal–Organic Framework (NiNH2BDC) and its reduced graphitic carbon (rGO) based hybrids for methanol (CH3OH) oxidation reaction (MOR) in an alkaline environment. In a thorough analysis of a solvothermally synthesized Metal–Organic Frameworks (MOFs) and its reduced graphitic carbon-based hybrids, functional groups detection was performed by FTIR, the morphological study by SEM, crystal structure analysis via XRD, and elemental analysis through XPS while electrochemical testing was accomplished by Chronoamperometry (CA), Cyclic Voltametric method (CV), Electrochemically Active Surface Area (EASA), Tafel slope (b), Electron Impedance Spectroscopy (EIS), Mass Activity, and roughness factor. Among all the fabricated composites, NiNH2BDC MOF/5 wt% rGO hybrid by possessing an auspicious current density (j) of 267.7 mA/cm2 at 0.699 V (vs Hg/HgO), a Tafel slope value of 60.8 mV dec−1, EASA value of 15.7 cm2, and by exhibiting resistance of 13.26 Ω in a 3 M CH3OH/1 M NaOH solution displays grander electrocatalytic activity as compared to state-of-the-art platinum-based electrocatalysts.

series was prepared by following the above-stated procedure. A clear solution of MOF obtained by the abovementioned scheme and appropriate amount rGO was sonicated for two hours to acquire a uniform blend and then transmitted to stainless steel autoclave with Teflon inner cavity for 24 h reaction at 393 K. A dry solid product was obtained after 3-4 times washing with DMF/ethanol via centrifugation followed by 24 h vacuum drying at 348 K (Fig. 1).

Materials characterization. For comprehensive characterization of as-synthesized samples, a Scanning
Electron Microscope (VEGA 3 TESCAN) was employed for exterior structure and morphology analysis. X-ray Powder Diffractometry (STOE Germany) was implemented for phase purity and crystalline nature scrutinization (Kα = Cu radiation, 0.1.54 0 A, scanning range = 5°-80°, step size = 4°/s at 5 mA and 20 kV) while validation of functional groups and metal-ligand strong interaction was corroborated through FTIR Spectrophotometer (Perkin spectrum) by selecting wavenumber range of 500-4000 cm -1 . Binding energy and composition records www.nature.com/scientificreports/ were collected by XPS (MI-600) respectively. The degree of defects and graphitization, as well as the extent of crystallinity, was ascertained via Raman Spectroscopy, and stability of material was observed through Thermo gravimetric analyses (TGA) by using a thermo-gravimetric analyzer (Perkin Elmer Pyris 1, Champaign, IL, USA). The temperature was increased from 20 to 500 °C at a heating rate (5 °C min −1 ) under an airflow rate of 20 ml min −1 .
Electrocatalytic measurements. An inclusive electrochemical evaluation was done in 3 M CH 3 OH/1 M NaOH mixture through the Gamry apparatus (Ref 3000/3000 AE). To get homogenous ink of electrocatalyst, 2.0 mg of sample (0.85 mg/cm 2 ), 97 µl ethanol, and 3 µl Nafion were sonicated for 40 min, and then 0.003 ml of electrocatalyst ink was plunged on a glassy carbon electrode (GCE = working electrode) by micropipette. Pt wire and Hg/HgO were chosen as auxiliary and reference electrodes correspondingly. The selected voltage window for electrocatalytic response through Cyclic Voltammetry (CV) and stability testing for 3600 s via chronoamperometry at a fixed potential of 0.69 V was − 0.1-0.7 V (vs Hg/HgO). Moreover, the frequency range of 1-1 × 10 5 Hz was picked to find out system resistance at an amplitude of 0.005 V.
In the FTIR spectrum of fabricated samples (Fig. 2a) the COO −1 group symmetric and asymmetric stretching vibrations generate resilient adsorption bands at the position of 1568 and 1374 cm −1 and the gap between these two bands designates the connection of the COO −1 group of the linker with nickel-metal through the bidentate mode of linking. The band at 1655 cm −1 besides 1250 cm −1 indicates the N-C group stretching mode of vibration as well as divulges the coordinated DMF group manifestation while NH 2 group stretching and bending vibrations bands appear at 3309 and 1684 cm −1 , respectively [45][46][47][48] . The region between 3300 and 3000 cm −1 was occupied by asymmetric & symmetric vibration bands of the H-N group 49 . C-H peak appears at 754 cm −1 while the evident peak at 587 cm −1 approves the presence of nickel and its coordination with the COOH group oxygen. Figure 2b represents the comparison of FTIR spectra of NiNH2BDC/5 wt% rGO composite before and after the stability test 50,51 .
In the reported samples XRD pattern (Fig. 3), less significant peaks within 2θ range of 42°-52° correspond to the Ni NH 2 BDC MOF distinctive peaks, while less intense peaks at 15°, 25°, 35°, 38°, 40°, and 61° ensure the development of Ni (OH) 2 during the reaction accompanied with DMF peak at 2θ, 7.5° and attributed to restricted hydrogen bonding leading to swelling effect and establishment of MOF pores (JCPDS. No:02-1216) 50,52 . The rGO characteristics peaks are positioned at 17° and 23° and the gradual incline in peak intensity from MOF to NiNH 2 BDC/5 wt% rGO composite reflects the successful inclusion of rGO and composites synthesis. The XRD pattern closely resembles the literature data 53,54 .
The particle size, crystalline shape, and morphology appraised at different magnifications via SEM study display the presence of hexagonal shape particles while Reduced Graphene Oxide sheets working as a MOF support not only control the size of MOF nanoparticles but also helps in a MOF NPs fine dispersal on its panes and prevents the agglomeration. During the optimization of the synthetic conditions, it was observed that the addition of a small amount of PVP further controls the growth, shape, and size of hexagonal particles. Furthermore in EDX analysis for elemental composition, the increase in carbon content from MOF towards 5 wt% rGO composite validates the successful synthesis of composites (S.I) 50,55 .
The information about elemental composition, binding energy, and metal oxidation state was obtained by XPS (Fig. 4). The C 1 s spectrum comprises 3 peaks at B.E (binding energy) value of 284, 286, and 288 eV stipulate the existence of C-C, C-O, and C = O groups, respectively while oxygen spectra deconvolution in 3 peaks with a binding energy value of 530, 531, and 532 eV specifies the existence of M-O, C = O, and C-O. Fragmentation of nitrogen XPS spectra into two peaks of 402 and 403 eV represents the presence of the amine and pyrrolic nitrogen, respectively 56 . In the nickel spectrum, the presence of Ni +2 /Ni +3 was verified by the peaks in the range of 853-875 eV. The peaks at 871.8 eV for Ni 2p 1/2 and 853.1 eV for Ni 2p 3/2 indicate the spin-orbital coupling of Ni (+ 2) while peaks positioned at binding energy value of 874.73 eV (Ni 2p 1/2 ) and 856.80 eV (Ni 2p 3/2 ) are due to spin-orbital coupling of Ni (+ 3). The energy difference of 17.6 eV not only confirms the existence of Ni +2 /Ni +3 but also supports the nickel hydroxide manifestation 55,[57][58][59][60][61][62] .
Raman spectrum obtained after interaction and scattering of electromagnetic radiation with matter gives an idea about crystalline and the defects rich nature of material [63][64][65][66][67] . Raman spectra of NiNH 2 BDC MOF rGO hybrids have been presented in Fig. 5. In the selected range of analysis (4000-500 cm −1 ) the G and D band appears due to graphitic carbon vibrations and carbon defects, respectively. The calculated I D /I G value is in following order; NiNH 2 BDC (0.95) < 1 wt% rGO composite (0.98) < 2 wt% rGO composite (1.003) < 5 wt% rGO composite (1.008). The (a) amplified I D /I G ratio in 5 wt% rGO composite due to rearrangements and structural defects promote the extensive π bonding and electron transfer from donor towards acceptor sites, and (b) ~ 5 and ~ 65-time shifting of G and D bands results in the smooth charge transfer process. Both of these factors are attributed to the enhanced catalytic activity of a 5 wt% reduced graphitic carbon (rGO) hybrid than parent MOF 68 .
The TGA (Thermo gravimetric analysis) was performed for the investigation of the stability of the electrocatalysts. The thermal behavior of MOF and 5 wt% rGO composite is divided into four domains of mass loss (Fig. 6). The first step at 75 °C corresponded to the solvated DMF molecules, (5% loss). A second step between 100-240 °C was assigned to surface adsorbed water loss (18%), The third step at 290 °C corresponds to the loss of coordinated DMF(23% loss), and after that 61% weight loss at 388 °C represents structure collapse with the linker decomposition ( Fig. 6)  The gradual increase in current density by increasing catalyst amount was due to easy access to abundant and exposed electroactive sites and after that decline in current density is due to; (i) restricted utilization of bottom layered material due to upper thick layer (ii) inferior charge transfer process, and (iii) active sites blockage by reaction intermediates (Fig. 7a) [73][74][75][76] . Likewise, during methanol concentration optimization (1-5 M), the 3 M concentration with the maximum delivered current was found to be the optimum amount. At low content of CH 3 OH, the boosted current response is owing to excess of available OH − ions owing to diffusion-controlled methanol transport process while at high CH 3 OH concentration, excess of methanol limit the OH − adsorption, and reaction intermediates block the active sites and consequently depress the catalytic activity, Fig. 7b [77][78][79] .
The electrocatalytic response of all samples in the presence and absence of CH 3 OH is compared and analyzed at 50 mV/s in Fig. 8a The Cyclic Voltametric investigations were executed at the scanning speed of 2, 5, 10, 25, and 50 mV/s by selecting the voltage window of − 0.1 to 0.7 to recognize the influence of scanning speed on the current density of the tested samples. At the highest scanning speed, the easy and maximum approach of electroactive species towards the electrode surface leads to maximum current density Fig. 8b-f 83,84 .
To get information about the diffusion-controlled process, a straight line obtained by directly relating the peak current density with (scan rate) 1/2 , provides a slop that is equivalent to diffusion coefficient (Fig. 9). Furthermore, the diffusion coefficient (D) is calculated by inserting the value of the α (charge transfer coefficient) in the Randles Sevcik equation 85 .
The EIS (Electrochemical Impedance Spectroscopy), an important parameter tends to explore the kinetics and reaction mechanism. The EIS evaluation was accomplished in 3 electrode systems within the selected frequency domain of 1-1 × 10 5 Hz in an alkaline solution. Figure 10a,b illustrate the Nyquist plot of the bare electrode and NiNH 2 BDC MOF/rGO composites. A depressed semicircle of NiNH 2 BDC/5 wt% rGO hybrid illustrates the lowermost resistance (highest conductivity) than other counterparts due to homogenous scattering of MOF NPs on rGO surface having high surface area, exposed electroactive sites with maximum OH − adsorption, facile CO oxidation, smooth charge, and mass transfer, and accelerated CH 3 OH oxidation process 86 . Besides, modification of interfacial structure as a result of rGO inclusion is also an important kinetic controlling factor [87][88][89] . The extracted EIS data obtained after fitting a suitable circuit represent the minimum contact resistance, electrolyte resistance, and substrate inherent resistance with consequent excellent conductivity and electrocatalytic activity of NiNH 2 BDC/5 wt% rGO (Table 2) 90,91 .
Tafel plot is another important parameter utilized for evaluation of methanol oxidation activity, reaction mechanism, and kinetics of catalytic process by co-relating ln current density (ln j) with overpotential (η) (Fig. 11). Over potential is the required potential greater than the requisite potential for a reaction to occur. The kinetic behavior of as-synthesized samples is determined by the given below Tafel equation 92 .    (Table 3). Two different informations are obtained by calculating the Tafel slope at low and high overpotential region as (a) Hydrogen removal (dehydrogenation) from CH 3 OH is the rate controlling step in low potential region while (b) CO exclusion during oxidation process occur in the escalated potential domain. The NiNH 2 BDC/5 wt% rGO composite lowermost Tafel slope value (57.3 mV/dec) reflects the fast removal of hydrogen from CH 3 OH during oxidation process [93][94][95][96][97] .
The utmost requirement for the practical application of electrocatalyst is its long-term stability under experimental conditions. A Chronoamperometry (i/t) experiment is conducted in N 2 saturated alkali solution at peak potential 0.69 V vs Hg/HgO in three electrodes set up for 60 min. The graphical response of the oxidation process can be elaborated as (i) initial maximum j (current density) is due to strong binding of catalyst at the electrode surface, minimum gas bubbles, and large available active sites. (ii) The rapid decline in current density after a short period is associated with (a) extreme gas release and reaction intermediates formation which block the electroactive sites, and(b) material detachment due to excessive bubbling (iii) finally, a steady-state is achieved  www.nature.com/scientificreports/ which persists for 3600 s due to the reaction intermediates passive adsorption [98][99][100][101] . According to chronoamperometry graph, the stability retained by NiNH 2 BDC/5 wt% rGO composite is 60.6% while the stability retained by remaining samples is; NiNH 2 BDC/2 wt% rGO 59.3%, NiNH 2 BDC/1 wt% rGO 59.0%, and NiNH 2 BDC MOF 57.3%, respectively (Fig. 12a,b). The minimum loss in current density of NiNH 2 BDC/5 wt% rGO composite attributed to (a) large specific surface area provided by 2D rGO sheets (b) tolerance towards poisonous reaction intermediates (c) fine scattering of small size MOF nanoparticles on rGO surface, and trivial charge transfer resistance 96 . The greater stability of NiNH 2 BDC/5 wt% rGO composite was further evaluated through Cyclic Voltamogram. The current density reserved by electrocatalyst after 200 cycles are presented in Fig. 13. The sample stability tends to decline after successive cycling due to the blockage of active sites owing to excessive bubbling during MOR with electrode surface coverage, inhibited transport of electrolyte toward electrocatalytic material, and decrease in Electrochemical Active Surface Area. This problem can be settled by refreshing the electrolyte via a subsequent cathodic reduction in reverse scan and by performing the CV for few cycles 98,102,103 .
The recommended mechanism for the CH 3 OH oxidation process is as under 104,105 .
In the case of a Nickel-based system, NiO smoothened the CO oxidation by providing required oxygen while the NiOOH group promotes the MOR by Ni +2 /Ni +3 oxidation/reduction process where + 2 to + 3 oxidation further promote CO oxidation 35,106 .
The mass activity of all electrocatalysts is determined from the ratio of the current density vs deposited mass NiOOH + (CHO) (ads) → Ni +2 (OH) 2 + (CO) ads  To calculate the electrocatalytic activity of as-synthesized samples, EASA is determined by dividing Cdl (double-layer capacitance) with Cs (specific capacitance). The specific capacitance is a constant factor for each specific system while double-layer capacitance is determined through multiple CV scans or EIS in the non-faradic region. The estimated EASA of NiNH 2 BDC MOF/1, 2, 5 wt% reduced graphitic carbon hybrids at voltage 0.19 was observed to be; 7.6 (pure MOF) < 9.1 (1 wt% rGO) < 14.4 (2 wt% rGO) < 15.7(5 wt% rGO), correspondingly. The comparatively heightened catalytically active surface area of NiNH 2 BDC/5 wt% rGO composite proves the superb electrocatalytic performance of material for MOR (Supplementary information Fig. 1). The given data also authenticate the CV, EIS, and Tafel results.
Moreover, the EASA is divided by the geometrical area of the electrode to calculate the R.F (roughness factor). It is a unitless factor, as it is a ratio.
Mass Activity = J/m R.F = EASA/Electrode geometrical area  www.nature.com/scientificreports/ The heightened roughness factor reflects the excellent catalytic performance of material due to the direct relationship between EASA and R.F. Roughness factor of NiNH 2 BDC MOF/1-5 wt% reduced graphitic carbon hybrids were found in the following order; 107, 128, 204, and 222 respectively (Table 4 and Fig. 14) 107 .
The comparative statement of the electrocatalytic response of tested materials with already reported materials is provided in Table 5 given below.

Conclusions
The NiNH 2 BDC MOF/1-5 wt% reduced graphitic carbon hybrids (NiNH 2 BDC/rGO) fabricated by sonicationassisted solvothermal approach were studied for the CH 3 OH oxidation process under alkaline condition. The NiNH 2 BDC MOF/5 wt% rGO composite by possessing auspicious current of 267.7 mA cm −2 at voltage 0.69, Tafel slope of 60.8 mV dec −1 , the resistance of 13.26 Ω, EASA 15.7 cm 2 , mass activity 168.7 mA/mg and roughness factor 222 in 3 M CH 3 OH/1 M NaOH solution displays better activity as compared to the state-of-the-art platinum-based materials and prove to be a proficient substitute of costly materials exploited for MOR in the direct CH 3 OH fuel cell.   Figure 14. The comparison of EASA, Mass activity, and Roughness factor of NiNH 2 BDC MOF/1, 2, 5 wt% reduced graphitic carbon hybrids.