Carbon black supported manganese phthalocyanine: Efficient electrocatalyst for nitrogen reduction to ammonia

Ammonia (NH3) is regarded as a renewable energy source as well as an important molecule for agricultural applications. The energy‐intensive Haber‐Bosch method produces large amounts of CO2 gas during ammonia production. As an alternative, there has recently been much interest in the electrocatalytic production of NH3 via the electrochemical nitrogen reduction reaction (ENRR) process utilizing renewable energy under ambient condition. Herein, we report a conducting carbon–supported manganese phthalocyanine electrocatalyst as an efficient electrocatalyst for ENRR applications. The MnPc electrocatalyst exhibited the activity with an ammonia production rate of 61.8 μg h−1mg−1cat with Faradaic efficiency (FE) of 31.3% @–0.4 V vs. RHE, respectively, under ambient condition in 0.1 M HCl solution whereas MnPc/C electrocatalyst exhibited an enhanced the productivity with an ammonia yield rate of 127.7 μg h−1mg−1cat with FE of 35.3% @–0.4 V vs. RHE, respectively. The reliability of N origin in ammonia formation is demonstrated by 1H‐NMR experiments and multiple control analysis. These results open the way for the further study of carbon‐supported transition‐metal phthalocyanine compounds for electrochemical nitrogen fixation to NH3.

is essential. 4,5One of the most frequently used compounds, ammonia (NH 3 ), is used to make medicines and agricultural fertilizers. 6,7Additionally, ammonia can be utilized as a novel hydrogen energy carrier with a high potential for automotive fuel replacement because of the high energy carrier, clean fuel properties, and high hydrogen capacity properties. 8,9Since about 100 years ago, NH 3 has primarily been produced from N 2 and H 2 in their elemental forms using the energy-intensive Haber-Bosch method at 350 • C-550 • C and 150-350 atm, often using catalysts made of iron. 10,11After using this method, the population of the world can increase significantly.Exothermic reactions (N 2 + 3H 2 ⇄ 2NH 3 , Δ f H 0 = -45.940kJ mol -1 , Δ f G 0 = -16.407kJ mol -1 ) at high pressure and temperatures require to maintain thermodynamically unfavorable chemical equilibrium, whereas high temperatures are required to produce tolerable reaction rates. 12Additionally, the primary method of supplying H 2 for synthesis gas involves the steam reforming of natural fossil gas, resulting in significant energy consumption and CO 2 emissions. 13,14Overall, this process requires about 2% of the world's energy usage and 1% of the emissions of greenhouse gases worldwide, respectively (for every ton of NH 3 , 1.87 tons of CO 2 is turn out). 15,16Due to the essential need for high-performance, economical, and environmentally friendly nitrogen fixation, this century-old industry requires necessary modifications.This provides exciting challenges in both basic research and engineering.As a result, scientists have looked at several techniques, including thermal catalysis, 17,18 photocatalysis, 19,20 biological nitrogenase catalysis, 21 non-thermal plasma (NTP) catalysis, 22 and, electrocatalysis [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] to produce ammonia under mild conditions.ENRR is a promising option for this in multiple aspects.The primary responsibility of an ENRR electrocatalyst is to boost the activation of N 2 molecules.2][43][44][45][46] There are still some significant challenges to overcome before electrochemical ENRR can be widely adopted as a scaled-up substitute for the Haber-Bosch method.NH 3 yield rate and FE are still limited due to competition; in particular, the hydrogen evolution reaction (HER) in watery electrolytes and the lower solubility (20 mg/L) of N 2 in electrolytes are the biggest obstacles for ENRR. 47,48Transition metal-based electrocatalysts can be a good choice for nitrogen reduction due to the synergistic effect that weakens the nitrogen triple bond due to reverse electron transfer through the metal vacant d-orbital to the nitrogen anti-bonding (π*) orbital. 491][52][53] Kong et al. developed FeN/C electrocatalysts that showed good NRR activity with a Faradaic efficiency of 15.8% and an NH 3 production rate of 24.8 μg h −1 mg cat −1 @−0.4 V. 54 He et al. 55 reported an electrocatalyst, FePc, that showed the highest NH 3 production rate of 10.25 μg h −1 mg −1 cat and Faradaic efficiency of 10.50% @-0.3 V. Xu and co-worker 56 designed an electrocatalyst, FePc/O-MWCNT to enhance N 2 reduction activity and selectivity with a NH 3 production rate of 36 μg h −1 mg −1 cat with Faradaic efficiency of 11.85%, respectively.Sun et al. 57 demonstrate Mn 3 O 4 as a nanocube catalyst for nitrogen conversion with an ammonia production rate and Faradaic efficiency of 11.6 μg h −1 mg −1 cat and 3.0% @-0.8 V, respectively.9][60][61][62][63] However, to the best of our knowledge, the study of manganese phthalocyanine (MnPc) used for electrochemical nitrogen reduction has not been reported in the scientific literature.
MnPc showed average electrical conductivity performance during NRR.To overcome this situation a certain amount of conducting carbon is added which can improve the NRR through various pathways.First, conductive carbon increases electron mobility.Second, electron entry into the carbon addition system plays a key role in increasing the current density.Adding carbon to the MnPc system also increases the nonmetallic content, which has the potential to significantly lower HER performance. 64,65In this study, we have prepared a carbon black supported metallic phthalocyanine-based transition metal system where the MnPc electrocatalyst exhibited the N 2 reactivity with ammonia production rate of 61.8 μg h −1 mg −1 cat with Faradaic efficiency of 31.3% @-0.4 V, respectively, whereas the MnPc/C electrocatalyst exhibited a good NRR activity with an NH 3 yield rate of 127.7 μg h −1 mg −1 cat and FE of 35.3% at −0.4 V, respectively.A control & an isotopic labeling experiment confirmed the validity of the nitrogen source in ammonia synthesis.

Synthesis and characterization
Certain amount of conducting carbon (C) was added to 30 mL of ethyl alcohol and the solution was then probe-sonicated for 6 h.After that, pristine MnPc was added maintaining a 1:1 ratio.The solution mixture was then stirred with a magnetic  1A) a characteristic B-band (Soret band) at 353 nm region.Also, a Q-bands spectrum showed at 715 nm (sharp peak) and 645 nm (small peak) for Mn (II) in MnPc/C system.The Q-band came because of the transition of π (a 1u ) to π* (e g ) for HOMO to LUMO and B-band transition came due to transition of π (a 2u ) to π*(e g ).A peak at 512 nm was shown because of charge-transfer (CT) excitation from unsaturated manganese (Mn) ions. 66Crystalline phases of carbon, MnPc, and MnPc/C were determined using the x-ray diffraction (XRD) technique (Figure S1) where all (h, k, l) indices for MnPc were aligned with ICDD card values (#02-063-3894). 67Various chemical bonds present in MnPc/C were recognized using FTIR spectroscopy (Figure 1B).The main FTIR vibration stretching frequency was shown at 902 cm −1 corresponding to the bond of Mn-N in MnPc/C.At 1200-1800 cm −1 , the peaks were observed due to in-plane vibration stretching of C-N and C-C.9][70] The morphology of MnPc/C was analyzed using transmission electron microscopy (TEM) analysis.Figure 1C shows the porous morphology where carbon was attributed with MnPc.The existence of Mn, C, and N in the MnPc/C system was confirmed by Figure 1D-F, which also included an analysis of the various chemical components present in the system.From disk centrifuge particle size analyzer Figure S2 shows a geometric mean particle size of carbon black is ∼0.221 μm.
The various elements in the MnPc/C catalyst were examined using x-ray photoelectron spectroscopy (XPS).Comprehensive XPS survey scans demonstrate the manganese, nitrogen, and carbon elements in the electrocatalyst (Figure 2A).The high-resolution (HR) C1s XPS spectrum (Figure 2B) observes at 284.5 eV.Also the peaks at 642.5 eV (2p 3/2 ) and 655 eV (2p 1/2 ) corresponding to Mn (II) species (Figure 2D).Two different types of N species at 398.77 eV (Figure 2C) for the pyridinic N and at 400.14 eV for the pyrrolic N are visible in HR N 1 s XPS spectrum.

Engineering aspect and mechanism for ENRR
A nitrogen molecule contains two covalently bonded nitrogen atoms (N≡N).To break the thermodynamically stable N≡N, it requires 941 kJ/mol energy.A high (10.82eV) energy band gap exists between the lowest unoccupied molecular Initially, the σ (2p) of N 2 shares its electron with the vacant d orbital of Mn and then returns to the anti-bonding (π*2p) orbital of nitrogen that breaks the bond order (Scheme 1B). 72he cloud of π-electron present in carbon black is key reason for the assembly of MnPc by π-π stacking interactions (Scheme 1C), which gives a path to transfer an electron from an electron donor to an acceptor, which further helps nitrogen reduction.Scheme 1D shows a probable mechanism for N 2 to NH 3 conversion where nitrogen is first adsorbed on the catalytic surface, which is further activated by applying a reduction potential.Then alternating protonation on *N forward the reaction.After a successful six protonation, two units of ammonia will be produced in the electrolyte solution. 58,73,74

Electrochemical activity
All ENRR analyzes were examined using an H-type cell (Figure 3A) in 0.1 N HCl electrolyte solution with the anode and cathode chambers by a Nafion 117 membrane that divides them.The membrane was treated with H 2 O 2 (5%) aqueous solution at 80 • C for 1 h before the test after being boiled in distilled water for 1 h.The membrane was once more heated in 0.5 M H 2 SO 4 for 2 h at 80 • C, and then it was boiled in distilled water for 6 h.Ammonia production (cathode chamber) can be reduced due to the use of an HCl electrolyte solution where Cl − ions first form Cl 2 (g) (anode chamber) @1.36 V (vs.RHE) which in presence of water forms an oxidizing agent HClO which reduces ammonia concentration due to further oxidation (NH 3 to N 2 ). 75But in our case, we applied voltage @-0.4 V vs. RHE which is not sufficient to produce Cl 2 from Cl − .Also, HClO is very unlikely to migrate from the anode through the Nafion membrane to the cathode chamber.Impedance spectroscopy was observed in Figure 3B where it was clearly observed that charge transfer resistance decreases when carbon is added to the MnPc system which can improve the electrochemical activity.The cyclic In the potential range of −0.0 to −1.0 V vs. RHE, the current density of MnPc/C in the nitrogen-saturated electrolyte is higher than that of MnPc and argon-saturated electrolyte, suggesting the effectiveness of NRR.The experiment was carried out to observe the amperometry behavior of MnPc/C at different voltages from −0.3 to −0.7 V vs. RHE (Figure 4B), the figure exhibits minute fluctuations in current density so evidence makes the electrocatalyst good strong and durable.Figure 4C,D for the MnPc and MnPc/C systems, respectively, show the UV-Vis spectra of the electrolyte's solution following an indophenol method analysis to evaluate the ammonia yield rate and FE (generated after the electrocatalytic process).According to the standard ammonia calibration curve (Figure S3), the MnPc electrocatalyst demonstrated NRR activity with an ammonia production rate of 61.8 μg h −1 mg −1 cat and Faradaic efficiency of 31.3% @-0.4 V, respectively, while the MnPc/C electrocatalyst demonstrated good NRR activity with ammonia production rate of 127.7 μg h −1 mg −1 cat and Faradaic efficiency of 35.3% (Figure 4E,F), respectively, which is comparable to the previously published results (Table S1).Several control studies were conducted at an ideal voltage of −0.4 V vs. RHE, using argon gas and nitrogen gas under various conditions, Ar saturated electrolyte solution, bare carbon paper as the working electrode, at open circuit potential, and N 2 as the reactant gas, to determine the precise source of nitrogen in ammonia.With MnPc and MnPc/C systems at −0.4 V, a significant quantity of ammonia was created during nitrogen purge in the electrolyte.This was not the case with argon gas saturated electrolyte, open-circuit voltage, and bare carbon paper (Figure 5A).However, an isotopic labeling measurement was done by 1 H NMR spectroscopy and the characteristic 15 NH 4 + signal was shown when 15 N 2 gas was used as the reactant gas (Figure 5B).The resulting 1 H-NMR spectra showed signature double peak for 15 NH 4 + telling us that the N 2 is only responsible for ammonia production.Three peaks were found in the 1 H NMR spectra of 14 NH 4 + when 14 N 2 gas was used as a reactant gas.No such NMR peak was observed when Ar was purged as the resulting gas during electrolysis.
Additionally, instead of ammonia production during the electrochemical nitrogen reduction, no by-products such as hydrazine (Figure 5C) and NO x (Figure 5D) (if produce in anodic chamber and migrate to cathodic chamber) are produced during the NRR reaction at varying potentials indicating the high selectivity of the electrocatalyst (all details explanation is given in supplementary section).Before starting the experiment, we tested any type of NO x contaminants present in  S4).After ENRR to ammonia, ammonia concentration in the electrolyte solution was also measured using ion chromatography (Eco IC Metrohm) method for further comparison with the UV-Vis data.A cationic column (Metrosep C 6-150/4.0) was utilized with an eluent stream rate of 0.9 mL/min at normal temperature.The eluent for IC consists of 1.7 mmol/L nitric acid.Ammonia peak observed at about 6.4 min of retention time with a pressure of 7.55 MPa.We have checked the catalyst stability for long run application.The Figures S5 (chemical structure) and S6 shows that the MnPc/C catalyst shows good stability up to 24 h of experiments. 76 standard ammonium calibration curve utilizing ammonium chloride (Merck) was carried out with varying the ammonia concentrations of 0.0, 0.2, 0.4, 0.6, 0.8, and 1 μg /mL (Figure 6A).A standard line curve (y = 0.219x−0.00169,R 2 = 0.997) was shown after plotting the peak area (y-axis) versus retention time (x-axis) from standard calibration curves (Figure 6B).It is shown that the linear correlation between the retention time (x-axis) and peak area (y-axis) at varying ammonium concentration solution.
Figure 6C shows that there is an ammonia peak signal when using N 2 gas during ENRR analysis at −0.4 V vs. RHE for MnPc/C but no such peak was observed in the case of Ar gas purging.The produced ammonia yield rate and FE after ENRR are comparable because there are not many variable data in both IC and UV-Vis methods (Figure 6D).Since we use conducting carbon black to increase the ammonia yield rate for MnPc, the carbon black itself shows a certain amount of ammonia production rate (Figures S7 and S8).

Materials
Manganese

Characterization techniques
FTIR spectrometer (Shimadzu IR Affinity-1S) was used for observing the various type of bonding in MnPc/C system.UV-Vis spectra were taken on a UV-Vis spectrophotometer (Shimadzu, UV 3600 Plus).All electrochemical NRR experiment was carried out by CHI 760E instrument.

Electrochemical set-up
In 0.1 M HCl solution with ultra-high quality nitrogen gas and argon gas being supplied, all electrochemical tests were measured.For the ENRR method, a three-electrode setup was utilized.The working electrode was a glassy carbon electrode with an area of 0.071 cm 2 , the reference electrode was saturated Ag/AgCl, and the counter electrode was Pt.Argon gas was purged with 0.1 M HCl solution 15 min before the start of the experiment to remove any possible contaminants.
Nitrogen gas was purged with 0.1 M HCl solution 30 min before the start of the experiment to saturate nitrogen gas in the electrolyte.All experimental potential data values were changed to RHE utilizing the equation of E (vs.RHE) = E (vs.Ag/AgCl) + 0.198 V + 0.059 × pH.Before the ENRR experiment, all gases were pre-purified with an basis trap of 0.1 M NaOH, an acidic trap of 0.05 M H 2 SO 4 , and a neutral trap of 0.05 M K 2 SO 4. 77 To exclude any false positive data, the following established protocol was maintained to obtain a reliable ENRR result. 78

Preparation of working electrode
5 mg of MnPc/C, 10 μL of a 5-wt% Nafion solution, and 100 μL of 2-propanol (Merck) were combined to create the catalytic ink.Then, the ink was uniformly suspended by ultra-sonication treatment for 5 min and vortex for 5 min.Finally, 2 μL of the prepared ink was loaded (∼1.3 mg/cm 2 ) onto the working electrode and it was dried in a vacuum oven at 80 • C for 4 h.The glassy carbon electrode was washed with distilled water and polished with alumina powder with a particle size of 0.3 μm before adding the catalyst.

CONCLUSIONS
In conclusion, a hybrid electrocatalyst MnPc and carbon black through π-π interaction is prepared in a simple and cost-effective process.The prepared MnPc/C electrocatalyst showed substantial electrocatalytic activity for nitrogen to ammonia conversion.The increased electrocatalytic activity of MnPc/C can be attributed to better interaction of MnPc with the electron cloud of carbon black, which provides better-facilitated electron transfer by the carbon support.By electrolysis, MnPc/C showed excellent selectivity and specificity with a maximum ammonia production rate of 127.7 μg h −1 mg −1 cat and Faradaic efficiency of 35.3% at −0.4 V, respectively, under ambient condition with aqueous 0.1 M HCl.Isotopic labeling tests and various control trials show that nitrogen supplies may be relied upon for NH 3 synthesis.By adding carbon black to the experiment, both the ammonia production and the FE were shown to be significantly increased.These findings pave the way for scientific investigations into carbon-supported others transition-metal phthalocyanine compounds for electrochemical nitrogen fixation to NH 3 .
spectrum of MnPc and MnPc/C; (B) FTIR spectrum of MnPc and MnPc/carbon; (C) microscopic image of MnPc/Carbon; (D-F) elemental distribution of MnPc/carbon containing Mn, N, and C elements bead at ∼80 • C for another 6 h.Finally, the sample was then collected from the solution mixture by a centrifuged machine at 12,000 rpm and dried at ∼110 • C for 12 h.UV-Vis spectrum of MnPc and MnPc/C electrocatalyst was analyzed in an ethyl alcohol medium.UV-Vis spectrum of MnPc/C showed (Figure U R E 2 (A) XPS survey scan of MnPc/C; (B) HR XPS scan of C1s; (C) HR XPS scan of N1s; and (D) HR XPS scan of Mn2p of MnPc/C orbital (LUMO, π* anti-bonding) and the highest occupied molecular orbital (HOMO, σ bonding). 71Scheme 1A, molecular orbital (MO) of nitrogen molecule, shows two electron pairs (opposite spin) present in the sigma (σ 2p) orbital and fully vacant anti-bonding (π*2p) orbital of a nitrogen molecule.Utilizing the vacant d orbital of Mn in MnPc, it can easily weaken the nitrogen triple bond by the synergic effect.
E M E 1 (A) MO of molecular N 2 , (B) Mn 2+ and N 2 orbital electronic transition, (C) MnPc and carbon black interaction (the black, green, cyan, pink, and red balls are labeled C, Mn, N, H, and O, respectively), and (D) probable N 2 to ammonia conversion mechanism.voltammetry (CV) curves of the MnPc and MnPc/C systems at various scan rates (20, 40, 60, 80, and 100 mV s −1 ) in the 0.1 M HCl electrolyte with potential varying from −0.5 to −0.1 V are shown in Figure 3C,E.Further, the electrochemical double-layer capacitance (C dl) was calculated from the CV plot @-0.3 V.The MnPc catalyst showed an electrical double-layer capacitance value (C dl) of 1.6 mF cm −2 and an electrochemical active surface area (ECSA) of 2.8 cm 2 where, ECSA = R f × S (R f = roughness factor of the working electrode & S = geometric area of electrode) (Figure 3D,F).For the MnPc/C system, an electrical double-layer capacitance value (C dl) was 3.3 mF cm −2 and ECSA was 5.7 cm 2 .The ECSA value suggests that MnPc/C may expose more active sites than MnPc, so MnPc/C has considerable NRR activity compared to MnPc.The linear sweep voltammetry (LSV) plots of MnPc/C (N 2 ), MnPc (N 2 ), and MnPc/C (Ar) are depicted in Figure 4A.
U R E 3 (A) Schematic representation of NRR cell design; (B) EIS plot of MnPc and MnPc/C; (C) the cyclic Voltammetry (CV) profiles of MnPc at the different sweep rates; (D) the double layer capacitance for MnPc catalyst @-0.3 V of CV profiles; (E) the cyclic voltammetry (CV) profiles of MnPc/C at the different sweep rates; and (F) the double layer capacitance for MnPc/C catalyst @-0.3 V of CV profiles LSV profile in Ar & N 2 saturated electrolyte; (B) amperometry curve of MnPc/C at different potential; (C) UV-Vis spectra of the solution at various potential of MnPc after electrolysis; (D) UV-Vis spectra of the solution at various potential of MnPc/C after electrolysis; (E) bar diagram of ammonia yield rate of MnPc & MnPc/C at various potential; and (F) bar diagram of FE of MnPc and MnPc/C at different potential feeding N 2 gas or air or in the electrolyte solution and the result showed a negative response (Figures S3 and