The increase of CO2
concentration in the atmosphere is thought to be one of the main causes of global climate change [1
]. In particular, the CO2
emission from the use of fossil fuels contributes to the increasing concentration because it establishes a continuous net increase in the natural cycle of the tropospheric carbon.
There are different strategies proposed to control this issue [2
] being the most extended the CO2
storage, and the CO2
transformation to other valuable products, together with the implementation of renewable energies.
On the other hand, renewable energy sources are supposed to be a replacement, but nowadays they are not producing the constant currents that fossil fuels provide. For this reason, the storage of surplus electrical energy produced during the peak production periods, and its release during peak demand periods, should be crucial. In this manner, extensive research effort has focused on battery storage [3
]. However, battery manufacturing requires a lot of resources, reducing their contribution to controlling CO2
emission, and its life is relatively limited. Furthermore, recycling of their components is also a challenge.
One possible option to address the problem of temporary storing and local surplus of renewable energy is the electro-catalytic reduction of CO2
to hydrocarbons in water [4
]. In this process, the water is split to provide the required hydrogen atoms, which react with CO2
to form hydrocarbons that can be used directly in the existing infrastructure of fuel transportation as well as in storing the renewable energy.
The direct electrochemical reduction of CO2
in aqueous solution has been typically studied with metal electrodes like Cu, Au, or Sn during the past few decades [5
]. Copper electrodes have been found to be quite good in the reduction of CO2
to hydrocarbons, although the Faradaic efficiency was still low as a result of the dissociation of H2
O to H2
]. More recently, metallic electrodes derived from corresponding metal oxides, like SnOx
, seemed to show promising results in certain catalytic performance for CO2
], and only a few transition-metal oxides such as TiO2
O have been reported as potential electro-catalysts for this application [14
Alternatively, the application of carbon materials in electro-catalytic CO2
reduction process is a plausible option, which has been tested with platinum catalysts supported on carbon nanotubes, carbon cloth or carbon black [15
], and even metal-free carbons [17
]. Centi et al. [15
] showed for the first time the possibility of electro-catalytically converting CO2
to hydrocarbons with carbon chains >C5, and with product distributions which do not follow the Anderson–Schulz–Flory distribution model typical for Fischer-Tropsch synthesis. Li et al. [17
] addressed nanoporous S-doped and S,N-codoped carbons as catalysts for electrochemical reduction CO2
to CO and CH4
, where the negative charge on the pyridinic nitrogen groups promotes electron–proton transfer to CO2
leading to COOH* intermediates, which are further reduced to CO.
Carbon gels doped with transition metals have also shown activity in this reaction [18
]. Although this CO2
reduction mechanism is still being studied [20
], the products obtained in the direct electrochemical reduction of CO2
to hydrocarbons can achieve several carbon atoms [21
]. Regarding the hydrocarbon selectivity, recently [22
], a high selectivity to C3-hydrocarbons among the detected products has been reported using Co- and Fe-carbon electrodes. Moreover, Fe-carbon electrodes have shown a well-fitted linear correlation between the average crystal sizes of iron and the faradaic efficiencies: the smaller the crystal size, the higher the faradaic efficiency [23
On the other hand, the large amounts of plastic residue is also a very important environmental problem, and the out-of-control combustion method should not be an option, because it would increase the CO2
atmospheric and cause other environmental pollution problems [24
]. Within these materials, polyethylene (HDPE or LDPE) based plastic bags represent a significant proportion. There are several propositions for the recycling of plastic waste, as their transformation in fuels [25
], the recovering of valuable components [26
] of their transformation in carbon-based materials or composites [27
]. However, the direct application of carbon-metal composites, obtained from real world plastic waste as CO2
electro-catalysts have not been reported yet. One important advantage of these composite materials as electro-catalysts compared to other proposed carbon based electro-catalysts is the low cost of the raw material since they can be obtained directly from the plastic waste. Therefore, with this proposal, we are focusing our actions on the CO2
problem twice, (i) researching in its electro-catalytic transformation to hydrocarbons; and (ii) proposing a way for the transformation of LDPE based residues in valuable products.
In the present work, we demonstrate the application of metal-carbon-carbon nanofibers composites obtained from real world plastic waste as promising electrodes in the electro-catalytic reduction of CO2 to hydrocarbons.
3. Materials and Methods
Three different composites of metal-carbon-carbon nanofibers (-CNF) were prepared by a catalyzed pyrolysis of urban plastic residues which were thermally pre-treated in a closed reactor. These residues were plastic bags that were used in several well-known supermarkets in Spain, in which the polymer composition mainly consisted of low-density polyethylene (LDPE). Firstly, 10 g of the above-mentioned plastic bags were dissolved in 100 mL of o-xylene at 80 °C, and then 2 g of the catalysts precursor was added. The catalyst precursors were the corresponding hydroxides of Fe, Co and Ni, and the resulting mixture was stirred for 4 h. After that, the o-xylene was evaporated, and the solid was heat treated at 350 °C in a closed reactor (Parr Instrument, Moline, IL, USA) (reactor ref. A1828HC2) for four hours. Finally, the so pre-treated solid was pyrolised under N2 flow (300 mL min−1) at 900 °C. Before the characterization, the composites were washed with cool water several times. The obtained composites were named as PFe, PCo and PNi, being Fe, Co and Ni the pyrolitic catalyst, respectively.
The metal contents of the composites were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using an ICP-OES PerkinElmer OPTIMA 8300 spectrometer (PerkinElmer, Madrid, Spain).
The samples were texturally characterized by physical adsorption of nitrogen, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and chemically characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) Linear sweep voltammetries (LSV) were also carried out.
N2 adsorption was carried out at −196 °C. Prior to this measuring process, the samples were outgassed overnight at 110 °C under high vacuum (10−6 mbar). The BET equation was applied to the N2 adsorption data obtaining the apparent surface area, SBET. The Dubinin-Radushkevich (DR) equation was applied to the N2 adsorption data to obtain the corresponding micropore volume (W0) and micropore mean width (L0). Total pore volumes (V0.95) were calculated from N2 adsorption isotherms at −196 °C and at 0.95 relative pressure.
SEM was carried out using a Zeiss SUPRA40VP scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany), equipped with a secondary electron detector, back-scatter electron detector and by using an X-Max 50 mm energy dispersive X-ray microanalysis system. All the samples were crushed before performing this analysis.
HRTEM was performed using a FEI Titan G2 60–300 microscope (FEI, Eindhoven, The Netherlands) with a high brightness electron gun (X-FEG) operated at 300 kV and equipped with a Cs image corrector (CEOS), and for analytical electron microscopy (AEM) a SUPER-X silicon-drift window-less EDX detector.
XRD analysis was carried out BRUKER D8 ADVANCE diffractometer (BRUKER, Rivas-Vaciamadrid, Spain) using CuKα radiation. JCPDS files were searched to assign the different diffraction lines observed. Diffraction patterns were recorded between 10° and 70° (2θ) with a step of 0.02° and a time per step of 96 s. The average crystal size was determined using the Scherrer equation.
XPS measurements of the metal-carbon-CNF composites were performed using a Physical Electronics ESCA 5701 (PHI, Chanhassen, MN, USA) equipped with a MgKα X-ray source (hυ = 1253.6 eV) operating at 12 kV and 10 mA and a hemispherical electron analyzer. The obtained binding energy (BE) values were referred to the C1s peak at 284.7 eV. A base pressure of 10−9 mbar was maintained during data acquisition. The survey and multi-region spectra were recorded at C1s, O1s, Fe2p, Co2p, Ni2p, K2p and Ca2p photoelectron peaks. Each spectral region was scanned enough times to obtain adequate signal-to-noise ratios. The spectra obtained after the background signal correction were fitted to Lorentzian and Gaussian curves to obtain the number of components, the position of each peak, and the peak areas.
Electro-catalytic reduction of CO2
to hydrocarbons was carried out in a three-electrode cell, working in batch mode at ambient temperature and pressure. The cell has 300 cm3
of total capacity. A Biologic VMP multichannel potentiostat (Bio-Logic Spain, Barcelona, Spain) was used to induce and control the electro-catalytic reaction by applying the selected potential differences over the electrodes. A platinum electrode was used as a counter electrode and Ag/AgCl as a reference electrode. The used electrolyte was 150 cm3
-saturated 0.1 M potassium bicarbonate aqueous solution. The setup was used in potentiostatic mode at −1.65 V, reproducing the voltage conditions of previous works [19
]. Prior to the electro-catalytic CO2
reduction, the liquid phase was saturated through bubbling with CO2
for 3 h. After saturation, the pH of the solution was 6.7. The CO2
feed and exit lines were closed off and the reactor was operated in the batch mode. The amount of composite used in the cathode as electro-catalyst (working electrode) was 80 mg which was homogeneously pasted on both faces of a graphite sheet with dimensions of 50 mm × 8 mm. In the preparation of the cathode, the metal-carbon-CNF composite was mixed with the corresponding amount of polytetrafluoroethylene (PTFE) in a weight ratio of (80:7) using a PTFE (60%) water solution. All working electrodes were kept in 0.1 M potassium bicarbonate aqueous solution overnight before being used in the electro-reactor. The samples were also tested as electro-catalysts carrying out the reaction under Ar-saturated solution and, therefore, using electrolytes free of CO2
The samples were also characterized by LSV (Bio-Logic Spain, Barcelona, Spain). The cathodic sweep analysis was conducted from the equilibrium electrode potential to negative electric potential of −2.0 V vs. Ag/AgCl, with a scan rate of 5 mV s−1, using the same experimental conditions and reactor set-up for the electro-catalytic reduction of CO2.
The hydrocarbons produced by the electro-chemical reduction of CO2
were analyzed from the gas phase using a gas chromatograph (GC) (Bruker Española, Rivas-Vaciamadrid, Spain), where the gases were directly injected into the GC column using a gas recirculating pump for low flows. The GC (carrier gas: He, column: Chrompack Poraplot Q, 50 m × 0.53 mm) was equipped with a FID and TCD detectors. The distribution of gaseous products can be expressed in terms of the carbon selectivity as the amount of carbon atoms (from CO2
) in a specific product relative to the total amount of carbon atoms in the detected hydrocarbons.
Here nCi represents the mol of product Ci, and i the number of carbon atoms in that product.
The liquid phase was also analyzed by Headspace Gas Chromatography-Mass Spectrometry using another GC equipped with a HP-INNOWax 30 m × 0.25 mm × 0.25 µm column, which was coupled to a MS-Triple quadrupole. The presence of carboxylic acids or alcohols of one to four carbon atoms were not detected.