Electrocatalytic Oxygen Reduction in Alkaline Medium at Graphene-Supported Silver-Iron Carbon Nitride Sites Generated During Thermal Decomposition of Silver Hexacyanoferrate
Silver-iron carbon nitride, which has been prepared by pyrolysis (under inert atmosphere) of silver hexacyanoferrate(II) deposited on graphene nanoplatelets, is considered here as electrocatalyst for oxygen reduction in alkaline medium (0.1 M potassium hydroxide electrolyte) in comparison to simple silver nanoparticles and iron carbon nitride (prepared separately in a similar manner on graphene nanoplatelets). The performance of catalytic materials has been examined using such electrochemical diagnostic techniques as cyclic voltammetry and rotating ring-disk electrode voltammetry. Upon application of the graphene nanoplatelet-supported mixed silver-iron carbon nitride catalyst, the reduction of oxygen proceeds at more positive potentials, as well as the amounts of hydrogen peroxide (generated during reduction of oxygen at potentials more positive than 0.3 V) are lower relative to those determined at pristine silver nanoparticles and iron carbon nitride (supported on graphene nanoplatelets), when they have been examined separately. The enhancement effect shall be attributed to high activity of silver toward the reduction/decomposition of H2O2 in basic medium. Additionally, it has been observed that the systems based on carbon nitrides show considerable stability due to strong fixation of metal complexes to CN shells.
KeywordsGraphene nanoplatelets Carbon nitride electrocatalyst Silver Iron Oxygen reduction reaction Hydrogen peroxide intermediate Alkaline fuel cell
Proton-Exchange Membrane Fuel Cells (PEMFCs) are promising environmentally friendly electrochemical devices for energy conversion and are still dominating research on low-temperature fuel cells, but, in recent years, substantial progress in fabrication and utilization of anion exchange membranes has driven growing interest in Alkaline Fuel Cells (AFCs) [1, 2]. Relative to acidic electrolytes, such general features as faster dynamics of the oxygen reduction reaction and larger availability of catalytic systems in alkaline media [3, 4] justify substitution of platinum and platinum group metals with more abundant and less expensive materials like silver [4, 5, 6, 7, 8, 9], transition metals (especially Fe, Co), particularly when coordinated with nitrogen [10, 11, 12, 13], transition metal oxides [14, 15], or even carbon nanostructures doped with heteroatoms (N, S, P, B) [16, 17, 18, 19].
Among the most widely studied oxygen reduction catalysts in alkaline media are the carbon-supported nitrogen-coordinated iron systems (abbreviated as FeNC) obtained via high-temperature pyrolysis of simple precursors. Structurally, the active sites are believed to resemble the centers existing in such macromolecular complexes as porphyrins or phthalocyanines. An interesting recent approach describes the carbon nitride-based electrocatalysts prepared through thermal decomposition of hexacyanometalates in which the carbon-based matrix embeds nitrogen atoms capable of coordination of metallic species [20, 21, 22, 23, 24, 25, 26, 27, 28]. Among advantages coming from the use of hexacyanometalates as starting compounds is the possibility of fabrication of materials of controlled stoichiometry with mixed metallic species (e.g., Fe, Co, Ni, Sn, Mn, Cu, Ag).
Due to appreciable electrocatalytic activity and reasonable stability during both oxygen and hydrogen peroxide reduction reactions [4, 5, 6, 7, 8, 9, 29, 30], silver can be considered as promising alternative to platinum for studies in alkaline media. The mechanism of oxygen reduction on Ag usually is reported to proceed predominantly to water. When it comes to structural dependence of the oxygen reduction kinetics, it tends to increase according to the following order of planes: (100) < (111) < (110) . It is also noteworthy that fairly active and stable materials have been obtained by combining silver with manganese oxides [32, 33], cobalt oxides , molybdenum oxides , and other transition metals (Co, Cu, Ni, Sn, Fe) [36, 37, 38, 39, 40].
Among important issues concerning preparation of the cathode materials for operation in low-temperature alkaline fuel cells is the need of utilization of carbon supports of high electrochemically accessible surface area and suitable porosity for proper mass transport. Special attention has been paid to high graphitization degree which determines electron conductivity and stability of the whole electrocatalytic system. Relative to the systems based on conventional carbon blacks [41, 42], various graphene-related materials with their unique properties have also been considered with an ultimate goal of increasing effectiveness and stability of catalysts [43, 44]. Nevertheless, the absence of surface functional groups and defects complicates utilization of pristine graphene as matrix for immobilization of the homogenously distributed largely dispersed active centers. Consequently, more porous and defected graphene oxide (GO) and reduced graphene oxide (rGO) have been more broadly studied as supports (or their precursors) capable of anchoring finely dispersed catalytic nanoparticles. However, stability of such materials and their electronic conductivity has often been reported as insufficient [42, 45]. On the other hand, it has recently been demonstrated that durability can be improved through application of the polyelectrolyte-modified graphene nanoplatelets (GNP) as supports for Pt nanoparticles. In this respect, the intrinsic high graphitization degree of GNP and the enhanced interactions between Pt and carbon (GNP) have been described as advantageous in Pt/GNP . Finally, irrespective of the choice of graphene-type material, the problem of gas permeability and water management (originating from so-called stacking effect) leads to lower intrinsic performance of cathodes during oxygen reduction reaction in relation to those based on other carbon nanostructures. In fact, graphene and its derivatives can be used as fairly effective additives to gas-barrier systems . The latter problem can be addressed by designing three-dimensional morphologies of composite assemblies in which additives of other carbonaceous materials act as spacers between the graphene-type sheets [42, 45, 48, 49, 50]. Durability of such hybrid systems seems to be significantly improved.
Nitrogenated derivatives of graphene-related materials have recently been also considered as supports or modifiers for silver-based electrocatalysts active toward oxygen reduction in alkaline environment [51, 52, 53, 54]. In the present work, we address fabrication and physicochemical identity of the graphene-nanoplatelet-supported silver-iron carbon nitride catalytic systems derived from thermal decomposition of silver(I) hexacyanoferrate(II) deposited onto polycarboxylate-functionalized GNP. The resulting material is elucidated for oxygen reduction reaction (ORR) in 0.1 M KOH, and the system’s activity is compared to the performance of pristine silver nanoparticles and iron carbon nitride (prepared separately in a similar manner on GNP and investigated under analogous conditions). Regarding strong fixation of metal centers coordinated within CN shells, the stability issue of examined systems is also discussed.
All chemicals were commercial materials of the highest available purity. HClO4, ethanol, 2-propanol, KOH, and K4[Fe(CN)6]·3H2O were from POCh (Poland). The solution of 5% Nafion-1100, as well as AgNO3, Fe2(SO4)3·6H2O, and the polycarboxylate-functionalized graphene nanoplatelets (GNP) were from Sigma-Aldrich. Vulcan XC-72R (C) was from Cabot (USA); graphene oxide sheets (GO) of 300–700 nm sizes (thickness, 1.1 ± 0.2 nm) were from Megantech; Pt(20%)/C was from E-Tek; 30% hydrogen peroxide was from Chempur (Poland); and the nitrogen, oxygen, and argon gases (purity 99.999%) were from Air Products (Poland). As a rule, the solutions were prepared from doubly distilled and subsequently deionized (Millipore Milli-Q) water.
Preparation of graphene nanoplatelets (GNPs) modified with silver hexacyanoferrate (AgI4[FeII(CN)6]), abbreviated as Ag4HCF/GNP, was achieved by precipitation method. In the actual procedure, the mixture of 15.1 cm3 of water, 2 cm3 of ethanol, and 50 mg of GNP was first placed for 1 h in an ultrasonic bath to obtain good dispersion. Then, 10.3 cm3 of 10 mmol dm−3 AgNO3 aqueous solution was added, and the slurry was left in the bath for about 15 min followed by mixing under magnetic stirring. After 10 min of mixing, 2.6 cm3 of 10 mmol dm−3 K4[Fe(CN)6] water solution was added, and the dispersion was left for ca. 5 h to assure full precipitation of the product (white sediment in a blank test). Total volume of mixture was 30 cm3. In the next step, the solution was centrifuged and removed. The sediment was washed three times with water and dried on a hot plate at about 60 °C. When taking into account the amounts of GNP and precursors used for synthesis, and assuming that the main precipitate is AgI4[FeII(CN)6], the overall content of Ag and Fe in the sample was on the level of 20% relative to GNP.
Preparation of GNPs modified with Prussian Blue (FeIII4[FeII(CN)6]), Fe4HCF3/GNP, was pursued in an analogous manner as for Ag4HCF/GNP, except that, in order to obtain the “insoluble” (largely K+-free) form of Prussian Blue postulated previously, Fe3+ was used in excessive (super stoichiometric amounts). Consequently, during the synthesis, 7.4 cm3 of 10 mmol dm−3 solution of Fe2(SO4)3 and 9.6 cm3 of 10 mmol dm−3 K4[Fe(CN)6] were added. The amount of water and ethanol solutions was adjusted to the level of 30 cm3 of the total volume. The resulting material (dark blue sediment in a blank test) was washed and dried. The content of Fe in the sample was on the level of 20% by weight relative to GNP.
Preparation of GNPs modified with silver nitrate (AgINO3), AgNO3/GNP, was accomplished by a simple impregnation method; namely, to 18.4 cm3 of the homogenized dispersion of GNP in water-ethanolic solution, 11.6 cm3 of 10 mmol dm3 AgNO3 was added. The slurry was subjected to magnetic stirring for several hours; later, the solvent was evaporated under ambient conditions (22 °C). The content of Ag in the sample was ca. 20% by weight relative to GNP.
All prepared samples were subjected to grinding in agate mortar and, subsequently, annealed in a silica tube under Ar atmosphere at 800 °C for 2 h (with the heating up rate of an oven, 4 °C min−1). The heat-treated samples are labeled AgFeCNx/GNP800, FeCNx/GNP800, and Ag/GNP800. For comparison, pristine GNP was also washed with water and annealed under the same conditions as described previously (it is labeled GNP800). The additional sample of AgFeCNx/GO800 was prepared.
The electrochemical measurements were performed with CH Instruments (Austin, TX, USA) Model 750D and 920D workstations. All electrochemical experiments were conducted at room temperature, 22 ± 2 °C. The mercury-mercury sulfate electrode (Hg/Hg2SO4) (in acid medium) and the saturated calomel electrode (SCE) (in alkaline medium) were used as reference electrodes. In the latter case, care was exercised to use a high-quality bridge to avoid contamination of the electrolyte with chlorides. As a rule, all potentials were recalculated by usual means and expressed against the standard RHE. Glassy carbon rod served as counter electrodes. The rotating ring-disk electrode (RRDE) experiments were performed using a variable speed rotator (Pine Instruments, USA). The electrode assembly utilized a glassy carbon disk (with geometric area of 0.247 cm2) and Pt ring. The collection efficiency (N) of the RRDE assembly was equal to 0.388, as determined from the ratio of ring and disk currents (at 1600 rpm) using the argon-saturated 0.005 mol dm−3 K3[Fe(CN)6] in 0.1 mol dm−3 K2SO4 solution. Before electrode layer preparation, working electrodes were polished with aqueous alumina slurries (grain size, 5–0.05 μm) on a Buehler polishing cloth. In the course of RRDE experiments, in order to oxidize H2O2 (generated at the disk) under convection-diffusional control, the potential of the ring electrode was kept at 1.23 V vs. RHE.
Electrode layers were deposited on a glassy carbon disk by dropping appropriate volumes of homogenized inks containing 5 mg of nanostructured catalyst admixed with Vulcan® XC-72R (labeled as C) in the mass ratio of 1 to 1, 500 μl of solvent (2-propanol) and 6 μl of binder (5% solution of Nafion®). They were subsequently dried at room temperature, 22 ± 2 °C. The final content of Nafion® was 5% in relation to the weight of catalyst and Vulcan. Catalyst loadings were 600 μg cm−2. In comparative measurements with the Pt(20%)/C standard, the Pt loading was 15 μg cm−2. The electrodes covered with the catalytic layers were typically washed out with the stream of water (in order to clean the surface from impurities) and, later, subjected to voltammetric potential cycling at the scan rate of 100 mV s−1 in the nitrogen-saturated 0.1 mol dm−3 KOH in the potential range from 0.05 to 1.05 V to activate the layers and receive reproducible responses. To assure reproducibility of the electrode preparation and its performance, we have conducted electrochemical experiments at least three times. The data were reproducible within ca. 5%.
In the experiments involving the preconditioning step (labeled P), the electrode was cycled in nitrogen-saturated 0.1 M HClO4 solution in the range of 0.05–1.05 V at the scan rate of 100 mV s−1 until stable (flat) response was obtained. The electrolyte was changed twice during the experiments.
In the additional experiments conducted in 0.5 M K2SO4, which were aiming at obtaining the electrochemical characteristics of precursors, Fe4HCF3/GNP, Ag4HCF/GNP, and the GNP carrier, the respective inks were prepared without Vulcan additive and loadings of the respective electrode layers were on the level of 300 μg cm−2.
Transmission Electron Microscopy (TEM) experiments were carried out with Libra 120 EFTEM (Carl Zeiss) operating at 120 kV. Scanning electron microscopic (SEM) measurements and energy-dispersive X-ray analysis were achieved using MERLIN FE-SEM (Carl Zeiss) equipped with EDX analyzer (Bruker). X-Ray diffraction (XRD) spectra were collected using Bruker D8 Discover equipped with a Cu lamp (1.54 Å) and Vantec (linear) detector.
Results and Discussion
General Physicochemical Characteristics
The EDX analysis of the materials generally reflects ratios and amounts of precursors used for their synthesis. In the case of Fe4HCF3/GNP (before annealing), the molar ratio of Fe to N is 0.5 (which is close to the value 0.4 expected from the stoichiometry of the FeIII4[FeII(CN)6]) compound); however, after pyrolysis (sample FeCNx/GNP800), the ratio of Fe to N has substantially changed to the value closer to 10 thus implying substantial loss of nitrogen species. The data obtained for Ag4HCF/GNP (before annealing) are consistent with the molar ratio of the elements Ag:Fe:N which is 3:1:6 and for AgFeCNx/GNP800 (after annealing), the composition changes into the order, 3:1:3, again implying the loss of N. The analytical data also imply the presence of carbon, oxygen, as well as some content of potassium. The EDX mapping of FeCNx/GNP800 (Fig. 2a) shows fairly homogenous distribution of N and Fe. In the case of AgFeCNx/GNP800 (Fig. 2b), the distribution of signals originating form Fe and Ag is less homogeneous: they tend to superimpose in certain areas thus implying the mixed nature of obtained nanoparticles.
Figure 4b presents cyclic voltammetric responses of (a) Ag/GNP800-C, (b) FeCNx/GNP800-C, (c) AgFeCNx/GNP800-C, and (d) GNP800-C layers deposited on glassy carbon electrode recorded in the de-aerated 0.1 M KOH. In the investigated range of potentials, basically all catalytic materials do not show redox transitions (i.e., they are not electroactive), and they are characterized by similar capacitive-type currents. Some current increases observed for FeCNx/GNP800-C at potentials lower than 0.3 V may reflect the presence of larger amounts of iron species (when compared to other compounds studied here). This redox pair was previously attributed to transition between Fe(OH)2 and Fe(OH)3 (coming from iron core) .
Oxygen Reduction Reaction
It is apparent from Fig. 5b (where the amount of the HO2− intermediate formed during electroreduction of oxygen has been monitored at the platinum ring electrode) that although Ag/GNP800-C (curve a), FeCNx/GNP800-C (curve b), and AgFeCNx/GNP800-C (curve c) are less selective toward the four-electron reduction of oxygen in comparison to the standard Pt(20%)/C (curve e), they exhibit much lower currents corresponding to oxidation of the HO2− intermediate relative to the unmodified GNP800-C carrier (curve d). In the case of Ag-containing samples (curves a and c in Fig. 5b), the HO2− oxidation currents have considerably decreased at potentials higher than 0.3 V relative to the silver-free system containing only iron (curve b in Fig. 5b).
Importance of Pretreatment in Acid Medium
The estimated EASA of Ag in Ag/GNP800-C (loading of Ag is about 60 μg cm−2) and AgFeCNx/GNP800-C (loading of Ag is about 53 μg cm−2) are as follows: 17.7 and 8.2 m2 g−1, respectively. A number of factors should be taken into account here: somewhat (about 11%) lower amounts of Ag (according to the synthesis procedure described in the “Experimental” section), higher average crystallite size (as it comes from the XRD analysis), as well as partial over-coating of catalytic centers by CN shells (as apparent from Fig. 1h) are applicable to the AgFeCNx/GNP800-C samples.
Consideration of Graphene Oxide as Support
It has been demonstrated for the first time that thermal decomposition under the inert atmosphere of silver(I) hexacyanoferrate(II), deposited on graphene nanoplatelets, produces nanomaterial containing silver and iron species coordinated to carbon nitride shells and exhibiting the remarkable efficiency toward oxygen reduction process in alkaline environment. It is important to note that, at the graphene-nanoplatelet-supported mixed silver-iron carbon nitrides, the reaction proceeds at more positive potential values in comparison to both bare silver nanoparticles and the iron-carbon nitride systems themselves (prepared separately under the same conditions on graphene-nanoplatelet carriers). Furthermore, amounts of the hydrogen peroxide-type intermediate (generated during reduction of oxygen in the potential range where cathode is expected to operate in a real fuel cell) are lower at the composite material mentioned previously. In this respect, high activity of silver toward decomposition of the undesirable intermediate (HO2−) seems to be of primary importance. Another important factor is that the systems based on carbon nitrides retain their electrocatalytic activity even after continuous polarization in acidic medium thus implying strong fixation of metal complexes to CN shells.
We acknowledge the European Commission through the Graphene Flagship—Core 1 project [Grant number GA-696656] and the Maestro Project [2012/04/A/ST4/00287 (National Science Center, Poland)].
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