Bifunctional Tailoring of Platinum Surfaces with Earth Abundant IronOxide Nanowires for Boosted Formic Acid Electro-Oxidation

To expedite the marketing of direct formic acid fuel cells, a peerless inexpensive binary FeOx/Pt nanocatalyst was proposed for formic acid electro-oxidation (FAO). +e roles of both catalytic ingredients (FeOx and Pt) were inspired by testing the catalytic performance of FAO at the FeOx/Au and FeOx/GC analogies. +e deposition of FeOx proceeded electrochemically with a postactivating step that identified the catalyst’s structure and performance. With a proper adaptation for the deposition and activation processes, the FeOx/Pt nanocatalyst succeeded to mitigate the typical CO poisoning that represents the principal element deteriorating the catalytic performance of the direct formic acid fuel cells. It also provided a higher (eightfold) catalytic efficiency than the bare Pt substrates toward FAO with a much better durability. Field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) were all employed to inspect, respectively, the surface morphology, bulk composition, and crystal structure of the catalyst. +e electrochemical impedance spectra could correlate the charge transfer resistances for FAO over the inspected set of catalysts to explore the role of FeOx in mediating the reaction mechanism.


Introduction
e global desire to sustain new green paths toward a low carbon future has motivated research and development in the industry of fuel cells (FCs) and hydrogen production. In 2016, the expansion and inroads of these industries surpassed expectations to realize additional uses in ground support equipment, drayage and long-haul trucks, passenger trains, delivery vans, carbon capture from natural gas-fired power generation and oil sands sites, and energy storage [1]. Basically, FCs are galvanic reactors combining a fuel continuously with oxygen to produce electricity, water, and heat. Unlike combustion technologies, FCs do not burn fuel, making the process quiet, pollution-free, and up to 2-3 times more efficient [1,2]. e H 2 /O 2 FCs (HFCs) received for long time the eminent effort since the invention by Sir William Robert Grove in 1839 passing by its real production by General Electric Company (1950) and utilization in Gemini space mission (1962) [3][4][5]. Right now, the HFCs operate a wide range of potential applications in transportation and in stationary and portable power services. Although satisfying the highest cleaning criteria particularly when H 2 is provided from renewable non-polluting sources, the HFCs failed to meet the safety standards related to the production, usage, transporting, and storing of the H 2 fuel which is highly flammable.
is, in addition to the low energy density of H 2 and the high cost of miniaturization of gaseous H 2 containers, encouraged the movement into realizing liquid FCs (LFCs). In this regard, different alcohols (methanol, ethanol, propanol, and ethylene glycol) and nonalcohols (formic acid, dimethyl ether, hydrazine, ammonia, borane, and sodium borohydride) were recommended as replacements for the H 2 fuel [6][7][8]. Nevertheless, formic acid (FA) which is inflammable and non-toxic (typically known as a food-additive) has shown a particular interest for the FCs' technology [9]. Indeed, the direct formic acid FCs (DFAFCs) that operate with the FA electro-oxidation (FAO) [10][11][12][13][14] enjoys several merits including the relatively low operating temperature, high practical power density, small crossover flux through Nafion membranes, rapid electrooxidation kinetics making them highly efficient, and high open circuit voltage (1.45 V) [15][16][17][18]. Yet, the commercialization of DFAFCs is encountered by the critical poisoning of the Pt catalyst (dedicated for FAO) with CO released with the non-faradaic dissociation of FA even at open circuit potential.
is poisoning inducts a catalytic impairment for Pt to support FAO directly at low potential which ultimately deteriorates the performance of DFAFCs [6,[19][20][21][22][23][24]. In order to prepare the DFAFCs for a proper competition with HFCs, the CO poisoning of the anodic Pt substrates has to be overcome. e essence of this overcoming lies in modulating the Pt-CO binding to mitigate the adsorption of CO on the Pt surface and enriching the Pt surface with oxygen containing moieties that facilitates the oxidative removal of poisoning CO at a relatively low potential. Interestingly, the use of hybrid nanocatalysts of Pt with Pd, Au, Ru, Rh, Bi, Sn, Co, Cu, and Ni or with transition metal oxides such as NiOx, CoOx, MnOx, and Cu 2 O for FAO succeeded to provide a proof for this conception [24][25][26][27][28][29][30]. e current investigation suggests a low-priced and efficient FeOx/Pt nanocatalyst for FAO. e modification of bare Pt electrodes with FeOx is intended to utilize a cheap (earth-abundant) modifier (Fe species) to boost the catalytic activity of conventional Pt anodes toward FAO. Generally, the utilization of transition metal and/or transition metal oxides is sought to mediate the mechanism of FAO to facilitate the charge transfer and ultimately improve the reaction kinetics. Interestingly, with a proper activation, a reasonable mitigation for CO poisoning was afforded while sustaining an improved catalytic performance toward FAO.

Materials and Pretreatment.
All chemicals utilized in this investigation were of analytical grades and used without prior treatments. Doubled distilled water was used to rinse the electrodes and to prepare the involved aqueous solutions. Iron (II) sulfate heptahydrate (FeSO 4 .7H 2 O) and sodium hydroxide pellets were purchased from Riedel-de Haen and Sigma-Aldrich, respectively. Polycrystalline platinum (Pt: d � 3.0 mm), gold (Au: d � 3.0 mm), and glassy carbon (GC: d � 6.0 mm) electrodes served as the working electrodes, while an Ag/AgCl/KCl (sat.) and a Pt spiral wire were used as reference and counter electrodes, respectively. All potentials will next be read in reference to the Ag/AgCl/KCl (sat.) electrode. Conventional cleaning treatments were applied to clean the Pt, Au, and GC electrodes which were mechanically polished with aqueous slurries of fine alumina powder and then sonicated and washed with doubly distilled water [31]. Moreover, the Pt electrode was electrochemically pretreated in 0.5 M H 2 SO 4 solution by cyclic potential between −0.2 and 1.3 V at 100 mV·s −1 until obtaining the characteristic cyclic voltammogram (CV) of a clean Pt surface. e Au electrode was similarly pretreated in 0.5 M H 2 SO 4 solution in the range from −0.2 to 1.5 V at 100 mV·s −1 for 10 min or until the CV characteristic for a clean poly-Au electrode was obtained.

Catalyst Preparation.
e electrodeposition of iron was pursued in 0.02 mole·L −1 FeSO 4 .7H 2 O solution by potential cycling technique between −0.855 and −1.205 V at a scan rate of 100 mV·s −1 (only two cycles were employed) [32]. e deposited iron was next subjected to an activation at constant potentials (−0.5, −0.9, and −1.3 V) for 10 min in 0.2 mole·L −1 NaOH solution. e catalyst was simply abbreviated based on the involved catalytic ingredients. For example, the FeOx/Pt catalyst indicates the electrodeposition of iron onto a poly-Pt electrode where the addition of a prefix "a-" to the catalyst's name outlines the activation process. e letter "x" may vary from 0 (for metallic Fe) to 1.5 (for Fe 2 O 3 ).

Electrochemical Characterization.
All catalysts were characterized electrochemically before and after modification with FeOx by measuring their characteristic CVs in 0.5 M H 2 SO 4 (Figure 1), where the corresponding real surface areas of the FeOx/Pt catalysts could be estimated assuming a reference value of 210 µC·cm −2 for H ads/des [33]. e electrocatalytic activity of the prepared catalysts toward FAO was examined in 0.3 M FA (pH � 3.5), where the pH was adjusted using a dilute solution of sodium hydroxide. Electrochemical and impedance measurements were performed at room temperature (25 ± 1°C) in a twocompartment three-electrode Pyrex glass cell (homemade) using an EG&G potentiostat (model 273) operated with E-Chem 270 software. Current densities were calculated for the FeOx/Pt catalysts on the basis of the real surface areas of Pt in the catalyst.

Materials Characterization.
In the meanwhile, the catalysts were inspected morphologically by field-emission scanning electron microscopy (FE-SEM, Zeiss Ultra 60) at an acceleration voltage of 8 kV and a working distance of 2.8-3.2 mm. e microscope was fitted with an energydispersive X-ray spectroscope (EDX) that reported the elemental composition of the catalyst. Structurally, the catalysts were analyzed using a glancing angle (θ � 5°) X-ray diffractometer (XRD, PANalytical, Empyrean) operated with Cu target (λ � 1.54Å), where 2θ varied from 20-120°at a scan rate of 29°s −1 . is associated the popular peaks for hydrogen adsorption and desorption (H ads / des ) that appeared between −0.2 and 0.1 V. ese peaks appeared split to distinguish the Pt (l00) (A1: at a lower potential) and the Pt (111) (A2: at a higher potential) crystal facets of the bare poly-Pt substrate. e same electrochemical features of Pt retained in the whole investigated the set of FeOx/Pt catalysts (before and after activation-curves B-E of Figure 1(a)). is, interestingly, reflected the partial coverage of the nano-FeOx on the Pt surface that ensures the surface exposure of Pt to the electrolyte (Pt is the base substrate for FA adsorption). Few more things may also be noticed upon the comparison of curves B-E with curve A in Figure 1(a).

Electrochemical Characterization.
(i) A new redox peak at 0.4-0.6 V appeared in Figure  1(a) (curves B-E) after the deposition of nano-FeOx which confirmed the successful deposition of Fe species [34]. is redox couple may likely be assigned to the Fe/Fe 2+ transformation [32].
(ii) e intensities of Pt ⟶ PtO, PtO ⟶ Pt, and H ads peaks (curve A) decreased with the nano-FeOx deposition (curve B) which agreed consistently with the shrinkage of the real Pt surface area. Consistently, the overall charge consumed in the H ads (A1 + A2) peaks of the FeOx/Pt catalyst decreased (∼21%) but the ratio of charges consumed in the H ads peaks (A1 : A2) at the Pt (100) and Pt (111) facets remained almost unchanged. is implies that nano-FeOx was equally deposited at both facets. Table 1   PtO ⟶ Pt (A)). We should emphasize here that calculations with the PtO ⟶ Pt (A) reduction peak always overestimate the real surface area of Pt as it sometimes overlaps the reduction peaks of dissolved oxygen and iron oxides. (iii) e data in Table 1  We paid attention to the phenomenon of surface reconstruction of Pt facets because it might impact strongly the catalytic efficiency of FAO. Grozovski et al. reported recently that Pt (100) domains of Pt nanoparticles were highly active toward FAO and CO poisoning more than the Pt (111) domains [35]. In addition, the modification of the Pt substrate with FeOx enriches the surface with hydroxyl groups that boost the indirect dehydration pathway of FAO [34].
On the contrary, Figures 1(b) and 1(c) track a similar evolution for the FeOx-modified catalysts (in an acidic medium as in Figure 1(a)) but on Au and GC substrates, respectively. We aimed from this inspection to evaluate the role of the substrate and nano-FeOx on the catalytic efficiency of FAO (see later this impact). Figure 1(b) shows clearly the typical characteristic electrochemical response of polycrystalline Au substrates in acidic media for all catalysts with the surface oxidation at the Au (110) and Au (111) facets of poly-Au substrate at ca. 1.2-1.4 V and the subsequent reduction at ca. 0.9 V [36,37]. e deposition of nano-FeOx resulted in the appearance of a new redox couple at ca. 0.47 V (anodic peak) and 0.39 V (cathodic peak) whose peak intensity depended on the activation potential [38]. e upper inset of Figure 1(b) that magnifies the surface oxidation of poly-Au substrate indicates the preferential deposition of nano-FeOx on the Au (110) with a major reconstruction prevailing the Au (111) orientation. e subsequent activation (all cases) retained mostly the domination of the Au (111) orientation. e intensity of the reduction peak at 0.9 V decreased with the nano-FeOx deposition which inferred its successful deposition. e activation at −0.5 V was odd (if compared to other activation voltages) where all of its faradaic processes appeared enhanced. For the FeOx/GC catalyst, a redox couple for Fe species was obvious at ca. 0.64 V (anodic peak) and 0.21 V (cathodic peak) whose peak intensities decreased upon activation (Figure 1(c)). e activation at −0.5 V resulted in the appearance of a second anodic peak at ∼80 mV that probably recommends the existence of iron in two different oxidation states. We were curious, therefore, to inspect the impact of activating the catalyst at −0.5 V morphologically and compositionally.

Material Characterization.
Morphologically, the FeOx/ Pt catalyst was inspected before and after activation at −0.5 V (Figure 2). It seems nano-FeOx was deposited mostly agglomerated in a sponge-like structure with few nanowires (ca. 20 nm in average diameter and 77 nm in average length, respectively). However, the agglomeration was not homogeneous along the entire Pt surface. Interestingly, upon activation, the agglomeration decreased and many nanowires appeared individually spanning the entire Pt surface.  Figure 3 confirms the successful deposition of all relevant components of the catalysts and estimates their relative proportions [39]. One may easily notice, with activation, the relative increase of the oxygen and Pt contents that accompanied a corresponding decrease of the Fe content. Recalling the discrepancy of the surface area measurements utilizing the H ads and PtO ⟶ Pt peaks in Figure 1(a) (see also Table 1) which dealt principally with surface measurements may open a plausible vision for the activation. Evoking that EDX is a bulk analysis (monitoring the composition along a depth in microns), we expect that activation at −0.5 V boosted the surface (Fe 2+ /Fe 3+ ) oxidation and the inward oxygen diffusion along the Pt surface. Meanwhile, it inducted a partial dissolution of iron specious that in solution may get readsorbed to subject Fe 3+ for a reduction in the cathodic-going scan (Figure 1(a)) within the same potential domain of PtO reduction. is, with activation at −0.5 V, increases the charge consumed in the reduction peak at 0.6 V while maintaining almost the same surface area of Pt (a minor change was noticed from the H ads measurements). e XRD analysis (Figure 4) failed to detect nano-FeOx rather to assign its crystal structure perhaps because of the low detection limit of the instrument or to the existence of nano-FeOx in a minute amount at the surface. Even, glancing the incident radiation at a small angle did not help. However, important information could be revealed by comparing the different diffraction peaks of bare Pt substrate. Basically, the Pt spectra reflected the diffractions at 2θ of 39.8, 46.3, 67.5, and 81.3°of the Pt (111), (200), (220), and (311) planes, respectively, but with different ratios (PDF card: 00-004-0802) [40]. It seems that nano-FeOx was preferentially deposited onto the Pt (111) plane and activation at −0.5 V inspired a surface reconstruction prevailing the Pt (200) diffraction plane.

Electrocatalysis of FAO.
e catalytic performance of catalysts was investigated by measuring the CVs of FAO in an aqueous solution of 0.3 M FA (pH 3.5) ( Figure 5). Interestingly, the bare Pt electrode ( Figure 5(a)) represented the expected behavior of Pt substrates in FA solution where FAO proceeded in two parallel pathways. e first was the direct (desirable) pathway involving the dehydrogenation of FA to CO 2 at ca. 0.26 V with a peak current density of I p d . e other was the indirect (undesirable) pathway involving the chemical dehydration of FA to poisonous CO that could be oxidized at ca. 0.65 V (with a peak current density I p ind )) after getting the Pt surface hydroxylated (Pt−OH) at ca. 0.5 V. Unfortunately, the adsorption of CO on the bare Pt surface in the low potential domain inducts a critical poisoning for the Pt catalyst deactivating most of its active sites and consuming a higher overpotential in undesirable oxidation route. In the backward cathodic-going scan, most of the poisonous CO intermediate has been released to expose a clean Pt surface for FAO through the dehydrogenation pathway. erefore, the current intensity in the backward scan (I b ) increases largely. e level of poisoning or, in other words, the catalytic activity of the catalyst can be evaluated from the relative  (Table 2). is highlighted the potential role of nano-FeOx in boosting the catalytic performance of the Pt catalyst. In fact, the I p d /I b varied in a narrow domain for the whole set of investigated catalysts. However, the activation of the FeOx/Pt catalyst influenced the activity of the catalyst toward FAO depending on the activation potential. Basically, employing the activation at −0.5 V yielded the best catalytic performance in terms of the I p d /I p ind (17.4, i.e., ∼ eightfold enhancement) ratio  Table 2). On the contrary, the FeOx/Au ( Figure 5(b)) and FeOx/ GC ( Figure 5(c)) catalysts either before or after activation exhibited no catalytic activity toward FAO with the experimental conditions of this investigation. e redox peak couple observed at ca. 0.4 V corresponds to the transformation of Fe species which again confirmed the inertness of not only the bare Au and GC substrates but also the nano-FeOx toward FAO.

Stability of the Binary Catalyst.
e stability of catalysts always concerns the industrial fuel cell manufacturer exactly the same as the catalysts' efficiencies. is is, definitely, a commercial view estimating the life-time of the real FCs. In this investigation, the modification of the Pt/GC electrode with nano-FeOx was intended to improve not only the catalytic activity but also the catalytic stability of the catalyst. e stability of the FeOx/Pt catalyst was evaluated by chronoamperometry measurements (Figure 6) at a potential of 0.2 V in a continuous electrolysis experiment which lasted for 3 h. e observed decay in the current density was normal for Pt-based catalysts which resulted either from a surface Pt reconstruction accumulating more poisoning CO   intermediate or from a mechanical detachment of surface modifiers (as nano-FeOx) that inspired the catalytic enhancement at the beginning. Fortunately, even with the decay, the a-FeOx/Pt exhibited the highest activity and stability after 3 h of continuous electrolysis.

Origin of Enhancement.
e electrochemical impedance spectroscopy (EIS) is an effective and attractive technique comparing the internal charge transfer resistance (R ct ) of a given process in different catalysts [29,30]. Figure 7 shows the Nyquist plots of the investigated catalyst at open circuit potentials in 0.3 M FA (pH � 3.5) in the frequency range from 10 mHz to 100 kHz. It was obvious that just the deposition of nano-FeOx on the Pt surface was not so effective in lowering R ct . However, with the activation of the catalyst (a-FeOx/Pt) at −0.5 V, a large decrease in the semicircle diameter and, hence, R ct was observed (Table 2). is recommended a potential role for the activated nano-FeOx in mediating the reaction mechanism of FAO in the way facilitating the charge transfer. It is believed that the existence of nano-FeOx in several oxidation states was behind the

Conclusion
A facile synthesis of a binary FeOx/Pt nanoanode for formic acid electro-oxidation (FAO) was presented. e investigation confirmed the substrate's dependence of FAO where a Pt surface was necessary but gold, glassy carbon, and nano-FeOx surfaces showed a complete inertness. e FeOx/ Pt catalyst exhibited a better (∼4 times) catalytic performance than the bare Pt electrode, and activating the catalyst at −0.5 V boosted the enhancement to ∼8 times. e EIS inspection confirmed the electronic role of nano-FeOx in the catalytic enhancement of FAO in the FeOx/Pt catalyst as it succeeded to mediate the reaction with the possible oxidation states it owned in such a way facilitating the charge transfer. e catalyst's activation was conceived to induct a surface reconstruction for the Pt surface sites in such a way enriching the favorable facets for FAO.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.   Journal of Nanotechnology