Formic acid electrooxidation

Kizhakevariam N, Weaver MJ. 1994. Structure and reactivity of bimetaUic electrochemical interfaces Infrared spectroscopy studies of carbon monoxide adsorption and formic acid electrooxidation on antimony-modified Pt(lOO) and Pt(lll). Surf Sci 310 183-197. 

Chen Y-X, Heinen M, Jusys Z, Behm RJ. 2006h. Kinetics and mechanism of formic acid electrooxidation—Spectro-electrochemical studies in a novel flow cell configuration. Angew ChemIntEd 45 981-985. 

Figure 6.31. Schematic of the dual path mechanism of the formic acid electrooxidation on a Pt or Pt alloy electrocatalyst.

With both reactions occurring at the same rate, a surface composition of CjHjOj would correspond to these strongly adsorbed species. However, other compositions with noninteger i, p, and q are likely, depending on the reactivity of each site. The oxidation of these intermediates is considered difficult, resulting in electrocatalyst deactivation 195). Formation of [CHO] and [COOH] has also been postulated for the reverse reaction, that is, the electroreduction of COj 202, and for formic acid electrooxidation 201, 203,204. One should remember that species [CHO] was also proposed as a surface intermediate of alkane oxidations (type I), reacting only with difficulty (Section IV,E,1). 

Z.Z. Zhu, Z. Wang, and H.L. Li, Functional multi-walled carbon nanotube/polyaniUne composite films as supports of platinum for formic acid electrooxidation Appl. Surf. Sci., 254, 2934-2940 (2008). 

Wang, J., Chen, Y., Liu, H., Li, R., Sun, X., 2010. Synthesis of Pd nanowire networks by a simple template-free and surfactant-free method and their application in formic acid electrooxidation. Electrochem. Commun. 12, 219—222. 

At electrode potentials for the onset of formic acid electrooxidation, only slow CO/ CO isotopic exchange was observed. The kinetics were markedly slower than those observed when using solution CO rather than formic acid. Altering the potential beyond the onset of formic acid electrooxidation yielded relatively rapid, albeit incomplete, isotopic exchange. The... 

Abstract Direct liquid fuel cells for portable electronic devices are plagued by poor efficiency due to high overpotentials and accumulation of intermediates on the electrocatalyst surface. Direct formic acid fuel cells have a potential to maintain low overpotentials if the electrocatalyst is tailored to promote the direct electrooxidation pathway. Through the understanding of the structural and environmental impacts on preferential selection of the more active formic acid electrooxidation pathway, a higher performing and more stable electrocatalyst is sought. This chapter overviews the formic acid electrooxidation pathways, enhancement mechanisms, and fundamental electrochemical mechanistic studies. 

Since around 1960, the formic acid electrooxidation mechanism has been investigated, resulting in several review articles [15-18]. Formic acid electrooxidation studies have been carried out on pure metal electrodes, such as platinum (Pt) [19], palladium (Pd) [20], gold [21-23], rhodium [24, 25], and iridium [26]. Studies have also been performed on alloys, intermetallics, and adatoms. The conversion efficiency is determined by the rate of a series of steps (a) reactant adsorption, (b) electrooxidation, and (c) product desorption. The electrooxidation... 

Fig. 3.1 Catalyst-mediated formic acid electrooxidation mechanisms (a) ensemble/ third-body effect and (b) bifunctional mechanism. The catalyst atoms (open circle) commonly Ft or Pd and (filled circle) secondary metal atom...

Formic acid electrooxidation has been studied on both Pt [39-41] and Pd [42] singlecrystal surfaces. Herein we compare the work of Iwasita et al. [41] and Hoshi et al. [42] on Pt and Pd single crystals, respectively. The cyclic voltammograms were... 

Park et al. compared methanol versus formic acid electrooxidation on polycrystalline Pt and on two sizes of carbon-supported Pt (2.0 nm vs. 8.8 nm) (Fig. 3.5) [44]. The potentials were referenced to a saturated calomel electrode (SCE) (RHE, 0.242 V) in 0.05 M H2SO4 at a scan rate of 50 mV s The cyclic voltammograms were normalized to a 1 cm Pt effective area. They observed a reduction in methanol activity for particles smaller that 4 nm and an opposite effect for formic acid see Fig. 3.5. The disparity between methanol and formic acid size-dependent performance trends is due to methanol preferentially adsorbing onto three adjacent Pt atoms found on Pt(lll)-faceted surfaces during the C-H bond dissociation step... 

Formic acid adsorption onto Pt requires either multiple sites for the dehydration pathway or (Mily one to activate C-H bond for the dehydrogenation pathway [46]. The onset of formic acid electrooxidation has been shown to be effected by both Pt particle size and reactant cmicentration (Fig. 3.5B, C). The dehydration pathway is favored on both the polycrystalline and 8.8 run Pt catalyst surfaces during the forward scan, as is apparent from the low currents and high overpotentials. The higher potentials are required to form the activated hydroxyl complexes required to oxidize the passivating CO moieties to CO2, similar to methanol. The formic acid... 

Demirci investigated the degree of segregation and shifting of d-band centers by metal alloy combinations to improve the direct liquid fuel cell catalyst activity through electronic promotion of the dehydrogenation pathway [57]. He focused on Pt- and Pd-based catalyst for formic acid electrooxidation and looked at the potential impact of surface adatom adsorption of other 3d, 4d, and 5d transition metals. The criteria he imposed for improved catalytic activity on Pt and Pd... 

Controlled electrochemical experiments are designed to probe select aspects of the formic acid electrooxidation reaction as a function of material selection and/or experimental conditions. Unfortunately, the selected experimental technique employed imposes deviations from a complex three-dimensional catalyst layer used in an operational DFAFC and thus results in inconsistencies between techniques. Assuming the current-potential relationship is always directly correlated to Faraday s law for charge and CO2 production, the assessment techniques can be broken down into three general categories (1) indirect correlation, (2) desorbed product detection, and (3) direct catalyst surface analysis. 

General Electrochemical Setup. Catalytic studies to probe formic acid electrooxidation efficiencies are commonly not performed in a complex fuel cell, but using a three-electrode electrochemical cell at room temperature, consisting of a working (catalyst of interest), a counter (Pt mesh), and a reference electrode. Potentials are typically referenced against an RHE, saturated calomel electrode (SCE), or sUver/silver chloride (Ag/AgCl). 

The supporting electrolyte type and concentration of formic acid impact the observed overpotentials. The two most commonly used supporting electrolytes are either H2SO4 or HCIO4. Specific bisulfate anion adsorption onto Pt surface sites from H2SO4 adversely increases the onset potential of formic acid electrooxidation. The top of Fig. 3.8 shows an unfavorable increase in the onset potential for OHads in the anodic cycle by 0.1 V on a Pt ( 2.3 nm)/C catalyst in the presence of 0.1 M H2SO4 versus 0.1 M HCIO4 [65]. In the presence of 0.5 M formic acid, the initial response in the forward anodic sweep at potentials below 0.4 V versus SCE is... 

Wang et al. compared formic acid electrooxidation EIS Nyquist plots for Pt/C and PtPd(aUoy)/C catalyst (Fig. 3.9a, b, respectively) [12]. The results were acquired in 0.5 M formic acid and 0.5 M HCIO4 over a range of applied dc potentials from 0.1 to 0.7 V versus Ag/AgCl (-1-0.199 V vs. RHE). The Nyquist... 

Electrochemical quartz crystal microbalance. To monitor adsorbate accumulation on catalyst surfaces from formic acid electrooxidation and advance mechanistic understanding, an electrochemical quartz crystal microbalance (EQCM) can be used to simultaneously measure current and mass [22, 66, 81-83]. The dampening of the vibration frequency (A/) of an AT-cut 9 MHz piezoelectric crystal is directly proportional to mass accumulation (Am) on the catalyst surface through the Sauerbrey equation (A/ = —/o 2(jUqPq) Am/A) [84], where /o is the base... 

Fig. 3.10 Cyclic voltammetry using a electrochemical quartz crystal microbalance of formic acid electrooxidation on a polycrystalline Pt surface in 0.2 M formic acid and 0.2 M HCIO4 at 50 mV s (a) current and (b) frequency (corresponding to negative mass changes) response. The upper potential limit is sequentially increased with each subsequent cycle [66]...

Several research groups have used differential electrochemical mass spectroscopy (DBMS) to monitor product conversion during formic acid electrooxidation [2, 21, 37, 86-88]. In Fig. 3.2, the origins of the CO2 product formation pathway is investigated by using isotopically labeled formic acid [37]. 

The in situ study in electrochemical cells of the catalyst surface is challenging due to low stuface sensitivity through the electrolyte. Several surface-sensitive techniques have been employed to probe the abundance and/or state of adsorbed surface species formed during formic acid electrooxidation broadband sum frequency generation [89, 90], surface-enhanced Raman spectroscopy [21], scanning tunneling microscopy [91], and Fourier transform infrared spectroscopy [19,26,27,31,32, 41,92-99],... 

Fourier transform infrared spectroscopy (FTIR) is a powerful technique to probe real-time adsorbed surface species (reactants, intermediates, products) and solution constituents due to selected molecular dipole bond vibrations induced by tuned incident radiation [100]. FTIR has been used to study the formic acid electrooxidation reaction mechanism in situ by stepping or scanning the potential where species of interest are generated, from either high potentials where the intermediate species are completely oxidized (a clean surface, >1 V vs. RHE) or low potentials where the intermediate species approaches the coverage limit (blocked surface, <0.05 V vs. RHE) [100]. The three observed reaction intermediates for formic acid electrooxidation are linearly bonded COl, bridge-bonded COb, and bridge-bonded formate (HCOOad) with vibrational bands at 2,052-2,080 cm 1,810-1,850 cm , and 1,320 cm , respectively [27, 98]. The vibration frequencies of the adsorbates are influenced by the electronic characteristics and electrochemical potential of the electrode surface. Additional peaks of lesser intensity are observed for the water adlayer and sulfate/bisulfate at the electrode interface [27, 98]. 

The type, structure, and electron density clearly determine the reactivity of a catalyst toward formic acid electrooxidation. The catalyst characteristics that promote reactant adsorption in the CH-down orientation exhibit enhanced activity through the dehydrogenation reaction pathway. Pd catalyst initially favors the dehydrogenation pathway but suffers for 30 % activity loss within only 3 h of continuous operation due to accumulation of reaction intermediates on the surface. 

Wang X, Hu J-M, Hsing IM (2004) Electrochemical investigation of formic acid electrooxidation and its crossover through a Nafion membrane. J Electroanal Chem 562 73-80... 

Kristian N, Yu Y, Gunawan P, Xu R, Deng W, Liu X, Wang X (2009) Controlled synthesis of Pt-decorated Au nanostructure and its promoted activity toward formic acid electrooxidation. Electrochim Acta 54 4916-4924... 

Fundamental anode catalyst research is imperative for improved direct formic acid fuel cell (DFAFC) performance and stability such that an intimate understanding of the interplay between structural, morphological, and physicochemical properties is needed. The primary base catalysts found to be active for formic acid electrooxidation are either platinum (Pt) or palladium (Pd). The cyclic voltammograms in Fig. 4.1 compare the activity of carbon-supported Pt to Pd towards formic acid electrooxidation. The anodic (forward) scan, relevant to DFAFC performance, is relatively inactive on Pt/C until the applied potential... 

Table 4.1 Groupings of secondary metals having been incorporated into Pt- or Pd-based anode catalysts for formic acid electrooxidation (adapted from [11])...

Catalyst activity towards formic acid electrooxidation is strongly influenced by preparation method and nanoparticle size. As discussed in the previous chapter, the optimal sizes for Pt/C and Pd/C are 4 nm and 5.2-6.5 imi, as determined by Park et al. [14] and Zhou et al. [15], respectively. This chapter is segregated into two sections bimetallic catalysts and catalyst supports. The section on bimetallic catalysts is subdivided into adatoms, alloys, and intermetallics. 

A common method for improving formic acid electrooxidation activity is through the incorporation of foreign adatoms in sub- or monolayer coverages onto metal electrocatalyst surfaces (substrates). Adatoms are usually deposited onto the metal surface either by under potential deposition (UPD) or by irreversible adsorption [17]. The two dominant reaction enhancement mechanisms for the direct dehydrogenation pathway, as described in Sect. 3.3 of the previous chapter for formic acid electrooxidation, are the third-body and electronic effects. The type of enhancement mechanism due to adatom addition is dependent on the substrate/adatom... 

To illustrate the primary effects of adatom addition, single-crystal electrodes are discussed here. Feliu and Herrero have extensively studied formic acid electrooxidation on Pt single-crystal substrates modified with an array of various adatoms. They have established a connection between the electronegativity of the adatoms in relation to Pt and the type of active enhancement mechanism incurred as a function of adatom coverage [42]. Their results support inhibition of the indirect pathway on Pt(lll) terraces and they have demonstrated that COads formation occurs at step and defect sites. For Pt(l 11) substrates decorated with electropositive adatoms, such as Bi, Pb, Sb, and Te, the electronic enhancement is extended to the second or third Pt atom shell from the adatom. While for electronegative adatoms, in respect to Pt, the third-body effect dominates with increased coverages, such as S and Se. 

Fig. 4.2 Plot of direct formic acid fuel cell performance at 0.6 V for Pt/C anodes as a function of Pb and Sb adatom coverages. The experimental data is compared to the two formic acid electrooxidation models proposed by Leiva (solid line) electronic enhancement and (dashed line) third-body effect [29]...

Wieckowski s group has studied formic acid electrooxidation on Pt nanoparticles decorated with controlled amounts of Pd and Pd-l-Ru adatoms [41]. They reported two orders of magnitude increase in the reactivity of the Pd-decorated catalyst compared to pure Pt towards formic acid oxidation. Also, it was concluded that the impact of COads on the Pt/Pd catalyst through the dual pathway mechanism is much lower even though the potential required to remove COads from the surface was the highest. 

Bi et al. boosted the performance of Pt nanoparticles towards formic acid electrooxidation by depositing sub-monolayer Au clusters [36]. The modified Pt nanoparticles exhibited a 23-fold increase in specific activity. This enhancement in... 

Bismuth has attracted significant interest as a Pt/C modifier for formic acid electrooxidation [21, 24, 26, 27]. A wide range of stable and well-characterized electrode surfaces modified by irreversible Bi adatom adsorption on Pt have been reported in the literature for a range of Bi coverages 6). Chen et al. have explored Bi adatom decoration on 81 nm tetrahexahedral Pt nanoparticles that while composed of (100) and (110) facets that are the least active for formic acid electrooxidation, they are boimd by 730 and vicinal high-index facets that are extremely active [18]. They have measured current densities of 10 mA cm for Bi coverages up to 0.9 at 0.4 V in 0.25 M formic acid and 0.5 M H2SO4 solution see Fig. 4.4. They also showed steady-state activity at 0.3 V of 2.8 mA cm after 1 min vs. 0.0003 mA cm for the non-modified Pt baseline. 

Kim et al. [24] reported a detailed analysis on formic acid electrooxidation on 3 nm Pt/C modified by irreversible adsorption of Bi. They ascribed the enhancement in catalytic activity to promotion of the direct pathway, which is dependent on the oxidation state and coverage of the Bi. For Bi coverage on Pt above 0.54, the oxidation rate of formic acid increased by a factor of 8. The amoxmts of CO and... 

Fig. 4.5 Anodic current density acquired during a cyclic voltammogram at 0.2 V vs. RHE for 3.4 nm and 2.4 mn Pt/C as a function of Bi coverage, in 0.1 M formic acid and 0.1 M HCIO4. Onset potential for formic acid electrooxidation of Bi-modified Pt electrodes (unpublished work from...

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