Electrocatalytic Activities towards the Electrochemical Oxidation of Formic Acid and Oxygen Reduction Reactions over Bimetallic , Trimetallic and Core – Shell-Structured Pd-Based Materials

The structural design of nanosized electrocatalysts is extremely important for cathodic oxygen reduction reactions (ORR) and anodic oxidation reactions in small organic compounds in direct fuel cells. While Pt is still the most commonly used electrode material for ORR, the Pd electrocatalyst is a promising alternative to Pt, because it exhibits much higher electrocatalytic activity towards formic acid electrooxidation, and the electrocatalytic activity of ORR on the Pd electrode is the higher than that of all other precious metals, except for Pt. In addition, the mass activity of Pt in a core–shell structure for ORR can be improved significantly by using Pd and Pd-based materials as core materials. Herein, we review various nanoscale Pd-based bimetallic, trimetallic and core–shell electrocatalysts for formic acid oxidation and ORR of polymer electrolyte fuel cells (PEFCs). This review paper is separated into two major topics: the electrocatalytic activity towards formic acid oxidation over various Pd-based electrocatalysts, and the activity of ORR on Pd-based materials and Pd core–Pt shell structures.


Introduction
Owing to their high electrocatalytic activity, Pt-based electrocatalysts have been extensively investigated as electrode materials for oxygen reduction reaction (ORR) and the electrooxidation of small organic compounds in polymer electrolyte fuel cells (PEFCs), which are potential alternatives to fossil fuel-based devices [1].However, Pt-based electrocatalysts suffer from several limitations.Since Pt is costly, rarely available and has high overpotential for ORR and the oxidation of small organic molecules, it is imperative to develop an electrocatalyst with little or no Pt for the commercialization of both PEFCs and direct methanol fuel cells (DMFCs).
In DMFCs, methanol in aqueous solution has an ideal energy density (4900 Wh L −1 ), however, the open circuit potential (OCP) is lower than that in PEFCs, due to the higher standard oxidation potential of methanol and the higher overpotential for electrocatalytic methanol oxidation.In addition, formic acid as a fuel shows a smaller crossover than methanol when Nafion ® is used as an ionomer, indicating that formic acid allows the use of electrolytes with high concentrations.Moreover, the standard oxidation potential for formic acid is approximately −0.25 V versus the reversible hydrogen electrode (RHE) [2], which is lower than that of the oxidation of methanol and H 2 .The theoretical electromotive force (emf) and OCP of direct formic acid fuel cells (~1.48 V versus RHE) is higher than that of PEFCs (1.23 V versus RHE) [2][3][4].Therefore, direct formic acid fuel cells are highly promising, and it is of great significance to develop an electrocatalyst with high activity towards the electrooxidation of formic acid.
Pt shows lower catalytic activity towards ORR in the presence of methanol (mixed in the electrolyte by methanol crossover) in acidic media.Therefore, various efforts have been made to develop electrocatalysts with little or no Pt for the oxidation of small organic compounds and ORR in fuel cells [5,6].Pd-based electrocatalysts are potential alternatives to Pt for ORR under both acidic and alkaline conditions because Pd has similar physicochemical characteristics (e.g., the crystal structure and being a group 10 element in the periodic table) to those of Pt.With the exception of Pt, Pd exhibits the highest electrocatalytic activity (among pure metals) towards hydrogen evolution reactions and ORR.In addition, the ORR kinetics of Pd-based electrocatalysts are not significantly affected by methanol because of their high tolerance to methanol under acidic conditions [7][8][9].While Pd is currently more expensive than Pt (Pd: 1232.5 USD/ozt and Pt: 796.66 USD/ozt on 5th December 2018), the prices of precious metals fluctuate by nature.Historically, Pt has been more expensive than Pd, therefore, we still believe that exploring Pt-free catalysts is valuable.In addition, to prepare metal nanoparticles, metal sources should be used as precursors.Pt precursors are still more expensive than their Pd analogues at most chemical vendors, such as Sigma-Aldrich.For example, the prices of Pd(II) acetylacetonate (99%) and Pt(II) acetylacetonate (99.98%) are currently 85 and 170 USD/g, respectively, at Sigma-Aldrich.
In the case of Pd electrocatalysts for ORR, under acidic conditions, the electrocatalytic activity of ORR over Pd appear to be lower than that of Pt because of a non-ideal d-band center value, which produces strongly absorbed oxygen during the initial ORR process (as mentioned in Section 3.1).Therefore, to realize highly active Pd-based electrocatalysts for ORR, it is significant to control their crystal structures and electronic conditions.On the other hand, in order to prepare Pt-based electrocatalysts with lower d-band centers and hence higher ORR activity, three-dimensional transition metals such as Fe, Co, Ni and Cu are used as alloying materials to modify the electronic state and crystal structure of Pt [10][11][12].This suggests that by modifying the electronic state and crystal structure of Pd with transition metal electrocatalysts, with activity comparable to that of Pt, a possibility that highly active Pd-based electrocatalysts may be achieved.
So far, there are several reviews on the activity of Pd-and Pt-based materials for PEFC catalysts [5,6,[13][14][15][16][17][18][19][20].Antolini has reported on the ORR activity of alloys and so-called ordered intermetallic compounds, which are atomically ordered stoichiometrically, unlike the random structure of alloy compounds.Ordered intermetallic compounds can be expected to realize a higher electrocatalytic activity when compared with random alloy compounds due to their long-term chemical ordering, resulting in intermetallic compounds that can form homogeneously active sites.In fact, most parts of ordered intermetallic compounds, such as Pt x Co y , Pt x Fe y , Pt x Ni y and Pd x Fe y , can exhibit a higher electrocatalytic activity towards ORR, compared with their random alloy compounds [14].Erikson has reviewed recent progress on the electrocatalytic activity towards ORR over Pd-based materials.Size, substrate, morphology and the crystal structure effect have been reviewed adequately [15].
In this review, we focus on the electrochemical oxidation of small organic compounds over Ptand Pd-based catalysts and compare the activities of Pd and various Pd-based electrocatalysts in terms of the electrooxidation of formic acid in Section 2. In Section 3, the ORR activities of Pd and Pd-based catalysts under both alkaline and acidic conditions are summarized.High-performance Pt-decorated Pd and Pd bimetallic alloy electrocatalysts for ORR are also discussed in this section.Effective alloying with intermetallic compounds and a core-shell structure can reduce Pd and Pt loading and increase electrocatalytic activity.

Electrocatalytic Activity for the Oxidation of Small Organic Compounds
Direct fuel cells, which use small organic compounds as fuel, have gained significant attention as next-generation energy conversion systems, owing to their high energy density and use of low-toxicity fuels.The electrochemical oxidation of small organic compounds can be catalyzed by Pt, Pd and their bimetallic compounds.Pt is widely used to catalyze the electrooxidation of methanol and ethanol in acidic media, owing to its strong adsorption and dissociation of methanol and ethanol [13,21].Cai et al., Bianchini et al., Ma et al. and Xie et al. have reviewed the catalytic activities of Pt-and Pd-based catalysts for the electrooxidation of ethanol in alkaline media and the electrooxidation of formic acid and the corresponding reaction mechanisms [13,16,17,19].The oxidation reaction proceeds via multiple pathways with direct and indirect processes.However, the methanol and ethanol oxidation reactions are kinetically limited because of the slow dissociative adsorption of methanol and ethanol and the formation of a CO ad species on the surface of Pt [22].Therefore, in the case of a Pt electrocatalyst, a high overpotential is required for the oxidation of ethanol and methanol.The onset potential for methanol oxidation over Pt is approximately 0.5 V versus RHE [21,23], because of which the effective voltage for DMFCs is significantly lower than that of hydrogen fuel cells (FCs).Kodiyath and coworkers showed that the electrocatalytic activity of intermetallic Pt 3 Ta towards ethanol oxidation is superior to that of pure Pt.The reaction mechanism of ethanol oxidation over Pt 3 Ta has been elucidated using potential-dependent in situ infrared reflection-absorption spectroscopy (IRRAS).It has been reported that ordered intermetallic Pt 3 Ta nanocrystals can promote not only C-C cleavage, but also the conversion of ethanol to CO 2 [24].
Since formic acid can be easily oxidized to CO 2 at low overpotentials, the oxidation of formic acid over Pt has also been investigated extensively.The electrooxidation of formic acid can proceed via two reaction pathways: The dehydrogenation and dehydration pathways.IRRAS studies have revealed that carbon monoxide is formed as an intermediate during the electrodecomposition of formic acid (because of its dehydration) [25][26][27].The direct two-electron pathway for the conversion of formic acid to CO 2 proceeds as a minor pathway on the surface of Pt electrocatalysts [27,28].Wang and coworkers proposed a reaction pathway for the electrochemical oxidation of formic acid over Pt (111), based on Vanderbilt-type ultra-soft pseudopotential calculations in both the gas and liquid phases [29,30].

Electrooxidation of Formic Acid on Pt-Based Electrocatalysts
Formic acid has gained immense attention as a fuel for direct fuel cells because the current density for the electrooxidation of formic acid over Pd and Pd-based materials is much higher than that of Pt-based electrocatalysts [6,31], because of their catalytically different pathways.The direct pathway for the oxidation of formic acid produces CO 2 (HCOOH → CO 2 + 2H + + 2e − ).In the CO pathway (HCOOH → CO + H 2 O), the dehydration of formic acid produces CO as an intermediate, which then oxidizes to CO It is well-known that Pd and Pt are the main electrocatalysts for the oxidation of formic acid.The electrooxidation of formic acid over Pt proceeds mainly via an indirect pathway, which involves the formation of CO via the dehydration reaction of formic acid.
The CO pathway proceeds preferentially when Pt is used as the electrocatalyst and the intermediate CO formed by the CO pathway is adsorbed on the Pt surface.The water activation reaction (2), which is necessary to remove CO from the surface of Pt, is difficult to proceed and hence reduces the electrocatalytic activity of Pt.An ideal formic acid oxidation electrocatalyst should function via the dehydrogenation pathway (direct pathway) while avoiding its poisoning.Till date, several efforts have been made to improve the direct pathway electrocatalytic oxidation of formic acid over Pt-based compounds.For example, CO 2 can be formed via the direct pathway by adding atomic Au to the Pt lattice [32].However, it is difficult to completely avoid the indirect pathway.
Abruña and coworkers have shown that ordered intermetallic PtBi and PtPb, which have a NiAs-type structure, exhibit electrocatalytic formic acid oxidation superior to that shown by Pt-Ru, Pd and Pt [33,34].This can be attributed to the CO tolerance of intermetallic PtPb [35].Moreover, CO is not formed on the surface of intermetallic PtPb during the electrooxidation of formic acid and methanol.This indicates that intermetallic PtPb electrooxidizes formic acid and methanol via the direct pathway, in which CO is not formed [36].These results indicate that the electrooxidation of formic acid over Pt and intermetallic PtPb catalysts proceeds via different reaction mechanisms (i.e., the direct pathway mainly proceeds on the intermetallic surface).In addition, ordered intermetallic PtPb and PtBi show enhanced electrocatalytic activity towards the oxidation of small organic compounds in alkaline media, and Pb and Bi in PtPb and PtBi can contribute to the electrooxidation of CO by forming Bi-(OH) ad [37].

Electrooxidation of Formic Acid on Pd Electrocatalysts
Since the physicochemical characteristics of Pd (such as the electronic properties and crystal structure) are similar to those of Pt, the electrooxidation of formic acid follows the same pathway on the surface of both Pd and Pt.However, Pd and Pd-based electrocatalysts show significantly higher catalytic activity towards the electrooxidation of formic acid than Pt electrocatalysts.This is because the electrooxidation of formic acid over Pd proceeds mainly via the direct pathway, with the CO pathway acting as the minor reaction [38,39].The intermediate CO produced upon the dehydration of formic acid reduces the electrocatalytic activity for formic acid oxidation due to CO poisoning of the Pt and Pd surfaces.CO formation by dehydration of formic acid proceeded via the carboxyl pathway (i.e., COOH*).Mavrikakis et al., calculated free-energy diagrams of formic acid oxidation on Pt and Pd electrocatalysts, including the carboxyl pathway, formate pathway (i.e., HCOO*) and indirect carboxyl-mediated mechanism [40].According to their calculations, the reaction mechanisms via formate and carboxyl have very similar onset potentials to Pd (111), in agreement with other reported results [41].On the other hand, the free-energy on Pt (111) is significantly higher for the formate pathway than the carboxyl pathway.CO poisoning on the Pd surface is less than that on the Pt surface because the HCOO* pathway can contribute to the overall activity.In addition, Melchionna has reported on Pd-based core-shell catalysts, which underline the possibility of constructing reversible catalysts for CO 2 reduction/HCOOH oxidation [42].Therefore, Pd is a promising catalyst for the oxidation of formic acid.However, as discussed previously, a small amount of formic acid is still oxidized via the indirect pathway [43].
Recently, Cai and coworkers proposed a novel reaction mechanism for the electrooxidation of formic acid over Pd that is different from the pathways discussed above (direct and CO pathways).They proposed (on the basis of high-sensitivity in situ surface-enhanced IR spectroscopy results) that CO is formed by the electrochemical reduction of CO 2 , rather than the direct dehydration of formic acid on the surface of Pd at potentials where adsorbed H species exist (Figure 1) [43][44][45].The formation of CO inactivates Pd catalysts, therefore, it is important to develop Pd-based electrocatalysts with a high CO tolerance.
formic acid over Pd that is different from the pathways discussed above (direct and CO pathways).They proposed (on the basis of high-sensitivity in situ surface-enhanced IR spectroscopy results) that CO is formed by the electrochemical reduction of CO2, rather than the direct dehydration of formic acid on the surface of Pd at potentials where adsorbed H species exist (Figure 1) [43][44][45].The formation of CO inactivates Pd catalysts, therefore, it is important to develop Pd-based electrocatalysts with a high CO tolerance.

Effect of Electronic State, Morphology and Structure on the Catalytic Activity of Pd towards Formic Acid Oxidation
Since Pd exhibits the highest electrocatalytic activity (among metals) towards the electrooxidation of formic acid, various efforts have been made to develop Pd-based compounds (Table 1).Co [47], Ni [48,49], Cu [49,50], Pb [31] and HoOx [51] have been widely used to modify Pd  Since Pd exhibits the highest electrocatalytic activity (among metals) towards the electrooxidation of formic acid, various efforts have been made to develop Pd-based compounds (Table 1).Co [47], Ni [48,49], Cu [49,50], Pb [31] and HoO x [51] have been widely used to modify Pd in order to improve its electrocatalytic activity towards formic acid oxidation (Table 1).Pd-based bi-and tri-metallic compounds show higher electrocatalytic activity towards formic acid oxidation than pure Pd.The electrocatalytic activity (towards formic acid oxidation) of Pd increases with a decrease in CO oxidation potential.For example, the dealloying of ordered intermetallic Pd 3 Pb and Pd 1.1 /WO 2.72 , which exhibit relatively high electrocatalytic activities towards formic acid oxidation, results in a lower peak potential for CO stripping.On the other hand, in the case of Pt-based bimetallic compounds, if the d-band center values are higher than those of pure Pt, the CO adsorption energies on their surface are lower than that on the surface of pure Pt [49,52].Hence, Au core-Pd shell (Au@Pd) structured nanoparticles (core-shell structures, such as a T core-M shell structure, will hereinafter be denoted as "T@M"), which have a high d-band center value [49], exhibit high CO tolerance, indicating that Au makes Pd CO tolerant [49,53].In fact, density functional theory (DFT) calculations have revealed that the CO adsorption energy on Pd/Au (111) surfaces is significantly lower than that on pure Pd [54].Therefore, bimetallic Pd-Au shows high activity towards formic acid oxidation because of its high CO tolerance.However, Au@Pd bimetallic compounds hardly show enhanced catalytic activity [49,53].Therefore, the d-band center values (i.e., CO adsorption energy on Pd) of Pd-based catalysts should be optimized.Hu et al. prepared Pd-based bi-and tri-metallic compounds, such as Ru@Pd, PdNi, PdCu, PdNiCu and Au@Pd, and evaluated their electrocatalytic activity towards the electrooxidation of formic acid [49,60].In addition, they also investigated the effect of the d-band center position (as determined by X-ray photoelectron spectroscopy) on their activity towards the electrooxidation of formic acid (Figure 2).In order to proceed via the direct pathway, HCOOad should be formed on Pd (i.e., HCOO-Pd).Therefore, the strength of the Pd-O bond is very important to realize the high catalytic activities of Pd-based catalysts.When the d-band center of Pd is away from the Fermi level, the bond between Pd and the intermediate becomes weak.On the other hand, when the d-band center is significantly close to the Fermi level, the bond between Pd and the intermediate becomes strong.Therefore, Pd shows electrochemical activity towards formic acid oxidation at the optimum d-band center position.
It is well-known that the electrocatalytic activities (towards ORR and the oxidation of organic compounds) of Pd-and Pt-based materials are significantly affected not only by their electronic states but also by their crystal structures.For example, Pt3Ni (111) shows a much higher electrocatalytic activity towards ORR than Pt3Ni (100) and Pt3Ni (110).Interestingly, Pt3Ni (110), which has a d-band center value is −2.70 eV, shows an ORR activity 2.5 times higher than that of Pt (111), which has a similar d-band center value (−2.76 eV) [61].This indicates that the electrochemical activities of Pdand Pt-based catalysts are affected more by their secondary material and surface structures than their electronic states.Therefore, in order to develop Pd-based materials with high electrochemical activities, it is very important to create ideal surfaces and morphologies.
Electrocatalysts with morphology-dependent activity have been developed as novel active electrocatalysts.Yang and coworkers developed Cu-Pd alloy multipods and evaluated their electrocatalytic activity towards formic acid oxidation [32].These multipods showed high catalytic activities, as illustrated in Figure 3, because of the presence of a large number of active edges/corner atoms in their unique multipod structure.The Cu-Pd alloy multipods were prepared by the galvanic replacement reaction between presynthesized Cu-template nanoparticles and Pd 2+ ion precursors.The electrocatalytic activities and electronic states of these Cu-Pd alloy multipods were compared with those of the Cu-Pd and Pd nanoparticles.The Cu-Pd alloy multipods showed a lower d-band center than the Cu-Pd and Pd nanoparticles, indicating that they show a higher electrocatalytic activity towards both the electrooxidation of formic acid and ORR, owing to their unique structure.The procedure proposed by Yang and coworkers for the synthesis of these multipods can be a In order to proceed via the direct pathway, HCOO ad should be formed on Pd (i.e., HCOO-Pd).Therefore, the strength of the Pd-O bond is very important to realize the high catalytic activities of Pd-based catalysts.When the d-band center of Pd is away from the Fermi level, the bond between Pd and the intermediate becomes weak.On the other hand, when the d-band center is significantly close to the Fermi level, the bond between Pd and the intermediate becomes strong.Therefore, Pd shows electrochemical activity towards formic acid oxidation at the optimum d-band center position.
It is well-known that the electrocatalytic activities (towards ORR and the oxidation of organic compounds) of Pd-and Pt-based materials are significantly affected not only by their electronic states but also by their crystal structures.For example, Pt 3 Ni (111) shows a much higher electrocatalytic activity towards ORR than Pt 3 Ni (100) and Pt 3 Ni (110).Interestingly, Pt 3 Ni ( 110), which has a d-band center value is −2.70 eV, shows an ORR activity 2.5 times higher than that of Pt (111), which has a similar d-band center value (−2.76 eV) [61].This indicates that the electrochemical activities of Pdand Pt-based catalysts are affected more by their secondary material and surface structures than their electronic states.Therefore, in order to develop Pd-based materials with high electrochemical activities, it is very important to create ideal surfaces and morphologies.
Electrocatalysts with morphology-dependent activity have been developed as novel active electrocatalysts.Yang and coworkers developed Cu-Pd alloy multipods and evaluated their electrocatalytic activity towards formic acid oxidation [32].These multipods showed high catalytic activities, as illustrated in Figure 3, because of the presence of a large number of active edges/corner atoms in their unique multipod structure.The Cu-Pd alloy multipods were prepared by the galvanic replacement reaction between presynthesized Cu-template nanoparticles and Pd 2+ ion precursors.
The electrocatalytic activities and electronic states of these Cu-Pd alloy multipods were compared with those of the Cu-Pd and Pd nanoparticles.The Cu-Pd alloy multipods showed a lower d-band center than the Cu-Pd and Pd nanoparticles, indicating that they show a higher electrocatalytic activity towards both the electrooxidation of formic acid and ORR, owing to their unique structure.The procedure proposed by Yang and coworkers for the synthesis of these multipods can be a promising approach to develop other bimetallic Pd-and Pt-based alloy multipods.Catalysts for formic acid electrooxidation should possess a high electrocatalytic activity and CO tolerance.In the case of Pt-based materials, structurally ordered intermetallic compounds, such as PtPb, PtBi, Pt3Ti, Pt3Hf and Pt3Ta, exhibit a high CO tolerance because of their electronic efficiency, including the d-band center [52,64,65].Herein, we have reviewed ordered intermetallic Pd3Pb electrocatalysts for formic acid oxidation [31].Figure 4 shows the electrocatalytic activity towards the formic acid oxidation of dealloyed ordered intermetallic Pd3Pb.The electrochemical treatment of intermetallic Pd3Pb results in the formation of an ordered intermetallic Pd3Pb core with a Pd-enriched surface (i.e., Pd3Pb@Pd).The first and second oxidation peaks for Pt and Pd correspond to the direct and CO oxidation pathways, respectively.On the other hand, no secondary oxidation peak was observed in the case of ordered intermetallic Pd3Pb, indicating that CO2 is formed directly on the surface of intermetallic Pd3Pb, without dehydration and CO adsorption.The CO binding energy of Pd3Pb (111) (1.05 eV) is lower than that of Pd (111) (1.29 eV).This is consistent with the predictions made on the basis of the measured and calculated d-band center values.This suggests that ordered intermetallic Pd3Pb/CB is more tolerant to CO adsorption and hence shows a higher formic acid Over the past few years, electrochemically dealloyed Pt-based bimetallic compounds have gained immense attention as ORR electrocatalysts.For example, the removal of Cu (by electrochemical treatment) from Pt-Cu results in the formation of a Pt-rich core-shell nanostructure, which shows a higher ORR activity than both nontreated Pt-Cu and Pt nanoparticles [52,54].A three to five layered Pt-rich shell, which can be obtained by a potential cycling of up to 5000 cycles, can enhance the ORR activity of Pt-based core-shell nanostructures [62,63].The electrochemical treatment used for dealloying the secondary material in Pt-based bimetallic alloy catalysts plays an important role in enhancing their electrocatalytic activities.
Catalysts for formic acid electrooxidation should possess a high electrocatalytic activity and CO tolerance.In the case of Pt-based materials, structurally ordered intermetallic compounds, such as PtPb, PtBi, Pt 3 Ti, Pt 3 Hf and Pt 3 Ta, exhibit a high CO tolerance because of their electronic efficiency, including the d-band center [52,64,65].Herein, we have reviewed ordered intermetallic Pd 3 Pb electrocatalysts for formic acid oxidation [31].Figure 4 shows the electrocatalytic activity towards the formic acid oxidation of dealloyed ordered intermetallic Pd 3 Pb.The electrochemical treatment of intermetallic Pd 3 Pb results in the formation of an ordered intermetallic Pd 3 Pb core with a Pd-enriched surface (i.e., Pd 3 Pb@Pd).The first and second oxidation peaks for Pt and Pd correspond to the direct and CO oxidation pathways, respectively.On the other hand, no secondary oxidation peak was observed in the case of ordered intermetallic Pd 3 Pb, indicating that CO 2 is formed directly on the surface of intermetallic Pd 3 Pb, without dehydration and CO adsorption.The CO binding energy of Pd 3 Pb (111) (1.05 eV) is lower than that of Pd (111) (1.29 eV).This is consistent with the predictions made on the basis of the measured and calculated d-band center values.This suggests that ordered intermetallic Pd 3 Pb/CB is more tolerant to CO adsorption and hence shows a higher formic acid electrooxidation than Pd NPs/CB.The higher electrocatalytic activity of intermetallic Pd 3 Pb can be attributed to the electrochemical dealloying process.In order to improve the electrochemical activity and durability of Pt-and Pd-based catalysts, electrochemically stable supporting materials, which modify the electronic structure of the catalysts, are extensively used [66,67].However, when carbon nanotubes (CNTs) are used as the supporting material for Pd nanoparticles (Pd/CNT), no significant improvement is observed in formic acid electrooxidation and CO stripping [68].
Recently, Raj et al., developed a novel electrocatalyst using unzipped (Uz) single-walled CNTs (SWCNTs) as a support for Pd nanoparticles.The unzipped SWCNTs were prepared via a one-pot synthesis method [69] without using harsh oxidizing agents.The Uz-CNT-Pd catalyst that was prepared showed a significantly higher formic acid electrochemical oxidation activity than the SWCNT/Pd and unsupported Pd nanoparticles (Figure 5).The onset potential for CO electrooxidation over Uz-CNT-Pd was −80 mV lower than that over Pd nanoparticles.This indicates that CO oxidation was facile in the case of Uz-CNT-Pd.In addition, the electrochemical surface area (as calculated from the magnitude of the peak area for CO oxidation) of Uz-CNT-Pd was approximately 2.5 times higher than that of the Pd nanoparticles.The electronic interaction between In order to improve the electrochemical activity and durability of Pt-and Pd-based catalysts, electrochemically stable supporting materials, which modify the electronic structure of the catalysts, are extensively used [66,67].However, when carbon nanotubes (CNTs) are used as the supporting material for Pd nanoparticles (Pd/CNT), no significant improvement is observed in formic acid electrooxidation and CO stripping [68].
Recently, Raj et al., developed a novel electrocatalyst using unzipped (Uz) single-walled CNTs (SWCNTs) as a support for Pd nanoparticles.The unzipped SWCNTs were prepared via a one-pot synthesis method [69] without using harsh oxidizing agents.The Uz-CNT-Pd catalyst that was prepared showed a significantly higher formic acid electrochemical oxidation activity than the SWCNT/Pd and unsupported Pd nanoparticles (Figure 5).The onset potential for CO electrooxidation over Uz-CNT-Pd was −80 mV lower than that over Pd nanoparticles.This indicates that CO oxidation was facile in the case of Uz-CNT-Pd.In addition, the electrochemical surface area (as calculated from the magnitude of the peak area for CO oxidation) of Uz-CNT-Pd was approximately 2.5 times higher than that of the Pd nanoparticles.The electronic interaction between the unzipped SWCNTs and Pd nanoparticles facilitated the electron transfer required for the electrooxidation of the formic acid.We believe that this approach can be used to develop graphitic carbon-coated electrocatalytically active Pd-and Pt-based materials.

ORR on Metal Catalysts
ORR, which uses Pt as the cathode catalyst, are one of the key types of reactions in the field of PEFCs.However, the slow reaction kinetics for ORR at the cathode and the use of the expensive Pt electrocatalyst limit the widespread commercialization of PEFCs.In acidic solutions, Pt is the best ORR electrocatalyst because of the optimal adsorption energy between molecular oxygen and Pt atoms.However, in order to reduce the cost of PEFCs, it is imperative to develop low-cost alternatives to Pt-based electrocatalysts.Hence, various efforts have been made to develop cost-effective electrocatalysts (using relatively abundant elements) for PEFCs.In his review, Raj discussed various Pt-free catalysts using non-precious transition metals, metal nitrides, carbides and nanoscale carbonbased metal-free electrocatalysts [5].According to Hammer and Stamenkovic et al. [70,71], the ORR electrocatalytic activity of metal catalysts is strongly affected by their electronic states, in particular, the position of the d-band center in the valence band, which is determined using DFT calculations and ultraviolet photoemission spectroscopy.Figure 6 shows the relationship between the d-band center value of various metals and their ORR activities under alkaline conditions, as reported by Lama and coworkers [72].As can be observed from the figure, a volcano-type relationship exists between the d-band center values of the pure metal catalysts and their electrocatalytic activities towards ORR.

ORR on Metal Catalysts
ORR, which uses Pt as the cathode catalyst, are one of the key types of reactions in the field of PEFCs.However, the slow reaction kinetics for ORR at the cathode and the use of the expensive Pt electrocatalyst limit the widespread commercialization of PEFCs.In acidic solutions, Pt is the best ORR electrocatalyst because of the optimal adsorption energy between molecular oxygen and Pt atoms.However, in order to reduce the cost of PEFCs, it is imperative to develop low-cost alternatives to Pt-based electrocatalysts.Hence, various efforts have been made to develop cost-effective electrocatalysts (using relatively abundant elements) for PEFCs.In his review, Raj discussed various Pt-free catalysts using non-precious transition metals, metal nitrides, carbides and nanoscale carbon-based metal-free electrocatalysts [5].According to Hammer and Stamenkovic et al. [70,71], the ORR electrocatalytic activity of metal catalysts is strongly affected by their electronic states, in particular, the position of the d-band center in the valence band, which is determined using DFT calculations and ultraviolet photoemission spectroscopy.Figure 6 shows the relationship between the d-band center value of various metals and their ORR activities under alkaline conditions, as reported by Lama and coworkers [72].As can be observed from the figure, a volcano-type relationship exists between the d-band center values of the pure metal catalysts and their electrocatalytic activities towards ORR.

ORR on Pd-Based Catalysts in Alkaline Media
Pd is a promising electrocatalyst that could act as an alternative for Pt for alkaline ORR because its ORR activity is comparable to that of Pt in alkaline media (Table 2).Recently, we reported that the ORR activity of Pd-M (M = Ni, Co, and Fe), prepared by electrochemical potential cycles, was higher than that of Pd after 1500 cycles because the d-band center value of Pd-M was reduced relative to pure Pd, based on hard X-ray photoemission spectroscopy and computational calculations [73].Several researchers have also reported that ordered intermetallic Pd3Pb has significantly increased electrocatalytic activity for ORR in alkaline media [8,[74][75][76][77].Electrocatalytic activity over intermetallic Pd3Pb strongly depends on the nanoparticle morphology.For ORR in alkaline media, spherically ordered intermetallic Pd3Pb nanoparticles were reported first by Cui et (110) facet results in a Pd-Pb local bonding unit that has an orbital configuration similar to that of Pt [75].This result indicates that the ordered intermetallic Pd3Pb can be used as an alternative catalyst to Pt. Pd is an excellent electrocatalyst for methanol oxidation in alkaline media, implying that methanol is easily adsorbed on the Pd surface, resulting in reduced ORR activity in alkaline media.Therefore, Pd-based electrocatalysts with tolerance for methanol adsorption must be developed.Recently, many researchers have developed Pd-based electrocatalysts with no electrocatalytic activity towards methanol oxidation in alkaline media.Lin et al. reported that ordered intermetallic Pd3Pb has high ORR selectivity in the presence of methanol in alkaline electrolytes [8].Chen et al. have demonstrated a novel approach for the preparation of Au-nanowire-@Pd bimetallic nanohybrids, functionalized by polyethylenimine (PEI), which have increased electrocatalytic activity towards ORR and increased methanol tolerance when compared with Pt electrocatalysts in alkaline media [78].According to their report, PEI exhibits a methanol-phobic behavior.Therefore, this result shows that highly active Pdbased materials for alkaline ORR can exhibit a methanol tolerance because of PEI functionalization.

ORR on Pd-Based Catalysts in Alkaline Media
Pd is a promising electrocatalyst that could act as an alternative for Pt for alkaline ORR because its ORR activity is comparable to that of Pt in alkaline media (Table 2).Recently, we reported that the ORR activity of Pd-M (M = Ni, Co, and Fe), prepared by electrochemical potential cycles, was higher than that of Pd after 1500 cycles because the d-band center value of Pd-M was reduced relative to pure Pd, based on hard X-ray photoemission spectroscopy and computational calculations [73].Several researchers have also reported that ordered intermetallic Pd 3 Pb has significantly increased electrocatalytic activity for ORR in alkaline media [8,[74][75][76][77].Electrocatalytic activity over intermetallic Pd 3 Pb strongly depends on the nanoparticle morphology.For ORR in alkaline media, spherically ordered intermetallic Pd 3 Pb nanoparticles were reported first by Cui et al. in 2016 [74] (110) facet results in a Pd-Pb local bonding unit that has an orbital configuration similar to that of Pt [75].This result indicates that the ordered intermetallic Pd 3 Pb can be used as an alternative catalyst to Pt. Pd is an excellent electrocatalyst for methanol oxidation in alkaline media, implying that methanol is easily adsorbed on the Pd surface, resulting in reduced ORR activity in alkaline media.Therefore, Pd-based electrocatalysts with tolerance for methanol adsorption must be developed.Recently, many researchers have developed Pd-based electrocatalysts with no electrocatalytic activity towards methanol oxidation in alkaline media.Lin et al. reported that ordered intermetallic Pd 3 Pb has high ORR selectivity in the presence of methanol in alkaline electrolytes [8].Chen et al. have demonstrated a novel approach for the preparation of Au-nanowire-@Pd bimetallic nanohybrids, functionalized by polyethylenimine (PEI), which have increased electrocatalytic activity towards ORR and increased methanol tolerance when compared with Pt electrocatalysts in alkaline media [78].According to their report, PEI exhibits a methanol-phobic behavior.Therefore, this result shows that highly active Pd-based materials for alkaline ORR can exhibit a methanol tolerance because of PEI functionalization.However, this results in a high coverage by oxygenated intermediates, which are slowly desorbed.On the other hand, in the case of metals with d-band centers away from the Fermi level (i.e., a low-band center), such as Ag and Au, it is difficult for oxygen and the metal catalyst to react, which results in poor electrocatalytic activity.Among all pure elements, Pt has the ideal d-band center for ORR under acidic conditions.Since the electronic state of Pt can be easily modified by alloying, various efforts have been made over the past few decades to develop Pt-based electrocatalysts.Stamenkovic and coworkers have shown that a volcano-type relationship exists between the ORR activity and d-band center values of Pt catalysts obtained by alloying with transition metals such as Co, Ni, Fe and Ti [10].Mentus and coworkers investigated the relationship between the d-band centers of bulk alloys of Pt with transition metals (M = Au, Pd, Cu, Bi and Pb) and the alloy composition.They reported that in Pt-M bimetallic compounds with a 1:1 ratio, Pb and Bi contribute to the decrease in the d-band center value [83].Although significant progress has been made in the field of PEFCs, the high cost of Pt-based catalysts is the major limitation to the widespread commercialization of PEFCs.Therefore, efficient and low-cost alternatives to Pt-based catalysts should be developed.Various studies have been carried out to develop ORR electrocatalysts using low-cost materials.
Pd-based catalysts have gained immense attention as potential alternatives to Pt for ORR.This is because with the exception of Pt, Pd shows the highest electrocatalytic activity towards ORR.Therefore, various Pd-based compounds with high electrocatalytic activity towards ORR in acidic media have been developed (Table 3).In acidic media, ORR proceeds via the four-and two-electron pathways, as shown below: The reaction mechanism of ORR strongly depends on the electrode materials, electrolyte and electrocatalyst.It is well known that Pt and Pd can induce a four-electron reduction in ORR.On the other hand, the production of H 2 O 2 by a two-electron reduction of oxygen also possesses a significant advantage.So far, there are several reports on the synthesis of H 2 O 2 through ORR.Pt polycrystalline with ammonium ions [84], Pt-Hg [85], Pd-Hg [85,86] Pd-Au [87] and Au [88] have been used as electrocatalysts for the synthesis of H 2 O 2 .In hydrogen-oxygen fuel cells, the four-electron transfer from ORR to H 2 O is highly preferred because of its high-power generation.In the case of ORR in acidic conditions, the electrocatalytic activity of Pd has a significantly lower electrocatalytic activity than that of pure Pt because Pd has a higher d-band center value than Pt, which results in the strong adsorption of oxygen during the initial stage of the ORR.Thus, to develop Pd-based electrocatalysts with high electrocatalytic activity towards ORR in acidic media, it is necessary to control their electronic crystal structures and properties.Recently, bimetallic Pd alloy compounds such as Pd-Fe, Pd-Co, Pd-Ni and Pd-Cu have gained immense attention as ORR catalysts because their activities are comparable to those of Pt and Pt-based catalysts [92][93][94][95][96][97][98][99][100][101].

The Development of Electrocatalysts for ORR Based on Theoretical Calculations
Lihui proposed a criterion (based on the negative alloy formation energy, negative surface segregation energy of Pd and oxygen binding ability) for screening Pd-based bimetallic electrocatalysts for ORR.The values of alloy formation energy (∆E alloy ), surface segregation energy (∆E segregation ) and oxygen binding energy (∆E O ) of Pd-based bimetallic alloy catalysts, obtained via numerical simulations, indicated that elements such as Fe, V, Zn, Nb, Mo, Ta and W enhance the ORR activity of the catalysts [102], as shown in Figure 7.However, the formation energies of Pd-Mo and Pd-W alloys are almost zero, indicating that the stability of these alloys is poor.Pd-Fe shows excellent electrocatalytic activity towards ORR in terms of the three factors (mentioned above), as predicted by DFT calculations.Although there are various reports on the effect of alloying elements such as Co, Cu, Cr, Fe and Ni on the ORR activity of Pd-based catalysts, the effect of Zn, Nb and Ta should also be investigated.a O2-saturated 0.6 M HClO4.
In the case of ORR in acidic conditions, the electrocatalytic activity of Pd has a significantly lower electrocatalytic activity than that of pure Pt because Pd has a higher d-band center value than Pt, which results in the strong adsorption of oxygen during the initial stage of the ORR.Thus, to develop Pd-based electrocatalysts with high electrocatalytic activity towards ORR in acidic media, it is necessary to control their electronic crystal structures and properties.Recently, bimetallic Pd alloy compounds such as Pd-Fe, Pd-Co, Pd-Ni and Pd-Cu have gained immense attention as ORR catalysts because their activities are comparable to those of Pt and Pt-based catalysts [92][93][94][95][96][97][98][99][100][101].

The Development of Electrocatalysts for ORR Based on Theoretical Calculations
Lihui proposed a criterion (based on the negative alloy formation energy, negative surface segregation energy of Pd and oxygen binding ability) for screening Pd-based bimetallic electrocatalysts for ORR.The values of alloy formation energy (Δalloy), surface segregation energy (Δsegregation) and oxygen binding energy (ΔO) of Pd-based bimetallic alloy catalysts, obtained via numerical simulations, indicated that elements such as Fe, V, Zn, Nb, Mo, Ta and W enhance the ORR activity of the catalysts [102], as shown in Figure 7.However, the formation energies of Pd-Mo and Pd-W alloys are almost zero, indicating that the stability of these alloys is poor.Pd-Fe shows excellent electrocatalytic activity towards ORR in terms of the three factors (mentioned above), as predicted by DFT calculations.Although there are various reports on the effect of alloying elements such as Co, Cu, Cr, Fe and Ni on the ORR activity of Pd-based catalysts, the effect of Zn, Nb and Ta should also be investigated.Computational studies have shown that Pd-Cu catalysts show ORR activity comparable to that of Pt.There are several possible structures for Pd-Cu electrocatalysts, such as B 2 , L1 2 , L1 0 , L1 1 -nonlayered and L1 1 -layered [103].The experimental results reported by various researchers have revealed that Cu-doped Pd electrocatalysts exhibit high electrocatalytic activities [50,[93][94][95].Numerical simulations have revealed that body-centered cubic structured Pd-Cu shows poor catalytic activity, while catalysts with a layered L1 1 surface show significantly faster ORR kinetics than pure Pd.On the other hand, Pd-Cu bimetallic catalysts with structurally dependent activities have been also been developed.Ordered intermetallic B2-type structured Pd-Cu nanoparticles have been prepared by a seed-mediated co-reduction method [93].Ordered Pd-Cu B 2 nanoparticles show higher ORR activity than disordered Pd-Cu nanoparticles.However, since the surface structure of bimetallic compounds can change under acidic conditions, it is necessary to consider a secondary material-free surface structure.
In one of our studies, we developed structurally ordered intermetallic electrocatalysts with excellent ORR activity in acidic media.Figure 8 shows the binding energy strength between the electrocatalyst surface and the dissociated O, as predicted by DFT calculations.The DFT calculations revealed that Pd-based catalysts can show electrocatalytic activities higher than those of Pt and Pt-based compounds.Interestingly, the binding energy between the dissociated O atoms and the Pd sites in PdCu 3 decreased significantly after electrochemical dealloying (i.e., the activity was increased by the electrochemical treatment).The binding energy of the dissociated O atoms (4.36 eV) on electrochemically dealloyed PdCu 3 is similar to that of Pt 3 Ni (111) [104].However, the measured (on the basis of the onset and half-wave potentials) ORR electrocatalytic activity decreased in the following order: Pt > dealloyed PdCu 3 NPs/CB > Pd NPs/CB.The difference between the ORR activities (measured using DFT calculations) of the dealloyed PdCu 3 and Pt can be attributed to the difference in the number of their active reaction sites [95].This study demonstrated that by carrying out electrochemical dealloying and modifying the electronic state of Pd-based electrocatalysts, electrochemical activity comparable to that of Pt and Pt-based electrocatalysts can be achieved.
Inorganics 2019, 7, x FOR PEER REVIEW 13 of 23 Computational studies have shown that Pd-Cu catalysts show ORR activity comparable to that of Pt.There are several possible structures for Pd-Cu electrocatalysts, such as B2, L12, L10, L11nonlayered and L11-layered [103].The experimental results reported by various researchers have revealed that Cu-doped Pd electrocatalysts exhibit high electrocatalytic activities [50,[93][94][95].Numerical simulations have revealed that body-centered cubic structured Pd-Cu shows poor catalytic activity, while catalysts with a layered L11 surface show significantly faster ORR kinetics than pure Pd.On the other hand, Pd-Cu bimetallic catalysts with structurally dependent activities have been also been developed.Ordered intermetallic B2-type structured Pd-Cu nanoparticles have been prepared by a seed-mediated co-reduction method [93].Ordered Pd-Cu B2 nanoparticles show higher ORR activity than disordered Pd-Cu nanoparticles.However, since the surface structure of bimetallic compounds can change under acidic conditions, it is necessary to consider a secondary material-free surface structure.
In one of our studies, we developed structurally ordered intermetallic electrocatalysts with excellent ORR activity in acidic media.Figure 8 shows the binding energy strength between the electrocatalyst surface and the dissociated O, as predicted by DFT calculations.The DFT calculations revealed that Pd-based catalysts can show electrocatalytic activities higher than those of Pt and Ptbased compounds.Interestingly, the binding energy between the dissociated O atoms and the Pd sites in PdCu3 decreased significantly after electrochemical dealloying (i.e., the activity was increased by the electrochemical treatment).The binding energy of the dissociated O atoms (4.36 eV) on electrochemically dealloyed PdCu3 is similar to that of Pt3Ni ( 111) [104].However, the measured (on the basis of the onset and half-wave potentials) ORR electrocatalytic activity decreased in the following order: Pt > dealloyed PdCu3 NPs/CB > Pd NPs/CB.The difference between the ORR activities (measured using DFT calculations) of the dealloyed PdCu3 and Pt can be attributed to the difference in the number of their active reaction sites [95].This study demonstrated that by carrying out electrochemical dealloying and modifying the electronic state of Pd-based electrocatalysts, electrochemical activity comparable to that of Pt and Pt-based electrocatalysts can be achieved.It is well-known that the ORR activity of Pt-and Pd-based electrodes significantly depends on their crystal structures.For example, Hoshi and coworkers evaluated the ORR activity of low-index faceted Pd, such as Pd (100), Pd (110) and Pd (111).The ORR activity increased in the following order: Pd (110) < Pd (111) < Pd (100).This trend is opposite to that shown by Pt [105].In particular, the ORR activity of Pd (100) is three times higher than that of Pt (110), which shows the highest activity among the low-index facets of Pt.Meku and coworkers reported that ordered intermetallic Pd3Fe It is well-known that the ORR activity of Pt-and Pd-based electrodes significantly depends on their crystal structures.For example, Hoshi and coworkers evaluated the ORR activity of low-index faceted Pd, such as Pd (100), Pd (110) and Pd (111).The ORR activity increased in the following order: Pd (110) < Pd (111) < Pd (100).This trend is opposite to that shown by Pt [105].In particular, the ORR activity of Pd (100) is three times higher than that of Pt (110), which shows the highest activity among the low-index facets of Pt.Meku and coworkers reported that ordered intermetallic Pd 3 Fe nanoparticles, in which Pd 3 Fe has a super lattice structure (Cu 3 Au type structure), show a much higher electrocatalytic activity towards ORR and durability than pure Pd and atomically disordered Pd-Fe (FCC type structure) [106].The dissolution of Pd under acidic conditions reduces the electrode stability of Pd and Pd-based compounds and consequently can decrease their ORR activities.

The Development of Electrocatalysts with Simultaneously Improved Activity and Durability
Wang and coworkers prepared Au-modified Pd-based ternary nanocatalysts (Au-Pd 6 CoCu) using the spontaneous replacement method, as shown in Figure 9.These catalysts showed durability comparable to that of Pt.The Au-modified Au-Pd 6 CoCu electrode showed higher ORR activity (comparable to that of pure Pt) than pure Pd.In addition, during the durability test (in which the catalysts were subjected to potential cycles), Au-Pd 6 CoCu showed a much lower MA loss (around 16%) [92] than the commercially available Pt (32%) [107].Only a trace amount of Au (Pd:Au = 100:1) is required to enhance the durability (for ORR) of Pd-based electrocatalysts.Wang et al. also demonstrated that trimetallic PdCuAu nanothorn assemblies have a large durability in acidic media because of their special morphology and the presence of Au [108].Thus, this approach can be used to develop Pd-based electrocatalysts with a durability higher than that of Pt and state-of-the-art Pt-based compounds.
Inorganics 2019, 7, x FOR PEER REVIEW 14 of 23 nanoparticles, in which Pd3Fe has a super lattice structure (Cu3Au type structure), show a much higher electrocatalytic activity towards ORR and durability than pure Pd and atomically disordered Pd-Fe (FCC type structure) [106].The dissolution of Pd under acidic conditions reduces the electrode stability of Pd and Pd-based compounds and consequently can decrease their ORR activities.

The Development of Electrocatalysts with Simultaneously Improved Activity and Durability
Wang and coworkers prepared Au-modified Pd-based ternary nanocatalysts (Au-Pd6CoCu) using the spontaneous replacement method, as shown in Figure 9.These catalysts showed durability comparable to that of Pt.The Au-modified Au-Pd6CoCu electrode showed higher ORR activity (comparable to that of pure Pt) than pure Pd.In addition, during the durability test (in which the catalysts were subjected to potential cycles), Au-Pd6CoCu showed a much lower MA loss (around 16%) [92] than the commercially available Pt (32%) [107].Only a trace amount of Au (Pd:Au = 100:1) is required to enhance the durability (for ORR) of Pd-based electrocatalysts.Wang et al. also demonstrated that trimetallic PdCuAu nanothorn assemblies have a large durability in acidic media because of their special morphology and the presence of Au [108].Thus, this approach can be used to develop Pd-based electrocatalysts with a durability higher than that of Pt and state-of-the-art Ptbased compounds.Electrocatalysts (for ORR) for DMFCs should also be methanol tolerant under acidic conditions, as methanol is used as a common fuel for these fuel cells.Therefore, for the commercialization of DMFCs, the ORR activity of the electrocatalyst should not be affected by the presence of methanol (which is mixed by the methanol crossover process) in the electrolyte.In the case of Pt electrocatalysts, the overpotential towards ORR under a methanol environment significantly increases to approximately 200 mV because methanol blocks hinder the ORR.The decrease in the ORR electrocatalytic activity of Pt and Pt-based electrocatalysts in the presence of methanol is caused by the adsorption of methanol molecules on the active sites for ORR.In contrast, the ORR half-wave and onset potentials of PdCu 3 and PdFeIr are not significantly affected by the presence of methanol [95,109].These results indicate that Pd-based compounds are potential electrocatalysts for DMFCs.

Pt-Decorated Pd and Pd Bimetallic Alloy Catalysts for ORR Electrocatalysts in Acidic Media
Recently, several efforts have been made to develop Pd-based ORR electrocatalysts with small amounts of Pt.The MA of Pt towards ORR can be improved by coating with Pt, as electrocatalytic reactions proceed on the surface of electrocatalysts.In particular, electrocatalysts with a Pd@Pt structure are both cost-effective and highly efficient.Wang and coworkers demonstrated that the ORR activity of Pd@Pt catalysts strongly depends on the Pt-Pt distance on monolayer Pt on various single crystal surfaces, which is measured by the extended X-ray absorption fine structure [110].They obtained a linear relationship between the lattice strain and the ORR activity of the catalysts.These results can be helpful for preparing highly active core-shell-type ORR electrocatalysts with short Pt-Pt distances.Xia and coworkers investigated the relationship between the ORR activity and thickness of Pt-shell-Pd nanocrystals [111].The Pd@Pt n shell (n = 1-6 layers)-type structure can enhance the electrocatalytic activity and durability during ORR in terms of the specific activity.A volcano-shaped relationship was obtained between the ORR specific activity and the number of Pt layers of the catalysts.Among all the Pd@Pt n catalysts, the Pd@Pt 2 layer showed the best ORR specific activity (even better than that of Pt).According to the DFT calculations, the enhanced ORR specific activity of the Pd@Pt 2 layer can be attributed to the weakening of OH bonds.This occurs because of the ligand and strain effects, which increase the rate of OH hydrogenation.The experimental results were consistent with those obtained computationally.These results suggest that Pd@Pt electrocatalysts show better performance than carbon-supported Pt.The Pd@Pt structure is promising for achieving the US Department of Energy specifications, which are as follows: A lifetime of 5000 h, a mass activity over 0.44 A mg −1 (per platinum group metal) and 10.1 A $ −1 [112].Indeed, the Adzic group has reported that the electrocatalytic activities, normalized by the price of Pt group metals, of Pd@Pt-type structures can surpass the standards specified by the US Department of Energy [113].Various efforts have been made to modify the Pt-Pt distance and electronic state of the surface of Pd-based compounds in Pd (Pd-M)@Pt shell electrocatalysts (Table 4).Abruña et al. and Wang et al. prepared electrocatalysts with a Pt-decorated Pd-M@Pd core-shell-type structure by the spontaneous displacement of Pd with Pt.This structure improved the MA of the catalysts by more than 10 times [9,119].The deposition of Pt on Pd and Pd-based compounds in order to create core-shell structures is strongly affected by particle size and the secondary materials used, such as Fe [119].Markovic and coworkers demonstrated that in acidic media, the specific activity towards ORR of Pt ( 111) is more than two times higher than that of Pt (100) [122].Moreover, Xia and coworkers also showed that Pt octahedral nanocages having Pt (111) facets have a higher activity towards ORR than Pt cubic nanocages with (100) facets, indicating that Pt nanostructures with (111) facets show good ORR electrocatalytic activity [102].However, no significant improvement is observed in the ORR activity of Pd nanostructures upon the incorporation of Pt ( 111) facets [123].
Li and coworkers developed ultra-thin icosahedral Pt-enriched nanocages through the selective removal of icosahedral Pd from Pd@Pt using a nitric acid solution [117], as shown in Figure 10.Among the nanocages with low-index facets, the icosahedral Pt-enriched nanocages with Pt (111) facets showed the highest electrocatalytic activity towards ORR.The as-prepared Pt-enriched nanocages showed between 7 and 10 times higher MA and specific activity, respectively, than JM Pt/C. the catalysts by more than 10 times [9,119].The deposition of Pt on Pd and Pd-based compounds in order to create core-shell structures is strongly affected by particle size and the secondary materials used, such as Fe [119].Markovic and coworkers demonstrated that in acidic media, the specific activity towards ORR of Pt ( 111) is more than two times higher than that of Pt (100) [122].Moreover, Xia and coworkers also showed that Pt octahedral nanocages having Pt (111) facets have a higher activity towards ORR than Pt cubic nanocages with (100) facets, indicating that Pt nanostructures with (111) facets show good ORR electrocatalytic activity [102].However, no significant improvement is observed in the ORR activity of Pd nanostructures upon the incorporation of Pt ( 111) facets [123].
Li and coworkers developed ultra-thin icosahedral Pt-enriched nanocages through the selective removal of icosahedral Pd from Pd@Pt using a nitric acid solution [117], as shown in Figure 10.Among the nanocages with low-index facets, the icosahedral Pt-enriched nanocages with Pt (111) facets showed the highest electrocatalytic activity towards ORR.The as-prepared Pt-enriched nanocages showed between 7 and 10 times higher MA and specific activity, respectively, than JM Pt/C.Recently, it has been reported that Pt-decorated ternary nanoparticles show enhanced ORR activity [96].Wang and coworkers prepared ternary PdxCoZn (x = 0, 4, 8 and 12) and Pt-decorated Pd8CoZn and evaluated their electrocatalytic activity and durability for ORR.They showed that a composition of Pd:Co:Zn = 8:1:1 was optimum for enhanced ORR activity.The ORR MA of Pt-coated Pd8CoZn nanoparticles at 0.9 V was about 27 times higher than that of the commercially available Pt/C.This approach can be helpful for the development of Pt-coated Pd-based ternary catalysts with a large number of active sites for ORR.
The use of Pd core materials in electrocatalysts can surpass conventional Pt-based electrocatalysts.Among the Pt-based electrocatalysts for ORR, the Pt-Ni bimetallic electrocatalyst is the best choice.For example, Stamenkovic et al. reported that the mass activity at 0.9 V versus RHE of a PtNi3 nanoframe is 5.7 A mg −1 Pt, much higher than that of state-of-the-art Pt nanoparticles [124].Recently, it has been reported that Pt-decorated ternary nanoparticles show enhanced ORR activity [96].Wang and coworkers prepared ternary Pd x CoZn (x = 0, 4, 8 and 12) and Pt-decorated Pd 8 CoZn and evaluated their electrocatalytic activity and durability for ORR.They showed that a composition of Pd:Co:Zn = 8:1:1 was optimum for enhanced ORR activity.The ORR MA of Pt-coated Pd 8 CoZn nanoparticles at 0.9 V was about 27 times higher than that of the commercially available Pt/C.This approach can be helpful for the development of Pt-coated Pd-based ternary catalysts with a large number of active sites for ORR.
The use of Pd core materials in electrocatalysts can surpass conventional Pt-based electrocatalysts.Among the Pt-based electrocatalysts for ORR, the Pt-Ni bimetallic electrocatalyst is the best choice.For example, Stamenkovic et al. reported that the mass activity at 0.9 V versus RHE of a PtNi 3 nanoframe is 5.7 A mg −1 Pt , much higher than that of state-of-the-art Pt nanoparticles [124].Moreover, the electrocatalytic activity of Pt 3 Ni is significantly improved by transition metal doping.In particular, Mo-doped Pt 3 Ni showed the best ORR electrocatalytic activity at that time (mass activity = 6.98A mg −1  Pt ) [125].Recently, Huang et al. presented Pd@Pt 3 Ni (Pt skin) nanoparticles (mass activity = 14.2A mg −1 Pt ), which exhibited the highest electrocatalytic activity towards ORR ever recorded, more than those of state-of-the-art Pt-Ni catalysts [121].

Conclusions
Electrocatalysts developed over recent years can be classified into three categories: (1) Pd-based materials for formic acid oxidation, (2) Pd-based materials for ORR, and (3) Pd-core-Pt shell-structured catalysts.Various advances made in the development of electronically modified and morphology-controlled Pd-based electrocatalysts for formic acid oxidation have been discussed in this review.Secondary materials such as Co, Ni, Cu, Pt and Au play an important role in the modification of the electronic states (i.e., tuning the d-band center) of Pd-based electrocatalysts.The formic acid oxidation activity of these catalysts depends on the downshift of the CO oxidation voltage.However, since HCOO ad should be formed on Pd sites in the initial stage of formic acid oxidation, the catalyst should have an optimum d-band center value for high electrocatalytic activity.The d-band center value can be tuned by the formation of Pd-based ternary materials.Electrochemical dealloying is an effective approach to prepare electrocatalysts with high activities.For example, electrochemically treated intermetallic Pd 3 Pb exhibits a significantly higher electrocatalytic activity than Pd NPs/CB.
Since Pd shows physicochemical characteristics similar to those of Pt and a higher methanol tolerance than Pt, Pd-based materials are potential catalysts for ORR under acidic conditions.Although Pd-Cu bimetallic compounds with electrocatalytic activity higher than that of Pt have not been realized experimentally yet, theoretical calculations have shown that Pd-based compounds are promising electrocatalysts.DFT calculations have revealed that the adsorption energy of dissociate O on Pd-based electrocatalysts is similar to that of Pt.Further investigation of Pd-based bimetallic catalysts for ORR is necessary.Although, the low stability of Pd during ORR under acidic conditions limits their application for PEFCs.Only a trace amount of Au is required to improve the ORR durability of Pd-based electrocatalysts.Intermetallic Pd 3 Pb has enhanced ORR activity in alkaline media when compared with pure Pd and Pt.Pt-based materials have realized activities higher than 10 A mg −1 Pt under acidic conditions, based on their mass activity.In contrast, the mass activities towards ORR in alkaline media remain less than 1 A mg −1  Pd when Pd-based materials are used as electrocatalysts.No published work has provided answers to the following two important questions: (i) Why can Pd-based materials with higher ORR activities than Pt-based materials under acidic conditions not be developed, although theoretical studies exhibit that Pd-based materials are promising electrocatalysts?Secondly, what is the key factor for improving the electrocatalytic activity towards ORR in alkaline media (i.e., why does the pure Pd electrocatalyst have almost the same activity as pure Pt towards ORR in alkaline media, despite Pd having a significantly higher d-band center and O adsorption energy than Pt)? Extensive research efforts, both experimental and theoretical, are required to answer these questions.We believe that these results will accelerate the development of novel electrocatalysts.

Figure 1 .
Figure 1.The reaction pathway for the electrooxidation of formic acid over Pd.Reproduced with permission [46].Copyright 2011, American Chemical Society.

Figure 1 .
Figure 1.The reaction pathway for the electrooxidation of formic acid over Pd.Reproduced with permission [46].Copyright 2011, American Chemical Society.

2. 4 .
Effect of Electronic State, Morphology and Structure on the Catalytic Activity of Pd towards Formic Acid Oxidation

5 )
The d-band center value of Pd-based materials can be tuned (for improving the catalytic activity) by forming PdCuNi trimetallic compounds.This indicates that Pd-based materials show electrocatalytic activity towards formic acid oxidation only at optimum d-band center values.This can be explained by the following reaction mechanism: HCOOH + Pd→ HCOO-Pd + H + + e − (4) HCOO-Pd → Pd-H + CO 2 (Inorganics 2019, 7, x FOR PEER REVIEW 6 of 23

Figure 2 .
Figure 2. Relationship between the d-band center values and current densities of various Pd-based catalysts at 0.5 V (versus RHE).Reproduced with permission [49].Copyright 2016, Elsevier.

Figure 2 .
Figure 2. Relationship between the d-band center values and current densities of various Pd-based catalysts at 0.5 V (versus RHE).Reproduced with permission [49].Copyright 2016, Elsevier.

Figure 3 .
Figure 3. High-resolution (HR)-TEM images (A-D) of Pd-Cu multipods with different pod numbers and the corresponding three-dimensional models.(E) Cyclic voltammograms and (F) chronoamperograms of Pd-Cu multipods, Pd-Cu nanoparticles and the Pd/C catalyst at 0.4 V in 0.1 M HClO4 and 1 M formic acid.Reproduced with permission [50].Copyright 2016, Royal Chemical Society.

Figure 3 .
Figure 3. High-resolution (HR)-TEM images (A-D) of Pd-Cu multipods with different pod numbers and the corresponding three-dimensional models.(E) Cyclic voltammograms and (F) chronoamperograms of Pd-Cu multipods, Pd-Cu nanoparticles and the Pd/C catalyst at 0.4 V in 0.1 M HClO 4 and 1 M formic acid.Reproduced with permission [50].Copyright 2016, Royal Chemical Society.

Figure 4 .
Figure 4. (A) Linear sweep voltammograms for oxidation of formic acid on (a) intermetallic Pd 3 Pb electrochemically treated for 50 cycles, (b) Pd, (c) Pt and (d) Pt-Ru alloy.(B) Cyclic voltammograms for CO stripping over (a) Pd 3 Pb electrochemically treated for 50 cycles and (b) Pd. (C) Experimentally measured (black square) and density functional theory (DFT) calculated (black circle) d-band centers for Pd and intermetallic Pd 3 Pb structures.Reproduced with permission [31].Copyright 2017, American Chemical Society.

Inorganics 2019, 7 ,Figure 6 .
Figure 6.Kinetic current (ik) at 0.80 V versus RHE for O2 reduction on carbon-supported metal nanoparticles in a 0.1 mol L −1 NaOH solution as a function of the metal d-band center.Reproduced with permission [72].Copyright 2007, American Chemical Society.
al. in 2016 [74].The electrocatalytic activities for ORR over ordered intermetallic Pd3Pb, having nanowire, tripod, and nanoplate morphologies, reported by Shi et al., Huang et al. and Luo et al., respectively, were more than three times higher than that of the spherical Pd3Pb nanoparticles [8,75-77].Huang et al. proposed that a coupled s-p-d exchange effect on the (110) facets of the Pd3Pb tripods improves the ORR catalytic activity in alkaline media.According to their first-principle calculations, the charge exchange between the Pd-4d and Pb-(sp) regions on the Pd3Pb tripods

Figure 6 .
Figure 6.Kinetic current (i k ) at 0.80 V versus RHE for O 2 reduction on carbon-supported metal nanoparticles in a 0.1 mol L −1 NaOH solution as a function of the metal d-band center.Reproduced with permission [72].Copyright 2007, American Chemical Society.
. The electrocatalytic activities for ORR over ordered intermetallic Pd 3 Pb, having nanowire, tripod, and nanoplate morphologies, reported by Shi et al., Huang et al. and Luo et al., respectively, were more than three times higher than that of the spherical Pd 3 Pb nanoparticles [8,75-77].Huang et al. proposed that a coupled s-p-d exchange effect on the (110) facets of the Pd 3 Pb tripods improves the ORR catalytic activity in alkaline media.According to their first-principle calculations, the charge exchange between the Pd-4d and Pb-(sp) regions on the Pd 3 Pb tripods

Figure 9 .
Figure 9. (A) Schematic of the preparation of Au-doped ternary PdCoCu.(B) Comparison of ORR polarization curves of various Pd-based catalysts in an O2-saturated 0.1 M HClO4 solution with a sweep rate of 5 mV s −1 .(C) Anodic sweep voltammograms for ORR on Pd6CoCu/C before and after 1000 potential cycles, and (D) the anodic sweep voltammograms for ORR on Au-Pd6CoCu/C before and after 10,000 potential cycles [92].

Figure 9 .
Figure 9. (A) Schematic of the preparation of Au-doped ternary PdCoCu.(B) Comparison of ORR polarization curves of various Pd-based catalysts in an O 2 -saturated 0.1 M HClO 4 solution with a sweep rate of 5 mV s −1 .(C) Anodic sweep voltammograms for ORR on Pd 6 CoCu/C before and after 1000 potential cycles, and (D) the anodic sweep voltammograms for ORR on Au-Pd 6 CoCu/C before and after 10,000 potential cycles [92].

Figure 10 .
Figure 10.(a) Linear sweep voltammograms for ORR over Pt nanoparticles, core-shell Pd-Pt and Ptenriched nanocages in an O2-saturated 0.1 M HClO4 solution with a sweep rate of 10 mV s −1 and (b) the corresponding Tafel plots.(c) Cyclic voltammetry curves in an N2-saturated 0.1 M HClO4 solution with a sweep rate of 10 mV s −1 and (d) the MA for these three catalysts at 0.9 V versus RHE.Reproduced with permission[117].Copyright 2016, American Chemical Society.

Figure 10 .
Figure 10.(a) Linear sweep voltammograms for ORR over Pt nanoparticles, core-shell Pd-Pt and Pt-enriched nanocages in an O 2 -saturated 0.1 M HClO 4 solution with a sweep rate of 10 mV s −1 and (b) the corresponding Tafel plots.(c) Cyclic voltammetry curves in an N 2 -saturated 0.1 M HClO 4solution with a sweep rate of 10 mV s −1 and (d) the MA for these three catalysts at 0.9 V versus RHE.Reproduced with permission[117].Copyright 2016, American Chemical Society.

Table 1 . Mass activities (MA) at 0.3 V versus the reversible hydrogen electrode (RHE) of various Pd-based catalysts for formic acid oxidation and CO stripping. Catalyst MA (A mg −1 Pd ) Concentration of Formic Acid (mol L −1 ) E for Peak Top of CO Stripping (V versus RHE) References
These electrocatalysts were measured by using Ag/AgCl as the reference electrode.The MAs in Ag/AgCl were calibrated to RHE using the following equation: E Ag/AgCl − E RHE = 0.0591pH + 0.21. a

Table 2 .
MA at 0.9 V (versus RHE) and half-wave potentials (E 1/2 ) for ORR by various Pd-based catalysts in an O 2 -saturated 0.1 M KOH aqueous solution.Electrocatalytic Activity of Pd-Based Materials towards ORR in Acidic Media Metals with high d-band center values, such as Ru, Ir and Rh, show low electrocatalytic activities because of the high adsorption strength of O 2 .These metals can easily dissociate the O-O bond.

Table 3 .
MA at 0.9 (0.85) V versus RHE and E 1/2 for ORR of various Pd-based catalysts in an O 2 -saturated HClO 4 aqueous solution.

Table 3 .
MA at 0.9 (0.85) V versus RHE and E1/2 for ORR of various Pd-based catalysts in an O2saturated HClO4 aqueous solution.

Table 4 .
MA and specific activity (SA) at 0.9 V versus RHE for the ORR of various Pd or Pd-based core-Pt shell catalysts in an O 2 -saturated 0.1 M HClO 4 aqueous solution.