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Article

An Enhanced Oxidation of Formate on PtNi/Ni Foam Catalyst in an Alkaline Medium

by
Antanas Nacys
*,
Dijana Šimkūnaitė
,
Aldona Balčiūnaitė
,
Aušrinė Zabielaitė
,
Daina Upskuvienė
,
Benjaminas Šebeka
,
Vitalija Jasulaitienė
,
Vitalij Kovalevskij
,
Eugenijus Norkus
and
Loreta Tamašauskaitė-Tamašiūnaitė
*
Center for Physical Sciences and Technology, Sauletekio Avenue 3, LT-10257 Vilnius, Lithuania
*
Authors to whom correspondence should be addressed.
Crystals 2022, 12(3), 362; https://doi.org/10.3390/cryst12030362
Submission received: 2 February 2022 / Revised: 6 March 2022 / Accepted: 7 March 2022 / Published: 8 March 2022
(This article belongs to the Special Issue Advances in Functional Inorganic Materials Prepared by Wet Chemical Methods (Volume II))
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Versions Notes

Abstract

:
In this study, a platinum-coated Ni foam catalyst (denoted PtNi/Ni foam) was investigated for the oxidation of the formate reaction (FOR) in an alkaline medium. The catalyst was fabricated via a two-step procedure, which involved an electroless deposition of the Ni layer using sodium hypophosphite as a reducing agent and the subsequent electrodeposition of the platinum layer. The PtNi/Ni foam catalyst demonstrated enhanced electrocatalytic activity for the FOR in an alkaline medium compared to the Ni/Ni foam catalyst and pure Pt electrode. Moreover, the PtNi/Ni foam catalyst promoted the FOR at more negative potentials than the Pt electrode. This contributed to a significant negative shift in the onset potential, indicating the high activity of the catalyst. Notably, in alkaline media with the PtNi/Ni foam catalyst, the FOR proceeds via a direct pathway mechanism without significant accumulation of poisonous carbonaceous species on the PtNi/Ni foam catalyst.

1. Introduction

The large-scale commercialization of formic acid (FA)/formate-based fuel cells is challenging because of the unavailability of highly active, durable, and low-cost catalysts for anodic oxidation reactions. The most promising are based on Pt or Pt-group materials (PGMs) [1,2,3,4,5]. Pd and Pd-based catalysts are the most widely studied and are currently regarded as state-of-the-art electrocatalysts for FA/formate oxidation because of their high catalytic activity and good anti-poisoning properties in acidic and alkaline media [6,7,8]. Combining Pd with other metals or metal nanoparticles, such as Ag [8,9,10], Cu [10,11], Ni [9], CeO2 [12], In [13], and B [14], improves the characteristics of the catalysts while simultaneously reducing the cost. In some cases, especially under alkaline conditions, Pd-based catalysts are even more active and less prone to deactivation by carbon monoxide (CO) than Pt-based catalysts [15]; however, their insufficient stability remains a major concern. Meanwhile, significantly less attention has been paid to Pt-based electrodes for the FOR. However, further development is required to mitigate the high cost and low performance of Pt-based catalysts owing to the fast deactivation of the in situ-generated poisonous carbonaceous intermediates, such as CO, and to develop CO-tolerant catalysts. Some efficient ways to overcome the present drawbacks are to decrease the Pt loading and fabricate nanostructured materials with Pt to exploit its mass activity and stability, or decorate and modify catalysts with Pt heteroatoms for better poison resistance [8,16,17]. Another way to enhance Pt electrocatalysis is to control the acidity of fuel solutions by maintaining a low concentration of FA and a high concentration of formate, which results in a smaller volume of FA being adsorbed. This contributes to a lower level of severe Pt poisoning by the adsorbed CO [18]. However, decreasing FA concentration is closely related to decreases in the current density of FA oxidation. Recently, free formate was demonstrated to promote anode oxidation in acidic solutions and suppress the poisoning pathway [18]. The FOR demonstrates faster reaction kinetics, a lower overpotential, and less poisoning of the anode catalyst in an alkaline media than in an acidic media [19,20,21,22].
Several conventional design principles for catalysts have been applied to improve their poisoning resistance. The following two strategies have been used: (1) Pt is coupled with other metals such as Ni [23,24,25,26], Au [8,27], Bi [28,29], Sb [30], and Rh [31], to prevent CO adsorption on the Pt surface through so-called ensemble and electronic effects, and (2) oxidative removal of adsorbed CO from the Pt surface is facilitated at low potentials by enriching the surface with oxygen-containing species via a bifunctional mechanism. This may be achieved by alloying Pt with oxophilic transition metals or metal oxides, such as NiOx [32,33,34,35], CoOx [32], Cu2O [36], FeOx [37], MnOx [38,39], and SnO2 [40]. The presence of oxophilic materials, including Ni-oxy species, in the catalyst composition can assist with the electrochemical dissociation of water to form OH– ions at a more negative potential than that required for OH– to be formed in the presence of bare Pt [32,41,42,43]. This phenomenon aids in the removal of adsorbed surface CO (COads) by oxidation, allowing for the development of CO-tolerant catalysts. In addition, the β-NiOOH phase, which originates from the oxidation of the lower oxidation state Ni or NiO through Ni(OH)2, is considered to be the active anti-poison site, and can act as a catalytic mediator through a reversible Ni (II)/Ni (III) redox system to facilitate a charge transfer for the FA oxidation reaction [32]. Moreover, Ni-based oxides/hydroxides are of significant interest as electrocatalysts because of their low price, nontoxicity, and earth-abundant resources. Recently, the spontaneous deposition of Pt on commercial Ni foams (Nifoam) was used to produce Pt-modified 3D Ni electrodes, which are perfect candidates for efficient catalysis owing to the high surface area and synergistic effects between the Pt nanoparticles and the supporting material [44]. This unique three-dimensional structure benefits significantly from the use of noble metals. As a result, Ni foams with a particular architecture and excellent conductivity have been successively used as supports for the preparation of catalysts for various electrochemical reactions, such as hydrogen evolution [45], water splitting in alkaline media [46], direct ammonia oxidation [47], methanol oxidation [48], ethanol oxidation [49], sodium borohydride hydrolysis [50], borohydride oxidation [51], hydrazine oxidation [52], and urea oxidation [53]. However, there are no reports of Ni foam as a support for FA oxidation.
Large surfaces are important for the good spacious dispersion of heterogeneous catalysts, as they provide an increased contact area for reactants to facilitate catalysis. Therefore, we deposited a Ni layer on the Ni foam surface (Ni/Nifoam) using the electroless deposition technique. Subsequently, a thin Pt layer was electrochemically deposited on the surface. The electrocatalytic activity of the PtNi/Nifoam catalysts for FOR was evaluated in alkaline media. Owing to the synergy between the active sites of Pt, the porous structure of the Ni-oxy species layer on the Ni foam, and the extremely high specific surface area of the catalyst, the prepared catalyst demonstrated advanced activity in alkaline media and pronounced resistance to poisoning by carbonaceous species.

2. Materials and Methods

The Ni foam with 20 pores/cm, a bulk density of 0.45 g cm−3, and a thickness of 1.6 mm was purchased from GoodFellow GmbH Supplier (Hamburg, Germany). The thin Ni layer was deposited on Ni foam, using sodium hypophosphite as a reducing agent described in detail in Ref. [50].
The Pt thin layer was electroplated on Ni/Nifoam using the electrolyte containing PtCl2(NH3)2, NH4NO3, NH4OH, and NaNO2 (pH 8) at the current density of 1 A dm−2 for 40 min. The temperature of the electrolyte was kept at a temperature of 95 °C.
The chemical composition of the samples was analyzed using the X-ray photoelectron spectroscopy method employing Kratos AXIS Supra+ spectrometer (Kratos Analytical, Manchester, UK, 2019) with monochromatic Al Kα (1486.6 eV) X-ray radiation powered at 225 W. The base pressure in the analysis chamber was less than 1 × 10−8 mbar and a low electron flood gun was used as a charge neutralizer. The survey spectra for each sample were recorded at a pass energy of 80 eV with a 1 eV energy step and high-resolution spectra (pass energy—10 eV, in 0.1 eV steps) over individual element peaks. The binding energy scale was calibrated by setting the adventitious carbon peak at 284.8 eV. XPS data were converted to VAMAS format and processed using the Avantage Software (Thermo Scientific, East Grinstead, UK).
The morphology and composition of the prepared coatings were investigated by scanning electron microscopy (SEM) using a SEM/FIB workstation Helios NanoLab 650 with an energy dispersive X-ray (EDX) spectrometer INCA Energy 350 X-Max 20. Pt loading was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). It was determined that the Pt loading in the PtNi/Nifoam catalyst was ca. 71 μg cm−2.
The oxidation of formate (FOR) was investigated using a Zennium electrochemical workstation (ZAHNER-Elektrik GmbH & Co.KG, Kronach, Germany). A conventional three-electrode cell was used for electrochemical measurements. The Ni/Nifoam and PtNi/Nifoam catalysts with a geometric area of 2.45 cm2 were employed as working electrodes. An Ag/AgCl/KCl (3 M KCl) electrode was used as a reference, Pt sheet with a geometric area of 4 cm2 was used as a counter electrode. For comparison, the Pt bulk electrode with a geometric area of 1 cm2. Cyclic voltammograms (CVs) were recorded at a potential scan rate of 50 mV s−1 from the open-circuit potential value in the anodic voltammetric scan up to +0.6 V unless otherwise stated in a 1 M NaOH solution containing FA concentration in the range of 0.1–0.7 M at a temperature of 25 °C. All reported potential values are referred to as “Ag/AgCl”.
Chronoamperometric measurements were carried out at a constant electrode potential of −0.6 V for 2600 s.

3. Results

3.1. Characterization of the Electrodes

X-ray photoelectron spectroscopy (XPS) was performed to analyze the electronic state of the surface composition of the as-prepared PtNi/Nifoam. The Pt 4f spectra reveal a doublet consisting of high-energy (Pt 4f5/2) and low-energy (Pt 4f7/2) bands (Figure 1a). Deconvolution of the latter reveals two peaks centered at 70.9 and 72.4 eV, indicating that Pt is in two different oxidation states, Pt (0) and Pt (II), respectively, and the lower binding energy (BE) peak at 70.9 eV is clearly dominant. This indicates that almost all Pt species grown on Ni/Nifoam exist in a metallic state. The higher BE peak (72.4 eV) represents PtO or Pt(OH)2 [54].
The Ni 2p3/2 XPS spectrum splits into three resolved peaks centered at 852.3 eV, 853.9, and 855.8 eV, indicating the presence of Ni, NiO, and Ni(OH)2 species, respectively, on the Nifoam surface (Figure 1b). The dominant BE peak at 855.8 eV indicates that the Ni(OH)2 species predominates in the catalyst composition. Moreover, the BE peak of Pt 4f7/2 for the PtNi/Nifoam catalyst shifted from 71.20 to 70.91 eV when compared to that for metallic Pt, suggesting that Pt undergoes electronic structural changes when it is coupled with Ni(OH)2 [55]. This shift implies that the Ni atoms occupy the Pt lattice and that the metallic grains are intermixed with Ni(OH)2 [56,57]. Furthermore, the O 1s XPS spectrum reveals three resolved peaks centered at 529.8, 531.3, and 532.8 eV (Figure 1c). The lowest energy contributions at 529.8 and 531.3 eV were assigned to oxide/hydroxide species such as NiO and Ni(OH)2, respectively [45]. The highest BE value (532.8 eV) generally corresponds to physically adsorbed water molecules [58,59]. The coexistence of Pt and Ni(OH)2 species was determined by XPS analysis and is supported by cyclic voltammetry measurements.
The cyclic voltammograms (CVs) of the bare Pt and PtNi/Nifoam electrodes in 1 M NaOH solution measured at a potential scan rate of 50 mV s−1 are displayed in Figure 2. The existence of a pair of coupled peaks at ca. +0.46 V is generally attributed to the reversible transformation of Ni (II)/Ni (III) species via the redox reaction between Ni(OH)2 and NiOOH, according to the following Equation (1) [60]:
Ni(OH)2 + OH− ↔ NiOOH + H2O + e−
The presence of Pt on the surface of the Ni/Nifoam electrode was confirmed by the cathodic peak located at ca. −0.4 V in the alkaline 1 M NaOH solution. The electrochemically active surface areas (ECSAs) of the prepared Pt and PtNi/Nifoam catalysts were determined from the CVs recorded in a deaerated 0.5 M H2SO4 solution at a scan rate of 50 mV s−1 (Figure S1, Supplementary Material) by calculating the charge associated with hydrogen adsorption at 210 μC cm−2 [61]. The ECSA for the PtNi/Nifoam electrode is 71 cm2, and that for the bare Pt substrate is 1.5 cm2. The above results reveal that the PtNi/Nifoam electrode has significantly higher current values and a larger surface area than the bare Pt substrate, and it outperforms the Pt electrode ca. 47-fold.
The electrochemical performance of the Pt and PtNi/Nifoam catalysts was evaluated by progressively increasing the anodic reverse potential limit (Era). A series of CVs for Pt and PtNi/Nifoam catalysts were recorded in a 1 M NaOH solution at different Era values at a scan rate of 50 mV s−1 (Figure 3). The CV curves of the bare Pt electrodes (Figure 3a) are consistent with those typically found for bare Pt electrodes in an alkaline solution. They are characterized by the following three potential regions: (a) a hydrogen adsorption/desorption region (−0.9 < E < −0.6 V), (b) a double layer region (−0.6 < E < −0.45 V), and (c) a hydroxide–oxide formation region (−0.45 < E < +0.6 V) [62,63,64]. The applied potential region can generate two types of oxygen-containing species, namely, PtOH and PtO [65]. This common potential domain, up to ca. −0.10 V, is attributed to OH− ion electroabsorption with charge transfer and the formation of surface hydroxides (OH− → OHad + e− in alkaline solutions) [63,66,67]. This potential domain is associated with reversible and weak OHad adsorption before the oxide layer formation. A more positive E domain represents the only somewhat reversible formation of strongly bound OHad species and high-valent oxides. The most positive potential region up to +0.6 V is attributed to stoichiometrically different Pt-OH species that transform into Pt-O−, Pt=O, or Pt2=O, depending on the surface coverage of strongly adsorbed OHad.
The cathodic peak generated in the backward (cathodic voltammetric) scan when the electrode potential was scanned at progressively more positive values was attributed to the reduction of Pt oxides, which increased and shifted to more negative potential values, indicating the formation of irreversible oxide species.
Meanwhile, the hydrogen adsorption/desorption peaks at the bare Pt electrode changed significantly less in response to the applied Era than those of Pt oxides. However, the processes mentioned above in an alkaline medium are not well separated in potential ranges, as in the case of an acidic medium [68,69]. The coupled H and OH adsorption/desorption processes in alkaline media are thought to occur reversibly up to ca. +0.65 V vs. RHE for Pt (100) and Pt (110), respectively. The OH− adsorption on Pt in an alkaline medium begins quite low at (ca. +0.35 V vs. RHE) [68,69], as soon as the desorption of hydrogen ceases. Notably, the presence of small quantities of adsorbed OHad species on the Pt surface was confirmed, even in the Hupd region in alkaline solutions [64,66]. Thus, the coupled competing adsorption/desorption processes of Hupd and OHad proceeded in the Hupd region.
Similarly, CVs were recorded for the PtNi/Nifoam electrode with a gradual increase in the upper Era (up to +0.6 V, Figure 3b). In contrast to the CVs determined for the Pt electrode, those for PtNi/Nifoam reveal a pronounced redox current peak pair at ca. +0.46 V, which was attributed to the reversible transformation of Ni (II)/Ni (III) species during the anodic and cathodic scans. With an increase in the upper Era, a slight negative shift of the Pt oxide formation/reduction potentials was observed, together with an increase in the corresponding current peak values during both anodic and cathodic scans. Meanwhile, almost complete suppression of the peak current was observed in the hydrogen adsorption/desorption domain, unlike in bulk Pt. The corresponding Pt oxide reduction current density peak values (jpc) for the Pt and PtNi/Nifoam catalysts determined in a 1 M NaOH solution with increasing Era are presented in Table 1. The results clearly reveal that jpc shifted to more negative potential regions. The reduction current density peak values for the PtNi/Nifoam catalysts increased by ca. 39.78 and 46.49 times compared to those for the bare Pt electrode, with increases in Era of +0.5 or +0.6 V, respectively. These data indicate that when the oxidation of Ni2+ to Ni3+ is achieved, the reduction of more Pt oxides to metallic Pt are revealed on the backward scan, demonstrating the synergy between the Pt and Ni species.

3.2. Electrocatalytic Evaluation for Formate Oxidation

The electrocatalytic activity of the Pt and PtNi/Nifoam catalysts in a 1 M NaOH solution containing 0.3 M or 0.5 M FA with different Era was evaluated by the CV measurements at a potential scan rate of 50 mV s−1 (Figure 4). Stabilized CVs of FOR for the Pt catalyst plotted in Figure 4a,b reveal one broad peak on the anodic scan, followed by another on the backward scan, regardless of the Era applied. Meanwhile, the first scans presented in Figure S2 (Supplementary Material) for different FA concentrations are significantly different from the stabilized ones. Three current peaks (labeled Peak I, Peak II, and Peak III) can be observed in the positive-going potential scan (solid lines), and one more peak (labeled Peak IV, dashed lines) appears in the negative-going potential scan. The nature of those peaks can be interpreted regarding the fact that all FA solutions with different concentrations studied in our work are strongly alkaline, and that the pKa of HCOOH is well above ~3.8; therefore, it is inevitable that a formate (HCOO−) oxidation reaction will occur [19]. A dual-pathway mechanism, analogous to FAO in acidic media, takes place for formate oxidation in alkaline media on polycrystalline Pt at pH ≈ 14 [19]. Formate typically undergoes oxidation through a reactive intermediate either directly to CO2, or indirectly through a strongly chemisorbed intermediate (COad) that is further oxidized to CO2 at higher potentials. Additionally, a third independent pathway, involving formate oxidation via the formation of less-active surface formate (HCOOads), and its oxidation to CO2 on the Pt surface at significantly higher potentials, should be considered [70].
The first peak (I) at ca. −0.5 V in Figure S2 was assigned to the direct oxidation of formate, and a second hardly discernible peak (II) at ca. −0.35 V was attributed to the indirect oxidation pathway via the strongly chemisorbed intermediate COads [19]. The third peak (III) at ca. +0.00 V indicates an independent pathway of formate oxidation via the formation of the less active surface formate (HCOOads) and its oxidation to CO2 [70]. At ca. −0.5 V, the last peak (IV) on the reverse potential scan developed in the electrode potential region where no surface hydrogen adsorption occurs. The oxide was reduced almost to completion. The properties of peak IV depend on several factors, such as the reduction of surface oxides, CO adsorption on the electrode surface, and mass transfer limitation [70]. As the catalyst active sites were cleaned of adsorbed intermediates at high potentials and the Pt oxides/hydroxides were reduced, this peak is most likely related to the direct oxidation pathway (dehydrogenation). Moreover, the presence of this peak and the potential region of peak I both suggest a similar reaction pathway. Peak IV might have been influenced by the oxidation of adsorbed residual species in this potential region, but this does not seem likely under the conditions of our experiment.
An increase in the number of applied scan cycles leads to a change in the slight shape transformation of the stabilized CVs, resulting in a broader formate oxidation current density peak on the bare Pt electrode (Figure 4a,b). Current density peaks for FO on forward (anodic voltammetric) and backward scans are denoted (jpf) and (jpb), respectively. It is impossible to propose an unambiguous treatment of the origin of the oxidation peak obtained in the case of stabilized CVs on the bare Pt electrode, as the overlapping of the peaks is not excluded, and the merging of direct and indirect FO pathways is presumable.
The stabilized CVs (Figure 4a,b) clearly reveal that a progressive increase in the upper anodic potential limit accelerates the formation of Pt oxides and results in higher Pt oxide reduction peak values on the negative-going potential scans and higher FO current density peak values on the forward-going potential scans of the Pt electrode. These results are similar to those obtained when the FA concentration is increased from 0.3 to 0.5 M.
The increase in the current density peak value for the Pt oxide reduction to metallic Pt occurs with a reduction potential shift to a more negative value. This indicates a more difficult reduction of Pt oxides. Meanwhile, the FO potential for both jpf and jpb remains almost unchanged regardless of the applied Era value. The jpf value triples if Era is changed from −0.2 to +0.6 V for FO at the Pt electrode in 0.3 M FA + 1 M NaOH solution (Figure 4a) and is ca. 1.40 times higher in 0.5 M FA + 1 M NaOH solution (Table 2).
The presence of an extremely clean metallic Pt surface thermodynamically improves the oxidation reaction of COads. However, the decrease in jpb with increasing Era in the forward scan in 0.3 M FA + 1 M NaOH solution is associated with the oxidation of residual incompletely oxidized carbon species on the catalyst surface rather than the oxidation of freshly chemisorbed species. Meanwhile, the absence of a clear correlation between the change in jpb value and the Era in 0.5 M FA + 1 M NaOH solution could be explained by the competitive oxidation between residual incompletely oxidized carbon species and freshly chemisorbed species due to the activation of the surface by the reduction of Pt oxides and/or hydroxides. The presence of Pt oxides and/or hydroxide oxides (such as PtO2) on the catalyst surface is thought to help oxidize the COad, and thus, reduce catalyst poisoning, similar to the case of PdO2 and Au2O3 in an alkaline media [6,70,71,72]. Nevertheless, the scope of the results indicates that the increase in the upper Era appears to favor the direct oxidation pathway in all cases for peaks I and IV.
Similarly, stabilized CVs were recorded for the PtNi/Nifoam catalyst with increasing Era (Figure 4c,d). One broad current density peak was generated in the negative potential region of the PtNi/Nifoam catalyst, characterized by FO via a direct pathway that was followed by a clear current density peak at positive potentials corresponding to the Ni(OH)/NiOOH surface transformation. The formate oxidation current density peak values increased tremendously with increases in the positively going potential limit. Current density peak values ca. 400 times higher than those obtained with the bare Pt electrode were observed, confirming that the ECSAs of PtNi/Nifoam are much larger and that there are many more active sites that facilitate significantly improved FO catalytic reactions owing to the synergy between the Pt and the porous Ni-coated foam substrate. The increase in FA concentration contributes similarly to current growth. A summary of the electrochemical measurements of the PtNi/Nifoam catalyst in 1 M NaOH with 0.3 or 0.5 M FA (Figure 4c,d) is presented in Table 3 and Table 4. The jpf value increased ca. 1.4 or 1.8 times for the PtNi/Nifoam catalyst in 1 M NaOH containing 0.3 M or 0.5 M FA, respectively, when the Era was changed from −0.2 to +0.6 V. The jpf/jpb ratio varied from ca. 1.0–0.8, confirming that the incorporation of Ni oxy-species effectively improves the electrocatalytic kinetics of the Pt-based catalysts and can prevent the accumulation of incompletely oxidized carbonaceous species (COads), directing the FOR toward the dehydrogenation pathway.
The influence of the first applied potential scan cycles recorded for the PtNi/Nifoam catalyst for different Era values in alkaline FA was investigated. For the sake of simplicity, only the positive-going potential CVs with two different Eras (−0.2 and +0.6 V) for the PtNi/Nifoam catalyst in a 1 M NaOH solution, containing different FA concentrations of 0.1, 0.3, and 0.5 M, are presented in Figure 5. Successive cycling in the potential region with an Era of −0.2 V resulted in an apparent jpf decrease at all FA concentrations applied to the PtNi/Nifoam catalyst. Meanwhile, when the Era was decreased to −0.6 V to include the potential window for the oxidation of Ni2+ to Ni3+, the jpf increased (unlike when Era = −0.2 V) and increased further with increasing FA concentration, indicating an improvement in FO. Changing the Era from −0.2 to +0.6 V resulted in increases of ca. 1.3, 1.4, and even 1.8 times for the Ipf values, for 0.1, 0.3, and 0.5 M FA containing alkaline solutions, respectively, on the PtNi/Nifoam catalyst. An increase in FA concentration from 0.1 to 0.5 M led to ca. 4.4–6.1-fold increased jpf values when the upper potential limit was −0.2 V or +0.6 V. Furthermore, a decrease in jpb values can be observed on the negative-going potential scans presented in Figure S3 (Supplementary Material), regardless of the Era value applied. Notably, when Era = −0.6 V, the forward and backward-going current density peak values become closer to each other, indicating better catalyst tolerance to the accumulation of carbonaceous species.
FO improves with increasing (more positive) Era because of the impact of the oxophilic transition metal component in the catalyst composition, which involves the potential region in which the Ni(OH)/NiOOH surface transformation takes place and provides an additional quantity of oxygen-containing species at low potentials, which contributes to the oxidation of adsorbed CO, thus decreasing the catalytic poisoning of the catalyst. In general, nickel hydroxide nanostructures play a crucial role in facilitating the dissociative adsorption of water molecules in alkaline media and the subsequent oxidative removal of carbonaceous poisons from neighboring platinum sites [33,34,35].
The electrocatalytic activity of the investigated catalysts toward FO on FA concentrations is shown in Figure 6. The insets in Figure 6a,b represent the linear relationship between jpeak and CFA, indicating the irreversible electrochemical process kinetics on both Pt and PtNi/Nifoam catalysts [73]. Moreover, the peak potential slightly shifted to the positive potential values with increasing FA concentration (Figure 6a,b). The reaction order of FO oxidation was determined by the slopes of lnjpeak vs. lnCFA (Figure 6c) using Equation (2) [73].
jpeak = zCβFA
where jpeak is the peak current density (mA cm−2), z is a constant, C refers to the FA concentration (mol L−1), and β the reaction order concerning CFA. The value of β is obtained as 1.11 on the PtNi/Nifoam catalyst and 1.19 on the bare Pt electrode (Figure 6c). The determined values corresponded to a first-order reaction.
Nevertheless, increasing the FA concentration from 0.3 to 0.5 M led to a ca. 1.8-times increase in jpf on both catalysts (Figure 6a,b). However, at all FA concentrations, the jpf on PtNi/Nifoam notably outperforms those on the bare Pt electrode. This result was more than ca. 390–420 times higher in alkaline media on PtNi/Nifoam than on bare Pt. The onset potential of the PtNi/Nifoam (ca. −0.9 V) electrode shifted significantly to a more negative potential region than that of the bare Pt electrode (ca. −0.7 V). The large difference in Eonset indicates higher PtNi/Nifoam electrocatalytic activity than that of Pt. Moreover, the obtained characteristics of the PtNi/Nifoam catalyst are consistent with those of the state-of-the-art PGM catalysts for formate oxidation. A comparison of their properties with those of Pt-and Pt-based electrocatalysts used for formate oxidation in alkaline media is presented in Table 5. The results reveal that the developed PtNi/Nifoam catalyst is competitive with state-of-the-art PGM catalysts and outperforms them in many cases. Such favorable results are attributed to the synergy between the Pt and the Ni-coated porous structure of the Nifoam substrate, which prevented the accumulation of incompletely oxidized carbonaceous species (COads), directing the FO reaction toward the dehydrogenation pathway.
To evaluate the mass activity of the investigated PtNi/Nifoam catalyst toward FO, the current density values for different FA concentrations in alkaline media were normalized with reference to the Pt loading for the PtNi/Nifoam catalysts (Figure 6d). The highest FO mass activity of the PtNi/Nifoam catalyst is ca. 990 mA mgPt−1, larger than Pt/C (ca. 225 mA mgPt−1) [40], Pt/C (391.2 mA mgPt−1) [11], and PtSnO2/C (ca. 600 mA mgPt−1) [40].
The stability of the PtNi/Nifoam catalyst was examined by chronoamperometry in 1 M NaOH solutions containing 0.1, 0.3, and 0.5 M FA (Figure 7). As illustrated, the FOR current density for the PtNi/Nifoam catalyst was the highest in 0.5 M FA + 1 M NaOH media; meanwhile, the current density disappeared completely after only 2000 s in a solution containing a lower concentration of FA. This indicates that an increase in the FA concentration increases the stability of the PtNi/Nifoam catalyst.
Additionally, the morphological measurements confirmed the stability of the PtNi/Nifoam catalyst. Figure S4 (Supplementary Material) shows an SEM view of a thin Ni layer deposited on the Nifoam substrate, using sodium hypophosphite as a reducing agent before the electrochemical deposition of Pt. The Ni layer accurately replicates the three-dimensional network structure of the Nifoam backbone with micro-open cages and wide flow channels inside them (Figure S4a). The SEM image of the Pt layer on the Ni/Nifoam surface after continuous electrode cycling in 0.5 M FA + 1 M NaOH in the potential region from –0.9 to +0.6 V vs. Ag/AgCl is displayed in Figure S4b,c (Supplementary Material). Bright spots of Pt the size of about 12–18 nm are evenly distributed over the entire Ni/Nifoam surface, as depicted in Figure S4c (Supplementary Material). The presence of Pt is further supported by EDX data indicating that the amount of deposited Pt in the PtNi/Nifoam catalyst reaches 39.63 at%. Meanwhile, the amount of Ni is 60.37 at%. Continuous cycling in FA solution does not significantly change the surface morphology of the catalyst, indicating the high stability and activity of the PtNi/Nifoam catalyst.
To confirm the better tolerance toward catalyst poisoning by adsorbed carbonaceous species on the PtNi/Nifoam catalyst compared to that on the bare Pt electrode, the CO stripping parameters were adjusted. The obtained data are shown in Figure 8.
A prominent COads oxidation current density peak was observed during the positive potential scan of the bare Pt electrode in 1 M NaOH. Concurrently, the same peak for the PtNi/Nifoam catalyst completely disappeared, indicating that the PtNi/Nifoam catalyst has almost total CO poisoning tolerance in an alkaline medium, unlike bare Pt. The results indicate that the oxidative removal of carbonaceous poisoning species from adjacent Pt sites by Ni(OH)2 was facilitated by the presence of a stable and reversible Ni (II)/Ni (III) redox transformation in alkaline media, as shown in Equation (1). The improvement in the formate oxidation reaction could be explained by the presence of a Ni-oxy species that can act as catalytic mediators via the abovementioned reaction by facilitating a charge transfer during the direct oxidation of formate to CO2, simultaneously oxidizing CO at a low potential throughout subsequent reactions [32,42,43].

4. Conclusions

A promising novel binary catalyst composed of a Pt-modified Ni layer coated on Nifoam was proposed for efficient formate oxidation in an alkaline medium. The PtNi/Nifoam catalyst was found to have a significantly high FO mass activity (990 mA mgPt−1). Moreover, it exhibited enhanced electrocatalytic activity toward formate oxidation via a direct pathway in an alkaline medium, unlike the pure Pt electrode. The prepared PtNi/Nifoam catalyst was tolerant to CO poisoning in an alkaline medium and demonstrated high poison removal ability. It shows rather negative onset potential on the bare Pt. The significant improvement in the Eonset to ca. −0.9 V for PtNi/Nifoam, compared to ca. −0.7 V for bare Pt, indicates that the PtNi/Nifoam electrode is competitive with the benchmark Pt and Pt-based electrocatalysts for formate oxidation in alkaline media. The improvement in electrocatalytic activity is a result of synergy between Pt and the porous structure of the Ni-oxy species layer on the Nifoam, which is intended to assist in the oxidative removal of the accumulated carbonaceous species from the surface by acting as a catalytic mediator for charge transfer in the oxidation process.
The PtNi/Nifoam catalyst appears to be a promising anode material for application in direct liquid FA fuel cells.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst12030362/s1, Figure S1: CVs of Pt (dotted lines) and PtNi/Nifoam (blue solid line) recorded in a 0.5 M H2SO4 solution at a scan rate of 50 mV s−1; Figure S2: CVs at the Pt electrode recorded in a 1 M NaOH solution (red solid line) and that containing 0.3, 0.5, and 0.7 M FA (anodic potential scan—solid lines and cathodic potential scan—dotted lines) at a scan rate 50 mV s−1; Figure S3: Negative-going potential CVs of PtNi/Nifoam recorded at anodic potential limit of 0.2 (a,c,e) and 0.6 V (b,d,f) in a 1 M NaOH solution containing 0.1 (a,b), 0.3 (c,d), and 0.5 (e,f) M FA at a scan rate of 50 mV s−1; Figure S4: SEM views of as-prepared Nifoam substrate coated by a Ni layer (a) and PtNi/Nifoam electrode at different magnifications (b,c) after continuous cycling in 0.5 M FA + 1 M NaOH in a potential region from −0.9 to +0.6 V vs. Ag/AgCl at a scan rate of 50 mV s−1. (d) The corresponding EDX spectra for PtNi/Nifoam electrode.

Author Contributions

This study was conducted through the contributions of all authors. Conceptualization, L.T.-T., A.N. and E.N.; Methodology, D.U., V.K. and V.J.; Investigation, B.Š., A.B. and A.Z.; formal analysis, B.Š. and A.B.; Validation, A.Z., D.Š.; Writing—Original Draft Preparation, A.N. and D.Š.; Writing—Review and Editing, L.T.-T. and E.N.; Visualization, D.U., V.K. and V.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by European Union Structural Funds project “Development of Doctoral Studies” under grant No. 09.3.3.-ESFA-V-711-01-0001.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 00362 g001 550
Figure 1. High-resolution XPS spectra of PtNi/Nifoam: (a) Pt 4f; (b) Ni 2p; (c) O1s.
Figure 1. High-resolution XPS spectra of PtNi/Nifoam: (a) Pt 4f; (b) Ni 2p; (c) O1s.
Crystals 12 00362 g001
Crystals 12 00362 g002 550
Figure 2. CVs of Pt (red dotted line) and PtNi/Nifoam (solid blue line) were recorded in 1 M NaOH solution at a scan rate of 50 mV s−1.
Figure 2. CVs of Pt (red dotted line) and PtNi/Nifoam (solid blue line) were recorded in 1 M NaOH solution at a scan rate of 50 mV s−1.
Crystals 12 00362 g002
Crystals 12 00362 g003 550
Figure 3. The CVs of Pt (a) and PtNi/Nifoam (b) catalysts at the different anodic potential limit (Era) were recorded in 1 M NaOH solution at a scan rate of 50 mV s−1.
Figure 3. The CVs of Pt (a) and PtNi/Nifoam (b) catalysts at the different anodic potential limit (Era) were recorded in 1 M NaOH solution at a scan rate of 50 mV s−1.
Crystals 12 00362 g003
Crystals 12 00362 g004a 550Crystals 12 00362 g004b 550
Figure 4. Stabilized CVs of the Pt (a,b) and PtNi/Nifoam (c,d) catalysts at different Era recorded in a 1 M NaOH solution containing different FA concentrations of 0.3 M (a,c) or 0.5 M (b,d) at a scan rate of 50 mV s−1. (a,b) The dotted lines indicate the stabilized CV curve of Pt recorded in a 1 M NaOH solution at a scan rate of 50 mV s−1. (c,d) Solid lines and dotted lines represent the anodic and cathodic potential scans, respectively, in the CVs.
Figure 4. Stabilized CVs of the Pt (a,b) and PtNi/Nifoam (c,d) catalysts at different Era recorded in a 1 M NaOH solution containing different FA concentrations of 0.3 M (a,c) or 0.5 M (b,d) at a scan rate of 50 mV s−1. (a,b) The dotted lines indicate the stabilized CV curve of Pt recorded in a 1 M NaOH solution at a scan rate of 50 mV s−1. (c,d) Solid lines and dotted lines represent the anodic and cathodic potential scans, respectively, in the CVs.
Crystals 12 00362 g004aCrystals 12 00362 g004b
Crystals 12 00362 g005a 550Crystals 12 00362 g005b 550
Figure 5. Positive-going potential scans of PtNi/Nifoam recorded at anodic potential limit of +0.2 (a,c,e) and +0.6 V (b,d,f) in a 1 M NaOH solution, containing 0.1 (a,b), 0.3 (c,d) and 0.5 (e,f) M FA, at a scan rate of 50 mV s−1.
Figure 5. Positive-going potential scans of PtNi/Nifoam recorded at anodic potential limit of +0.2 (a,c,e) and +0.6 V (b,d,f) in a 1 M NaOH solution, containing 0.1 (a,b), 0.3 (c,d) and 0.5 (e,f) M FA, at a scan rate of 50 mV s−1.
Crystals 12 00362 g005aCrystals 12 00362 g005b
Crystals 12 00362 g006 550
Figure 6. Positive-going potential scans of Pt (a) and PtNi/Nifoam (b) for different FA concentrations with the corresponding jpeak vs. E (the insets) and lnjpeak vs. lnCFA (c). (d) The current density peak values normalized with reference to the Pt loading for PtNi/Nifoam.
Figure 6. Positive-going potential scans of Pt (a) and PtNi/Nifoam (b) for different FA concentrations with the corresponding jpeak vs. E (the insets) and lnjpeak vs. lnCFA (c). (d) The current density peak values normalized with reference to the Pt loading for PtNi/Nifoam.
Crystals 12 00362 g006
Crystals 12 00362 g007 550
Figure 7. CA data for the PtNi/Nifoam catalyst were recorded in a 1 M NaOH solution containing 0.1, 0.3 and 0.5 M FA, at a constant potential value E = −0.6 V for 2600 s.
Figure 7. CA data for the PtNi/Nifoam catalyst were recorded in a 1 M NaOH solution containing 0.1, 0.3 and 0.5 M FA, at a constant potential value E = −0.6 V for 2600 s.
Crystals 12 00362 g007
Crystals 12 00362 g008 550
Figure 8. CVs for the oxidative CO stripping from the Pt (a) and PtNi/Nifoam (b) catalysts surface in 1 M NaOH at a scan rate of 50 mV s−1. CO was adsorbed at −0.9 V from 1 M NaOH for 15 min; a potential sweep was carried out in an N2-saturated solution.
Figure 8. CVs for the oxidative CO stripping from the Pt (a) and PtNi/Nifoam (b) catalysts surface in 1 M NaOH at a scan rate of 50 mV s−1. CO was adsorbed at −0.9 V from 1 M NaOH for 15 min; a potential sweep was carried out in an N2-saturated solution.
Crystals 12 00362 g008
Table
Table 1. Comparison of electrochemical activities for the bare Pt and PtNi/Nifoam catalysts in a 1 M NaOH solution with respect to different anodic potential limits (Era).
Table 1. Comparison of electrochemical activities for the bare Pt and PtNi/Nifoam catalysts in a 1 M NaOH solution with respect to different anodic potential limits (Era).
Era, VEpc, Vjpc, mA cm−2Epc, Vjpc, mA cm−2jpc PtNi/Nifoam/jpc Pt
PtPtNi/Nifoam
+0.1−0.205−0.069−0.235−2.15130.77
+0.2−0.221−0.079−0.256−2.50831.39
+0.3−0.252−0.090−0.288−2.98833.13
+0.4−0.281−0.105−0.321−3.71135.32
+0.5−0.303−0.125−0.356−5.00439.78
+0.6−0.332−0.139−0.389−6.48946.49
Table
Table 2. Summary of electrochemical measurements at Pt catalyst in 1 M NaOH + 0.5 M FA from the data in Figure 4b.
Table 2. Summary of electrochemical measurements at Pt catalyst in 1 M NaOH + 0.5 M FA from the data in Figure 4b.
Era, VEpf, Vjpf, mA cm−2Epb, Vjpb, mA cm−2jpf/jpbjpf at X V/jpf at +0.0 V
+0.0−0.4930.1185−0.4970.09131.301.00
+0.1−0.4900.1322−0.5000.09761.351.12
+0.2−0.5000.1423−0.5070.09781.461.20
+0.3−0.4950.1501−0.5010.09431.591.27
+0.4−0.4990.1537−0.5070.08751.761.30
+0.5−0.4930.1549−0.5090.07851.971.31
+0.6−0.5070.1661−0.5120.09891.681.40
Table
Table 3. Summary of electrochemical measurements at PtNi/Nifoam catalyst in 1 M NaOH + 0.3 M FA from the data in Figure 4c.
Table 3. Summary of electrochemical measurements at PtNi/Nifoam catalyst in 1 M NaOH + 0.3 M FA from the data in Figure 4c.
Era, VEpf, Vjpf, mA cm−2Epb, Vjpb, mA cm−2jpf/jpbjpf at X V/jpf at −0.2 V
−0.2−0.53026.75−0.59125.991.031.00
−0.1−0.56329.30−0.59729.570.991.10
+0.0−0.57035.38−0.58240.100.881.32
+0.5−0.54937.77−0.56548.040.791.41
+0.6−0.55236.80−0.55747.220.781.38
Table
Table 4. Summary of electrochemical measurements at PtNi/Nifoam catalyst in 1 M NaOH + 0.5 M FA for the data in Figure 4d.
Table 4. Summary of electrochemical measurements at PtNi/Nifoam catalyst in 1 M NaOH + 0.5 M FA for the data in Figure 4d.
Era, VEpf, Vjpf, mA cm−2Epb, Vjpb, mA cm−2jpf/jpbjpf at X V/jpf at −0.2 V
−0.2−0.46139.48−0.47838.401.031.00
−0.1−0.44648.73−0.49049.550.981.23
+0.0−0.45454.00−0.48362.530.861.37
+0.5−0.45169.91−0.48191.950.761.77
+0.6−0.44970.32−0.49688.610.791.78
Table
Table 5. Comparison of Pt and the Pt-based electrocatalysts for formic acid/formate oxidation at ambient conditions in an alkaline media with respect to the onset potential (Eonset) and maximum current density (Imax, normalized by electrode geometric area) obtained on anodic peaks (at a peak potential (Epeak)). The data are adopted from [4,8,9,11,12,14,22,70,74,75,76,77].
Table 5. Comparison of Pt and the Pt-based electrocatalysts for formic acid/formate oxidation at ambient conditions in an alkaline media with respect to the onset potential (Eonset) and maximum current density (Imax, normalized by electrode geometric area) obtained on anodic peaks (at a peak potential (Epeak)). The data are adopted from [4,8,9,11,12,14,22,70,74,75,76,77].
CatalystConditions of ExperimentEonset
(V vs. RHE)		Imax (mA cm−2)
at Epeak (V vs.
RHE)		Ref.
Pt disk0.2 M HCOONa + 1 M NaOH
(pH ≈ 14.0) 20 mV s−1		+0.30.1 mA cm−2
(+0.5 V vs. RHE)		[19]
Pt disk0.4 M HCOO- + 1 M KNO3
(pH 13) 5 mV s−1		+0.53.1 mA cm−2
(+1.1 V vs. RHE)		[74]
Pt
nanoparticles
(Pt black)		0.5 M HCOOK + 0.5 M KOH
(pH ≈ 13.7) 50 mV s−1		+0.20.2 mA cm−2
(+0.5 V vs. RHE)		[70]
Pt (40%)/
Vulcan carbon		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 20 mV s−1		+0.414.6 mA cm−2
(+0.6 V vs. RHE)		[22]
Pt (50%)/C0.5 M HCOOH + 0.5 M KOH
(pH ≈ 13.7) 50 mV s−1		+0.833.9 mA cm−2
(+1.2 V vs. RHE)		[11]
PtAg alloy
nanoballoon
nanoassembly		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		+0.232.6 mA cm−2
(+0.7 V vs. RHE)		[17]
Pd black0.5 M HCOOH + 0.5 M KOH
(pH ≈ 13.7) 50 mV s−1		+0.827.3 mA cm−2
(+1.2 V vs. RHE)		[11]
Pd/C1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		+0.1108.8 mA cm−2
(+0.7 V vs. RHE)		[4]
Pd (20%)-H/Vulcan
carbon		0.5 M HCOOK + 1 M KOH
(pH ≈ 14.0) 20 mV s−1		+0.271.0 mA c cm−2
(+0.8 V vs. RHE)		[76]
Pd (20%)/C1 M HCOONa + 1 M NaOH
(pH ≈ 14.0) 20 mV s−1		+0.240.0 mA cm−2
(+0.8 V vs. RHE)		[75]
Pd (25%)/Vulcan
carbon		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		+0.24.6 mA cm−2
(+0.7 V vs. RHE)		[8]
Pd (40%)/Vulcan
carbon		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 20 mV s−1		+0.2102.0 mA cm−2
(+1.0 V vs. RHE)		[22]
Pd/Vulcan
carbon		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		+0.423.0 mA cm−2
(+0.8 V vs. RHE)		[77]
Pd (20%)/Reduced
graphene oxide		1 M HCOONa + 1 M NaOH
(pH ≈ 14.0) 20 mV s−1		+0.257.0 mA cm−2
(+0.8 V vs. RHE)		[75]
Pd54Ag46 (core-shell)1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		+0.231.0 mA cm−2
(+0.7 V vs. RHE)		[8]
Pd70Cu30/Vulcan
carbon		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		+0.24.3 mA cm−2
(+0.6 V vs. RHE)		[12]
Pd72Ce28/Vulcan
carbon		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		+0.119.4 mA cm−2
(+0.6 V vs. RHE)
Pd2.3Co/Vulcan
carbon		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		+0.338.0 mA cm−2
(+0.8 V vs. RHE)
PdNi/Vulcan carbon1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		0.274.0 mA cm−2
(+0.8 V vs. RHE)
PdNi/Ketjen carbon1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		0.2117.0 mA cm−2
(+0.8 V vs. RHE)		[77]
Pd60Ag20Ni20
(alloyed)		1 M HCOOK + 1 M KOH
(pH ≈ 14.0) 50 mV s−1		0.299.6 mA cm−2
(+0.8 V vs. RHE		[9]
Pd2Ag1 aerogel0.5 M HCOOK + 0.5 M KOH
(pH ≈ 13.7) 50 mV s−1		0.227.5 mA cm−2
(+0.8 V vs. RHE)		[8]
Pd2Ag1Pt0.25 aerogel0.5 M HCOOK + 0.5 M KOH
(pH ≈ 13.7) 50 mV s−1		0.160.0 mA cm−2
(+0.7 V vs. RHE)
Pd (interstitial B)0.5 M HCOOK + 1 M KOH
(pH ≈ 14.0) 100 mv s−1		0.290 mA cm−2
(+0.8 V vs. RHE)		[14]
PdCuPt (hierarchical zigzag-branched urchin-like superstructure0.5 M HCOOH + 0.5 M KOH
(pH ≈ 13.7) 50 mV s−1		0.5102.4 mA cm−2
(+1.2 V vs. RHE)		[11]
Pt0.5 M HCOOH + 1 M NaOH
(pH ≈ 14.0) 50 mV s−1		0.340.17 mA cm−2
(+0.53 V vs. RHE)		Our work
PtNi/Nifoam 0.5 M HCOOH + 1 M NaOH
(pH ≈ 14.0) 50 mV s−1		0.1470.32 mA cm−2
(+0.59 V vs. RHE		Our work
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Nacys, A.; Šimkūnaitė, D.; Balčiūnaitė, A.; Zabielaitė, A.; Upskuvienė, D.; Šebeka, B.; Jasulaitienė, V.; Kovalevskij, V.; Norkus, E.; Tamašauskaitė-Tamašiūnaitė, L. An Enhanced Oxidation of Formate on PtNi/Ni Foam Catalyst in an Alkaline Medium. Crystals 2022, 12, 362. https://doi.org/10.3390/cryst12030362

AMA Style

Nacys A, Šimkūnaitė D, Balčiūnaitė A, Zabielaitė A, Upskuvienė D, Šebeka B, Jasulaitienė V, Kovalevskij V, Norkus E, Tamašauskaitė-Tamašiūnaitė L. An Enhanced Oxidation of Formate on PtNi/Ni Foam Catalyst in an Alkaline Medium. Crystals. 2022; 12(3):362. https://doi.org/10.3390/cryst12030362

Chicago/Turabian Style

Nacys, Antanas, Dijana Šimkūnaitė, Aldona Balčiūnaitė, Aušrinė Zabielaitė, Daina Upskuvienė, Benjaminas Šebeka, Vitalija Jasulaitienė, Vitalij Kovalevskij, Eugenijus Norkus, and Loreta Tamašauskaitė-Tamašiūnaitė. 2022. "An Enhanced Oxidation of Formate on PtNi/Ni Foam Catalyst in an Alkaline Medium" Crystals 12, no. 3: 362. https://doi.org/10.3390/cryst12030362

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