Fast Cryomediated Dynamic Equilibrium Hydrolysates towards Grain Boundary-Enriched Platinum Scaffolds for Efficient Methanol Oxidation

Although platinum nanocrystals have been considered as potential electrocatalysts for methanol oxidation reaction (MOR) in fuel cells, the large-scale practical implementation has been stagnated by their limited abundance, easy poisoning, and low durability. Here, grain boundary-enriched platinum (GB-Pt) scaffolds are produced in large scale via facilely reducing fast cryomediated dynamic equilibrium hydrolysates of platinum salts. Such plentiful platinum grain boundaries are originated from the fast fusion of short platinum nanowires during reduction of the individually and homogeneously dispersed platinum intermediates. These grain boundaries can provide abundant active sites to efficiently catalyze MOR and meanwhile enable to oxidize the adsorbed poisonous CO during the electrocatalytic process. As a consequence, the as-synthesized GB-Pt scaffolds exhibit an impressively high mass activity of 1027.1 mA mgPt−1 for MOR, much higher than that of commercial Pt/C (345.2 mA mgPt−1), as well as good stability up to 5000 cycles. We are confident that this synthetic protocol can be further extended to synthesize various grain boundary-enriched metal scaffolds with broad applications in catalysis.


Introduction
Direct methanol fuel cells (DMFCs) have been demonstrated as one of the most promising power sources for electronic mobile devices and electric vehicles due to their ultrahigh energy densities and low pollution [1][2][3]. However, their wide applications have stagnated owing to the notoriously sluggish kinetics of methanol oxidation reaction (MOR) at the anode [4][5][6][7][8]. Thus, it is inevitable to develop high active precious metal electrocatalysts like platinum (Pt) to reduce the energy barriers of MOR [9][10][11]. From the perspective of Pt atomic efficiency [12], the precisely controlled synthesis of Pt nanocrystals such as Pt irregular nanoparticles [13,14], nanowires [15][16][17][18], and nanorods [19] is highly desirable since they have inherent anisotropic morphologies with abundant low-coordinated surface atoms [20,21], enabling to slow down the ripening process and increase the electrocatalytic activities for methanol oxidation.
Very recently, it is demonstrated that Pt nanocrystals with twin defects or dislocations have shown unique electrocatalytic behaviors for methanol oxidation, differentiating from coarsely gained grains, single crystals, or particles [21][22][23], since Pt atoms close to defects have decreased the number of neighbors in the first coordination shell, and are favorable to forcefully adsorb the reactants and catalyze related bond-breaking reactions during the catalytic process [24,25]. Thus, the emergence of abundant defects, dislocations, or grain boundaries not only reduce the activation energy for methanol oxidation but also afford abundant catalytic sites for the oxidation of the adsorbed CO intermediate, significantly improving their stabilities [26,27]. Hence, there is great interest in Pt nanocrystals with a high density of grain boundaries for electrocatalytic MOR [20,26]. Unfortunately, to date, it remains a big challenge to synthesize grain boundary-enriched Pt nanocrystals via a facile and cost-efficient approach [28,29].
Here, 3D grain boundary-enriched Pt scaffolds (3D GB-Pt scaffolds) are facilely produced via reducing fast cryogenic hydrolysates of ammonium hexachloroplatinate ((NH 4 ) 2 PtCl 6 ) aqueous solution. These grain boundaries are originated from the rapid coalescence of short Pt nanowires (~13.2 nm) during reduction of the individually and homogeneously dispersed platinum hydrolysates. These grain boundaries can provide abundant active sites to efficiently catalyze MOR and meanwhile enable to oxidize the adsorbed poisonous CO during the electrocatalytic process. Coupled to the 3D interconnected networks, both mass transport and electron transfer during MOR are fast in 3D GB-Pt scaffolds. As a consequence, the as-synthesized 3D GB-Pt scaffolds show an ultrahigh mass activity of 1027.1 mA mg Pt −1 for MOR, much higher than that of commercial Pt/C (345.2 mA mg Pt −1 ), and long-term durability up to 5000 cycles (only 9.8% loss of the initial activity). Figure 1 and Figure S1, 3D GB-Pt scaffolds were synthesized through reduction of fast cryogenic hydrolysates of platinum salts in aqueous solution. Specifically, (NH 4 ) 2 PtCl 6 (5 mg) was dissolved in 10 ml deionized water, forming a paleyellow solution. After standing for 10 h, the hydrolysis of (NH 4 ) 2 PtCl 6 reaches a dynamic equilibrium as follows [30,31]:

Preparation of 3D GB-Pt Scaffolds. As illustrated in
where the hydrolysis equilibrium could be adjusted via tuning the pH values and the concentration of Cl − during the hydrolysis process ( Figure S2). After fast cryogenic treatment (the treatment temperature is -196°C), a yellow foam with equilibrium hydrated [PtCl 6−n (OH) n ] 2− , NH 4 + , Cl − , and nonhydrolytic [PtCl 6 ] 2− was formed. Then, the foam was reduced at 200°C under a mixed gas of H 2 and Ar with a v/v ratio of 1 : 9 for 2 h, affording 3D GB-Pt scaffolds. In contrast, Pt rods were generated through reduction of the slow cryogenic hydrolysates, since hydrolytic equilibrium of (NH 4 ) 2 PtCl 6 went reversely back during the slow cryogenic process.

Effect of Fast Dynamic Equilibrium Hydrolysates to
Guide 3D GB-Pt Scaffold Formation. To identify the dynamic equilibrium hydrolysates of (NH 4 ) 2 PtCl 6 during our fast cryogenic treatment, UV-vis absorption measurement was conducted owing to its high sensitivity to ligand-to-metal (Cl − ➜Pt) charge transfer of [PtCl 6 ] 2− and [PtCl 6−n (OH) n ] 2− complexes [32,33]. In principle, by adjusting the pH values of (NH 4 ) 2 PtCl 6 solution from 0 to 12, the different dynamic equilibriums could be achieved, as demonstrated by their UV-vis absorption spectra with different absorbance intensities at~262 nm (Figure 2(a)), which were originated from the charge transfer involving orbitals with Cl ligand π-character [30,33]. Notably, while adjusting the pH value to 14, all the (NH 4 ) 2 PtCl 6 was conversed to [Pt(OH) 6 ] 2− , Cl − , and NH 4 + , where the hydrolytic equilibrium was completely broken [34]. Through fast cryogenic treatment, the hydrolysates at the different dynamic equilibrium states could be well maintained. This could be clearly demonstrated via their UV-vis absorption spectra (Figure 2(b)). On the contrary, through slow cryogenic treatment (the treatment temperature is -5°C), all the adsorption bands of the hydrolysates at the different dynamic equilibrium states showed the similar absorption intensities (Figure 2(c)). It is suggested   Figure 1: Schematic illustration of the synthesis of 3D GB-Pt scaffolds. The procedure for preparing 3D GB-Pt scaffolds involves two steps: (1) fast cryogenic treatment of the (NH 4 ) 2 PtCl 6 solution to produce fast cryogenic hydrolysates; (2) hydrogen reduction of the obtained fast cryogenic hydrolysates.
To further evaluate the crystalline phases of fast cryogenic intermediates, X-ray diffraction (XRD) measurement was conducted and is shown in Figure 2(d). Interestingly, in the case of fast cryogenic intermediates, except for the characterization peaks of (NH 4 ) 2 PtCl 6 , there are four additional XRD peaks detected at 22.9°, 32.6°, 46.9°, and 58.3°( marked with blue diamonds in Figure 2 Figure S4) reveal that the product NH 4 Cl behaves like a matrix to accommodate the homogeneous dispersion of the fast cryogenic Ptcontaining intermediates with sizes of 3-5 nm. Hence, through initial hydrogen reduction treatment, numerous individual Pt nanocrystalline structures could be produced, derived from Pt-containing intermediates. Meanwhile, NH 4 Cl would be gradually decomposed and leave gaps between the Pt nanocrystalline structures. With further increase of the reduction time, such gaps allow the infusion between adjacent Pt nanocrystalline structures, generating grain boundary-enriched Pt scaffolds as demonstrated in Figure 3 and Figures S5 and S6. The typical TEM images (Figure 3(b)-3(e) and Figure S7) clearly disclose that these scaffolds are constructed from short Pt crystalline wires with average lengths of~13 nm and diameters of~4.2 nm. HRTEM images (Figures 3(d) and 3(e) and Figure S8) show clearly the grain boundaries between two short Pt wires and an obvious interlayer spacing of 0.223 nm, corresponding to d(111) of Pt [35,36]. In contrast, different levels of hydrolysate [PtCl 6−n (OH) n ] 2− and Cl − were recrystallized to single-crystalline (NH 4 ) 2 PtCl 6 during the slow cryogenic treatment, without the detection of equilibrium products of the hydrolysis (Figure 2(d) and Figure S9). After hydrogen reduction treatments of slow cryogenic intermediates, only Pt rods were obtained (Figures S10 and S11). This may be ascribed to the low level of NH 4 Cl in the slow cryogenic intermediates, which are unable to prevent the fast growth of big Pt crystalline structures. Moreover, pH values of the (NH 4 ) 2 PtCl 6 solution also affect the formation of 3D GB-Pt scaffolds. As shown in Figures S12 and S13, while the pH value is 2 and 7, the resultant GB-Pt scaffolds show interconnected 3D networks. As the pH value is 0, all the nanowires or particles are strongly aggregated, owing to the less amount of the hydrolysis product NH 4 Cl, which cannot efficiently prevent the aggregation during reduction process. In contrast, as the pH value arrives to 12 and 14, all the Pt nanocrystalline structures are separated, without formation of good networks. This should be ascribed to the excessive hydrolysis product NH 4 Cl after fast cryogenic treatment, which prevents the infusion between the formed Pt nanocrystalline structures during the reduction process. These results manifest that the appropriate amount of NH 4 Cl plays a key role to the formation of 3D grain boundary-enriched Pt scaffolds. To further confirm this hypothesis, two chloroplatinates without ammonium (Na 2 PtCl 6 and K 2 PtCl 6 ) were selected to substitute the platinum precursor (NH 4 ) 2 PtCl 6 during our fabrication processes. As shown in Figure S14a-c, only Pt nanoparticles were generated after reduction of the fast cryoequilibrium hydrolysates of K 2 PtCl 6 . If we deliberately added an amount of NH 4 Cl into the K 2 PtCl 6 system, 3D Pt scaffolds could be formed again (Figure S14d-f and Figure S15). This phenomenon appears again as Na 2 PtCl 6 was used as the Pt precursor (Figures S16 and S17). Therefore, the presence of a moderate amount of NH 4 Cl is the essence of the production of 3D grain boundaryenriched Pt scaffolds. In this manner, various grain boundary-enriched metal (e.g., Pd) scaffolds could be fabricated via our fast cryogenic treatment with the presence of a NH 4 Cl additive, and the detailed characterizations of 3D GB-Pd scaffolds are shown in the Supplementary Materials ( Figure S18-S21).
It should be noted that the grain boundary density of the sample can be well controlled through tuning of the hydrolysis equilibrium by adjusting the concentrations of (NH 4 ) 2 PtCl 6 solution before fast cryogenic treatment. As shown in Figure S22, a higher concentration of (NH 4 ) 2 PtCl 6 leads to an increase of the length of Pt wires. Based on the TEM analysis, we quantitatively measured the average Pt grain length of the GB-Pt scaffolds. It is shown that the average grain length of GB-Pt scaffolds-5 is only 13.2 nm (the grain boundary density of 75.9 μm −1 ), which is the highest grain boundary density among all the GB-Pt scaffolds samples. (For details, see the Supplementary Materials.) In this manner, the grain boundary density of 3D GB-Pd scaffolds was 64.1 μm −1 ( Figure S23).

Structural and Compositional Analysis of 3D GB-Pt
Scaffolds. To gain further insight into the crystalline properties of 3D GB-Pt scaffolds, XRD, XPS, and nitrogen adsorption/desorption isotherm were carried out. As revealed in Figure 4(a) and Figures S24 and S25, in the case of 3D GB-Pt scaffolds, there are three prominent XRD peaks at 39.8°, 46.2°, and 67.7°, corresponding to the (111), (200), and (220) planes of face-centered-cubic (fcc) platinum (JCPDS no. 87-0647) [37], respectively, in good accordance with the above HRTEM and SAED analyses. The XPS survey demonstrates that there are only two main species of Pt and O detected in our 3D GB-Pt scaffolds, without other impurities (Figure 4(b) and Figures S26 and S27). N 2 adsorption-desorption data indicates a high specific surface area of 50.6 m 2 g −1 , much higher than that of the Pt rods (21.1 m 2 g −1 ) sample (Figure 4(c)). ) (Table S1). Accurately, as shown in Figures 5(b) and 5(c), an ultrahigh peak with current density of 203.8 mA cm −2 is achieved, corresponding to 1027.1 mA mg Pt −1 for 3D GB-Pt scaffolds in mass activity, much higher than that of the Pt/C sample (345.2 mA mg Pt −1 ) ( Figure S29). Moreover, the onset potential of 3D GB-Pt scaffolds is only 217 mV, much lower than those of Pt rods (257 mV) and the Pt/C catalyst (356 mV). Associated with the above TEM and HRTEM analyses, such high electrocatalytic activities of 3D GB-Pt scaffolds should be attributed to the large presence of grain boundaries of Pt. This can be further demonstrated by the different 3D GB-Pt scaffolds with tunable intensities of grain boundaries, in which their activities are linearly proportional to the densities of grain boundaries of Pt ( Figure 5(d)). The electrocatalytic activity of 3D GB-Pt scaffolds obtained at different pH values was also systematically investigated ( Figure S30). Among them, 3D GB-Pt scaffolds-pH(7) exhibit a very high value of 1027.1 mA mg Pt −1 , much higher than other samples (368.9, 586.9, 485.6, and 166.6 mA mg Pt −1 for 3D GB-Pt scaffolds-pH(0), (2), (12), and (14), respectively).
To further investigate the electrocatalytic stability of 3D GB-Pt scaffolds for MOR, we conducted a stability test in 0.5 M H 2 SO 4 and 1 M methanol. Remarkably, even after 5000 cycles, there is only 9.8% loss of the initial activity for 3D GB-Pt scaffolds ( Figure 5(e)). This value is much lower than that of the Pt/C catalyst (66.5% loss of the activity), clearly confirming the excellent durability of 3D GB-Pt scaffolds for electrocatalytic MOR. To gain insight into the reason of the excellent durability performance of 3D GB-Pt scaffolds, CO-stripping measurement was conducted [38]. As demonstrated in Figure S31, the peak potential of 3D GB-Pt scaffolds is only 0.54 V (vs. SCE), much lower than that of Pt/C (0.61 V vs. SCE), suggesting that 3D GB-Pt scaffolds have an enhanced antipoisoning (CO) property [39]. This should ascribe to the presence of large Pt grain boundaries that can efficiently oxidize the adsorbed CO ads [40]. Furthermore, after durability measurement, both the configurations and grain boundaries of 3D GB-Pt scaffolds were well preserved ( Figure S32). The electrochemical impedance spectroscopy (EIS) of the 3D GB-Pt scaffolds, Pt rods, and Pt/C electrocatalysts was carried out to investigate the kinetics of methanol oxidation. As shown in Figures S33  and S34, typical methanol oxidation behaviors catalyzed by the Pt-based catalyst are observed at a series of potentials from 0.1 to 1.0 V [11]. Among them, 3D GB-Pt scaffolds have a much smaller semicircle diameter than Pt rods and Pt/C electrocatalysts at 0.4 V ( Figure 5(f)), demonstrating the high methanol oxidation rate of 3D GB-Pt scaffolds [41], in accordance with the electrocatalytic MOR results ( Figure 5(b)). In addition, 3D GB-Pd scaffolds also exhibit remarkably high activity of 1117.9 mA mg Pd −1 ( Figure S35) and good stability (9.0% loss of the initial activity after 5000 CV cycles) towards MOR ( Figure S36).

Discussion
In summary, grain boundary-enriched platinum scaffolds were produced in large scale via simply reducing fast cryomediated dynamic equilibrium hydrolysates of (NH 4 ) 2 PtCl 6 . Such plentiful platinum grain boundaries are ascribed to the fast fusion of short platinum nanowires during reduction of the individually and homogeneously dispersed platinum-containing intermediates in the NH 4 Cl matrix, which is the essence of the production of grain boundary-enriched Pt scaffolds. These grain boundaries  ). Moreover, the mass activities of 3D GB-Pt scaffolds are linearly proportional to the densities of grain boundaries of Pt. And these grain boundaries enable oxidization of the adsorbed poisonous CO during the electrocatalytic process, leading to a long stability up to 5000 cycles. We are confident that such a simple synthetic protocol can be extended to produce various grain boundary-enriched metal scaffolds with broad applications for catalysis and sensors.

Synthesis of 3D GB-Pt Scaffolds.
For the synthesis of 3D GB-Pt scaffolds, a certain amount of (NH 4 ) 2 PtCl 6 was initially added in DI water (10 ml) to form a pale-yellow solution. The solution was stirred for 60 min and then frozen at -196°C in liquid nitrogen for 30 min; after that, the obtained frozen product was further dried under vacuum conditions at -60°C for 48 h, and a three-dimensional yellow foam was obtained. The foam was further reduced at 200°C for 2 h under 10% H 2 /Ar gas, generating 3D GB-Pt scaffolds. 3D GB-Pt scaffolds-X were fabricated, where X represents the amount of (NH 4 ) 2 PtCl 6 in the 10 ml solution before fast cryogenic treatment. 3D GB-Pt scaffolds-pH(Y) were also fabricated, where Y represents the pH value of the (NH 4 ) 2 PtCl 6 solution before fast cryogenic treatment.

Synthesis of Pt
Rods. The synthetic procedure of Pt rods was similar to that of 3D GB-Pt scaffolds except that the fast cryogenic treatment was replaced by slow cryogenic treatment (the freezing temperature changed form -196°C to -5°C).

Supplementary Materials
(m-o) 3D GB-Pt scaffolds-pH (14). Figure S13: TEM images of GB-Pt scaffolds derived from hydrogen reduction of fast cryoequilibrium hydrolysates with different pH values.
(d-f) SEM images of reduced fast cryoequilibrium hydrolysates of K 2 PtCl 6 and NH 4 Cl solution. Figure S15: (a-c) TEM images of reduced fast cryoequilibrium hydrolysates of K 2 PtCl 6 with additional NH4Cl, with marked grain boundaries in (c). Figure  S16: SEM images of reduced fast cryoequilibrium hydrolysates of Na 2 PtCl 6 without and with additional NH 4 Cl.
(a-c) SEM images of reduced fast cryoequilibrium hydrolysates of Na 2 PtCl 6 solution.
(d-f) SEM images of reduced fast cryoequilibrium hydrolysates of Na 2 PtCl 6 and NH 4 Cl solution. Figure S17: (a-c) TEM images of reduced fast cryoequilibrium hydrolysates of Na 2 PtCl 6 with additional NH 4 Cl, with marked grain boundaries in (c). Figure Figure  S19: (a-d) HRTEM images of 3D GB-Pd scaffolds with marked grain boundaries. Figure S20: XRD patterns of the 3D GB-Pd scaffolds and Pd/C. Figure S21: (a) nitrogen adsorption/desorption isotherm of 3D GB-Pd scaffolds. (b) Nitrogen adsorption/desorption isotherm of Pd/C. Figure  S22: grain length distribution of the 3D GB-Pt scaffolds.
(d) Nyquist plots of 3D GB-Pt scaffolds, Pt rods, and Pt/C electrocatalysts for methanol oxidation at 0.4 V. Figure S34: (a-c) Nyquist plots of 3D GB-Pt scaffolds, Pt rods, and Pt/C electrocatalysts for methanol oxidation before and after stability test. Figure S35: (a, b) comparative Pd specific activity and mass activity of 3D GB-Pd scaffolds and Pd/C (10 wt%) in 1 M KOH and 1 M methanol electrolyte. Figure S36: CVs of 3D GB-Pd scaffolds before and after 5000 cycles in 1 M KOH and 1 M methanol electrolyte.