Sustainable Furfural Biomass Feedstocks Electrooxidation toward Value‐Added Furoic Acid with Energy‐Saving H2 Fuel Production Using Pt‐Decorated Co3O4 Nanospheres

Here, furfural oxidation was performed to replace the kinetically sluggish O2 evolution reaction (OER). Pt‐Co3O4 nanospheres were developed via pulsed laser ablation in liquid (PLAL) in a single step for the paired electrocatalysis of an H2 evolution reaction (HER) and furfural oxidation reaction (FOR). The FOR afforded a high furfural conversion (44.2%) with a major product of 2‐furoic acid after a 2‐h electrolysis at 1.55 V versus reversible hydrogen electrode in a 1.0‐M KOH/50‐mM furfural electrolyte. The Pt‐Co3O4 electrode exhibited a small overpotential of 290 mV at 10 mA cm−2. As an anode and cathode in an electrolyzer system, the Pt‐Co3O4 electrocatalyst required only a small applied cell voltage of ∼1.83 V to deliver 10 mA cm−2, compared with that of the pure water electrolyzer (OER∥HER, ∼1.99 V). This study simultaneously realized the integrated production of energy‐saving H2 fuel at the cathode and 2‐furoic acid at the anode.


Introduction
The electrochemical oxidation and hydrogenation of biomass feedstocks are emerging as environmentally friendly and sustainable processes for producing high-value-added products.The electrocatalytic process occurs by the applied potential, which can be driven under substantially mild conditions.[3] Recently, the electrocatalytic half-cell reactions of biomassderived feedstock oxidation and reduction with a typical water electrolyzer system have attracted interest.They involve two half-reactions, namely, HER and OER, respectively.6][7][8][9][10] The half-reaction of OER in OWS is continuously a bottleneck for the electrolysis process, owing to its high theoretical potential (1.229 V vs RHE).This consumes a large portion of input electrical energy and is a kinetically sluggish reaction. [5]Apart from being the outcome of OER, O 2 is not a considerably attractive high-value chemical.Therefore, the coupling of HER with other oxidation reactions of biomass-derived molecules is highly desirable for input electrical energy-saving, simultaneously produces H 2 fuel during cathodic reduction, and upgrades value-added products during anodic oxidation. [1,5]he refinement of biomass is focused on accomplishing a carbon-neutral economy.The utilization of biomass (the biggest natural carbon source) will not change the ecosystem carbon balance because biomass feedstocks supply contemporary carbon. [4,11]Furfural, which is most commonly observed as a versatile biomass feedstock, can be derived by the dehydration/hydrolysis of carbohydrates, such as bagasse and corncobs.In addition, furfural is electrocatalytically oxidized to furoic acid as a major product.[14] Thus, recently, a few attempts have been dedicated to investigating the electrocatalytic oxidation of furfural coupled with HER, for example, Ni 3 N-V 2 O 3 , [15] Cu nanoparticles (NPs), [16] Pt/Au, [17] Cu 3 P nanosheets, [18] Ni 2 P/Ni arrays, [4] and NiFe oxide. [1]The efficiency of electrochemical reactions (HER and furfural oxidation reaction (FOR)) and the product yield of H 2 and furoic acid mainly depend on the electrocatalyst selected.[21] Advanced synthetic methods have been extensively used for the synthesis of highly electro-active catalysts.Here, the pulsed laser ablation in liquids (PLAL) technique in an aqueous solution was employed for the fabrication of Pt-decorated Co 3 O 4 nanospheres (Pt-Co 3 O 4 ) in a single-step reaction.The PLAL process is an innovative synthetic method that produces high-purity catalytic nanomaterials in a short time.[24][25][26] Conventional synthetic routes such as chemical reduction, hydro, and solvothermal methodologies are typically time-and energyconsuming and generally require expensive precursors and toxic reducing agents, stabilizing agents, or surfactants to control the surface and size, and frequently afford by-products. [27,28]Thus, the synthesized Pt-Co 3 O 4 electrocatalyst was proficient in catalyzing FOR to 2-furoic acid and HER to H 2 in an alkaline medium.Interestingly, the assembled FOR-integrated electrolyzer with Pt-Co 3 O 4 required only ∼1.83 V cell voltage to deliver 10 mA cm −2 , which was substantively less than that of the OWS electrolyzer (∼1.99 V).This study established the feasibility of constructing a two-electrode (Pt-Co 3 O 4 kPt-Co 3 O 4 ) electrolyzer with the Pt-Co 3 O 4 catalyst acting as the anode and cathode and simultaneously realized the integrated production of value-added 2-furoic acid and energy-saving H 2 fuel.

Structural Analyses
The schematic demonstration for the synthesis of Pt-Co 3 O 4 nanospheres via PLAL is shown in Figure 1a.The Pt-Co 3 O 4 nanospheres were synthesized by the ablation of the Co plate in 10 mL of 50-μM aqueous platinum salt solution (Figure 1a (stage-i)).Subsequently, during PLAL (stage-ii), the focused laser generates a plasma plume at high temperature (∼2000 K) and pressure (∼100 atm) containing vaporized metal ions and nuclei (M x+ ) on the surface of the Co plate.Simultaneously, laser-induced decomposition of solvent (water molecule).The plasma plume then collapses, releasing M x+ species in the solvent, where they interact with the solvated electrons, • H and • OH radicals, and undergo the following transformation: M x+ + OH − → M-(OH) x → M y O x , giving Co 3 O 4 as a product. [29,30]Additional reduction of Pt 2+ ions in 50-μM K 2 PtCl 4 aqueous solution to Pt NPs occurs under the combined effect of photo-irradiation from the laser and • H radicals after the rapid quenching of the plasma plume (Figure 1a (stage-ii)).Therefore, the simultaneous process of Co 3 O 4 formation and Pt 2+ ions reduction in the same environment during PLAL is highly beneficial for the establishment of strong metal-support interactions between Pt and Co 3 O 4 in Pt-Co 3 O 4 composites obtained after PLAL (Figure 1a (stage-iii)).The precise control of the low-weight percent precious metal decoration is of great interest for scale-up industrial applications as performance comparable with pure metallic composites can be achieved with doping as low as ∼5 wt.%.Hence, it is possible to disperse less noble metal onto the bare electrocatalyst to construct low-cost and highefficiency advanced catalysts.Therefore, we prepared ∼5-6 wt.% Pt NPs decorated with Co 3 O 4 nanospheres with twice the performance of bare Co 3 O 4 .A further increase in Pt loading affects the formation of Co 3 O 4 during the PLAL process and also decreases the yield of the final products, possibly due to the more acidic nature of the Pt precursor.
Initially, the samples were analyzed by powder XRD, and the patterns are shown in Figure 1b.Both samples exhibited sharp peaks at 31.3°, 36.9°, and 59.4°corresponding to the (220), (311), and (511) crystallographic planes, respectively.The peak positions and relative intensities correlated with the JCPDS card no.00-042-1467 of cubic Co 3 O 4 . [31,32]In addition, the Pt-Co 3 O 4 sample exhibited a faint peak of cubic Pt NPs at 39.9°, corresponding to the most prominent (111) crystal plane according to the JCPDS card no.70-2057. [5]The percentage of relative crystallinity was estimated from the ratio between the areas of the characteristic XRD peaks of Co 3 O 4 grown in the absence and presence of Pt ions.The calculated Pt-Co 3 O 4 crystallinity was about ∼74.59% compared with the bare Co 3 O 4 due to lattice distortions occurring during the simultaneous formation of Co 3 O 4 crystallines and the reduction of Pt ions to Pt NPs. [33]However, peak positions and symmetry are maintained before and after the experiment indicating phase purity without the presence of alloy or bimetallic particles.Therefore, undesirable side reactions such as the alloying of Co and Pt did not occur.The Raman spectra of the samples were recorded in the 100-1000 cm −1 range, where all the Raman active modes corresponded to the normal spinel structure of Co 3 O 4 .Co 3 O 4 belongs to the O h [7] symmetry group and crystallized with Co 2+ and Co 3+ in tetrahedral and octahedral sites, respectively. [33]This structure exhibited five Raman active modes, namely, A 1g , E g , and three F 2g modes, which were observed in the Raman spectra of the bare Co  [34] These results were supported by the ICP-OES analysis, and the full data are provided in Table S1, Supporting Information.Furthermore, HR-TEM revealed the crystal planes in both samples, with characteristic interplanar distances of 2.86, 2.  3b). [35,36]  water. [37,38]In the Pt 4f spectrum of Pt-Co 3 O 4 sample, the 4f 7/2 at 71.6 eV and 4f 5/2 at 74.6 eV components exhibit asymmetric shape, which are characteristics of Pt NPs in the metallic state.Furthermore, the fitting of XPS spectrum reveals the presence of faint signals of Pt (II) and Pt(IV) from PtO and PtO 2 at 73.5 and 75.7 eV, respectively which caused due to the increase in surface oxygen concentration in Pt NPs in Pt-Co 3 O 4 composite. [39,40]The results of XPS analysis manifest the successful synthesis of pure Co 3 O 4 and simultaneous decoration of the surface with Pt NPs having strong and proper interaction with Co 3 O 4 in Pt-Co 3 O 4 composite.Thus, the migration of charge was facilitated. [41]Furthermore, the degree of activity of the catalytic sites for the electrocatalysts was estimated by TOF values of −0.1, −0.2, and −0.3 V versus RHE (Figure 4e).To evaluate the TOF, the active site number was estimated from the area under the backward CV profile recorded at 50 mV s −1 for each electrocatalyst (Figure S2, Supporting Information).The TOF of Pt-Co 3 O 4 at the aforementioned potentials were 0.02, 0.06, and 0.19 s −1 , respectively, which were nearly threefold the TOF values estimated for the bare Co 3 O 4 (0.002, 0.04, and 0.07 s −1 ).Hence, a great number of molecules reacted at each active site of Pt-Co 3 O 4 .Another important parameter is the ECSA value, which is a quantitative analysis of the area available for a particular reaction to occur on the electrode material.The C dl value is directly connected to the ECSA, and the C dl is in direct ratio to the slope of the linear dependency of the anodic and cathodic residual current versus the scan rate of the CV curves performed at diverse scan rates in the nonfaradaic region (Figure S3, Supporting Information).As shown in Figure 4f, the assessed C dl values of 6.7 and 5.1 mF cm −2 for Pt-Co 3 O 4 and bare Co 3 O 4 showed that the decoration of Co 3 O 4 with Pt NPs provided additional sites and increased the number of active centers for the electrochemical reaction to occur. [42]Bulk electrolysis was performed to test the stability of the optimal Pt-Co 3 O 4 electrocatalyst during HER at −0.3 V versus RHE for 10 h (Figure S4, Supporting Information).Continuous H 2 gas evolution and a negligible current loss (∼7% of initial current density) confirmed the strong physical and chemical properties of the Pt-Co 3 O 4 electrocatalyst.The decoration of Co 3 O 4 supports with high surface energy metals such as platinum favors hydrogen activation, which increases the HER kinetics and enhances the H 2 production rate at low overpotentials. [43]Thus, a small amount of Pt NPs on the surface of Co 3 O 4 lowers the overpotential value from 464 to 290 mV.In addition, the metallic nature of Pt NPs accelerates the charge transfer due to the low resistance of highly crystalline metals, which can be clearly seen in the R ct values obtained from impedance spectroscopy.These results revealed that the Pt decoration in Co 3 O 4 was beneficial for the enhancement of its electrochemical application, as further confirmed during FOR.

Furfural Oxidation Reaction
Bare Co 3 O 4 and Pt-Co 3 O 4 were tested during furfural electrooxidation in 1.0-M KOH to afford the value-added product, 2-furoic acid.First, LSV curves of the bare electrolyte were recorded for both electrocatalysts, and simple OER occurred at given oxidation potentials (Figure 5a,b, black curves).Similarly, LSV curves were recorded for 1.0-M KOH/ 50-mM furfural, where the current density rapidly increased at low potentials, which specified that the oxidation of organic furfural to 2-furoic acid is a more energetically favorable reaction, compared with water oxidation (Figure 5a,b, red and blue curves for Co 3 O 4 and Pt-Co 3 O 4 , respectively). [4]Comparative current densities at 1.35, 1.4, 1.45, and 1.5 V versus RHE are presented in Figure 5c, which shows a sixfold current density in the presence of 50-mM furfural.Furthermore, the performance of Pt-Co 3 O 4 was better under oxidizing potentials; it required a low potential of ∼1.6 V versus RHE to attain 10 mA cm −2 in 1.0-M KOH and ∼1.45 V versus RHE in 50-mM furfural, compared with the bare Co 3 O 4 (1.66 and 1.62 V vs RHE).To quantify the furfural conversion to 2-furoic acid, continuous FOR at a constant potential (1.55 V vs RHE) was performed for 120 min.Subsequently, the sample underwent HPLC at a 30-min interval (Figure S5, Supporting Information).The 2-furoic acid yield was estimated from the calibration curve of standard 2-furoic acid with concentrations of 1-50 mM measured prior to the FOR (Figure S6, Supporting Information).
A constant decrease in the intensity (i.e., the integrated area under the HPLC curve) of the furfural signal after ∼4 min and an increase in the characteristic peak of 2-furoic acid after 6.2 min during HPLC suggested the continuous conversion during FOR with the bare Co 3 O 4 and Pt-Co 3 O 4 (Figures S7 and S8, Supporting Information, respectively). [44]ccordingly, the furfural conversion and 2-furoic acid production in mM concentration range were estimated, and the results are shown in Figures S9 and S10, Supporting Information.A total of 16.5 mM (∼33%) from the initial furfural concentration (50 mM) was converted within 120 min of FOR over the Co 3 O 4 electrocatalyst, resulting in 6.4 mM of 2-furoic acid with FE of 34.8% (Figure 5d, Figure S9 and S10a, Supporting Information).Compared with the optimal Pt-Co 3 O 4  S2, Supporting Information).[47][48] Similarly, Co 3 O 4 possesses such active centers to drive the FOR in alkaline media.However, the selectivity of the Co metal toward the single FOR product is limited.51][52] Oppositely, the high oxidation capability of Pt NPs was reported to play an important role in the oxidation of biomass-derived organic molecules, such as furfural and hydroxymethylfurfural. [41,42]herefore, the deposition of Pt NPs on Co 3 O 4 nanospheres being advantageous for performance enhancement is the focus of the present study.The Pt NP active centers provided extra sites for the furfural adsorption on the electrocatalysts toward the selective conversion into 2-furoic acid with an FE enhancement of the overall process.Moreover, these Pt NPs can exhibit affinity toward H 2 O or OH − , which may further participate in the formation of Co-based reactive intermediates for FOR in alkaline media. [43]The XRD analysis of the Pt-Co 3 O 4 working electrode was performed after 2 h of continuous bulk electrolysis, confirming the formation of a thin layer of reactive species on the surface of the Pt-Co 3 O 4 working electrode during FOR (Figure S12, Supporting Information).The obtained XRD pattern revealed the presence of Pt-Co 3 O 4 peaks with additional sharp diffraction peaks of the resulting reactive Co(OH) 2 (JCPDS card no.00-030-0443) and CoOOH (JCPDS card no.01-073-1213) phases. [30,53]In addition, the XPS analysis of Pt-Co 3 O 4 working electrode materials was also performed after the electrochemical stability test to confirm the chemical stability, as shown in Figure S13, Supporting Information.The resulting Co 2p, O 1s, and Pt 4f core-level spectra obtained after the electrolysis test were identical to those before electrolysis (Figure 3), except for minor changes related to the formation of surface reactive species.Considering the formation of intermediate Co-based active species and Pt NP active sites on the Pt-Co 3 O 4 surface, furfural oxidation can be described by the following reaction: where F denotes the furanic ring, Pt* is the Pt NP active site, and Pt-ox is the intermediate state of the Pt active site under a high oxidation potential, which can be eventually recovered as Pt*. [47]nitially, the surface of the electrocatalysts will change to form a thin layer of reactive intermediates, i.e., CoOOH.Simultaneously, the aldehyde molecule transforms into the corresponding di-ion upon the attack of excess hydroxide from the solution, followed by a surface chemical reaction such as the adsorption of di-ions on CoOOH and Pt* centers.Next, the strong electron-donating behavior of two O − in the di-ion forces the aldehydic hydrogen with its electron pair to attack another furfural molecule, leaving behind the salt of the 2-furoic acid, furoate (i.e., F-COO − − -K + ).[56] Considering the substantial performance of Pt-Co 3 O 4 in HER and FOR, a two-electrode electrolyzer separated by a proton exchange membrane was fabricated using Pt-Co 3 O 4 as the cathode and anode (Figure 6a). [16]For comparison, OWS (water electrolysis) was initially performed in pure 1.0-M KOH, where Pt-Co 3 O 4 achieved 5 and 10 mA cm −2 at 1.86 and 1.99 V, respectively, which demonstrated the ability to generate H 2 and O 2 (Figure 6b).Upon adding 50-mM furfural in the anodic compartment, the electrolyzer cell voltage shifted to lower values, which only required 1.58 and 1.83 V to achieve 5 and 10 mA cm −2 , respectively, where the cell voltages were ∼ 280 and 160 mV lower than those of the water electrolyzer (Figure 6c). Figure 6d shows the current density comparison of the pure water and FOR-integrated electrolyzers at different voltages in the range of 1.4-1.8V.The electrolyzer coupled with HER and FOR revealed low cell voltages, indicating the energy-saving simultaneous production of highly desired H 2 and 2-furoic acid.

Conclusions
Pt-Co 3 O 4 nanospheres were synthesized by PLAL as robust bifunctional electrocatalysts.The Pt and Co 3 O 4 nanospheres enhanced the accessible ECSA.The half-cell FOR performed in an H-cell system afforded a high furfural conversion (∼44.2%) with a high yield of 2-furoic acid (13.2 mM).For HER, the Pt-Co 3 O 4 electrocatalyst required a low η (290 mV) at 10 mA cm −2 in an alkaline medium.The two-electrode (Pt-Co 3 O 4 kPt-Co 3 O 4 ) electrolyzer system in an H-cell for FOR and HER generated 2-furoic acid and H 2 fuel at the anode and cathode, respectively, and attained 10 mA cm −2 at a low voltage (∼1.83 V).
PLAL-assisted synthesis of Pt-decorated Co 3 O 4 nanospheres: Pt-decorated Co 3 O 4 nanospheres were synthesized by PLAL in a single step.In a typical synthesis, the PLAL of a Co target placed in 10 mL of a 50-μM aqueous platinum salt solution was performed using the Nd:YAG (Surellite-II) pulsed laser.The pulsed laser with a fundamental wavelength of 1064 nm with 100 mJ/pulse power was focused on the surface of the Co target by focusing the lens with a 30-mm focal length.After 30 min of PLAL, the obtained colloidal solution was centrifuged at 22 430 × g and washed well with water and ethanol many times.Thereafter, the obtained Pt-Co 3 O 4 nanosphere sample was dried at 100 °C overnight.The detailed sampling process for all characterization techniques is described in Appendix S1, Supporting Information.To compare and study the synergy between Pt and Co 3 O 4 , pure Co 3 O 4 nanospheres were synthesized using the same PLAL procedure of the Co plate in 10 mL of water.Electrochemical measurements: An electrochemical analyzer (CHI708E) was used for the electrochemical tests in a three-electrode assembly for HER and FOR.A half-cell HER was performed in a single-cell compartment with Hg/HgO, a graphite rod, and an electrocatalyst coated on carbon cloth (CC) as the reference electrode (RE), counter electrode (CE), and working electrode (WE), respectively, in a 1.0-M KOH electrolyte.For the WE fabrication, 1 mg of the synthesized electrocatalyst was mixed with water (45 μL), isopropyl alcohol (45 μL), and Nafion solution (10 μL) with ultrasonication for 20 min.First, the carbon cloth was pretreated by soaking in 0.5 M H 2 SO 4 aqueous solution for 24 h.It was then thoroughly rinsed with absolute ethanol and DI water and dried overnight at 100 °C.The pretreatment process helps to remove any impurities or organic moieties trapped in the carbon matrix and inert the surface.Additionally, the pretreatment process also increases the wettability of the carbon cloth substrate, thereby proper adhesion nature increases between the surface of the carbon cloth and the active material.Thereafter, the obtained electrocatalyst ink was drop coated on the CC (1 × 1 cm 2 ) and dried at 60 °C for 2 h.A half-cell FOR was performed in an H-cell compartment with Hg/HgO, a Pt wire, and an electrocatalyst coated on CC as the RE, CE, and WE, respectively.For a half-cell FOR, 1.0-M KOH was utilized as a catholyte and 50-mM furfural in 1.0-M KOH was utilized as the anolyte.The potential measured with respect to the Hg/HgO electrode was referred to as the reversible hydrogen electrode (RHE) potential conforming to the Nernst equation. [8]EIS was performed at a frequency between 10 −1 and 10 5 Hz for the fabricated electrodes.The C dl values were estimated from cyclic voltammetry (CV) profiles recorded in the non-faradaic region at diverse scan rates.Thus, the C dl was directly relative to the electrochemical active surface area (ECSA) of the electrocatalysts.The intrinsic property of the electrocatalysts was investigated by turnover frequency (TOF) measurements, according to a previous report. [57]n addition, the two-electrode electrolyzer system was assembled using the Pt-Co 3 O 4 electrode as the cathode and anode in an H-cell setup for the OWS (OER and HER) and furfural-assisted electrolyzer (FOR and HER) measurements.The cathode and anode compartments were separated using a Nafion 117 membrane.The 1.0-M KOH was utilized as both the catholyte and anolyte for OWS, while for the furfural-assisted electrolyzer, the 1.0-M KOH was used as the catholyte, and the 1.0-M KOH/50-mM furfural solution was used as the anolyte.According to the previous reports, higher concentrations of furfural (>100 mM) can saturate the organics on the electrode surface and initiate the buildup of a polymer. [54,58,59]Furthermore, stability is a significant factor for any catalyst.Here, the stability of the fabricated Pt-Co 3 O 4 electrocatalyst toward half-cell HER and FORintegrated two-electrode electrolysis were assessed using chronopotentiometry and bulk electrolysis techniques, respectively.
Quantitative analysis of product: The FOR and 2-furoic-acid product yield were analyzed using a 1260 Infinity II LC HPLC system furnished with an ultraviolet (UV)-visible (VIS) detector (Agilent Technologies) and a 4.6 × 150 mm 5-μM C18 column (Agilent ZORBAX Bonus-RP).Here, during bulk electrolysis, 100 μL of aliquot was diluted with 3.9 mL of water.Wavelengths of 240 and 276 nm were applied for the detection of 2-furoic acid and furfural, respectively.As a mobile phase, 5 mM of aqueous ammonium formate and methanol at a ratio of 8:2 was utilized at a flow rate of 1 mL min −1 .The separation time was set at 18 min each.The quantification of 2-furoic acid was performed according to the standard calibration curves of 2-furoic acid with known concentrations.The quantitative analysis of furfural conversion, pseudo-first-order apparent rate constant k of FOR, Faradaic efficiency (FE) to 2-furoic acid production, and carbon balance was performed based on the following Equations (1-4): Faradaic efficiency 2ÀFuroic acid ¼ nC where C 0 and C t represent the concentration of furfural at the initial and at a given time of FOR, respectively.C FA and C FF are the concentrations of 2furoic acid and furfural in 50 mL of anolyte after 2 h of bulk electrolysis, respectively.n is the number of electrons (2e − in FOR), F is the Faradaic constant (96485.33C mole −1 ), and Q is the number of charges.

Figure 1 .
Figure 1.a) Stages of pure Co 3 O 4 and Pt-Co 3 O 4 nanosphere synthesis via PLAL: (i) Co metal plate immersed in a 50-μM aqueous solution of Pt precursor prior to the reaction.(ii) Interaction of Co ions discharged from the Co metal plate and Pt ions with • H and • OH radicals in the reaction media after water decomposition in plasma plume during PLAL.(iii) Formation of Pt-Co 3 O 4 nanospheres in colloidal form.b) XRD patterns and c) Raman spectra of Pt-Co 3 O 4 and Co 3 O 4 samples.
3 O 4 and Pt-Co 3 O 4 in Figure 1c.The position of characteristic bands of Co 3 O 4 was maintained in the Raman spectra of pure and Pt-Co 3 O 4 .However, the intensity of peaks appeared slightly lesser in Pt-Co 3 O 4 , which could be attributed to the less crystalline nature due to the lattice distortions of the Pt-decorated Co 3 O 4 structure.To investigate the surface features and composition of the electrocatalysts, FE-SEM with EDS mapping and HR-TEM were performed.FE-SEM images and the elemental mapping of Pt-Co 3 O 4 and Co 3 O 4 are shown in Figure 2a (i-iv),c (i-iii), respectively, where the shape of the particles in both samples was spherical with the approximate size of the Co 3 O 4 nanospheres in the range of ∼500-700 nm.The Pt NPs in Pt-Co 3 O 4 were spherical, homogeneously deposited, and evenly dispersed on the surface of big Co 3 O 4 nanospheres.The elemental mapping shows the presence of Co and O in both samples and the additional signal of Pt in the Pt-Co 3 O 4 sample (Figure 2a (iiv),c (i-iii)).This confirmed the existence and purity of the assynthesized samples.The bare sample contained 85.24 and 14.76 wt.% of Co and O only, respectively.However, Pt-Co 3 O 4 exhibited 81.52, 13.75, and 4.73 wt.% of Co, O, and Pt, respectively.The wt.% ratio of Co and O in both materials were close to that calculated from the Co 3 O 4 stoichiometric formula.
44, 1.56, and 1.43 Å of Co 3 O 4 and 2.24 Å of the Pt crystal plane appearing for Pt-Co 3 O 4 in Figure 2b,d.The measured lattice distances correlated with the pronounced XRD peaks.Additionally, the average Pt NP size was estimated by TEM to be ∼4.16AE 0.05 nm (Figure S1, Supporting Information).XPS was employed to identify the chemical composition on the interface of bare Co 3 O 4 and Pt-decorated Co 3 O 4 .Figure 3a-d presents the overall XPS survey spectra, Co 2p, O 1s, and Pt 4f core level spectra of Co 3 O 4 and Pt-Co 3 O 4 , respectively.All spectra were calibrated on the basis of C 1s peak position.In Figure 3a, the overall spectrum of Pt-Co 3 O 4 contains the characteristic signal of Pt NPs, which was not existed in bare Co 3 O 4 .In Figure 3b, the Co 3 O 4 spectrum exhibits a pair of pronounced peaks at 779.9 eV and 796 eV corresponding to Co 2p 3/2 and Co 2p 1/2 spin orbit splitting, respectively.Co 2p 3/2 was deconvoluted into two peaks at 779.8 eV and 782.1 eV ascribed to Co 2+ and Co 3+ oxidation states related to the spinel Co 3 O 4 structure.XPS spectrum of Pt-Co 3 O 4 undergoes a positive shift toward higher binding energy, i.e., deconvoluted Co 2+ peak was located at 780.8 eV and Co 3+ peak at 783 eV.This shift in peak positions was induced by both metal/support interaction and the size of Pt NPs deposited on the Co 3 O 4 surface (≈4.2 nm) (Figure O 1 score level spectra of materials are presented in Figure 3c.The peaks at 529.78 and 529.8 eV in the spectra of Co 3 O 4 and Pt-Co 3 O 4 are attributed to the lattice oxygen present in Co-O bonds; on the other hand, the peaks at 531.08 eV (Co 3 O 4 ) and 531.2 eV (Pt-Co 3 O 4 ) are attributed to the defective oxygen in

Figure 2 .
Figure 2. Morphological and elemental analysis by FE-SEM with EDS mapping and HR-TEM of a) (i-iv) and b) (i-ii) for Pt-Co 3 O 4 and c) (i-iii) and d) (i-ii) for bare Co 3 O 4 .
HER represents the half-cell reaction of water electrolysis, where the reduction of H + occurs.The recorded linear sweep voltammetry (LSV) curves for Pt-Co 3 O 4 and Co 3 O 4 in 1.0-M KOH are shown in Figure 4a.The observed low overpotential (∼290 mV) and small Tafel slope (∼185 mV dec −1 ) of Pt-Co 3 O 4 revealed its enhanced electrocatalytic HER performance, compared with that of the bare Co 3 O 4 (568 mV and 619 mV dec −1 , respectively) (Figure 4b,c).Furthermore, the charge-transfer resistance (R ct ) values measured from the EIS Nyquist plot at −0.3 V versus RHE for Co 3 O 4 and Pt-Co 3 O 4 were ∼28.32 and 5.19 Ω, respectively.The Pt-Co 3 O 4 with the lowest resistance value exhibited better intrinsic properties, such as fast charge transfer and high conductivity (Figure 4d).It may have been that the metallic Pt NPs on the surface of Co 3 O 4 in Pt-Co 3 O 4 contributed to the enhanced conductivity of the bare Co 3 O 4 because of the strong metal-support interaction between Pt and Co 3 O 4 .

Figure 5 .
Figure 5. FOR activity of Co 3 O 4 and Pt-Co 3 O 4 in 1.0-M KOH: LSV curves recorded in 1.0-M KOH with and without 50-mM furfural for a) Co 3 O 4 and b) Pt-Co 3 O 4 .c) Comparison of the current density of bare Co 3 O 4 and Pt-Co 3 O 4 at 1.35, 1.4, 1.45, and 1.5 V versus RHE in 1.0-M KOH/50-mM furfural.d) Faradaic efficiency to 2-furoic acid and carbon balance of FOR over Co 3 O 4 and Pt-Co 3 O 4 electrocatalysts.

Figure 6 .
Figure 6.Electrolyzer coupled with HER and FOR: a) real image of H-cell setup for the assembled FOR-coupled electrolyzer using Pt-Co 3 O 4 as the anode and cathode, b) polarization curves, c) cell voltage at 5 and 10 mA cm −2 , and d) current density obtained from 1.4 to 1.8 V in the absence and presence of 50-mM furfural in the anodic compartment.