ORIGINAL PAPER

Fabrication of ordered porous silicon nanowires electrodemodified with palladium-nickel nanoparticlesand electrochemical characteristics in direct alkaline fuel cellof carbohydrates

Bairui Tao1,2 & Keyang Zhao1 & Fengjuan Miao1 & Zaishun Jin2& Jianbo Yu2

&

Paul K. Chu3

Received: 8 January 2016 /Revised: 28 March 2016 /Accepted: 17 April 2016 /Published online: 26 April 2016# Springer-Verlag Berlin Heidelberg 2016

Abstract A Pd-Ni nanoparticle modified silicon-based anodeis fabricated and the possibility of using it for the direct alka-line fuel cell of carbohydrates has been investigated by elec-trochemical method. Upright and porous ordered silicon nano-wires (SiNWs) arrays are prepared by wet etching. The Pd-Ninanoparticles are covered to the SiNWs uniformly by chemi-cal deposited successively. Using six kinds of common carbo-hydrate, including glucose, fructose, maltose, lactose, sucrose,and starch, as testing subjects, the performance of electrocat-alytic oxidation is studied. Experiment results show that theelectrochemically active surface area of Pd-Ni/SiNWs elec-trode electrochemically active surface area is 53.482 cm2,and higher electrocatalytic activity and stability is displayedfor the direct oxidation of glucose, fructose, maltose, and lac-tose. Firstly, the Pd-Ni/SiNWs electrode has better electro-chemical performance for carbohydrates and is promisingfor applications in direct alkaline fuel. Secondly, more kindsof carbohydrates might potentially use as energy source fordirect alkaline fuel.

Keywords Fuel cells . Carbohydrates . Pd-Ni/SiNWselectrode

Introduction

For meeting energy demand, exploring alternative energysources and more efficient energy consumption, the fuel celltechnology is thus attracting much attention [1]. As organicfuels in fuel cells beginning with hydrazine, a series of organiccompounds including alcohols, organic acids, and dimethylether have been investigated widely and some of them havebeen commercialized preliminarily under this circumstances[2–6]. Nevertheless, constantly seeking more and better po-tential energy material is still an unstoppable task. Ideally, theoptimal ingredients as fuel should be non-toxic, green (renew-able), easily stored, portable, readily manufactured, and high-energy-density. In this respect, not only do carbohydrates havemany favorable attributes mentioned above but they also pos-sess immense commercial potential.

Benefiting from their characteristic properties, variouscomposite and nanostructured functional materials such asnanotubes, nanowires, nanopores, nanoclusters, graph-eme, and some doped material have been applied to aseries of energy equipments and sensors as electrode ma-terials over the past few years [7–12]. Particularly, thesilicon nanowires (SiNWs) with the unique one-dimensional (1D) structure and controllable propertieshave been investigated extensively [13, 14]. Numerousrecent studies have intensively focused on the efficiencyof SiNWs used in electrodes modified with metal nano-particles (NPs) and appropriate electrocatalytic activity ofvarious organic or inorganic substances for fuel cells andsensors [15–19]. Amidst of frequently-used loads,

Bairui Tao and Keyang Zhao are common first authors.

* Bairui Taotbr_sir@163.com

* Fengjuan Miaomiaofengjuan@163.com

1 College of Communication and Electronic Engineering, QiqiharUniversity, Qiqihar, Heilongjiang 161006, China

2 Mudanjiang Medical University, Mudanjiang, Heilongjiang 157000,China

3 Department of Physics and Materials Sciences, City University ofHong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Ionics (2016) 22:1891–1898DOI 10.1007/s11581-016-1717-y

precious metal catalysts show a promising applicationforeground on account of their higher activity and milderreaction conditions [20]. For direct alkaline fuel cells, apalladium-based catalyst is sometimes used instead ofplatinum and platinum-based catalysts because of the low-er price and the ability of avoiding the CO poisoning[21–23], it has also been reported that Pd has quickeroxidation kinetics and higher catalytic activity for the ox-idation of alcohols and glucose in alkali electrolytes [21,22, 24–27].

The virtues of SiNWs and Pd can be combined incatalytic oxidation of carbohydrates and exhibit excellentactivity. In this study, to prepare a prototype of carbohy-drates fuel cell, the ordered aligned SiNWs arrays areemployed as the backbone of electrode and Pd-Ni alloynanoparticles are dispersed on the sidewalls of them viachemical deposition. By improving the fabrication pro-cess of the Pd-Ni/SiNWs electrode, monolayer self-assembly technology is utilized for the surface pretreat-ment of SiNWs before the electroless nickel plating [28,29]. Six kinds of common carbohydrates including glu-cose, fructose, lactose, maltose, sucrose, and starch areadopted to assess the performance of the Pd-Ni/SiNWselectrode in an alkaline medium.

Experimental details

The fabrication procedure for the Pd-Ni/SiNWs is shownin Fig. 1 concisely. The <100>-single-side polished N-type silicon wafers with resistivity of 10–50 Ω cm werecut into rectangular pieces with dimensions of about1 × 0.5 cm, cleaned using the standard RCA protocol,and served as the substrate for fabrication of SiNWs.After that, the SiNWs arrays were prepared by wetchemica l e t ch ing w i th AgNO3 (25 mM) : HF(15 %) = 1:1 (v/v) and the detailed fabrication process

can be found elsewhere [30]. Deionized (DI) water anddiluted nitric acid (~30 %) was used to rinse the siliconsamples to remove residues attached to the surface of theSiNWs.

The reactions which related to the above wet etchingare driven by galvanic cell formation. In aqueous HFsolution, the redox potentials of Ag+/Ag and Si4+/Si cou-ple is 0.799 and −1.37 V, respectively. The bare siliconcan do displacement reaction with silver ions, whichcause the deposition of Ag at the localized surface ofSi. The silver particles gain electrons and at a low po-tential, conversely, the surface of Si where is not coveredby silver loses electron at a high potential and holes areformed. Thus, the galvanic cell formation is formed attiny local area. The interface of solution-Ag is the equalof the cathode of galvanic cell, and the interface ofsolution-Si is the equal of the anode. The total reactioncan be expressed by following equation [31]:

Siþ 6F− þ 4Agþ ¼ SiF62− þ 4Ag ð1Þ

The above reactions will continue if the adequate silverions are provided. The Si which under Ag has negative poten-tial because of the contact of Si and Ag particles, thus, it isdifficult to be oxidized. On the other hand, the Si which ex-posed in solution is oxidized to SiO2 because of the electron ofSi atom is transferred to Ag and more holes are formed, sub-sequently dissolved by HF [32]. As a result, the upright andordered SiNWs arrays are prepared.

In order to obtain composite materials with better adhesion,the nickel plating process comprises four steps as described inthe following. (a) Hydroxylation: After removing oil andgrease, the fabricated SiNWs were put in a piranha solutionfor 4 h at 90 °C. (b) Coupling: The hydroxylated SiNWs wereprocessed with absolute ethyl alcohol solution of γ-aminopropyltriethoxysilane. (c) Activation: The SiNWs sam-ples were dipped in a solution to activate and the formulation

Fig. 1 Fabrication procedure for the Pd-Ni/SiNWs

1892 Ionics (2016) 22:1891–1898

of the activating solution is listed in Table 1. (d) After soakingin 10 % HCl solution for 1 min, electroless nickel platingcommenced and the detailed process is described in Ref.[33], and then the procedures of palladium plating and anneal-ing were performed. Afterward, the Pd-Ni/SiNWs wasannealed at 400 °C for 300 s in a rapid thermal annealingsystem under argon atmosphere. Copper wires were connect-ed to the Pd-Ni/SiNWs chips by silver conductive adhesiveand the junction was sealed with epoxy resin.

The morphologies of the SiNWs and Pd-Ni/SiNWs werecharacterized by scanning electron microscopy (SEM), trans-mission electron microscopy (TEM), X-ray diffraction (XRD)and energy dispersive spectroscopy (EDS). The adhesionstrength between the coating and SiNWs was assessed byadhesive tape stripping and scribing, and the electrochemicaltests were performed by three-electrode system. Pt foil (Pt251,5 × 5 mm) and saturated Hg/HgO (0.1 M KOH) served as thecounter and reference electrode, respectively. The modifiedPd-Ni/SiNWs constituted the working electrode. All thechemical reagents used in the experiments were analyticalreagents and used as received without purification. The solu-tions were prepared with DI water and all the vessels andcontainers were cleaned ultrasonically prior to use. The exper-iments were conducted at room temperature (~25 °C).

Results and discussion

Figure 2 exhibits the characterization images of theSiNWs and Pd-Ni/SiNWs nanocomposite. Figure 2a, bdisplays the top-view and cross-sectional of the SiNWsvia SEM, whereas Fig. 2c, d depicts the correspondingmagnified pictures of the top-view and cross-sectional af-ter palladium-nickel plating. The SiNWs has bundle-likemorphology and are aligned vertically to the substratewith a clear interface between the SiNWs and bulk sili-con. The length of the Pd-Ni/SiNWs is about 65 μm andthe diameter of nanowires varies between 80 and 150 nm.After chemical plating, the nanowires are still uniformlydistributed, isolated and parallel to each other. The TEMimage of the Pd-Ni/SiNWs surface is shown in Fig. 2e.The dense layer should be Ni and the agglomerated

particles which size is about 3 nm may contain Pd. Thepattern of SAED in Fig. 2f is a cluster of concentric ringswhich have some diffraction spots; it could signify thatthe structure of materials in selected area is polycrystal-line mostly. The EDS results in Fig. 2g show that thecomposite mainly consist of Ni, Pd, and O. No character-istic peaks of Si can be found, confirming that the layercovers the surface of Si entirely. Multiple areas are probedby EDS and similar spectra are obtained, demonstratingbetter corrosion resistance in alkaline media [34].

To determine the optimal processing conditions in elec-troless nickel plating, the relationship between pH of theplating bath and the quality of deposits is explored. As thepH is increased by addition of ammonia, the nickel plat-ing layer is more prone to cracking. No corrugation, ab-scission, and cracking are found from the Ni coatings bytape stripping and scribing at a pH of 7.5, indicating thatthe state is very well between the coatings and theSiNWs. After completion of preparation process, the sta-bility of Pd-Ni/SiNWs composite electrode has been eval-uated in 1 M KOH at a scanning rate of 50 mV s−1 from−1.2 to 0.55 V. Figure 3 reveals that the relative standarddeviation of the current response is 4.52 % after 50 cycles.Two oxidation peaks can be observed from the positivescans in Fig. 3 corresponding to different electrochemicalprocesses on the surface of the Pd-Ni/SiNWs. The peak Ioccurs in the potential range of −1.0 to −0.75 V and iscaused by adsorption of hydrogen, whereas the peak IInear −0.08 V corresponds to the formation of an oxidelayer on the surface of the electrode. The three reductionpeaks III, IV, and V arise from reduction of Ni2+, Ni3+,and Pd2+ produced during the positive sweep [24, 35–37],respectively. After leaving the composite electrode in thesolution or air for about 1 month, the current responseshows no obvious change and the surface of electrodeshows no cracks and delamination. The results providefurther evidence about the strong adhesion and good elec-trochemical stability.

In addition, the electrochemically active surface area(EASA) of catalysts is one of the most important parametersthat have a direct influence on the activity of a catalyst.Compared with the CVs of Pd-Ni/Si in the background, the

Table 1 Formulas andprocessing conditions of theactivating solutions

Chemicals name Concentration Preparation conditions

Solution A HCl 10 mL (37 %) Continue to stir at 70 °C for 20 minPdCl2 0.02 g

SnCl 2 g

H2O 10 mL

Solution B NaCl 12 g 60 °C for 20 min and then mixsolutions A and BH2O 100 mL

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Pd-Ni/SiNWs has a bigger EASA obviously. For Pd-basedcatalysts, the EASA is usually estimated according to the fol-lowing equation [38–40]:

EASA ¼ Q pdoð ÞS

ð2Þ

Where BQ(PdO)^ is the quantity of electric charge involvedin the reduction of PdO monolayer and the value can be reck-oned through integrating the appropriate area in CVs. BS^ is aproportionality constant used to relate charge per unit areawhich associated to the reduction of PdO monolayer is com-monly taken as 424 μC/cm2. A high EASA of 53.482 cm2 iscalculated for Pd-Ni/SiNWs, which can be attributed to thelarge surface to volume ratio of SiNWs and the wonderfuldispersion of Pd-Ni nanoparticles.

The contrastive catalytic effect between Pd-Ni/SiNWs andPd-Ni/Si electrodes towards glucose is displayed in the insetof Fig. 3. As can be seen by the cyclic voltammograms, Pd-Ni/

Fig. 2 SEM images: a top-viewand b cross-section of the SiNWsbefore electrodeposition. c, dCorresponding magnified pic-tures of the top and cross-sectionafter palladium deposition. eTEM image of the Pd-Ni/SiNWssurface. f SAED patterns of thePd-Ni/SiNWs. g EDS spectrum ofthe Pd-Ni/SiNWs

Fig. 3 Cyclic voltammograms of a Pd-Ni/SiNWs and b Pd-Ni/Si in 1 MKOH solution at a scanning rate of 50 mV s−1, and the inset is the cyclicvoltammograms of a Pd-Ni/SiNWs and b Pd-Ni/Si obtained in 1MKOHwith 1 M glucose at a scanning rate of 50 mV s−1

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SiNWs show vastly higher electrocatalytic activity for glucoseoxidation than that of Pd-Ni/Si under the same conditions.This phenomenon that can be attributed to the SiNWs is ofhigh specific surface area which ensures the nanoparticlesdispersed better, thus exposing a large number of active siteand achieving synergistic effect of Pd-Ni nanoparticles.

Figure 4a, b depicts the cyclic voltammograms of thePd-Ni/SiNWs electrode in 1 M KOH solutions containingdifferent kinds of carbohydrates at a scanning rate of50 mV s−1. The concentration of glucose, fructose, lac-tose, maltose, and sucrose is 1 M and that of starch is10 g/L. Figure 4a shows an obvious oxidation peak inboth the forward and reverse scans on each responsecurve of the tested reducing sugars. The first oxidationpeak in positive scan is caused by Pd nanoparticles whichhave some ability of electrocatalytic oxidation for thesefour reducing sugars and the other re-oxidation peak incathodic sweep corresponds to removal of the incompleteoxidation products formed in the forward scan. These re-sults indicate that the reducing sugars can undergo a de-hydrogenation reaction with the aid of the palladium-based catalyst. The oxidation current density by and largedecreases in the following order: glucose, fructose, malt-ose, and lactose. However, there is no obvious catalytic

oxidation reaction towards either of the two types of non-reducing sugars in our experiments as shown in Fig. 4b.

Carbohydrates can go through a variety of complex reac-tions in NaOH and KOH solution. Lobry de Bruyn-vanEkenstein transformation will occur in a diluted alkali solu-tion, which causes the isomerization and rearrangement ofaldoses or ketoses [41, 42]. A more concentrated alkalinesolution is able to transform the carbohydrates into isomerizeddeoxy sugars acid [43]. Moreover, an alkaline solution canbreak down aldoses and ketoses into smaller fragments bythe inverse acetal reaction.

There is no solid evidence of the palladium-catalyzed oxi-dation mechanism of carbohydrates in alkaline solution thathas been reported except glucose. Herein, we attempt to ex-plore and discuss it through known principle.

The mechanism of the phenomenon shown in Fig. 4 ispostulated as follows. In the beginning, with a rough com-parison of molecular construction between glucose andfructose, it is distinct that glucose has an aldehyde groupwhich fructose does not and means that fructose can notbe oxidized directly. Nevertheless, tautomerism of α-hydroxy ketose exists in the alkaline environment whichleads to the formation of the aldose structure and part offructose can be converted into glucose. Given the aboveand referenced oxidation of glucose [44], the reactionprocesses of fructose are described as follows:

glucose !OH−1

enediol !OH−1

fructose ð3Þ

C6H12O6 þ 0:5O2 !Pd=OH−1

!Pd=OH‐1

C6H12O7þH2Oglucoseð Þ gluconic acidð Þ

ð4Þ

The reason why the current density of glucose is larger thanfructose in an identical condition could be also interpreted bymentioned above, and our observation also coincides withthose reported previously [45].

Two kinds of reactions may bear upon the tested reducingdisaccharides in alkali condition. Firstly, maltose and lactosecan be oxidized with oxygen and generate the correspondingaldobionic acids or salts [46, 47]. Secondly, maltose could beconverted to glucose, erythrose through two parallel mecha-nisms: shydrolysis and enolization [48, 49]. The similar reac-tion also occurs for lactose and the main products are galac-tose and tagatose [47, 49–51]. And then, these aldoses mayundergo further oxidation reactions with the involvement ofPd, or by saying that the existence of Pd may accelerate theaforementioned reactions.

In addition, sucrose has no hemiacetal hydroxyl and so theopen chain does not form in the solution. Notwithstanding,there is one hemiacetal hydroxyl in the reducing end of thestarch chain configuration; the proportion is so small that noeffective catalytic oxidation occurs on the surface of the com-posite electrode.

Fig. 4 Cyclic voltammograms of Pd-Ni/SiNWs in 1 M KOH solutioncontaining different carbohydrates at a potential scanning rate of50 mV s−1 (a glucose, fructose, lactose, and maltose. All of theconcentrations are 1 M. b The concentration of sucrose is 1 M and thatof starch is 10 g/L)

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Taking fructose as an example, the effect of KOH concen-tration on the current response is shown in Fig. 5. At a fructoseconcentration of 1 M, the oxidation current increases initiallyand then decreases with increasing KOH concentrations. Thepeak current reaches a maximum at a KOH concentration of2 M while the peak potential tends to be more negative. Itimplies that more OH− can promote the fructose oxidationreaction within appropriate concentration range, but in thepresence of too much KOH, the system has higher internalresistance and the hydroxyl ions take up more active sitesavailable on the Pd-Ni/SiNWs electrode thus inhibiting elec-trocatalytic oxidation [52]. This phenomenon that the peakcurrent has the trends of increasing first followed by decreas-ing and the peak potential shifts negatively with increasingconcentration of KOH is similar to that observed from otheralkaline direct fuel cells [53].

Taking glucose and maltose for instance, the influence ofcarbohydrates concentration on catalytic effect is displayed inFig. 6. These results show that the simple molecular structureof glucose is more easily to be oxidized than the complexmolecular structure of fructose. Furthermore, in 2 M KOHsolution, there is a relatively obvious tendency that the oxida-tion current for glucose and maltose increasing first and thendecreasing with increasing carbohydrates concentrations. Theexplanation of this phenomenon may be that a competitiverelation is formed between the adsorption of carbohydratesand hydroxyl. The adsorption of carbohydrates has graduallybecame the dominant one and hampered the adsorption ofhydroxyl ions when the carbohydrates concentration reachesa certain line (1 M for glucose, 0.5 M for maltose). What ismore, we have easily observed that all the potential of oxida-tion peaks and re-oxidation peaks have a continuous positive-shift when the concentration of glucose increases from 0.1 to

2.5M now that the pH of reaction system has been changed byreaction products [54].

The stability of the catalytic activity for carbohydrates ox-idation is investigated by chronoamperometry measurementsat an oxidation potential of −0.2 V in solution of 2 M KOHcontaining 1 M glucose, fructose, maltose, and lactose asshown in Fig. 7. Taking 300 s as a cutoff point, there is aprecipitous decline of all the current densities before it, andwhich can attribute to the adsorption of poisoning intermedi-ates. In the interval of 300 to 800 s, the downtrend is relatively

Fig. 5 Cyclic voltammograms acquired from various concentrations ofKOH with 1 M fructose at a potential scanning rate of 50 mV s−1 and theinset is the anodic peak current and potential for fructose electrooxidationversus KOH concentrations

Fig. 6 Cyclic voltammograms acquired from 2 M KOH with variousconcentrations of carbohydrates at a potential scanning rate of50 mV s−1 (a glucose, b maltose). Inset of b, magnified view of re-oxidation peak in the cathodic sweep

Fig. 7 Chronoamperogram of electroactivity of Pd-Ni/SiNWs electrodeat an oxidation potential −0.2 V in 2 M KOH/1 M glucose (a), fructose(b), maltose (c), and lactose (d) solution at 25 °C

1896 Ionics (2016) 22:1891–1898

weak and the attenuation values are 9.85, 10.07, 11.58, and9.73 %, respectively. This is because that the most poisoningreaction products can be oxidized and removed continuously,which makes the electrode to maintain its catalytic activityrelatively well. To sum up, the conclusion has shown thatPd-Ni/SiNWs composite electrode has a remarkable electro-catalytic stability.

Conclusion

In this work, upright and ordered SiNWs arrays are preparedby wet etching. Pd-Ni/SiNWs nanocomposite electrodes withexcellent adhesion are fabricated by chemical deposited suc-cessively. Electrochemical assessment reveals that the Pd-Ni/SiNWs electrode has large effective surface area and highelectrocatalytic activity to oxidation of glucose, fructose, malt-ose, and lactose. The corresponding mechanisms of the reac-tions have also been explored and described tentatively. Thiskind of electrode is suitable for direct carbohydrates fuel cell,and the simple fabrication process which is compatible with Siintegrated circuit makes it easy to commercialize andpopularize.

Acknowledgments This work was jointly supported by the NationalNatural Science Foundation of China (Grant No. 61204127, 81172204,81271628), Natural Science Foundation of Heilongjiang Province (GrantNos. F201332 and F201438), and City University of HongKongAppliedResearch Grant (ARG) No. 9667085.

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