Manuscript submitted March 27, 2013; revised manuscript received May 7, 2013. Published May 21, 2013.
Alkaline fuel cells are among the most efficient low-temperatureoperating fuel cells.1 The catalysts have traditionally been platinum-group metals or their alloys, but these quick-to-start devices are,however, highly sensitive to fouling (e.g., CO2), and require cost-prohibitive, high purity H2 for long term stability. Group 4 metalcompounds, sought as a low cost alternatives, show significant activ-ity toward the oxidation of many aliphatic species in alkaline mediaand are not prone to poisoning.2–4
Early work on nickel-phosphides, specifically for corrosion re-sistance, is a well-studied field of materials.5 Preparation of thesealloys relies on elevated temperature and low pH for the deposition ofa variety of stoichiometries, from hypophosphite and phosphine gasprecursors. More recent analyzes of nickel phosphorous materials em-phasizes the catalytic capabilities of these materials, among these arephosphides for hydroprocessing6 and phosphates for electrocatalysis.7
Metals such as nickel, manganese, and cobalt show electrochem-ical activity in alkaline environments toward both oxidation andreduction.8 The local environment of the electrodeposited metal,whether in an complex (e.g., citrate)9 or a co-deposited species (e.g.,with Co, Fe or Cu),2,10 has been shown to greatly affect catalyticproperties and longevity of the catalyst system. Recently, work oncobalt complexes has shown that inclusion of phosphate increaseslongevity of the system at high potentials, resulting in a ‘self-healing’catalyst.11,12 However, no conclusive evidence indicating the role ofthe phosphorous in the catalyst is given. That is, the use of phosphatein preparing nickel complexes for catalytic materials has been studied,but the inclusion of phosphate as either an active component of thecatalyst or active in depositing the metal during deposition, has not.
Nickel-DNA complexes have more recently been explored ashigh-activity nickel complexes for fuel oxidation in alkaline systems,specifically for selective methanol oxidation.13 In a recently publishedstudy, we analyzed such Ni-DNA species. These ‘bioelectrocatalysts’display increased catalytic activity to various aliphatic alcohols andsugars for applications in alkaline fuel cells.14 The catalytic mecha-nism and the role of DNA remain unclear. It has been hypothesizedthat these effects are attributed to the phosphate backbone of DNA, inthat DNA provides both a physical scaffold and a conductive pathway(i.e., the phosphate backbone) for the nickel electrocatalyst. However,given the relatively high electric field experienced during electro-deposition protocols (105 V cm−1), it is likely that DNA is denatured atthe heterogeneous interface and that merely the chemical constituentsof DNA are complexing with nickel. It is hypothesized here that theDNA is merely acting as a source of phosphate for the complexation ofnickel-phosphate aggregates analogous to the aforementioned cobaltphosphate species.15
∗Electrochemical Society Student Member.∗∗Electrochemical Society Active Member.
To explore this theory, we have focused on nickel-phosphate aggre-gates, using nickel-phosphate based electrocatalysts for analyte oxi-dation of methanol, formaldehyde, and formate in alkaline solutions.We demonstrate increased sensitivity of a nickel species co-depositedfrom phosphate solutions versus chloride analogs, and have comparedthem to similarly prepared Ni-DNA complexes. The electrochemicalresponse of nickel-phosphate is different from co-deposited chloridecontrols at the 99.99% confidence level. The role of the phosphate isconsidered, but empirical evidence is not forthcoming. Characteriza-tion of the catalytic surface is performed with both electrochemicalanalysis, including voltammetry and amperometry, and spectroscopicanalysis via X-ray photoelectron spectroscopy (XPS). To provide ad-ditional information to elucidate structure-function / composition-function relationships of the catalytic surface, characterization of thesamples were performed via atomic force microscopy (AFM).
Experimental
Nickel electrocatalyst coated electrodes are prepared via electro-deposition from electrolyte solutions of either 0.1 M NaCl (control),0.1 M K2HPO4 solutions, or 0.1 mg/mL DNA(Calf thalamus DNA,Sigma). Electrodes are prepared on wet-proofed Toray paper, TGP-H-060 (Fuel Cell Earth, Stoneham, MA). The Toray electrodes area standard 1 cm2 geometric working area, where a wax coating isapplied to define the surface area.
Solutions containing either 1 M methanol (Fisher Sci, HPLCgrade), 0.1 M formaldehyde (Aldrich), or 0.1 M sodium formate(Fisher Sci) are used to collect voltammetric data. Electrodes areimmersed in solution to equilibrate for approximately five minutesbefore evaluation. (The sodium formate solution is degassed with N2
to prevent spontaneous formate oxidation.)Electrodeposition occurs in a 0.5 mg/mL solution of NiCl2
(Aldrich, anhydrous, 99.99%). Control electrodes were deposited witha 0.1 M NaCl in 18 M� water (Millipore, MilliQ). Ni-DNA electrodesare prepared from 0.5 mg/mL NiCl2 and 0.1 mg/mL DNA solutions.K2HPO4 (Fisher) solutions of 10, 50, 100, 250 and 500 mM wereused for phosphate optimization. An electrode array is used for si-multaneous deposition as described in Reference 3. All depositionsand statistical evaluations utilize n = 3 (different) electrodes. Elec-trodeposition of the Ni2+ occurs for 1800 seconds at 1.8 V versus aAg|AgCl reference electrode, consistent with M-DNA electrodeposi-tion protocols.13,14 The counter electrode used is a large Pt mesh. Elec-trodeposition and electrochemical analysis are done using a Digi-Ivy2300 bipotentiostat with the same three-electrode setup. All potentialsare referenced versus Ag|AgCl.
XPS and AFM samples are deposited in the same manner as Toraybased electrodes, except glassy carbon plates (2 cm × 0.3 cm ×0.1 cm) are substituted instead of Toray paper. The glassy carbonis polished with 1 and 0.05 μm alumina and rinsed with copiousamounts of DI before deposition. AFM, run on a Bruker Dimension
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H464 Journal of The Electrochemical Society, 160 (8) H463-H468 (2013)
Icon, analyzed surface areas of 1.19 × 106 nm2 for Ni-Pi and 1.03 ×106 nm2 for Ni-Cl. XPS and elemental analysis were performed usinga Kratos Axis Ultra DLD, utilizing a corrected binding energy of 0.27/ eV for Ni-Cl sample and 0.22 / eV for Ni-Pi sample. A sixty secondargon sputtering was performed to remove surface adsorbates.
Results and Discussion
Demonstrating the greatest activity in alkaline environments, allelectrochemical analysis of nickel deposited electrodes use solutionsof 0.1 M NaOH and 18 M� water in addition to appropriate organicadditives. The voltammetric characterization requires an initial cyclingto achieve a steady-state response. Here, this response is achievedafter 10 cycles. The generally accepted reaction scheme for nickelelectrocatalysts in alkaline solutions is as follows:4,16
OH− + Ni(OH)2 ⇀↽ NiOOH + H2O + e−
An initial formation of nickel oxyhydroxide is followed by sub-strate adsorption:
RCH2OHsol ⇀↽ RCH2OHadsorbed
An intermediate complex that is associated with a proton shift tothe catalytic surface then occurs:
Finally, product desorption occurs, which is accompanied by theregeneration of the heterogeneous catalytic surface:
[RCHOHadsorbed + Ni(OH)2] ⇀↽ Ni(OH)2 + RCHOO− or RCHOOH
Figure 1 shows the cyclic voltammetric response (swept initiallyin the oxidative direction here, and throughout the study) of the nickelelectrodes in a 0.1 M NaOH solution, deposited from 0.1 M sodiumchloride as control (Ni-Cl), 0.1 M monobasic phosphate (Ni-Pi), and0.1 mg/mL Ni-DNA. Chloride was chosen here as control as the nickelcounter ion source, NiCl2, contributes to an approximate chlorideconcentration of 0.1 mM in all deposition solutions. Chloride ion wasspeculated by Horkans as a possible competitive deposition specieswith nickel; however, this is not a well understood process.17
The phosphate concentration was analyzed between 10 and500 mM. The response of these films is given in Figure 2; the in-lay of 250 and 500 mM phosphate are separated for clarity. Concen-trations above 100 mM phosphate in the deposition baths leads tothe loss of nickel activity (i.e., Ep ox ≈ 0.560 V vs. Ag|AgCl). Whenthe phosphate concentration in the deposition bath is increased to500 mM, the current response at the characteristic nickel oxidation
Figure 1. CVs of nickel electrodes in a 0.1 M NaOH solution. The gray curvecorresponds to nickel electrodes formed by electrodeposition in a 0.1 M NaClsolution, the black curve corresponds to nickel electrodes formed by electrode-position in a 0.1 M phosphate solution, and the dashed gray corresponds toNi-DNA; scan rate of 50 mV/sec. The peaks correspond to the oxidation andreduction of Ni2+/3+, while the current at 0.8 V corresponds to water oxidation.
Figure 2. Effects of phosphate concentration on electrocatalytic response;representative cyclic voltammograms for electrochemical response at50 mV/sec in 0.1 M NaOH, concentrations labeled in the figure.
peak potential (i.e., Ep ox ≈ 0.560 V vs. Ag|AgCl), again increases,but as a purely capacitive current. This current response does not cor-relate to nickel deposition or to the electrocatalytic capabilities of thedeposited films. In effect, a traditional faradaic analysis does not ac-curately correlate deposition current to the resulting electrocatalysisof substrate. Differences in current densities between Ni-Cl and Ni-Pideposited films are found in Table I. Where again, averages accountfor n = 3 (different) electrodes. Statistical evaluations include tradi-tional Spooled and Tcalc analyzes, with values compared to two-tailedStudent T values.
Table II gives the effective oxidative current response for Ni2+/3+
by varying the concentration of phosphate between 10 and 500 mM.The current response of the Ni-DNA complex is also given. The Ni-DNA films give highly variable current response. However, if themass concentration of phosphate within DNA is approximately 10%,the current response of the Ni-DNA system agrees with the trendseen for Ni-Pi complexes at the low concentration end. The reasonfor the current response at the high end for Ni-DNA complexes isless forthcoming. It could be as simple as increased capacitance fromelectrodeposited DNA not participating in any electrocatalytic activity.
Table I. Oxidative response of fuels in alkaline solution at Ni-Cland Ni-Pi electrodes, peak ratio of Ni3+/fuel oxidation.
Table II. Voltammetric current response for deposition of nickelin varying [phosphate] deposition baths; currents taken from Niip ox for catalysts prepared between 0 and 100 mM phosphate; for250 and 500 mM deposition baths, currents measured at 0.56 V vs.Ag|AgCl when no nickel signal is observed.
Journal of The Electrochemical Society, 160 (8) H463-H468 (2013) H465
Figure 3. Electro-oxidation of methanol: a) Representative cyclic voltammo-grams at 50 mV/sec in a 0.1 M NaOH and 1 M methanol solution: Ni-Cl(gray), Ni-Pi (black); inlay: representative chronoamperometry, Ni-Cl (gray),Ni-Pi (black); b) calibration curves of Ni-Cl (gray) and Ni-Pi (black); linear fitof y = 3.63 × 10−5x –2.67 with an R2 = 0.98 for chloride (gray) and −2.18× 10−4x + 8.89 with an R2 = 0.98 for phosphate (black).
Given these observed trends, the Ni-Pi complexes were furtherexplored for their oxidative catalytic ability toward a variety ofhydrocarbons and are compared to chloride controls. Voltammetriccharacterization shows the enhanced activity of phosphate depositedelectrodes for both nickel oxidation and reduction, as well as the on-set for water oxidation (oxygen evolution), seen in Figure 1, versuschloride controls. This is rationalized as the formation of catalytichydroxyl groups on the surface of the Ni2+ catalyst, as given in themechanism above. After stabilizing, the enhanced Ni2+ oxidation sig-nal at 0.52 V is increased by 85% versus Ni-Cl controls. This increaseis statistically significant at the 99% confidence level from Ni-Cl. A68% increase in water oxidation current, at 0.8 V vs. Ag|AgCl, is alsostatistically significant at the 99% confidence level, while Ni3+ reduc-tion is increased by 84% at 0.44 V vs. Ag|AgCl, which is statisticallysignificant at the 99% confidence level.
Activity of nickel complexes in alkaline solutions for oxidation ofvarious aliphatic alcohols has led to the following accepted oxidationmechanism.4 For methanol oxidation, a six electron process occurs asfollows with carbonate as the final oxidation product.
CH3OH → H2CO + 2e− → HCO−2 + 2e− → CO2−
3 + 2e−
Increased affinity for hydrocarbons helps to explain the oxidativecharacter of the current response for the system in 1 M methanol and0.1 M NaOH. The voltammograms seen in Figure 3a and tabulated inTable I show the oxidation of methanol at nickel electrodes electrode-posited in phosphate electrolyte and chloride co-deposited controls.The current response at 0.8 V vs. Ag|AgCl shows a statistically sig-nificant increase of 68% at the 99% confidence level.
Figure 4. Electro-oxidation of formaldehyde: a) Representative cyclicvoltammograms at 50 mV/sec in a 0.1 M NaOH and 0.1 M formaldehydesolution: Ni-Cl (gray), Ni-Pi (black); inlay: representative chronoamperome-try, Ni-Cl (gray), Ni-Pi (black); b) calibration curves of Ni-Cl (gray) and Ni-Pi(black); The linear fit for Ni-Cl (gray) is y = −8.88 × 10−5x –3.39 × 10−4,with R2 = 0.99, while for Ni-Pi (black) the fit is −3.21 × 10−4x + 6.65× 10−4 with R2 = 0.99.
Chronoamperometry demonstrates catalytic activity towardsolution-based analytes over a range of concentrations. A calibra-tion curve was generated by standard addition of each analyte in anelectrolyte solution. The corresponding curves show strong linearity(R2 = 0.99) for most systems. All chronoamperometric measurementsuse a potential bias of 0.65 V vs. Ag|AgCl, with run times of 300 sec-onds. Samples are introduced to the stirring system every 30 seconds;the current measurements are taken after 10 seconds of mixing. Rep-resentative amperometric data is given in Figures 3–5 for methanol,formaldehyde, and formate analytes, along with the correspondingcalibration curves (and relative error). The amperometric calibrationcurve for methanol, with additions of 25 μL of 1 M methanol (in0.1 M NaOH) every 30 seconds, gives a linear plot. Sensitivity in-creases by nearly an order of magnitude for Ni-Pi. The responses aredifferent at the 95% confidence level. Relative standard deviations forchloride and phosphate are 17 and 22%, respectively, with no baselinecorrection.
Formaldehyde analyzed at 0.1 M in 0.1 M NaOH solutions, givessimilar current responses to analyte oxidation, yet surprisingly lacksthe nickel character of alkaline and methanol solutions. (The reasonfor this is not known, but is reproducible.) The oxidative wave offormaldehyde at Ni-Pi (black) shows a statistical increase of 32%over Ni-Cl at the 99% confidence level. The amperometric responseto standard additions in formaldehyde solutions is given in the inlay ofFigure 4a and tabulated in Table I, where 250 μL of 0.1 M formalde-hyde in 0.1 M NaOH was added to solution every 30 seconds. Thecorresponding calibration curve is seen in Figure 4b. Again, the differ-ence in response is significant at the 99% confidence level and more
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H466 Journal of The Electrochemical Society, 160 (8) H463-H468 (2013)
Figure 5. Electro-oxidation of formate: a) Representative cyclic voltammo-grams at 50 mV/sec in a 0.1 M NaOH and 0.1 M formate solution: Ni-Cl(gray), Ni-Pi (black); inlay: representative chronoamperometry, Ni-Cl (gray),Ni-Pi (black); b) calibration curves of Ni-Cl (gray) and Ni-Pi (black); withfits of y = −8.8 × 10−6x–6.29 × 10−5 for Ni-Cl (gray) at R2 = 0.97, andy = −9.62 × 10−6–8.83 × 10−4 with R2 = 0.99 for Ni-Pi (black).
sensitive for Ni-Pi by 260%. Relative error for Ni-Cl is 5.6% and 9.9%for Ni-Pi.
When considering the complete oxidation of aliphatic alcohols(e.g., methanol) to CO2, the next logical analyte is formate. Formate,however, does not exhibit the enhanced sensitivity at Ni-Pi as in thecases of methanol and formaldehyde. Although the current response at0.8 V vs. Ag|AgACl is enhanced again by 24% at the 99% confidencelevel, at the potential chosen here for amperometric analysis (0.65V), the increased sensitivity (i.e., slope) is not present (Figure 5). Thecalibration curve, generated in the same manner as formaldehyde,shows an increase in sensitivity of only 9%, different at the 90%confidence level. However, if baseline is normalized for increasedcurrent response at 0 mM formate, this difference is negated. Thisresult implies that selectivity is present in the case of Ni-Pi catalystfor the first two oxidative steps in methanol oxidation.
Nickel electrodeposits into nanoparticle aggregates on electrodesurfaces. Surface analysis of nickel deposited substrates by atomicforce microscopy (AFM) in Figure 6 show a relative increase of 40%(4.3 to 7.3 nm) in the half-height of deposited aggregates when nickelis co-deposited with phosphate. Elemental analysis via X-ray pho-toelectron spectroscopy (XPS), given in Table III, reveals increaseddeposited nickel content (3x increase); this may account for the differ-ence. However, changes to inter-aggregate structure are not revealed.(Ni-DNA aggregates were analyzed in Reference 14.)
As was true with the cobalt system studied by Nocera et. al.,11,12
increased Ni content may be attributed to a deposition mechanism thatrelies on phosphate present in the system as seen in Table I. In thecobalt-phosphate system, phosphate is speculated as acting as a protonaccepting electrolyte; in that during water oxidation, the abundance of
Figure 6. AFM image of nickel deposited surfaces: a) Ni-Pi with average filmhalf-height of 7.3 nm and b) Ni-Cl with average film half height of 4.3 nm.Larger particulate remnants present in a) due to adventitious material. (RMSroughness of polished glassy carbon ≈ 0.6 nm).
phosphate in the system prevents electrodeposited catalyst leachingfrom the surface. Other common electrolytes, including sulfate, nitrateand hypochlorite did not demonstrate the same properties.
DNA is a phosphate rich system, comprised of approximately 10%(by wt) phosphate. For Ni-DNA complexes, it is not without reason tohypothesize that DNA acts as a phosphate source, encouraging metaldeposition. Gileadi offers significant insight on the theory of metaldeposition.18 Analogous to metal deposition, the evolution of gassesat metal electrodes first requires the formation of modified layers, e.g.,the formation of PtO before oxygen evolution; a process that requiresa larger overpotential than necessary. Gileadi also offers a theoryon charge transfer, in short, that adsorbed species (such as anions)
Table III. XPS data for Ni-Cl and Ni-Pi electrode compositions.
Peak Electrode Atomic% Mass%
O 1s Ni-Cl 1.81 2.32Ni-Pi 8.38 9.75
N 1s Ni-Cl 1.40 1.57Ni-Pi 1.08 1.10
C 1s Ni-Cl 95.55 92.03Ni-Pi 86.06 75.24
Cl 2p Ni-Cl 0.96 2.73Ni-Pi 0.57 1.48
Ni 2p Ni-Cl 0.29 1.36Ni-Pi 0.96 4.10
P 2p Ni-Pi 0.13 0.29
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Journal of The Electrochemical Society, 160 (8) H463-H468 (2013) H467
Figure 7. CV of Ni-Cl (solid gray), Ni-Pi (dash black), and Ni from a two-stepdeposition (solid black); 0.1 M NaOH, v = 50 mV/sec.
at the inner Helmholtz plane facilitate electron transfer between theelectrode and metal in solution which acts to lower or eliminate theactivation energy.19 However, as Gileadi states, this theory has notbeen treated in literature and no experimental evidence exists.
Likewise, Conway and Bockris extensively analyzed mechanismsfor the electrolytic deposition of metals.20 The authors propose thatdeposition occurs through a series of steps, including ion transfer (i.e.,metal ion) from solution to the electrode surface, surface diffusion tomore favorable sites, loss of the solvation shell (dehydration), andultimately the redox process from metal ion (Mn+) to deposited metal(M0). Direct deposition and redox is unlikely, as prohibitively high�H values exist. The analysis specifically speculates on the possibilityof intermediate ion states assumed during deposition, and their roleon facilitating a decrease in the energy of activation. However, energycalculations for these processes by Conway and Bockris are basedupon initial and final states of metal species, based upon enthalpies orchemical potential energy. Neither Gileadi nor Conway and Bockrisoffer empirical evidence for this deposition mechanism. Conway andBockris, do in a later work,21 indicate that current exchange densitiesfor Ni2+ deposition are low (relative to Ag+ and Cu2+) due to theinstability of Ni+ ions in aqueous solutions.
We hypothesized that phosphate acts to stabilize this intermediatespecies, providing an environment for Ni+. In a recently publishedstudy on the electrolyte effects of manganese oxygen reduction elec-trocatalysts, it was observed that the electrolyte deposition bath has asubstantial effect on the electrocatalytic properties of the manganese.22
The trends in the manganese study are also unclear, as it does not obeythe trends of phosphate, nor metal-oxygen coordination.
To test our hypothesis and determine if phosphate facilitates thedeposition of nickel through the formation of a surface modified layer,glassy carbon electrodes were surface modified through three differ-ence mechanisms. Glassy carbon was used here to lend higher con-fidence to the known surface area. Before deposition (as describedabove) glassy carbon electrodes were polished with alumina powder,rinsed, and sonicated in DI to remove any adsorbates. Two deposi-tions proceeded as above, from 0.5 mg/mL NiCl2 solutions containingeither 0.1 M NaCl or K2HPO4. The third variation required two depo-sitions: first, a 3 minute, preferential phosphate deposition from 0.1 MK2HPO4 solution. And second, 30 minute deposition for a 0.5 mg/mLNiCl2 in 0.1 M NaCl solution. As can be seen in the CV in Figure 7,a drastic change in both the nickel character and nickel content of theNi2+/3+ peak in 0.1 M NaOH is observed.
Further analysis by XPS into the oxidation state of the nickelreveals no difference in nickel character between Ni-Cl and Ni-Pideposited systems. Analyzed as per Smart and Biesinger,23,24 fittingthe nickel peak shows that the deposited nickel species are primarilymetallic in nature. Spectra of the Ni 2p signal, shown in Figure 8,reveals increased nickel in the Ni-Pi system; these values are given inTable III. Typically, electrodeposited nickel is considered to be diva-
Figure 8. XPS spectra for Ni 2p signal: a) Ni-Pi, b) Ni-Cl; for both spectra:Ni 2p spectra (black), overall fit (red), Ni0 (metallic, gray), satellite 1 (green),satellite 2 (orange), background (blue).
lent (e.g., NiO) in alkaline solutions.4 Analyzes on planar or ‘massive’nickel metal surfaces do not generate the same catalytic character asthe deposited species.25 The phosphorus, it appears, enables bothgreater nickel content to be deposited, and deposited in a manner(structural) that enables greater catalytic activity.
Conclusions
We have analyzed electrodeposited nickel complexes that functionas electrocatalysts for water and aliphatic alcohol oxidation. Complex-ing agents have been shown to increase the activity of these nickelspecies, among them DNA.13 However, the role of the DNA in thesecomplexes was unclear. Therefore, it was hypothesized here that theDNA component of these ‘biocatalysts’ serves merely as a source ofphosphate, and are not an active component of the complex. To testthis theory, nickel-phosphate complexes were formed through the de-position of nickel in K2HPO4 electrolyte solutions and were comparedto previously studied to Ni-DNA complexes.14 These Ni-Pi complexesdemonstrated increased sensitivity to methanol and formaldehyde ox-idation versus chloride controls. For the Ni-DNA species, at approx-imately 10% phosphate (m/m), the current response correlates wellto the concentration of phosphate in solution. However, the Ni-DNAcomplexes are highly variable in their electrochemical behavior. Theincorporation of phosphate through HPO4
2− produces both increasedactivity and decreased variance versus the DNA complex system. Ad-ditionally, these results show that DNA is not needed for improvedelectrocatalysis when phosphate acts as a more consistent and cheaperalternative.
The relationship between increased catalytic activity and phos-phate incorporation could be one of many: increased surface area (asshown by AFM), increased Ni content (as shown by XPS), or a changein the carbon surface layer that facilities nickel deposition possibly
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H468 Journal of The Electrochemical Society, 160 (8) H463-H468 (2013)
through double-layer disruption or an electron tranport mechanism.As is often the case, this analysis raises more questions, specificallythe mechanism by which phosphate facilitates increased nickel de-position. Further research will focus on understanding the detailedstructure/function relationship of nickel electrocatalysts formed byelectrodeposition of nickel in phosphate electrolytes and anion effectson metal deposition.
Acknowledgment
The authors thank the Utah Science and Technology Initiative(USTAR) and the National Science Foundation for financial support.
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