Generating Huge Magnetocurrent by Using Spin-Dependent

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Generating Huge Magnetocurrent by Using Spin-Dependent Dehydrogenation Based on Electrochemical System Haiping Pan, Xin Xiao, Bin Hu, Yan Shen, and Mingkui Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11357 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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The Journal of Physical Chemistry

Generating Huge Magnetocurrent by Using Spin-dependent Dehydrogenation Based on Electrochemical System Haiping Pan1, Xin Xiao1, Bin Hu1,2, Yan Shen1 and Mingkui Wang1* 1. Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China 2. Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT:

Systems featuring large magneto-current (MC) at room temperature are attractive owing to their potential for application in magnetic field sensing. Usually, the magnetic materials are exploited to achieve large MC effect. Here, we report a huge MC of up to 150% in a non-magnetic system based on the electrochemical oxidation of hydrazine at room temperature. The huge MC is ascribed to the spin-dependent N-H bond cleavage and re-formation through dehydrogenation during the oxidation of hydrazine. Specifically, the N-H bond cleavage generates singlet radical pairs. An external magnetic field can accelerate the spin evolution from singlet to triplet in spincorrelated radical pairs by perturbing spin precessions. Increasing the amount of triplet radical pairs can largely reduce the N-H bond recovery and significantly enhance the oxidation current of hydrazine. As a consequence, the spin-dependent bond formation through dehydrogenation can provide a new approach to generate huge MC in electrochemical cells.

TOC GRAPHICS

KEYWORDS: Magnetocurrent, radical pairs, spin evolution, electrochemical oxidation, spin chemistry


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Magneto-current (MC) is defined as changes caused by an applied magnetic field in the electrical current. The MC, also known as magneto-resistance (MR), has been extensively studied in organic semiconductor devices [1,2,3]. Examples include giant MR of 40% at 11 K in organic spin valve generated by spin-polarized carriers injection and transport [3], and tunnel MR due to electron spin-polarized tunneling [4,5,6,7,8]. Both organic spin valve and tunnel MR devices contain ferromagnetic electrodes. Here, we explore an entirely different approach of generating huge MC in a non-magnetic system at room temperature by using the spin-dependent N-H bond formation through dehydrogenation based on the oxidation of hydrazine. This possible new approach depends on the following experimental arguments. Firstly, the anodic oxidation of hydrazine occurs predominantly through a dehydrogenation of hydrazine molecule on platinum electrodes

[9,10]

, owing to the relatively active electrocatalytic effect of platinum on hydrazine

oxidation. The dehydrogenation process undergoes catalytic breaking of the N-H bond [11,12,13,14] and generate singlet radical pairs (N-H→ [N•↑…↓•H]1) due to Pauli exclusion principle

[15]

.

Secondly, an applied magnetic field can facilitate the spin flipping from singlet to triplet in spincorrelated radical pairs [16,17,18]. Furthermore, the singlet radical pairs can recombine to form new N-H bond and inhibit the oxidation process of hydrazine. However, the recombination of triplet radical pairs is spin-forbidden due to the Pauli exclusion principle

[19]

. Hence the triplet pairs

tend to dissociate into escape products and largely accelerate the oxidation rate of hydrazine. An external magnetic field can increase the amount of triplet pairs by magnetic field-induced intersystem crossing process

[20,21,22,23]

and thus largely enhance the oxidation current in the

electrochemical cells. As a consequence, a huge MC is generated by spin-dependent N-H bond formation through dehydrogenation based on the oxidation process of hydrazine. Our previous work has indicated that the oxidation current of tertiary amines could be increased by an external


3


magnet field and was ascribed to spin-dependent α-C–H bond cleavage and re-formation [24]. Herein, we further investigated magnetic field effect on the oxidation process of hydrazine, which has a different chemical structure compared with tertiary amines and involves the N-H bond cleavage during the oxidation process of hydrazine. Understanding the reactivity of hydrazine in an electrochemical environment is important for various applications, including fuel cells, electrochemical detection, the use as reducing agent and anti-corrosion agent

[11,14]

. Meanwhile, the oxidation reactions of hydrazine on electrode

surface are interesting for knowing the complex multistep electrocatalytic processes involving electron and proton transfer

[14]

. Previous studies have disclosed that the splitting off the first

hydrogen is the rate determining step during the oxidation process of hydrazine [9]. Here, we found that an external magnetic field can largely accelerate this rate determining step by enhancing singlet→triplet spin evolution in radical pairs and thus generate huge MC. Therefore, one can use the huge MC effect to overcome the rate determining step during the electrochemical

H H

NN

H H

80

acetonitrile water ethanol

3 2

MC

40

1

0

B

0

15

30 45 Time (s)

60

0

4

160 b

acetonitrile

120 80

water ethanol

MC

40

B

3 2 1

0 0

30

60 90 Time (s)

120

Magnetic field (T)

120

Magnetic field (T)

4

160 a

Current change (%)

oxidation process of hydrazine and greatly increase the oxidation rate of hydrazine.

Current change (%)

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0

Figure 1. Magneto-current is shown for hydrazine dissolved in three different solvents (acetonitrile, ethanol and water). The inset is the molecular structure of hydrazine. The three solutions were prepared by separately combining 20 mM hydrazine and 0.1 M TBAP in acetonitrile and ethanol, 20 mM hydrazine and 0.1 M KOH in water. (a) Current change is shown as a function of time in a triangular wave of magnetic field. (b) Current change is shown as a function of time in a rectangular wave of magnetic field.


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Figure 1 shows the MC for 20 mM hydrazine dissolved in three different solvents (acetonitrile, water and ethanol) based on the electrochemical oxidation process. The applied electrode potentials were set at 0.4 V, 0.2 V and 0.4 V in acetonitrile, water and ethanol, respectively. We found the electrical currents quickly increased in these three systems and huge MC was generated by applying an external magnetic field. The maximum amplitude of MC (MCmax) reaches about 151%, 90%, and 35.5% in acetonitrile, water, and ethanol systems (Figure 1a), respectively, at the peak magnetic field of 0.9 Tesla. It should be noted that the MC curves exhibited approximately symmetric characteristics when a symmetrical rectangular wave of magnetic field was applied (Figure 1b). Furthermore, the MC increases rapidly as the applied magnetic field increases, and still maintains a large value during the period of constant magnetic field. These observations indicate that the application of an external magnetic field can dramatically change the electrical current, generating a huge MC in the electrochemical oxidation of hydrazine. For electrochemical systems, the MC can be usually generated by spin-dependent redox process

[24]

or magneto-convection effects induced by Lorentz force effect

[ 25 ]

. Here in the

present system, the magnetic field is applied parallel to the charge transport direction (Figure 2a). This configuration can minimize the influence of Lorentz force effect on charge transport. In addition, when an applied electrode potential was set less than -0.6 V for acetonitrile, -0.7 V for water and -0.4 V for ethanol, the MC was too low to be observed (as shown in Supporting Information Figure S1b, S2b and S3b), presumably due to a slow oxidation rate of hydrazine (Figure S1a, S2a and S3a). These results suggest that the MC is generated by magnetic fieldsensitive oxidation process of hydrazine rather than the magneto-convection effect.


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0.3 c 0.2V 0V

0.2 0.1

≤-0.7V

0.0 210 240 270 300 330 Wavelength (nm)

90 d 60

Current change (%)

Current change (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Absorbance (a.u.)


12 9 6 3 0 -3

30

0

50 100 150 200 Magnetic field (mT)

0 0.0 0.2 0.4 0.6 0.8 Magnetic field (T)

Figure 2. (a) Experimental setup for magneto-current measurements. (b) Schematic diagram to show spin-dependent dehydrogenation of hydrazine induced huge magneto-current in electrochemical cells. (c) The absorption spectra of intermediates produced by electrochemical oxidation of 20 mM hydrazine solution at different applied potentials. (d) Magneto-current is shown at high and low (the inset) magnetic field for 20 mM hydrazine and 0.1 M KOH dissolved in water at 0.2 V applied potential.

We further propose a possible mechanism for oxidation process of hydrazine as shown in Figure 2b to elucidate the huge MC based on hydrazine system. The oxidation of hydrazine experiences the catalytic breaking of N-H bond [9,10,11,13,14], generating singlet radical pairs (NH→[N•↑…↓•H]1) due to Pauli exclusion principle. An applied magnetic field can facilitate the spin

evolution

of

singlet→triplet

[16,

26

,

27

]

in

spin-correlated

radical

pairs

([N•↑…↓•H]1→[N•↑…↑•H]3). The singlet radical pairs can either recombine to form new N-H bonds or dissociate into escape products. However, the recombination of triplet radical pairs is


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spin-forbidden due to the Pauli exclusion principle. Therefore the triplet radical pairs tend to totally dissociate into oxidation products (Figure 2b). In addition, the intermediates produced by electrochemical oxidation of hydrazine were characterized with the absorption spectroscopy as shown in Figure 2c. The peak at 230 nm corresponds to the characteristic absorption spectrum of hydrazyl radical •N2H3 [12]. This confirms the generation of hydrazyl radical •N2H3 during the oxidation process of hydrazine. The dissociation of radical pairs involves both electron and proton transfer process. Before the application of a magnetic field, the dissociation of radical pairs and electron transfer between radical pairs and the working electrode reach a dynamic balance with a stable electrical current over time (Figure S1c, S2c and S3c). An applied magnetic field could disturb the dynamic balance and largely enhance the electrical current by facilitating singlet→triplet spin evolution [16,26,27]

in spin-correlated radical pairs. Indeed, after an external magnetic field is applied, the

singlet→triplet inter-conversion rate in radical pairs can be largely enhanced by magnetic fieldperturbed spin precession

[16,17,18]

. In general, the precession of the electron spin vector in a

radical can be controlled by both the internal magnetic field and the applied magnetic field B [27]. The difference between precessional frequencies of the spin vectors in a radical pair is given by [16,27,28,29]

, ∆ω = (µ B / h )[∆gB ] + ∆ ∑ ai mi

(1)

where ∆gB arises from the Zeeman interaction for cases in which the g factors of the two electrons are different (∆g mechanism) and Σaimi arises from the difference in the hyperfine field experienced by electrons (hyperfine coupling mechanism). When the two radicals in a pair experience different spin precession frequencies (ω1≠ω2), the singlet↔triplet (S↔T) conversion


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occurs

[29, 30 ]

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. The ∆g mechanism and hyperfine coupling mechanism induced S↔T

interconversion rate (kISC) is approximately given by [31], k ISC ~ 3 ×106 ( ∆gB + ∑ a1i M 1i − ∑ a2 j M 2 j ) i

j

(2)

where kISC is in s-1, B and a are in gauss, Mij is the spin states of nucleus j on radical i. Here in our system, one part of the radical pair generated by N-H bond cleavage is hydrogen (Figure 2b), which has a g-factor of 2.0114[32]. Hence the ∆g is evaluated to be 0.0079 for the g-factor of the other part of the radical pair is about 2.0035[33]. Thus, the kISC induced by ∆g mechanism (kISC-∆g) was estimated to be about 2.13×108 s-1 at our maximal magnetic field of 9000 G. Furthermore, the kISC induced by hyperfine coupling mechanism (kISC-HFC) is about 1.8×107 s-1 for the hyperfine coupling constants of hydrogen (aH) and nitrogen (aN) being 11 G and 11.5 G[34] and the spin state of hydrogen and nitrogen being 1/2 and 1[35]. Thus the kISC-∆g is much larger than kISC-HFC at high magnetic field up to 9000 G. Therefore, we suggest the ∆g mechanism is the dominant mechanism for the singlet→triplet interconversion at high magnetic field in this system. However, at much weaker magnetic field, e.g. B≦760 G, the kISC-∆g was estimated to be less than 1.8×107 s-1 (kISC-HFC). This indicates the hyperfine coupling mechanism is dominant for the spin mixing from singlet to triplet at low magnetic field (≦760 G). Furthermore, the Zeeman Effect could suppress the spin mixing induced by hyperfine coupling mechanism at low magnetic field, leading to negative MC. When the applied magnetic field increases, the ∆g mechanism-enhanced spin mixing dominates and MC gradually changes from negative to positive (Figure 2d).


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According to equation 2, increasing the strength of the applied magnetic field can further enhance the spin mixing and magnify the singlet→triplet interconversion rate. In addition, the singlet radical pairs can re-form new N-H bonds while the recombination of triplet radical pairs is spin-forbidden due to Pauli exclusion principle. Thus the triplet radical pairs have a much larger dissociation probability than that of the singlet radical pairs (Figure 2b). Consequently, the magnetic field-induced singlet→triplet interconversion in radical pairs can significantly enhance the electrical current in the oxidation of hydrazine owing to the much larger dissociation probability of the triplet radical pairs. Now we further investigate the spin-dependent N-H bond formation through dehydrogenation in generating huge MC based on the hydrazine system. We can see in Figure 1a the MC based on hydrazine oxidation exhibit significant different MCmax amplitudes of 151%, 90% and 35.5% for acetonitrile, water and ethanol systems, respectively. Here, we should notice that acetonitrile is an aprotic solvent while water and ethanol are protic solvents (shown in Supporting Information Table 1). Generally, the molecules of a protic solvent can easily donate protons (H+) to reactants while an aprotic solvent is more difficult to donate protons. Therefore the dehydrogenation rate is suppressed in protic solvents due to the fact that the proton diffusion from the reaction zone to the bulk solution is reduced. On the contrary, the dehydrogenation rate is largely enhanced in an aprotic solvent for it is readily to receive protons. Consequently, a huge MC in the aprotic solvent (acetonitrile) and a much smaller MC in protic solvents (water and ethanol) are observed (Figure 1). The remarkable difference on MC in protic and aprotic solvents gives further evidence that spin-dependent N-H bond formation through dehydrogenation is the mechanism responsible for the observed huge MC in hydrazine-based electrochemical systems.


9

1V 0.8V

18 12

0.6V 0.4V

6 0

≤0V

0.0

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pH=8.5

pH=6.5 Current change (%)

30 a 24

50 b 40

1V 0.8V

30 0.6V

20

0.2V

10

0.2 0.4 0.6 0.8 Magnetic field (T)

-0.2V

0 0.0


pH=13 Current change (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Current change (%)


90 c 0.2V 0V

60

-0.2V

30 -0.4V ≤-0.7V

0 0.0


Figure 3. Magneto-current for 20 mM hydrazine dissolved in three aqueous solutions with different pH values. (a) 20 mM N2H4 +0.1 M NaH2PO4 (pH=6.5); (b) 20 mM N2H4 +0.1 M NaH2PO4+0.1M KOH (pH=8.5); (c) 20 mM N2H4 +0.1M KOH (pH=13).

To further confirm this new method of using spin-dependent N-H bond formation through dehydrogenation to generate huge MC, we have also studied the MC of hydrazine dissolved in water with different pH values. Figure 3 shows that the hydrazine solution with different pH leads to very different MC with different applied electrode potentials. Firstly, when the applied electrode potentials were set less than 0 V, -0.2 V and -0.7 V in hydrazine solution with pH=6.5, 8.5 and 13, hardly any MC was observed. Because the oxidation and dehydrogenation rate of hydrazine is very slow at these potentials (the small oxidation current showed in Figure S4). These results further confirm that the MC is generated by the oxidation of hydrazine and Lorentz force-induced magneto-convection effect can be excluded. Furthermore, the amplitudes of MC increase accordingly as applied electrode potentials increase in these three different solutions


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(Figure 3). Since enhancing the applied electrode potentials can increase the oxidation and dehydrogenation rate of hydrazine (the oxidation current increases when applied potentials increase, as showed in Figure S4), thus the MC amplitudes increase. Besides, in the hydrazine solution of pH=13, the MCmax reaches 93% while the MCmax are only about 52% for pH=8.5 solution and 28% for pH=6.5 solution. The higher the pH value of the solution, the easier it is to receive protons. Therefore, the dehydrogenation rate increases as the pH value of the solution increases since the diffusion of the protons from the reaction zone to the bulk solution is accelerated as the pH value increases. Thus the MCmax increases as the pH value of the solution increases. As a consequence, the significantly different MC in the solutions with different pH values further confirm that the spin-dependent N-H bond formation through dehydrogenation is the mechanism responsible for the generation of huge MC based on the hydrazine system. In summary, we report a huge MC of up to 150% in a nonmagnetic system at room temperature based on the oxidation of hydrazine. The observed huge MC indicates that the spindependent N-H bond formation through dehydrogenation can provide a new approach to overcome the rate determining step during the oxidation process of hydrazine and generate huge MC. This new method can be verified by using different solvents with different dehydrogenation rates. It is found that an aprotic solvent could largely enhance the spin-dependent dehydrogenation rate and generate huge MC. Nevertheless, a protic solvent could reduce the spin-dependent dehydrogenation rate and lead to much smaller MC. In short, we observed a huge MC of up to 150% in the aprotic solvent (acetonitrile) while a much smaller MC of 90% and 35.5% in protic solvents (water and ethanol). The remarkable difference on MC between protic and aprotic solvents further confirms that the spin-dependent N-H bond cleavage and re-


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formation through dehydrogenation can provide a new approach to generate huge MC in electrochemical systems. EXPERIMENTAL METHODS: Chemical hydrazine, potassium hydroxide (KOH), sodium dihydrogen phosphate (NaH2PO4), tetra-n-butylammonium perchlorate (TBAP), anhydrous ethanol and acetonitrile were all purchased from Aldrich and used as received. The final solution concentrations were 20 mM hydrazine and 0.1M TBAP in acetonitrile and ethanol, 20 mM hydrazine and 0.1M KOH/0.1M NaH2PO4/ 0.1M KOH+0.1M NaH2PO4 in water. Here TBAP, KOH and NaH2PO4 were used as the supporting electrolyte. The experimental setup of MC measurement was the same as in ref. 24. Magneto-current (MC) was measured by current change as a function of magnetic field, MC= (IB-I0)/I0×100%, where IB and I0 are the current with and without a magnetic field, respectively. The absorption spectra of hydrazine solution at different applied electrode potentials were characterized by UV-VIS spectrophotometer SPEKOL 2000, using a spectroelectrochemical cell produced by ALS Company. Electrode potential was supplied and the current was recorded by CHI 750D electrochemical workstation. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The Cyclic voltammograms and magnetocurrent with different applied electrode potentials are provided to exclude Lorentz force and magneto-convection effect in our system (PDF).

AUTHOR INFORMATION Corresponding Author *Email: [email protected]


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Financial support from the National Basic Research Program of China (2014CB643506), the NSFC (51661135023, 21103057), the Natural Science Foundation of Hubei Province (ZRZ2015000203), Technology Creative Project of Excellent Middle & Young Team of Hubei Province (T201511) and the Wuhan National High Magnetic Field Center (2015KF18) are acknowledged. The authors also thank the facility support of the Center for Nanoscale Characterization & Devices (CNCD), WNLO of HUST. REFERENCES

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(33) Nelsen, S.; Landis, R. Diazenium cation-hydrazyl equilibrium. Three-electron-two-center. pi. systems. J. Am. Chem. Soc. 1974, 96, 1788-1793. (34) Adams, J.; Thomas, J. Electron paramagnetic resonance of a hydrazine radical ion. J. Chem. Phys. 1963, 39, 1904-1906. (35) Hayashi, H. Introduction to dynamic spin chemistry: magnetic field effects on chemical and biochemical reactions; World Scientific Publishing Co Inc: Singapore, 2004; Vol. 8.


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Generating Huge Magnetocurrent by Using Spin-Dependent

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