A low component count, microcontroller-based potentiostat circuit was developed through the use of operational amplifiers arranged in different feedback configurations. This was developed to alleviate the cost burden of equipment procurement in low-cost and budget applications. Simplicity was achieved in the design by the use of the microcontroller’s native functionalities and a low-cost R/2R resistor ladder digital-to-analogue converter. The potentiostat was used to investigate the Ni2+/Ni(s) redox couple in a 3-electrode cell with a silver/silver chloride reference electrode and graphite counter and working electrodes. Linear sweep voltammograms were ob-tained at scan rates of 10, 20, 30 and 40 mV/s. The analysis of the peak current versus (scan rate)1/2 plot indicated that the Ni2+/Ni(s) reduction, though conforming to the Randles-Sevcik equation, was a non-reversible redox reaction.a
The potentiostat has remained the work-horse of the electrochemistry laboratory ever since the development of the first 3-electrode cell by Hickling in the early 1940s. Using Hickling’s original ideas, many incarnations of the device have been produced for different specialized applications over the years from basic electrochemical redox investigations [
Potentiostat circuits are of widely varying complexities, often dictated by the intended use. A key distinguishing feature is the current measurement range. It is almost impossible to have a single system capable of current measurement across a wide range, from the sub pico-ampere up to the ampere range due to the different circuit architectures required. Auto-ranging resistors have been employed as a solution to achieve multi-current measurement capabilities [
The potentiostat is fast ceasing to be the prime preserve of the electrochemistry laboratory. It is quickly making inroad into industrial and consumer devices. Potentiostat circuits can be found embedded in various gas sensors [
The demand for “embedded” and specialized potentiostat circuits is bound to continue. This has inspired the development by many workers of simplified, low cost instrumentation that do away with the bells and whistles, while retaining core potentiostat functionalities. A field-portable, low-foot print design is described in [
In the simplified design reported here, otherwise expensive components are replaced with low-cost, simplified circuit architecture. It is believed that the low component count, and low cost of this design will contribute further to making the potentiostat ubiquitous in low-resource electrochemical laboratories and in budget products and applications.
A schematic representation of the potentiostat system is presented in
A C-Sharp based software interface was developed in-house for data acquisition and control of the circuit. The data acquired were exported to spread sheet programs for detailed analysis.
In summary the main parts of the potentiostat circuit are: 1) The potentiostat control circuit; 2) Digital-to-Ana- logue-Converter circuit; 3) PIC18F452 Microcontroller (MicroChip Inc., Arizona USA); 4) AC/DC full wave rectifier circuit; 5) Voltage level shifters; and 6) Current-to-voltage converter.
The potentiostat control circuit is presented in
1) Module A
Module A is a differential amplifier. The operational amplifier (op-amp) amplifies the voltage difference between the non-inverting input and the inverting input. This can be described by the following equations:
At the summing point,
If V2 = 0, then
If V1 = 0, then
When R1 = R2 and R3 = R4, then
If all the resistors are of the same value, that is R1 = R2 = R3 = R4, then the circuit becomes a unity gain differential amplifier.
2) Module B
In Module B, the voltage output from module A is fed into the non-inverting input of op-amp 2 while the op-amp’s inverting input was directly connected to its output. Op-amp 2 was configured as a voltage follower. This effectively isolates the input from the output to prevent loading of the input signal. The voltage output from op-amp 2 was connected to the CE.
3) Module C
Module C comprises op-amp 3 configured as a voltage follower. This is an additional measure to prevent current flow through the RE. Current flow through the RE will polarise it, rendering it unreliable. Point Z in this module is a connection to a level shifter circuit. The level shifter’s task is to transform the voltage at Z into the 0 to +5 V range required by the microcontroller for the measurement of the RE’s potential.
4) Module D
Module D is the current measurement module based on the feedback ammeter principle. Here, the input current flows through the feedback resistor (Rf = 100 Ω). The low offset current of the op-amp changes the current (Iin) by a negligible amount. Thus, the output voltage is a measure of the input current, and sensitivity is determined by Rf. Op-amp 4 is tied to real ground; voltage perturbations coming from the WE forces the op-amp to output a voltage of equal but opposite polarity through the 100 Ω resistor. This action forces the potential values at the op-amps inputs to the same value of zero. This feedback voltage is also measured at Y. From this point it is transmitted to the level shifter for eventual connection to the microcontroller’s ADC.
A typical R/2R resistor ladder is illustrated in
the microcontroller’s I/O ports are connected to
In this manner, the voltage value from the microcontroller’s on-board program (converting voltage value into binary equivalents of 0 s and 1 s) is converted to its actual analogue equivalent. This physical equivalent, called the control voltage, represents the actual voltage value desired. It is this voltage that is connected to point X in Module A (see
An investigation of the Ni2+/Ni reduction reaction was carried out by using linear sweep voltammetry (LSV). This was done in a 3-electrode cell with a silver/silver chloride reference electrode and graphite working and counter electrodes. The surface area of the working electrode was 0.12568 cm2. The experiment was carried out by using a 1 M solution of nickel sulphate (BDH Chemicals, Poole England) prepared with distilled water. The surfaces of the graphite counter and working electrodes used were polished to rid it of any adsorbed films and rinsed in de-ionised water. LSV was carried out at four different scan rates of 10, 20, 30 and 40 mV/s within a potential window of between −0.1 and −1.5 V (vs. Ag/AgCl).
The complete potentiostat circuit is presented in
The potential (with respect to ground) at both the working and reference electrodes is equal in magnitude but opposite that applied to the counter electrode by op-amp 1. The potential applied to the counter electrode by op-amp 1 appeared at the working electrode through conduction by charged ionic species in the test solution. In order to have the same potential on the working electrode, the applied potential at the counter electrode caused a quantity of charged species (ions), equivalent to the applied potential, to be produced or consumed at the counter electrode. The ionic species produced were the current conductors in the solution and were conducted towards the working electrode, under the influence of potential difference. At the working electrode, the ionic conductors were exchanged for electrons at the electrode’s surface.
In the circuit, provision was made for measuring the small exchange current. This was accomplished via op-amp 2, which was configured as a feedback ammeter. The feedback ammeter produces an output voltage equivalent of the exchange current. Op-amp 3 is used as a voltage follower and acts as a buffer for the reference electrode to prevent current flow through it. The buffer was essential to keep the potential at the reference electrode constant. This was naturally achieved in op-amp 3 due to its inherently large input impedance (several mega Ohms). Thus, current could not flow from the input to the output of the op-amp. It instead, followed the path of least resistance, which was through the working electrode. It also acted as an inverting voltage follower, producing an inverted output of the reference electrode potential for onward summing at point A, for the attainment of virtual ground.
The microcontroller is capable of producing voltage in the range of 0 - 5000 mV through the R/2R ladder network. It was then level-shifted to between −2500 mV and +2500 mV. This gives a voltage output range of 5000
mV. The microcontroller’s 10 bit ADC reads this voltage at the following resolution:
Also, in Module D (
The current measurement resolution depends on the voltage read resolution of the microcontroller’s ADC (5 mV calculated in Equation (12)), and the value of Rf (i.e. 100 Ω). The minimum current detectable by the potentiostat circuit was thus determined using Ohm’s law as follows:
Or, considering positive and negative voltage perturbations,
The maximum current measured by the circuit is similarly determined by applying Ohm’s law to the maximum voltage value measurable (2500 mV) and the value of RF. Hence, the maximum current measurable by the potentiostat is,
Considering positive and negative voltage perturbations,
The incorporation of 16-bit and higher precision ADC units would seem to suggest an automatic attainment of higher resolution. While this prospect is attractive, the actual reality is however different. At 16-bit resolution and a Full Scale Range of 5 V (5000 mV), the voltage resolution is about 76 µV. This opens upnumerous problems related to circuit instability and noise. Guaranteeing this sort of accuracy with digital switching current emanating from a microcontroller is not a task easily achieved without costs. The use of a higher precision ADC (in a microcontroller) comes with the added responsibility of additional circuit components such as the incorporation of a separate chip with separate filtered power, and stringent circuit assembly protocol. Even ordinarily mundane tasks such as soldering and the physical sizes of resistors become very serious considerations that are necessary to ensure stability. Also, environmental considerations such as temperature and interference from nearby electronics or electrical installations have to be factored-in because interferences from them can be sources of noise and unreliability. If these considerations are not properly executed then higher precision may actually cause more problems that it sets out to solve. Though the considerations are solvable, they come with costs-component cost, production costs, circuit complexity and labour cost. While these costs may be easily absorbed in high-end commercial developments, it goes against the focus of the current study which has as its thrust cost minimization and availability of potentiostat in low-resource and constrained budget environments.
In the potentiostat circuit, no current is allowed to pass to the RE so as to allow the reference potential to remain unchanged during the experiment. To achieve this, the RE input impedance should be very high. The potentiostat control circuit in
Linear sweep voltammograms of Ni2+/Ni redox couple at different scan rates are presented in
Since potentials were measured with respect to the Ag/AgCl reference electrode, equivalent values versus the standard hydrogen electrode (SHE) were obtained using the equation below [
Scan rate (V/s) | Peak current density (mA/mg) | D | (Vs. Ag/AgCl) | (V) (Vs. SHE) | |
---|---|---|---|---|---|
0.01 | 0.10 | 2.06 | 4.65 × 10−8 | −1.060 | −0.806 |
0.02 | 0.14 | 2.39 | 3.13 × 10−8 | −1.129 | −0.816 |
0.03 | 0.17 | 2.93 | 3.14 × 10−8 | −1.295 | −0.855 |
0.04 | 0.20 | 3.47 | 3.30 × 10−8 | −1.285 | −0.865 |
where E (Vs SHE) is the electrode potential relative to the SHE, E (Vs reference) is the potential measured in the experiment relative to the Ag/AgCl reference and Ereference (Vs SHE) is the potential of the Ag/AgCl reference relative to the SHE. For 3 M NaCl (aq) filling solution (used in this work), Ereference (Vs SHE) is 209 mV [
It would be seen from the table that Ni2+ reduction occurs at increasingly negative potential as the scan rate increases. These values of the reduction potential of Ni2+ differ from the standard value of −0.25 V (Vs. SHE). The deposition of nickel on graphite requires high nucleation overpotential.
Nucleation overpotential of 0.3 V has been reported for the deposition of nickel on graphite [
The relationship between the peak current and concentration of analyte’s ionic species is given by the Randles-Sevcik equation:
where ip is the peak current (in amperes), n is the number of moles of electrons appearing in half-reaction for the redox couple, ν is the scan rate at which the potential window is scanned (V/sec), F is Faraday constant (96,485 C/mol), A is the electrode’s exposed surface area (cm2), R is the Universal gas constant (8.314 J/mol K), T is the absolute temperature, and D is the analyte’s diffusion coefficient (cm2/sec).
If ambient temperature was taken to be 25˚C (298.15 K), the equation becomes:
If the concentration of the analyte (in this case, nickel sulphate), the peak current, scan rate and number of moles of electrons (n) are known, the Randles-Sevcik equation can be used to determine the diffusion coefficient of the analyte ions.
The reduction of nickel (II) ions is according to following equation:
Here n = 2. Hence using this value of n, together with other known variables in the Randles-Sevcik equation, the diffusion coefficients (D) at the different scan rates were calculated. These are presented in
The plot of peak current vs. (scan rate)1/2 is presented in
The non-zero intercept can be attributed to the contribution of non-faradaic currents to the peak current [
This work has described the development steps and circuit architecture for a low-cost simplified potentiostat circuit. The microcontroller-based unit is able to demonstrate key potentiostat functionalities in the investigation of the Ni2+/Ni redox couple. The possible range of functions is expandable with expanded software algorithms. Applications for simplified potentiostat circuits transcend low-resource environments. Such simplified
circuits can also find application in budget or low-cost monitoring and sensing instrumentation.
BolajiAremo,MosobalajeOyebamiji Adeoye,Imoh BasseyObioh,Odunayo AdetolaAdeboye, (2015) A Simplified Microcontroller Based Potentiostat for Low-Resource Applications. Open Journal of Metal,05,37-46. doi: 10.4236/ojmetal.2015.54005