(Invited) Silicon a New Electrode for Electrochemical Reduction of Nitrates Comparison with Boron-Doped Diamond
Authors: Levy-Clement Claude , Bastide Stephane 
Published: December 2024

Nitrate (NO3
-) ions are one of the world’s most widespread water pollutants. Previous studies on electroreduction of nitrate at cathodes were aimed to selectively convert NO3
- to inert N2. Extensive studies on boron-doped diamond (BDD) have shown that this promising electrode reduces very efficiently the nitrates. Under highly negative potentials, H2 evolution is strongly impeded confirming that the competition between H2 formation and NO3
- reduction at BBD electrode is weak (1). N2 was obtained in acidic solution (2, 3). This result was confirmed at pilot scale (3). However, since several years the goal has changed toward the clean production of a useful product such as ammonia which is formed following the 8 electrons process: NO3
- + 10 H+ + 8 e- à NH4
+ +3 H2O. Optimizing the conditions of nitrate electroreduction at BDD electrodes Einaga and al obtained ammonia (4).

One drawback is the cost of the BDD electrode and research of new efficient electrodes toward nitrate reduction is highly desirable. Silicon (Si) and diamond (C) belong to the same column of the periodic table. They share numerous properties including crystallographic structure and formation of metal-hydrogen bonds (Si-H or C-H bonds) at the surface. This led us to undertake preliminary studies using monocrystalline heavily doped P-type Si wafer as a new cathode for nitrate reduction. The cyclic voltammetry in HCl and HNO3 solutions showed that Si similarly to BDD electrode exhibited a slow kinetics for H2 evolution illustrated by a very high overvoltage for water decomposition (Fig. 1).

The products formed during NO3
- reduction were analyzed following a procedure developed during the experiments with BDD electrodes. Electrolysis were done during 6 hours under constant potential using an electrochemical cell with a double compartment separated by a Nafion membrane. The starting solution was 1M NO3
- solution at pH 0.5 permitting to avoid the formation of toxic NO2
- nitrite species. Reduction products in solution and gases were analyzed after the electrolysis, using spectrophotometry and gas chromatography. The potential applied to the cathode varied from -3 to -5 V/SCE. The amount of reduced nitrates during the electrolysis increased linearly with the cathodic polarization for both Si and BDD electrodes. The electrolysis efficiency with the Si electrode was ~70 % that of BDD. The faradic efficiency of H2 production at the Si electrode decreased from 65 to 15 % at -3 and -5 V, respectively with a 5 % minimum at -4V. NH4
+ was found to be the major product formed at -4V. The stability of the Si electrode was fairly good with an etching limited to a thickness of 1 µm/cm2 per electrolysis. That may be due to a local formation of OH- (reduction of H2O). The interest of using Si as an alternative electrode for the reduction of nitrate deserves further investigations.

References

1). C. Lévy-Clément, N.A. Ndao, A. Katty, M. Bernard, A. Deneuville, C. Comninellis, A. Fujishima, Diamond and Related Materials 12, 606–612, 2003.

2).V. Georgeaud, F. Omnes, J.-P Gauthier, C. Lévy-Clément, Nitrates in water: electro-reduction to nitrogen gas with BDD electrodes. Optimization of reaction conditions and development of great size electrodes. RITEAU (French national research program), Nitradiam report n°103 2 90 6067, 2008

3).V. Georgeaud, A. Diamand, D. Borrut, D. Grange and M. Coste , Water Science and Technology, 63-2, 2011

4).P. Kuang, K. Natsui, C. Feng, Y. Einaga, Chemosphere 251, 126364 (2020) and references therein

Figure 1