Influence of Magnetic Fields on the Behavior of Single Hydrogen Bubbles Generated via Water Electrolysis

F. Karnbacha,b, M. Uhlemanna, X. Yangc,d, K. Eckertc, D. Baczyzmalskie, C. Cierpkae, G. Mutschkec,d, A. Geberta

aInstitute of Complex Materials, IFW Dresden, Germany bFaculty of Mechanical Science and Engineering, TU Dresden, Germany cInstitute of Fluid Mechanics, TU Dresden, Germany dInstitute of Fluid Dynamics, HZDR, Germany eInstitute of Fluid Mechanics and Aerodynamics, UniBW München, Germany

Motivation …in applied magnetic field → MHD → electrolyte convection

• Change in energy policy to renewable energies [1] Nucleation: discharging Planar electrode : Bulk convection: • Differences between production and consumption adsorption B = 0 mT B = 70 mT - hydrodynamic drag → Development of novel methods for energy ex ex supersaturation - coalescence storage:

- improved desorption classical Hydrogen production via water electrolysis Growth: mass transport in applied magnetic fields: coalescence [f(electrolyte, Localized convection: ► Understanding of single bubble behavior electrode, size)] - secondary flow ► Vizualisation of bulk and near-bubble - lift force generated Detachment: interfacial tension

electrolyte flow micro - reduced bubble size contact angle - improved desorption AIM: Increase of efficiency by minimizing f(pH, wettability) overpotential and ohmic drop → fast detachment and reduced bubble size Setup … Microelectrode → Single hydrogen bubble

Electrochemical cell: Measurement parameters: Principal bubble behavior: Without B , EMSE = -1.5 V

- Electrolyte: 1 M H2SO4 ► Potentiostatic characterisation : Nucleation – Growth - Detachment - External magnetic field: ► Flow field around detached bubble - Electromagnet: B ≤ 1 T [2] - Permanent magnet: B ≤ 0.8 T (PIV) : - Potentiostatic characterisation: -1.5 V, -2.0 V vs. MSE - Video imaging by high speed camera: Particle image/tracking velocimetry (PIV/PTV), A B Astigmatism particle tracking velocimetry (APTV): ► High speed imaging Polystyrene particles as tracer (Ø 1µm, 15 µm) allows exact correlation of D F the current-time-signal with ► torus-like electrolyte flow the evolution of the single Two different B-field configurations: ► rise velocity: 120 ± 20 mm/s bubble: ► complete electrolyte refreshment perpendicular (┴) and parallel (II) B perpendicular to WE surface B parallel to WE surface

B No B B No B B No B B No B Potentiostatic characterisation: EMSE = -2.0 V, B 30 s on/30 s off

B No B B No B B No B B No B

Bulk flow (APTV)[3]:

► FL-driven electrolyte convection reaches far into bulk ► highest flow velocity in the vicinity of the bubble, azimuthal flow acts only in one direction ► secondary flow is established in the bulk that

becomes larger with increasing FL

B → MHD → Bubble diameter + lifetime B → MHD → Bubble diameter + lifetime Conclusions Magnetic field perpendicular to WE: Magnetic field parallel to WE: ► B-field leads to slowed down bubble growth and detachment ► B-field leads to accelerated bubble growth and detachment ► Change of the convection behavior in the vicinity of the bubble foot ► Bubble diameter and lifetime decrease with increasing B-field strength

Mechanism of bubble growth and detachment differs Faster detachment and increase of efficiency from planar electrodes of H2 production

[1] J. A. Koza et al., Electrochem. Commun. 11(2) (2009), 425-429. [2] X. Yang, F. Karnbach et al., Langmuir 31 (2015), 8184-8193. [3] D. Baczyzmalski et al., J. Electrochem. Soc. (2016), under review.