12 Feb 2026
How reactor geometry, mass transport, and instrumentation turn electrosynthesis from a promising paper into a repeatable plant step.
Electrochemistry has always carried two reputations at once. In the hands of a specialist, it is one of the cleanest ways to do redox chemistry: electrons are the reagent, and the by-products can be as simple as hydrogen or oxygen. In the hands of a general synthetic team under time pressure, it can feel unforgiving: a reaction that worked on one afternoon becomes stubbornly irreproducible a week later, and the post-mortem rarely points to a single root cause.
In the last decade, what has changed is not only the chemistry but the way we run it. Flow has given electrosynthesis a set of engineering “handles” that batch experiments do not naturally provide. The most important handle is that a flow electrochemical cell is, unambiguously, a reactor with a defined geometry: a defined interelectrode gap, a defined surface-to-volume ratio, a defined hydraulic residence time, and an electrical operating point that can be held constant without relying on a technician’s touch.
When I train new teams at Amar Flow Laboratory, I start with a mental model that looks almost too simple: an electrochemical transformation is a sequence of steps—transport from bulk to surface, adsorption, electron transfer, desorption, and transport back to bulk. The overall rate, and often the selectivity, are governed by whichever of those steps is slowest. The moment you accept that, the cell stops being “a box with wires” and becomes a piece of process equipment with the same status as a heat exchanger or a distillation column.
The attached literature makes the same point directly: mass transport phenomena sit alongside electron-transfer kinetics as key design constraints for electrochemical reactors, and understanding the engineering principles is critical to exploit the full potential of flow electrochemistry. That perspective is exactly what industry needs, because it reframes troubleshooting from superstition (“electrochemistry is temperamental”) to a solvable problem (“our transport and electrical fields are not where we think they are”).
One of the biggest hidden costs in batch electrochemistry is “making the electricity behave.” In a beaker, the distance between electrodes is often millimeters to centimeters, and the resistance of the liquid phase penalizes you with an ohmic drop (IR drop). The conventional fix is to add supporting electrolyte to raise conductivity. That solves the electrical problem, but it introduces downstream work: separation, recycling, and, for regulated pharma routes, an extra story around impurity control.
Flow electrochemistry changes the geometry. When the interelectrode gap is reduced from the centimeter scale to hundreds of micrometers, the ohmic drop drops with it. The attached fundamentals paper explains the relationship explicitly: the voltage loss scales with current and gap distance, and can be mitigated by increasing conductivity or by shrinking the gap. Shrinking the gap is the more elegant solution because it attacks the problem at its source; it often allows you to reduce electrolyte loading without destabilizing the cell.
This is not an academic detail. Supporting electrolyte is frequently the reason an otherwise attractive electrosynthesis is abandoned in development: it complicates work-up, adds cost, and can introduce trace impurities that matter in pharmaceutical intermediates. If the electrochemical geometry lets you run at lower electrolyte load, you have created value before you have even optimised yield.
At Amar, this is the engineering logic behind our microflow electrochemistry chip platform. The chip form factor is intentionally narrow-gap and low-holdup. It is built for method development, rapid screening, and “get it to work” problems where transport and IR drop dominate. If a chemistry is sensitive to potential control, or if it becomes messy when you add electrolyte, the microflow chip is often the most honest first experiment: it reveals whether the reaction is fundamentally compatible with efficient charge delivery and short diffusion distances.
A narrow gap alone is not enough. In microreactors, flow is almost always laminar, which means you do not get “free mixing” from turbulence. Transport to the electrode is then dominated by diffusion (and, for charged species, migration), and diffusion length scales become your clock. The fundamentals paper offers a useful back-of-the-envelope reminder: molecules diffuse only tens of micrometers in a second, but centimeters over a day. If your electrode gap and diffusion path are on the order of tens to hundreds of micrometers, diffusion ceases to be a bottleneck; if you leave the gap at millimeters or centimeters, diffusion becomes the bottleneck unless you inject energy through stirring or high electrolyte concentration.
This is why flow electrochemistry often feels “more reproducible” than batch. The reactor geometry forces the boundary conditions to be similar every time, and the transport regime is easier to hold constant. It also explains why many early batch electrochemistry protocols show scattered results: a small change in stirring, electrode position, or electrode surface condition changes the thickness of the diffusion layer and therefore the effective reaction rate at the electrode interface.
Mass transport becomes even more interesting when gases or second phases are involved. The fundamentals paper discusses segmented flow and the formation of toroidal vortices inside liquid slugs—an effect that can significantly enhance mixing and mass transfer to electrode surfaces. This is not just a flow-chemistry curiosity. In practice, it means that a two-phase system that is sluggish and diffusion-limited in batch may become fast and controllable in flow, provided you choose a cell geometry that tolerates the phases and you manage pressure and residence time properly.
A recurring mistake in early electrochemistry projects is to treat “the cell” as an interchangeable accessory. In reality, the cell is the reactor, and reactor choice encodes assumptions about transport, pressure drop, fouling tolerance, heat removal, and operability. We keep three form factors in our portfolio because their sweet spots are different, and a serious development program often benefits from moving through them in a disciplined sequence.
Microflow chips are our precision tools. They are designed to give tight electrical control through narrow gaps, high electrode surface-to-volume ratio, and minimal holdup volume. They shine when you need to map feasibility quickly: current density versus selectivity, mediator loading, electrolyte minimisation, and the effect of residence time on overoxidation or overreduction. They are also the natural platform when you want to couple electrochemistry with inline analysis—UV-Vis, IR, conductivity, or simple electrochemical diagnostics—because small holdup gives you fast response and clean signal interpretation.
Plate cells are our workhorse development reactors. Compared with microchips, plate cells provide larger channels and higher throughput, with greater tolerance for practical streams: higher viscosity, modest solids, and longer continuous campaigns. The gap is still controlled and far smaller than a typical batch setup, but the hydraulic design is more forgiving. For many pharma intermediates and specialty chemicals, this is where the lab recipe becomes a stable, day-long (and then multi-day) run: a level of boring reliability that operators appreciate and QA teams can document.
Modular stacks are how we scale without losing the physics. Flow electrochemistry does not scale like a stirred tank; it scales by preserving interfacial area and gap while increasing total electrode area. A stack is essentially numbering-up of electrochemical plates with common utilities and a repeatable electrical architecture. This approach aligns with what the literature highlights on industrial electrochemical practice: large-scale electrochemical processes are often arrays of many cells rather than a single giant cell. In other words, stacks scale in the direction that electrochemistry already “wants” to scale.
To keep this case broadly applicable, I will avoid naming a client molecule and focus on the failure mode. A development team came to us with an anodic oxidation that worked in batch at small scale but generated a persistent impurity that tracked with the amount of supporting electrolyte. Their batch protocol used a wide electrode spacing and relied on high electrolyte loading to keep the cell voltage manageable. Work-up was unpleasant, and the impurity profile was uncomfortable for a route headed toward regulated supply.
We translated the reaction into a microflow chip campaign with a narrow gap and stable temperature control. The initial goal was not yield; it was electrical honesty. With the shorter gap, we could reach the desired current density at a lower cell voltage, and we had headroom to reduce supporting electrolyte without the cell becoming unstable. This followed the same relationship described in the fundamentals paper: reduce gap and you reduce the ohmic penalty, which reduces the need to purchase conductivity with electrolyte.
The second change was transport-focused. In the batch setup, current efficiency collapsed when the team pushed current, because the reaction became mass-transfer-limited at the electrode interface. In the microflow chip, diffusion distances were short and the boundary layer was inherently thinner. By tuning flow rate (and therefore the residence time and mass-transfer regime), we could maintain stable conversion while suppressing the overoxidation pathway that had been feeding the impurity.
By the end of the screening, the electrolyte load was substantially lower, the impurity fell below the reporting threshold, and the route looked like a process step rather than a scientific curiosity. What mattered in the debrief was not “microreactors are better.” The lesson was that the impurity problem was not an intrinsic chemical limitation; it was a geometric and electrical artifact. Once that was understood, the next platform choice—moving into a plate cell for higher throughput—became straightforward: preserve the controlled gap and transport behavior, increase area, and keep the same electrical set points.
