Perspective—Trends in the Miniaturization of Photosynthetic Power Cell towards Improved Power Density

While still in its early stages, the μPSC holds a lot of potential for a carbon-negative power supply. Based on the literature, the μ-PSC designs can produce open-circuit voltages in the range of 100–993 mV and short circuit currents in hundreds of μA, current densities in the range of 0.2–3000 mA m⁻², and power densities ranging from 0.0004 to 715 mW m⁻². ^{8,16,23,34,41–43} Thus far, the maximum power reported of a single μPSC device reported is 715 mW m⁻², ⁴² which involved the use of gold nanoparticles in the anode chamber to enhance photon absorption as well as electron transport from the microorganism (algae - Chlamydomonas reinhardtii) to the anode surface. Although this represents a significant advancement for micro-biological solar cells generally, considerable work remains to be done to fulfill commercial requirements. For more information on the development of the μPSC technology, readers are referred to Refs. 4, 8, 42–45.

Some common parameters for quantifying the performance of the μPSC include, open-circuit voltage (OCV), current density (CD), power (P), power density (PD), polarization and power curves, quantum yield, and Coulombic efficiency, ${\varepsilon }_{C}.$ ^5,24 The OCV quantifies is the maximum potential obtainable from the cell, measured from the terminals of the cell in the absence of an external load or electric current - as opposed to the maximum potential attainable in the cell (i.e., the ideal voltage). The ideal voltage (~1.2 V in an air-cathode MFC: 0.805 V from the cathode and −0.42 V from the anode ⁵ ), which corresponds to the cell emf (${V}_{emf}$)/Nernst reversible voltage (NRV), comes in the absence of internal losses (such as ohmic, concentration and activation losses). The current divided by the total electrode area gives the CD. It reflects the inherent rates of charge transfer between analytes and the electrode, providing insights into the structure and interrelation of the analyte and the current collector. Measured cell voltage ${V}_{cell}$ is often a linear function of the current, and simply described as $OCV\,=\,{V}_{cell}\,-\,I{R}_{int},$ where $I{R}_{int}$ is the summation of all internal losses of the μ-PSC, proportional to the system's internal resistance ${R}_{int}$ and generated current $I.$ Power is estimated $I{V}_{cell};$ the voltage is taken across a fixed external resistor ${R}_{ext},$ while the current $I$ is estimated from ${V}_{cell}/{R}_{ext}.$ Hence, power is estimated normally as ${V}_{cell}^{2}/{R}_{ext}.$ The PD is useful for comparing the power yield of different systems. Power is normalized to a characteristic feature of the generating system, usually the effective anode surface area, as it is where the biological reactions take place. ^4,8,45 Further discussions on the PD will be presented in the next section. The Coulombic efficiency basically describes the efficiency of the system in harnessing electrons from the photosynthetic organisms; in terms of the ratio of electrons recovered to the amount available in the system if all the microorganisms produced current. ⁴⁶

The PD is basically a measure of the power generated by a device divided by the size of the device. At the macrolevel, it presents a straightforward approach to compare any power product towards efficiency enhancement as well as for thermal considerations. In the BES settings, it quantifies the performance in terms of power output per electrode size (active surface area, in particular), with a common unit of measurement of PD in milliwatts per meter square (mA/m²). Hence, one simple approach to increasing PD is reducing component size (that is, miniaturization). It principally provides the benefit of higher surface-to-volume ratio, hence, heat—a critical efficiency parameter of any electrical/mechanical system—is dissipated faster than stored; and a system that releases energy quickly can also recharge quickly. In addition to this, there is also the advantage of component integration. Energy supply from components small enough to fit inside a cell phone or camera flash, for instance, and sufficient to power the device makes an ideal high-power density system. Similarly, in BECs, in addition to the benefits of reduced anolyte volume and shorter charge transport length scales, the thermal performance of the system is enhanced by the dominance of heat dissipation over heat storage, resulting in greater efficiency, measurable through the PD. This is highlighted by our assessment of the several types of PSC designs that have been reported during the past two decades, which is summarized in Fig. 2. For ease of classification and a fair comparative basis, devices that implemented external performance boosters like gold or silver nanoparticles, as in Refs. 42, 47, and the like are left out.

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As earlier indicated, the main goal of this work is to examine the relationship between size reduction and power output from the several different uPSC designs that have been reported during the past two decades. In essence, miniaturization enhances the physical proximity of the biocatalyst organisms to the anode surface, shortening the length scale of charge transfer and reducing resistive loss in the electrolyte as a result. By contrast, in macro BECs, where charges must travel longer distances, factors including ohmic polarization, reaction kinetics, and concentration (mass transport) polarization are known to have significant limiting impacts on power outputs. ^4,22

Figure 2 presents an overview of the Power Densities of intrinsic μPSCs (that is, with no external energy booster) from recent literature data (details in supplementary Table A). While other factors such as biocatalyst agents' photosynthetic capacity, electrode material, light intensity and so on all play a role in the device's electrical output, increasing the surface-to-volume ratio of the working unit through miniaturization offers great prospects for optimizing the overall performance of the μPSC technology. There are also instances where extrinsic energy inputs have been used to obtain significant power density boosts. ^42,43,48 Nevertheless, a comprehensive review of the literature, summarized in Fig. 2, reveals a noteworthy connection between miniaturization (measured in terms of anode surface area) and gains in power density. Clearly, most of the reported works have shown lower power density with higher anode area (highlighted by the rectangular box in the figure). The green ellipse, on the other hand, demonstrates the higher power densities amongst all the listed μPSCs. The studies, in other words, demonstrated relatively optimum anode surface areas. This in all emphasizes the need for optimal anode area design in order to increase the PSC's power density.

Subsequently, to push the μPSC technology to levels suitable for real-world applications, two or more units of the μPSC device can be arrayed in diverse configurations to obtain an optimized total operating voltage and current, sufficient to meet the demands of low-power devices. ⁴⁹ The various array configurations and their power output have been investigated and it was suggested to design the array configurations based on the power requirement. To this end, the beauty of miniaturization, which offers a huge promise of power enhancement through optimal work area design and the combining of many small units, becomes even more tangible in our quest to push green energy to the greatest heights.