tion of preimplantation genetic diagnosis
remains in a “state of suspended animation”
(12). Professional self-regulation likely
works well when the interested professions reflect a well-established public consensus. But when it comes to empowering
parents to decide what sort of children they
have beyond questions of serious childhood
diseases, professional organizations cannot even agree on the appropriate ethical
framework. Scholars have articulated alternatives: whether parental discretion should
hold near-absolute sway (13); whether a
child should have the prospect of “a decent
chance of a happy life” (14); whether the
welfare of the child-to-be should come first
(15); and whether models emphasizing the
deep interrelationship of parent and child
should be considered (16).
Legislation, regulation, and professional
guidelines depend on widely shared pub-
lic values for their legitimacy. Technologies
affecting reproduction have powerful reso-
nance, as the response to the 23andMe pat-
ent confirms, even as the lives of children and
their parents are affected far more by eco-
nomic opportunity and security, education,
and community resources.
Discussion of the ethics of mitochondrial
manipulation cannot be postponed indefinitely. With little prospect of sensible legislation in the near term, and conflicting guidance
from professional organizations, a national
conversation about current and emerging
technologies shaping the choices that parents will have is urgent. The UK conducted a
similar exercise a decade ago that combined
polling, focus groups, and the Internet (17).
This is a task for the U.S. Presidential Commission for the Study of Bioethical Issues to
pursue, given that its mission is to ensure that
scientific research, health care delivery, and
technological innovation are conducted in a
socially and ethically responsible manner. It
will not be easy to avoid the quicksand of the
abortion debate, but it would be a great public
service to provide a sober assessment of the
choices that would-be parents increasingly
face, and to encourage a respectful dialogue
about the meaning of parenthood and the
worth of a child so that parents and children
can flourish together.
2. A. Wojcick et al., U.S. Patent 8,543,339 (2013).
3. “A 23andMe Patent,” The 23andMe Blog, available from
4. Committee on Ethics, Obstet. Gynecol. 109, 475 (2007).
5. Ethics Committee of the American Society for Reproductive Medicine, Fertil. Steril. 75, 861 (2001).
6. Ethics Committee of the American Society for Reproductive Medicine, Fertil. Steril. 100, 54 (2013).
7. M. E. Fallat et al., Pediatrics 131, 620 (2013).
8. D. W. Bianchi et al., N. Engl. J. Med. 370, 799 (2014).
9. A. R. Gregg et al, J. Am. Coll. Med. Genet. 15, 395
10. J. O. Kitzman et al., Sci. Transl. Med. 6, 137ra76 (2012).
11. S. Baruch, D. Kaufman, K. L. Hudson, Fertil. Steril. 89,
12. K. L. Hudson, Fertil. Steril. 85, 1638 (2006).
13. J. A. Robertson, J. Med. Ethics 29, 213 (2003).
14. B. Steinbock, R. McClamrock, Hastings Cent. Rep. 24, 15
15. J. Glover, Choosing Children: Genes, Disability, and
Design (Clarendon, Oxford/New York, 2007).
16. T. H. Murray, The Worth of a Child (Univ. of California
Press, Berkeley, 1996).
17. Human Fertilisation and Embryology Authority, Sex
Selection: Options for Regulation (2003); available
Batteries keep our devices working throughout the day—that is, they have a high energy density—but they can
take hours to recharge when they run down.
For rapid power delivery and recharging (i.e.,
high power density), electrochemical capacitors known as supercapacitors (1) are used.
One such application is regenerative braking, used to recover power in cars and electric
mass transit vehicles that would otherwise
lose braking energy as heat. However, supercapacitors have low energy density.
Batteries and supercapacitors both rely
on electrochemical processes, although sepa-
rate electrochemical mechanisms determine
their relative energy and power density. Dur-
ing the past 5 to 7 years, the energy storage
field has witnessed a dramatic expansion in
research directed at materials that might com-
bine the high energy density of batteries with
the long cycle life and short charging times
of supercapacitors (2). However, the blurring
of these two electrochemical approaches can
cause confusion and may lead to unwarranted
claims unless careful attention is paid to fun-
damental performance characteristics.
The electrochemical processes occurring
in batteries and supercapacitors give rise to
their different charge-storage properties. In
lithium ion (Li+) batteries, the insertion of Li+
that enables redox reactions in bulk electrode
materials is diffusion-controlled and can be
slow. Supercapacitor devices, also known as
electrical double-layer capacitors (EDLCs),
store charge by adsorption of electrolyte ions
onto the surface of electrode materials (see
the figure, panels A to D). No redox reactions
are required, so the response to changes in
potential without diffusion limitations is rapid
and leads to high power. However, the charge
is confined to the surface, so the energy den-
sity of EDLCs is less than that of batteries (3).
As shown in the figure, panels E to H, super-
capacitors can be distinguished from batter-
ies by both potentiostatic and galvanostatic
methods. The different methods for achieving
double-layer capacitance are characterized
by classic rectangular cyclic voltammograms
(panel E) and a linear time-dependent change
in potential at a constant current (panel G). In
batteries, the cyclic voltammograms are char-
acterized by faradaic redox peaks, often with
rather large voltage separation (greater than
0.1 to 0.2 V) between oxidation and reduc-
tion because of phase transitions (panel F)
(4). The presence of two phases is indicated
by the voltage plateau in galvanostatic experi-
ments (panel H).
In the 1970s, Conway and others recognized that reversible redox reactions occurring at or near the surface of an appropriate electrode material lead to EDLC-like
electrochemical features but the redox processes lead to much greater charge storage
Where Do Batteries End
and Supercapacitors Begin?
Patrice Simon,1 Yury Gogotsi,2 Bruce Dunn,3
Electrochemical measurements can distinguish
between different types of energy storage
materials and their underlying mechanisms.
1Université Paul Sabatier–Toulouse III, CIRIMAT UMR-CNRS
5085, and RS2E, FR CNRS 3459, 118 Route de Narbonne,
31062 Toulouse, France. 2Department of Materials Science
and Engineering and A. J. Drexel Nanomaterials Institute,
Drexel University, Philadelphia, PA 19104, USA. 3
Department of Materials Science and Engineering and California
NanoSystems Institute, University of California, Los Angeles, CA 90095, USA. E-mail: firstname.lastname@example.org;